High calcium diet augments vascular potassium relaxation in hypertensive rats

Effects of oxonic acid-induced hyperuricemia on mesenteric artery tone and cardiac load in experimental renal insufficiency

Background

Recent studies suggest a causal role for increased plasma uric acid in the progression of chronic renal insufficiency (CRI). However, uric acid also functions as an antioxidant with possible beneficial effects.

Methods

We investigated the influence of hyperuricemia on mesenteric arterial tone (main and second order branch) and morphology in experimental CRI. Forty-four Sprague–Dawley rats were 5/6 nephrectomized (NX) or Sham-operated and fed 2.0% oxonic acid or control diet for 9 weeks.

Results

Oxonic acid feeding elevated plasma uric acid levels 2.4 and 3.6-fold in the NX and Sham groups, respectively. Plasma creatinine and urea were elevated 2-fold and blood pressure increased by 10 mmHg in NX rats, while hyperuricemia did not significantly influence these variables. Right and left ventricular weight, and atrial and B-type natriuretic peptide mRNA content were increased in NX rats, but were not affected by hyperuricemia. In the mesenteric artery, hyperuricemia did not influence vasoconstrictor responses in vitro to norepinephrine or potassium chloride. The small arteries of NX rats featured hypertrophic remodeling independent of uric acid levels: wall to lumen ratio, wall thickness and cross-sectional area were increased without changes in lumen diameter. In the main branch, vasorelaxations to acetylcholine were impaired in NX rats, but were not affected by hyperuricemia. In contrast, relaxations to the large-conductance Ca²⁺-activated K⁺-channel (BK_Ca) opener NS-1619 were reduced by oxonic acid feeding, whereas responses to nitroprusside were not affected.

Conclusions

Experimental hyperuricemia did not influence cardiac load or vascular remodeling, but impaired BK_Ca -mediated vasorelaxation in experimental CRI.

High blood pressure-lowering and vasoprotective effects of milk products in experimental hypertension

Milk casein-derived angiotensin-converting enzyme (ACE)-inhibitory tripeptides isoleucine-proline-proline (Ile-Pro-Pro) and valine-proline-proline (Val-Pro-Pro) have been shown to have antihypertensive effects in human subjects and to attenuate the development of hypertension in experimental models. The aim of the present study was to investigate the effect of a fermented milk product containing Ile-Pro-Pro and Val-Pro-Pro and plant sterols on already established hypertension, endothelial dysfunction and aortic gene expression. Male spontaneously hypertensive rats (SHR) with baseline systolic blood pressure (SBP) of 195 mmHg were given either active milk (tripeptides and plant sterols), milk or water ad libitum for 6 weeks. SBP was measured weekly by the tail-cuff method. The endothelial function of mesenteric arteries was investigated at the end of the study. Aortas were collected for DNA microarray study (Affymetrix Rat Gene 1.0 ST Array). The main finding was that active milk decreased SBP by 16 mmHg compared with water (178 (sem 3) v. 195 (sem 3) mmHg; P < 0·001). Milk also had an antihypertensive effect. Active milk improved mesenteric artery endothelial dysfunction by NO-dependent and endothelium-derived hyperpolarising factor-dependent mechanisms. Treatment with active milk caused mild changes in aortic gene expression; twenty-seven genes were up-regulated and eighty-two down-regulated. Using the criteria for fold change (fc) <  0·833 or > 1·2 and P < 0·05, the most affected (down-regulated) signalling pathways were hedgehog, chemokine and leucocyte transendothelial migration pathways. ACE expression was also slightly decreased (fc 0·86; P = 0·047). In conclusion, long-term treatment with fermented milk enriched with tripeptides and plant sterols decreases SBP, improves endothelial dysfunction and affects signalling pathways related to inflammatory responses in SHR.

