Decreased calcium pump adenosine triphosphatase in red blood cells of hypertensive subjects

Morphological and functional alteration of erythrocyte ghosts and giant unilamellar vesicles caused by Vipera latifi venom

Snake bites are an endemic public health problem in Iran, both in rural and urban area. Viper venom as a hemolytic biochemical "cocktail" of toxins, primarily cause to the systemic alteration of blood cells. In the sixties and seventies, human erythrocytes were extensively studied, but the mechanical and chemical stresses commonly exerted on red blood cells continue to attract interest of scientists for the study of membrane structure and function. Here, we monitor the effect of Vipera latifi venom on human erythrocytes ghost membranes using phase contrast and fluorescent microscopy and changes in ATPase activity under snake venom influence in vitro. The ion pumps [Na⁺,K⁺]-ATPase and (Ca²⁺+Mg²⁺)-ATPase plays a pivotal role in the active transport of certain cations and maintenance of intracellular electrolyte homeostasis. We also describe the interaction of Vipera latifi (VL) venom with giant unilamellar vesicles (GUVs) composed of the native phospholipid mixtures visualized by the membrane fluorescence probe, ANS, used to assess the state of membrane and specifically mark the phospholipid domains.

Calcium and Magnesium ATPase Activities in Women with Varying BMIs

OBJECTIVE

Intracellular calcium (Ca) is increased in obese humans, and magnesium (Mg)-ATPase activity is increased in monosodium glutamate-induced obese rats. The aims of this study were to test the hypotheses that Ca-ATPase activity is negatively correlated with BMI, and that Mg-ATPase activity is positively correlated with BMI and Ca-ATPase activity in obese women.

RESEARCH METHODS AND PROCEDURES

Thirty healthy adult women, with BMIs of 20 to 40, donated a single sample of whole blood and were interviewed as to medical history and family history of obesity. Erythrocyte membranes were isolated and assayed for Ca-ATPase and Mg-ATPase. Weight and height were self-reported. Regression analysis was used to determine relationship between BMI and enzyme activity. Family history of obesity served as a covariant.

RESULTS

Ca-ATPase was negatively correlated with increasing BMI (r = - 0.38, p = 0.02). The relationship between BMI and Ca-ATPase remained valid after controlling for family history of obesity (r = -0.36, p = 0.03). There was a positive correlation between Mg-ATPase activity and Ca-ATPase (r = 0.42, p = 0.024), and this relationship remained valid after controlling for BMI and family history of obesity (r = 0.41, p = 0.03).

DISCUSSION

Ca-ATPase activity decreases as BMI increases. Decreased ATPase activity may contribute to increased intracellular calcium, previously reported in obese persons. Further studies are needed to determine whether a drop in Ca-ATPase activity can serve as a marker for the development of obesity.

Plasma membrane calcium pumps in smooth muscle: from fictional molecules to novel inhibitors

Plasma membrane Ca2+pumps (PMCA pumps) are Ca2+-Mg2+ATPases that expel Ca2+from the cytosol to extracellular space and are pivotal to cell survival and function. PMCA pumps are encoded by the genes PMCA1, -2, -3, and -4. Alternative splicing results in a large number of isoforms that differ in their kinetics and activation by calmodulin and protein kinases A and C. Expression by 4 genes and a multifactorial regulation provide redundancy to allow for animal survival despite genetic defects. Heterozygous mice with ablation of any of the PMCA genes survive and only the homozygous mice with PMCA1 ablation are embryolethal. Some PMCA isoforms may also be involved in other cell functions. Biochemical and biophysical studies of PMCA pumps have been limited by their low levels of expression. Delineation of the exact physiological roles of PMCA pumps has been difficult since most cells also express sarco/endoplasmic reticulum Ca2+pumps and a Na+-Ca2+-exchanger, both of which can lower cytosolic Ca2+. A major limitation in the field has been the lack of specific inhibitors of PMCA pumps. More recently, a class of inhibitors named caloxins have emerged, and these may aid in delineating the roles of PMCA pumps.Key words: ATPases, hypertension, caloxin, protein kinase A, protein kinase C, calmodulin.

Evolving role of calcium antagonists in the management of hypertension

What, if anything, should physicians make of these apparently disparate hazards in a group of drugs that they prescribe so widely? The authors, and many of our colleagues, still believe that most patients clearly benefit from the judicious use of calcium antagonists. Reports of several large, powered prospective, randomized outcome studies including ALLHAT commend the use of long-acting calcium antagonists that, by virtue of their ability to attain more gradual and sustained plasma levels, do not evoke reactive sympathetic activation and consequently are safe. Concomitantly, such formulations should promote increased patient compliance and thereby favorably influence hypertension-related morbidity and mortality.

Low Ca(2+) pump activity in diabetic nephropathy

Elevated cell Na(+)-H(+) exchange (NHE) activity characterizes diabetic nephropathy (DN), but the mechanisms of this abnormality are unclear. Recent evidence suggests that NHE and the Ca(2+) pump share similar regulatory pathways, but whether abnormalities in Ca(2+) metabolism characterize DN is not known. We investigated Ca(2+) efflux rates, NHE activity, cytosolic Ca(2+) ([Ca(2+)](i)) concentrations, and intracellular pH (pH(i)) in human skin fibroblasts from 20 patients with type 1 (insulin-dependent) diabetes and nephropathy; 20 patients with diabetes with normoalbuminuria matched for age, sex, and duration of diabetes; and 10 individuals without diabetes. Ca(2+) pump-mediated Ca(2+) efflux was significantly lower in patients with nephropathy than in patients with normoalbuminuria and individuals without diabetes (0.074 +/- 0.01 versus 0.115 +/- 0.01 versus 0.131 +/- 0.02 nmol.mg(protein)(-1).min(-1); analysis of variance [ANOVA], P = 0.015). Elevated maximal velocity of the Na(+)-H(+) exchanger was confirmed in fibroblasts from patients with nephropathy (14.4 +/- 1.2 versus 7.1 +/- 0.7 versus 8.0 +/- 1.2 mmol H(+).l cell(-1).min(-1); ANOVA, P < 0.0001). A reverse correlation between Ca(2+) pump activity and NHE rates could be shown. Adjustment for glycated hemoglobin and plasma lipid levels did not affect these findings. Finally, [Ca(2+)](i) concentrations and pH(i) were normal in all patients. Low Ca(2+) pump activity is a concomitant event of elevated NHE rates in DN; the molecular dysfunction(s) underlying these abnormalities remains to be established.

