Dahl salt-susceptible and salt-resistant rats. A review

Hypertensive strains and normotensive 'control' strains. How closely are they related?

The spontaneously hypertensive rat and the Dahl salt-sensitive rat are the most widely studied genetic models of hypertension. Many investigators have attempted to study the pathogenesis of hypertension by comparing these strains with their respective normotensive "controls," the Wistar-Kyoto rat and the Dahl salt-resistant rat. However, the genetic relation between each of these hypertensive strains and its corresponding normotensive control has never been clearly defined. Based on an analysis of DNA "fingerprint" patterns generated with six multilocus probes, we found that the spontaneously hypertensive rat (Charles River Laboratories, Inc.) is genetically quite different from its normotensive Wistar-Kyoto control: these strains only share approximately 50% of their DNA fingerprint bands in common. The inbred Dahl salt-sensitive rat (SS/Jr strain) (Harlan Sprague Dawley, Inc.) and the Dahl salt-resistant rat (SR/Jr strain) share approximately 80% of their DNA fingerprint bands in common. To the extent that the genes identified by DNA fingerprint analysis are representative of loci dispersed throughout the rodent genome, the current findings provide evidence of extensive genetic polymorphism between these commonly used hypertensive strains and their corresponding normotensive controls, particularly in the spontaneously hypertensive rat model. These findings, together with the fact that an enormous number of biochemical and physiological differences have been reported between these hypertensive and normotensive strains, suggest that continued comparison of spontaneously hypertensive rats with Wistar-Kyoto rats or Dahl salt-sensitive with salt-resistant rats will have limited value for investigating the pathogenesis of hypertension.

Maternal involvement in the development of cardiovascular phenotype

Over the past 20 years, laboratory studies of genetically defined animal models of human essential hypertension have provided valuable information on the pathophysiology of this disturbance in cardiovascular regulation. Relatively fewer studies have examined the impact of preweaning factors on the developing cardiovascular system of hypertensive animals. In our laboratory studies, we have utilized two inbred genetically hypertensive models: the spontaneously hypertensive (SHR) rat and its Wistar/Kyoto (WKY) normotensive control strain as well as the Dahl hypertension-sensitive (SS/Jr) and hypertension-resistant (SR/Jr) strains. To manipulate the preweaning maternal environment, we have employed the technique of reciprocal cross-fostering of litters between hypertensive and matched normotensive mothers. Our findings to date point to the maternal environment as a powerful influence on the development of high blood pressure in genetically hypertensive rats. In general, hypertensive rats reared by normotensive foster mothers have significant reductions in arterial blood pressure in adulthood. Thus, the progression of hypertensive disease is not strictly predetermined by genotypic factors. Rather, a genetic predisposition to hypertension interacts with preweaning environmental factors to determine an animal's cardiovascular phenotype in adulthood.

Aldosterone metabolism in the isolated perfused liver of R and S hypertension-prone Dahl rats

The effect of a chronic oral salt load on hepatic metabolism of aldosterone was examined in isolated livers of salt-resistant (R) and salt-sensitive (S) hypertension-prone male Dahl rats perfused with d-[4-14C]aldosterone (10(-9) M). Aldosterone was analyzed by radioimmunoassay and [4-14C]aldosterone radiometabolites by high-performance liquid chromatography. In salt-loaded S rats, systolic blood pressure was 30 mmHg higher than in the other three groups (P less than 0.01). In S rats, on standard and high-salt diets, plasma renin activity was 64% (P less than 0.001) and 50% (P less than 0.01) lower, and, on the standard diet, plasma aldosterone was 50% (P less than 0.01) lower than in R rats. Salt loading suppressed plasma renin activity by 42% (P less than 0.05) in R rats and plasma aldosterone by 66 and 33% (P less than 0.01) in R and S rats, respectively. In isolated perfused liver, hepatic function did not differ between various groups. Hepatic clearance of aldosterone in R rats given water and saline and in S rats given water did not differ, whereas hepatic clearance of aldosterone was 28 and 35% higher in salt-loaded S rats when compared with S rats on water (P less than 0.01) and with salt-loaded R rats (P less than 0.001), respectively. Polar and reduced metabolites of [4-14C]aldosterone were released into the circulation in livers of R and S rats on both diets, but highest relative levels of polar metabolites of aldosterone were found in salt-loaded S rats. In all groups, only polar metabolites of aldosterone were excreted in bile.(ABSTRACT TRUNCATED AT 250 WORDS)

High salt diet down-regulates proximal tubule Na+, K(+)-ATPase activity in Dahl salt-resistant but not in Dahl salt-sensitive rats: evidence of defective dopamine regulation

We examined the regulation of Na+,K(+)-ATPase activity in proximal tubule segments during a high salt diet in prehypertensive Dahl salt-sensitive and salt-resistant rats. Rats were placed on normal salt or high salt diets (0.9% saline as drinking water). During the normal salt diet, Na+,K(+)-ATPase activity was not different between Dahl salt-sensitive and salt-resistant rats. After 2 days and 10 days on a high salt diet, Na+,K(+)-ATPase activity in Dahl salt-resistant rats significantly decreased when compared to Dahl salt-resistant rats on a normal salt diet (P less than 0.01). The decreased Na+,K(+)-ATPase activity in Dahl salt-resistant rats during a high salt diet was reversed by treatment with an inhibitor of aromatic L-amino acid decarboxylase (dopamine synthesizing enzyme), benserazide. In contrast, Na+,K(+)-ATPase activity did not decrease during the high salt diet and benserazide had no effect on Na+,K(+)-ATPase activity in Dahl salt-sensitive rats. These results indicate that Dahl salt-sensitive rats do not have the capacity to down-regulate the proximal tubule Na+,K(+)-ATPase activity during a high salt diet. Indirect evidence suggests that the regulation of Na+,K(+)-ATPase activity by locally produced dopamine is absent in Dahl salt-sensitive rats.

Insulin resistance and blood pressure in Dahl rats and in one-kidney, one-clip hypertensive rats

In normal rats, sucrose feeding results in insulin resistance and an elevation of arterial pressure. The purpose of this study was twofold: 1) to evaluate the effect of sucrose feeding on blood pressure in a genetic model and in an acquired model of hypertension and 2) to determine whether these models of hypertension are associated with insulin resistance. In Dahl salt-resistant (Dahl-R) rats on both a 0.45 and a 3% NaCl intake, systolic blood pressures were higher (P less than 0.01) in sucrose-drinking than in water-drinking animals. In contrast, blood pressure was not affected by dietary sucrose in Dahl salt-sensitive (Dahl-S) rats. Blood pressure was also not affected by sucrose in the one-kidney, one-clip (1K,1C) hypertensive rat. In response to an oral glucose load, serum glucose was similar in Dahl-R and Dahl-S rats, although serum insulin was higher (P less than 0.05) in Dahl-S rats, suggesting that Dahl-S rats are insulin resistant. In contrast, glucose and insulin responses were similar in hypertensive 1K,1C animals and normotensive controls. In conclusion, sucrose feeding increased blood pressure in Dahl-R but not in Dahl-S or 1K,1C hypertensive rats. Additionally, Dahl-S rats, but not hypertensive 1K,1C rats, are insulin resistant.

L-arginine abrogates salt-sensitive hypertension in Dahl/Rapp rats

This study examined the contribution of nitric oxide (NO) to the susceptibility or resistance to the hypertensive effects of high sodium chloride (8.0% NaCl) intake in young Dahl/Rapp salt-sensitive (SS/Jr) and salt-resistant (SR/Jr) rats. Using NG-monomethyl-L-arginine (L-NMMA) as a probe for NO production in vivo, we found that increasing dietary sodium chloride increased NO activity in salt-resistant rats, but not in salt-sensitive rats. Exogenous L-arginine, the substrate for NO synthesis, decreased blood pressure to normotensive levels in salt-sensitive rats made hypertensive for 2 wk from 8.0% NaCl chow. D-arginine had no effect on blood pressure of these rats and L-arginine did not change blood pressure of salt-resistant rats. Intraperitoneal injections of L-arginine and its precursor, L-citrulline, and oral L-arginine, but not D-arginine, prevented the increase in blood pressure in salt-sensitive rats on the high salt chow over 2 wk of observation. In contrast, L-arginine did not alter the development of hypertension in spontaneously hypertensive rats. Mean urinary cGMP levels were higher in salt-sensitive rats on oral L-arginine than salt-sensitive rats on D-arginine. Infusion of L-NMMA acutely decreased, whereas intravenous L-arginine rapidly increased, urinary cGMP in both groups. L-arginine and L-citrulline increased production of NO and prevented salt-sensitive hypertension in Dahl/Rapp rats.

