Effects of high or low sodium intake 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.

Gut Microbiome, Intestinal Permeability, and Tissue Bacteria in Metabolic Disease: Perpetrators or Bystanders?

The emerging evidence on the interconnectedness between the gut microbiome and host metabolism has led to a paradigm shift in the study of metabolic diseases such as obesity and type 2 diabetes with implications on both underlying pathophysiology and potential treatment. Mounting preclinical and clinical evidence of gut microbiota shifts, increased intestinal permeability in metabolic disease, and the critical positioning of the intestinal barrier at the interface between environment and internal milieu have led to the rekindling of the "leaky gut" concept. Although increased circulation of surrogate markers and directly measurable intestinal permeability have been linked to increased systemic inflammation in metabolic disease, mechanistic models behind this phenomenon are underdeveloped. Given repeated observations of microorganisms in several tissues with congruent phylogenetic findings, we review current evidence on these unanticipated niches, focusing specifically on the interaction between gut permeability and intestinal as well as extra-intestinal bacteria and their joint contributions to systemic inflammation and metabolism. We further address limitations of current studies and suggest strategies drawing on standard techniques for permeability measurement, recent advancements in microbial culture independent techniques and computational methodologies to robustly develop these concepts, which may be of considerable value for the development of prevention and treatment strategies.

GUT Microbiome-GUT Dysbiosis-Arterial Hypertension: New Horizons

Arterial hypertension is a progressive cardiovascular syndrome arising from complex and interrelated etiologies. The human microbiome refers to the community of microorganisms that live in or on the human body. They influence human physiology by interfering in several processes such as providing nutrients and vitamins in Phase I and Phase II drug metabolism. The human gut microbiota is represented mainly by Firmicutes and Bacteroidetes and to a lesser degree by Actinobacteria and Proteobacteria, with each individual harbouring at least 160 such species. Gut microbiota contributes to blood pressure homeostasis and the pathogenesis of arterial hypertension through production, modification, and degradation of a variety of microbial-derived bioactive metabolites. Animal studies and to a lesser degree human research has unmasked relative mechanisms, mainly through the effect of certain microbiome metabolites and their receptors, outlining this relationship. Interventions to utilize these pathways, with probiotics, prebiotics, antibiotics and fecal microbiome transplantation have shown promising results. Personalized microbiome-based disease prediction and treatment responsiveness seem futuristic. Undoubtedly, a long way of experimental and clinical research should be pursued to elucidate this novel, intriguing and very promising horizon.

The gut microbiome and hypertension

PURPOSE OF REVIEW

The mammalian mucosal surfaces are densely inhabited by a diverse microbial ecosystem termed the microbiota. Among these highly heterogeneous populations, the largest and richest is the gut microbiota, recently suggested to affect various physiological traits and susceptibility to disease. Novel metagenomic and metabolomic approaches, which have been developed in the past decade, have enabled the elucidation of the contribution of the microbiota to metabolic, immunologic, neurologic and endocrine homeostasis.

RECENT FINDINGS

Dysbiosis, the alteration in the gut microbiota composition and function, has been lately associated with the pathogenesis of multifactorial diseases such as obesity, diabetes and cardiovascular disorders. Recent studies have also suggested associations between dysbiosis and essential hypertension, a common chronic medical condition affecting 20% or more of the adult population worldwide, which is considered a major causative factor for heart disease, stroke, chronic renal failure, blindness and dementia.

SUMMARY

In this review, we discuss the accumulating research pointing to possible interplays between the gut microbiome and hypertension and highlight future prospects by which utilization of microbiome-related techniques may be incorporated into the diagnosis and therapeutic arsenal of hypertension management.

Altered urinary sodium excretion response after central cholinergic and adrenergic stimulation of adult spontaneously hypertensive rats

In this study, we hypothesized that blunting of the natriuresis response to intracerebroventricularly (i.c.v.) microinjected cholinergic and adrenergic agonists is involved in the development of hypertension in spontaneously hypertensive rats (SHR). We evaluated the effect of i.c.v. injection of cholinergic and noradrenergic agonists, at increasing concentrations, and of muscarinic cholinergic and α1 and α2-adrenoceptor antagonists on blood pressure and urinary sodium handling in SHR, compared with age-matched Wistar Kyoto rats (WR). We confirmed that CCh and NE microinjected into the lateral ventricle (LV) of conscious rats leads to enhanced natriuresis. This response was associated with increased proximal and post-proximal sodium excretion accompanied by an unchanged rate of glomerular filtration. We showed that cholinergic-induced natriuresis in WR and SHR was attenuated by previous i.c.v. administration of atropine and was significantly lower in the hypertensive strain than in WR. In both groups the natriuretic effect of injection of noradrenaline into the LV was abolished by previous local injection of an α1-adrenoceptor antagonist (prazosin). Conversely, LV α2-adrenoceptor antagonist (yohimbine) administration potentiated the action of noradrenaline. The LV yohimbine pretreatment normalized urinary sodium excretion in SHR compared with age-matched WR. In conclusion, these are, as far as we are aware, the first results showing the importance of interaction of central cholinergic and/or noradrenergic receptors in the pathogenesis of spontaneous hypertension. These experiments also provide good evidence of the existence of a central adrenergic mechanism consisting of α1 and α2-adrenoceptors which works antagonistically on regulation of renal sodium excretion.

CNS neuroplasticity and salt-sensitive hypertension induced by prior treatment with subpressor doses of ANG II or aldosterone

Although sensitivity to high dietary NaCl is regarded to be a risk factor for cardiovascular disease, the causes of salt-sensitive hypertension remain elusive. Previously, we have shown that rats pretreated with subpressor doses of either ANG II or aldosterone (Aldo) show sensitized hypertensive responses to a mild pressor dose of ANG II when tested after an intervening delay. The current studies investigated whether such treatments will induce salt sensitivity. In studies employing an induction-delay-expression experimental design, male rats were instrumented for chronic mean arterial pressure (MAP) recording. In separate experiments, ANG II, Aldo, or vehicle was delivered either subcutaneously or intracerebroventricularly during the induction. There were no sustained differences in BP during the delay prior to being given 2% saline. While consuming 2% saline during the expression, both ANG II- and Aldo-pretreated rats showed significantly greater hypertension. When hexamethonium was used to assess autonomic control of MAP, no differences in the decrease of MAP in response to ganglionic blockade were detected during the induction. However, during the expression, the fall was greater in sensitized rats. In separate experiments, brain tissue that was collected at the end of delay showed increases in message or activation of putative markers of neuroplasticity (i.e., brain-derived neurotrophic factor, p38 mitogen-activated protein kinase, and cAMP response element-binding protein). These experiments demonstrate that prior administration of nonpressor doses of either ANG II or Aldo will induce salt sensitivity. Collectively, our findings indicate that treatment with subpressor doses of ANG II and Aldo initiate central neuroplastic changes that are involved in hypertension of different etiologies.

Super, red palm and palm oleins improve the blood pressure, heart size, aortic media thickness and lipid profile in spontaneously hypertensive rats

BACKGROUND

Oleic acid has been shown to lower high blood pressure and provide cardiovascular protection. Curiosity arises as to whether super olein (SO), red palm olein (RPO) and palm olein (PO), which have high oleic acid content, are able to prevent the development of hypertension.