Coexisting independent sodium-sensitive and sodium-insensitive mechanisms of genetic hypertension in spontaneously hypertensive rats (SHR)

Some essential hypertensive patients and genetic hypertensive rat strains have less than the normal levels of Mg2+ tightly bound to the plasma membranes of their erythrocytes and other cells, i.e., the magnesium binding defect (MgBD). This binding defect appears to cause increased passive permeability of the membrane to Na+ and thereby its increased intracellular concentration, particularly if the Na+-extrusion enzyme systems of the cell are also defective. The Na+-Ca2+ exchange system in the cell membrane exports Na+ and imports Ca2+, increasing the tone of the smooth muscle cell and thus producing hypertension (HTn). This HTn is Na+-sensitive. Evidence supporting this postulate was obtained by determining the intraerythrocyte total concentrations of Na+, Ca2+, K+, and Mg2+ in two strains of spontaneously hypertensive rats (SHR and SS/Jr rats, having the MgBD together with the other requisites of the Na+-sensitive pathway) and their respective controls (WKY and SR/Jr rats, in which this complete pathway is absent). The Na+ and Ca2+ concentrations in the hypertensive rats were increased, and that of K+ was decreased. The concentrations of these cations were very similar in the two hypertensive strains. The level of membrane tightly bound Ca2+ in SHR erythrocyte membranes was significantly higher than those in the other three rat strains, which were not statistically different from each other. These results support previously reported evidence of the existence of a novel HTn-generating mechanism in the SHR rat, in which the intracellular Ca2+ concentration is increased as the result of the enhanced diffusion of this ion into the cell and the accompanying deficiency of the Ca2+ extrusion enzyme systems. This pathway is therefore Na+-insensitive, i.e., Ca2+-sensitive.Key words: essential hypertension, Na+-sensitive hypertension, Na+-insensitive hypertension, Ca2+-sensitive hypertension.

Combined SSCP and heteroduplex analysis of the human plasma membrane Ca(2+)-ATPase isoform 1 in patients with essential hypertension

In recent theories concerning the pathogenesis of essential hypertension, altered calcium homeostasis plays an important role. Increased intracellular Ca(2+) levels have repeatedly been reported in different cell types of hypertensive subjects. In vascular smooth muscle cells the plasma membrane Ca(2+)-ATPase (PMCA) represents the most important Ca(2+)-ejection system. Modifications of this pump therefore have been assumed to increase contractile tone of small vessels. For this reason, the purpose of this study was to determine if genetic alterations in the hPMCA1 gene might be associated with arterial hypertension. For detection of polymorphisms all 22 PMCA1 exons from 44 patients with essential hypertension (based on rigorous clinical data in addition to a positive family history) and from 40 normotensives without a family history of hypertension were PCR amplified and subsequently subjected to combined single-strand conformation polymorphism (SSCP) and heteroduplex (HTX) analysis. Despite the high sensitivity of almost 100%, differences could not be identified between hypertensives and normotensives within the two groups. These data indicate that at least in this population PMCA1 polymorphisms are presumably not related to common forms of essential hypertension. Furthermore, the high degree of evolutionary conservation of the PMCA1 gene underlines the pivotal role of this ATPase for cell physiology.

Serum calcium: a new, independent, prospective risk factor for myocardial infarction in middle-aged men followed for 18 years

BACKGROUND

Primary hyperparathyroidism (HPT) is a disease characterized by hypercalcemia, and associated with an increased mortality in cardiovascular diseases. However, serum calcium levels within the normal range have not been evaluated as a prospective cardiovascular risk factor.

METHODS

A cohort of males aged 50 (n = 2183) were investigated in 1970-1973 for serum calcium and known cardiovascular risk factors. They were then followed up over the next 18 years.

RESULTS

During the follow-up period, 180 subjects experienced a myocardial infarction (MI). The serum calcium levels were significantly elevated at the baseline (2.37 +/- 0.09 SD versus 2.35 +/- 0.09 mmol/l, p < 0.03) in the subjects who developed a MI when compared with the rest of the cohort. Also blood pressure, body mass index (BMI), fasting insulin, serum cholesterol, serum triglycerides, and the atherogenic index were significantly elevated in the MI group (p < 0.01), while HDL-cholesterol was lower at the baseline investigation (p < 0.01). Cox's proportional hazard analysis showed that only serum calcium (p < 0.01), BMI (p < 0.0003), diastolic blood pressure (p < 0.0009), and the atherogenic index (p < 0.002) were significantly independent risk factors for MI. The range of serum calcium levels from the mean value, -2 SDs to the mean value +2 SDs corresponds to a variation in estimated risk for MI ranging from 0.06 to 0.15.

CONCLUSIONS

Serum calcium was found to be an independent, prospective risk factor for MI in middle-aged males suggesting a role for extracellular calcium levels in the atherosclerotic process.

Blood pressure reduction after parathyroidectomy for secondary hyperparathyroidism: further evidence implicating calcium homeostasis in blood pressure regulation

A link between plasma calcium, dietary cations, and blood pressure has been suspected for some time, with human, experimental animal, and epidemiological data adduced to support this hypothesis. We identified 21 patients receiving regular maintenance hemodialysis, but not receiving any regular antihypertensive treatment, who had undergone 22 surgical removals of the parathyroid glands in the period 1978 to 1992. These patients' records were then scrutinized. The group preparathyroidectomy mean systolic blood pressure (BP) was 142.6 +/- 19.4 mm Hg. After the operation, the mean systolic BP was 133.6 +/- 21.9 mm Hg (P = 0.004). Plasma calcium decreased from 2.72 +/- 0.18 mmol/L to 2.52 +/- 0.19 mmol/L (P < 0.001). There was a correlation between the decreases in systolic blood pressure (SBP) (9.4%) and plasma calcium (7.3%); r = 0.60, P = 0.012. The decrease in SBP was not immediate, but delayed some months and complete by approximately 9 months after the operation. Furthermore, using ambulatory BP monitoring in a group of long-term hemodialysis patients, we found that parathyroidectomized patients had lower BP and pulse rates than those with intact parathyroid glands (SBP, 122.9 +/- 16.3 mm Hg v 102.9 +/- 9.9 mm Hg; pulse rates, 87.5 +/- 12.7 v 72.0 +/- 7.5 beats/min, P < .001, nonparathyroidectomy v postparathyroidectomy, both comparisons). These data support a link between plasma calcium and BP in patients receiving maintenance hemodialysis.

