I. INTRODUCTION

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There is general agreement that the relatively high prevalence of hypertension (~15-20%) in most developed countries has a significant impact on the risk of cardiovascular, renal, and other diseases. According to World Health Organization statistics, cardiovascular diseases remain the major cause of death in all developed countries. The understanding of early changes preceding clinical manifestations of hypertension or of other cardiovascular diseases and the knowledge of how cardiovascular risk factors change in the course of time are essential for improving the prevention measures. Although the full manifestation of severe cardiovascular disturbances usually occurs in adulthood and/or senescence, the roots of several polygenic cardiovascular diseases can be traced back to early ontogeny (26, 416, 427).

This review summarizes contemporary information on early abnormalities in the structural and functional development of the cardiovascular system (including their neurohumoral control) and their impact on the subsequent development and maintenance of genetic hypertension. Alterations in neurohumoral regulatory mechanisms [sympathetic nervous system (SNS), renin-angiotensin system (RAS), and endothelium-derived vasoactive factors], cellular growth control, steroid action, lipid metabolism, cell membrane function (ion transport, signal transduction, and cell Ca²⁺ handling) belong to the most probable candidates for the "primary defects" in the pathogenesis of hypertension. Our attention is focused on the rat as a principal model for the study of ontogenetic aspects of hypertension development. The main issue is to clarify whether the defined stimuli acting in the respective critical periods (developmental windows) can alter the further fate of predisposed individuals. Cardiovascular abnormalities might thus be considered as late consequences of altered development of the cardiovascular apparatus that was affected by certain stimuli operating in earlier stages of ontogeny. There are two seemingly contrasting approaches for how to investigate this hypothesis. Hypertension development in the genetically predisposed organism can be attenuated or prevented by interventions in particular ontogenetic stages (see sects. VI and VII) or the hypertensive process can be induced in the normotensive organism by appropriate stimuli applied in corresponding developmental periods (see sect. IX). Both approaches might well represent the two sides of the same coin.

	II. RAT AS A MODEL FOR STUDYING DEVELOPMENTAL ASPECTS OF HYPERTENSION

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In both rats and humans, the development of hypertension and its subsequent complications take place in characteristic ontogenetic periods. An effective study of particular cardiovascular alterations emerging in the course of the developmental process (from their early onset until their late consequences) requires the use of adequate animal models. They should be genetically well-defined, accessible for techniques used in cardiovascular physiology, and economically acceptable; in addition, their life span must be short enough compared with that of the investigators. Indeed, a major part of the experimental hypertension research has been carried out in rodents, namely, in the rat (197), which is very convenient for the study of cardiovascular physiology. Although the mouse seems to be a more appropriate tool for genetic manipulations (29, 106, 668), its small size precluded physiological measurements for a long time. The information about the rat genome is still not extensive enough, but multiple inbred rat strains with genetic hypertension (192, 758) represent valuable material for modern genetic analysis ranging from gene-phenotype relationships over quantitative trait loci cosegregation to congenic or transgenic strain construction (286, 591, 611, 637).

The detailed knowledge of numerous aspects of developmental physiology in the rat together with the possibility to induce or to prevent the hypertensive process by specific interventions in particular stages of ontogeny make this animal species a useful tool for hypertension research. In the rat, a wide spectrum of experimental hypertension models is available that differ in the contribution of genetic and environmental factors to the elevation of blood pressure (BP). In some cases, e.g., salt-sensitive Dahl or Sabra rats, the developmental interaction of both the above components is of fundamental importance for hypertension development.

The worldwide availability of spontaneously hypertensive rats (SHR) has made it possible not only to identify numerous cardiovascular abnormalities in this model but also 1) to estimate the degree of their genetic determination (see sect. IV), 2) to study their ontogenetic aspects in detail (see sect. V), and 3) to evaluate their role in the pathogenesis of hypertension by means of various interventions (see sects. VI and VII). Depending on the age at which such interventions were initiated and on their duration, research could not only be focused on the therapy of established hypertension but especially on its prevention. Sophisticated attempts to modify the natural course of hypertension development by various transient interventions (pharmacological, nutritional, etc.) have allowed the identification of well-defined periods of remarkable sensitivity of the developing organism to a wide spectrum of exogenous or endogenous factors that can attenuate or accelerate further development of hypertension. An example of this is the long-term attenuation of hypertension development observed in SHR subjected to early transient antihypertensive treatment (7, 70, 263, 472, 600).

The results obtained in SHR represent a major part of our knowledge on genetic hypertension (>90% of papers available in this field). Altogether, SHR studies offer such a complexity of information that could never have been attained in any other strain of genetically hypertensive rats. The frequent use of SHR can be criticized for reasons ranging from the heterogeneity of various breeding colonies (394, 516, 621) to inadequate use of normotensive control strains (588, 678) and to the inappropriateness of SHR as a model of human essential hypertension (139, 477). Nevertheless, the rational use of this hypertensive model provides a lot of valuable information. Genetically hypertensive rats, which are often used for targeting specific hypertensive mechanisms by distinct classes of antihypertensive drugs (successfully employed in the treatment of essential hypertension), might become a valuable tool in the search for specific age periods in which transient drug treatment could prevent abnormal cardiovascular development.

Particular forms of experimental hypertension in the rat represent long-term alterations in the development of the cardiovascular system and its regulations. Such alterations are highly dependent on the genotype, but they can be modified by various environmental factors (nutrition, stress, or exercise) applied during specific ontogenetic periods of increased susceptibility to their action (see sect. III). To a certain extent, this must also be true for human essential hypertension, which is a polygenic disease with an important genetic component, but its progress can also be influenced by some of the above-mentioned environmental risk factors. One of the main drawbacks of past research on essential hypertension was the fact that a considerable effort had been concentrated on the effects of particular risk factors in adults with established hypertension who had already passed the presumed critical periods (developmental windows) of high susceptibility.

	III. LATE CARDIOVASCULAR EFFECTS OF INTERVENTIONS IN CRITICAL DEVELOPMENTAL PERIODS

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If cardiovascular diseases were considered as late consequences of abnormal cardiovascular development (Fig. 1), the adequate research strategy should be based on 1) detailed knowledge of cardiovascular changes occurring in particular stages of ontogeny, 2) precise definition of critical age periods characterized by increased susceptibility of the developing organism to the action of defined endogenous factors and/or exogenous stimuli, and 3) recognition of differences in the mechanisms by which the organism responds to a given stimulus during or beyond the respective critical period, leading to either permanent transformation or only transient modification of the cardiovascular phenotype.

View larger version (113K): [in this window] [in a new window]   Fig. 1. Gene-environmental interaction during blood pressure ontogeny and hypertension development. E1En represent environmental stimuli affecting expression of genetic information (G1Gn) occurring in particular critical periods (developmental windows).

The rat has become an invaluable species for studying the developmental aspects of hypertension because its distinct developmental periods are relatively brief and well-defined compared with most other laboratory animal species. The classification of particular developmental periods in the life of the rat is based on essential changes in the mother-offspring relationship, nutrition and digestion, water and electrolyte metabolism, and gonadal activity (for review, see Refs. 379, 380, 790).

