Hemodynamic and metabolic evidence of salt sensitivity in spontaneously hypertensive rats

The Interaction of Kidneys and Gut in Development of Salt-Sensitive Hypertension

The incidence of salt-sensitive hypertension is quite common and varies between 30-60% in hypertensive patients. Regarding the causal role of high salt intake in the development of salt-sensitive hypertension, recent evidence has demonstrated that the gut through its microbiota plays a significant role in its genesis. Besides the gut, the kidneys also play important role in salt-sensitive hypertension and there is clinical and experimental evidence of an interrelationship between the gut and the kidneys in the development of salt-sensitive hypertension through the so-called "gastro-renal axis." The gut besides being an absorptive organ, it is also a hormonal secretory organ involving the secretion of gastrin, dopamine, norepinephrine, angiotensin, and aldosterone which through their action with the kidneys are involved in the development of salt-sensitive hypertension. In addition, the kidneys exert a protective role against the development of hypertension through the secretion of prostaglandins and their vasodilatory action. To assess the current evidence on the role of high salt intake and the interplay of the gut and kidneys in its development, a Medline search of the English literature was contacted between 2012 and 2022, and 46 pertinent papers were selected. These papers together with collateral literature will be discussed in this review.

The role of gut microbiota in the development of salt-sensitive hypertension and the possible preventive effect of exercise

INTRODUCTION

The aim of the present study is to analyze the data indicating an association between high salt intake and the gastrointestinal microbiota in the development of salt-sensitive hypertension in animals and men. It is also, to discuss the preventive effects of exercise on gut-induced hypertension by favorably modifying the composition of gut microbiota.

AREAS COVERED

Salt sensitivity is quite common, accounting for 30%-60% in hypertensive subjects. Recently, a novel cause for salt-sensitive hypertension has been discovered through the action of gut microbiota by the secretion of several hormones and the action of short chain fatty acids (SCFAs). In addition, recent studies indicate that exercise might favorably modify the adverse effects of gut microbiota regarding their effects on BP. To identify the role of gut microbiota on the incidence of hypertension and CVD and the beneficial effect of exercise, a Medline search of the English literature was conducted between 2018 and 2023 and 42 pertinent papers were selected.

EXPERT OPINION

The analysis of data from the selected papers disclosed that the gut microbiota contribute significantly to the development of salt-sensitive hypertension and that exercise modifies their gut composition and ameliorates their adverse effects on BP.

Sodium Accumulation and Blood Capillary Rarefaction in the Skin Predispose Spontaneously Hypertensive Rats to Salt Sensitive Hypertension

Recent studies in humans and rats suggested that increased Na⁺ storage in the skin without parallel water retention may predispose to salt-sensitive hypertension. In the current studies, we compared tissue Na⁺ storage in salt sensitive spontaneously hypertensive rats (SHR) versus salt resistant normotensive Brown Norway (BN-Lx) rats. After salt loading (10 days drinking 1% NaCl solution), the SHR showed significant parallel increase in Na⁺-to-water as well as (Na⁺+K⁺)-to-water ratios suggesting increased storage of osmotically inactive Na⁺ in the skin while no significant changes in skin electrolyte concentrations were observed in BN-Lx rats. SHR rats after salt treatment exhibited a nonsignificant decrease in skin blood capillary number (rarefaction) while BN-Lx rats showed significantly increased skin blood capillary density. Analysis of dermal gene expression profiles in BN-Lx rats after salt treatment showed significant up-regulation of genes involved in angiogenesis and proliferation of endothelial cells contrary to the SHR. Since the skin harbors most of the body’s resistance vessels it is possible that blood capillary rarefaction may lead to increased peripheral resistance and salt sensitivity in the SHR.

Voiding behavior in awake unrestrained untethered spontaneously hypertensive and Wistar control rats

The spontaneously hypertensive rat (SHR), a genetic model of high blood pressure, has also been studied as a potential model of overactive bladder. In vivo studies have confirmed the presence of surrogate markers of overactive bladder, including detrusor overactivity, increased urinary frequency, decreased bladder capacity and voided volume (VV), and afferent hypersensitivity to bladder irritation. However, these observations were during awake cystometry using implanted bladder catheters tethered to an infusion pump and artificially filled. We conducted experiments in awake unrestrained untethered age-matched female SHRs and Wistar rats to quantify naïve consumption and voiding behavior and the effect of capsaicin desensitization on consumption and voiding behavior. Food and water consumption, body weight, voiding frequency, and VV were recorded. Rats were placed in metabolism cages for 24 h, up to twice a week, from 17 to 37 wk of age. Compared with Wistar rats, SHRs exhibited decrease in VV and did not exhibit diurnal variation in VV between light and dark periods, suggesting that SHRs may have bladder hypersensitivity. Furthermore, SHRs may also have smaller bladder capacities, as they consumed less water, voided less volume (regardless of light cycle), and had equal urinary frequencies compared with age-matched Wistar rats. We detected no change in SHR voiding behavior following capsaicin desensitization, which was in contrast to a prior awake in vivo cystometry study describing increased VV and micturition interval in SHRs and suggests that C-fiber activity may not contribute to bladder hypersensitivity in SHRs.NEW & NOTEWORTHY We characterized the long-term (20 wk) voiding, defecation, and consumption behavior of age-matched spontaneously hypertensive and Wistar rats without the influence of anesthesia or catheters. Spontaneously hypertensive rats exhibited bladder hypersensitiviy that persisted for the 20-wk duration and was unaffected by capsacin desensitization.

Renal denervation and CD161a immune ablation prevent cholinergic hypertension and renal sodium retention

Cholinergic receptor activation leads to premature development of hypertension and infiltration of proinflammatory CD161a⁺/CD68⁺ M1 macrophages into the renal medulla. Renal inflammation is implicated in renal sodium retention and the development of hypertension. Renal denervation is known to decrease renal inflammation. The objective of this study was to determine the role of CD161a⁺/CD68⁺ macrophages and renal sympathetic nerves in cholinergic-hypertension and renal sodium retention. Bilateral renal nerve denervation (RND) and immune ablation of CD161a⁺ immune cells were performed in young prehypertensive spontaneously hypertensive rat (SHR) followed by infusion of either saline or nicotine (15 mg·kg^-1·day^-1) for 2 wk. Immune ablation was conducted by injection of unconjugated azide-free antibody targeting rat CD161a⁺. Blood pressure was monitored by tail cuff plethysmography. Tissues were harvested at the end of infusion. Nicotine induced premature hypertension, renal expression of the sodium-potassium chloride cotransporter (NKCC2), increases in renal sodium retention, and infiltration of CD161a⁺/CD68⁺ macrophages into the renal medulla. All of these effects were abrogated by RND and ablation of CD161a⁺ immune cells. Cholinergic activation of CD161a⁺ immune cells with nicotine leads to the premature development of hypertension in SHR. The effects of renal sympathetic nerves on chemotaxis of CD161a⁺ macrophages to the renal medulla, increased renal expression of NKCC2, and renal sodium retention contribute to cholinergic hypertension. The CD161a⁺ immune cells are necessary and essential for this prohypertensive nicotine-mediated inflammatory response.NEW & NOTEWORTHY This is the first study that describes a novel integrative physiological interaction between the adrenergic, cholinergic, and renal systems in the development of hypertension, describing data for the role of each in a genetic model of essential hypertension. Noteworthy findings include the prevention of nicotine-mediated hypertension following successful immune ablation of CD161a⁺ immune cells and the necessary role these cells play in the overexpression of the sodium-potassium-chloride cotransporter (NKCC2) in the renal medulla and renal sodium retention. Renal infiltration of these cells is demonstrated to be dependent on the presence of renal adrenergic innervation. These data offer a fertile ground of therapeutic potential for the treatment of hypertension as well as open the door for further investigation into the mechanism involved in inflammation-mediated renal sodium transporter expression. Taken together, these findings suggest immune therapy, renal denervation, and, possibly, other new molecular targets as having a potential role in the development and maintenance of essential hypertension.

