Effect of chronic dietary treatment with L-tryptophan on the maintenance of hypertension in spontaneously hypertensive rats

Chronic dietary administration of L-tryptophan at 2.5 and 5.0% by weight reduced the elevated systolic blood pressure of spontaneously hypertensive (SH) rats. Blood pressure was reduced significantly by 3 weeks after initiation of treatment and continued to fall during the course of the 15 weeks of treatment. Body weights of the treated rats were not affected significantly by treatment, nor were daily food and fluid intakes and urine outputs. SH rats, treated with the higher dose of tryptophan, also significantly reduced their urinary outputs of epinephrine and norepinephrine compared with SH controls, while both doses of tryptophan increased urinary outputs of dopamine significantly above that of SH controls. Treatment with tryptophan increased significantly the specific binding of [125I]angiotensin II (Ang II) to membranes from the diencephalon in a dose-dependent manner. Measurement of catecholamine concentration of the supernatant from homogenates used for the Ang II binding assay revealed a significant correlation between the specific binding of Ang II to brain membranes of the two tryptophan-treated groups and the concentration of norepinephrine in the supernatant. There was also a significant correlation between the specific binding of Ang II and the concentration of dopamine in the supernatant of the control group and the group treated with the higher dose of tryptophan. These results show that chronic dietary administration of tryptophan can reduce the elevated blood pressure of SH rats and support the possibility that this neutral amino acid may act via its effect on the concentration of the neurohormones, norepinephrine and dopamine, in the diencephalon to regulate the binding of Ang II to its receptors.     

Sexual dimorphism of blood pressure in spontaneously hypertensive rats is androgen dependent

Intact male and female spontaneously hypertensive rats showed a progressive increase in blood pressure with growth; male attained systolic blood pressure levels of 244 +/- 6 mmHg, and females 205 +/- 3 mmHg at age 22 weeks. Orchidectomy at age 4 weeks significantly attenuated the systolic blood pressure elevation in the male (195 +/- 4 mmHg at age 22 weeks), but ovariectomy at age 4 weeks had no effect on the development of hypertension in the female. The pattern of development of hypertension in orchidectomized males was the same as that in intact and ovariectomized females. Administration of testosterone propionate to gonadectomized rats of both sexes conferred a male pattern of blood pressure development. These results indicate that the sexually dimorphic pattern of hypertension in the spontaneously hypertensive rat is androgen dependent, rather than estrogen dependent. Plasma norepinephrine levels did not differ between the sexes, nor were they altered by gonadectomy or testosterone replacement, suggesting that the higher blood pressures in the intact male and androgen treated male and female SHR are not dependent on increased sympathetic outflow in the established phase of hypertension. Stores of norepinephrine in the posterior hypothalamic region were significantly greater in intact male rats and testosterone treated rats of both sexes than in intact or ovariectomized females, and were higher in the pons of intact female rats than in all other groups. These alterations in central catecholamine stores were not correlated with blood pressure. Further study is needed to assess the functional significance of these androgen mediated alterations in posterior hypothalamic neurons as a determinant of the androgen mediated sexual dimorphism of blood pressure in the spontaneously hypertensive rat.     

Dietary sodium restriction and blood pressure response to sympathetic blockade in young versus adolescent spontaneously hypertensive rats

The effects of dietary sodium restriction on the maintenance of blood pressure (BP) by sympathetic tone were evaluated in young versus more mature spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY). Sympathetic activity was assessed by BP responses to alpha 1-receptor blockade (prazosin), central inhibition of sympathetic outflow (clonidine), and by ganglionic blockade (hexamethonium). On regular sodium intake, SHR showed elevated BP and increased BP responses to sympathetic blockade at both 10 and 16 weeks of age. Sodium restriction to 9 or 17 mumols Na+/g food prevented or blunted development of hypertension in SHR when started at 4 weeks of age but did not affect BP when started at 10 weeks of age. Sodium restriction initiated in young SHR also prevented development of increased BP responses to sympathetic blockade. However, sodium restriction in more mature SHR did not decrease the increased BP responses to sympathetic blockade. We conclude that prevention of development of sympathetic hyperactivity in young SHR represents a major mechanism in the antihypertensive effect of sodium restriction in young SHR.     