The intrauterine (prenatal) period varies from 21 to 23 days and is terminated by birth, when the fetal circulation, nutrition, and other physiological functions are profoundly rearranged. In the rat, postnatal life begins with a 14-day period of milk consumption as the only source of water and nutrients, i.e., the suckling period. At the end of the second postnatal week, rat pups open their eyes, change their thermoregulation, begin to move actively, start to consume solid food, and later to drink water. Spontaneous milk consumption decreases up to the age of 28 days in parallel with changes in the nutritional pattern. The third and fourth postnatal weeks represent the weaning period in which pups gradually become independent of their mothers. It should be noted that some controversial results obtained in animals from various colonies could result from the different duration of the mother-pup interaction in this developmental period because some breeders wean pups prematurely, i.e., at the age of 19-23 days. Thereafter, weanling rats proceed through prepuberty, which lasts ~2 wk and is followed by a period of sexual maturation (puberty), that begins around the age of 45 days and is terminated in animals aged 60-70 days. Qualitative changes in the structure and function of particular systems seen during maturation are followed by quantitative changes occurring in adulthood, since body growth of the rat proceeds throughout its whole life span. In contrast to the relatively precise definition of the above developmental periods, the onset of senescence in the rat is not yet clearly defined. Rats older than 24 mo can be regarded as aged, although there are differences in longevity between various rat strains (202).

Each developmental period is associated with characteristic transformation of the cardiovascular system. The development of the cardiovascular system represents a dynamic sequence of events leading to the expression of genomic information responsible for the structure, function, and regulation of particular regions of the cardiovascular apparatus.

The morphogenesis of heart and vessels occurs mainly during the prenatal period, but it continues partially even after birth. Extensive hyperplasia of the rat heart, which is typical for the fetal period, still persists shortly after birth. In this period, the heart grows more rapidly than the body so that the heart weight-to-body weight ratio attains its maximum at 4-5 days of postnatal life and then declines (110). Despite the diminished cellular division, the number of cardiomyocytes in the heart doubles during the first 3-4 wk of life. The diameter of rat cardiomyocytes increases from ~5-6 µm at birth to 14-18 µm in adulthood (391, 773). The postnatal increase of work load and metabolic demands leads to cardiac enlargement mainly by cell hypertrophy, although the increased growth rate of rat pups from reduced litters fed by a single mother is accompanied by enhanced proliferation of cardiomyocytes (587).

Despite the fact that conduit arteries already achieve their adult number of smooth muscle layers during embryonic development (743), the postnatal thickening of the media is due to the intensive production of connective tissue, cell proliferation, and hypertrophy (450, 545). In the rat, suckling and weaning periods represent the time of intensive maturation of vascular structure (240) and its sympathetic innervation (639), which develops concomitantly with the major ontogenetic changes of the SNS (666, 776) and the RAS (324, 576), both of which are involved in the regulation of growth and proliferation of vascular smooth muscle cells.

In the suckling period, rat dams can influence the development of the cardiovascular phenotype of their pups not only by behavioral interactions and milk composition but also by eliciting the characteristic cardiovascular response, i.e., BP elevation and heart rate acceleration, which occurs in the pups in response to maternal milk delivery (515).

Prepuberty and sexual maturation represent the stage when profound readjustment of central hemodynamics occurs in the rat (9). High cardiac output and low systemic resistance (per unit of body mass) are characteristic hemodynamic features of weanling rats (584). The subsequent developmental rise of systemic resistance, which is accompanied by the decrease of cardiac output, results in the mature pattern of hemodynamic mechanisms maintaining BP in adults (8, 667). The structural resetting of resistance vessels is responsible for the major part of the increase in vascular resistance and reactivity (1, 201), whereas the developmental changes in vascular sensitivity to vasoactive agents are less important for the maturational rise in systemic resistance. The intense maturation of the baroreceptor reflex also occurs in these developmental periods (10, 680).

Considerable cardiovascular changes accompany senescence in both humans and animals (for reviews on cardiovascular aging, see Refs. 159, 202, 463). The most prominent senescent alterations concern the structure and function of arteries that accumulate collagen and become less elastic (450, 742). Higher vascular stiffness leads to a further BP increase and might also affect baroreceptor sensitivity. Furthermore, the balance in the production of endothelium-derived relaxing and constricting factors is substantially altered in senescent animals (670, 705).

Detailed knowledge of the ontogenetic changes occurring in the cardiovascular system during particular critical periods (developmental windows) cannot only facilitate the search for primary genetic defects (responsible for subsequent development of high BP) but also the effective targeting on discrete hypertensive mechanisms in appropriate stages of ontogeny. There are numerous examples that particular stimuli influencing the organism during relatively short critical periods of its early development might result in long-term permanent alterations that often become apparent in adulthood only.

A low protein intake of pregnant rats is associated with increased BP of their offspring (401). Reduced placental activity of 11-hydroxysteroid dehydrogenase (11-HSD), an enzyme metabolizing maternal glucocorticoids, increases the access of these steroids into the fetus of rats fed low-protein diets during pregnancy (411). This indicates that glucocorticoids might participate in programming maternal diet-induced hypertension in the rat (405). Indeed, this type of hypertension can be prevented by a pharmacological blockade of maternal glucocorticoid synthesis (403). Furthermore, fetal exposure to exogenous glucocorticoids during the last trimester of gestation initiates the development of hypertension manifested in adult rats (431). Similarly, the inhibition of placental 11-HSD activity by carbenoxolone in pregnant rats with normal protein intake elevates BP of their adult offspring (404, 446).

Another example is altered cholesterol metabolism in adult animals that is partially dependent on the nutrition during early postnatal life (for reviews, see Refs. 246, 308). Maternal milk represents a high-fat nutrition for rat pups, which is gradually exchanged for a high-carbohydrate diet in the course of weaning period, i.e., between the 15th and 28th day of postnatal life (21). When this stepwise dietary change occurs abruptly as a part of premature weaning, e.g., at the age of 16-18 days after birth, plasma cholesterol falls rapidly in weanling rats (248). The persistent changes in the activity of some liver and gut enzymes involved in cholesterol metabolism (252), which are induced in the weaning period, explain why in adulthood plasma cholesterol levels of prematurely weaned rats are dependent on the diet consumed between postnatal days 15 and 30. Lowest plasma cholesterol was found in animals weaned to a high-fat diet, whereas highest values were observed in rats weaned to a high-carbohydrate diet (250, 251). Plasma cholesterol of adult rats (including the response to high-fat or atherogenic diets) could be modified not only by premature weaning to different diets (249) but also by overfeeding of pups following litter size reduction (247) as well as by an increased dietary fat intake before the natural weaning occurring at the end of the first postnatal month of life (111). This dependence of adult metabolic responsiveness on early nutritional experience might explain the variety of differences between particular breeding colonies and laboratories performing hypertension research.