Na+-Cl- cotransporter-mediated chloride uptake contributes to hypertension and renal damage in aldosterone-infused rats

Recently, in addition to epithelial sodium channel alpha-subunit (αENaC), the thiazide-sensitive sodium-chloride cotransporter (NCC) and pendrin, also known as sodium-independent chloride/iodide transporter, were reported to be activated by aldosterone. Here, we investigated whether chloride (Cl^-) is responsible for hypertension, inflammation, and renal damage in aldosterone-infused rats. Following left nephrectomy, 8-wk-old male Sprague-Dawley rats were allocated into four groups: 1) drinking 1.0% sodium chloride solution with aldosterone infusion (Aldo/NaCl rats); 2) drinking 1.44% sodium bicarbonate solution with aldosterone infusion (Aldo/NaHCO₃ rats); 3) drinking distilled water with aldosterone infusion (Aldo/water rats); and 4) drinking distilled water without aldosterone infusion (sham rats). Additionally, heminephrectomized rats with aldosterone infusion were fed a 0.26% NaCl diet (control); 8.0% NaCl diet (high-Na/high-Cl); or a 4.0% NaCl 6.67% sodium citrate diet (high-Na/half-Cl). Last, Aldo/NaCl rats were treated with or without hydrochlorothiazide. Blood pressure in the Aldo/NaCl rats was significantly higher than in the Aldo/NaHCO₃ rats, which was associated with the increased expression of NCC. Expression of markers of inflammation (CD3, CD68, interleukin-17A) and fibrosis (α-smooth muscle actin, collagen 1) were also increased in Aldo/NaCl rats. Similarly, aldosterone-infused rats fed a high-Na/half-Cl diet had lower blood pressure than those fed a high-Na/high-Cl diet, with a reduction of phosphorylated NCC, but not αENaC and pendrin. NCC inhibition with hydrochlorothiazide attenuated interleukin-17A protein expression along with the phosphorylation of NCC in Aldo/NaCl rats. These findings suggest that NCC-mediated Cl^- uptake plays important roles in the development of aldosterone-induced hypertension and renal injury.

Effects of kimchi supplementation on blood pressure and cardiac hypertrophy with varying sodium content in spontaneously hypertensive rats

We tested the effects of dietary intake of freeze-dried Korean traditional fermented cabbage (generally known as kimchi) with varying amounts of sodium on blood pressure and cardiac hypertrophy in spontaneously hypertensive rats (SHRs). Wistar-Kyoto rats (WKY), as a control group, received a regular AIN-76 diet, and the SHRs were divided into four groups. The SHR group was fed a regular diet without kimchi supplementation, the SHR-L group was fed the regular diet supplemented with low sodium kimchi containing 1.4% salt by wet weight, which was provided in a freeze-dried form, the SHR-M group was supplemented with medium levels of sodium kimchi containing 2.4% salt, and the SHR-H group was supplemented with high sodium kimchi containing 3.0% salt. Blood pressure was measured over 6 weeks, and cardiac hypertrophy was examined by measuring heart and left ventricle weights and cardiac histology. SHRs showed higher blood pressure compared to that in WKY rats, which was further elevated by consuming high sodium containing kimchi but was not influenced by supplementing with low sodium kimchi. None of the SHR groups showed significant differences in cardiac and left ventricular mass or cardiomyocyte size. Levels of serum biochemical parameters, including blood urea nitrogen, creatinine, glutamic-oxaloacetic transaminase, glutamic-pyruvic transaminase, sodium, and potassium were not different among the groups. Elevations in serum levels of aldosterone in SHR rats decreased in the low sodium kimchi group. These results suggest that consuming low sodium kimchi may not adversely affect blood pressure and cardiac function even under a hypertensive condition.

Current challenges and unresolved problems in hypertensive disease

Over the past four or five decades, hypertension and cardiovascular medicine has experienced dramatic and innovative changes that have significantly reduced morbidity and mortality. A vast array of new antihypertensive compounds have been developed, which are able to inhibit many pathophysiologic mechanisms of the disease and prevent many of the outcomes in patients with hypertension. Much of this series of therapeutic breakthroughs have been the result of active participation of clinical scientists with tremendous and remarkable knowledge of and experience with the fundamental mechanisms of disease. In more recent years, much new information has appeared concerning the basis genetic and biologic mechanisms involved in cardiovascular and renal diseases. What remains of utmost importance is for members of the academic community with a wide spectrum of experience and points of view to continue to work with the fundamental problems and mechanisms of the diseases.

Salt loading produces severe renal hemodynamic dysfunction independent of arterial pressure in spontaneously hypertensive rats

We have previously shown that salt excess has adverse cardiac effects in spontaneously hypertensive rats (SHR), independent of its increased arterial pressure; however, the renal effects have not been reported. In the present study we evaluated the role of three levels of salt loading in SHR on renal function, systemic and renal hemodynamics, and glomerular dynamics. At 8 wk of age, rats were given a 4% (n = 11), 6% (n = 9), or 8% (n = 11) salt-load diet for the ensuing 8 wk; control rats (n = 11) received standard chow (0.6% NaCl). Rats had weekly 24-h proteinuria and albuminuria quantified. At the end of salt loading, all rats had systemic and renal hemodynamics measured; glomerular dynamics were specially studied by renal micropuncture in the control, 4% and 6% salt-loaded rats. Proteinuria and albuminuria progressively increased by the second week of salt loading in the 6% and 8% salt-loaded rats. Mean arterial pressure increased minimally, and glomerular filtration rate decreased in all salt-loaded rats. The 6% and 8% salt-loaded rats demonstrated decreased renal plasma flow and increased renal vascular resistance and serum creatinine concentration. Furthermore, 4% and 6% salt-loaded rats had diminished single-nephron plasma flow and increased afferent and efferent arteriolar resistances; glomerular hydrostatic pressure also increased in the 6% salt-loaded rats. In conclusion, dietary salt loading as low as 4% dramatically deteriorated renal function, renal hemodynamics, and glomerular dynamics in SHR independent of a minimal further increase in arterial pressure. These findings support the concept of a strong independent causal relationship between salt excess and cardiovascular and renal injury.

Development of hypertension in a pyelonephritis-induced model: the effect of salt intake and inability of renal sodium handling

The role of the kidney in the control of blood pressure has been convincingly demonstrated by several studies. Recent evidence has suggested that subtle acquired tubulointerstitial injury may cause a defect in sodium excretion function, thus leading to salt-sensitive hypertension. There are no reports, however, examining the effect of experimental chronic pyelonephritis on renal sodium handling and arterial pressure. Thus, to examine the influence of salt intake and unilateral nephrectomy, unanesthetized, unrestrained rats were randomly assigned to one of two separate groups: sham-operated rats (CO) or chronic unilateral pyelonephritic rats (CP). After twenty one days, the pyelonephritic group was subdivided in two: one subgroup continued with water intake (CPw), while the other was changed to 0.9% NaCl intake (CPs), like the control group (COs). After seven days, all rats were submitted to unilateral nephrectomy of the left normal kidney. Data presented herein show that chronic pyelonephritis produced an increase in mean arterial pressure (CO: 121.4 +/- 1.0 mmHg to CP: 127.0 +/- 0.9 mmHg, p = 0.000) that was enhanced by saline ingestion (COs: 121.6 +/- 1.4 mmHg; CPw: 127.0 +/- 1.8 mmHg; CPs: 132.1 +/- 1.2 mmHg, p = 0.000) and further aggravated by unilateral nephrectomy (CO: 125.2 +/- 2.6 mmHg; CPw: 127.5 +/- 0.9 mmHg; CPs: 139.2 +/- 1.1 mmHg, p = 0.000). Unchanged blood pressure measurements (120.2 +/- 2.3 mmHg) were observed beyond 21 days in control rats maintained on water regimen when compared with saline-drinking groups. These changes in mean arterial pressure were observed despite an increased fractional sodium excretion in the CPs group compared to the other groups before uninephrectomy (COs: 0.125 +/- 0.025%; CPw: 0.045 +/- 0.013%; CPs: 0.292 +/- 0.046%; p = 0.000), as compared to CPw after uninephrectomy (COs: 0.249 +/- 0.077%; CPw: 0.062 +/- 0.011%; CPs: 0.363 +/- 0.195%, p = 0.019). In addition, it was shown that daily liquid intake was higher in CPs than in CPw but similar to COs, both before uninephrectomy (COs: 42.8 +/- 2.6 ml/d; CPw: 34.3 +/- 3.5 ml/d; CPs: 51.8 +/- 3.7 ml/d, p = 0.006) and after uninephrectomy (COs: 40.9 +/- 5.5 ml/d; CPw: 33.8 +/- 1.4 ml/d; CPs: 53.0 +/- 3.5 ml/d, p = 0.004). The current data suggest that chronic pyelonephritis promotes an inability of renal tubules to handle sodium excretion when exposed to sodium overload and aggravated by uninephrectomy, thus constituting a model for salt-sensitive hypertension.