METHODOLOGY/PRINCIPAL FINDINGS

Four-week-old male spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats were fed 15% SO, RPO or PO supplemented diet for 15 weeks. After 15 weeks of treatment, the systolic blood pressure (SBP) of SHR treated with SO, RPO and PO were 158.4±5.0 mmHg (p<0.001), 178.9±2.7 mmHg (p<0.001) and 167.7±2.1 mmHg (p<0.001), respectively, compared with SHR controls (220.9±1.5 mmHg). Bradycardia was observed with SO and PO. In contrast, the SBP and heart rate of treated WKY rats were not different from those of WKY controls. The SO and PO significantly reduced the increased heart size and thoracic aortic media thickness observed in untreated SHR but RPO reduced only the latter. No such differences, however, were observed between the treated and untreated WKY rats. Oil Red O enface staining of thoracic-abdominal aorta did not show any lipid deposition in all treated rats. The SO and RPO significantly raised serum alkaline phosphatase levels in the SHR while body weight and renal biochemical indices were unaltered in both strains. Serum lipid profiles of treated SHR and WKY rats were unchanged, with the exception of a significant reduction in LDL-C level and total cholesterol/HDL ratio (atherogenic index) in SO and RPO treated SHR compared with untreated SHR.

CONCLUSION

The SO, RPO and PO attenuate the rise in blood pressure in SHR, accompanied by bradycardia and heart size reduction with SO and PO, and aortic media thickness reduction with SO, RPO and PO. The SO and RPO are antiatherogenic in nature by improving blood lipid profiles in SHR.

Long-term exercise attenuates blood pressure responsiveness and modulates kidney angiotensin II signalling and urinary sodium excretion in SHR

Observations have been made regarding the effects of long-term exercise training on blood pressure, renal sodium handling and renal renin-angiotensin-aldosterone (RAS) intracellular pathways in conscious, trained Okamoto-Aoki spontaneously hypertensive rats (SHR) and Wistar Kyoto (WKy) normotensive rats, compared with appropriate age-matched sedentary SHR and WKy. To evaluate the influence of exercise training on renal function and RAS, receptors and intracellular angiotensin II (AngII) pathway compounds were used respectively, and lithium clearance and western blot methods were utilised. The current study demonstrated that increased blood pressure in SHR was blunted and significantly reduced by long-term swim training between the ages of 6 and 16 weeks. Additionally, the investigators observed an increased fractional urinary sodium excretion in trained SHR (SHR(T)) rats, compared with sedentary SHR (SHR(S)), despite a significantly decreased creatinine clearance (C(Cr)). Furthermore, immunoblotting analysis demonstrated a decreased expression of AT1(R) in the entire kidney of T(SHR) rats, compared with S(SHR). Conversely, the expression of the AT2(R), in both sedentary and trained SHR, was unchanged. The present study may indicate that, in the kidney, long-term exercise exerts a modulating effect on AngII receptor expression. In fact, the present study indicates an association of increasing natriuresis, reciprocal changes in renal AngII receptors and intracellular pathway proteins with the fall in blood pressure levels observed in T(SHR) rats compared with age-matched S(SHR) rats.

Hypothalamic SOCS-3 expression and the effect of intracerebroventricular angiotensin II injection on water intake and renal sodium handling in SHR

In rats, the acute central dipsogenic and natriuretic action of angiotensin II (AngII) seems to be independent of the hemodynamic effects of the peptide; however, in genetically hypertensive models, this relationship has not yet been investigated. It has been demonstrated that AngII induces the suppressor of cytokine signaling (SOCS-3) expression in the brain that, in turn, modulates further activation of the pathway, leading to desensitization to AngII stimuli with regard to its dipsogenic effect. This study investigates age-related Janus kinase (JAK-2) and SOCS-3 hypothalamic expression, by immunoblotting, and the involvement of SOCS-3 expression in urinary sodium handling and dipsogenic response in spontaneously hypertensive rats (SHR), compared with age-matched Wistar-Kyoto (WKy) rats. The intracerebroventricular (i.c.v.) application of AngII significantly enhanced the dipsogenic response, reduced C(Cr), and reciprocally promoted increased absolute and fractional rates of excretion of sodium in WKy rats. The central AngII-induced dipsogenic effect in WKy and SHR was significantly attenuated by prior i.c.v. administration of DUP753. In addition, the magnitude of the dipsogenic and renal response to AngII was significantly attenuated in age-matched SHR. Blocking of hypothalamic SOCS-3 expression by an antisense oligonucleotide resulted in partial reversal of the refractory nature of AngII in thirst responses in SHR. The altered centrally applied AngII response in SHR associated with increased hypothalamic JAK-2/SOCS-3 expression may suggest that abnormal regulation of the central angiotensin pathways may contribute to dysfunction of water-electrolyte homeostasis in SHR.

Altered renal sodium handling in spontaneously hypertensive rats (SHR) after hypertonic saline intracerebroventricular injection: role of renal nerves

The mechanism by which blood pressure rises in the SHR strain remains to be elucidated. Also, there is a surprising lack of experimental data on the natriuretic mechanisms induced by intracerebroventricular (ICV) injection of hyperosmotic saline (HoS) in SHR. In normotensive animals ICV injection of HoS causes coordinated responses including natriuresis and inhibition of renal sympathetic nerve activity. In the present study, we hypothesized that presumable blunting of the sympathoinhibitory response to centrally injected HoS may contribute to a lack of suppression of efferent renal nerve outflow in SHR. To test this hypothesis, the present study evaluates the influence of renal denervation after central HoS injection at increasing concentration on urinary sodium handling in SHR compared with age-matched normotensive WKy rats. The study confirmed previous data showing pronounced natriuretic response to centrally HoS stimuli but also demonstrated that the creatinine clearance (C(Cr)) and fractional sodium excretion responses diminished as graded NaCl concentrations were increased in WKy rats but not in SHR. In SHR, increased FE(Na) obtained by central administration of 0.90 M NaCl was produced by increases in proximal (FEP(Na)) and post-proximal fractional urinary sodium rejection without changes in C(Cr), indicating a direct tubular effect. Renal denervation caused significant antinatriuresis by decreased C(Cr) and increased FEP(Na) reabsorption in WKy but not in SHR. This study suggests that natriuresis observed only after higher centrally HoS stimuli with a rightward shift of dose-response curve provides evidence of a down-regulation of target organ responsiveness of periventricular areas of genetic hypertensive rats.

Chloride-Dominant Salt Sensitivity in the Stroke-Prone Spontaneously Hypertensive Rat

We tested the hypothesis that in the stroke-prone spontaneously hypertensive rat (SHRSP), the Cl- component of dietary NaCl dominantly determines its pressor effect (salt-sensitivity). We telemetrically measured systolic aortic blood pressure (SBP) in SHRSP loaded with: nothing (CTL); NaCl alone (NaCl) (44 mmol/100 grams chow); KCl (KCl) alone (44 mmol); NaCl (44 mmol) combined with KHCO3 (77 mmol) (NaCl/KBC) or with KCl (77 mmol) (NaCl/KCl). Across all groups, from age 10 to 15 or 16 weeks, SBP increased linearly (mm Hg/week) (dp/dt, change in SBP as a function of time): CTL, 5.6; NaCl, 9.5; KCl, 8.8; NaCl/KBC, 9.1; and NaCl/KCl, 14.6. Thus, the value of dp/dt in KCl matched that in NaCl. The value of dp/dt in NaCl/KCl exceeded that in NaCl in direct proportion to the greater Cl- load. Across all groups, only Cl- load bore a direct, highly linear relationship with dp/dt. Strokes occurred only, but always with SBP >250 mm Hg, a value observed almost exclusively in NaCl/KCl. Thus, Cl- dominantly determined the pressor effect induced with dietary NaCl, both with NaCl loaded alone and combined with either KCl or KHCO3, and thereby likely determined the occurrence of stroke with NaCl loading. Over the initial 3-day period of NaCl loading and exacerbating hypertension, external balance of Na+ increased similarly among all groups. However, within 24 hours of initiating NaCl loading, urinary creatinine excretion decreased in direct proportion to dp/dt and urinary Cl- excretion. We conclude that in the SHRSP, the Cl- component of a dietary NaCl dominantly determines salt sensitivity and thereby phenotypic expression. We suggest that Cl- might do so by inducing renal vasoconstriction.