The major importance of critical periods for the later BP rise is namely indicated by the research of salt-dependent forms of experimental hypertension (for details, see sect. IX). Salt intake can be increased in rats of various ages (ranging from weaning period up to senescence), but the most pronounced BP increase is elicited when high salt intake begins in prepuberty (790). It is evident that the BP response to this hypertensive stimulus might involve only those mechanisms that are available at a given stage of development in which the stimulus is applied. Moreover, this stimulus represents a different load for immature and adult organisms. Consequently, the activation of distinct pathogenetic mechanisms in young and adult animals exposed to such environmental stimuli could elicit completely different long-term cardiovascular effects (322, 381, 790). The greater response and severe late consequences induced in immature animals can be ascribed to the obvious plasticity and/or vulnerability of the developing organism that is prone to considerable adaptations during early ontogeny.

We assume that the same principles are also true for the development of genetic hypertension. Primary genetic defects lead to the full manifestation of cardiovascular diseases in adulthood or senescence through a cascade of intermediate phenotypes (47). The onset of particular abnormal intermediate phenotypes seems to be confined to distinct developmental periods. The expression of altered genetic information must apparently occur at a very early stage of development because a moderate BP elevation has already been detected in SHR shortly before birth (19). Gray (240) therefore suggested that some primary abnormalities should be present even in fetal SHR. Such early abnormalities (e.g., sympathetic hyperactivity or membrane defects) might serve as important stimuli for subsequent abnormal development of the cardiovascular system and/or its regulation. Their possible correction by interventions in early critical periods should normalize cardiovascular development and prevent the development of genetic hypertension (for details, see sect. VI).

It is evident that the adult cardiovascular phenotype results from the complex interactions between developmental processes and environmental factors during critical developmental periods. Both the intensity of developmental processes and the probability of their modification by environmental factors decrease with progressing age. It is therefore not surprising that cardiovascular abnormalities usually originate from alterations occurring during early stages of ontogeny.

	IV. CARDIOVASCULAR ABNORMALITIES LINKED TO GENETIC HYPERTENSION

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Up to now, hundreds of abnormalities have been revealed in genetically hypertensive rats by comparing hypertensive and normotensive strains. It is, however, evident that the majority of such differences plays a minor role, if any, in the pathogenesis of hypertension. In the last decade, a wide spectrum of genetic methods has been used to elucidate this problem. More than 200 papers concerned the relationship of candidate genes to particular cardiovascular abnormalities as well as the cosegregation of BP with various quantitative traits. F₂ hybrid and back-cross populations together with recombinant inbred and congenic strains represent the principal tools for such analysis (285, 637). Despite the tremendous progress in molecular genetics and genetic analysis, the dissection of particular genetic components of complex cardiovascular traits, such as hypertension, is still difficult because of multiple gene-gene and gene-environment interactions (611, 635).

Table 1 shows some quantitative traits or candidate genes that have been found to be linked to BP in segregating populations. Although this list might focus our attention on some important mechanisms, the results of genetic analysis cannot be used as the sole indicative criterion. There are several reasons why both positive and negative results should be considered with caution.

The adult phenotype (e.g., BP level) is a result of multiple interactions of genetic and environmental factors occurring during ontogeny (Fig. 1). The expression of genetic information during particular developmental windows can be substantially affected by specific environmental stimuli that may influence the organism in corresponding critical ontogenetic periods. If this concept is true for hypertension development, it must also be valid for the above-mentioned genetic studies. Indeed, the degree of genetic determination of BP decreases with advancing age of animals (304). Moreover, distinct chromosomal loci might have essentially different BP effects in the very same set of F₂ hybrids when BP was determined in animals of various age (619).

Thus the age of examined F₂ or back-cross animals might be of crucial importance for the outcome of genetic analysis because significant linkage of certain quantitative traits can be found in particular developmental periods only. A classical example of this is altered renal blood flow and glomerular filtration rate, which cosegregated with BP in F₂ rats aged 11 wk, but this was not found at the age of 4 or 16 wk (258). This observation suggests that transient changes appearing at particular age periods might be essential for subsequent BP development. Indeed, the diameter of distal afferent arterioles found in 7-wk-old F₂ rats correlated inversely with their BP at the age of 23 wk, but media thickness or media cross-sectional area of renal arterioles was no phenotypic predictor of hypertension development (534). On the contrary, the media-to-lumen ratio (M/L) of mesenteric resistance vessels or left ventricular mass of 9-wk-old F₂ rats had no significant relationship to BP of 20-wk-old animals (259), although both the mesenteric M/L (due to changes in media thickness) and relative heart weight cosegregated with BP in adult F₂ rats (501). Similarly, there was no relationship between relative heart weight of newborns and BP of adult animals of the Prague recombinant inbred strains (158). To our knowledge, significant cosegregation of relative heart weight with BP was never reported in animals younger than 16 wk (238, 390, 501, 777), but it was often demonstrated in older F₂ rats in which heart hypertrophy could already develop after a sufficiently long exposure to elevated BP (163, 238, 255, 501, 721). It seems that cardiovascular hypertrophy develops in parallel with the hypertensive process but is not the primary event in the pathogenesis of genetic hypertension as might be concluded from cosegregations reported in adult animals.

Environmental conditions, under which the studied F₂ animals are reared, often play a permissive role for the demonstration of genetic linkage. The most typical example is the high salt intake that is sometimes used to augment BP elevation (107, 445, 520). For example, the angiotensin-converting enzyme (ACE) genotype cosegregated with systolic BP only after salt loading of adult F₂ rats (287, 520). The same was true for the associations of relative heart weight with diastolic BP (163) or of the angiotensinogen genotype with pulse pressure (449). On the other hand, the cosegregation of the 27-kDa heat shock protein genotype with relative heart weight (255) or of the renin genotype with adrenal renin mRNA (314) were no longer significant after salt loading. Furthermore, substantial differences may exist between animals subjected to a relatively short-term increase of salt intake in adulthood [the above-mentioned genetic studies in SHR or stroke-prone SHR (SHRSP)] and those that were raised on a high-salt diet since weaning (all studies on Dahl rat in Table 1). Generally speaking, the age-dependent interactions of nutritional or other environmental factors with BP development have almost completely been ignored in the genetic analysis of hypertension.

Further problems, complicating the evaluation of the importance of candidate genes for the pathogenesis of cardiovascular abnormalities exclusively on the basis of genetic analysis, concern the influence of genetic background on the phenotypic expression of particular genes. The same genetic defect might have quite opposite phenotypic effects that depend on the second progenitor strain used for generation of a given F₂ population. Consequently, numerous genetic studies with salt-sensitive (SS/Jr) Dahl rats demonstrated a wide range of BP effects for SS/Jr alleles of the renin gene (590), ACE gene (140), atrial natriuretic peptide (ANP) receptor gene (140), inducible nitric oxide (NO) synthase gene (137), and the endothelin (ET) system genes (135). Thus the absence of cosegregation might also be due to the inappropriate choice of F₂ cross used for linkage analysis.

It is becoming clear that the future progress of hypertension research requires a combination of the genetic analysis with ontogenetic and intervention studies. This would facilitate 1) search for specific cardiovascular abnormalities in developing rats predisposed to genetic hypertension, 2) detailed characterization of corresponding pathogenetic mechanisms, and 3) verification of their importance for high BP development by means of the selective interference with particular system(s) in corresponding critical developmental periods.