The role of sodium in hypertension is more complex than simply elevating arterial pressure

Excessive salt intake exacerbates hypertension and further increases left-ventricular mass in clinical essential and experimental hypertension. Additionally, a growing body of evidence strongly suggests that high dietary salt loading exerts detrimental cardiac effects independently of its hemodynamic load. The clinical evidence of cardiac structural and functional alterations associated with salt is, however, scarce. In order to explore the purported beliefs in humans, in this review we draw on our experimental studies in naturally occurring hypertension and discuss the clinical implications of the nonhemodynamic mechanisms underlying these salt-related changes.

Sodium directly impairs target organ function in hypertension

PURPOSE OF REVIEW

A large body of epidemiologic evidence has been amassed attesting to the relation of increased salt ingestion to the prevalence of hypertension; however, only a minority of patients with essential hypertension are salt sensitive. This report discusses the hypothesis that salt sensitivity need not be demonstrated exclusively by a marked rise in arterial pressure with salt loading; it may also be manifested by evidence of impaired target organ structure and function.

RECENT DEVELOPMENTS

This discussion summarizes the authors' recent experience with the spontaneously hypertensive rat, the best experimental model for naturally occurring hypertension, which demonstrates that salt loading precipitates the common structural and functional cardiac and renal changes associated with long-standing hypertension.

SUMMARY

As a result of salt loading, left ventricular diastolic dysfunction and impaired renal excretory function with massive proteinuria occur. Both are associated with marked ischemia and fibrosis and only a small additional increase in arterial pressure.

Cardiac structural and functional responses to salt loading in SHR

Increased dietary salt intake induces cardiac fibrosis in the spontaneously hypertensive rat (SHR), yet little information details its effects on left ventricular (LV) function. Additionally, young normotensive rats are more sensitive to the trophic effect of dietary sodium than older rats. Thus cardiac responses to salt loading were evaluated at two ages in the SHR; LV collagen content was also examined. SHR (8 or 20 wk of age) were given an 8% salt diet; their age-matched controls received standard chow. Echocardiographic indexes, arterial pressure, and LV hydroxyproline concentration were measured at 16 and 52 wk in the younger and older SHR groups, respectively. In most SHR, salt excess increased arterial pressure, LV mass, and hydroxyproline concentration and impaired LV relaxation manifested by prolonged isovolumic relaxation time, decreased early and atrial filling velocity ratio (V(E)/V(A)), and slower propagation velocity of E wave (V(P)). LV systolic function remained normal. However, one-quarter of the young salt-loaded SHR developed cardiac failure with systolic and diastolic dysfunction associated with greater LV mass and ventricular fibrosis. They also had lower arterial pressure, decreased fractional shortening, and a restrictive pattern of mitral flow. Moreover, the shorter deceleration time of the E wave and increased V(E)/V(P), an index of LV filling pressure, indicated increased LV stiffness in these rats. These findings demonstrated that sodium sensitivity in SHR is manifested not only by further pressure elevation but also by significant LV functional impairment that most likely is related to enhanced ventricular fibrosis. Moreover, the SHR are more susceptible to cardiac damage when high dietary salt is introduced earlier in life.

Ganglionic tyrosine hydroxylase and norepinephrine transporter are decreased by increased sodium chloride in vivo and in vitro

The present study tested the hypothesis that, in normal male rats, chronic changes in salt intake alter the levels of tyrosine hydroxylase and the norepinephrine transporter in sympathetic ganglia. Increasing dietary salt (from 0.02% to 1%, 4% or 8% NaCl in rat chow) decreased (p<0.05) the mRNA levels of tyrosine hydroxylase and the norepinephrine transporter in the adrenal gland, superior cervical ganglia and celiac ganglia. In addition, tyrosine hydroxylase and norepinephrine transporter protein levels were decreased (p<0.05) in the adrenal gland. To test the hypothesis that NaCl acts directly on postganglionic neurons to suppress the expression of these proteins, it was determined if increases in NaCl concentrations, of a magnitude achieved during increases in dietary salt in vivo, suppress expression of tyrosine hydroxylase and the norepinephrine transporter in cultured sympathetic neurons in vitro. Increased dietary salt increased plasma NaCl concentrations each by up to 4-6 mEq l(-1) (p<0.05), with the greatest increases occurring at night when the rats consume most of their food. In addition, NaCl added to cultured neurons decreased tyrosine hydroxylase and norepinephrine transporter protein and mRNA levels, and norepinephrine uptake; however, the NaCl concentration increases required were 15-30 mEq l(-1). These data suggest that increased dietary salt can influence the activity of the sympathetic nervous system by suppressing the levels of tyrosine hydroxylase and the norepinephrine transporter. While increased NaCl levels can act directly on neurons to suppress these proteins, this action may occur in vivo only in severe pathophysiological states, but not during increases in dietary salt without the synergistic effect of other factors.

Involvement of brain mineralocorticoid receptor in salt-enhanced hypertension in spontaneously hypertensive rats

We recently showed that brain mineralocorticoid receptors (MRs) are involved in blood pressure and kidney function control in normotensive Wistar rats. We now assessed the involvement of brain MRs in spontaneously hypertensive rats (SHR), in which the presence of adrenocorticoids has been shown to be required for the development of hypertension. The effect of a single intracerebroventricular (ICV) injection of an MR antagonist (RU28318) on systolic blood pressure (SBP) and renal function was examined in conscious adult SHR and Wistar-Kyoto rats (WKY) maintained on a standard-sodium diet (0.4% Na(+)). In WKY, a long-lasting decrease in SBP was caused by the ICV injection of 10 ng RU28318 as previously reported in Wistar rats, associated with increased urinary excretion of water and electrolytes. In SHR maintained on the standard diet, the ICV injection of RU28318 (10 or 100 ng) had no effect on cardiovascular and renal functions. However, the ICV injection of 10 ng RU28318 in SHR after 3 weeks of high sodium intake (8% Na(+)) caused a long-lasting decrease in SBP. The effect was present at 8 hours (DeltaSBP 34+/-2 mm Hg), persisted at 24 hours (DeltaSBP 29+/-1 mm Hg), and disappeared at 48 hours after the injection. The hypotension was not associated with changes in heart rate, urinary excretion of water and electrolytes, and plasma renin activity, whereas renal denervation did not affect the decrease in SBP. A more pronounced decrease in SBP (49+/-3 mm Hg at 8 hours) was observed with 100 ng RU28318. This dose of the antagonist was without effect after subcutaneous administration. Thus, brain MRs appear to participate in the maintenance of hypertension in conscious adult SHR sensitized by sodium loading.

Sodium-induced rise in blood pressure is suppressed by androgen receptor blockade

Our objective was to test the hypothesis that 1) a high Na (HNa, 3%) diet would increase blood pressure (BP) in male Wistar-Kyoto (WKY) and spontaneously hypertensive Y chromosome (SHR/y) rat strains in a territorial colony; 2) sympathetic nervous system (SNS) blockade using clonidine would lower BP on a HNa diet; and 3) prepubertal androgen receptor blockade with flutamide would lower BP on a HNa diet. A 2 x 4 factorial design used rat strains (WKY, SHR/y) and treatment [0.3% normal Na (NNa), 3% HNa, HNa/clonidine, and HNa/flutamide]. BP increased in both strains on the HNa diet (P < 0.0001). There was no significant decrease in BP in either strain with clonidine treatment. Androgen receptor blockade with flutamide significantly decreased BP in both strains (P < 0.0001) and normalized BP in the SHR/y colony. Neither heart rate nor activity could explain these BP differences. In conclusion, a Na sensitivity was observed in both strains, which was reduced to normotensive values by androgen blockade but not by SNS blockade.