Circadian rhythm of plasma sodium is disrupted in spontaneously hypertensive rats fed a high-NaCl diet

High-NaCl diets elevate arterial pressure in NaCl-sensitive individuals, and increases in plasma sodium may trigger this effect. The present study tests the hypotheses that 1) plasma sodium displays a circadian rhythm in rats, 2) the plasma sodium rhythm is disturbed in spontaneously hypertensive rats (SHR), and 3) excess dietary NaCl elevates plasma sodium concentration in SHR. The results demonstrate that plasma sodium has a circadian rhythm that is inversely related to the circadian rhythm of arterial pressure. Although the plasma sodium rhythms of SHR and control rats are nearly identical, the plasma sodium concentrations are significantly higher in SHR throughout the 24-h cycle. Maintenance on a high-NaCl diet increases plasma sodium concentration similarly in both SHR and control rats, but it blunts the plasma sodium rhythm only in SHR. These results demonstrate that in rats, plasma sodium has a circadian rhythm and that high-NaCl diets increase plasma sodium concentration.     

Insulin-exacerbated hypertension in captopril-treated spontaneously hypertensive rats: role of sympathoexcitation

Insulin excess exacerbates hypertension in spontaneously hypertensive rats (SHR). This study examined the relative contribution of the renin-angiotensin system and the sympathetic nervous system in this phenomenon. In SHR, daily subcutaneous injections of insulin were initiated either before short-term angiotensin-converting enzyme inhibition with captopril or after lifetime captopril treatment. Insulin treatment resulted in significant increases in mean arterial pressure and heart rate and captopril treatment lowered arterial pressure, but captopril did not lower arterial pressure more in the insulin-treated compared with control rats. To test the contribution of the sympathetic nervous system to this form of hypertension, each rat was intravenously infused with either a ganglionic blocker (i.e., hexamethonium) or a centrally acting alpha2-adrenergic receptor agonist (i.e., clonidine). Administration of either agent largely eliminated the differences in mean arterial pressure and heart rate between the insulin-treated and saline-treated SHR, irrespective of captopril treatment. These data indicate that in SHR, the ability of insulin to increase blood pressure is closely related to sympathoexcitation, which is unresponsive to blockade of angiotensin-converting enzyme.     

Dietary vitamin B6 supplementation attenuates hypertension in spontaneously hypertensive rats

In spontaneously hypertensive rats (SHRs) excess endogenous aldehydes bind sulfhydryl groups of membrane proteins, altering membrane Ca²⁺ channels, increasing cytosolic free calcium and blood pressure. N-acetyl cysteine normalizes elevated blood pressure in SHRs by binding excess endogenous aldehydes. It is known that dietary vitamin B6 supplementation can increase the level of endogenous cysteine. Our objective was to investigate whether a dietary supplementation of vitamin B6 can prevent hypertension and associated changes in SHRs. Starting at 7 weeks of age, animals were divided into three groups of six animals each. Animals in WKY-control group and SHR-control group were given a normal vitamin B6 diet; and SHR-vitamin B6 group, a high vitamin B6 diet (20 times the recommended dietary intake; RDA) for the next 14 weeks. After 14 weeks, systolic blood pressure, platelet [Ca²⁺]_i and liver, kidney and aortic aldehyde conjugates were significantly higher in SHR controls compared to WKY controls. These animals also showed smooth muscle cell hyperplasia in the small arteries and arterioles of the kidneys. Dietary vitamin B6 supplementation attenuated the increase in systolic blood pressure, tissue aldehyde conjugates and associated changes. These results further support the hypothesis that aldehydes are involved in increased systolic blood pressure in SHRs and suggest that vitamin B6 supplementation may be an effective antihypertensive.     

Anesthetic requirement of isoflurane is reduced in spontaneously hypertensive and Wistar-Kyoto rats

The isoflurane requirement to keep 50% of rats (Rattus norvegicus) unresponsive to noxious stimuli (MAC) was determined in age matched Sprague-Dawley (SD, n = 8), Spontaneously Hypertensive (SHR, n = 8) and Wistar-Kyoto (WKY, n = 8) strains. Following induction and orotracheal intubation, each rat received isoflurane (1.65% end-tidal) for 120 minutes. Physiologic parameters were similar except for expected differences in mean arterial pressure (148 +/- 13mmHg-SHR group, 101 +/- 10mmHg-SD group and 94 +/- 12mmHg-WKY group [mean +/- standard deviation]). Anesthetic equilibration was verified by infrared analysis of end-tidal gases. MAC was then determined in each rat by the tail clamp method and a group MAC calculated.     