Early altered renal sodium handling determined by lithium clearance in spontaneously hypertensive rats (SHR): Role of renal nerves

The mechanism by which blood pressure rises in the SHR strain remains to be elucidated. Since the long-term changes in renal sodium tubule handling associated with genetic hypertension have not been examined in detail, we hypothesized that SHR hypertension development may result from sustained renal sympathetic nerve overactivity and consequently decreased urinary sodium excretion. To test this hypothesis, we assessed renal sodium handling and cumulative sodium balance for 10 consecutive weeks in unanesthetized renal-denervated SHR, performed prior to the start of the entire 10-week metabolic studies, and their age-matched normotensive and hypertensive controls. The present investigation shows that SHR excreted less sodium than Wistar-Kyoto (WKy) rats during the initial 3-week observation period (p <0.05). This tendency was reversed when SHR were 10-wk old. Fractional urinary sodium excretion (FENa+) was significantly lower in 3 and 6-wk-old SHR when compared with the WKy age-matched group, as follows: SHR3-wk-old: 0.33 +/- 0.09% and WKy3-wk-old: 0.75 +/- 0.1% (P <0.05); SHR(6-wk-old): 0.52 +/- 0.12% and WKy6-wk-old: 0.83 +/- 0.11%. The decreased FENa+ in young SHR was accompanied by a significant increase in proximal sodium reabsorption (FEPNa+) compared with the normotensive age-matched control group (P <0.01). This increase occurred despite unchanged creatinine clearance (CCr) and fractional post-proximal sodium excretion (FEPPNa+)in all groups studied. The decreased urinary sodium excretion response in SHR up to the age of 6 weeks was significantly eradicated by bilateral renal denervation of SHR3-wk-old: 0.33 +/- 0.09% and SHR6-wk-old: 0.52 +/- 0.12% to DxSHR 3-wk-old: 1.02 +/- 0.2% and DxSHR 6-wk-old: 0.94 +/- 0.2% (P <0.01), in renal denervated rats. The current data suggest that neural pathways may play an instrumental role on renal sodium reabsorption as result of sustained sympathetic nervous system overexcitability.

Hypertension and the metabolic syndrome: closely related central origin?

In primary hypertension a mild hyperresponsiveness of hypothalamic, sympatho-hormonal centres to psychosocial stimuli forms a major pathogenetic element, although high salt intake in some subjects may contribute via volume expansion. Hypertension is often associated with another "civilisation" disorder, the metabolic syndrome, defined as abdominal obesity, insulin resistance and dyslipidaemia. According to recent research, the metabolic syndrome has in all likelihood a central neuroendocrine origin in the form of enhanced engagement of the hypothalamic-pituitary-adrenal (HPA) axis. Here the peripheral endocrine perturbations act as triggers for both central obesity and the metabolic abnormalities. The reaction pattern characterising early primary hypertension is identical with, or closely related to, the "defence reaction", while that leading to the metabolic syndrome is similar to that of the "defeat reaction". Both belong to the primitive survival reactions, common to all mammals, though man can control, or at least mask, his outward-behavioural part but not the neuro-hormonal expressions. Animal experiments show how frequent or chronic mental challenges are capable of engaging these limbic-hypothalamic centres, affecting blood pressure regulation as well as endocrine-metabolic regulation. Furthermore, these centres are tightly coupled functionally, and their signals to the periphery often combined. On a long-term basis their engagements appear to be decisive for the development of both primary hypertension and the metabolic syndrome, as suggested by intervention studies. In both these "disorders of civilisation", observations strongly indicate that psychosocial stress, socioeconomic handicaps, lack of exercise, abuse and also psychiatric traits are involved. Such factors, characteristic of current competitive society, probably cause mixed engagements of the two above-mentioned neuro-hormonal patterns, and thereby, with time, primary hypertension and the metabolic syndrome, with end-points such as coronary artery disease, diabetes mellitus type2 and stroke. Susceptibility to such developments is probably enhanced by genetic factors. This overview of recent developments therefore serves to emphasise how both primary hypertension and the metabolic syndrome seem to have a common central origin. Central regulatory factors are often overlooked, partly because it is not realised that limbic-hypothalamic centres are the major regulators of both circulatory and metabolic events, and partly because of the long period of time required before these disease end-points are reached.

Ontogenetic aspects of hypertension development: analysis in the rat

In this review, we attempt to outline the age-dependent interactions of principal systems controlling the structure and function of the cardiovascular system in immature rats developing hypertension. We focus our attention on the cardiovascular effects of various pharmacological, nutritional, and behavioral interventions applied at different stages of ontogeny. Several distinct critical periods (developmental windows), in which particular stimuli affect the further development of the cardiovascular phenotype, are specified in the rat. It is evident that short-term transient treatment of genetically hypertensive rats with certain antihypertensive drugs in prepuberty and puberty (at the age of 4-10 wk) has long-term beneficial effects on further development of their cardiovascular apparatus. This juvenile critical period coincides with the period of high susceptibility to the hypertensive effects of increased salt intake. If the hypertensive process develops after this critical period (due to early antihypertensive treatment or late administration of certain hypertensive stimuli, e.g., high salt intake), blood pressure elevation, cardiovascular hypertrophy, connective tissue accumulation, and end-organ damage are considerably attenuated compared with rats developing hypertension during the juvenile critical period. As far as the role of various electrolytes in blood pressure modulation is concerned, prohypertensive effects of dietary Na+ and antihypertensive effects of dietary Ca2+ are enhanced in immature animals, whereas vascular protective and antihypertensive effects of dietary K+ are almost independent of age. At a given level of dietary electrolyte intake, the balance between dietary carbohydrate and fat intake can modify blood pressure even in rats with established hypertension, but dietary protein intake affects the blood pressure development in immature animals only. Dietary protein restriction during gestation, as well as altered mother-offspring interactions in the suckling period, might have important long-term hypertensive consequences. The critical periods (developmental windows) should be respected in the future pharmacological or gene therapy of human hypertension.

Effects of calcium and potassium supplements on arterial tone in vitro in spontaneously hypertensive rats

1. Calcium and potassium intakes inversely correlate with blood pressure in experimental hypertension. Therefore, we examined the effects of calcium and potassium supplements alone and in combination on arterial tone in spontaneously hypertensive rats (SHR). Wistar-Kyoto (WKY) rats served as normotensive controls. Calcium and potassium contents in the control diet were both 1%, while those in supplemented chows were 3% and 3.5%, respectively. The sodium content of all diets was moderately elevated to 1.1%. 2. After 12 weeks of the study systolic blood pressures in SHR on high calcium and on high potassium diets were markedly lower (about 53 and 58 mmHg, respectively) than in hypertensive controls, while combined supplementation of these cations reduced blood pressure even further (about 69 mmHg). 3. Responses of mesenteric arterial rings in vitro were examined at the end of the study. Both high calcium and high potassium diets improved the impaired relaxation to acetylcholine (ACh) in SHR, while the combination of these supplements completely normalized this response. Cyclo-oxygenase inhibition by diclofenac augmented the relaxation to ACh in hypertensive controls but not in the other groups. Nevertheless, enhanced endothelium-mediated dilatation was still observed in the presence of diclofenac and the nitric oxide synthase inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME) in all supplemented groups. Interestingly, additional blockade of Ca2+-activated K+ channels by tetraethylammonium abolished the improved relaxation to ACh in SHR on high calcium and on high potassium, but distinct responses were still observed in WKY rats and SHR on the combined supplement. 4. When hyperpolarization of smooth muscle was prevented by precontraction of the preparations with 50 mM KCl, only marginal differences were observed in the diclofenac and L-NAME-resistant relaxations to ACh between the study groups. Finally, endothelium-independent vasorelaxations of noradrenaline-precontracted rings to nitroprusside, isoprenaline and cromakalim were comparably augmented by all supplements. 5. In conclusion, the vascular mechanisms underlying the antihypertensive effect of high calcium and high potassium diets during moderately elevated sodium intake in SHR may involve enhanced arterial hyperpolarization, increased smooth muscle sensitivity to nitric oxide and decreased production of vasoconstrictor prostanoids. The administration of these cations in combination was more effective than either of them alone in reducing blood pressure and restoring arterial tone.