Antihypertensive effects of an aromatase inhibitor in inbred salt-sensitive rats

Rats susceptible to the hypertensive effect of dietary salt (SS/Jr) have excess urinary 19-nordeoxycorticosterone compared with salt-resistant control rats (SR/Jr). 19-Nordeoxycorticosterone is a hypertensinogenic mineralocorticoid, but whether it contributes to the salt sensitivity of SS/Jr is unknown. This study sought to evaluate the contribution of 19-nordeoxycorticosterone to the salt sensitivity of SS/Jr by lowering its production with an aromatase inhibitor, 10-propargyl-androst-4-ene,3,17-dione (19-acetylenic-androstenedione, 19-AA). This aromatase inhibitor also preferentially inhibits nonaromatizing adrenal 19-hydroxylation, an essential step in the formation of 19-nordeoxycorticosterone. To test this hypothesis, inhibitor (120 mg) or vehicle pellets were implanted into male and female weanling SS/Jr at 42 days of age. A high salt diet (8% NaCl) was started and two additional pellets were implanted at 52 and 62 days of age. Systolic blood pressure was measured in all animals and urinary corticosteroids in males. Compared with vehicle, the inhibitor lowered blood pressure at 50 days of age (when it could first be measured) until 64 days of age in females and 71 days of age in males. Corticosterone and aldosterone levels were not different between 19-AA- and vehicle-treated SS/Jr. 19-Nordeoxycorticosterone levels, however, were mildly reduced with the inhibitor (0.05 less than p less than 0.10). After 28 days of high salt diet all 23 of the 19-AA-treated SS/Jr were alive, whereas almost one half of the control animals had died. These data demonstrate that 19-AA attenuates the hypertension in SS/Jr; this effect may be through reduction in 19-nordeoxycorticosterone production.(ABSTRACT TRUNCATED AT 250 WORDS)

Enhanced chloride reabsorption in the loop of Henle in Dahl salt-sensitive rats

This study examines the nephron segments contributing to the blunted pressure-natriuretic response in Dahl salt-sensitive rats. Urine and late proximal and early distal tubular fluid samples were collected from 16-20-week-old, inbred Dahl salt-sensitive (DS/Jr) and salt-resistant (DR/Jr) rats, and Dahl salt-sensitive (DS) and salt-resistant (DR) rats from the Brookhaven colony, that were maintained from birth on a low (0.3%) sodium chloride diet. Urine flow and sodium and chloride excretions were 65% less in the DS/Jr than in the DR/Jr rats when their kidneys were perfused at an equal renal perfusion pressure of approximately 150 mm Hg. The percentages of the filtered load of water and chloride remaining at the end of the proximal tubule were significantly greater in DS/Jr rats than in DR/Jr rats; however, the percentages of the filtered load of water and chloride reaching the early distal tubule were significantly less, by 29% and 77%, respectively. Fractional reabsorption of water and chloride in the loop of Henle of DS/Jr rats was twice that observed in DR/Jr rats. Similar results were obtained in DS and DR rats of the Brookhaven strain. Urine flow and sodium and chloride excretions were 60% lower in DS than in DR rats at a renal perfusion pressure of 135 mm Hg. Proximal tubular reabsorption of water and chloride was similar in DS and DR rats; however, the percentages of the filtered load of water and chloride reabsorbed in the loop of Henle were greater in DS than in DR rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Dual hemodynamic mechanisms for salt-induced hypertension in Dahl salt-sensitive rats

Cardiac output, blood volume, total peripheral resistance, and renal blood flow were measured in awake salt-sensitive and salt-resistant Dahl rats on normal rat chow (1% NaCl) and on high salt (8% NaCl) diets. Rats were studied after 4, 8, and 46 weeks on a 1% NaCl diet and after 4 and 8 weeks on an 8% NaCl diet. Salt-sensitive rats on 8% NaCl for 4 weeks developed systolic hypertension; by 8 weeks they developed greater systolic and also diastolic hypertension. Salt-resistant rats on 8% NaCl remained normotensive throughout the studies, although renal resistance decreased (p less than 0.05). At 4 weeks, hypertension in salt-sensitive rats on 8% NaCl was caused by increased blood volume and cardiac output (p less than 0.05), with normal total peripheral resistance. At 8 weeks, hypertension was due to increased total peripheral resistance (p less than 0.05); cardiac output was below normal despite persistent elevation of blood volume (p less than 0.05). Salt-sensitive rats on 1% NaCl for 46 weeks were hypertensive, with elevated total peripheral resistance (p less than 0.05); cardiac output decreased (p less than 0.05), whereas blood volume remained unchanged. Salt-resistant rats on 1% NaCl remained normotensive with no charges in hemodynamics. Salt-sensitive rats on 8% NaCl for 4 weeks had an increase in renal vascular resistance but no significant change in nonrenal resistance or total peripheral resistance. The increased total peripheral resistance in salt-sensitive rats on 8% NaCl for 8 weeks and on 1% NaCl for 46 weeks was a reflection of increases of both renal and nonrenal vascular resistance.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of chronic intraventricular sodium on blood pressure and fluid balance

To examine if chronic sodium loading on the brain produces sustained increases in blood pressure, water intake, and sodium excretion, hypertonic (0.5 M and 1.5 M) and isotonic (0.15 M) NaCl solutions were infused into the third ventricle of Sprague-Dawley rats at a rate of 5.5 microliters/hr for 7 days. Intracerebroventricular infusion of 1.5 M NaCl significantly increased systolic blood pressure during the entire infusion period (+23 +/- 5 mm Hg on day 1 and +15 +/- 2 mm Hg on day 7, n = 10, mean +/- SEM). Blood pressure rose insignificantly in the 0.5 M NaCl group, whereas it remained at the baseline levels in the 0.15 M NaCl group. The increases in water intake (day 2), positive water balance (day 2), and negative sodium balance (day 3) were observed in the 1.5 M NaCl group. On day 7, the 1.5 M NaCl group showed hyponatremia and low plasma osmolality and had higher plasma norepinephrine but not vasopressin compared with the 0.15 M NaCl group. In another series of study, depressor response to intravenous hexamethonium (20 mg/kg) in the 1.5 M NaCl group was greater than that in the 0.15 M NaCl group on both day 1 and 7. The depressor response to d(CH2)5Tyr(Me)-arginine vasopressin (10 micrograms/kg) in the 1.5 M NaCl group was greater on day 1 but not on day 7. These results indicate that sustained sodium stimulus on the central nervous system causes mild hypertension and alters water and sodium balance. The sympathetic nervous system but not vasopressin may play an important role in the chronic phase of central NaCl-induced hypertension.

Adrenergic innervation of blood vessels in Dahl rats with salt hypertension

The adrenergic innervation of major arteries and veins was examined in Dahl salt-sensitive (DS) and salt-resistant (DR) rats using a histochemical fluorescent technique to detect the intraneuronal catecholamine content. Under the conditions of low salt intake the density of adrenergic plexus tended to be higher in DR than in DS rats. A decrease of catecholamine content with a subsequent reduction in the density of visible adrenergic plexus was observed in mesenteric and renal but not in femoral vascular beds of salt hypertensive DS rats. The adrenergic innervation was more altered in veins than in respective arteries. Pronounced alterations of vascular sympathetic innervation induced by high salt intake in DS rats contrasted with negligible changes occurring in DR animals. Observed changes of adrenergic innervation in particular vascular beds of salt hypertensive DS rats could reflect the enhanced catecholamine turnover and sympathetic hyperactivity which is important for the pathogenesis and/or maintenance of salt hypertension in Dahl salt-sensitive rats.