Role of Na+/K+-ATPase in the high extracellular calcium-induced impairment of rabbit aorta contractile activity

The present study was designed to prove whether the activation of the sarcolemmal Na+/K+-ATPase in the rabbit aorta could explain the decreased contraction caused in this tissue by high extracellular calcium. To demonstrate this hypothesis, we evaluate the modification in the contractile responses to KCl and alpha1-adrenoceptor agonists (methoxamine and phenylephrine) produced by a high extracellular Ca2+ concentration (10 mM) in isolated rabbit aorta rings when the Na+/K+-ATPase is inhibited with ouabain. Ouabain 10(-4) M caused an initial rapid increase in tone in the rabbit aorta rings, which could be linked to the release of catecholamines provoked when the Na+/K+-ATPase in the nerve terminal was blocked. This glycoside also caused a delayed contractile response in the preparations that could be linked to the inhibition of the Na+/K+-ATPase in the sarcolemma of the smooth muscle. The maximum inhibition of the sarcolemmal pump was fixed 2 h and 15 min after ouabain 10(-4) M administration. Both responses were smaller with the 10-mM Ca2+ concentration than with the 2.5-mM Ca2+ concentration. The contractions elicited by KCl and the alpha1-adrenoceptor agonists were higher in the aorta ring preparations incubated with the 2.5-mM Ca2+ solution than in the aorta ring preparations incubated with the 10-mM Ca2+ solution. When the Ca2+ concentration in the organ bath was 2.5 mM, 10(-4) M ouabain administration caused a decrease in the responses to KCl and alpha1-adrenoceptor agonists. By contrast, when the Ca2+ concentration in the organ bath was 10 mM, 10(-4) M ouabain did not modify these responses. As a consequence, the contractions elicited by KCl were very similar in all the ouabain-treated preparations and those elicited by the alpha1-adrenoceptor agonists in ouabain-treated preparations were even higher when the Ca2+ concentration in the organ bath was 10 mM than when the Ca2+ concentration in the organ bath was 2.5 mM. The results of this study suggest that the increase in extracellular Ca2+ concentration may facilitate the functioning of the Na+/K+-ATPase in the vascular smooth muscle (VSM) and produces opposite effects to ouabain. This effect of high extracellular Ca2+ concentration on the sarcolemmal pump may explain the decrease in the contractile responses elicited by depolarization and alpha1-adrenoceptor stimulation observed in rabbit aorta ring preparations.

Alpha-adrenoceptor-mediated pressor responses in pithed rats fed diets with different calcium contents

Results of many clinical and experimental studies indicate an inverse relationship between dietary calcium and the prevalence of hypertension. Our study was designed to evaluate the alterations in arterial blood pressure and the changes in alpha-adrenoceptor-mediated vascular reactivity in normotensive Sprague-Dawley and spontaneously hypertensive rats (SHR) fed from weaning (3 weeks of life) three diets: normal calcium (Ca 1%), low calcium (Ca 0.1%), and high calcium (Ca 2.5%). The systolic and the diastolic arterial blood pressures were measured weekly by the tail cuff method. The plasma calcium levels in the animals were also measured regularly by colourimetric methods, and the alpha-adrenoceptor-mediated vascular reactivity was evaluated by measuring the pressor responses to alpha-adrenoceptor agonists in pithed rats. These determinations were carried out at the end of the feeding periods (9 weeks of life in Sprague-Dawley rats and 20 weeks of life in SHR) and also at the moments when maximal differences in arterial blood pressure were observed between the conscious animals fed the normal calcium diet and those fed the other two diets. Dietary calcium deficiency increased arterial blood pressure in both strains but calcium supplements were effective to lower this only in hypertensive animals. The plasma calcium levels were altered in both strains when calcium administration was not normal. The low-calcium diet did not modify the pressor responses to either the alpha(1)-adrenoceptor agonist, methoxamine, or the alpha(2)-adrenoceptor agonist, B-HT 920 (5-allyl-2-amino-5,6,7, 8-tetrahydro-4H-thiazolo-(4,5-D)-acepin-dihydrochloride, talixepole), in the normotensive and the hypertensive rats. On the contrary, the high-calcium diet caused a definite decrease in alpha(1)- and alpha(2)-adrenoceptor-mediated vascular reactivity in both strains. The changes in the alpha-adrenoceptor-mediated vasoconstrictor responses were observed in pithed 9-week old Sprague-Dawley rats and in pithed 20-week old SHR, but none were observed in pithed 15-week old SHR, although at this age maximal differences in arterial blood pressure between the animals fed the high- and the normal calcium diet were observed. The results of this study suggest that the mechanisms implicated in the effects of dietary calcium supplements on arterial blood pressure are clearly different from the mechanisms, which bring about changes in arterial blood pressure when the diet is deficient in calcium. The results of this study also show that calcium administration causes variations in alpha-adrenoceptor-mediated vascular reactivity, but this is probably not the only mechanism implicated in the calcium effect on arterial blood pressure.