Influence of dietary salts on the cardiovascular effects of low-dose combination of ramipril and felodipine in spontaneously hypertensive rats

1 In spontaneously hypertensive rat (SHR) we examined over a 4-week period the influence of control low sodium diet, common salt-enriched diet (sodium chloride 6% of the dry weight of the chow) and a novel mineral salt-enriched diet (potassium-, magnesium-, and l-lysine-enriched mineral salt added at a 75% higher level of 10.5% to produce the same sodium chloride concentration of 6%) on the cardiovascular effects produced by a low-dose combination of an angiotensin converting enzyme inhibitor ramipril (0.25 mg kg(-1) day(-1) in the food) and a calcium channel blocker felodipine (0.4 mg kg(-1) day(-1) subcutaneously via an osmotic minipump). 2 Common salt, but not the mineral salt, accelerated the development of hypertension and induced left ventricular and renal hypertrophy in SHR. Neither common salt nor mineral salt significantly affected heart rate. 3 The combination of ramipril and felodipine decreased systolic blood pressure and prevented the development of left ventricular hypertrophy effectively during the common salt diet without any significant effect on the heart rate. The cardiovascular effects of the drug combination were improved by the low sodium diet or by replacement of high common salt in the diet by mineral salt. 4 Responses of endothelium-intact mesenteric arterial rings in vitro were examined at the end of the four-week study. The combination of ramipril and felodipine markedly improved the endothelium-dependent vascular relaxation responses to acetylcholine and enhanced the endothelium-independent vascular relaxation responses to sodium nitroprusside in SHR on control and common salt diets. Replacement of common salt in the diet by mineral salt improved the endothelium-dependent vascular relaxation responses to acetylcholine. The drug combination attenuated the alpha-adrenoceptor-mediated vascular contractile responses to noradrenaline during the common salt diet. 5 Ramipril and felodipine in combination increased plasma renin activity by 1.9-3.2 fold without affecting serum aldosterone levels. 6 Our findings suggest that the cardiovascular effect of the low-dose combination of ramipril and felodipine was maintained during high salt intake. However, salt restriction or replacement of common salt in the diet by the potassium- and magnesium-enriched mineral salt improved the cardiovascular effects of the drug combination. In the face of a high intake of sodium, a part of the beneficial cardiovascular effects of the drug combination is apparently mediated by improved endothelium-dependent and endothelium-independent vascular relaxation responses and attenuated alpha-adrenoceptor-mediated vascular contractile responses.

NaCl sensitivity of essential hypertensive patients is related to insulin resistance

OBJECTIVE

To evaluate insulin sensitivity of essential hypertensive patients with different salt sensitivities of blood pressure in the absence of confounding factors such as obesity, glucose intolerance and the inclusion both of normotensive and of hypertensive subjects that have affected most previous studies.

PATIENTS

Ninety-nine patients with untreated mild or moderate essential hypertension, World Health Organization class I-II, participated in the study.

METHODS

Salt sensitivity was estimated using the Weinberger protocol with minor modifications and the patients were classified into tertiles of salt sensitivity.

RESULTS

Patients with high NaCl sensitivities were slightly older and had somewhat higher blood pressures than did subjects with low salt sensitivities. Plasma renin activity significantly decreased with increasing salt sensitivity. There were no differences among the three groups in terms of body mass index, fasting blood glucose and insulin plasma levels. There were no differences among the groups in the integrated glucose and insulin response to a standard oral-glucose tolerance test However, there was a significant difference in insulin sensitivity between two subgroups of the upper and lower tertile of salt sensitivity, the salt-sensitive hypertensives having a markedly lower utilization of glucose than did the salt-resistant ones, with a minor overlap (5.4 +/- 0.6 versus 7.4 +/- 0.3 mg/kg per min, P < 0.01).

CONCLUSIONS

This study showed that essential hypertensive patients with high NaCl sensitivities were relatively insulin resistant compared with those with low NaCl sensitivities, independently of confounding factors such as age, obesity and glucose intolerance. Insulin resistance was not associated with overt hyperinsulinaemia among these patients.

Cardiovascular and renal effects of the combination of felodipine and metoprolol during a high-salt and a moderate-salt diet in spontaneously hypertensive rats

In the present study the influence of dietary salt on the cardiovascular and renal effects of the calcium channel blocker felodipine (1.2 mg/kg sc) and the beta 1-adrenoceptor blocking drug metoprolol (250 mg/kg po), alone and in combination, was examined in the spontaneously hypertensive rats (SHRs) in a 4-week study. In addition, the influence of different diet and drug regimens on vascular functions was assessed by measuring the vascular relaxation and contractile responses of mesenteric arterial rings in vitro at the end of the experimental period. In SHRs, a high-salt diet caused a marked rise in blood pressure, impaired the endothelium-dependent vascular relaxation responses to acetylcholine and induced left ventricular hypertrophy (LVH) and renal hypertrophy. Metoprolol had little if any effect on salt-induced changes in blood pressure, endothelium-dependent vascular relaxation or renal hypertrophy, but it partially prevented the development of salt-induced LVH. Felodipine during the high-salt diet lowered blood pressure to normotensive level and completely prevented salt-induced left ventricular and renal hypertrophy as well as endothelial dysfunction. Felodipine produced tachycardia, especially at the beginning of drug treatment. The combination of felodipine and metoprolol abolished the effects of the individual drugs on heart rate. The drug combination also completely prevented the detrimental cardiovascular and renal effects induced by a high salt intake. Although salt restriction did not further enhance the profound antihypertensive effect of the combination of metoprolol and felodipine, it enhanced the effects of the drug combination on LVH and renal hypertrophy. Our findings indicate that felodipine treatment, alone and in combination with metoprolol, normalizes blood pressure and prevents the development of salt-induced LVH and renal hypertrophy. During the high-salt diet the beneficial vascular effects of felodipine as well as those of the combination of felodipine and metoprolol are mediated, at least in part, by prevention of salt-induced endothelial dysfunction. The only apparent benefit from the use of metoprolol in combination with a relatively high dose of felodipine was the prevention of tachycardia.

Effects of dietary sodium and magnesium on cyclosporin A-induced hypertension and nephrotoxicity in spontaneously hypertensive rats

Arterial hypertension, nephrotoxicity, and magnesium loss are common side effects of the immunosuppressive agent cyclosporin A (CsA). In the present study, the effects of dietary sodium and magnesium on CsA toxicity were examined in spontaneously hypertensive rats. A 6-week treatment with CsA during a moderately low-sodium diet (Na 0.3%, Mg 0.2% of the dry weight of the chow) raised blood pressure only slightly, without evidence of nephrotoxicity. By contrast, CsA during a high-sodium diet (Na 2.6%) produced a pronounced rise in blood pressure as well as marked nephrotoxicity, comprising decreased creatinine clearance, increased levels of serum creatinine and urea, and increased urinary protein excretion. During the high-sodium diet, CsA decreased myocardial and bone magnesium concentration and increased myocardial and renal calcium concentration. Magnesium supplementation (Mg 0.6%) protected against the CsA-induced hypertension and nephrotoxicity during the high-sodium diet. Magnesium supplementation also completely prevented the CsA-induced myocardial magnesium depletion and calcium accumulation in the heart and kidney during the high-sodium diet. Our findings indicate a detrimental interaction between increased sodium intake and CsA treatment and a marked protection by concomitant oral magnesium supplementation.

