Increased kidney xanthine oxidoreductase activity in salt-induced experimental hypertension

Clinical and experimental studies have established an association between high sodium intake and arterial hypertension. The renal mechanisms resulting in impaired sodium excretion in hypertension-prone subjects are not clear. In hypertension-prone rats, high blood pressure results in increased renal mass and hemodynamic changes, both of which may alter renal oxygen distribution. Xanthine oxidoreductase (XOR) oxidizes ATP metabolites hypoxanthine and xanthine to urate. Because XOR is induced by hypoxia, we assessed kidney XOR activity in 2 models of salt-sensitive hypertension, spontaneously hypertensive rats (SHR) and Dahl salt-sensitive (Dahl S) rats. Increasing sodium intake from basal (0.08%) to high (2.56% wt/dry wt in the diet) increased renal XOR activity dose-dependently from 68+/-8 to 143+/-21 microU/mg protein in the Dahl S (P<0.05) but not in Dahl salt-resistant (Dahl R) rats. On basal and high sodium diets, SHR had higher renal XOR activity (101+/-10 and 134+/-26 microU/mg protein, respectively) than normotensive Wistar-Kyoto rats (55+/-2 and 58+/-6 microU/mg protein, P<0.05). Sodium restriction (0.02% wt/wt) downregulated kidney XOR activity in both Dahl S and R rats by nearly 40%. In SHR, allopurinol treatment totally inhibited renal XOR activity, but neither systolic blood pressure nor renal mass changed. The results suggest that renal XOR induction is a consequence of increased salt intake or the resulting hypertension. However, further studies on renal XOR activity during the development of hypertension are needed to assess the importance of XOR in the pathophysiology of arterial hypertension.

Cardiovascular effects of dietary salts and isosorbide-5-mononitrate in spontaneously hypertensive rats

The influence of isosorbide-5-mononitrate (IS-5-MN) on the cardiovascular effects of high dietary salt intake (NaCl, 6.6% of dry weight of food) and that of a potassium, magnesium and l-lysine-enriched salt alternative (Pansalt 10.5%, producing a 6.6% content of NaCl) was studied in spontaneously hypertensive rats in an 8-week experiment. Common salt produced a marked rise in blood pressure and induced cardiac and renal hypertrophy, while the salt alternative, although containing the same amount of NaCl, neither increased blood pressure nor caused any significant cardiac hypertrophy. IS-5-MN treatment at a daily dose of approximately 60-70 mg/kg (mixed with food) attenuated the rise in blood pressure induced by common salt, but did not prevent the cardiac or renal hypertrophy. IS-5-MN did not offer any additional benefit to the use of the salt alternative diet alone in treatment of high blood pressure. Mesenteric arterial responses in vitro were examined at the end of the study. IS-5-MN treatment during the moderately low-salt (NaCl 0.7%) control diet tended to decrease the contractile response to noradrenaline and increase the relaxation to acetylcholine. Common salt, but not the salt alternative, induced a 50% increase in the 24-h urinary excretion of cyclic GMP. Both salt supplements induced an 8-9-fold increase in the excretion of calcium, and about a 2-fold increase in the excretion of phosphorus. Common salt also increased the excretion of magnesium by 50%. IS-5-MN treatment had no significant effect on the excretion of the mineral elements. Our findings show that increased intake of potassium and magnesium reduces the harmful effects of common salt. Pressure-independent mechanisms are involved in salt-induced left ventricular and renal hypertrophy, since they remained unaffected despite the prevention of the salt-induced rise in blood pressure by IS-5-MN treatment.

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.