Erythrocyte Na+, K+ and Ca2+, Mg(2+)-ATPase activities in hypertensives on angiotensin-converting enzyme inhibitors

OBJECTIVE

To investigate erythrocyte membrane Na+, K(+)- and Ca2+, Mg(2+)-ATPase activities in newly diagnosed hypertensive patients before and after 2, 4, and 6 months of treatment with enalapril or captopril as monotherapy.

METHODS AND RESULTS

Na+, K(+)-ATPase activity (nmol ATP hydrolysed/min per mg protein) rose by 6 months of treatment in both groups when values were compared in each treated group over time (4.5 +/- 0.8 to 9.9 +/- 1.2; 4.9 +/- 0.8 to 10.5 +/- 1.7, respectively, p < 0.001 for both). When the treated groups were compared with controls at each period of time, Na+, K(+)-ATPase activity was higher at months 4 and 6 (p < 0.001) for both groups, respectively). Ca2+, Mg(2+)-ATPase activity (nmol ATP hydrolyzed/min per milligram protein) in the absence and in the presence of calmodulin increased in the enalapril (6.4 +/- 0.7 to 8.9 +/- 0.95, p < 0.05; 13.4 +/- 1.2 to 17.2 +/- 1.2, p < 0.05, respectively) and captopril (7.0 +/- 0.6 to 8.5 +/- 0.7; 14.4 +/- 1.1 to 16.0 +/- 1.0, p < 0.05, respectively) groups after 6 months of treatment compared within each treated group over time. When patient groups were compared with controls at time 0, 2, 4, and 6 months, the pump activity was higher in the treated groups at 6 months.

CONCLUSION

The long-term enhancement of cell membrane Na+, K(+)-and Ca2+, Mg(2+)-ATPase activity associated with enalapril and captopril therapy may represent a specific effect of these agents or alternatively, a nonspecific outcome of blood pressure reduction.

The influence of blood pressure on intracellular Ca2+ content in erythrocytes: effects of cadmium chloride and nifedipine

Cellular abnormalities associated with elevated Ca2+ concentrations have been postulated to be involved in the pathogenesis of hypertension. The present study was undertaken to investigate the effects of blood pressure changes on cytosolic Ca2+ levels in erythrocytes. Cadmium, which has been implicated in the etiology of hypertension was used as the hypertensive agent and the classical blocker of voltage-operated calcium channels nifedipine was used to treat hypertension. 10 weeks old male rats were divided into four groups; control, CdCl2, CdCl2 and nifedipine, nifedipine groups. CdCl2 caused elevations in blood pressure and in the cytosolic erythrocyte Ca2+ levels both of which were reduced after nifedipine administration. After nifedipine alone, cytosolic Ca2+ levels were increased. These findings suggest that cytosolic Ca2+ content decreasing action of nifedipine in the CdCl2 and nifedipine applied group could be secondary to the antihypertensive action.

Calcium-ATPase and insulin in adolescent offspring of essential hypertensive parents

A number of abnormalities in calcium homeostasis have been reported in patients with essential hypertension. IN turn, insulin has been shown to influence the activity of the Ca(2+)-ATPase. We have previously shown that normotensive offspring of essential hypertensive individuals have an exaggerated insulin response to a glucose overload. Therefore, the aim of the present study was to evaluate basal and calmodulin-activated Ca(2+)-ATPase in red blood cells and its relationship to the insulin response during an intravenous glucose tolerance test in 27 normotensive adolescents with a family history of essential hypertension (F+) (mean age, 13.9 +/- 0.5 years) and in 10 control subjects matched for age and body mass index with no family history of hypertension (F-). The results (mean +/- SD) were as follows (mumol Pi/[mg protein/h]10(-1)): basal Ca(2+)-ATPase, 4.5 +/- 1.2 in F+ and 5.1 +/- 1.6 in F- (P = NS); calmodulin-activated Ca(2+)-ATPase, 13.6 +/- 3.9 in F+ and 16.2 +/- 1.7 in F- (P < .04). The insulin area under the curve after the glucose load was 3413 +/- 1674 microU/mL per hour in F+ and 2752 +/- 928 in F- (P = NS). Calmodulin-activated Ca(2+)-ATPase showed a negative correlation with the insulin area under the curve (r = -.59, P < .005) and cholesterol levels (r = -.38, P < .03). Urinary calcium excretion was 1.82 +/- 0.9 mmol/d in F+ and 2.47 +/- 0.9 mmol/d in F- (P = NS).(ABSTRACT TRUNCATED AT 250 WORDS)

Alterations in sodium metabolism as an etiological model for hypertension

An adequate matching for race, sex, stage of the menstrual cycle, family history of hypertension, and the amount of sodium and other electrolytes in the diet should be a prerequisite for valid conclusions when interpreting the erythrocyte concentration and fluxes of sodium in essential hypertensive patients in comparison with normal subjects. Alterations in intracellular sodium concentration and transmembrane sodium transport systems as causes of essential hypertension are postulated. This review article describes how this abnormal sodium and calcium metabolism translates into increased systemic vascular resistance through altered vasoactive responses and/or vasculature structural changes.

Preeclampsia and calcium adenosine triphosphatase activity of red blood cell ghosts

OBJECTIVE

The current work was undertaken to study the calcium adenosine triphosphatase activity of red blood cell membranes from pregnant women with preeclampsia.

STUDY DESIGN

Six normotensive and six preeclamptic pregnant women at 38 to 39 weeks of gestation were studied. The diagnosis of preeclampsia was made on the basis of blood pressure (> 140/90 mm Hg), proteinuria (> 0.5 gm of urinary protein per day), or edema. Hemoglobin-free red blood cell ghosts were prepared from the heparinized blood samples and were used to determine the calcium adenosine triphosphatase activity.

RESULTS

It was found that the calcium adenosine triphosphatase activity of preeclamptic women is diminished by about 50% compared with that of normotensive pregnant women.