Correlative evidence of hypertension and altered cochlear microhomeostasis: electrophysiological changes in the spontaneously hypertensive rat

The spontaneously hypertensive rat model has been used to show that hypertension is an important pathophysiological risk factor in age-related hearing loss. In the present study, compound action potential (CAP), electrochemical potential (ECP), and potassium concentration (CK+) measurements were taken from the cochlea of genetically predisposed, spontaneously hypertensive rats (SHR) and from normotensive Wistar-Kyoto (WKY) rats. In the SHR model, as the duration of hypertension increased with the animal's age (from 3 to 8 months), CAP thresholds increased, ECP increased in marginal cells only, and CK+ increased in both endolymph and marginal cells. Collectively, the data suggest that ionic alternations of cellular potentials are involved in hearing changes in the hypertensive state. Ultimately, such data may assist in understanding hearing loss in individuals who are diagnosed with hypertension.

Protective effects of ME3221 on hypertensive complications and lifespan in salt-loaded stroke-prone spontaneously hypertensive rats

1. A comparison was made on the protective effects of the following: ME3221, a competitive angiotensin AT1 receptor antagonist; losartan, in which a major active metabolite is a non-competitive angiotensin AT1 receptor antagonist; and enalapril, an angiotensin-converting enzyme inhibitor, using the salt-loaded stroke-prone spontaneously hypertensive rats (SHRSP). 2. SHRSP received orally ME3221 (3 and 10 mg/kg per day), losartan (10 mg/kg per day) and enalapril (10 mg/kg per day) from the 6th to the 20th week of age. All the control rats showed rapid elevation of systolic blood pressure (SBP), accompanied by hypertensive complications, and died by 15 weeks of age. 3. ME3221, losartan and enalapril suppressed the elevation of SBP in the salt-loaded SHRSP to a comparable degree. ME3221 and losartan increased the survival rate to > 90%, and diminished hypertensive complications such as cerebral apoplexy (stroke), renal injury (increased proteinuria, and total N-acetyl-beta-D-glucosaminidase activity) and heart failure (cardiac hypertrophy and pleural effusion). 4. Competitive (ME3221) and non-competitive (losartan) angiotensin AT1 receptor antagonists showed comparable efficacy against the complications and mortality of the salt-loaded SHRSP; both were more potent than enalapril in the protective effect.

Association of salt sensitivity in rats with genes of the major histocompatibility complex

Dietary sodium intake has long been considered an important factor in the genesis and maintenance of hypertension in both humans and experimental animals. To identify the possible association between salt sensitivity and genes of the major histocompatibility complex (RT1 complex), we studied the blood pressure response to an 8% NaCl diet in normotensive Lewis rats, spontaneously hypertensive rats (SHR), and Lewis.1K congenic rats (congenic to Lewis with the SHR main histocompatibility complex RT1). During the first 4 weeks of a high salt diet, the blood pressure increase was the same in SHR and Lewis.1K congenic rats. Thus, the presence of a small segment of SHR chromosome 20 with genes of the RT1 complex (and closely related genes) in the Lewis genome sensitized the blood pressure of these animals to the hypertensive effects of a high salt diet. Genes of the RT1 complex influenced the salt-induced increase of relative kidney weight more than that of relative heart weight. Our results support the hypothesis that some alleles within or close to the RT1 complex might be responsible for the higher sensitivity of hypertensive individuals to certain environmental stressors, including high salt intake.

High calcium diet prevents baroreflex impairment in salt-loaded spontaneously hypertensive rats

To investigate the role of the sympathetic control mechanism in the antihypertensive effect of dietary calcium supplementation, we examined whether a high calcium diet affected mean arterial pressure, renal sympathetic nerve activity, heart rate, and overall and central properties of the arterial baroreceptor reflex in salt-loaded young spontaneously hypertensive rats (SHR). Six-week-old SHR were fed either a normal (0.66%) or high (8.00%) salt diet with either a normal (1.17%) or high (4.07%) calcium content for 4 weeks. The arterial baroreceptor reflex was elicited with rats under halothane anesthesia by altering mean arterial pressure with nitroprusside or phenylephrine. The overall property of the arterial baroreceptor reflex was assessed by the median mean arterial pressure (MAP50) and maximal gain (Gmax) of the relation between mean arterial pressure and renal sympathetic nerve activity and between mean arterial pressure and heart rate. The central property of the arterial baroreceptor reflex was assessed by reflex inhibition of renal sympathetic nerve activity and heart rate elicited by electrical stimulation of the aortic depressor nerve. Compared with the control group fed a normal salt/normal calcium diet, the high salt/normal calcium group had significantly higher mean arterial pressure and renal sympathetic nerve activity but not heart rate. Moreover, the arterial baroreceptor reflex was impaired in the latter group, as evidenced by an increase in MAP50 and decrease in Gmax of the two relations and an attenuation of reflex inhibition of renal sympathetic nerve activity by aortic depressor nerve stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of high salt intake on control of hindlimb vascular resistance by arterial baroreflex and vagal afferents in spontaneously hypertensive rats

1. This study aimed to examine whether a high salt diet alters control of vascular resistance by arterial baroreflex and vagal afferents in spontaneously hypertensive rats (SHR) and Wistar Kyoto rats (WKY). 2. SHR and WKY aged 8 weeks were fed either high (8%) or normal salt (0.4%) diet for 4 weeks. Arterial baroreflex control of hindlimb vascular resistance was assessed by examining reflex-induced vasodilation and vasoconstriction in response to phenylephrine and nitroprusside, respectively, in the constant-flow perfused hindlimb of urethane-anesthetized rats. 3. Tonic influence of the cardiopulmonary vagal afferents was evaluated by examining the effects of vagotomy on hindlimb vascular resistance and on the gain of arterial baroreflex control of hindlimb vascular resistance. 4. The gain of the arterial baroreflex control of hindlimb vascular resistance in response to both phenylephrine and nitroprusside were not significantly different between SHR receiving high and normal salt diets, and between WKY receiving high and normal salt diets. 5. Vagotomy increased hindlimb vascular resistance in all four groups of rats. However the high salt diet than those in SHR on a normal salt diet but similar between the two groups of WKY. Vagotomy increased the slope of arterial baroreflex control of hindlimb vascular resistance in SHR receiving a normal salt diet and the two groups of WKY but not in SHR receiving a high salt diet.(ABSTRACT TRUNCATED AT 250 WORDS)

Renal corticosterone 6 beta-hydroxylase in the spontaneously hypertensive rat

Excess 6 beta-OH-corticosterone production by family 3A cytochromes P-450 may play a role in genesis of hypertension in the spontaneously hypertensive rat (SHR), by producing a renal defect in Na+ excretion. Renal cytochromes P-450 may be a causal factor in this genetic model. Since family 3A P-450 is present in rat kidney (collecting duct), the renal family 3A catalytic (6 beta-OHase) and immunoreactive activities were compared in SHR and normotensive control (Wistar-Kyoto; WKY) rats. Corticosterone 6 beta-hydroxylation is markedly higher in SHR than in WKY renal microsomal preparations. Western blot analysis with antibodies to rat and rabbit liver family 3A isoforms demonstrated related proteins. Densitometry revealed greater relative intensity of staining in SHR compared to WKY with both antibodies. Both antibodies inhibited corticosterone 6 beta-hydroxylation by SHR renal microsomes. Increased renal 6 beta-OH-corticosterone production by increased renal family 3A cytochromes P-450 may play a role in the blood pressure elevation 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.