Cardiovascular effects of chronic inhibition of nitric oxide synthesis and dietary salt in spontaneously hypertensive rats

The cardiovascular effects of chronic inhibition of nitric oxide synthesis and dietary salt were studied in 9-wk-old spontaneously hypertensive rats (SHR). N omega-nitro-L-arginine methyl ester (L-NAME, 0.025% in food, about 20 mg/kg/d) was given to rats receiving diets containing low, moderate, and high salt levels (NaCl 0.2%, 1.1%, and 6.0% of the dry weight of the chow) for 3 wk, L-NAME increased systolic blood pressure by 50 to 60 mmHg in all treated groups, as compared with an average rise of 10 to 20 mmHg in the control SHR. The high-salt diet did not further increase blood pressure. L-NAME also induced cardiac and renal hypertrophy, and these changes were aggravated by the high-salt diet. In addition, 19 of the 30 rats treated with L-NAME suffered strokes and all of them had several myocardial infarctions and renal damage, while the rats not treated with L-NAME had no evidence of stroke or myocardial or renal injury. Responses of mesenteric arterial rings in vitro were studied at the end of the experiment. The vascular contractile responses to noradrenaline were increased, and the relaxation responses to acetylcholine were inhibited in the L-NAME treated groups. In addition, the high-salt diet alone tended to inhibit the response to acetylcholine. Plasma renin activity was markedly increased by L-NAME treatment and decreased by the high-salt diet. The 24-h urine protein excretion was increased both by the L-NAME treatment and by the high-salt diet. The combination of L-NAME and the high-salt diet markedly raised the serum creatinine concentration. Our findings show that the coronary and renal functions are particularly vulnerable in SHR during impaired nitric oxide synthesis, and that the end-organ damage is worsened by an increased intake of dietary salt. We suggest that dysfunction of the endothelium is the primary cause of the effects observed in this study.

Influence of age on cardiovascular effects of increased dietary sodium and angiotensin-converting enzyme inhibition in normotensive Wistar rats

Recent studies have shown that increased intake of dietary sodium chloride produces blood pressure-independent increase in cardiac and renal mass even in young normotensive rats. With advancing age the harmful cardiovascular effects of increased dietary sodium are not so well known. In the present study the influence of advancing age on the cardiovascular effects of increased intake of sodium (control diet, 0.3% and high-sodium diet, 2.6% sodium in the chow) were examined in young and aged (3 and 18 months old, respectively, at the beginning of the experiment) male normotensive Wistar rats in a six-week study. Moreover, the potential role of renin-angiotensin system in ageing during normal and a high-sodium intake was studied using a pharmacological tool, angiotensin converting enzyme (ACE) inhibitor ramipril. Ageing did not significantly modify basal systolic blood pressure measured by the tail cuff method. A high intake of sodium chloride increased blood pressure significantly only in aged rats, while in young rats it increased renal weight. Left ventricular weight was not affected by high-sodium diet in either age group. The ACE inhibition during control diet lowered blood pressure and decreased left ventricular weight in young rats only and these effects were completely blocked by a high-sodium diet. The maximal vascular contraction force of mesenteric arterial rings to noradrenaline was decreased with ageing while endothelium-dependent and -independent relaxation responses were unaltered with ageing. The sensitivity to sodium nitroprusside was impaired by the high-sodium diet in young rats. In both age groups the urinary excretion of calcium was increased during the high-sodium diet. In conclusion, the increased intake of sodium produced different changes in cardiovascular function in normotensive rats depending on age. With advancing age, the sensitivity to sodium-induced increase in blood pressure was increased. In aged rats a high intake of dietary sodium elevated blood pressure, while in young rats it increased renal mass without increase in blood pressure. In both age groups sodium did not affect left ventricular hypertrophy. Both high-sodium intake and ageing attenuated or even abolished the cardiovascular effects of ACF inhibition.