	V. MATURATION OF CARDIOVASCULAR PHENOTYPES IN GENETIC HYPERTENSION

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The exact role of particular cardiovascular intermediate phenotypes genetically linked to BP (Table 1) can be elucidated according to their appearance during the hypertensive process, as well as on the basis of their response to defined antihypertensive treatment and/or targeting of specific hypertensive mechanisms. The same approach can be used for the evaluation of multiple candidate genes, the products of which seem to be associated with numerous neurohumoral dysregulations described in rats with genetic hypertension. Despite the substantial effort in the last two decades, it is still difficult to make a precise definition of the primary defect (genetic linkage to BP, early appearance before hypertension development, relevance to BP regulation, and BP modification by specific targeting of the defect) and to demonstrate all its attributes for any of the abnormalities investigated. The polygenic nature of genetic hypertension, the existence of multiple systems substituting each other in the control of particular phenotypes, and the difficult dissociation of primary events from the consequences of elevated BP often preclude the possibility to draw clear-cut conclusions concerning the actual importance of particular cardiovascular alterations. However, our selection of principal abnormalities of the developing cardiovascular system in genetically hypertensive rats has largely adhered to the above-mentioned criteria for primary defects.

A.  Functional and Structural Alterations

1.  BP

The early postnatal BP increase is very rapid even in normotensive rats, in which the mean arterial pressure of newborns is ~15-25 mmHg and it progressively increases to 80-95 mmHg within the first 3-4 postnatal week (241, 399, 447). Although the BP of SHR newborns was usually found to be significantly higher compared with that of Wistar-Kyoto rats (WKY) (67, 109, 241), characteristic acceleration of the BP rise in SHR mainly occurs between the 3rd and 10th week of age when their BP rapidly increases by ~30% above that of WKY. The BP of WKY reaches adult levels by ~10 wk of age, but in SHR, it continues to rise at least until the age of 20 wk (Fig. 2). One of the aims of our review is to document that BP development in SHR can be modified especially by the stimuli (including antihypertensive treatment), influencing the animals during the phase of accelerated BP rise that seems to coincide with the period of augmented pressure-dependent vascular hypertrophy (see also sect. VI).

View larger version (20K): [in this window] [in a new window]   Fig. 2. Ontogeny of mean arterial blood pressure (MAP), cardiac output (CO), and total peripheral resistance (TPR) in spontaneously hypertensive rats (SHR) [expressed as percentages of Wistar-Kyoto rat (WKY) controls]. (Data adapted from References 8, 584, 667, 756.)

Vascular resistance seems to be reduced in newborn and 2-wk-old SHR compared with age-matched WKY (348). At the age of 4-6 wk, the unchanged systemic resistance in young SHR is accompanied by increased cardiac output (186, 454, 667), while the progression of hypertension is associated with elevated systemic resistance and almost unchanged cardiac output (Fig. 2). The early increase of cardiac output is not a necessary prerequisite for the later BP rise because the chronic -blockade from conception until the age of 12 wk does not prevent hypertension development in SHR (572).

The elevation of peripheral vascular resistance in primary hypertension is partially caused by the decrease of arteriolar lumen diameter due to media thickening or remodeling. In animals with genetic hypertension, the hypertrophy of blood vessel walls parallels the developmental rise in BP and systemic resistance, which is maximal between the 3rd and 10th week of age. On the other hand, the relative reduction of arteriolar lumen size is almost age independent (Figs. 2 and 3). Arteriolar rarefaction is usually absent in young SHR and develops later (56, 583). These structural alterations of resistance vessels are reflected by increases in both minimal vascular resistance (representing diminished luminal cross-sectional area of the resistance vessels at complete relaxation) and maximal vascular contraction (representing active smooth muscle contractile contribution).

View larger version (25K): [in this window] [in a new window]   Fig. 3. Development of systolic blood pressure (BP) and changes of structural and functional properties of mesenteric resistance vessels (M, media thickness; L, lumen diameter; M/L, media-to-lumen ratio; AWT, active wall tension) in SHR (expressed as percentages of WKY controls). (Data adapted from References 327, 328, 503, 504, 726, 727.)

It is difficult to estimate the exact role of increased BP in the onset of structural alterations of blood vessels. Although neither BP elevation nor vascular alterations could be demonstrated in some SHR breeding colonies before the age of 4 wk (399, 418, 423), other investigators reported significant vascular changes in fetal (177) and newborn SHR (239, 348). If the BP of SHR were really increased before birth (19, 241), higher wall stress in utero might be the stimulus for altered development of the arterial wall (309). Increased M/L was repeatedly found in different vascular beds of SHR during the first postnatal month (177, 532, 546), together with a higher number of smooth muscle cell layers (418). Media hypertrophy in young SHR can be ascribed to increased smooth muscle cell size and greater amounts of connective tissue (726). The increased elastin content in the aortic wall of developing SHR becomes manifest as an increased number and/or greater thickness of elastic laminae (309, 546). Altered biochemical composition of the vessel wall (increased elastin and collagen) (141) and vascular wall hypertrophy also play an important role in hypertension-induced changes of wall distensibility. During the early phases of hypertension development, wall distensibility might even be augmented in SHR (348, 774), but long-term hypertension decreases arterial distensibility (122, 243, 774).

Functional changes of blood vessels based on structural alterations (increased slope and higher maximum of contraction response, reduced wall distensibility) were already detected in young SHR aged 4-5 wk (201, 372, 500, 601), and they develop in parallel with the progression of hypertension and associated structural resetting of the vasculature (197, 375, 502). On the other hand, the increased sensitivity of vascular contraction to extracellular Ca²⁺ and the higher vascular sensitivity to norepinephrine (NE) (unmasked by cocaine inhibition of NE uptake) are characteristic intrinsic abnormalities of SHR vasculature since prehypertensive stages, but they do not change with advancing age (503, 504).

Early structural changes of the vasculature have sometimes been attributed to primary, pressure-independent genetically determined factors, e.g., increased noradrenergic innervation seen in prehypertensive SHR (5, 277, 418). One of the main arguments for this opinion was that the treatment of SHR dams and their pups with the vasodilator hydralazine prevented BP elevation, but SHR still showed hypertrophy in the mesenteric vascular bed and became rapidly hypertensive after drug withdrawal (663). It is well known that hydralazine treatment augments the activity of both SNS and RAS. In fact, the treatment of SHR dams and pups with the ACE inhibitor captopril completely normalized not only BP but also the media cross-sectional area in the aorta and renal arteries (283). Similarly, neonatal sympathectomy prevented the development of hypertension and substantially reduced vascular alterations in SHR (200, 374, 420).

Further arguments for the contribution of elevated BP to vascular alterations were obtained in experiments with the protection of regional circulation against high BP. The partial constriction of the femoral artery in immature rats aged 3-5 wk, which had normalized local transmural pressure, restored the altered structural and functional development of femoral resistance vessels in SHR (69, 199). It remains to be ascertained whether early regional hypotension can also abolish other vascular alterations found in young SHR, such as diminished -adrenoceptor-mediated relaxation (210) or augmented myogenic tone due to increased Ca²⁺ influx through L-type channels (14) and/or low Ca²⁺ sequestration in the sarcoplasmic reticulum (371).