The borderline hypertensive rat (BHR) as a model for environmentally-induced hypertension: a review and update

In recent years, the burgeoning disciplines of health psychology and behavioral medicine have renewed interest in the important role that environmental factors can play in the disease process. Nowhere is this concern more well-founded than in the area of cardiovascular disorders, particularly hypertension. Epidemiologists and clinicians have long suspected that stressful life events can be a sufficient trigger for the expression of hypertension in some individuals. To understand better the ways in which these variables interact in the disease process, researchers have tried, with limited success, to produce experimental hypertension in animals by exposing them to stressful environmental paradigms. Additionally, recent investigations using the borderline hypertensive rat (BHR) have demonstrated the important role genetic factors can play in mediating both the behavioral and cardiovascular responses to environmental stressors. The current paper will review these attempts and discuss recent data from experiments using a relatively new animal model that appears to be especially appropriate for the study of environmental-genetic factors in the elaboration of essential hypertension. We will also discuss potential mechanisms by which environmental stress influences arterial pressure and suggest avenues for further inquiry into the stress-disease relationship.

Chromosomal mapping of two genetic loci associated with blood-pressure regulation in hereditary hypertensive rats

The spontaneously hypertensive rat and the stroke-prone spontaneously hypertensive rat are useful models for human hypertension. In these strains hypertension is a polygenic trait, in which both autosomal and sex-linked genes can influence blood pressure. Linkage studies in crosses between the stroke-prone spontaneously hypertensive rat and the normotensive control strain Wistar-Kyoto have led to the localization of two genes, BP/SP-1 and BP/SP-2, that contribute significantly to blood pressure variation in the F2 population. BP/SP-1 and BP/SP-2 were assigned to rat chromosomes 10 and X, respectively. Comparison of the human and rat genetic maps indicates that BP/SP-1 could reside on human chromosome 17q in a region that also contains the angiotensin I-converting enzyme gene (ACE). This encodes a key enzyme of the renin-angiotensin system, and is therefore a candidate gene in primary hypertension. A rat microsatellite marker of ACE was mapped to rat chromosome 10 within the region containing BP/SP-1.

Cyclic nucleotides and calmodulin-phosphodiesterase activator: potential biochemical markers of salt sensitivity

Salt sensitivity affects a fraction of hypertensive and normotensive subjects, but biochemical markers that target salt-sensitive individuals are not available at present. The aim of these studies was to establish if calmodulin-phosphodiesterase (CaM-PDE) activator (J Clin Invest 82: 276, 1988) and platelet cyclic nucleotides could serve as potential markers of salt sensitivity. The results demonstrated that CaM-PDE activator was increased in the heart of Dahl salt-sensitive rats (DS/JR) compared to Dahl salt-resistant (DR/JR) animals fed a 1% sodium diet. Normotensive male Wistar rats given a low (0.15%) or high (3.5%) sodium diet from age 7 weeks to 11 weeks presented significant increases (p less than 0.01) of three parameters: blood pressure (from 106 +/- 4 to 128 +/- 8 mmHg); platelet aggregation in response to ADP and thrombin; and CaM-PDE activator levels (from 1.57 +/- 0.14 to 2.8 +/- 0.18). Basal as well as stimulated cyclic nucleotide levels were significantly reduced in rats fed the high sodium diet. Since the degree of stimulation by the agonists was unaltered, the results are compatible with augmented PDE activity. These preliminary data suggest that CaM-PDE activator should be explored as a potential marker of salt sensitivity.

Peripheral taste responses in genetically hypertensive rats

In two-bottle preference-aversion tests, the spontaneously hypertensive rat (SHR) tolerates higher concentrations of NaCl than the normotensive Wistar-Kyoto (WKY). In contrast, the inbred Dahl salt-sensitive (S/JR) and inbred Dahl salt-resistant (R/JR) rat show similar preferences for NaCl. In order to determine if taste receptor function was also altered between the hypertensive rat and its normotensive control, we recorded electrophysiological taste responses from the chorda tympani (CT) nerve in SHR, WKY, S/JR and R/JR rats. Responses to a concentration series (0.05 M to 0.5 M) of NaCl, NaAcetate, KCl, NH4Cl and CaCl2 were recorded before and after lingual application of amiloride hydrochloride, an epithelial sodium transport blocker. When expressed relative to the 0.5 M NH4Cl response, responses to the majority of stimuli were equivalent between the SHR and WKY. By comparison, relative responses to NaCl were greater in the R/JR than S/JR; however, the magnitude of amiloride suppression was equivalent between these two strains. Relative responses to the majority of the remaining salts did not differ between the S/JR and R/JR. These results suggest that taste receptor function may be equivalent between the hypertensive rat and its normotensive control.

Dietary sodium loading elevates blood pressure in baroreceptor denervated rats

We have examined the possibility that defective baroreflex function may contribute to the hypertensive effect of a high dietary sodium intake. In a preliminary study we found that, after interrupting the baroreflex by sino-aortic denervation (SAD), feeding a high-sodium diet to normotensive Wistar/Kyoto rats (WKY) caused their tail-cuff blood pressure to rise to hypertensive levels. In the present study, SAD and sham-operated WKY were fed diets with either a low or a high sodium content. An increase in the sensitivity of blood pressure to sodium after baroreceptor denervation was confirmed by direct measurement of mean arterial pressure (MAP) in conscious rats via indwelling aortic catheters. After 8 weeks the MAP in SAD rats on the high sodium diet was 35 mm Hg higher than in SAD rats on the low sodium diet. Ganglion blockade reduced MAP to a similar level in all treatment groups, but pressor responses to phenylephrine were greater in SAD rats on the high sodium diet, suggesting that the hypertensive effect of sodium in this group might have been due to increased sympathetic vasoconstriction.

The early ontogeny of sodium appetite in spontaneously hypertensive vs. normotensive rats

Rats of various strains differ widely in their appetite for sodium. For example, the Okamoto-Aoki spontaneously hypertensive rat (SHR) exhibits a much greater salt appetite than its normotensive control strains. Developmental observations of salt appetite in this strain have been collected only for rats at or above weanling age. In this experiment, the salt appetite of the SHR was compared with that of normotensive Sprague-Dawley and Wistar-Kyoto rats, at 3, 6 and 12 days after birth. The results show that the increased salt appetite of the SHR is a fundamental behavioral trait that appears very early in life. This observation is consistent with a defective central angiotensin system in SHR rats.

Alpha-adrenoceptor properties in rat strains sensitive or resistant to salt-induced hypertension

Cerebral and renal alpha 2-adrenoceptors are implicated in the control of sympathetic activity and of sodium reabsorption respectively. In addition, sodium ions play an important role in the regulation of either alpha 2-adrenoceptor densities and affinities for adrenergic agonists. In the present study, alpha-adrenoceptor properties were investigated in genetically predetermined salt-sensitive and salt-resistant Dahl and Sabra rats. Cerebral alpha 2-adrenoceptor densities were higher in salt-resistant than in salt-sensitive Dahl and Sabra rats. In contrast, renal alpha 2-adrenoceptor density was higher in salt-sensitive than in salt-resistant rats. No difference in cerebral and renal alpha 1-adrenoceptor densities was observed between Dahl and Sabra substrains. Noradrenaline content in cerebral and renal cortex were also similar in both these rat substrains. Sodium ions markedly increased cerebral and renal high-affinity alpha 2-adrenoceptor densities in salt-sensitive but not in salt-resistant rats. Cerebral and renal alpha 1-adrenoceptor densities were unchanged in salt-sensitive and salt-resistant substrains of Dahl and Sabra rats. In addition, sodium ions reduced the affinity of adrenaline for renal alpha 2-adrenoceptors in salt-sensitive rats but not in salt-resistant rats. We can conclude that there exist genetically determined differences in the densities and properties of cerebral and renal alpha 2-adrenoceptors between salt-sensitive and salt-resistant rat strains. Abnormal densities of alpha 2-adrenoceptors may play a primary role in the role in the development of hypertension in salt-sensitive animals. These results also suggest an association between absence of sodium regulation of alpha 2-adrenoceptors and resistance to salt-induced hypertension. The absence of sodium regulation in salt-resistant rats may be linked either to a particular receptor conformation or to an abnormal structure of the receptor system. This property may represent a genetically-mediated change responsible for the resistance to the development of salt-induced hypertension.