Differential protein expression in hypertrophic heart with and without hypertension in spontaneously hypertensive rats

Although cardiac hypertrophy in hypertension has been well recognized, the molecular mechanisms for the development of hypertrophy are still largely unknown. In this study, the protein expression profiles of left ventricular myocardia in spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats at different ages were analyzed using 2-DE in combination with MALDI-TOF/TOF MS/MS. The results showed that 20 proteins were modulated in the hypertrophic myocardium. Out of these modulated proteins, 13 proteins presented significant changes in SHR at an early stage prior to the development of sustained hypertension, while the changes of the other 7 protein expressions occurred only at a late stage in SHR when the blood pressure was significantly elevated, and were largely reversible by treatment with rennin-angiotensin-aldosterone system inhibitors losartan or enalapril. These data demonstrate that the changes in energy metabolism in the hypertrophied heart favor an increase in glycolysis and a decrease in oxidation of fatty acid and glucose, which occur at an early stage in SHR without hypertension. Our results also provide evidence to support the hypothesis that oxidative stress plays an important role in the development of hypertensive cardiac hypertrophy.     

In spontaneously hypertensive rats alterations in aortic wall properties precede development of hypertension

In hypertension arterial wall properties do not necessarily depend on increased blood pressure alone. The present study investigates the relationship between the development of hypertension and thoracic aortic wall properties in 1.5-, 3-, and 6-mo-old spontaneously hypertensive rats (SHR); Wistar-Kyoto rats (WKY) served as controls. During ketamine-xylazine anesthesia, compliance and distensibility were assessed by means of a noninvasive ultrasound technique combined with invasive blood pressure measurements. Morphometric measurements provided in vivo media cross-sectional area and thickness, allowing the calculation of the incremental elastic modulus. Extracellular matrix protein contents were determined as well. Blood pressure was not significantly different in 1.5-mo-old SHR and WKY, but compliance and distensibility were significantly lower in SHR. Incremental elastic modulus was not significantly different between SHR and WKY at this age. Media thickness and media cross-sectional area were significantly larger in SHR than in WKY, but there was no consistent difference in collagen density and content between the strains. Blood pressure was significantly higher in 3- and 6-mo-old SHR than in WKY, and compliance was significantly lower in SHR. The findings in this study show that in SHR, in which hypertension develops over weeks, alterations in functional aortic wall properties precede the development of hypertension. The decrease in compliance and distensibility at a young age most likely results from media hypertrophy rather than a change in intrinsic elastic properties.     

Vascular changes in cyclosporine A-induced hypertension and nephrotoxicity in spontaneously hypertensive rats on high-sodium diet.

Functional and morphological changes of blood vessels in cyclosporine A (CsA)-induced hypertension and nephrotoxicity were studied in spontaneously hypertensive rats (SHR). The role of the L-arginine-nitric oxide (NO) pathway and the importance of oxidative stress in CsA toxicity were also assessed. SHR (7-8 week old) on a high-sodium diet were treated with CsA (5 mg kg(-1) d(-1) s.c.) for 6 weeks. A proportion of the rats were treated concomitantly with the NO precursor L-arginine (1.7 g kg(-1)d(-1) p.o.). CsA elevated blood pressure and caused renal dysfunction and morphological nephrotoxicity. CsA also impaired mesenteric and renal arterial function and caused structural damage to intrarenal and extrarenal small arteries and arterioles. Medial atrophy of the mesenteric resistance vessels and decreased viability of smooth muscle cells of the thoracic aorta were observed. Renal and arterial damage was associated with the presence of inflammatory cells. CsA did not affect markers of the L-arginine-NO pathway (urinary cyclic GMP excretion or endothelial or inducible NO synthase expression in kidney, aorta or heart) or oxidative stress (urinary excretion of 8-isoprostaglandin F2alpha, plasma urate concentration or total radical trapping capacity). Concomitant L-arginine treatment did not affect CsA-induced changes in blood pressure or histological findings but tended to alleviate the arterial dysfunction. The renal and cardiovascular toxicity of CsA was associated with arterial dysfunction and morphological changes in small arteries and arterioles in SHR on a high-sodium diet. The findings did not support the role of oxidative stress or a defect in the L-arginine-NO pathway.     