Myogenic tone of small resistance vessels is increased in immature SHR but not in adult animals with established hypertension (320). However, myogenic tone of young SHR is not significantly enhanced within the physiological range of distending pressures. It should also be noted that the reduction of lumen diameter (clearly demonstrated in SHR arteries under passive conditions) disappears in the presence of myogenic tone (319). Under isobaric conditions, when the vessels are studied at physiological distending pressures, increased wall-to-lumen ratio is not accompanied by enhanced contractility of distal mesenteric arteries even in adult SHR with established hypertension and pronounced vascular structural alterations (319). Thus further detailed in vitro experiments, which will respect the in vivo pressure conditions, are required to clarify the exact contribution of structural and functional abnormalities of resistance vessels to BP rise occurring during the development of genetic hypertension.

It is evident that in rats with developing genetic hypertension there is a parallel increase of systemic resistance and vascular wall hypertrophy that is most intensive during the rapid phase of BP elevation, i.e., at the age of 3-10 wk. We suggest that this age period might be of fundamental importance for further hypertension development because the vicious circle between increasing distending pressure and progressing hypertrophy of the arteriolar wall is established in this particular developmental window. The attenuation of both pressure-induced and humorally mediated components of vascular smooth muscle hypertrophy by the transient antihypertensive treatment in this juvenile critical period postpones the subsequent BP rise, ameliorates the severity of hypertension, and lowers the incidence of various organ complications (for details, see sect. VI).

The importance of the kidney in the development and maintenance of genetic hypertension is obvious. This can best be documented by BP effects of renal cross-transplantation between normotensive and hypertensive strains because the genetic predisposition to hypertension can be transferred with the donor kidney to the recipient. This was demonstrated not only in adult rats with established hypertension (49, 126, 350, 597) but also in young prehypertensive animals (127, 204, 373, 595). Although the influence of hypertensive renal damage should always be considered after transplantation of adult kidney (596), several structural and/or functional abnormalities in the immature kidney of genetically hypertensive rats have been proposed to play a primary role in the pathogenesis of hypertension. They range from altered glomerular hemodynamics to abnormal regulation of renal Na⁺ transport due to possible membrane defects.

Lower Na⁺ and water excretion were indeed reported in Milan hypertensive rats (MHS) or SHR aged 3-7 wk. This abnormality is most important in the weaning period and prepuberty because, as the BP rises during sexual maturation (8-9 wk of age), urinary Na⁺ excretion reaches values found in normotensive controls (30, 46, 517). It has been proposed that abnormal properties of the Na⁺-K⁺-2Cl cotransport system in the ascending limb of Henle's loop might represent the underlying hypertensive mechanism transferred with the MHS kidney into a normotensive recipient (47). Analogous alterations of this transport system in MHS erythrocytes also persist after bone marrow cross-transplantation (48), indicating that basic alterations of the Na⁺-K⁺-2Cl cotransport system in MHS are fully genetically determined. The common denominator of ion transport alterations in MHS seems to be point mutation in -adducin, which is one of cytoskeleton membrane proteins (50). It should be noted that both Na⁺-K⁺-2Cl cotransport activity (48) and -adducin genotype (50) cosegregate with BP in F₂ hybrids. The presence of membrane defect could explain why a transplanted kidney retains its functional properties despite the opposite phenotypic properties of the recipient.

The dopaminergic control of renal Na⁺ handling is also altered during the development of genetic hypertension. Impaired transduction of the renal dopamine D₁ receptor signal has been proposed to be responsible for the diminished natriuretic response to acute volume expansion in SHR (86). In contrast to WKY, dopamine fails to inhibit the renal tubular Na⁺-K⁺ pump and Na⁺/H⁺ exchanger in SHR (85, 225) due to impaired stimulation of adenylyl cyclase, phospholipase C, and protein kinase C (87, 341, 361). D₁ receptor-mediated natriuresis is also defective in salt-sensitive Dahl rats (256), which have a similar defect of dopaminergic control of the Na⁺-K⁺ pump (528) and D₁ receptor/adenylyl cyclase coupling (542) as SHR. A decreased ability of dopamine and/or D₁ agonists to stimulate D₁ receptors in proximal tubules of SHR was already demonstrated at the age of 3 wk (361). As the stimulation of adenylyl cyclase activity through D₁ receptors rises with age in normotensive but not in hypertensive rats, the coupling defect becomes more prominent in adult SHR (190). Recently, the importance of altered D₁ receptor coupling for the pathogenesis of genetic hypertension has been confirmed in F₂ hybrids of SHR and WKY (6).

On the other hand, at least in immature SHR, the impaired Na⁺ and water excretion results from reduced glomerular filtration rather than from enhanced proximal tubular reabsorption (148). Elevated renal vascular resistance in young SHR is predominantly due to the increased resistance of preglomerular vessels (148, 234) that can be partially normalized by early antihypertensive treatment (346). At the age of 4-6 wk, the diameter of afferent arterioles is smaller in SHR than in WKY and undergoes even further reduction in SHR aged 18-20 wk (222, 358). Antihypertensive treatment of young SHR reversed these arteriolar changes, suggesting that the narrowing of afferent arterioles might represent a mechanism protecting glomeruli from direct damage by high BP (359). Nevertheless, renal vascular wall thickening is not only a secondary effect of high BP because it also develops in SHR that had been made normotensive by hydralazine treatment since conception (663). Reduced afferent arteriole diameter might be a good predictor of hypertension development because its value determined at the age of 7 wk cosegregated with BP of 23-wk-old F₂ rats (534). This could also explain why BP of adolescent F₂ hybrids cosegregated with altered renal blood flow and glomerular filtration rate (258).

Enhanced renal sympathetic nerve activity might also contribute to the pathogenesis of genetic hypertension (142, 363, 553) by increasing renal vascular resistance (38, 113) and/or augmenting tubular Na⁺ reabsorption (284). A high renal NE content in young SHR aged 4-7 wk (617) is accompanied by low density of ₁- and ₂-adrenoceptors, the number of which increases in the kidney during the later development of spontaneous hypertension (623). Nevertheless, BP did not cosegregate with the density of either ₁- or ₂-adrenoceptor in the kidney of adult F₂ rats (486). There was also no significant linkage of BP to ₂-adrenoceptor gene polymorphism (366, 685).

It seems that BP effects of renal denervation strongly depend on the age of animals used. Chronic bilateral renal denervation of 4-wk-old SHR (repeated every 3 wk for 16 wk) blocked 30-40% of the expected progressive BP rise, whereas the BP of WKY was not affected (533). Kidney denervation in young SHR shifted the pressure-filtration curve to the left without affecting the pressure-flow relationship. These findings are compatible with the enlarged preglomerular and diminished postglomerular lumen diameters in the denervated kidney (702). Thus renal nerves might affect the structural development of renal vasculature in SHR. In contrast to young animals, renal denervation did not reduce BP of adult SHR (740), suggesting that the kidneys of adult SHR had been reset to maintain a permanently elevated BP.