Elevated 18-hydroxy-corticosterone in inbred salt-sensitive rats

Rats susceptible to the hypertensive effect of dietary salt (SS/Jr) have excess 18-hydroxydeoxycorticosterone (18-OH-DOC) and 19-nor-DOC compared to control rats (SR/Jr). This may be caused by an abnormal adrenal 11 beta-hydroxylase, which catalyzes the 11 beta, 18, and 19-hydroxylations of DOC. A comparison of the urinary products of this enzyme including 18-OH-DOC, 19-nor-DOC, corticosterone (B), and 18-OH-B have not been described in the SS/Jr. Therefore, these steroid products were measured at 7 and 12 weeks of age in 36 weanling male and female, SS/Jr and SR/Jr (n = 9 in each group), on a low-salt diet. In both the male and female SS/Jr urinary free levels of 18-OH-DOC, 19-nor-DOC, and 18-OH-B were elevated, while B was not different at 6 and 10 weeks of age. The largest increases were in 18-OH-B levels, and these levels correlated with 18-OH-DOC and B but not 19-nor-DOC. The high degree of correlation between these steroids probably reflects their closely related dependence on adrenal 11 beta-hydroxylase biosynthesis.

Reduction in aortic smooth muscle beta-adrenergic responsiveness results in enhanced norepinephrine responsiveness in the Dahl salt-sensitive rat

In an earlier report we observed alterations in aortic smooth muscle alpha- and beta-adrenergic responsiveness in the Dahl rat. The present study was designed to define the time course of these changes and further characterize the alterations in this model of hypertension. Four-week-old male Dahl salt-sensitive (DS) rats were placed on either a normal (DSN) or high salt diet (8% NaCl; DSH). Aortic smooth muscle responsiveness was studied at 3 and 6 weeks of the dietary treatment. No differences were seen between the 2 groups in either the contractile response to KCl or the relaxation responses to sodium nitrite and acetylcholine. Isoproterenol-induced relaxation was significantly attenuated and contractile response to norepinephrine (NE) were enhanced at 3 weeks of treatment in DSH rats, however, no differences were seen between the 2 groups at 6 weeks. Since alterations were seen only at 3 weeks of treatment, aortic smooth muscle responsiveness to the specific alpha-agonists phenylephrine (alpha 1), guanfacine (alpha 2) and contractile responses to NE in the presence of propranolol were evaluated at three weeks. No differences were observed between the 2 groups with any of these treatments. Thus, it appears that the increased NE responsiveness seen in aortic smooth muscle of DSH rats during the developmental stage of the hypertension is the result of a decrease in beta-adrenergic responsiveness and not an increase in alpha-adrenergic responsiveness.

Characterization of the adrenal 11 beta-hydroxylase in inbred salt-sensitive and resistant rats

Rats have been bred for susceptibility and resistance to the hypertensive effect of dietary salt (S/JR & R/JR). S/JR have an abnormal adrenal steroid 11 beta,18-hydroxylase activity resulting in increased production of 18-OH-DOC. S/JR also produce increased quantities of 19-nor-DOC, which may be related, since the 11 beta,18-hydroxylase also catalyzes the 19-hydroxylation of DOC, a pivotal step in 19-nor-DOC biosynthesis. The purpose of the present studies was to further characterize the mutant S/JR adrenal steroid 11 beta,18-hydroxylase. Preliminary studies are also presented on assessing the renal 19-desmolase, the last step in 19-nor-DOC biosynthesis. Adrenal glands were harvested from R/JR and S/JR and prepared for incubation studies, protein immunoblotting, and RNA analysis. Kidneys from Sprague-Dawley rats were also used for isolated renal perfusion studies. Both S/JR and R/JR strains had a single immunostaining band for 11 beta,18-hydroxylase at 51,000 molecular weight which were equal in intensity. Both strains had a single RNA transcript at 4.3 kilobases which hybridized with equal intensity to the bovine cDNA (pB11-9). The Km for 11 beta- and 18-hydroxylation was identical within strains but was different between strains. The Km for 19-hydroxylation was different between S/JR and R/JR, and was much greater than 11 beta- and 18-hydroxylation in both strains. This suggests that the catalytic site for 19-hydroxylation is different than that for 11 beta- and 18-hydroxylation and that the S/JR enzyme binds the substrate with higher affinity than the R/JR enzyme. In the isolated perfusion studies the rat kidneys converted 80% of 19-oxo-DOC to either 19-oic-DOC or 19-nor-DOC. These data demonstrate that the difference in S/JR enzyme activity is probably due to a point mutation in the enzyme or to a change in a regulatory protein.

Hypertension in the spontaneously hypertensive rat is linked to the Y chromosome

The objective of our study was to determine the genetic influence on blood pressure in spontaneously hypertensive rats (SHR), and normotensive Wistar-Kyoto (WKY) rats using genetic crosses. Blood pressure was measured by tail sphygmomanometry from 8 to 20 weeks of age. Blood pressure was significantly higher from 12 to 20 weeks in the male offspring derived from WKY mothers x SHR fathers as compared with male offspring derived from SHR mothers X WKY fathers (180 +/- 4 versus 160 +/- 5 mm Hg, p less than 0.01). There was no significant difference between the blood pressure of the F1 females, further supporting Y chromosome linkage and not parental imprinting. The blood pressure data from F2 males derived from reciprocal crosses of parental strains were consistent with the presence of a Y-linked locus, but not with an X-linked locus controlling blood pressure. The data strongly suggest that hypertension in the SHR has two primary components of equal magnitude, one consisting of a small number of autosomal loci with a second Y-linked component.

Kinetics of Na+ and K+ transport in red blood cells of Dahl rats. Effects of age and salt

Blood pressure response to chronic high salt intake and kinetics of red blood cell Na+ and K+ (Rb+) transport were studied in salt-sensitive (DS) and salt-resistant (DR) Dahl rats fed a high salt diet (8% NaCl) for 7 weeks from the fifth (young), 12th (adult), or 23rd (old) week of age. The kinetics of ouabain-sensitive Rb+ uptake and Na+ extrusion were determined in Na+ media as a function of both intracellular Na+ (Na+i, 2-8 mmol/l cells) and extracellular Rb+ (Rb+o). In addition, the kinetics of furosemide-sensitive Rb+ uptake (related to Rb+o) and the magnitude of the Na+ and Rb+ leaks were assessed. High salt induced hypertension in young and adult but not in old DS rats although red blood cell Na+ was slightly increased in all age groups of DS rats fed a high salt diet. The kinetic parameters of the Na(+)-K+ pump were similar in DS and DR rats fed a low salt diet. Ouabain-sensitive transport rates were not suppressed in erythrocytes of salt hypertensive Dahl rats. Maximal velocities of the Na(+)-K+ pump (related to Na+i) decreased significantly with age in all groups except in DS rats fed a high salt diet. This was compensated by an age-dependent increase in the affinity for Na+i so that no substantial differences in transport rates between young and old rats were seen at physiological cell Na+ and plasma K+ levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of dietary salt on the skeletal muscle microvasculature in Dahl rats

The purpose of this study was to identify microvascular alterations that could contribute to increased peripheral vascular resistance in the Dahl salt-sensitive rat with salt-induced hypertension. Intravital microscopy was used to study the spinotrapezius muscle arteriolar network in anesthetized salt-sensitive rats fed either a high salt (7% sodium chloride) or low-normal salt (0.45% sodium chloride) diet for 4 weeks. Age-matched Dahl salt-resistant rats on high and low-normal salt diets served as controls. The high salt diet had no effect on arterial pressure in salt-resistant rats but increased arterial pressure in salt-sensitive rats. Mean resting diameter of arcade arterioles in salt-sensitive rats on high salt diet was reduced by 25% compared with salt-sensitive rats on low salt or salt-resistant rats on either diet. After abolition of vascular tone with 10(-3) M adenosine, arcade diameters were comparable in all groups. No difference among groups was found in either resting or passive diameter of the more distal transverse arterioles. Measurement of vessel lengths and numbers in cleared muscle specimens revealed no differences among groups in the anatomic density of either arcade or transverse arterioles. These data suggest that a reduction in the resting diameter of arcade, but not transverse, arterioles may increase spinotrapezius muscle vascular resistance in hypertensive salt-sensitive rats. The similarity in vascular densities among groups indicates that structural rarefaction of arterioles does not contribute to any increase in spinotrapezius muscle resistance at this stage of salt-induced hypertension.