Effects of isradipine or enalapril on blood pressure in salt-sensitive hypertensives during low and high dietary salt intake. MIST II Trial Investigators

This large multicenter study, tested the antihypertensive effects of isradipine, a dihydropyridine calcium channel blocker and enalapril, an angiotensin-converting enzyme inhibitor, in salt-sensitive hypertensive patients under low and high salt intake diets. After a 3-week (weeks -9 to -6) of ad lib salt diet, those patients who had a sitting diastolic blood pressure (SDBP) of > or =95 but < or =115 mm Hg qualified to enter a 3-week (weeks -6 to -3) placebo run-in low salt diet (50 to 80 mmol Na+/day). Then high salt (200 to 250 mmol Na+/day) was added to the placebo treatment for 3 weeks (weeks -3 to 0). Those patients who demonstrated an increase in SDBP > or =5 mm Hg from the low to high salt diet were considered salt sensitive and were randomized into a 4-week (weeks 0 to 4) double-blind treatment period of either isradipine 2.5 to 10 mg twice a day, enalapril 2.5 to 20 mg twice a day, or placebo. Then they entered a 3-week (weeks 4 to 7) placebo washout phase of low salt diet (50 to 80 mmol Na+/day). After week 7 and while the low salt diet was continued the patients were restarted on their double-blind treatment for 4 more weeks (weeks 7 to 11) and the study was completed. Of 1,916 patients screened, 464 were randomized into the double-blind treatment phase and 397 completed the study. Both isradipine and enalapril decreased the sitting systolic blood pressure (SSBP) and SDBP during the high salt diet, to a similar degree, whereas enalapril caused a greater reduction in SSBP and SDBP than isradipine during the low salt diet (11.3 +/- 1.2/7.7 +/- 0.7 mm Hg v 7.7 +/- 0.9/4.8 +/- 0.6 mm Hg, mean +/- SEM, respectively, P < .02). Within drugs, the effect of isradipine on blood pressure (BP) was higher during the high than the low salt diet (14.9 +/- 1.5 v 7.6 +/- 1.3 mm Hg for SSBP and 10.1 +/- 0.6 v 4.8 +/- 0.9 mm Hg for SDBP, P < .001), but enalapril exerted a similar effect during both diets. Because salt restriction lowered both SSBP and SDBP, the lowest BP achieved with both drugs were during the salt restriction phase.

The effect of hypertension on the response to blood loss in a rodent model

OBJECTIVE

Hypertensive patients having higher baseline peripheral resistance and sympathetic tone than normotensive patients may have aberrant responses to hemorrhage. In an attempt to further characterize this clinical observation, the authors compared the hemodynamic and metabolic responses to hemorrhage between spontaneously hypertensive rats (SHR) and normotensive rats (NTR).

METHODS

Twenty adult rats (10 NTR and 10 SHR) were anesthetized with althesin via the intraperitoneal route. Femoral arteries were cannulated by cutdown. Twelve (6 SHR and 6 NTR) rats underwent controlled catheter hemorrhage of 25% of their total blood volumes. Eight rats (4 SHR and 4 NTR) served as nonhemorrhage controls. Mean arterial pressure (MAP) and base excess (BE) were measured prehemorrhage and then every 15 minutes for the next 120 minutes. Data were reported as mean +/- standard error of the mean (SEM). Group comparisons were analyzed by ANOVA with repeated values post-hoc by Bonferroni. Statistical significance was defined by an alpha = 0.05.

RESULTS

Immediately after hemorrhage, the SHR group experienced a significantly (p < 0.001) greater drop in MAP of 70 +/- 4% in the SHR vs 40 +/- 6% in the NTR. Blood pressure in the NTR returned to control values 15 minutes after hemorrhage, but the SHR remained relatively hypotensive for the entire length of the experiment. Base excess in the SHR decreased significantly (p < 0.004) by 8.2 +/- 2 mmol/L from control values, as compared with no changes in BE for the NTR.

CONCLUSIONS

The authors observed significant differences in the response to hemorrhage between hypertensive and normotensive rats. Hypertensive rats experienced a more profound hemorrhagic shock insult than normotensives for the same degree of blood loss.

Dietary-sodium-induced cardiac remodeling in spontaneously hypertensive rat versus Wistar-Kyoto rat

OBJECTIVE

To study the effects of short-term and long-term high sodium intake on cardiac mass and design in sodium-sensitive spontaneously hypertensive rats versus sodium-resistant Wistar-Kyoto rats.

METHODS

Young spontaneously hypertensive rats and Wistar-Kyoto rats were randomly allocated to control diet, 2 or 8% dietary sodium for 2-12 weeks and changes in resting hemodynamics, cardiac angiotensin II level, sympathetic activity and cardiac structure evaluated. Sympathetic activity was assessed by measuring levels of plasma catecholamines, responses of blood pressure to ganglionic blockade, and rates of cardiac turnover of norepinephrine.

RESULTS

High sodium intake for 4 weeks increased left ventricle weight of Wistar-Kyoto rats aged 4 weeks (by 11 and 25% for 2 and 8% NaCl diets, respectively). This hypertrophic response was temporary, however, had already diminished after 6 weeks, and was absent after 12 weeks of a high sodium intake. However, after prolonged exposure concentric remodeling occurred (i.e. left ventricle wall thickness : radius ratio increased with no change in left ventricle mass). High sodium intake did not affect resting blood pressure, cardiac index, cardiac angiotensin II level, and general sympathetic activity of Wistar-Kyoto rats. Short-term high sodium intake did not increase left ventricle mass of young spontaneously hypertensive rats, unless sodium intake was so high (8% NaCl) that blood pressure and general sympathetic activity increased, too. However, a prolonged moderate (2%) increase in sodium intake also caused concentric remodeling in spontaneously hypertensive rats without increasing left ventricle mass, blood pressure, cardiac index, and general and cardiac sympathetic activities.

CONCLUSIONS

The blood pressure in young Wistar-Kyoto rats is sodium-insensitive but the heart structure is sodium-sensitive and high dietary sodium intake causes an early hypertrophic response, and then concentric remodeling. In contrast, hypertrophic response appears to occur after the response of blood pressure in spontaneously hypertensive rats, whereas the remodeling is similar to that in Wistar-Kyoto rats.

Vascular remodeling: the role of angiotensin-converting enzyme inhibitors

Chronic sustained hypertension leads to structural changes of the small and large arteries. These alterations consist of smooth-muscle hypertrophy, increased deposition of collagen, and "dilution" or destruction of elastin fibers. In addition, there may be no growth at all, but a "rearrangement" of vascular wall material termed "remodeling." These changes serve to increase wall thickness and the media-to-lumen ratio and to decrease the external and internal diameter of the vessel--all of which contribute to increased systemic vascular resistance by the small arteries and increased impedance by the larger arteries. It has been suggested that these structural changes are an adaptive effort by the vessel to maintain a constancy of wall tension, but the end result is detrimental in that the effect is a further increase in systemic vascular resistance and blood-flow impedance, which lead to left ventricular hypertrophy and its consequences. The stimuli for these changes are stretch stimuli, mediated through stretch receptors on the arterial wall, and trophic stimuli mediated at the tissue level through the action of angiotensin II, aldosterone, and catecholamines. Angiotensin-converting enzyme inhibitors, especially those with effective tissue penetration, are ideal drugs to reverse these structural changes ("reverse remodeling"), decrease the systemic vascular resistance, and increase the vascular compliance. These agents exert their effects through suppression, at the tissue level, of angiotensin II, aldosterone, catecholamines, endothelins (ET1, ET3), and transforming growth factor-beta1 (TGF-beta1) and through an increase in local levels of kinins, prostaglandins, and nitric oxide, which have antigrowth effects. Although this is a class effect, it appears to be stronger with those angiotensin-converting enzyme inhibitors providing the greatest tissue penetration.

Polymorphisms in the carboxy-terminus of the epithelial sodium channel in rat models for hypertension

OBJECTIVE

To investigate whether mutations in the C-terminus of the three subunits of the rat epithelial sodium channel (alphabetagamma-rENaC) contribute to the hypertensive phenotype in five rat models for essential hypertension.

DESIGN

We sequenced the C-terminal regions of alpha-, beta- and gamma-rENaC genes in five different hypertensive rat strains [spontaneously hypertensive rats (SHR), Dahl salt-sensitive (SS/Jr) rats, Milan hypertensive (MHS) rats, Sabra hypertensive (SBH) rats and Lyon hypertensive rats (LHR)] and their normotensive controls [Wistar-Kyoto (WKY) rats, Dahl salt-resistant (SR/Jr) rats, Milan normotensive (MNS) rats, Sabra normotensive (SBN) rats and Lyon normotensive rats (LNR)]. Identified polymorphisms were tested for cosegregation with blood pressure as well as for increased epithelial sodium channel (ENaC) activity.

METHODS

Genomic DNA extracted from hypertensive and normotensive rat strains was amplified by the polymerase chain reaction and polymerase chain reaction fragments were sequenced. Cosegregation analysis was performed to test for correlations between blood pressure and different genotypes. The effects of a polymorphism on ENaC activity were assessed by functional expression in Xenopus laevis oocytes. The chromosomal location of the gene for gamma-ENaC was determined by linkage analysis in an F2 (MHS x MNS) population.