CONCLUSION

A diminution of the calcium adenosine triphosphatase activity of erythrocytes in preeclampsia might be an indication that the in vivo activity of the calcium pump of these cells is diminished, which could, in turn, drive the cells to increase their cytoplasmic free calcium concentration.

In vivo effect of calcitriol on calcium transport and calcium binding proteins in the spontaneously hypertensive rat

The abnormal intestinal Ca2+ transport reported in spontaneously hypertensive rats (SHR) has been attributed to decreased responsiveness to calcitriol. We reexamined this hypothesis by studying the calcitriol regulation of SHR duodenal calbindin-D9K and calmodulin and the relation of calcitriol to Ca2+ uptake by isolated enterocytes. SHR and normotensive Wistar-Kyoto (WKY) rats were injected with either 50 ng/d calcitriol (vit-D) or vehicle alone (control) for 3 days. Decreased calbindin-D9K (P < .001) and cellular Ca2+ flux (P < .001) were observed in control SHR. Calcitriol increased total cell and brush border calbindin-D9K (P < .0001); this variation paralleled plasma calcitriol levels in both strains. In contrast, Ca2+ flux, which increased in vit-D animals, remained lower in SHR for plasma calcitriol levels similar to those in WKY rats. Immunoreactive calmodulin was similar in both strains whether assayed in total cell or brush border membranes. In contrast, when measured by ligand blotting (45Ca), calmodulin was lower in SHR than in WKY rats (P < .01), suggesting the existence of a calmodulin pool with reduced Ca2+ binding capacity in the hypertensive strain. Calcitriol had no effect on calmodulin in either strain. In conclusion, Ca2+ binding protein regulation by calcitriol is normal in the SHR, and decreased hormone responsiveness cannot account for the defective duodenal calcium transport of this experimental model of hypertension.

Measurement of Ca2+ pump-mediated efflux in hypertension

Ca2+ homeostasis has been a prominent research area in the study of hypertension. There is convincing evidence that hypertension in spontaneously hypertensive rats is characterized by enhanced Ca2+ influx in various cell types. It is, however, still unclear whether hypertension is associated with reduced or enhanced Ca2+ efflux. Reduced Ca2+ efflux would augment the effects of enhanced Ca2+ influx. However, enhanced Ca2+ extrusion may occur as an adaptive process to minimize the effects of Ca2+ overload. This question remains unanswered because of inconsistent results obtained using a variety of experimental techniques. In this article we have reviewed the research findings and discuss existing and possible new techniques to assess Ca2+ efflux in hypertension, with particular attention to vascular smooth muscle. We have focused mainly on studies using the spontaneously hypertensive rat and discuss its appropriateness as a model for essential hypertension.

Insulin resistance--not hyperinsulinemia--is pathogenic in essential hypertension

A correlation between essential hypertension and insulin resistance/hyperinsulinemia is well documented, and there is adequate reason to believe that this association is causal. The common presumption that hyperinsulinemia mediates this connection is based on studies demonstrating various pressor effects of insulin, such as sodium retention, activation of the sympathetic nervous system, and stimulation of renin output. However, a consideration of physiological parameters in essential hypertensives indicates that these insulin-mediated pressor effects are unlikely to play a crucial pathogenic role in most cases of essential hypertension. Moreover, physiological elevation of insulin following a meal is typically associated with a reduction of blood pressure in hypertensives and the elderly. Euglycemic insulin clamps tend to reduce blood pressure in elderly subjects, and prolonged maintenance of hyperinsulinemia in animals does not raise blood pressure. In fact, insulin has long been known to have direct vasodilatory or antipressor effects on resistance vessels, and there is recent evidence that insulin reduces vascular resistance in skeletal muscles to facilitate glycogen storage after a meal. I propose that essential hypertensives experience a net deficit of insulin activity in vascular muscle, and that, in conjunction with other genetic or acquired defects of electrolyte transport, this leads to an increase in basal vascular tone and a hypersensitivity to pressor agents. Correction of insulin resistance usually aids blood pressure control, and in addition may mitigate the excess cardiovascular risk associated with hypertension.

Salt sensitivity in hypertension. Renal and cardiovascular implications

The mechanisms responsible for the increase in blood pressure response to high salt intake in salt-sensitive patients with essential hypertension are complex and only partially understood. A complex interaction between neuroendocrine factors and the kidney may underlie the propensity for such patients to retain salt and develop salt-dependent hypertension. The possible role of vasodilator and natriuretic agents, such as the prostaglandins, endothelium-derived relaxing factor, atrial natriuretic factor, and kinin-kallikrein system, requires further investigation. An association between salt sensitivity and a greater propensity to develop renal failure has been described in certain groups of hypertensive patients, such as blacks, the elderly, and those with diabetes mellitus. Salt-sensitive patients with essential hypertension manifest a deranged renal hemodynamic adaptation to a high dietary salt intake. During a low salt diet, salt-sensitive and salt-resistant patients have similar mean arterial pressure, glomerular filtration rate, effective renal plasma flow, and filtration fraction. On the other hand, during a high salt intake glomerular filtration rate does not change in either group, and effective renal blood flow increases in salt-resistant but decreases in salt-sensitive patients; filtration fraction and glomerular capillary pressure decrease in salt-resistant but increase in salt-sensitive patients. Salt-sensitive patients are also more likely than salt-resistant patients to manifest left ventricular hypertrophy, microalbuminuria, and metabolic abnormalities that may predispose them to cardiovascular diseases. In conclusion, salt sensitivity in hypertension is associated with substantial renal, hemodynamic, and metabolic abnormalities that may enhance the risk of cardiovascular and renal morbidity.