Another abnormality, which is present in the kidney of immature SHR, is the exaggerated renal vascular reactivity to various vasoconstrictors such as angiotensin II or thromboxane A₂ (TxA₂) (83). This is based not only on increased glomerular density of angiotensin type I (AT₁) (271) and TxA₂ receptors (80) but especially on the altered ability of PGE₂ or PGI₂ to counterbalance the effects of the above vasoconstrictors on renal vasculature (81). The impaired ability of vasodilator prostaglandins to buffer the renal vasoconstriction in young SHR is due to the defect in G_s protein-dependent cAMP generation (82, 84). However, there is no information about its possible linkage to BP.

The resetting of kidney function occurs very early in ontogeny because the relationship between Na⁺ excretion and renal perfusion pressure is already shifted to the right in 3-wk-old SHR compared with WKY (605). The shift of pressure-natriuresis curve was eliminated when young SHR were treated with captopril (478) or hydralazine (365), although the latter treatment did not prevent the structural alterations of renal vasculature (663). Altered renal medullary hemodynamics in young SHR (607) may also contribute to the shift of pressure-natriuresis curve, i.e., to decreased fractional Na⁺ excretion at a given level of renal perfusion pressure. Reduced papillary blood flow, especially prominent in SHR aged 6-9 wk, is the principal abnormality of renal medullary hemodynamics during the development of spontaneous hypertension (607). The renal medulla exerts an important antihypertensive action because its destruction in prepubertal SHR substantially elevated BP of adult animals (40).

The mechanisms controlling the pressure-natriuresis relationship also involve the action of 20-hydroxyeicosatetraenoic acid (20-HETE) produced by renal cytochrome P-450 monooxygenases from arachidonic acid. Individual members of the cytochrome P-450 gene family (CYP4A) are overexpressed in the kidney of young SHR (383, 638) in which increased 20-HETE production was demonstrated (307, 547, 615). 20-HETE participates in renal blood flow autoregulation and tubuloglomerular feedback control through its potent vasoconstrictor and tubular effects. The inhibition of renal cytochrome P-450 activity lowered BP in young (7-wk-old) but not in adult SHR (430, 616). Chronic inhibition of this enzyme by SnCl₂ treatment of young SHR prevented hypertension development, and BP reduction persisted even if this treatment was withdrawn at the age of 15 wk (185). Because cytochrome P-450 inhibitors are capable of decreasing high preglomerular resistance in deep nephrons of young SHR (307), 20-HETE may contribute to the resetting of pressure-natriuresis curve through alterations in papillary blood flow.

An attempt was made to test whether BP of adult rats may depend on renal cytochrome P-4504A genotype (675). Under the conditions of normal salt intake, CYP4A genotype did not cosegregate with BP of F₂ rats, but this genotype had a significant relation to the BP response elicited by a high salt intake in F₂ hybrids of SHR and Brown Norway rats (675). The latter finding is compatible with the role of CYP4A2 gene in the pathogenesis of salt hypertension in salt-sensitive Dahl rats (606, 674).

It is evident that the alterations of renal blood flow, glomerular filtration rate, tubular Na⁺ reabsorption, and pressure-natriuresis relationship can already be demonstrated in young genetically hypertensive rats before the rapid phase of BP increase. Most of these alterations cosegregate with BP in F₂ hybrids, and they can often be corrected by early treatment with ACE inhibitors (for details, see sect. VIB). At present, it cannot be excluded that early renal abnormalities could represent primary defect(s) in the pathogenesis of genetic hypertension as it has been suggested on the basis of renal cross-transplantation studies.

B.  Abnormal Neurohumoral Regulation

1.  SNS

The crucial role of the SNS in the pathogenesis of hypertension is mediated not only by increased resting sympathetic tone leading to enhanced vasoconstriction but also by trophically induced cardiovascular hypertrophy which, at least in SHR, may even precede the rise of BP (1). The importance of the sympathoadrenal system for the development of high systemic resistance and cardiovascular hypertrophy in rats with genetic hypertension has convincingly been demonstrated by neonatal sympathectomy of SHR (125, 200, 374, 420) (for details, see sect. VIA).

Although SNS plays a dominant role in BP maintenance of both normotensive and hypertensive animals, there is still sparse convincing evidence for the genetic linkage of SNS abnormalities to high BP (Table 1). However, important SNS alterations occur so early in ontogeny that they cannot be revealed by linkage studies performed in adulthood (usually at the age of 12-18 wk).

Ontogeny of the neural component of BP control as well as the maturation of the functional unit consisting of sympathetic nerve terminals and their target tissues (i.e., vascular smooth muscle) is most intensive in the first 3-5 wk of postnatal life of the rat (166, 470, 666, 710). In certain vascular beds of SHR, enhanced sympathetic innervation was observed before the onset of hypertension (for review, see Ref. 277) and impairment of the vascular wall developed in parallel with enhanced sympathetic activity (666). Thereafter, the development of SNS alterations in SHR slows down but continues until the age of ~24 wk (337).

The early increase of sympathetic innervation in SHR seems to be related to elevated levels of the nerve growth factor (NGF) that were found in the mesenteric artery and aorta of young SHR aged 3-8 wk but not in those of adult animals (165, 713, 776). Nerve growth factor mRNA in the mesenteric vessels of SHR is already increased at birth (187). In addition, NGF gene is linked to the BP in F₂ hybrids (342). As a trophic protein, NGF also takes part in vascular hypertrophy mediated by enhanced sympathetic innervation of blood vessels. Such interaction seems to be most intensive during the weaning period, i.e., in the third and fourth postnatal week (776). In fact, chronic treatment of newborn normotensive rats with NGF for 2 wk had no effect on vascular morphology (421), whereas NGF treatment prolonged until puberty increased vascular sympathetic innervation and caused a hyperplastic response of vascular smooth muscle cells (similar to those seen in SHR), although it did not elevate BP (775).

In hypertension, the postsynaptic ₁-adrenergic functions become dominant, whereas -adrenergic functions are attenuated. This is reflected by a reduction in the number of -adrenoceptors and in the production of its second messenger, cAMP, whereas the number of ₁-adrenoceptors remains unchanged or is increased, and the production of their second messengers, inositol trisphosphate and diacylglycerol, is enhanced in cardiac and vascular tissues (for review, see Ref. 131). However, ontogeny of altered adrenergic functions in genetically hypertensive rats is still not fully understood.

The increased sympathetic innervation of resistance vessels (5, 277, 418) and the enhanced BP response to ganglionic blockade (666) in young SHR suggest augmented vasoconstriction mediated by ₁-adrenoceptors. This might be not only due to a greater NE release from sympathetic nerve fibers of SHR (733) but also due to the hyperreactivity of resistance vessels (caused by well-known structural changes) or their supersensitivity to ₁-adrenoceptor agonists (unmasked by cocaine inhibition of neuronal catecholamine uptake) (372, 503).