RESULTS

We found no polymorphisms at the C-terminus of alpha- and beta-rENaC in the five rat models tested. We identified two polymorphisms at the C-terminus of the gamma-subunit, one leading to an amino acid change. Milan strains (MNS and MHS) were polymorphic for this mutation. By cosegregation analysis we could exclude the possibility that there was a correlation between blood pressure and this polymorphism. Functional expression of the polymorphism caused no increase in ENaC activity assessed by measurement of the amiloride-sensitive sodium current in Xenopus oocytes. The gene for the gamma-ENAC was located on rat chromosome 1.

CONCLUSIONS

No polymorphisms at the C-terminus of the three subunits of the epithelial sodium channel cosegregating with blood pressure were detected in five different genetic rat models for hypertension. If an altered ENaC activity contributes to the pathogenesis of hypertension in these rats, it must thus arise from mutations in other parts of the protein, from mutations outside the coding region impairing the proper regulation of one of the subunits or from mutations in an ENaC-associated protein.

Brain 'ouabain' in the median preoptic nucleus mediates sodium-sensitive hypertension in spontaneously hypertensive rats

Pressor responses to an acute increase in cerebrospinal fluid sodium and exaggeration of the hypertension and sympathetic hyperreactivity in spontaneously hypertensive rats (SHR) by high sodium diet involve release of brain "ouabain" and subsequent activation of the brain renin-angiotensin system. In the present study, we determined whether release of "ouabain" in the median preoptic nucleus participates in these responses. In conscious Wistar rats, the pressor and heart rate responses to central hypertonic saline (0.3 mol/L NaCl, 3.8 microL/min over 10 minutes) and ouabain (0.6 microgram) were compared after median preoptic nucleus injection of either gamma-globulins or Fab fragments binding ouabain and brain "ouabain" with high affinity. Microinjection of Fab fragments into the median preoptic nucleus abolished the pressor and tachycardic responses to central hypertonic saline and significantly reduced the pressor response to central ouabain. In SHR on high sodium, microinjection of Fab fragments into the median preoptic nucleus significantly decreased baseline blood pressure to a level not different from that in SHR on regular sodium (149 +/- 7 versus 145 +/- 5 mm Hg), whereas the enhanced responses to air stress were not affected. Our results support the concept that blood pressure responses to central hypertonic saline and exaggeration of the hypertension in SHR by high sodium diet depend on release of brain "ouabain" in the median preoptic nucleus.

Diurnal blood pressure variation and dietary salt in spontaneously hypertensive rats

We have previously reported that high dietary salt exposure significantly increases daytime mean arterial pressure in spontaneously hypertensive rats (SHR) but not in normotensive Wistar-Kyoto (WKY) controls. In the present study, we used a telemetry monitoring system to evaluate the effects of high dietary salt exposure on diurnal variation of mean arterial pressure and heart rate in SHR and WKY rats. After implantation of a radio frequency transducer, SHR and WKY rats were maintained on either high (8%) or basal (1%) salt diets. Hemodynamic values were then analyzed for diurnal variation with the use of a nonlinear data-fitting program. After 2 weeks of dietary exposure, high salt-fed SHR had significantly greater 24-hour mean arterial pressure (156 +/- 3 mmHg) than SHR receiving basal (135 +/- 2 mmHg) and WKY rats receiving high (100 +/- 2 mmHg) or basal (100 +/- 1 mmHg) salt diets. Rhythm analysis indicated significant increases in both daytime and nighttime mean arterial pressure during high salt exposure in SHR. In WKY rats, high salt exposure increased nighttime but not daytime mean arterial pressure, with no net effect on 24-hour mean arterial pressure. High dietary salt exposure significantly decreased heart rate in both SHR and WKY rats, and it did not significantly alter the pattern of diurnal blood pressure or heart rate variation. These results indicate that WKY rats manifest an acute sensitivity to salt ingestion but have compensatory mechanisms sufficient to prevent sustained increases in mean arterial pressure; such mechanisms are lacking in SHR.

Effects of salt intake on blood pressure and heart rate responses to footshock stress in SHR, BHR, and WKY rats

The SHR shows chronic elevations in blood pressure in response to stress or a high salt diet, at least in some studies. Stress and salt have also been combined in studies in the SHR. Tonic levels of blood pressure are not clearly elevated by superimposing acute stress on top of a chronic high salt diet. The BHR is a new model with lower resting blood pressure and marked sensitivity to environmental stressors such as stress and dietary salt intake. In the present study, SHR, BHR, or WKY were placed on a normal or high salt (8% in chow) diet. During the 8th week of the appropriate diet, blood pressure and heart rate were monitored during rest and footshock stress. Salt elevated the resting blood pressure in all three strains, but only marginally in the WKY. Stress did not further elevate the effect seen with salt, although it had a differential effect on heart rate in the three strains. In SHR, the salt group had a higher heart rate, although in BHR it was no different, and in WKY it was lower, than that seen in same-strain normal diet groups. The results are discussed in terms of the ability of the combination of stress and chronic high salt intake to alter baroreflex function in SHR, although only marginally affecting it in BHR. WKY, on the other hand, do not show evidence of altered baroreflex function when an acute stressor is superimposed on a high-salt diet.

Brain ouabain-like activity and the sympathoexcitatory and pressor effects of central sodium in rats

Intracerebroventricularly infused hypertonic saline elicits sympathoexcitatory and pressor effects. To clarify the mechanisms mediating these effects, we evaluated blood pressure (BP), heart rate (HR), and renal sympathetic nerve activity (RSNA) responses to intracerebroventricular administration of 0.3 M NaCl, ouabain, and rat hypothalamic and pituitary extracts containing ouabain-like activity (OLA) in conscious Wistar rats, before and after intracerebroventricular preinjection of digoxin-specific antibody Fab (DAF) fragments. To exclude modulatory effects of arginine vasopressin (AVP), treatment with DAF fragments was in all experiments preceded by intravenous injection of the AVP antagonist [beta-mercapto-beta,beta-cyclopentamethylenepropionyl1,o- Me-Tyr2,Arg8]AVP. After AVP antagonist pretreatment, 0.3 M NaCl i.c.v. at 3.8 microliters/min for 10 minutes caused simultaneous increases in BP, RSNA, and HR. After AVP antagonist pretreatment, intracerebroventricular injections of 0.3 and 1.0 microgram/l microliter ouabain or the OLA equivalent to 1 microgram ouabain/2 microliters elicited similar significant increases in BP, HR, and RSNA. After pretreatment with AVP antagonist and DAF fragments (66 micrograms/4 microliters i.c.v.), BP, HR, and RSNA responses to 0.3 M NaCl, ouabain, and OLA were all significantly diminished. In contrast, combined AVP blockade and DAF fragments did not affect the BP response to intracerebroventricular angiotensin II, the BP, HR, and RSNA response to intracerebroventricular carbachol and to air stress, or the HR and RSNA responses to intravenous sodium nitroprusside. Intracerebroventricularly injected gamma-globulins (66 micrograms/4 microliters) did not affect the responses to 0.3 M NaCl, ouabain, or OLA.(ABSTRACT TRUNCATED AT 250 WORDS)

Brain ouabain and central effects of dietary sodium in spontaneously hypertensive rats

High sodium intake (HNa) increases brain ouabainlike activity (OLA) in rats. In spontaneously hypertensive rats (SHR), HNa exaggerates development of hypertension and pressor and sympathoexcitatory responses to stress. To investigate whether dietary sodium-induced changes in brain OLA play a functional role, responses of mean arterial pressure (MAP), heart rate (HR), and renal sympathetic nerve activity (RSNA) to intracerebroventricular ouabain and to mental stress and intracerebroventricular alpha 2-adrenoceptor agonist guanabenz alone or preceded by intracerebroventricular ouabain were recorded in conscious SHR and Wistar-Kyoto (WKY) rats maintained from 4 to 8 weeks of age on different sodium diets: 1) low sodium intake (LNa, 17 mumol), 2) normal sodium intake (NNa, 101 mumol), and 3) HNa (1,370 mumol). SHR on NNa showed significantly higher MAP and RSNA compared with WKY rats on NNa. HNa or LNa significantly increased or decreased MAP but had no effects on resting RSNA in SHR and had no effects on resting MAP and RSNA in WKY rats. Intracerebroventricular ouabain induced dose-dependent increases in MAP, RSNA, and HR. In both SHR and WKY rats, LNa significantly enhanced these responses. In contrast, HNa significantly attenuated these responses only in SHR. Air stress increased and intracerebroventricular guanabenz decreased MAP, HR, and RSNA. The magnitudes of increases and decreases were significantly larger in SHR than in WKY rats. In WKY rats, dietary sodium did not change these responses. In contrast, in SHR, HNa significantly enhanced MAP, HR, and RSNA responses to air stress or intracerebroventricular guanabenz.(ABSTRACT TRUNCATED AT 250 WORDS)