Decreased cytosolic calcium and prostaglandin synthesis in prehypertensive rats

The capacity of cultured renal medullary interstitial cells derived from Dahl salt-sensitive and salt-resistant rats to synthesize prostaglandin E2 (PGE2) was compared. Basal and arginine vasopressin (AVP)-induced PGE2 production by interstitial cells from salt-resistant rats was fourfold to fivefold higher than corresponding values of those from the salt-sensitive rats. Similarly, basal and AVP-responsive release of [3H]arachidonate were twofold higher by interstitial cells from salt-resistant compared with salt-sensitive rats. Differences in PGE2 production were abolished by the calcium inophore A23187 or the addition of exogenous arachidonate. The latter findings suggested a role for altered availability of endogenous arachidonate, possibly mediated by reduced calcium-responsive lipase activity. Basal and AVP-induced increases in cytosolic free calcium concentration, assessed by the aequorin method, were significantly lower in interstitial cells from salt-sensitive versus salt-resistant rats, further supporting a possible role for altered cellular calcium homeostasis. Studies of the potential contribution of various phospholipases and of triglyceride lipase to the release of arachidonate for PGE2 synthesis in interstitial cells implicated phospholipase A2 activity as a major pathway. When assessed in vitro in cell cytosolic fractions at identical calcium concentration, phospholipase A2 activity was lower in interstitial cells from salt-sensitive versus salt-resistant rats. Thus, both reduced cytosolic free calcium and phospholipase A2 activity of interstitial cells from salt-sensitive rats may contribute to the diminished capacity of these cells to liberate endogenous arachidonate for PGE2 synthesis.

Sodium intake modulates renal vascular reactivity to endothelin-1 in Dahl rats

1. The systemic and renal haemodynamic responses to endothelin-1 (ET1) were evaluated and compared to Angiotensin II (AII) in anaesthetized Dahl salt-sensitive (DS) and salt-resistant (DR) rats on either low (0.1% NaCl in diet) or high (8% NaCl in diet) salt intake. 2. Baseline mean arterial pressure on low salt diet was similar in both strains, while on high salt diet it was 73 +/- 4 mmHg in DR rats and 119 +/- 8 mmHg in DS rats (P less than 0.05). Baseline renal blood flow (RBF) and renal vascular resistance (RVR) were similar in all groups. 3. AII in bolus injection induced a short, dose-dependent increase in blood pressure and renal vascular resistance and a fall in renal blood flow. The maximal pressure increase was significantly greater in DS rats on high salt diet than that in each of the other groups (P less than 0.05). The fall in renal blood flow and the increase in renal vascular resistance were attenuated in both strains on low salt diet. 4. ET1 induced an initial decrease followed by a prolonged increase in blood pressure; both phases were similar in all groups. However, renal vascular reactivity to ET1 was markedly modulated by salt intake. On low salt diet, following a bolus injection of ET1 (1 nmol/kg), RBF decreased by 34% in DR and by 20% in DS rats, while on high salt diet RBF decreased by 76% in DR and by 80% in DS rats (P less than 0.05 high vs low salt).(ABSTRACT TRUNCATED AT 250 WORDS)

Role of the kidney in the pathogenesis of primary hypertension

Primary hypertension in animals and humans probably represents several different pathophysiological states rather than being a uniform nosological entity. Among other factors, renal mechanisms may be primarily and secondarily involved. The availability of genetically homologous animal models for hypertension has greatly promoted studies on the etiology and pathogenesis of high blood pressure disease. In particular, renal transplantation studies between genetically hypertensive and normotensive rats from three different models have provided strong evidence for a primary role of the kidney in genetic hypertension. Other factors, such as vascular, neural, and humoral mechanisms have also been shown to be involved and may be particularly effective in increasing blood pressure, when they act through the kidney. Several functional and biochemical differences have been identified between kidneys from genetically hypertensive and normotensive animals. However, the relative contribution of each of these factors to the development of primary hypertension remains to be determined. Evidence from studies on human renal graft recipients also indicates that, among other factors, the kidney plays an important role in the development of primary hypertension in humans.

Effect of dietary tryptophan on the development of hypertension in the Dahl salt-sensitive rat

Dahl salt-sensitive rats are inbred on the basis of their tendency to develop hypertension when placed on a high salt diet. The present study investigated the effects of chronic dietary tryptophan (trp) at 50 g/kg food on the development of hypertension in these animals under conditions of both normal and elevated dietary salt. Dietary trp attenuated the development of hypertension in inbred Dahl salt-sensitive (DS/JR) rats and had no effect upon the patterns of development of systolic blood pressures in the normotensive controls, the inbred Dahl salt-resistant (DR/JR) rat and the outbred parental Sprague Dawley (SD) rat. Cardiac hypertrophy, which is associated with Dahl salt-induced hypertension, was blocked by the high trp diet. Further work will be necessary to elucidate the mechanisms by which dietary trp protected against the development of hypertension and cardiac hypertrophy in DS/JR rats.

Effects of nisoldipine on atrial natriuretic peptides, blood pressure and cardiac hypertrophy in Dahl rats

The role of the calcium antagonist nisoldipine and the arteriolar vasodilator minoxidil on plasma levels of atrial natriuretic peptides (ANP), systolic blood pressure and heart weight was estimated in inbred Dahl salt sensitive (S) rats and inbred Dahl resistant (R) rats in long-term experiments. S rats develop quickly malignant hypertension, cardiac hypertrophy and have increased ANP plasma levels when fed a high salt diet (8% NaCl), while R rats on a high salt stay normotensive. In S rats 5 weeks on a high salt diet, therapeutic treatment with nisoldipine for 5 weeks not only decreased blood pressure but also produced a regression in cardiac hypertrophy and a reduction in elevated ANP plasma levels in comparison to the untreated salt-loaded S controls. Similar results were achieved in a preventive trial. In contrast with nisoldipine, therapeutic treatment with minoxidil in salt-loaded S rats lead to no reduction in cardiac hypertrophy and produced an additional increase in plasma ANP despite a reduction in blood pressure. The increase in plasma ANP level in this model of hypertension and its modulation by antihypertensive treatment with a calcium antagonist or an arteriolar vasodilator show that the changes in ANP plasma levels are probably secondary to hypertensive disease and the associated cardiac volume overload.

Sodium regulation of alpha 2-adrenoreceptors in Dahl rats. Effect of feeding a low or high salt diet

Sodium regulation of alpha 2-adrenoreceptors was investigated in inbred salt-sensitive (S) and inbred salt-resistant (R) rats fed a high or low salt diet. The systolic blood pressure was higher in S rats than in R rats, and this difference was obviously greater on a high salt diet. In rats fed a low or high salt diet, S rats had higher alpha 2-adrenoreceptor density in the kidneys compared with R rats as measured by [3H]yohimbine binding and Scatchard analysis. The affinity of the receptors in the kidney for the antagonist, yohimbine, was nearly the same in these two strains either on a low or high salt diet. In the brain, the affinities or the numbers of receptors were not significantly different whether these two strains were fed a low or high salt diet. Inclusion of NaCl up to 80 mM in the assay medium did not alter the in vitro binding of [3H]yohimbine in the kidney or brain. On the other hand, inclusion of NaCl in the assay medium reduced the ability of epinephrine in competing with [3H]yohimbine for the receptor sites in the kidney and in the brain, and this effect of NaCl was the same in a given tissue between S and R rats, whether they were fed a low or high salt diet. These results suggest that: (1) in the kidneys, the receptor density and not the receptor affinity was different between S and R strains whether they were fed a low or high salt diet; (2) in the brain, the receptor density and affinity were the same between S and R rats regardless of the diet (low or high salt), indicating that the sodium salt diet modulates the peripheral but not the central alpha 2-adrenoreceptors; and this modulatory effect was observed only in S rats; (3) Na+ was able to reduce the affinity of the agonist (epinephrine) for the receptors in both S and R rats, and this effect of Na+ on central and peripheral alpha 2-adrenoreceptors was similar in prehypertensive rats and rats with salt-induced hypertension; and (4) the resistance of R rats to salt-induced hypertension was not due to the absence of Na+ binding component involved in the regulation of alpha 2-adrenoreceptor-adenylate cyclase complex.