The reduced density of -adrenoceptors in the aorta (68) and heart (441, 755) of adult SHR contrasts with unchanged values in prehypertensive SHR (55, 58, 755). However, the unaltered -adrenoceptor density in young SHR might involve lowered ₁-adrenoceptor density counterbalanced by increased density of ₂-adrenoceptors (487). The early increase of myocardial NE content precedes the downregulation of ₁-adrenoceptors and the increase of G_i protein expression leading to desensitization of cardiac adenylyl cyclase in SHR (58). These changes were detected in the heart and aorta of young SHR before the development of spontaneous hypertension (461).

Presynaptic modulation of NE release from sympathetic nerve endings is an important mechanism by which numerous vasoactive substances or drugs can affect vascular tone (707). Diminished feedback inhibition and increased feedback facilitation, which enhance NE release per nerve impulse, seem to be augmented in genetic hypertension. Spontaneously hypertensive rats are characterized by a reduced negative feedback of NE release mediated by presynaptic ₂-adrenoceptors. This impairment is usually greater in younger (4-10 wk old) than in older SHR (218, 687, 706). Early alteration of this feedback control is compatible with elevated plasma NE levels observed only in young but not in adult SHR (561, 687). The dysfunction of presynaptic ₂-adrenoceptors reveals the facilitatory effects of ₂-adrenoceptors in the vasculature of SHR. Enhanced facilitation of NE release (mediated by presynaptic ₂-adrenoceptors) results in augmented pressor responses to periarterial nerve stimulation that precedes hypertension development in SHR (709). In contrast to normotensive rats, which are characterized by an age-related loss of both vascular - and -adrenoceptor responsiveness, SHR exhibit an age-related loss of vasodilator responsiveness mediated by -adrenoceptors despite the persisting high vasoconstrictor responsiveness to -adrenoceptor agonists (60).

The modulation of sympathetic neurogenic vasoconstriction through presynaptic ₂-adrenoceptors stimulated by epinephrine also explains how the adrenal medulla could be involved in the early development of spontaneous hypertension. Hypertension development was attenuated only in those SHR that were subjected to adrenal demedullation at 6 wk of age or younger. Furthermore, hypertension development in young demedullated SHR was restored by chronic epinephrine supplementation (59).

The enhanced sympathetic innervation due to elevated NGF levels is a characteristic finding in the vasculature of immature SHR before and during the rapid phase of BP rise. Augmented vasoconstrictor response to ₁-adrenoceptor agonists, the downregulation of ₁-adrenoceptors, and enhanced NE release from sympathetic nerve endings (due to high sympathetic nerve traffic together with decreased feedback inhibition and increased feedback facilitation of neurotransmitter release mediated by presynaptic ₂- and ₂-adrenoceptors) were also demonstrated in weanling and prepubertal SHR.

The importance of this system for the pathogenesis of genetic hypertension is underlined by the fact that the blockade of RAS by early gene therapy (318, 452, 575) or by pharmacological interventions with ACE inhibitors or AT₁ receptor antagonists in the juvenile critical period (207, 262, 498, 538, 749) may attenuate or even prevent the development of hypertension.

The genetic evidence for the participation of RAS in the pathogenesis of hypertension is summarized in Table 1. Thus the renin gene (592), ACE gene (140), as well as AT_1A and AT_1B receptor genes (135, 136) cosegregate with BP in F₂ populations derived from salt-sensitive Dahl rats. On the other hand, the ACE gene (287, 520) but not the renin gene (445) or angiotensinogen gene (298) was linked to BP in SHRSP × WKY F₂ rats, whereas BP was significantly associated with the renin gene (395, 686) and ACE gene (779) but not with the angiotensinogen gene (449) in SHR × WKY F₂ hybrids. However, recent experiments with the renin gene transfer do not support the importance of this candidate gene in the pathogenesis of high BP in either SHR (677) or Dahl rats (329, 679). As far as the functional parameters of peripheral RAS are concerned, BP cosegregated with renal renin secretion (642) but not with plasma levels of ANG II (299). The latter observation is not surprising because local tissue RAS activity might be more important than circulating ANG II.

Recent experiments with gene therapy of immature SHR (318, 452, 575) have demonstrated that the perinatal delivery of AT₁ receptor antisense cDNA gene caused a long-term attenuation of hypertension development that lasted up to 7 mo of age. The early expression of AT₁ receptor antisense transcript in various tissues resulted in persistent selective attenuation of the respective cellular ANG II actions. This was documented by reduced AT₁ receptor mRNA levels in the mesenteric artery and adrenal glands, decreased ANG II binding to AT₁ receptors in the heart, diminished contractile response of aorta and renal resistance arterioles to ANG II, attenuated pressor response to ANG II administration, and lowered dipsogenic effects of ANG II. Furthermore, it restored the endothelium-dependent vasorelaxation to ACh and corrected the contractile response to phenylephrine or KCl. Finally, this early gene therapy prevented the alterations of vascular Ca²⁺ homeostasis characteristic for untreated SHR. All these changes were still observed 3-7 mo after a single AT₁ receptor antisense gene delivery to SHR pups aged 5 days (223, 452, 464). Because losartan exerted no significant effects in SHR subjected to early gene therapy (318, 452), the above antisense delivery seems to have similar cardiovascular effects as chronic treatment of young SHR with AT₁ receptor antagonists (456, 498, 538). It is of interest that such early gene therapy lowered the BP just at the age of 4-12 wk, i.e., in the developmental window (critical period) in which transient antihypertensive therapy is capable of inducing long-term BP reduction that persists even after drug withdrawal (see also sect. VIB and Fig. 5). The above observations should attract our attention to RAS abnormalities detected in SHR during the first three postnatal months.

Multiple abnormalities of renal and circulating RAS have been described in immature rats with genetic hypertension, the kidney of which is characterized by increased reactivity to ANG II. Preglomerular vessels of young SHR are hyperresponsive to ANG II (723). Angiotensin II-stimulated Na⁺ reabsorption in proximal tubules is substantially enhanced in 5-wk-old SHR (693). An elevated renal ANG II content and increased ANG II receptor binding capacity in tubular brush-border membranes were indeed found in immature SHR (465), together with a greater number of glomerular (117, 271) and proximal tubular AT₁ receptors (92). Renal RAS seems to be more active in young SHR compared with age-matched WKY. Increased renin mRNA levels (620), higher kidney renin activity (417, 652), and enhanced renal renin secretion (11, 282) might contribute to the elevation of plasma renin activity occasionally reported in young SHR (641), although the last observation was not confirmed by later studies (22, 357, 649).

There are also signs of hyperactive brain RAS in immature SHR. Higher ANG II levels (574) together with increased ANG II receptor binding capacity (618) and augmented neuronal response to ANG II (524) were reported in the hypothalamus of young SHR. The levels of angiotensinogen and angiotensinogen mRNA are also elevated in the brain of immature SHR (279, 691).

Enhanced activity of renal and brain RAS together with the greater responsiveness of the kidney and hypothalamus of immature SHR to ANG II explain why the early impairment of AT₁ receptors causes long-term attenuation of the development of genetic hypertension. On the other hand, the contribution of direct and indirect effects of ANG II to the elevation of vascular tone and to the hypertrophy of the resistance vessel wall in rats with developing genetic hypertension still remains to be determined.