Cerebroventricular dilation in spontaneously hypertensive rats (SHRs) is not attenuated by reduction of blood pressure

In previous studies, we found that spontaneously hypertensive rats (Okamoto-Aoki SHRs) suffer progressive postnatal dilation of the brain ventricles. In the present study we examined intracerebroventricular pressure and blood pressure as possible mechanisms of ventricular dilation in SHRs. We found that intracerebroventricular pressure was not elevated in SHRs. The role of blood pressure was examined in SHRs treated chronically with the antihypertensive drug, captopril, beginning in utero, and in renal hypertensive Sprague-Dawley rats (SDs). Although our experimental treatments produced significant changes in mean arterial pressures, they did not alter brain ventricular size: SDs with experimental hypertension had normal-sized brain ventricles and SHRs with pharmacologically reduced blood pressure had enlarged ventricles. These results suggest that neither increased intraventricular pressure nor high blood pressure is the sole cause of hydrocephalus in SHRs.

The effect of dietary fish oil and long-term salt loading on blood pressure and eicosanoid metabolism in spontaneously hypertensive rats

1. Dietary suppression of prostanoid synthesis with fish oils has had little effect on blood pressure in models of experimental hypertension in rats. However, a pressor effect of dietary fish oils was observed in spontaneously hypertensive rats (SHR) subject to 1 week of salt loading. 2. Animals were allocated to semisynthetic diets containing either 10% by weight Max EPA fish oil or a control diet of coconut oil, and studied after receiving 1.5% saline for 4 weeks. 3. Within the first week of salt loading, SHR-fed fish oil showed an increase in blood pressure (mean = 9 mmHg) relative to controls. This effect was transient, and after the first week of salt loading there was little difference in blood pressure between the two dietary groups. 4. Following dietary treatment there were substantial changes in plasma fatty acid composition with a 48% decrease in arachidonic acid content of fish oil-fed rats compared with control animals. Rats on the fish oil diet showed a threefold decrease in serum thromboxane generation. Prostacyclin production by incubated segments of aorta was reduced by more than 50% compared with the coconut oil-fed control group. 5. SHR on the fish oil diet showed increased urine volume and sodium excretion, presumably due to increased fluid and salt intake. 6. This study shows that dietary suppression of prostacyclin synthesis is associated with only a minor effect on blood pressure in long-term salt loading of SHR.

Spontaneously hypertensive rats exhibit reduced hypothalamic noradrenergic input after NaCl loading

Spontaneously hypertensive rats (SHR) of the Okamoto strain exhibit a significant exacerbation in severity of hypertension when fed diets high in NaCl. To examine the hypothesis that abnormalities in the monoaminergic innervation of the hypothalamus and brainstem contribute to the NaCl-induced exacerbation of hypertension, the monoamine and monoamine metabolite contents of specific hypothalamic and brainstem regions thought to be involved in the pathogenesis of hypertension were determined in SHR fed a diet containing 8% or 1% NaCl for either 2 or 6 weeks beginning at age 8 weeks. SHR maintained on the 8% NaCl diet for 2 weeks displayed significant decreases in norepinephrine in both the anterior and posterior hypothalamic regions but not in other brainstem or hypothalamic regions, as compared with animals consuming 1% NaCl. In addition, stores of the principal terminal norepinephrine metabolite 3-methoxy-4-hydroxyphenylglycol were reduced in the anterior hypothalamic region of SHR fed an 8% NaCl diet for 2 weeks. After 6 weeks on the diets, SHR fed 8% NaCl showed small but statistically nonsignificant reductions in norepinephrine stores of the anterior hypothalamic region as compared with SHR fed a basal diet, while WKY fed 8% NaCl had significantly elevated norepinephrine stores in the anterior hypothalamic region as compared with WKY fed a basal diet. There was a significant group X diet interaction (p less than 0.05). After 6 weeks on the 8% NaCl diet, SHR (but not WKY) displayed a significant reduction in norepinephrine content of the posterior hypothalamic region. No NaCl-induced differences in norepinephrine stores were found in the pons or medulla of either strain.(ABSTRACT TRUNCATED AT 250 WORDS)

Dietary sodium intake and age in spontaneously hypertensive rats: effects on blood pressure and sympathetic activity

Spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY) were placed on sodium restricted diets (9 and 17 mumol/g) or on a regular sodium diet (101 mumol/g) at 2, 4, 7, or 10 weeks of age, and continued until 16 weeks of age. Severe sodium restriction (9 mumol/g) initiated at 2 or 4 weeks of age prevented hypertension development in SHR and severely retarded growth. Hypertension development was attenuated when 9 mumol/g was initiated at 7 weeks of age, and was not affected when started at 10 weeks of age. Mean arterial pressure (MAP) in WKY receiving 9 mumol Na/g initiated at 2 and 4 weeks of age was below normal, but was not affected when this diet was given at 7 or 10 weeks of age. Less severe sodium restriction (17 mumol Na/g) resulted in a reduction in hypertension development when initiated at 2, 4, and 7 weeks of age, but not at 10 weeks of age. MAP was normal in WKY receiving 17 mumol Na/g at all ages of diet initiation. When the 9 or 17 mumol Na/g diet were initiated at 2, 4, and 7 weeks of age, the response of blood pressure to hexamethonium administration was blunted in SHR relative to both WKY receiving the same diet, and to control SHR receiving 101 mumol Na/g. We conclude that both WKY and SHR require a minimum amount of dietary sodium for normal growth and for the achievement of normal BP in WKY, and hypertension in SHR. This sodium requirement decreases with age. SHR and WKY exhibit similar sensitivities to sodium intake with respect to body weight, but the effects on BP are more pronounced in SHR. The BP lowering effects of dietary sodium restriction may be due to a blunting of the pressor effectiveness of the sympathetic nervous system.

Blood-brain barrier leakage and brain edema in stroke-prone spontaneously hypertensive rats. Effect of chronic sympathectomy and low protein/high salt diet

Brain edema associated with severe chronic hypertension was studied in stroke-prone spontaneously hypertensive rats (SHRSP), 5 to 9 months of age. Blood-brain barrier (BBB) leakage sites and intracerebral spreading pathways for plasma proteins were delineated by an intravenously (i.v.) injected exogenous dye tracer (Evans blue), known to form a complex with albumin in blood, and by immunohistochemical visualization of extravasated endogenous plasma proteins. The tissue content of edema fluid was estimated by measuring the specific gravity of selected brain regions, stained or unstained by the tracer dye, on a bromobenzene-kerosene gradient column. Multifocal BBB leakage sites were macroscopically detected within the cerebral cortex and the deep gray matter after i.v. circulation of Evans blue-albumin for 30 min. After 24 h of i.v. circulation the dye tracer had spread not only locally in the gray matter but also into the adjacent white matter, where it was widely distributed. Immunohistochemically visualized plasma proteins showed similar distribution. Unilateral superior cervical ganglionectomy performed at 4 weeks of age neither increased the incidence of major BBB opening to Evans blue-albumin nor altered the specific gravity of the ipsilateral cerebral hemisphere in grown-up SHRSP, furthermore, the blood pressure remained unchanged. The lack of significant effect on BBB function may possibly be attributed to the extensive reinnervation of the cerebral arteries, verified in the grown-up SHRSP using the Falck-Hillarp fluorescence method for visualization of catecholaminergic nerve fibers. In SHRSP raised on a low-protein and high-salt diet the mean arterial blood pressure was 212 mm Hg compared to 195 mm Hg in controls (P less than 0.05) and the incidence of BBB opening was 72% compared to 25% in controls (P less than 0.05). After 24 h of i.v. circulation of Evans blue-albumin, brain regions stained by the dye tracer showed significantly reduced specific gravity (P less than 0.001), while unstained regions had normal values. Thus the brain edema fluid spread, as revealed by specific gravity measurements, corresponded to the intracerebral distribution of extravasated plasma proteins.