Insulin attenuates agonist-mediated calcium mobilization in cultured rat vascular smooth muscle cells

Insulin has been shown to attenuate pressor-induced vascular contraction, but the mechanism for this vasodilatory action is unknown. This study examines the effect of insulin on angiotensin II (ANG II)-induced increments in cytosolic calcium in cultured rat vascular smooth muscle cells (VSMC). 20-min incubations with insulin (10 microU/ml to 100 mU/ml) did not alter basal intracellular calcium concentration ([Ca2+]i), but inhibited the response to 100 nM ANG II in a dose-dependent manner (ANG II alone, 721 +/- 54 vs. ANG II + 100 mU/ml insulin, 315 +/- 35 nM, P < 0.01). A similar effect of insulin on ANG II action was observed in calcium poor buffer. Moreover, insulin did not effect calcium influx. ANG II receptor density and affinity were not affected by 24-h incubation with insulin. To further clarify the mechanisms of these observations, we measured ANG II-induced production of inositol 1,4,5-triphosphate (IP3), and IP3-releasable 45Ca. Insulin treatment did not alter ANG II-stimulated IP3 production. However, IP3-stimulated release of 45Ca in digitonin permeabilized cells was significantly reduced after 5-min incubations with 100 mU/ml insulin. Thapsigargin induced release of calcium stores was also blocked by insulin. Thus, insulin attenuates ANG II-stimulated [Ca2+]i primarily by altering IP3-releasable calcium stores. Insulin effects on ANG II-induced [Ca2+]i were mimicked by preincubation of VSMC with either sodium nitroprusside or 8-bromo-cGMP. As elevations in cGMP in vascular tissue lower [Ca2+]i, it is possible that insulin affects IP3 release of calcium by a cGMP-dependent mechanism that would contribute to its vasodilatory effects.

Ca2+ ATPase activity in essential and renal hypertension

In 15 patients with essential hypertension, 16 patients with renal hypertension and in 12 healthy subjects Ca2+ ATPase activity was determined in red blood cells both in the basal state and after maximal stimulation with calmodulin. Normal subjects showed a basal and maximal activity of 7.1 +/- 3.6 and 16.0 +/- 2.3 pmol phosphate/min.10(6) RBC, respectively. Renal hypertensives had a similar basal Ca2+ ATPase activity (5.4 +/- 4.1 pmol phosphate/min.10(6) RBC) and a lowered maximal Ca2+ ATPase activity (9.8 +/- 5.4 pmol phosphate/min.10(6) RBC, p < 0.05). In essential hypertensives basal and maximal Ca2+ ATPase activity was 9.0 +/- 5.3 and 35.4 +/- 14.4 pmol phosphate/min.10(6) RBC, respectively, the latter being significantly increased (p < 0.01). This finding, which is in contrast to earlier results indicating a lowered Ca2+ ATPase activity in essential hypertension, may be explained as a consequence of an increased Ca2+ influx in essential hypertension. A lowered Ca2+ ATPase activity does not seem to be involved in the pathogenesis of essential hypertension.

Alterations in cellular calcium handling as a result of systemic calcium deficiency in the developing chick embryo: I. Erythrocytes

Chick embryos rendered calcium (Ca) deficient by shell-less (SL) culture develop hypertension and tachycardia. Since hypocalcemia is accompanied by hypernatremia systemically but not by lower cellular Ca (Koide and Tuan, 1989), we speculate that cellular Ca handling may be altered in the SL embryo, perhaps involving Na transport. Using erythrocytes (RBC) from day-14 SL and normal (NL) embryos as the experimental cell, cellular Ca handling was studied under varying extracellular osmotic and ionic conditions by analyzing 45Ca uptake and cell volume regulation. Two agents, p-chloromercuriphenylsulfonate (PCM), and inosine/iodoacetamide (INI) were used to treat the RBCs to modify plasma membrane ion permeability and to deplete cellular ATP, respectively. Other cellular functions and activities related to Ca homeostasis, including ATP content and Ca(2+)-ATPase activity, were also analyzed. These analyses showed: (1) in NaCl, Ca uptake was similar in NL and SL cells, except after INI treatment, which resulted in slower Ca uptake by the SL cells, (2) in choline and sucrose, Ca uptake by SL RBCs was higher, (3) Ca uptake by RBCs of both embryos changed depending on the osmotic agent (Na < K < or = choline < sucrose), (4) Ca(2+)-ATPase activity was higher in SL RBC, although there was no change in the size or charge of the enzyme, and (5) in any osmotic agent, cellular Na was significantly lower, whereas cellular K was higher, in SL RBC. Based on these results, three features of RBC Ca handling were apparent: (1) Na-Ca exchange was functional and was more active in SL RBCs, (2) Ca uptake was dependent on the total ionic electrochemical gradient but not on bulk H2O movement, and (3) Ca pumping out capacity was directly correlated with Ca(2+)-ATPase activity. Elevated Ca uptake in sucrose-treated SL RBC is therefore indicative of its greater ion permeability. Taken together, these findings indicate that cellular Ca handling of the RBCs of SL chick embryos is characterized by a more active Na-Ca exchange system, greater ion permeability, and higher Ca pumping out capacity, thereby suggesting an up-regulated Ca handling function in the SL RBCs. The abnormal cellular Ca handling may be a direct result of the systemic Ca deficiency of the SL chick embryo and may be functionally related to its hypertension and tachycardia.

Assay of the Ca pump ATPase activity of intact red blood cells

An assay for the Ca pump ATPase of intact human red blood cells (RBCs) was developed. The assay utilized a small volume (typically 10 microliters) of packed RBCs in 1 ml of a buffer of known composition. The assay was based on the exposure of intact RBCs to the ionophore, A23187, in the presence of Ca. Such exposure caused a rapid degradation of ATP in RBCs. This degradation process is modeled in a numerical simulation in a companion paper (Vincenzi, F. F. and Hinds, T. R. (1992) Biochim. Biophys. Acta 1105, 63-70). The loss of ATP followed pseudo-first-order kinetics, and the rate constants for ATP degradation was taken as a measure of the capacity of the Ca pump ATPase. A number of variables were examined to optimize the activity of the ATPase. These variables included the concentrations of Ca and A23187. Because A23187 can promote loss of cellular Mg, it was necessary to include MgCl2 in the incubation medium to optimize ATPase activity. Likewise, it was determined that inclusion of iodoacetic acid optimized the rate of ATP loss, presumably by preventing the resynthesis of ATP from ADP and inorganic phosphate. Cobalt inhibited the ionophore-dependent loss of ATP by apparent competition with Ca for binding to A23187. Results of many assays demonstrated substantial differences in the rate constant for ATP loss in RBCs from different individuals. RBCs were selected according to density. Density associated loss of Ca pump ATPase activity was observed both by the intact RBC assay, and by assay of Ca pump ATPase activity in saponin lysates of RBCs. The correlation coefficient between the two assays was 0.93. It is suggested that the rate constant for ATP loss in intact RBCs exposed to A23187 and Ca can be taken as a measure of the Ca pump ATPase activity. This may be useful when isolated membrane ATPase assays fail (e.g., dog RBCs). The intact cell assay can also be carried out on very small volumes of cells and may be of particular value when RBC volumes are limited.