Influence of different dietary salts on the cardiovascular and renal effects of moxonidine in spontaneously hypertensive rats

The influence of common salt (NaCl) and a novel potassium-, magnesium-, and L-lysine-enriched mineral salt on the cardiovascular and renal effects of the selective imidazoline I1-receptor agonist moxonidine was examined in spontaneously hypertensive rats (SHR). Common salt was added at the level of 6% of the dry weight of the chow, and mineral salt at a 75% higher level of 10.5% thereof to produce the same NaCl concentration of 6% as in the common salt group. During the control diet an 8-week oral treatment with moxonidine (117 mg/1000 g of the dry weight of the chow producing an approximate daily dose of 10 mg/kg), lowered blood pressure by 13 mmHg. The common salt diet alone raised blood pressure by 27 mmHg. Moxonidine lowered blood pressure by 21 mmHg during the common salt diet, but the blood pressure remained 19 mmHg higher than in the moxonidine-treated SHR receiving the control diet (P<0.05). Unlike common salt, mineral salt alone did not raise blood pressure nor did it interfere with the antihypertensive effect of moxonidine. Moxonidine showed a kidney-protective effect during the control diet measured as decreased urinary protein excretion, but it did not affect the development of left ventricular hypertrophy. Moxonidine increased plasma renin activity during the control diet and it raised the serum aldosterone level both during the control and mineral salt diets. The vascular relaxation responses of the mesenteric arterial rings to both acetylcholine (an indicator of endothelium-dependent vascular relaxation) and nitroprusside and nitroprusside (an indicator of endothelium-independent vascular relaxation) were attenuated by the common salt diet alone but maintained during the moxonidine treatment. Our findings are consistent with the concept that moxonidine is able to improve the excretion of sodium. This effect might explain the maintenance of normal vascular relaxation during a high intake of common salt. These effects may partly account for the antihypertensive effect of moxonidine.

Influence of age and dietary sodium on the cardiovascular and renal effects of ramipril in spontaneously hypertensive rats

The aim of the present study was to investigate the influence of age and an increased intake of dietary sodium on the cardiovascular and renal effects of the angiotensin converting enzyme inhibitor, ramipril. Male spontaneously hypertensive rats (SHR) aged 10 and 60 weeks received either control or a high level of sodium (0.3% vs. 2.6% Na) and ramipril (2 mg/kg/day) mixed in the chow for 6 weeks. Blood pressure was measured weekly by tail-cuff method. Arterial functions were determined by measuring vascular contractile and relaxation responses of mesenteric arterial rings in vitro at the end of the study. An age-related increase in systolic blood pressure, left ventricular (LVH) and renal hypertrophy (RH) as well as proteinuria were found in SHR. The vascular relaxation to nitroprusside was impaired in aged SHR. The high sodium intake accelerated the development of hypertension only in young SHR but increased LVH and RH in both age groups. Ramipril effectively lowered blood pressure in both age groups, but decreased the LVH significantly only in young rats. Ramipril markedly improved the vascular relaxation to acetylcholine and nitroprusside only in young rats. The vascular contractile responses to noradrenaline and potassium chloride were not affected by age, sodium intake or ramipril treatment. The high sodium intake markedly attenuated the cardiovascular effects of ramipril. The high-sodium diet enhanced the urinary excretion of cyclic GMP in both age groups, while it increased urinary excretion of protein in young SHR only. In conclusion, the cardiovascular effects of ramipril were impaired with advanced age even in the presence of a control intake of sodium. A high sodium intake attenuated or even abolished the cardiovascular effects of ramipril in both young and aged SHR.

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.

Cardiovascular effects of a low-dose combination of ramipril and felodipine in spontaneously hypertensive rats