Renal Na+, K+-ATPase in SHR: studies of activity and gene expression

The mechanism by which increased dietary intake of calcium reduces blood pressure in the spontaneously hypertensive rat is unknown. The present studies were designed to determine if there were alterations in the activity of the major membrane ion translocating pump, sodium, potassium-ATPase (NKA), in the kidneys of hypertensive rats and whether increased dietary calcium intake affected the activity of this enzyme. Fifteen-week old SHR's were found to have lower total ATPase activity in microsomal preparations from the kidney than age matched Wistar-Kyoto animals. Both the ouabain-sensitive component (NKA) and the ouabain-insensitive component were lower in SHR. Increasing dietary calcium intake from 1% to 3% elevated both components of the ATPase activity in SHR, but was without effect in WKY. Measurement of membrane phospholipid composition suggested that altered phospholipid composition did not account for the reduced ATPase activity observed, but indicated a reduced density of ATPase in SHR. A technique has been devised for qualitative and quantitative analysis of Na, K-ATPase alpha isoforms using RT-PCR. This technique reveals that the alpha 1 isoform is the sole catalytic isoform present in the nephron. Accurate and precise quantification of the amount of gene expression in individual nephron segments is reported and will be applied to determine whether dietary calcium influences blood pressure by a mechanism which alters nephron NKA gene expression.

Oral calcium supplementation reduces intraplatelet free calcium concentration and insulin resistance in essential hypertensive patients

We evaluated the effect of oral calcium supplementation on blood pressure, calcium metabolism, and insulin resistance in essential hypertension. After receiving a standard diet with 500 mg of calcium per day during a 4-week period, 20 nondiabetic, essential hypertensive patients were randomized in a double-blind fashion to receive oral calcium supplementation (1500 mg of calcium per day) or placebo for 8 weeks. At the end of the 4-week period of low-calcium diet and after the 8-week period of intervention, we measured blood pressure (by both office and 24-hour ambulatory blood pressure monitoring), calcium-regulating hormones [urinary hydroxyproline and serum osteocalcin, parathormone, and 1,25(OH)2-vitamin D3], intraplatelet free calcium concentration, fasting plasma glucose and insulin levels, and the insulin-sensitivity index (euglycemic-hyperinsulinemic clamp). Compared with patients maintained at low calcium intake, essential hypertensive patients under oral calcium supplementation significantly reduced serum osteocalcin (from 22.2 +/- 1.9 to 17.9 +/- 2.0 micrograms/L; P = .0015), parathormone (from 4.20 +/- 0.38 to 3.30 +/- 0.36 pmol/L; P = .0003), and 1,25(OH)2-vitamin D3 (from 98.0 +/- 11.0 to 61.6 +/- 5.7 pmol/L; P = .0062). Likewise, we found a significant reduction in intraplatelet free calcium concentration (from 35.9 +/- 1.2 to 26.5 +/- 0.8 nmol/L; P = .0005) and fasting plasma insulin levels (from 71.8 +/- 5.9 to 64.6 +/- 6.2 pmol/L; P = .05) and a significant increase in the insulin-sensitivity index (from 2.89 +/- 0.77 to 4.00 +/- 0.95 mg.kg-1.min-1; P = .0007). None of these parameters were significantly modified in patients maintained at low calcium intake. Office and 24-hour mean values of systolic and diastolic blood pressure did not change after 8 weeks of oral calcium supplementation or placebo.