Plasma ionized calcium and cardiovascular risk factors in mild primary hyperparathyroidism: effects of long-term treatment with active vitamin D (alphacalcidol)

Primary hyperparathyroidism (HPT) has been associated with hypertension, hyperinsulinaemia, hypertriglyceridaemia and hyperuricaemia. In the present study, plasma ionized calcium (Ca2+) was studied in relation to cardiovascular risk factors in 20 subjects with mild hypertension. Plasma Ca2+ was found to be negatively correlated with fasting serum insulin, triglycerides and urate, and with diastolic blood pressure (DBP). However, after the interaction of the different risk factors had been taken into account in the multiple regression analysis, only the relationship between Ca2+ and serum insulin was significant (r = 0.55, P less than 0.01). In a previous double-blind, placebo-controlled study 1 micrograms alphacalcidol, a synthetic analogue of 1,25 dihydroxy-vitamin D3, induced a decrease in blood pressure in mild HPT subjects. In the present study, the highest dose that did not further aggravate the hypercalcaemia was given in a long-term study over a 12-month period to 18 mild HPT subjects (average dose, 1.75 micrograms daily). The treatment induced a reduction in body weight of 0.9 kg (P less than 0.05) and an increase in serum urate from 330 +/- 92 to 380 +/- 104 mmol l-1 (P less than 0.01). A reduction in blood pressure was only observed at the end of the study, from 142 +/- 17/86.6 +/- 9.1 to 139 +/- 13/82.9 +/- 8.9 mmHg (P less than 0.05 for DBP). The reduction in systolic blood pressure was significantly correlated with the reduction in body weight induced by treatment (r = 0.63, P less than 0.02). No consistent changes in glucose or lipid metabolism were induced by treatment.

Control of the erythrocyte free Ca2+ concentration in essential hypertension

Since Ca2+ ions seem to directly participate in the control of erythrocyte membrane structure and deformability and because cell Ca2+ metabolism has been repeatedly proposed to be modified in hypertension, the intracellular calcium ion concentration ([Ca2+]i) was investigated in red blood cells from hypertensive and normotensive subjects. [Ca2+]i was measured by using the fluorescent Ca2+ chelator fura-2. Red blood cell [Ca2+]i was increased in hypertensive compared with normotensive subjects in the whole population and further increased when hypertensive were compared with age-matched normotensive subjects. An inverse relation between age and [Ca2+]i was observed when calculated with blood pressure adjusted. In hypertensive patients, high [Ca2+]i values were associated with a reduced erythrocyte deformability. The initial rate of 45Ca2+ uptake did not differ between the two blood pressure groups. Similarly, when the extracellular Ca2+ concentration was elevated from 1 to 2 mmol/l, [Ca2+]i increased by 16 +/- 4% (p less than 0.03) in red blood cells from both groups, thus maintaining a significant difference between hypertensive and normotensive subjects. Under these conditions, the addition of 10(-7) mol/l nicardipine, a dihydropyridine Ca2+ antagonist, decreased [Ca2+]i by 15 +/- 4% (p less than 0.05) and 7 +/- 5% in erythrocytes from hypertensive and normotensive subjects, respectively, thereby reducing the difference in [Ca2+]i observed between these two groups. This nicardipine effect was positively correlated to the initial [Ca2+]i. In the presence of 5 mumol/l W7, a calmodulin antagonist, [Ca2+]i increased significantly only in erythrocytes from hypertensive patients (26 +/- 6%, p less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Purification and analysis of erythrocyte membrane Ca(2+)-ATPase from small samples of patient blood: application to cystic fibrosis

A method is presented for the micro-scale isolation and characterization of erythrocyte membrane Ca(2+)-ATPase from small samples (7 mL) of whole human blood. Ca(2+)-ATPase isolated by this technique was more than 92% pure and showed calcium-activation characteristics similar to enzyme purified by standard macroscale procedures--viz maximal velocity of activation (VCA2+) = 15.5 +/- 1.2 mumol ATP hydrolysed/mg/min, and reciprocal of apparent affinity (KCa2+) = 0.73 +/- 0.15 microM free calcium (mean +/- SEM; n = 9). Using the isolation procedure described, purified Ca(2+)-ATPase could be prepared and assayed in a single working day. When the calcium-activation kinetics of cystic fibrosis erythrocyte membrane Ca(2+)-ATPase were reassessed using enzyme purified by this technique, VCa2+ and KCa2+ were not significantly different from normal values.

Increased whole blood viscosity combined with decreased erythrocyte fluidity in untreated patients with essential hypertension

Erythrocyte fluidity and other haemorheological variables were studied in 22 patients with essential hypertension and compared with age- and sex-matched healthy controls. Hypertensive patients displayed a significantly lower erythrocyte fluidity (P less than 0.001). Similarly, significantly elevated values for haematocrit, plasma and whole blood viscosity, as well as aggregation tendency were observed compared to controls. Although differing in these respects from controls, there were no obvious relationships between these rheological variables and either systolic or diastolic blood pressure. The significantly lower erythrocyte fluidity and other changes in haemorheological variables of red blood cells found in hypertensive patients may be explained by an enlarged metabolic pool of free calcium ions in these red blood cells. It is suggested that the molecular mechanisms underlying the evolution of essential hypertension are multifactorial rather than being based on a single molecular derangement. Primary events resulting in altered physicochemical properties of the red blood cells may work in concert in the development of essential hypertension, in addition to the increased availability of calcium ions and their potential role in smooth muscle contraction.