1. Cardiovascular effects of submaximal antihypertensive doses of the angiotensin converting enzyme inhibitor, ramipril (0.25 mg kg-1 day-1 in the food), and the calcium channel blocker, felodipine (0.4 mg kg-1 day-1 subcutaneously by osmotic minipump), both alone and in combination, were examined in spontaneously hypertensive rats (SHR) in a four-week study. 2. Both ramipril and felodipine as monotherapy decreased systolic blood pressure. The antihypertensive effect of the drug combination was more than that of ramipril treatment alone, but not significantly better than that of felodipine monotherapy. Ramipril or felodipine treatments did not significantly affect the heart rate, either alone or in combination. 3. The beneficial effect of ramipril monotherapy on left ventricular hypertrophy was more prominent than that of felodipine. The cardioprotective effect of felodipine was improved when combined to ramipril. The systolic blood pressure at the end of the experimental period correlated only weakly with left ventricular hypertrophy. 4. Responses of mesenteric arterial rings in vitro were examined at the end of the four-week study. Ramipril and felodipine monotherapies as well as their combination markedly improved the endothelium-dependent vascular relaxation responses to acetylcholine. The combination of ramipril and felodipine slightly enhanced the endothelium-independent vascular relaxation responses to sodium nitroprusside. Ramipril treatment alone slightly diminished the vascular contractile responses to noradrenaline. Neither ramipril nor felodipine alone or in combination affected the vascular contractile responses to potassium chloride. 5. Ramipril treatment, both alone and in combination with felodipine, caused a three fold increase in plasma renin activity. Serum aldosterone, fasting blood glucose level, serum insulin and the 24 hour urinary excretions of sodium, potassium, magnesium, calcium, phosphorus or protein were not significantly affected by the drug treatments. 6. Our findings suggest that a better overall control of hypertension and end-organ damages, without an increase in adverse effects, can be achieved by the combination of submaximal antihypertensive doses of felodipine and ramipril than by monotherapy with either drug alone.

Delayed increase in blood pressure induced by spontaneously hypertensive rat plasma after high sodium intake

Plasma from spontaneously hypertensive rats (SHR) and from patients with essential hypertension has been suggested to contain a substance with a delayed pressor effect, parathyroid hypertensive factor (PHF), associated with salt-sensitive forms of hypertension. In order to study whether high sodium intake would increase plasma levels of PHF-like activity, determined by bioassay, SHR received either a high-sodium (2.6% Na in the chow) or a normal-sodium (0.3% Na) diet for 6 weeks. Intravenous injection of plasma from SHR on a high-sodium diet (6 ml/kg) to urethane-anaesthetized Wistar rats induced a delayed increase in mean arterial blood pressure 70-80 min after bolus injection. No delayed pressor effects could be demonstrated by plasma from SHR or Wistar rats on a normal-sodium diet. It is concluded that a factor with a delayed blood pressure-increasing effect appears to be present in plasma from SHR on a high-sodium diet but not in plasma from normotensive Wistar rats or SHR on a normal-sodium diet. Further studies to characterize this factor and its possible relation to salt-induced hypertension are warranted.

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.

Age-related delay in recovery of beta-adrenergic sensitivity after abrupt propranolol withdrawal in rats

We compared cardiovascular responses to various adrenergic agonists in conscious 3-month and 12-month old rats that had been treated with propranolol daily for 7 days, to determine whether changes in beta-adrenergic hypersensitivity induced by abrupt propranolol withdrawal would differ with age. Depressor and tachycardic responses elicited by beta-adrenergic stimulation with isoproterenol were still reduced during the first 3 days following propranolol withdrawal, but were restored to pretreatment levels, more slowly in 12-month than in 3-month-old rats. Opposite pressor and bradycardic responses to alpha-adrenergic stimulation with phenylephrine did not differ between age groups, either before or after propranolol withdrawal. By contrast, pressor and bradycardic responses produced by combined alpha- and beta-adrenergic stimulation with epinephrine after propranolol withdrawal, though unaltered in 3-month-old rats, were enhanced in 12-month-old rats. Hence after sudden propranolol withdrawal beta-adrenergic sensitivity in conscious rats was gradually restored, rather than being enhanced, but more slowly at 12 than at 3 months of age. These results suggest that following abrupt cessation of prolonged propranolol treatment, restoration of normal beta-adrenergic sensitivity becomes delayed in older rats.