Hypertension produced by a high sodium diet in the borderline hypertensive rat (BHR)

The effect of high dietary sodium (8%) on blood pressure in spontaneously hypertensive (SHR), borderline hypertensive (BHR), and normotensive Wistar-Kyoto (WKY) rats was determined weekly by tail cuff plethysmography for one week of baseline and four weeks of diet. After 4 weeks, significant elevations in systolic blood pressure were found in SHR and BHR groups, but not in WKY. BHR studied an additional 4 weeks showed a further progression of hypertension, reaching levels nearly equal to control SHR. Direct measurement of arterial pressure in conscious animals in their home cage confirmed the elevation in pressure in both SHR and BHR groups. Metabolic studies revealed that the high sodium diet reduced body weight in SHR and BHR strains, but not in WKY. Although both urinary volumes and sodium excretion values were significantly lower in SHR and BHR compared with WKY, this effect disappeared when adjustments for body weight were made. Plasma norepinephrine determinations revealed a significant response to cold stress in all groups. Plasma epinephrine was elevated in all strains in response to cold stress; however, a consistent statistical elevation was seen only in WKY. The BHR is discussed as a model for determining the triggers responsible for environmentally-induced hypertension.

Genetic analysis of blood pressure and sodium balance in spontaneously hypertensive rats

Blood pressure and parameters of sodium balance were measured during the first 16 weeks of life in male Okamoto spontaneously hypertensive rats (SHR, n = 22), Wistar-Kyoto rats (WKY, n = 25), and the F1 (n = 27) and F2 (n = 81) hybrids of the SHR and WKY. Genetic analysis revealed that blood pressure in SHR was controlled by approximately four independent genetic loci and the degree of genetic determination was 64.5%. No difference in blood pressure was discernible before 12 weeks of age between those F2 rats that at 16 weeks had blood pressures either higher or lower than one standard deviation from the mean. Exchangeable sodium was measured sequentially in individual rats of all populations by determining their whole-body radioactivity while receiving 37.5 mM 22Na/23NaCl drinking fluid of constant specific activity as their sole source of sodium. The SHR had consistently higher exchangeable sodium levels than WKY and showed evidence of relative sodium retention during the early developmental phase of hypertension. Sodium intake was higher in SHR than WKY from 4 to 16 weeks of age, although saline preference was the same in both strains. None of these parameters of sodium balance were found to correlate with blood pressure in the F2 population. It is concluded that the heritable abnormalities of sodium balance in SHR appear to represent coincidental inbred characteristics controlled by genetic loci that are unrelated to those loci responsible for the expression of hypertension in this model.

Nonpharmacological approaches to the control of high blood pressure. Final report of the Subcommittee on Nonpharmacological Therapy of the 1984 Joint National Committee on Detection, Evaluation, and Treatment of High Blood Pressure

This report reviews a variety of nonpharmacological approaches used to control arterial blood pressure. Of all the modalities considered, only three had sufficient scientific support to warrant recommendation for inclusion in hypertension treatment programs. Each of these three modalities--weight control, alcohol restriction, and sodium restriction--was found to be capable not only of independently controlling blood pressure (particularly in patients with mild hypertension) but also of reducing the number and dosage of prescribed pharmacological agents, should their prescription be indicated. Weight reduction was found to reduce the risk from elevated arterial pressure as well as overall cardiovascular morbidity and mortality. However, because the rate of recidivism was exceedingly high in these studies, close and continuous patient follow-up is considered necessary. Excessive alcohol intake is associated in many studies with proportionally higher arterial pressures and an increased prevalence of hypertension. Therefore, the recommendation of moderation in alcohol consumption to less than 2 oz of ethanol daily for patients with hypertension is supported. Restriction of dietary sodium to less than 2 g/day was the only other nonpharmacological approach with sufficient support to be recommended as a treatment for hypertension. Although long-term studies are sorely lacking, sodium restriction has been shown to be manageable and safe and probably will benefit those hypertensive patients who are sodium-sensitive.

Progressive postnatal dilation of brain ventricles in spontaneously hypertensive rats

Cross-sectional areas of the forebrain ventricles were measured from coronal sections in spontaneously hypertensive rats (SHRs) 4, 8, 12, 16, 21 and 56 weeks of age and in age-matched Wistar--Kyoto (WKY) and Sprague--Dawley (SD) normotensive rats. Progressive ventricular dilation and associated attrition of brain tissue was observed in SHRs of both sexes after 4 weeks of age, and was present in animals obtained from two different suppliers. In some SHRs, ventricle size was increased to 270% of control. Hence, it seems likely that some systemic and behavioral signs which are concomitant with hypertension in the SHR may be attributable to hydrocephalus and its neuropathological correlates.

Biochemical aspects of salt-induced, pressure-independent left ventricular hypertrophy in rats

In an attempt to investigate the effect of chronic salt loading on heart size and biochemical composition, sixty 3-month-old male Wistar-Kyoto rats (WKY) were equally divided into two groups: One group was given 1% NaCl in drinking water, and the other group was given tap water as the control. After 7 months, five randomly selected rats from each group were examined for body weight (BW), indirect blood pressure (BP), hematocrit (Hct), and organ weights. Ventricles of the heart, aorta, and mesenteric arteries were biochemically analyzed for collagen (C) and noncollagenous protein NC) and deoxyribonucleic acid (DNA). Although here was no difference between the salt and control groups in BP (129 +/- 3 mmHg vs. 125 +/- 4 mmHg, mean +/- SE), BW (376 +/- 8 g vs. 372 +/- 5 g) and Hct (42.8% +/- 1.0% vs. 44.3% +/- 0.8%), left ventricular weight (857 +/- 19 mg vs. 788 +/- 8 mg) and kidney weight (2.59 +/- 00.9 g vs 2.31 +/- 0.05 g) were both significantly (P less than 0.01) greater in the salt group, whereas the weights of the right ventricle, aorta, and adrenal glands were equal. Biochemical analysis showed significant increase in NC/DNA ratio and total collagen content as well as decreased DNA concentration in the left ventricle of the salt group compared with the control, suggesting hypertrophy rather than hyperplasia of myocardial cells with concomitant activation of collagen synthesis. Since the manner of reactive collagen production appeared different from that reported for spontaneously hypertrophic rats (SHR) or in hypoxia- or aortic constriction-induced cardiac hypertrophy, some unique mechanism may be involved in salt-induced cardiac hypertrophy.

Accelerated natriuresis induced by synthetic atrial natriuretic polypeptide in spontaneously hypertensive rats

Effects of synthetic alpha human atrial natriuretic polypeptide (alpha-hANP) on diuresis, natriuresis and mean arterial blood pressure (MAP) were compared between 4-5 month-old male spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY) under ether anesthesia. In both groups, the peptide injected (0.5 micrograms/100g body weight, i.v.) caused potent (about ten fold), rapid and short-acting (for 15 min) increases in sodium (Na+) and chloride excretions and also an increase in urine flow and potassium excretion with lesser magnitude. Although ratios of the maximum response to basal value were much the same, net increases in urine flow and Na+ output were significantly greater in SHR than in WKY. As to the effect on MAP, a rapid (within 2 min) fall observed in the two groups. These results suggest that the atrial natriuretic peptide may be involved in the altered regulatory mechanism of fluid and electrolyte balance in SHR models with genetic hypertension.

Sodium and hypertension: effect of dietary calcium supplementation on blood pressure

Normotensive female Wistar Kyoto rats were studied to examine the effects of replacing drinking water with highly palatable saline solution at a concentration (0.5%) close to the maximum preferred concentration in these animals. A further group of animals was offered a calcium-supplemented diet in addition to substitution of drinking water with saline. Fluid consumption in animals drinking tap water was constant at between 35-40 ml/day throughout the six-month study period. In contrast, animals drinking 0.5% saline consumed 75-85 ml/day throughout the study period, irrespective of the calcium content of the diet. Voluntary consumption of these quantities of saline was associated with the development of a moderate hypertension, measured by tail cuff plethysmography, after two months of study. However, in animals consuming calcium-supplemented diets the hypertensive response disappeared after 3 months of study. Blood pressures were validated at the conclusion of the study by direct arterial cannulation and confirmed the presence of hypertension in saline drinking animals only when diets lacked calcium supplementation. No changes in blood ionized sodium concentration were associated with saline consumption; however, blood ionized calcium was significantly reduced in animals drinking saline, but not when calcium-supplemented diet was available. These studies suggest an interaction between sodium and calcium in the genesis of sodium-dependent hypertension.