Effect of salt-loading on erythrocyte deformability in spontaneously hypertensive and Wistar-Kyoto rats

Effects of salt-loading on erythrocyte and erythrocyte ghost deformabilities were measured by laser diffractometry using a flat cell and a helium-neon laser in spontaneously hypertensive rats (SHR) and age-matched normotensive Wistar-Kyoto rats (WKY). Salt-loading did not affect the deformability of erythrocytes in SHR and WKY, although a significantly reduced deformability was observed in salt-loaded SHR compared with values in control WKY and salt-loaded WKY (p less than 0.05, p less than 0.05, respectively). In contrast, salt-loading significantly reduced the deformability of erythrocyte ghosts in WKY and SHR (p less than 0.05, p less than 0.05, respectively). Our results suggest that salt-loading reduces erythrocyte membrane viscoelasticity in both WKY and SHR, and that the observed reduction of ghost deformability induced by salt-loading may influence the peripheral circulation.

Maternal behavior of SHR rats and its relationship to offspring blood pressures

In Part I of this study maternal behavior of 15 spontaneously hypertensive (SHR) and 11 Wistar-Kyoto (WKY) rat mothers was recorded throughout the preweaning period. SHR mothers were observed: 1) in arched-and blanket-nursing postures more often, and in passive-nursing posture less often, 2) licking their pups more often, and 3) resting less often. Further, SHR mothers retrieved their pups more quickly than did WKY. In the second part of this study we found that a composite maternal behavior score, derived from summing occurrences of mother/pup contact, arched-nursing, and pup-licking, was positively correlated with the offspring blood pressures. This relationship, which accounted for 43.5% of the variance in adult blood pressure, suggests that certain types of interactions between mothers and their pups may contribute to individual differences in cardiovascular system development.

Exaggerated depressor response to 6-iodoamiloride in NaCl-sensitive spontaneously hypertensive rats

The current study tested the hypothesis that NaCl-sensitive hypertension may result from increased membrane sodium channel activity. The effect of 6-iodoamiloride, and analog of the sodium channel blocker amiloride, on mean arterial pressure (MAP) was examined in conscious, freely moving NaCl-sensitive spontaneously hypertensive rats (SHR-S) fed high (8%) or normal (1%) NaCl diets. SHR-S and age-matched NaCl-resistant SHR (SHR-R) and normotensive Wistar-Kyoto (WKY) control rats were studied at 9 weeks of age after 2 weeks on either high (8%) NaCl or control (1%) NaCl diets. 6-iodoamiloride was infused intravenously in doses of 0.38 or 0.76 mg/100 g body weight, and MAP and heart rate (HR) were monitored from a femoral arterial cannula for 2 hours. The 8% NaCl diet caused a significant elevation in MAP in SHR-S but not in SHR-R or WKY. Administration of 6-iodoamiloride (both doses) produced a significant, sustained decrease in MAP in both SHR-S and SHR-R. Maximal depressor responses to high dose 6-iodoamiloride were significantly enhanced in SHR-S fed 8% NaCl (31.2 +/- 3.7 mm Hg) compared to SHR-S fed 1% NaCl (14.8 +/- 2.4 mm Hg) or SHR-R fed either 8% or 1% NaCl diets (15.6 +/- 4.2 and 10.2 +/- 3.0 mm Hg, respectively). In contrast, feeding an 8% NaCl diet had no significant effect on the depressor responses to 6-iodoamiloride in either SHR-R or WKY rats. In WKY, these doses of 6-iodoamiloride had no significant effect on MAP in either diet group. 6-iodoamiloride had no significant effect on heart rate in any group. These results support the hypothesis that the exacerbation of hypertension in SHR-S fed a high NaCl diet may result from increased membrane sodium channel activity.

High NaCl diet increases anterior hypothalamic alpha 2-adrenoceptors in SHR

Previous studies carried out in our laboratories demonstrated that dietary NaCl supplementation induced an increase in blood pressure associated with a reduction in noradrenaline release in the anterior hypothalamic region of NaCl-sensitive spontaneously hypertensive rats (SHR). The present study tested the hypothesis that this reduction in noradrenaline release induces a compensatory increase in alpha 2-adrenoceptor number in the anterior hypothalamic region of SHR. Low affinity p-[3H]aminoclonidine (alpha 2-adrenoceptor agonist) binding in the anterior hypothalamic region was increased significantly in SHR, but not in normotensive Wistar-Kyoto (WKY) rats on a high (compared to a basal) NaCl diet. Increased p-[3H]aminoclonidine binding was present at 1 and 2 weeks following initiation of the diets. In contrast, in the posterior hypothalamic region of SHR, alpha 2-adrenoceptors were significantly reduced following 1 week on the high (compared to basal) NaCl diet, and no difference between groups was observed following 2 weeks on the diets. The high NaCl diet did not alter alpha 2-adrenoceptors in the medulla, nor did it affect alpha 1- or beta-adrenoceptors in any brain region studied in either SHR or WKY. In SHR, dietary Ca2+ supplementation diminished the blood pressure elevation associated with high NaCl diets, and simultaneously prevented the NaCl induced increase in alpha 2-adrenoceptors in the anterior hypothalamic region. These data support the hypothesis that, in susceptible individuals, changes in noradrenergic transmission in the anterior hypothalamic region contribute to the cardiovascular effects of dietary NaCl loading and Ca2+ supplementation.

Renal cortical and papillary blood flow in spontaneously hypertensive rats

The present study examined whether an alteration in renal medullary hemodynamics is associated with the development of hypertension in the spontaneously hypertensive rat (SHR). The relationships between whole kidney, cortical and papillary blood flows, and renal perfusion pressure were compared in 3- to 5-, 6- to 9-, and 12- to 16-week-old SHR and Wistar-Kyoto rats (WKY). Cortical and papillary blood flows were similar in the different age groups of SHR and WKY over most of the range of perfusion pressure studied. Control papillary blood flows, determined at a renal perfusion pressure equal to the mean arterial pressure of each animal, were not significantly different in the 3- to 5- and 12- to 16-week-old SHR in comparison to values observed in age-matched WKY. In contrast, the control papillary blood flow was 30% lower in 6- to 9-week-old SHR in comparison to the value observed in WKY. Papillary blood flows were significantly less in all age groups of SHR than the corresponding flows measured in WKY when they were compared at equivalent renal perfusion pressures. These findings indicate that medullary vascular resistance is elevated even in very young SHR and suggest that alterations in vasa recta hemodynamics may participate in the development of hypertension by shifting the pressure-natriuresis relationship toward higher pressures.

High salt diet decreases longevity in the spontaneously hypertensive rat

1. Blood pressure, body weight and mortality rate were examined in female spontaneously hypertensive rats (SHR) of the Okamoto strain maintained on a lifelong high (5% NaCl w/w) or low (0.1% NaCl w/w) salt diet. 2. A high salt diet increased mortality and decreased body weight in SHR, but failed to significantly alter blood pressure up to 24 months of age. 3. Hence, a low salt diet does not appear to be a long-term risk factor and may protect the SHR from cardiovascular accidents.