The cosegregation of BP with restriction fragment length polymorphism marking the kallikrein gene family of SHR in Prague recombinant inbred strains (581) indicates the importance of this system in genetic hypertension (74, 462). Further evidence was provided by human tissue kallikrein gene delivery that lowered BP in various hypertensive models including SHR and salt-sensitive Dahl rats (77). A single injection of the respective cDNA construct to newborn SHR attenuated the development of genetic hypertension as evidenced by the BP decrease found at the age of 9-17 wk (78). In contrast, systemic kallikrein gene delivery to adult SHR lowered their BP for 6 wk only (78, 725).

The important role of the kallikrein-kinin system for BP development was further supported by the studies on the early blockade of bradykinin B₂ receptors. It was demonstrated in normotensive rats that bradykinin B₂ receptor blockade during prenatal and/or early postnatal periods causes BP elevation in adulthood, whereas the same treatment of adult animals has no BP effects (457). Such a protective role of bradykinin in BP ontogeny is missing in SHR (182) in which reduced urinary kallikrein excretion has been demonstrated from the age of 4 wk, the abnormality being more pronounced in adult rats (3, 580). The association of low urinary kallikrein excretion with BP elevation has also been confirmed in inbred low-kallikrein Wistar rats that have higher BP and augmented salt-induced BP rise compared with Wistar rats with normal urinary kallikrein excretion (458). Similar findings were obtained in kininogen-deficient Brown Norway rats (349a).

The information on the changes of tissue kinins during the development of genetic hypertension is still rather contradictory. The kininogenase activity in the aorta was considerably suppressed in both prehypertensive and hypertensive SHR (560). The renal kallikrein content is also decreased in Milan hypertensive rats (23) in which this abnormality is present since birth (188). Although the tissue active kallikrein content in the renal cortex is also reduced in SHR since birth, its kininogenase activity (expressed per mg protein) is elevated in SHR from the age of 4 wk and decreases in senescence only (193, 580). Elevated bradykinin levels were indeed found in various tissues (kidney, adrenal, lung, and heart) of young SHR aged 6 wk, but this was not seen in older animals (72). The brain kallikrein-kinin system is hyperactive especially in SHR aged 5-6 wk, although elevated kinin levels together with a higher kallikrein level and augmented kininogenase activity partially persisted in the cerebrospinal fluid of adult 18-wk-old SHR compared with age-matched WKY (355).

The available data suggest that the impairment of the kallikrein-kinin system plays an important, age-dependent role in the pathogenesis of genetic hypertension, because the early (perinatal) bradykinin "deficiency" is associated with the later BP elevation, and vice versa.

Atrial natriuretic peptide represents an important component of chronic BP regulation, because it can effectively decrease BP by its natriuretic and vasorelaxant effects. Indeed, chronic blockade of endogenous ANP by a specific monoclonal antibody accelerated hypertension development in SHRSP (312). On the other hand, chronic inhibition of ANP degradation, which increases plasma ANP levels, prevented salt-induced acceleration of hypertension development in SHR (330). Furthermore, salt-induced exacerbation of hypertension was prevented in SHR by long-term ANP infusion that elevated plasma ANP to levels seen in WKY fed a high-salt diet (331).

Atrial natriuretic peptide also participates in central baroreflex control of sympathetic nerve activity. Acting on forebrain structures [anterior hypothalamic area (AHA)], ANP elevates BP, whereas its action on hindbrain sites [nucleus tractus solitarii (NTS)] lowers BP (551). Consequently, a microinjection of ANP antibody into AHA caused a dose-dependent BP decrease in SHR but not in WKY (763), whereas antibody administration into NTS elevated BP and restored impaired baroreflex control of the heart rate and sympathetic nerve activity in SHR (764, 781). In young SHR, a reduced ANP content and lower number of ANP binding sites in the forebrain (25, 473) seem to be less important for hypertension development than the decreased ANP content combined with diminished ANP binding in hindbrain structures (25, 614).

The genetic analysis revealed that the ANP locus is linked to BP in SHR × WKY F₂ hybrids (778, 780). Furthermore, the ANP receptor (guanylate cyclase A) gene was also found to cosegregate with BP in some F₂ populations derived from New Zealand genetically hypertensive rats (265) or salt-sensitive Dahl rats (140).

The age-dependent involvement of ANP in the pathogenesis of genetic hypertension is still not clear, but the overexpression of human ANP gene decreased BP only in young (4-wk-old) but not in adult (12-wk-old) SHR (443). Although immature SHR are lacking sufficient ANP secretion, they are able to respond adequately to its vasorelaxant and natriuretic action. Impaired ANP release might be a reason why plasma ANP levels are usually not elevated in immature SHR, but they increase with advancing age and/or with the progress of hypertension (245, 306, 766). Moreover, the atrial ANP content is reduced in adult but not in immature SHR (245, 689, 766). Less effective atrial ANP release after saline infusion and/or atrial pressure elevation has already been demonstrated in young SHR, the difference between SHR and WKY being augmented with increasing age (613). An exaggerated natriuretic response to exogenous ANP was reported in 6- but not 11-wk-old SHR (578). Similarly, vascular relaxation and cGMP response to ANP were enhanced in mesenteric arteries of 4-wk-old SHR but not in vessels of animals aged 12-16 wk (629).

All available data indicate that the insufficient ANP release in young genetically hypertensive rats decreases the potency of ANP system to lower BP. The reduction of vasorelaxant and natriuretic ANP effects in adult genetically hypertensive rats further impairs the antihypertensive role of ANP during the later hypertensive stages.

The importance of the endothelium and its multiple vasoactive products in the regulation of vascular tone under normal and pathological conditions has intensively been investigated in the last decade (for review, see Refs. 162, 196, 531, 627). It is beyond the scope of this review to describe all complex regulatory interactions between circulating and locally produced vasoactive factors as well as their influence on vascular smooth muscle in various forms of genetic hypertension. Our attention will therefore be focused on the developmental abnormalities of principal endothelium-derived vasodilating and vasoconstricting factors including NO and endothelins.

In fact, there is only little evidence that NO or ET alterations are linked to genetic hypertension. The respective genetic analysis has never been performed in SHR in which the development of endothelial dysfunction was studied in detail. The only available information has been obtained in Dahl rats in which particular forms of NO synthase (NOS) and various components of the ET system were tested for cosegregation with BP. Under the conditions of high salt intake, ET-3 (100) as well as ET-2 and ET_B receptor loci (135) were suggested to be linked to BP in F₂ hybrids derived from salt-sensitive Dahl rats. Surprisingly, neither constitutive endothelial NOS (Nos3 gene) nor neuronal NOS (Nos1 gene) was found to be linked to BP, but the locus for inducible NOS (containing Nos2 gene) cosegregated with BP in at least two different F₂ populations (137, 138). Nevertheless, subsequent genetic analysis of this broad region of rat chromosome 10 excluded Nos2 as a candidate gene (170).

The lack of genetic linkage between NO and high B