Angiotensin-(1-7) prevents development of severe hypertension and end-organ damage in spontaneously hypertensive rats treated with L-NAME

We examined the influence of chronic treatment with ANG-(1-7) on development of hypertension and end-organ damage in spontaneously hypertensive rats (SHR) chronically treated with the nitric oxide synthesis inhibitor L-NAME (SHR-L-NAME). L-NAME administered orally (80 mg/l) for 4 wk significantly elevated mean arterial pressure (MAP) compared with SHR controls drinking regular water (269 +/- 10 vs. 196 +/- 6 mmHg). ANG-(1-7) (24 microg x kg(-1) x h(-1)) or captopril (300 mg/l) significantly attenuated the elevation in MAP due to L-NAME (213 +/- 7 and 228 +/- 8 mmHg, respectively), and ANG-(1-7) + captopril completely reversed the L-NAME-dependent increase in MAP (193 +/- 5 mmHg). L-NAME-induced increases in urinary protein were significantly lower in ANG-(1-7)-treated animals (226 +/- 6 vs. 145 +/- 12 mg/day). Captopril was more effective (96 +/- 12 mg/day), and there was no additional effect of captopril + ANG-(1-7) (87 +/- 5 mg/day). The abnormal vascular responsiveness to endothelin-1, carbachol, and sodium nitroprusside in perfused mesenteric vascular bed of SHR-L-NAME was improved by ANG-(1-7) or captopril, with no additive effect of ANG-(1-7) + captopril. In isolated perfused hearts, recovery of left ventricular function from 40 min of global ischemia was significantly better in ANG-(1-7)- or captopril-treated SHR-L-NAME, with additive effects of combined treatment. The beneficial effects of ANG-(1-7) on MAP and cardiac function were inhibited when indomethacin was administered with ANG-(1-7), but indomethacin did not reverse the protective effects on proteinuria or vascular reactivity. The protective effects of the ANG-(1-7) analog AVE-0991 were qualitatively comparable to those of ANG-(1-7) but were not improved over those of captopril alone. Thus, during reduced nitric oxide availability, ANG-(1-7) attenuates development of severe hypertension and end-organ damage; prostaglandins participate in the MAP-lowering and cardioprotective effects of ANG-(1-7); and additive effects of captopril + ANG-(1-7) on MAP, but not proteinuria or endothelial function, suggest common, as well as different, mechanisms of action for the two treatments. Together, the results provide further evidence of a role for ANG-(1-7) in protective effects of angiotensin-converting enzyme inhibition and suggest dissociation of factors influencing MAP and those influencing end-organ damage.     

Evidence that renal arginine transport is impaired in spontaneously hypertensive rats

Low renal nitric oxide (NO) bioavailability contributes to the development and maintenance of chronic hypertension. We investigated whether impaired l-arginine transport contributes to low renal NO bioavailability in hypertension. Responses of renal medullary perfusion and NO concentration to renal arterial infusions of the l-arginine transport inhibitor l-lysine (10 μmol·kg(-1)·min(-1); 30 min) and subsequent superimposition of l-arginine (100 μmol·kg(-1)·min(-1); 30 min), the NO synthase inhibitor N(G)-nitro-l-arginine (2.4 mg/kg; iv bolus), and the NO donor sodium nitroprusside (0.24 μg·kg(-1)·min(-1)) were examined in Sprague-Dawley rats (SD) and spontaneously hypertensive rats (SHR). Renal medullary perfusion and NO concentration were measured by laser-Doppler flowmetry and polarographically, respectively, 5.5 mm below the kidney surface. Renal medullary NO concentration was less in SHR (53 ± 3 nM) compared with SD rats (108 ± 12 nM; P = 0.004). l-Lysine tended to reduce medullary perfusion (-15 ± 7%; P = 0.07) and reduced medullary NO concentration (-9 ± 3%; P = 0.03) while subsequent superimposition of l-arginine reversed these effects of l-lysine in SD rats. In SHR, l-lysine and subsequent superimposition of l-arginine did not significantly alter medullary perfusion or NO concentration. Collectively, these data suggest that renal l-arginine transport is impaired in SHR. Renal l-[(3)H]arginine transport was less in SHR compared with SD rats (P = 0.01). Accordingly, we conclude that impaired arginine transport contributes to low renal NO bioavailability observed in the SHR kidney.     