Antihypertensive therapy and arterial function in experimental hypertension

1. Alterations in the function of the endothelium and arterial smooth muscle may be important in the establishment of hypertension. Thus, the possible favorable influences of blood pressure-lowering agents on vascular responsiveness may be important in the chronic antihypertensive actions of these compounds. 2. A number of reports have suggested that ACE inhibitors can improve arterial function in hypertension, whereas the knowledge about the vascular effects of other antihypertensive drugs, like beta-blockers, calcium channel blockers, and diuretics remains rather limited. 3. In this article, the effects of antihypertensive therapy on arterial function in human and experimental hypertension are reviewed.

Dietary calcium and magnesium supplements in spontaneously hypertensive rats and isolated arterial reactivity

1. High calcium diet attenuates the development of hypertension but an associated undesirable effect is that Mg2+ loss to the urine is enhanced. Therefore, we studied the effects of high calcium diet alone and in combination with increased magnesium intake on blood pressure and arterial function. 2. Forty-eight young spontaneously hypertensive rats (SHR) were allocated into four groups, the dietary contents of Ca2+ and Mg2+ being: 1.1%, 0.2% (SHR); 2.5%, 0.2% (Ca-SHR); 2.5%, 0.8% (CaMg-SHR); and 1.1%, 0.8% (Mg-SHR), respectively. Development of hypertension was followed for 13 weeks, whereafter electrolyte balance, lymphocyte intracellular free calcium ([Ca2+]i), and mesenteric arterial responses in vitro were examined. Forty normotensive Wistar-Kyoto (WKY) rats were investigated in a similar manner. 3. Calcium supplementation comparably attenuated the development of Lypertension during normal and high magnesium intake in SHR, with an associated reduced lymphocyte [Ca2+]i and increased Mg2+ loss to the urine. 4. Endothelium-dependent arterial relaxation to acetylcholine was augmented in Ca-SHR and CaMg-SHR, while the relaxations to isoprenaline and the nitric oxide donor SIN-1 were similar in all SHR groups. Relaxation responses induced by the return of K+ to the organ bath upon precontractions in K(+)-free solution were used to evaluate the function of arterial Na+, K(+)-ATPase. The rate of potassium relaxation was similar in Ca-SHR and CaMg-SHR and faster than in untreated SHR. 5. Contractile responses to high concentrations of potassium and noradrenaline, and the ability of vascular smooth muscle to sequester Ca2+, which was evaluated by eliciting responses to caffeine or noradrenaline after loading periods in different Ca2+ concentrations, were comparable in all SHR groups. In SHR with increased magnesium intake, and in WKY rats with calcium or magnesium supplementation, no detectable effects on blood pressure and arterial function were observed.6. In conclusion, high calcium diet attenuated the development of hypertension in SHR, with an associated augmented endothelium-dependent relaxation, promoted recovery rate of ionic gradients across the cell membrane via Na+, K+-ATPase, and reduced basal [Ca2+ ]i. Dietary magnesium supplementation, whether combined with normal or high calcium intake, had no beneficial effects on blood pressure or arterial function.

Calcium supplementation attenuates an enhanced platelet function in salt-loaded mildly hypertensive patients

We designed this study to evaluate the effect of low versus high calcium intake on platelet function in salt-loaded patients with mild hypertension. After a 7-day period of dietary salt restriction, 19 patients were placed on a high salt (300 mmol/d), low calcium (6.25 mmol/d) diet for 7 days; 10 of these patients were given 54 mmol/d of supplementary calcium, and 9 patients were given placebo. At the end of the low and high salt regimens, we evaluated changes in blood pressure, platelet aggregation, and the platelet release reaction measured as plasma beta-thromboglobulin and platelet factor 4 levels. With high salt intake, significant increases in mean blood pressure (P < .02), red blood cell sodium (P < .01), and platelet aggregation induced by 3 mumol/L ADP (P < .01) and by 3.0 mg/L epinephrine (P < .05) were observed in the placebo-treated patients but not in the calcium-supplemented ones. Compared with the placebo-treated patients, calcium-supplemented patients had a smaller weight gain (P < .05) but excreted more sodium and calcium (P < .01) at the end of the high salt regimen. Calcium supplementation resulted in decreases in beta-thromboglobulin (P < .05), platelet factor 4 (P < .01), and plasma and urinary excretions of norepinephrine (P < .02) during the high salt, low calcium regimen. The decrease in plasma norepinephrine correlated positively with the decreases in beta-thromboglobulin (r = .72, P < .02) and platelet factor 4 (r = .85, P < .01).(ABSTRACT TRUNCATED AT 250 WORDS)