Neural control of renal function: cardiovascular implications

The innervation of the kidney serves to function of its component parts, for example, the blood vessels, the nephron (glomerulus, tubule), and the juxtaglomerular apparatus. Alterations in efferent renal sympathetic nerve activity produce significant changes in renal blood flow, glomerular filtration rate, the reabsorption of water, sodium, and other ions, and the release of renin, prostaglandins, and other vasoactive substances. These functional effects contribute significantly to the renal regulation of total body sodium and fluid volumes with important implications for the control of arterial pressure. The renal nerves, both efferent and afferent, are known to be important contributors to the pathogenesis of hypertension. In addition, the efferent renal nerves participate in the mediation of the excessive renal sodium retention, which characterizes edema-forming states such as congestive heart failure. Thus, the renal nerves play an important role in overall cardiovascular homeostasis in both normal and pathological conditions.

High-salt diet elevates baroreceptor pressure thresholds in normal and Dahl rats

Dahl Salt Sensitive (DS) rats rapidly develop high blood pressure when exposed to a high-salt diet. Recent studies suggest that DS rats have poorly functioning baroreceptor afferents and baroreflexes even when salt intake is restricted. This study examines baroreceptor pressure- and mechano-transduction in DS, Dahl Resistant (DR), and Sprague-Dawley (SD) rats during low- and high-salt conditions. Single unit, regularly discharging baroreceptors were studied using an in vitro aortic arch-aortic nerve preparation. Pressure thresholds and suprathreshold pressure sensitivities were determined from responses to slow ramps of pressure. Pressure-diameter relations measured in each rat were used to transform pressure threshold and pressure sensitivity values to their mechanical equivalents in terms of aortic wall strain. A total of 407 unit baroreceptors were studied from 49 rats. Tail systolic blood pressures were significantly higher only in DS during high salt. Pressure threshold was similar for all groups on low salt. Exposure to a high-salt diet increased the mean pressure threshold for all three groups. Pressure threshold for high-salt diet was highest in DS and lowest in DR. Pressure sensitivities were lowest in DS and highest in DR on low salt. High salt had no significant effect on pressure sensitivity. The differences in threshold apparent when expressed in terms of pressure were eliminated by conversion to their mechanical equivalents (strain threshold and strain sensitivity). The results suggest that baroreceptors in the two Dahl rat strains represent two extremes from normal baroreceptor function. DS tend to be less pressure responsive than normal (SD), and DR tend to be somewhat more responsive to pressure.(ABSTRACT TRUNCATED AT 250 WORDS)

Sequential changes of cerebrospinal fluid sodium during the development of hypertension in Dahl rats

The role of sodium retention and consequent changes in cerebrospinal fluid sodium concentration in the genesis of hypertension in Dahl rats was evaluated. Dahl salt-sensitive (DS, n = 7), Dahl salt-resistant (DR, n = 7), and Sprague-Dawley (n = 6) rats were housed in metabolic cages and instrumented with a stainless steel cannula in the cisterna magna and a femoral arterial catheter. A blood sample was drawn daily (200 microliters), and cerebrospinal fluid was collected by continuous 24-hour withdrawal (200 microliters/day). Daily sodium, potassium, and water balances were also determined. Rats were studied sequentially on 0.4%, 4%, and 8% sodium diets (7 days per sodium level). Mean arterial pressure increased with 4% NaCl from 107 to 120 mm Hg (p less than 0.05) over 24 hours in DS rats and remained at about that level until the NaCl was increased to 8%, which resulted in a gradual rise of mean arterial pressure over the next 7 days to 135 mm Hg. Cerebrospinal fluid sodium was unchanged in DR and Sprague-Dawley rats fed 4% or 8% sodium, but in DS rats rose from 152.3 to 155.2 +/- 0.6 meq/l on the third day at 4% sodium and remained elevated over the next 2 weeks of study. Blood sodium was unchanged throughout the study in all groups. On the first day only of the 4% and 8% sodium diets, both DS and DR rats exhibited a similar net retention of sodium, which was greater than the Sprague-Dawley rats (p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Sympathetic-adrenal medullary responses to acute stress in Dahl hypertensive (S/JR) rats

Sympathetic-adrenal medullary responses to acute footshock stress were assessed in inbred Dahl salt-sensitive (S/JR) and salt-resistant (R/JR) rats by measuring plasma levels of norepinephrine (NE) and epinephrine (EPI). Ten-week-old S/JR and R/JR rats were surgically prepared with indwelling tail artery catheters which permitted direct measurements of mean arterial pressure (MAP, mmHg) and heart rate (HR, beats/min) and remote sampling of blood. Two days after surgery, S/JR and R/JR rats were subjected to an acute stress paradigm. Blood samples were collected before and 3 minutes after transfer of rats to a shock chamber, after 1 minute of intermittent footshock, and again 5 minutes later. S/JR rats had significantly higher resting MAP's compared to R/JR rats. In contrast, baseline heart rates were similar for rats of the two strains. Basal plasma levels of NE and EPI were also similar in S/JR and R/JR rats. Upon transfer from the home cage to a shock chamber, S/JR rats exhibited significant increases in plasma levels of both catecholamines, while R/JR rats maintained circulating levels of NE and EPI that were near baseline values. However, S/JR and R/JR rats had similar increments in plasma NE and EPI following acute footshock stress. Five minutes after footshock, levels of NE and EPI returned toward baseline values for R/JR's, but remained significantly elevated above baseline in hypertensive S/JR rats. These data suggest that S/JR rats are more responsive than R/JR controls to the mild stress of transfer, but exhibit comparable responses to the more intense stress of inescapable footshock.(ABSTRACT TRUNCATED AT 250 WORDS)

Failure of salt loading to inhibit tissue norepinephrine turnover in prehypertensive Dahl salt-sensitive rats

To determine if alterations of electrolyte balance or sympathetic nervous system activity are present in Dahl salt-sensitive rats (DS) before the onset of hypertension, we compared electrolyte balances, extracellular fluid volume (inulin space), plasma volume (radiolabeled albumin), and norepinephrine turnover in peripheral tissues (heart and interscapular brown fat) in prehypertensive DS and Dahl salt-resistant rats (DR). Animals were maintained for 5 to 7 days on either a "normal" or high NaCl diet. Tissue norepinephrine turnover was evaluated by measuring the rate at which norepinephrine content decreased following tyrosine hydroxylase inhibition with alpha-methyl-p-tyrosine. Blood pressure was higher (p less than 0.05) in DS (135 +/- 2 [SE] mm Hg) than in DR (129 +/- 2 mm Hg) and was not affected by the diets. Extracellular fluid volume and net Na+ and Cl- balances did not differ between DS and DR. However, plasma volume was greater in DS than in DR (p less than 0.05). In both fat and heart, norepinephrine turnover was decreased by dietary NaCl loading in DR (p less than 0.01), but not in DS. Thus, the tendency of the DS to become hypertensive with high NaCl intake may be related to the combined effects of an increased plasma volume and the failure of high dietary NaCl to inhibit peripheral sympathetic nervous system activity.

Function of the hypertensive kidney during calcium flux manipulation

Dihydropyridine calcium channel agonists and antagonists elicit exaggerated glomerular and circulatory responses from kidneys isolated from Dahl rats genetically programmed to develop NaCl-induced hypertension (Dahl S rats). These differential responses are further magnified by NaCl loading. In contrast, "chemical sympathectomy" with 6-hydroxydopamine enhances renal vascular responses to calcium channel agonists in a manner that depends on the antecedent dietary NaCl intake, and is independent of genetic predilection to develop NaCl-induced hypertension. These findings are consistent with the hypothesis that aberrations of vascular and perhaps glomerular calcium entry modulation may be determinants of altered renal hemodynamics in NaCl-sensitive hypertension. The latter may be responsible for the enhanced responsiveness to calcium channel antagonists observed in NaCl-sensitive hypertension in humans.