Waning cardiovascular responses to adrenergic drugs in conscious ageing rats

Blood pressure and heart rate responses to various drugs were recorded in three groups of conscious rats at ages 3-, 12- and 26-months to determine whether cardiovascular responsiveness changes selectively with age. Basal mean pressures were higher while heart rates were lower in 26-month-old rats than in others. Phenylephrine, as an alpha-adrenergic agonist, produced significantly smaller pressor and bradycardic responses in 26-month-old than in younger rats. By contrast, pressor and bradycardic responses to angiotensin did not differ between age groups. Depressor responses produced by isoproterenol, as a beta-adrenergic agonist, were unaffected by age, but the accompanying tachycardia was significantly weaker in 26-month-old than in younger rats. On the other hand, combined alpha- and beta-adrenergic stimulation with epinephrine elicited pressor and bradycardic responses that were significantly smaller in 12- and 26-month-old rats than in 3-month-old rats. Thus, our results show that while vascular alpha-adrenergic and myocardial beta-adrenergic responses diminished with age, cardiovascular responses to angiotensin were essentially unaltered. Considered collectively these results suggest that ageing impairs responsiveness to alpha- and beta-adrenergic stimuli selectively without affecting that to other vasoactive drugs probably because as endogenous catecholamines increase with age, receptor occupancy also increases and the respective vascular and myocardial receptors become saturated.

Tail-cuff detection of systolic hypertension in different strains of ageing rats

To identify rat strains suitable for studying age-related development of hypertension we compared pressures measured with the tail-cuff method in different groups of ageing Fischer 344, Wistar, or Sprague-Dawley rats. Preliminary experiments to establish optimal frequency of chronic blood pressure measurement in ageing rats showed that tail-cuff systolic pressures did not differ significantly whether taken weekly or monthly. Repeated tail-cuff measurements were comparable even when a common cuff size was used in different groups of rats with varying tail diameters. Additional studies were then carried out in 1-year old male Wistar and Sprague-Dawley rats to measure tail-cuff pressures monthly during the second year of age. Systolic and mean pressures increased progressively with age in both strains, as did body weight and heart rate, but the incidence of hypertension was higher in Sprague-Dawley than in Wistar rats. Elevations in mean pressures were sometimes more pronounced than those in systolic pressure. Two months after the last tail-cuff measurement, the presence of hypertension in Wistar rats was verified by the elevated mean pressures that were recorded from femoral artery catheters. Our results overall suggest that the predisposition to hypertension was higher in Sprague-Dawley than in Wistar or Fischer 344 rats of the same age, and also in males than in females of the same strain. Of all the different strains and sexes we compared, therefore, male Sprague-Dawley rats from 20 to 24 months of age may be the best model for studying the development of systolic hypertension with age.

Enhanced sympathetic mediation of chronotropic baroreflexes in old Sprague-Dawley rats

We compared reflex heart rate responses elicited during intravenous infusions of phenylephrine or sodium nitroprusside in conscious 4- and 24-month-old male Sprague-Dawley rats to determine whether baroreflex regulation changes with age. Underlying neural mechanisms were assessed by repeating baroreflex tests following cholinergic blockade with methylatropine or beta-adrenergic blockade with propranolol. Basal blood pressures always tended to be higher, while corresponding heart rates were lower, in old than in young rats. Reflex bradycardia (but not tachycardia) was initially weaker in 24-month-old rats as were reductions in both reflex bradycardia and tachycardia after cholinergic blockade. On the other hand, the reduction in reflex tachycardia following beta-adrenergic blockade in old rats was more pronounced and almost equal to that produced by combined cholinergic and beta-adrenergic blockade. From these results we conclude that with old age in male Sprague-Dawley rats, just as has been shown previously in Fischer 344 rats, predominant efferent pathways for regulating heart rate reflexes are also altered to become almost exclusively beta-adrenergic or sympathetic.