Enhancement of arterial relaxation by long-term atenolol treatment in spontaneously hypertensive rats

1. The effects of long-term atenolol (25 mg kg-1 day-1) therapy on arterial function were studied in spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats. The 14-week treatment attenuated the increase in blood pressure by approximately 30 mmHg in SHR, but did not affect blood pressure in WKY rats. 2. Responses of mesenteric arterial rings in vitro were examined at the end of the study. The relaxation to acetylcholine was similar in WKY rats and atenolol-treated SHR and more pronounced than in untreated SHR, whereas the relaxation to the nitric oxide donor 3-morpholinosydnonimine (SIN-1) was comparable in all study groups. Moreover, after maximal relaxations to acetylcholine, marked recontractions developed in untreated SHR but not in the other groups. Vasorelaxation to isoprenaline was also attenuated in SHR and was moderately improved by the atenolol therapy. 3. Arterial relaxation induced by return of potassium to the organ bath upon precontractions elicited by potassium-free solution were used to evaluate vascular smooth muscle Na+, K+-ATPase. The rate of potassium relaxation was fastest in WKY rats and was also faster in atenolol-treated than in untreated SHR. 4. The ability of vascular smooth muscle to sequester calcium was evaluated by eliciting responses to caffeine or noradrenaline after loading periods in different organ bath calcium concentrations. The subsequent contractions were lower in untreated SHR than in WKY rats, and augmented in SHR by the atenolol treatment. 5. Smooth muscle contractions to noradrenaline were comparable in SHR and WKY rats, while atenolol treatment slightly increased the maximal response to this agonist in SHR. Responses to potassium chloride were not affected by atenolol and contractions following cumulative re-addition of calcium to the organ bath after precontraction with potassium chloride and noradrenaline in calcium free solution were comparable in all study groups.6. In conclusion, the moderate antihypertensive effect of atenolol in SHR was accompanied by enhancement of beta-adrenoceptor-mediated and normalization of endothelium-dependent arterial relaxation.Furthermore, ability to sequester calcium into cellular stores, and function of Na+,K+-ATPase were augmented in vascular smooth muscle. Therefore, the present results suggest that the long-term blood pressure-lowering action of atenolol in this type of genetic hypertension is accompanied by improved arterial relaxation and normalization of endothelial function.

Dietary calcium and blood pressure in experimental models of hypertension. A review

More than 80 studies have reported lowered blood pressure after dietary calcium enrichment in experimental models of hypertension. The evidence presented here suggests that dietary calcium may act concurrently through a number of physiological mechanisms to influence blood pressure. The importance of any given mechanism may vary depending on the experimental model under consideration. Supplemental dietary calcium is associated with reduced membrane permeability, increased Ca(2+)-ATPase and Na,K-ATPase, and reduced intracellular calcium. These results suggest that supplemental calcium may limit calcium influx into the cell and improve the ability of the VSMC to extrude calcium. This could be a direct effect of calcium on the VSMC or an indirect effect mediated hormonally. The calcium-regulating hormones have all been found to have vasoactive properties and therefore may influence blood pressure. Furthermore, CGRP and the proposed parathyroid hypertensive factor are both vasoactive substances that are responsive to dietary calcium. Therefore, diet-induced variations in calcium-regulating hormones may influence blood pressure. Modulation of the sympathetic nervous system is another important way that dietary calcium can influence blood pressure. There is evidence of altered norepinephrine levels in the hypothalamus as a consequence of manipulations of dietary calcium as well as changes in central sympathetic nervous system outflow. Dietary calcium has also been shown to specifically modify alpha 1-adrenergic receptor activity in the periphery. In some experimental models of hypertension, dietary calcium may alter blood pressure by changing the metabolism of other electrolytes. For example, the ability of calcium to prevent sodium chloride-induced elevations in blood pressure may be attributed to natriuresis. However, natriuresis does not account for all of the interactive effects of calcium and sodium chloride on blood pressure. Sodium chloride-induced hypertension may be due in part to calcium wasting and subsequent elevation of calcium-regulating hormones. Chloride is an important mediator of this effect because it appears that sodium does not cause calcium wasting when it is not combined with chloride. More attention to the central nervous system effects of dietary calcium is needed. Not only can calcium itself influence neural function, but many of the calcium-regulating hormones appear to affect the central nervous system. The influence of calcium and calcium-regulating hormones on central nervous system activity may have important implications for blood pressure regulation and also may extend to other aspects of physiology and behavior.