Effects of gene-environment interactions on cardiovascular risk factors in Chinese adolescent twins

A population-based sample of 73 male and 77 female monozygotic (MZ), and 41 male and 33 female dizygotic (DZ) Chinese adolescent twin pairs were studied to assess effects of gene-environment interactions of systolic blood pressure (SBP), diastolic blood pressure (DBP), serum cholesterol and triglyceride levels. Intrapair concordance in BP levels was found to be significantly associated with the interaction of zygosity and salty foods preference and also with that of zygosity and vegetable preference. A consistently positive and statistically significant association was observed between the intrapair difference in serum cholesterol and the interaction of zygosity and animal organ preference; while intrapair concordance in serum cholesterol was associated with the interaction of zygosity and milk consumption. Intrapair difference in serum triglycerides was associated with the interaction of zygosity and fish preference, and a significant association was also found between the intrapair concordance in serum triglycerides and the interaction of zygosity and sweets preference. These observations suggest that the impact of these environmental agents may be influenced by the genotype.

Environmental effects on cardiovascular risk factors in Chinese adolescent monozygotic twins

The monozygotic (MZ) cotwin control method was employed to elucidate possible environmental determinants of systolic blood pressure (SBP), diastolic blood pressure (DBP), serum cholesterol and triglyceride levels. A population-based twin sample of 73 male and 77 females MZ twin pairs was recruited from 12 junior high schools in Taipei city. Intrapair differences in blood pressure were negatively associated with intrapair difference in vegetable preference, attaining significance for DBP in males and SBP in females. Cholesterol was positively associated with milk consumption and preference for sweets, fried foods, meat and fish. A negative association was also observed between cholesterol and vegetable preference. These associations for cholesterol were significant in males only. Triglyceride level negatively associated with preferences for sweets and vegetable, attaining significance for vegetables in both males and females and for sweets in males only.

Achievements in hypertension: a 25 year overview

Only 25 years ago, the field of hypertension was challenged by retrospective clinical data and epidemiologic information suggesting that an elevated arterial pressure is a major risk factor for enhanced cardiovascular morbidity and mortality. Not only was antihypertensive therapy looked on by many as dangerous and fraught with severe and undesirable side effects, but its validity in reversing the course of disease was not yet demonstrated. This review discusses the dramatic new information amassed over the past 25 years that points to the new physiologic and clinical concepts concerning hypertension. It considers impressive new diagnostic techniques and methods designed to identify secondary forms of hypertension and target organ involvement. In summary, it outlines the feasibility of reversing overall (and cardiovascular) morbidity and mortality with an array of antihypertensive agents that provide the therapeutic ability to suppress most pathophysiologic pressor mechanisms of hypertensive disease. The lesson is clear: hypertension provides the greatest available challenge to the new era of preventive cardiology in the 21st century.

Stress and high sodium effects on blood pressure and brain catecholamines in spontaneously hypertensive rats

The following experiments were designed to determine if territorial stress, dietary sodium (Na), or the combination of stress and Na effect the rate of development of hypertension in the spontaneously hypertensive rat (SHR 4-18 wks) and if central catecholamines (C) were altered by these treatments. BP was significantly elevated from 2-8 weeks of stress treatment as compared to SHR controls. Norepinephrine (NE) levels in the nucleus tractus solitarius and amygdala (A), and dopamine (D) levels in the hippocampus and A showed significant elevations in the stressed group. High Na (3%) treatment combined with stress treatment produced an even further BP increase and elevated D levels in the amygdala, and elevated NE levels in the area postrema as compared to control SHR's. Selected brain C variables were able to correctly classify animals into high and low BP groups with 90-100% accuracy. Our data support the concept that there are important stress and Na effects upon brain neurochemistry which influence the development of hypertension in the SHR.

Contribution of the sympathetic nervous system to the hypertensive effect of a high sodium diet in stroke-prone spontaneously hypertensive rats

In stroke-prone spontaneously hypertensive rats (SHRSP) plasma norepinephrine levels and vascular reactivity to norepinephrine are increased and intravascular volume is reduced during the developmental phase of hypertension. Since the accelerated rise in blood pressure following sodium-loading in SHRSP cannot be attributed to the volume-retaining properties of sodium, the effects of an increased dietary intake of sodium on biochemical parameters of sympathetic vascular tone were investigated. The following results were obtained. First, the increased reactivity of vascular smooth muscle was further augmented in sodium-treated SHRSP; the degree of supersensitivity was positively correlated to the plasma sodium concentration. After blockade of the neuronal uptake by 30 microM cocaine, no difference in vascular reactivity to norepinephrine was detected between SHRSP on a normal and SHRSP on a high-sodium diet. Second, the inactivation of norepinephrine by the neuronal uptake was impaired in rats on a high-sodium diet, the impairment being more pronounced in SHRSP than in Wistar-Kyoto (WKY) rats. This decreased inactivation could be expected to cause higher concentrations of the neurotransmitter at the receptor site if the transmitter release from the nerve ending remains constant. Third, the release of norepinephrine and epinephrine into the plasma was increased in sodium-loaded SHRSP but not in sodium-loaded WKY. Cold exposure exaggerates these differences between normotensive and hypertensive rats. These findings suggest that a high-sodium intake modifies the transmission of sympathetic impulses at the level of the nerve terminal in both WKY and SHRSP. In the normotensive rats, moderate impairment of norepinephrine inactivation, however, was balanced by an appropriate reduction in central sympathetic discharge following sodium-loading. In the hypertensive rats, the peripheral disturbance in norepinephrine inactivation due to sodium-loading was obviously not balanced by an adequate withdrawal of central sympathetic discharge. The resultant hemodynamic change was a further increase in the sympathetically mediated vasoconstriction, which is regarded as at least one of the main mechanisms of the sodium-dependent acceleration of hypertension in SHRSP.

Age-dependent salt hypertension in Brattleboro rats: a hemodynamic analysis

The hemodynamic effects of 0.6% saline, consumed either from youth (4th week of age) or from adulthood (12th week of age), were studied in unanesthetized, unoperated, and uninephrectomized homozygous female Brattleboro rats. Long-term saline drinking induced a general decrease of blood pressure in unoperated rats which was more pronounced in rats drinking it from youth. The relation of low systemic resistance and high cardiac output (observed at the age of 10-15 weeks) to the high mortality of these rats was discussed. Two phases were recognized in the development of salt hypertension in uninephrectomized rats drinking saline from youth. The increased systemic resistance played a major role during the early phase (13-15 weeks), while changes of body fluids as well as altered arterial compliance contributed to the elevation of systolic blood pressure in the late phase of salt hypertension (20-30 weeks of age). In uninephrectomized rats drinking saline from adulthood, the late blood pressure response was only slightly attenuated in comparison with uninephrectomized rats drinking saline from youth. The absence of increased arterial rigidity in the former group was the only major hemodynamic difference between these two groups of uninephrectomized rats aged 20-30 weeks.

Blood pressure and sodium excretion in the sinoaortic denervated rat during chronic high and low sodium intake and acute sodium loading

We studied the effects of dietary sodium on the magnitude of hypertension in sinoaortic denervated (SAD) rats. Groups of SAD rats and sham operated (SO) controls drank tap water and received chows with different amounts of sodium: low (0.08%), regular (0.4%), high (3%) or very high (7%) sodium; other groups, some after unilateral nephrectomy, received regular chow and 1% saline to drink. These various sodium regimens were started before operations and were continued for at least 12 weeks after SAD and SO. Weekly systolic tail-cuff pressures of SAD rats were significantly higher throughout the 12 week postoperative period than those of SO rats regardless of sodium regimen (p less than 0.05 to less than 0.01). Analysis of variance indicated no significant differences between pressures of SAD rats on regular or low sodium chows and those receiving any of the high sodium regimens. When SAD rats were switched from regular to high sodium diets no significant change was induced in systolic pressures. We then examined renal sodium excretion in response to oral sodium loading or to intravenous saline infusion in groups of SAD and SO rats. Both types of studies revealed that SAD rats excreted the extra sodium significantly faster than SO rats. We conclude that hypertension induced by SAD is not dependent on the amount of sodium in the diet and that the magnitude of hypertension is not increased by chronic high sodium intake. The rapid excretion of sodium suggests SAD rats have an enhanced sensitivity to activation and/or to effects of neural and/or humoral factors affecting renal sodium excretion.