Plasma concentrations of endothelin-1 in spontaneously hypertensive rats and DOCA-salt hypertensive rats

To investigate the possible involvement of endothelin-1 (ET-1), an endothelium-derived potent vasoconstrictor peptide, in the pathophysiology of hypertension, plasma ET-1 levels in 15-week-old spontaneously hypertensive rats (SHR) and DOCA-salt hypertensive rats were measured with a sandwich-type enzyme immunoassay. The vasocontractile effect of ET-1 in aortic helical preparations was significantly more sensitive in DOCA-salt hypertensive rats than in control sham-operated rats, but plasma levels of ET-1 did not differ between them. Plasma ET-1 levels in genetically hypertensive rats (SHR and stroke-prone SHR) were significantly lower than those in age-matched normotensive Wistar-Kyoto (WKY) rats. The plasma concentrations of big ET-1, a precursor of ET-1, in both SHR and SHR-SP were significantly lower than those of WKY, suggesting that the production of ET-1 is decreased in rats of genetic hypertension. Although the vascular reactivity to ET-1 increased in both DOCA-salt hypertensive and genetically hypertensive rats, present findings of the plasma ET-1 levels suggest that the role of ET-1 in the vascular control system may be different in DOCA-salt hypertensive rats and genetically hypertensive rats.     

Antihypertensive effects of onion on NO synthase inhibitor-induced hypertensive rats and spontaneously hypertensive rats

This study was designed to show the effects of onion on blood pressure in N(G)-nitro-L-arginine methyl ester (L-NAME) induced-hypertensive rats and stroke prone spontaneously hypertensive rats (SHRSP) using dried onion at 5% in their diets. For the experiment with L-NAME induced-hypertensive rats, male 6-weeks-old Sprague-Dawley rats were given tap water containing L-NAME to deliver 50 mg/kg BW/day. In this experiment, we found distinct antihypertensive effects of onion on the L-NAME induced-hypertensive rats and the SHRSP. Dietary onion decreased the thiobarbituric acid reactive substances (TBARS) in plasma in these hypertensive rats. Also, onion increased the nitrate/nitrite (products of nitric oxide (NO)) excreted in urine and the NO synthase (NOS) activity in the kidneys in SHRSP. These results suggested that the increased NO caused by the greater NOS activity, and additionally by the increased saving of NO by the antioxidative activity of onion, was one of the cause of the antihypertensive effect of onion in SHRSP. In the L-NAME induced hypertensive rats, onion did not significantly block the inhibition of NOS activity by L-NAME, and decreased nitrate/nitrite excretion in urine was not restored. The mechanism of the antihypertensive effect of onion probably involves increased saving of NO by antioxidative activity of onion in L-NAME induced-hypertensive rats.     

Delayed development of hypertension and increased aortic relaxation in spontaneously hypertensive rats after neonatal sympathectomy

The effect of neonatal sympathectomy on vasodilator responses to acetylcholine (ACh) and cAMP has been studied in aortic rings of spontaneously hypertensive rats (SHR) and normotensive animals. The relaxation of intact SHR aorta in response to ACh and cAMP was 20-35% lower than that of normotensive rats. Sympathectomy in normotensive rats did not affect the level of blood pressure and aorta reactivity to Ach. In SHR, sympathectomy caused a decrease in blood pressure, while relaxation in response to ACh and cAMP increased, as compared to intact SHR, but remained lower than in normotensive rats. The data obtained suggest that the decrease in arterial pressure of sympathectomized SHR is a result not only of the reduction in sympathetic effects but also of the increase in smooth muscle relaxation.     

The effects of hydrogenated coconut oil, safflower oil, and evening primrose oil on development of hypertension and sodium handling in spontaneously hypertensive rats

Spontaneously hypertensive rats (SHR) were fed a basal regular diet (BD) or three different fat-supplemented diets which contained 10% hydrogenated coconut oil (HCO), 10% safflower oil (SFO), or 10% evening primrose oil (EPO). The rats received these four different diets from 4 weeks to over 24 weeks of age. The development of hypertension in SHR was significantly retarded in the EPO-supplemented animals. The blood pressure was lower in the SFO group animals as compared with the BD and HCO groups, but this did not reach significance. Sodium excretion rate in young SHR was increased in the EPO group compared with the HCO and SFO groups, and the urinary K/Na ratio was decreased in the EPO group compared with the HCO and EPO groups. Water intake and urine volume were increased in the SFO group as compared with the HCO and EPO groups. Sodium concentration in erythrocytes was decreased in the rats receiving SFO. Pressor responses to norepinephrine and angiotensin II were enhanced in the EPO and SFO groups as compared with the basal chow group. These data suggest that a dietary supplementation of EPO which contains a substantial amount of gamma-linolenic acid consistently lowers blood pressure in SHR. The mechanism is uncertain, but the effects on sodium handling may in part be responsible for the retardation of the development of hypertension. There was a difference between the EPO and the SFO groups in sodium--water handling, and to some extent in the blood pressure development in SHR.