Comparison of cumulative and non-cumulative administration of vasoactive agents in arterial smooth muscle responses in vitro

Two methods of determining concentration-response curves were compared in isolated endothelium-intact mesenteric arterial rings from Wistar rats: arterial contractile and relaxation responses were elicited by adding compounds cumulatively or introducing a single concentration at a time (non-cumulative method). The contractile responses induced by high concentrations of K+ (20-125 mM) were comparable between the two methods, whether or not the responses were elicited in the presence of phentolamine (10 microM) and atenolol (10 microM). Noradrenaline (1 nM-10 microM) likewise induced similar contractions regardless of method of administration, the only exception being the highest concentration (100 microM) which produced lower contractile force when added directly upon resting tension than after cumulative administration. This difference was abolished by atenolol (10 microM). Arterial smooth muscle relaxations induced by endothelium-dependent (acetylcholine 1 nM-10 microM) and -independent agents (nitroprusside 1 nM-1 microM, isoprenaline 10 nM-100 microM) were similar whether the relaxants were added in a cumulative fashion or in a single concentration introduced upon each precontraction. Thus, cumulative and non-cumulative administration of contractile and relaxing agents give quite comparable results. We conclude that the cumulative method is a reliable and time-saving way of studying vascular smooth muscle responses in vitro.

Quinapril treatment and arterial smooth muscle responses in spontaneously hypertensive rats

1 The effects of long-term angiotensin-converting enzyme inhibition with quinapril on arterial function were studied in spontaneously hypertensive rats, Wistar-Kyoto rats serving as normotensive controls. 2 Adult hypertensive animals were treated with quinapril (10 mg kg-1 day-1) for 15 weeks, which reduced their blood pressure and the concentrations of atrial natriuretic peptide in plasma and ventricular tissue to a level comparable with that in normotensive rats. 3 Responses of mesenteric arterial rings in vitro were examined at the end of the study. Compared with normotensive and untreated hypertensive rats, responses to noradrenaline were attenuated in hypertensive animals on quinapril, both force of contraction and sensitivity being reduced. Quinapril also attenuated maximal contractions but not sensitivity to potassium chloride. Nifedipine less effectively inhibited vascular contractions in normotensive and quinapril-treated than in untreated hypertensive rats. 4 Arterial relaxation responses by endothelium-dependent (acetylcholine) and endothelium-independent (sodium nitrite, isoprenaline) mechanisms were similar in normotensive and quinapril-treated rats and more pronounced than in untreated hypertensive rats. 5 Cell membrane permeability to ions was evaluated by means of potassium-free solution-induced contractions of endothelium-denuded denervated arterial rings. These responses were comparable in normotensive and quinapril-treated rats and less marked than in untreated hypertensive rats. 6 Intracellular free calcium concentrations in platelets and lymphocytes, measured by the fluorescent indicator quin-2, were similar in normotensive and quinapril-treated rats and lower than in untreated hypertensive rats. 7 In conclusion, quinapril treatment improved relaxation responses and attenuated contractions in arterial smooth muscle of hypertensive rats. These changes may be explained by diminished cytosolic free calcium concentration, reduced cell membrane permeability, and alterations in dihydropyridine-sensitive calcium channels following long-term angiotensin-converting enzyme inhibition.