Sodium sensitivity in human subjects. Hemodynamic and hormonal correlates

To investigate factors associated with sodium sensitivity, 157 subjects were studied while receiving 10 and 200 mEq sodium diets. Measurements included blood pressure (BP), forearm vascular resistance, plasma renin activity (PRA), and plasma aldosterone. Sodium repletion was associated with a greater than 5% increase in mean BP in 16% of the normotensive subjects and 29% of the borderline hypertensive subjects. Sodium-sensitive subjects were compared with sodium-resistant subjects in both the normotensive (n = 92) and borderline hypertensive (n = 65) groups. Forearm vascular resistance was significantly higher (p less than 0.05) during sodium loading in the sodium-sensitive subgroups of both the normotensive and borderline hypertensive groups (35.8 +/- 29 vs 23.8 +/- 20 [SD] and 37.5 +/- 29 vs 22.5 +/- 14 mm Hg/ml/min/100 g, respectively. Venous capacitance was lower in the sodium-sensitive than in the sodium-resistant borderline hypertensive subjects (0.8 +/- 0.21 vs 1.69 +/- 0.24 ml/100 g). During sodium restriction, PRA was significantly lower (p less than 0.01) in the sodium-sensitive subsets (2.56 +/- 1.6 vs 4.04 +/- 2.6; 2.65 +/- 2.1 vs 3.88 +/- 2.6 ng angiotensin I/ml/hr). Aldosterone was lower (p less than 0.01) during sodium depletion in the sodium-sensitive subsets (17.3 +/- 12 vs 26.3 +/- 16; 18.5 +/- 18 vs 27.9 +/- 17 ng/ml). A significant inverse correlation existed between change in BP with sodium repletion and change in PRA or level of PRA during sodium depletion (p less than 0.003).(ABSTRACT TRUNCATED AT 250 WORDS)

Renal disease and the development of hypertension in salt-sensitive Dahl rats

To elucidate the role of the kidneys in the development of hypertension in Dahl salt-sensitive (S), as compared to resistant (R) rats of the JR strain, we analyzed functional and morphological changes before and after the administration of an 8% NaCl diet and the onset of hypertension. The diet was begun at six weeks of age and was continued until 12 weeks of age. At six weeks, blood pressure was not different between S and R rats. Hypertension occurred in S rats receiving the 8% NaCl diet at week 8, and in S rats receiving 0.9% NaCl at week 10. Albuminuria and proteinuria were found in S rats prior to the 8% NaCl diet and progressed regardless of diet. Electron microscopy of glomeruli revealed segmental loss of epithelial foot processes in S rats at six weeks prior to the 8% NaCl diet. Mesangial widening, arteriolar myo-intimal cell hyperplasia and interstitial fibrosis occurred in all S rats. Inulin and PAH clearances in S rats decreased with time, the changes being accelerated by the 8% NaCl diet. Micropuncture of S and R rats prior to the 8% NaCl diet revealed no glomerular hypertension in S rats. The number of glomeruli in S and R rats were not different. We conclude that prehypertensive S rats of the JR strain already have albuminuric glomerular disease not associated with reduced number of glomeruli or glomerular hypertension. The renal pathology is accelerated once hypertension develops. A lower NaCl intake delays, but does not prevent renal disease in S rats.

Renal nerves and the development of Dahl salt-sensitive hypertension

Several experimental forms of hypertension require intact renal innervation for the development or maintenance (or both) of the elevated arterial pressure. We determined the relationships between urinary sodium and water excretion and arterial pressure in Dahl salt-sensitive rats (DS) with innervated (n = 6) and denervated (n = 7) kidneys after switching from a low to a high sodium diet. Arterial pressure significantly increased in both groups within 48 hours after they began to eat an 8% sodium chloride diet. This hypertension increased to 188 +/- 9 and 190 +/- 7 mm Hg, respectively, in rats with innervated and denervated kidneys after 12 days. Mean arterial pressures were not significantly different between groups on any day. The rise in arterial pressure of DS placed on a high sodium intake was associated with an elevation of urine flow rate and urinary sodium excretions in rats with either innervated or denervated kidneys. Urine flow rates and urinary sodium excretions were greater in denervated than in innervated rats on Days 4 through 7 after beginning the high sodium diet. This diuresis and natriuresis in rats with denervated kidneys were associated with greater water and sodium intakes on Days 4 to 7 of the high sodium diet when compared with rats with innervated kidneys. These results demonstrate that, following exposure to a high sodium intake, DS have increased arterial pressure within 24 hours. The development of this arterial hypertension is not dependent on intact renal innervation. In conscious DS, the renal innervation does participate in the regulation of urinary sodium excretion by promoting renal sodium and water reabsorption.(ABSTRACT TRUNCATED AT 250 WORDS)

Stress increases renal nerve activity and decreases sodium excretion in Dahl rats

The effects of a stressful environmental stimulus (air stress) on mean arterial pressure, renal sympathetic nerve activity, and renal function were examined in conscious Dahl salt-sensitive (DS) and Dahl salt-resistant rats (DR) on low (0.4%) and high (8%) NaCl diets. Air stress increased renal sympathetic nerve activity and decreased urine flow rate and urinary sodium excretion in conscious Dahl rats on a high sodium diet, but it had no effect in rats on a low sodium diet. Mean arterial pressure did not change during air stress in any group. Renal denervation prevented the antidiuretic and antinatriuretic responses to air stress in DS and DR on a high NaCl diet. An increased renal tubular reabsorption of sodium and water appeared to mediate the antinatriuretic and antidiuretic responses to air stress, since glomerular filtration rate and renal plasma flow were unchanged. Thus, environmental stress increases renal sympathetic nerve activity and decreases urinary sodium excretion more in Dahl rats on a high NaCl diet than on a low NaCl diet. On a high NaCl diet, these responses are greater in DS than in DR.

Ganglionic immunoreactive atrial natriuretic factor in rat experimental hypertension

Because previous data have suggested a dependence of ganglionic atrial natriuretic factor (ANF) content on preganglionic cholinergic input, we investigated the possibility that the increased neural activity observed in spontaneously hypertensive rats (SHR) may be reflected by ganglionic immunoreactive ANF levels. Four-week-old normotensive SHR had celiac ganglionic immunoreactive ANF values comparable to those of Wistar-Kyoto rats (WKY). When they became hypertensive, however, at 12 weeks of age, the SHR manifested higher immunoreactive ANF levels in celiac ganglia than the WKY group (25.3 +/- 2.6 vs 14.5 +/- 1.7 pg/ganglion; p less than 0.01), but there were no differences in levels in the superior cervical and nodose ganglia. The values in celiac ganglia were quadrupled on the average in hypertensive Dahl salt-sensitive rats under the influence of an 8% salt intake for 5 weeks, but no difference was noted in any of these ganglia between this group and their salt-resistant partners. The celiac and superior cervical ganglionic immunoreactive ANF content in normotensive Sprague-Dawley rats was higher with high salt than with normal salt intake. Hypertensive rats treated with deoxycorticosterone acetate (DOCA)-salt and sham-treated controls showed immunoreactive ANF concentrations in celiac ganglia similar to those detected in Dahl rats but, again, no differences were found between groups. Thus, hypertensive SHR, compared to WKY, have higher celiac ganglionic immunoreactive ANF levels, unlike Dahl salt-sensitive and DOCA-salt animals relative to their respective controls. This increase is unique to SHR (although all three models have elevated plasma immunoreactive ANF when they are hypertensive) and to the celiac ganglia.(ABSTRACT TRUNCATED AT 250 WORDS)

Brain binding sites for atrial natriuretic factor (ANF): alterations in prehypertensive Dahl salt-sensitive (S/JR) rats

The binding of radioiodinated atrial natriuretic factor (125I-ANF-28) to discrete areas of brain in 7 week old, inbred Dahl salt-sensitive (S/JR) and salt-resistant (R/JR) rats was studied utilizing quantitative film autoradiography. At this age, S/JR rats exhibit systolic blood pressures that are prehypertensive and tend to be slightly higher than systolic blood pressures of age-matched R/JR rats. Scatchard analysis of 125I-ANF-28 binding in forebrain revealed that S/JR rats have a significantly increased number of binding sites for 125I-ANF-28 in the subfornical organ as compared to R/JR controls. In contrast, values for 125I-ANF-28 binding capacity in the choroid plexus and area postrema were similar for both strains, and binding affinity constants for 125I-ANF-28 binding revealed no strain differences in any brain area examined. The elevation in the number of binding sites for atrial natriuretic factor may serve as a compensatory mechanism acting in part to lower fluid volume and sodium levels prior to the precipitous increase in blood pressure which occurs in S/JR rats by 10 weeks of age.