Impaired calcium metabolism associated with hypertension in Zucker obese rats

Recent data from our laboratory indicate that reduced membrane Ca-adenosine triphosphatase (ATPase) activity in non-insulin-dependent diabetics may be responsible for increases in intracellular calcium and, consequently, for elevated vascular resistance. Since obesity is frequently associated with hypertension, even before the development of overt diabetes, we evaluated blood pressure and erythrocyte cation levels and membrane Na/K-ATPase and Ca-ATPase in Zucker obese rats and their lean controls (n = 10 per group). Intra-arterial blood pressure, determined via a femoral cannula, demonstrated elevated systolic and diastolic pressure in the obese rats (P less than .05). There were no significant differences in Na/K-ATPase between groups, but there was a decrease in Ca-ATPase (P less than .01) in the obese rats and an increase in tissue and cellular calcium content (P less than .05). These data demonstrate a specific impairment in membrane Ca-ATPase activity in obese rats they may have caused the observed increase in cellular calcium and, consequently, increased blood pressure. These phenomena may result from impaired insulin activation of membrane Ca-ATPase in these insulin-resistant animals.

Membrane transport of ions in hypertension

A variety of disturbances in transmembrane monovalent and divalent cation fluxes has been described in blood cells from hypertensive patients. Other membrane properties, such as fluidity and calcium binding, are also altered. It is now abundantly clear that some of the inconsistencies in this field are due to poor matching of patients and controls. However, even when careful matching is carried out, differences in membrane functions are still seen. It is suggested that these are due to a disturbance in the physicochemical properties of the cell membrane, related to changes in cell membrane phospholipid fluidity. This change could maintain peripheral resistance either by directly or indirectly increasing tone or by predisposing to resistance vessel hypertrophy. Recent evidence emphasizes the role of the latter rather than the former in experimental hypertension. It is postulated that overactivity of the phosphoinositide second messenger system as a result of alteration in all membrane properties predisposes genetically susceptible individuals to resistance-vessel hypertrophy and hypertension.

Erythrocyte Ca2(+)-ATPase: activation by enzyme oligomerization versus by calmodulin

The subject of our studies is the mechanism of activation of the erythrocyte Ca2(+)-ATPase. Using purified, detergent solubilized enzyme it was found that equivalent maximal Ca2(+)-ATPase activity is obtained either upon addition of calmodulin or upon increase of enzyme concentration. Three independent methods, including Ca2(+)-ATPase activity, polarization of the enzyme modified with an external fluorescent probe, and efficiency of fluorescence resonance energy transfer between enzyme molecules have established that the concentration dependent activation is due to enzyme oligomerization. The oligomers bind calmodulin with a lower stoichiometry (0.5 mol calmodulin/mol Ca2(+)-ATPase), higher Ca2+ affinity (KCa = pCa 7.4), and higher cooperativity for Ca2+ (nH = 2.6) than the monomeric form (stoichiometry = 1 mol calmodulin/mol Ca2(+)-ATPase, KCa = pCa 7.0, nH = 1.1). The Ca2+ dependence of calmodulin binding and activation of monomers indicates that calmodulin binds before the Ca2(+)-ATPase activity is exhibited, demonstrating that the activation of this enzyme form is totally dependent on calmodulin. In contrast, oligomers reveal very similar Ca2+ dependence for calmodulin binding and for Ca2(+)-ATPase activity as well as for Ca2+ binding (assessed by tryptophan fluorescence), and for the oligomerization process (assessed by fluorescence energy transfer). The calmodulin antagonist drug 48/80 inhibits the calmodulin dependent activity of the monomers (I50 = 1.4 micrograms/ml) but has no effect on the activity of oligomers, confirming that calmodulin plays no role in the activation of the oligomeric enzyme. Our studies indicate that the erythrocyte Ca2(+)-ATPase can be activated by its high affinity, Ca2+ dependent binding of calmodulin or by a Ca2+ dependent oligomerization process which may involve calmodulin binding site.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of cellular calcium metabolism in abnormal glucose metabolism and diabetic hypertension

The prevalence of hypertension in patients with non-insulin-dependent diabetes mellitus (NIDDM) is considerably higher than in the non-diabetic population. Insulin resistance may contribute to this increased prevalence. Abnormal cellular calcium (Ca2+) homeostasis may link insulin resistance and high blood pressure in patients with NIDDM. Observations of abnormal cellular Ca2+ homeostasis in animal models of NIDDM and obesity as well as in diabetic patients are consistent with this hypothesis. Abnormalities in cellular Ca2+ homeostasis are also found in hypertensive animals and humans. Alterations in cell membrane phospholipid content and distribution may be the primary cause of abnormal plasma membrane Ca2+ fluxes in patients with NIDDM and hypertension.

Distribution of proteins in erythrocyte membranes from patients with hypertension

The distribution of characteristic proteins in erythrocyte membranes was studied in patients with essential hypertension (EH) (n = 44), secondary hypertension of renal genesis (n = 42), and healthy persons (n = 44). Densitograms of gels analyzed after electrophoresis of erythrocyte ghosts showed a twofold increase in the amount of band 4.5 (Mw = 52-59 kD) and band 6 (Mw = 35 kD) polypeptides in EH patients as compared to that in healthy persons. Radioimmunoassay with monoclonal antibodies obtained to the sarcoplasmic reticulum (SR) of heart muscle has demonstrated that the amount of the antibodies bound to fragmented erythrocyte membranes from EH persons is greater by at least 28% than that in healthy people. Patients with secondary arterial hypertension of renal genesis did not reveal a significant difference in binding of monoclonal antibodies as compared to the control group. Thus, erythrocyte membranes from EH subjects are different from those taken from the blood of healthy people by the increased amount of bands 4.5 and 6 proteins.

Relation of serum calcium concentration to metabolic risk factors for cardiovascular disease

Data from a health screening survey with over 18,000 adult participants were used to determine the relations between serum calcium concentration and the cardiovascular risk factors hypertension, hyperglycaemia, and hyperlipidaemia. Blood pressure and serum glucose and cholesterol concentrations were all positively related to each other independent of age, sex, kidney function, and obesity. Similar relations between the risk factors were found in subjects with hypertension or hyperglycaemia independent of the degree of overweight. These results suggested that there might be a metabolic syndrome of cardiovascular risk factors. Serum calcium concentration was positively related to systolic and diastolic blood pressures and serum glucose and cholesterol concentrations. Thus a common feature in the syndrome is an increased serum calcium concentration. The relations between serum calcium concentrations and the cardiovascular risk factors were not limited to the upper parts of the distribution, being seen over a wide range. Changes in calcium metabolism seem to be related to a metabolic syndrome of hypertension, impaired glucose tolerance, and hyperlipidaemia.