Effects of high calcium diet on arterial smooth muscle function and electrolyte balance in mineralocorticoid-salt hypertensive rats

1. The effects of a high calcium diet (2.5%) on blood pressure, electrolyte balance, plasma and tissue atrial natriuretic peptide (ANP), cytosolic free Ca2+ concentration ([Ca2+]i), and arterial smooth muscle responses were studied in one-kidney deoxycorticosterone (DOC)-NaCl hypertensive Wistar rats. 2. Calcium supplementation for 8 weeks markedly attenuated the development of DOC-NaCl hypertension and the associated cardiac hypertrophy, and prevented the DOC-NaCl-induced sodium-volume retention as judged by reduced plasma Na+, and decreased plasma and ventricular ANP concentrations in high calcium-fed DOC-NaCl rats. However, calcium supplementation did not affect the DOC-NaCl-induced rise in platelet [Ca2+]i. 3. Smooth muscle contractions of isolated mesenteric arterial rings in response to depolarization by K+ (20-30 mM) were enhanced in DOC-NaCl-treated rats, this enhancement being abolished by concurrent oral calcium loading. The Ca2+ entry blocker nifedipine (10 nM) inhibited the contractions induced by K+ (30-125 mM) more effectively in DOC-NaCl rats than in controls, while the inhibition in calcium-loaded DOC-NaCl rats was significantly greater than in controls only with 30 mM K+. 4. The contractions of mesenteric arterial rings induced by omission of K+ from the organ baths were used to evaluate cell membrane permeability to ions. In chemically denervated rings the onset of the gradual rise in contractile force in K(+)-free medium occurred earlier, and the rate of the contraction was faster in DOC-NaCl-treated rats than in controls and high calcium-fed DOC-NaCl rats. Smooth muscle relaxation induced by 0.5 mM K+ upon K(+)-free contractions was clearly slower in DOC-NaCl rats than in controls and calcium-supplemented DOC-NaCl rats. 5. The functions of arterial smooth muscle Na+, Ca2+ exchange and Ca(2+)-ATPase were evaluated by the aortic contractions elicited by low Na+ medium, and the subsequent relaxation responses induced by Ca(2+)-free solution (in the presence of 5 mM caffeine, 1 microM nifedipine and 10 microM phentolamine). The rate of aortic low Na+ contractions (evaluating Ca2+ influx via Na+, Ca2+ exchange), as well as that of subsequent relaxations was slower in DOC-NaCl-treated rats than in controls, whether the relaxation was induced in normal (144.0 mM) or low (1.2 mM) organ bath Na+ concentration (reflecting Ca2+ extrusion by both Ca(2+)-ATPase and Na+, Ca2+ exchange, and by Ca(2+)-ATPase alone, respectively). However, in calcium-supplemented DOC-NaCl rats the aortic responses did not differ from control. The difference between the relaxation rate in normal and low Na+ concentration in each aortic ring,representing the contribution of Na+, Ca2+ exchange in these relaxations, was comparable in all groups.6. In conclusion, calcium supplementation clearly attenuated the development of hypertension, cardiac hypertrophy, and sodium retention induced by the DOC-NaCI treatment. However, the associated rise in platelet [Ca2+], was not prevented, suggesting that in this form of experimental hypertension increased dietary calcium does not lower blood pressure by reducing [Ca2+]i. The results from vascular responses in vitro suggest that in arterial smooth muscle the DOC-NaCl treatment increased contractile sensitivity to depolarization, voltage-dependent Ca2+ entry and cell membrane permeability to ions, and attenuated relaxation responses and vascular Na+, K+-ATPase function. The results further suggest reduced ability of the cell membrane to transport Ca2+ (possibly via Ca2+-ATPase) in DOC-NaCl hypertension. The high calcium diet opposed these alterations. The present results thus provide evidence that the antihypertensive effect of a high calcium diet in mineralocorticoid-salt hypertension is mediated by its beneficial effects on systemic sodium balance and arterial smooth muscle function.