Differential ontogeny of alpha 1-adrenergic and cholinergic receptor sites in the atria and ventricles of the inbred Dahl hypertension-sensitive (S/JR) and -resistant (R/JR) rat

The ontogeny of atrial and ventricular alpha 1-adrenergic and muscarinic cholinergic receptor sites was investigated in inbred Dahl hypertension-sensitive (S/JR) and -resistant (R/JR) rats between 5 and 150 days of age. The density of sites in both cardiac regions was generally greater in the neonate than mature rat. A marked proliferation of sites was observed in neonatal and young adult rats that occurred in the following order: ventricular cholinoceptors----ventricular adrenoceptors----atrial cholinoceptors----atrial adrenoceptors. The density of ventricular adrenoceptors was greater in the S/JR rat than the R/JR rat at 5 days of age. At 150 days of age, the density of sites was less in the S/JR rat than the age-matched R/JR rat or the normotensive 50-day-old S/JR rat. The development of atrial adrenoceptors was similar between the strains, regardless of the blood pressure. The density of ventricular cholinergic receptors was greater in the S/JR strain at 5 and 15 days of age. However, the density of atrial cholinergic sites was consistently greater in the S/JR strain throughout development. The results of this study suggest that: (1) significant prenatal receptor development occurs in the heart; (2) receptor development may precede the functional maturation of postganglionic autonomic efferents; and (3) distinguishing differences in the regional density of alpha 1-adrenergic and muscarinic cholinergic binding sites are present between S/JR and R/JR rats at much earlier points in development than previously shown.

Sensitization of aortic baroreceptors by high salt diet in Dahl salt-resistant rats

High salt diet alters neural cardiovascular control. This influence has been attributed to central neural or efferent mechanisms. To test the hypothesis that a high salt diet might alter afferent baroreceptor function, Dahl salt-resistant (DR) and salt-sensitive rats (DS) were fed a high or a low salt diet. Blood pressure was measured intra-arterially in unanesthetized animals. Aortic baroreceptor function was then evaluated during urethane anesthesia by recording multifiber aortic depressor nerve activity during a phenylephrine-induced blood pressure ramp. Mean arterial pressure in the conscious state was elevated (155 +/- 5 [SE]mm Hg) in DS fed a high salt diet but was normal in DS fed a low salt diet and in DR. Slopes of linear regressions relating aortic nerve discharge to mean arterial pressure were 71% higher in DR fed a high salt diet than in DR fed a low salt diet (p less than 0.025), indicating that high salt potentiated baroreceptor function in DR. In contrast, high salt diet produced no significant effects on baroreceptor function in DS. No salt-induced changes in dynamic or static aortic distensibility (assessed from pressure-volume curves of the in situ isolated arch) were detectable in either rat strain. Absence of salt-induced baroreceptor sensitization in DS was not due to the hypertensive state because the sensitization also failed to occur in separate groups of DS in which salt-induced hypertension had been prevented by chemical sympathectomy with 6-OH-dopamine.(ABSTRACT TRUNCATED AT 250 WORDS)

Hypertension-induced alterations in copper and zinc metabolism in Dahl rats

It has been suggested that one risk factor in the development of hypertension and vascular disease may be abnormal copper and zinc metabolism. In the current study we tested the hypothesis that hypertension itself may result in alterations in the metabolism of these essential elements. Dahl salt-sensitive rats were fed diets containing 0.4 or 8.0% NaCl for 32 days. At the conclusion of the study, blood pressure was significantly higher in the rats fed a high NaCl diet than in controls. Liver, kidney, and heart copper concentrations were significantly lower in the rats fed a high NaCl diet compared with controls, while plasma copper levels were higher. In contrast, tissue zinc levels were higher in the rats fed a high NaCl diet than in controls, while plasma zinc levels were lower. It is hypothesized that alterations in copper and zinc metabolism may be one factor underlying tissue damage in these animals.

Binding characteristics of atrial natriuretic factor and the production of cyclic GMP in kidneys of Dahl salt-sensitive and salt-resistant rats

The binding of atrial natriuretic factor (ANF) was studied in kidney membranes of inbred salt-sensitive (S) and inbred salt-resistant (R) rats on high or low salt diet. Important differences between strains were seen in the rate of dissociation of ANF from its renal receptor(s) and this was dependent on salt (NaCl) intake. On low salt diet ANF dissociation rates were similar between strains. R rats responded to high salt diet with a decrease in the rate of ANF dissociation from its renal receptor, but ANF dissociation in S rats was not altered by dietary salt. Receptor density was similar between strains. Basal cGMP production was slightly higher for renal membranes of S rats, but ANF stimulation of cGMP production was similar between strains and was not influenced by salt intake in either strain. Since strain-related salt-induced changes in ANF-receptor binding kinetics were not reflected in any strain-related salt-induced changes in ANF stimulated cGMP production, it is tentatively concluded that the ANF receptor likely to be different between S and R strains is the ANF receptor not linked to cGMP production.

Response of isolated Dahl rat kidney to calcium antagonists

We studied the responses of isolated perfused kidneys from prehypertensive, salt-sensitive (DS) and salt-resistant (DR) Dahl rats to nitrendipine or verapamil, after norepinephrine vasoconstriction. The perfusion pressure was kept constant. Superimposition of these calcium antagonists upon norepinephrine increased DS GFR by 155% and DR GFR by 58% (P = 0.03), with verapamil increasing the GFR more than nitrendipine (P = 0.02). Nitrendipine and verapamil also partially reversed norepinephrine induced increases in renal vascular resistance, but did not decrease vascular resistance or increase GFR in the absence of norepinephrine. During the increase in GFR produced by calcium antagonists, DR sodium excretion increased, but DS sodium excretion did not. Therefore, calcium antagonists disproportionately increased DS kidney GFR but did not correct DS kidney sodium retention. These data raise the possibility that the DS rat kidney possesses an abnormality of cell calcium regulation affecting glomerular dynamics, and provide evidence that the renal perfusion pressure is more critical than the GFR in adjusting DS rat sodium-excretion.

Sympathetic responses of the heart and adrenal medulla in developing Dahl hypertensive rats

Functional development of the sympathetic nervous system was examined in inbred Dahl salt-sensitive (S/JR) and salt-resistant (R/JR) rats by assessing cardiac and adrenal medullary responses to insulin-induced hypoglycemia at 2, 4, 8, 12, and 16 days of age. Heart ornithine decarboxylase (ODC) activity and adrenal catecholamine content were measured in pups of the two strains 3 hours after administration of either saline or insulin. The centrally mediated increase in sympathetic outflow caused by insulin-induced hypoglycemia was attended by induction of heart ODC activity and depletion of adrenal epinephrine (EPI). No significant differences were found overall between R/JR and S/JR strains with regard to either heart ODC activity or adrenal epinephrine. This was true for basal values obtained from saline-injected pups as well as for measures from insulin-injected pups. Functional innervation of the heart was present in pups of both strains as early as 2 days of age, while in the adrenal medulla a significant response to stimulation was not detected until 8 days of age. While the susceptibility for hypertension in the salt-sensitive animals may well be linked to increased sympathetic tone, the present findings indicate that S/JR rats do not have an accelerated development or a hyperresponsiveness of sympathetic input to either the heart or the adrenal medulla during the pre-weanling period.