Physiologic and hemodynamic considerations in blood pressure control while maintaining organ perfusion

Essential hypertension remains a significant contributor to mortality, despite treatments effective in decreasing blood pressure. A major reason mortality remains high may be that the conventional stepped-care approach does not always reduce blood pressure in a physiologic manner, which may result in metabolic derangements and elicitation of undesirable reflex mechanisms. Use of the stepped-care approach does not take into account fundamental hemodynamic differences associated with a hypertensive patient's age, race and weight. A nonphysiologic approach in the use of antihypertensive agents may be associated with progression of left ventricular hypertrophy, metabolic derangements, arrhythmias, sudden death, and increased atherosclerotic and nephrosclerotic complications. Considerations of the alterations in cardiovascular structure that can occur in light of the activity of the renin-angiotensin system, the sympathetic nervous system and transcellular calcium flux are discussed as to how they affect long-term survival.

Salt sensitivity and systemic hypertension in the elderly

Aging in industrialized societies is accompanied by increases in the incidence and prevalence of hypertension, with a disproportionately greater increase occurring among aging blacks than among aging whites. This geriatric hypertension is generally of a salt-sensitive nature with a disproportionate frequency of isolated systolic hypertension. Although salt-taste acuity declines with age, salt sensitivity among the elderly does not appear to result from a compensatory increase in salt intake. Rather, age-related increases in salt sensitivity result, in part, from a reduced ability to appropriately excrete a salt load, which is due to a decline in renal function and to a reduced generation of natriuretic substances such as prostaglandin E2 and dopamine. Age-associated declines in the activity of membrane sodium/potassium-adenosine triphosphatase (Na/K-ATPase) may also contribute to geriatric hypertension because this results in increased intracellular sodium that may cause reduced sodium-calcium exchange and thereby increase intracellular calcium and vascular resistance. Reductions in cellular calcium efflux due to reduced calcium-ATPase activity may similarly cause an increase in intracellular calcium and vascular resistance. Increasing dietary calcium intake may represent an effective nonpharmacologic treatment for some salt-sensitive persons because it appears to reduce intracellular calcium by (1) suppressing parathyroid hormone-mediated calcium influx, (2) increasing Na/K-ATPase activity, and (3) reducing intravascular volume due to calcium-induced natriuresis.

A role of calcium in altered sodium ion transport of hypertensives?

Research on the etiology of essential hypertension has led to many reports of altered ion transport in cells from hypertensive patients and animal models. Abnormalities in sodium and calcium ion gradients and transport in vascular smooth muscle, neuronal tissue, cardiac muscle as well as erythrocytes have been extensively investigated. It is not clear whether these abnormalities are of primary or secondary nature. The current knowledge of sodium and calcium ion transport in essential hypertension is briefly reviewed here. Furthermore, evidence is presented which suggests a role of calcium in the regulation of sodium transport activity.

Two forms of vasoconstriction in systemic hypertension

Clinical, pharmacologic and biochemical evidence characterizes essential hypertension as a heterogeneous spectrum of pathophysiologic substances rather than the single entity it has long been presumed to be. Although the causes of essential hypertension remain obscure, 2 different mechanisms for long-term vasoconstriction that sustain diastolic hypertension in the experimental and clinical forms of primary aldosteronism and renovascular hypertension can also be identified and quantified among patients with essential hypertension. The first mechanism is renin independent, requires antecedent sodium retention and appears related to abnormal membrane transport of calcium. This vasoconstriction is identified by low plasma renin and ionized calcium and is correctable by sodium depletion or calcium channel or alpha blockade. The second vasoconstrictor mechanism is renin mediated and involves an increase in cytosolic calcium. This mechanism is quantifiable by the plasma renin level or the hypotensive response to an antirenin-system drug (converting enzyme inhibitor, beta blocker or saralasin). Depending on the state of sodium balance, these 2 mechanisms contribute reciprocally to maintenance of arteriolar tone in experimental models, in both normal and hypertensive people, and in patients with congestive heart failure. In these situations, at the extremes of the range of plasma renin values, one or the other mechanism predominates, whereas in the medium range of renin values both mechanisms can be operative. These interrelations provide a basis for applying more precisely tailored therapy and for stratifying patients pathophysiologically for further study.

Red blood cells Ca2+ pump is not altered in essential hypertension of humans and Kyoto rats

The kinetic parameters of the Ca2+ pump were assessed in red blood cells of essential hypertensive subjects as compared to their respective controls. Uphill Ca2+ efflux was investigated in Ca2+ -saturated intact red blood cells using a new method recently developed for human red cells (Dagher,G. and Lew, V. J. Physiol. (London), in the press). 45Ca-equilibrated cells were obtained using ionophore A23187 and Ca2+ efflux was assessed after addition of excess CoCl2 which totally inhibits Ca2+ influx and thus exposes uphill Ca2+ extrusion by the pump. The results comprise methodological aspects of the use of this technique in rat red blood cells. The determination of the maximal velocity and the Ca2+ concentration for half-maximal stimulation (KCa 0.5) did not reveal any alteration in essential hypertensives and spontaneously hypertensive rats as compared to their controls.

The calcium paradox of essential hypertension

Three disparate observations--that calcium mediates vascular smooth muscle contraction, that calcium channel blockers lower blood pressure, and that increased dietary calcium intake can also ameliorate hypertension--constitute somewhat of a paradox. The complex interrelationships between calcium metabolism and essential hypertension are discussed in this review. Recent evidence suggests possible defects in intracellular calcium transport or in calcium binding in essential hypertension. This evidence, and the paradoxical therapeutic efficacy of both calcium channel blockers and supplemental dietary calcium, can be integrated into a single theoretic construct.

Theoretical mechanisms of dietary calcium's antihypertensive action

Theoretical mechanisms underlying dietary calcium's antihypertensive action are reviewed. Based upon known defects in the regulation of membrane Ca2+ transport and regulation of intracellular free Ca2+ concentration, we conclude that maneuvers that favorably modify calcium homeostasis such as dietary calcium supplementation or 1,25 (OH)2 vitamin D3 administration appear to favorably modify these defects. Consequent improvements in vascular smooth muscle function may mediate the reductions in blood pressure that follows chronic dietary calcium supplementation in experimental hypertension and in the clinical setting.