The effect of high calcium intake on intracellular free [Ca2+] and Na(+)-H+ exchange in DOC-NaCl-hypertensive rats

The effects of calcium supplementation on blood pressure, intracellular free calcium concentration ([Ca2+]i) and rate of Na(+)-H+ exchange were studied in DOC-NaCl-hypertensive rats. All the animals were uninephrectomized and divided into two main groups: the first group received deoxycorticosterone (DOC) (25 mg/kg, s.c.) once a week and had 0.7% NaCl as drinking fluid while the other received equal volumes of saline and tap water to drink. The animals were further divided according to dietary calcium intake: in the Control and DOC groups the chow contained 1.1% calcium, in the Calcium and DOC+Calcium groups, 2.5%. After 6 and 8 weeks, blood pressure in the DOC group was higher than in the Control group; on the other hand, the development of hypertension was attenuated in the DOC+Calcium compared with the DOC group. The Control and Calcium groups did not differ from each other. Platelets and lymphocytes were used as experimental models to study changes in the regulation of [Ca2+]i, evaluated by fluorescent indicators indo-1 and quin-2. In lymphocytes, basal [Ca2+]i was highest in the DOC group, but similar in DOC+Calcium and Control groups. In platelets, both basal and thrombin-stimulated [Ca2+]i were higher in the DOC and DOC+Calcium groups than in the Control group. In both cell types [Ca2+]i was similar in Control and Calcium groups. In addition, platelets were used to study the ability of the cells to recover from intracellular acidification by first blocking the Na(+)-H+ exchange in a Na(+)-free medium and then restarting the exchange mechanism by increasing the extracellular Na+ concentration at constant speed.(ABSTRACT TRUNCATED AT 250 WORDS)

Contractions induced by potassium-free solution and potassium relaxation in vascular smooth muscle of hypertensive and normotensive rats

1. Vascular contractions induced by K(+)-free solution and relaxation responses following the return of K+ to the organ bath were studied in mesenteric arterial rings from spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY) with particular focus on the role of vascular adrenergic nerve-endings and endothelium. 2. In endothelium-denuded rings the omission of K+ from the incubation medium resulted in gradual contractions, the rate of which was slower in SHR than WKY. Nifedipine (1 microM) inhibited the contractions more effectively in SHR than WKY. 3. Adrenergic denervation in vitro with 6-hydroxydopamine reduced the contractions induced by the K(+)-free medium in endothelium-denuded rings. The remaining contractions after denervation were markedly greater in SHR than WKY. 4. The presence of intact vascular endothelium attenuated the K(+)-free contractions in both strains, the attenuation being smaller in SHR than WKY. NG-nitro-L-arginine methyl ester (L-NAME, 0.1 mM) and methylene blue (10 microM), but not indomethacin (10 microM), abolished the attenuating effect of endothelium on the K(+)-free contractions. L-Arginine (1 mM) reversed the effect of L-NAME in WKY but not in SHR. 5. The re-addition of K+ after full K(+)-free contractions dose-dependently relaxed the rings. The rate of this K(+)-induced relaxation was significantly slower in SHR than WKY at all K+ concentrations (0.1-5.9 mM) studied, whether the endothelium or functioning adrenergic nerve-endings were present or not. Ouabain (1 mM) totally inhibited the K+ relaxation in SHR but only partially in WKY.6. Vascular smooth muscle contractions induced by high concentrations of potassium were comparable between the strains. The EC50 for noradrenaline-induced contractions was lower in SHR than WKY, but the maximal forces did not differ significantly.7. In conclusion, the contractile response in K+-free solution more clearly differentiates vascular rings from SHR and WKY than the responses induced by the classical contractile agents noradrenaline and high concentrations of potassium. The depressant effect of the presence of intact endothelium on the K+-free contractions, which was smaller in SHR than WKY, is mediated via the endothelium-derived relaxing factor. Neurotransmitter release from vascular adrenergic nerve-endings participates less in the K+-free contractile response in SHR than WKY. Moreover, the contractile response is more dependent on calcium entry through nifedipine-sensitive calcium channels in SHR than WKY. The greater K+-free contractions of denervated endothelium-denuded rings and the reduced K+ relaxation rate in SHR when compared to WKY suggest increased cell membrane permeability and decreased activity of vascular Na+, K+-ATPase, respectively, in this type of genetic hypertension.