The neural basis of salt sensitivity in the rat: altered hypothalamic function

Dietary NaCl supplementation in NaCl-sensitive spontaneously hypertensive rats (SHR-S) elevates blood pressure, increases peripheral sympathetic nervous system activity and depresses endogenous noradrenaline stores and noradrenaline release in the anterior hypothalamus. NaCl-resistant spontaneously hypertensive rats (SHR-R) and normotensive Wistar Kyoto (WKY) rats are resistant to the NaCl-induced alterations in blood pressure and central and peripheral noradrenergic activity, suggesting that the alterations observed in the SHR-S during NaCl loading are genetically mediated. The anterior hypothalamus is a major cardiovascular regulatory region, and depressor responses elicited by pharmacologic (alpha 2 adrenoceptor) stimulation of this area are exaggerated in SHR-S fed a high NaCl diet compared with SHR-S fed a basal diet and compared with SHR-R and WKY fed a high or basal NaCl diet. Membrane-binding techniques confirm that alpha 2 adrenoceptors in the anterior hypothalamic area are increased in number in SHR-S fed a high NaCl diet, presumably reflecting upregulation in response to reduced local noradrenaline release. These findings are consistent with the hypothesis that decreased noradrenergic activity of sympathoinhibitory neurons in the anterior hypothalamic area may mediate the exacerbation in hypertension that occurs in SHR-S during dietary NaCl supplementation.

High NaCl diet reduces hypothalamic norepinephrine turnover in hypertensive rats

The current study tested the hypothesis that high NaCl diets elevate blood pressure in NaCl-sensitive spontaneously hypertensive rats (SHR-S) by reducing noradrenergic input to depressor neurons in the anterior hypothalamus. SHR-S were studied at 7 weeks of age, and age-matched salt resistant SHR (SHR-R) and normotensive Wistar-Kyoto rats (WKY) were controls. Rats were fed either high (8%) NaCl or control (1% NaCl) diets for 2 weeks, following which norepinephrine turnover in hypothalamus (anterior, posterior, and ventral regions), brainstem (pons and medulla), and thoracic spinal cord was assessed using the dopamine beta-hydroxylase inhibitor 1-cyclohexyl-2-mercapto-imidazole (CHMI). Regional brain catecholamines were measured by high performance liquid chromatography with electrochemical detection following intraperitoneal injection of CHMI or vehicle. Disappearance of norepinephrine following CHMI was used as an index of noradrenergic neuronal activity. The 8% NaCl diet caused a significant elevation in blood pressure in SHR-S but not in SHR-R or WKY. Endogenous norepinephrine levels and turnover were lower in the anterior hypothalamus of SHR-S fed 8% NaCl than in those fed 1% NaCl but were not significantly different in other groups. Endogenous norepinephrine levels and turnover were greater in pons of 8% NaCl--fed SHR-S than in those fed 1% NaCl but were not significantly different in other groups. These observations support the hypothesis that reduced noradrenergic input to depressor neurons in the anterior hypothalamus and increased noradrenergic input to neurons in the pons are related to NaCl sensitivity in the SHR-S.

Hypertensinogenic factors and renal alpha-adrenoceptors in young SHR and WKY rats

The effects of high sodium intake (drinking 1% NaCl), DOCA and DOCA + 1%NaCl for 6 weeks on renal alpha 1- and alpha 2-adrenoceptors and on systolic blood pressure (SBP) were examined in young spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY). On normal sodium intake, SHR rats had higher renal alpha 1 (p less than .001) and alpha 2-adrenoceptor densities (p less than .001) and SBP (p less than .001) than WKY rats. Although, WKY rats given either 1% NaCl, DOCA, and DOCA + 1% NaCl developed hypertension after 6 weeks of treatment, only 1% NaCl administration for the same period produced an increase in the alpha 1- and alpha 2-adrenoceptor densities when compared to the control (p less than .01 and p less than .001, respectively). In the SHR rats, to the contrary, ingestion of 1% NaCl and DOCA + 1% NaCl increased the already elevated alpha 2-adrenoceptor density (p less than .001) and SBP even more in this strain after 6 weeks of treatment. Equilibrium dissociation constants (KD), however, were similar for both classes of receptors in experimental and control rats. This study indicates that postweaning exposure of the WKY and SHR rats to a high salt treatment and DOCA can influence the renal alpha-adrenoceptor densities. Although the functional significance of the changes is unclear, it is reasonable to speculate that postweaning exposure to a hypertensinogenic stimuli such as a 1% NaCl and/or DOCA may ultimately interfere with the functional development of the kidney differently in rats genetically predisposed to hypertension (SHR) from normotensive (WKY) rats.

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.

Altered pressure-natriuresis relationship in young spontaneously hypertensive rats

In the present study, the pressure-natriuretic responses of 3- to 5- and 6- to 9-week-old spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY) were characterized to determine whether the relationship between sodium excretion and renal perfusion pressure is altered during the development of hypertension. Differences in the neural tone and renal hormone levels in SHR and WKY were minimized by denervating the kidney and fixing plasma concentrations of aldosterone, vasopressin, norepinephrine, and cortisol by intravenous infusion. Renal perfusion pressure was varied using adjustable occluders on the lower aorta. The slopes of the relationships between sodium excretion and renal perfusion pressure were not significantly different in 3- to 5-week-old SHR and WKY (0.31 +/- 0.05 vs 0.42 +/- 0.06 microEq/min/g kidney weight); however, the x-intercept of this relationship was significantly shifted to the right by 15 mm Hg in SHR compared to WKY. Blood pressure was moderately elevated even in the 3- to 5-week-old SHR in comparison to WKY (98 +/- 5 vs 81 +/- 6 mm Hg). As the degree of hypertension became more severe, the slope of the pressure-natriuresis relationship became significantly lower in 6- to 9-week-old SHR compared to the corresponding slope observed in age-matched WKY (0.16 +/- 0.02 vs 0.31 +/- 0.04 microEq/min/g kidney weight). These results indicate that the relationship between sodium excretion and renal perfusion pressure is altered even in very young SHR. Thus, the resetting of kidney function occurs very early and may be necessary for the development of hypertension in SHR.

Double-blind, placebo-controlled trial of potassium chloride in the treatment of mild hypertension

Epidemiological and experimental data suggest blood pressure-lowering effects of dietary potassium. A randomized, double-blind clinical trial was used to assess blood pressure response to orally administered potassium, 120 mEq/day, and to placebo in 101 adults with mild hypertension. Blood pressure was measured with a random-zero sphygmomanometer every 2 weeks of this 8-week trial. Systolic blood pressure in the potassium-treated group decreased by 6.4 +/- 13.7 (SD) mm Hg (p less than or equal to 0.025) compared with 0.11 +/- 13.0 mm Hg in the placebo-treated group (p = 0.96). Diastolic blood pressure in the potassium-treated group decreased by 4.1 +/- 8.3 mm Hg (p less than or equal to 0.05) compared with a 1.6 +/- 6.5 mm Hg decrease in placebo-treated subjects (p = 0.09). Baseline blood pressure of potassium-treated subjects was unexpectedly higher than that of controls. After correcting for baseline variation, blood pressure still decreased 3.4/1.8 mm Hg more in potassium recipients than in placebo recipients (p = 0.14 and 0.24, respectively). Blood pressure decreased by 19/13 mm Hg in five blacks taking potassium versus a 1/0 mm Hg increase in seven blacks taking placebo. Compliance with the potassium regimen was 91.5% by pill count; only one subject discontinued treatment because of side effects. In conclusion, 120 mEq/day of microencapsulated potassium chloride was well tolerated in adults with mild hypertension. An antihypertensive effect of potassium cannot be ruled out despite the fact that there was no statistically significant difference between potassium-treated and placebo-treated subjects after adjustment for differences in baseline blood pressure.(ABSTRACT TRUNCATED AT 250 WORDS)