Contrasting dopaminergic patterns in two forms of genetic hypertension

Dopaminergic mechanisms in genetic hypertension were explored via the measurement of catecholamine (CA) turnover, tissue concentration and urinary excretion of dopamine (DA) and its metabolites. In salt-sensitive (S) Dahl rats, the tissue concentration and urinary excretion of DA and its metabolites were decreased in response to salt loading, while adrenal dopamine-beta-hydroxylase (D beta H) activity and aldosterone responsiveness to angiotensin II (A II) were increased. In contrast, spontaneously-hypertensive rats (SHR) showed elevated tissue levels and urinary excretion of DA and its metabolites, adrenal DA turnover and ganglionic DA generation following cholinergic stimulation, but D beta H activity and aldosterone responsiveness to A II were diminished. These two patterns, the hypodopaminergic state in Dahl S rats and the hyperdopaminergic state in SHR, account for two distinct mechanisms of hypertension and precede its development. We detected striking dopaminergic activity-related similarities between Dahl S rats and low-renin essential hypertension (EH) on the one hand, and SHR and non-modulator EH patients on the other.

Dietary sodium intake and urinary dopamine and sodium excretion during the course of blood pressure development in Dahl salt-sensitive and salt-resistant rats

Recent studies have suggested that dopamine (DA) formed within the kidney may play an important role in promoting sodium excretion, and that renal production and excretion of DA is determined by dietary sodium intake. Inasmuch as increased sodium consumption produces hypertension in Dahl salt-sensitive (DS) rats but not in Dahl salt-resistant (DR) rats, the present study was designed to examine the relationship between sodium consumption and urinary excretion of DA in these rats. DS and DR rats were placed on either high sodium chloride (8%) or low sodium chloride (0.4%) diets at 4 weeks of age and their systolic blood pressure (SBP), urine volume, urinary sodium and catecholamine excretion were measured once every week for the next 4 weeks. High sodium chloride diet increased SBP in DS rats at 6 weeks of age and SBP continued to rise until they were 8 weeks old. The SBP of DR rats did not reach hypertensive levels when they were given high sodium chloride diet. The SBP of DS rats on low sodium chloride diet was significantly higher than DR rats on the same diet. The urinary DA excretion increased with age in all four groups of rats and was similar when they were 8 weeks old. However, both DS and DR rats on high sodium chloride diet excreted greater amounts of sodium and had increased urine volume compared to the DS and DR rats on low sodium chloride diet. There were no significant differences in urinary NE or E excretion in these four groups of rats. Kidney levels of DA and NE were significantly lower in DS compared to DR rats on high sodium chloride diet. These results show that although there are no differences in urinary DA excretion between rats on low and high sodium intake, both DS and DR rats on high sodium chloride diet are able to exhibit a natriuretic response. The DS rats eliminate sodium at the expense of an elevated SBP whereas DR rats stay normotensive. Therefore, it appears that alterations in mechanisms controlling renal vascular resistance rather than sodium excretion are responsible for the development of hypertension in DS rats.

Salt level in weaning diet affects saline preference and fluid intake in Dahl rats

Weanling Dahl salt-sensitive (DS) and salt-resistant (DR) rats were used to compare effects of feeding high or low NaCl diets on taste preference for, and intake of, a wide range of saline concentrations. The DS and DR were fed either 8.0 or 0.4% dietary NaCl for 4 weeks. Then, with all animals fed the 0.4% NaCl diet, their taste preferences for 0.0001 to 0.56 M saline were assessed using three 24-hour two-bottle preference tests of each solution versus distilled deionized water. Saline preference and intake were influenced by concentration and its interaction with genotype, with DS exhibiting higher preferences than DR for hypotonic saline. The DS preexposed to 8.0% dietary NaCl showed elevated consumption levels of water and total fluid (saline + water) that persisted throughout the 5-week test period, despite transfer to the 0.4% NaCl diet before the initiation of preference testing. Findings indicate that genotype, dietary NaCl levels in weaning diet, and saline concentration of preference test solutions interact to influence saline preference and saline and water intake in Dahl rats.

The ontogeny of renal alpha 1- and alpha 2-adrenoceptors in the Dahl rat model of experimental hypertension

[3H]prazosin (PRAZ) and [3H]rauwolscine (RAUW) were used to examine the ontogeny of renal alpha 1- and alpha 2-adrenoceptors in the inbred Dahl hypertension-sensitive (S/JR) and -resistant (R/JR) rat. PRAZ and RAUW each bound to a single population of non-interacting sites. The binding of each ligand was saturable and reversible. The greatest proliferation of each receptor subtype occurred between 5 and 25 days of age. During this period, a 4 to 5-fold increase in the density of each was observed. Adult levels of each were reached by 50 days of age. The alpha 2-adrenoceptor was the predominant subtype present in renal tissue. However, its ratio to the alpha 1 subtype was influenced by strain and age: the ratio was greatest in the S/JR strain and decreased with age in both strains. The profile of alpha 1-adrenoceptor development was similar in S/JR and R/JR rats. In contrast, the density of alpha 2-adrenoceptors was similar in S/JR and R/JR rats at 5 and 15 days of age but significantly greater in the S/JR rat between 25 and 150 days of age. The elevated density of alpha 2-adrenoceptors could not be explained by strain-related differences in blood pressure or alterations in the affinity of the receptor. The results suggest that a relationship may exist between elevated renal alpha 2-adrenoceptor density and the genetic predisposition to hypertension in the S/JR rat. However, because this relationship is not apparent during the neonatal period of development, the possibility that the elevated density of sites may be secondary to some other event should also be considered.

Up-regulation of renal prostaglandin receptors in genetic salt-dependent hypertension

Development of hypertension in Dahl salt-sensitive rats (DS) is accompanied by reduced renomedullary prostaglandin synthesis, which may be responsible for their lower natriuretic capacity. To examine the changes in renomedullary prostaglandin E2 synthesis, the effects of high (8.0%) and normal (0.6%) NaCl diets were examined in DS and in Dahl salt-resistant rats (DR). In response to an 8.0% NaCl diet, the number of prostaglandin E2 receptors in the renal outer medulla of DR increased (2.97 +/- 0.2 vs 2.18 +/- 0.2 pmol/mg on 0.6% NaCl diet) while no change was noted in their affinities (Kd, 9.5 +/- 0.2 vs 9.4 +/- 0.3 nM). Receptor number and affinity in the renal cortex, inner medulla, and liver of DR were not affected. In contrast, renomedullary receptors of DS had a lower affinity than those of age-matched DR (Kd, 13.9 +/- 0.2 nM on 0.6% NaCl diet and 14.0 +/- 0.3 nM on 8.0% NaCl diet) and did not increase in number after a high salt diet. This apparent inability of DS to modulate prostaglandin receptors may contribute to their susceptibility to salt-induced hypertension.

Pre- and neonatal exposure of the Dahl rat to NaCl: development and regional distribution of myocardial alpha 1-adrenergic and cholinergic receptor sites

The prenatal and/or postweaning effects of a hypertensinogenic high NaCl-containing diet (8.0% NaCl, w/w) on (1) the regional distribution of alpha 1-adrenoceptors and muscarinic cholinergic receptor sites in the heart and (2) the predisposition/resistance to hypertension (HT) were assessed in the inbred Dahl HT-sensitive (S/JR) and HT-resistant (R/JR) rat. The density of alpha 1-adrenoceptors was reduced in the left ventricle but not consistently affected in the ventricular septum, right ventricle, or atria of S/JR offspring with NaCl-induced HT. Both normotensive and hypertensive S/JR rats also displayed a significantly greater density of cholinergic receptor sites in the atria but few consistent alterations in other regions of the heart, compared to R/JR rats. Maternal diet had no effect on the predisposition/resistance to salt-induced HT and little effect on the regional development of alpha 1-adrenoceptors and cholinergic receptor sites. The results of this study suggest that the reduced density of ventricular alpha 1-adrenoceptors in the S/JR strain is a consequence of HT while the elevated density of cholinergic receptors in the atria may be related to the genetic predisposition/resistance to HT.