Calcium and sodium transport and vitamin D metabolism in the spontaneously hypertensive rat

Calcium appetite, blood pressure and electrolytes in spontaneously hypertensive rats

Appetite for solutions of 0.01 M-0.1 M calcium chloride or calcium lactate were assessed using the two-bottle choice technique in spontaneously hypertensive (SHR) and Wistar-Kyoto (WKY) normotensive rats fed calcium replete diets. SHR exhibited a marginally increased preference for calcium chloride and a significantly increased preference for calcium lactate (p less than 0.02). In SHR, but not in WKY, 28 days of calcium exposure via the preference test solutions significantly affected systolic blood pressure (p less than 0.03). The group of SHR ingesting calcium chloride had a lower mean systolic blood pressure, and that ingesting calcium lactate had a higher mean systolic blood pressure than the control group receiving no exposure to calcium in preference tests. Blood pressures of individual rats, however, were not related to cumulative milliequivalents of Ca consumed, body weight, whole blood or serum ionized Ca, K or Na concentrations, or total serum calcium. Strain differences in chemosensory sensitivity might play a role in mediating the enhanced self-selection of calcium by SHR.

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.

Bone disorders in spontaneously hypertensive rat

The bones of adult (26 weeks old) spontaneously hypertensive rats (SHR) were examined chemically and histologically by comparing them with those of the corresponding normotensive Wistar-Kyoto (WKY) rats. The mean cortical thickness, the mean ash weight per unit bone volume, and the ash as percentage of dry weight of femur were significantly lower in SHR than in WKY. Besides, the percent cortical area measured on tibial cross-section was also reduced in SHR compared with WKY. These findings strongly suggest that the development of osteoporotic bone disorders exists in adult spontaneously hypertensive rats.

Calcium uptake by intestinal brush border membrane vesicles. Comparison with in vivo calcium transport

In prior studies, we examined kinetics of steady state in vivo transepithelial calcium transport in rat and hamster. The present studies related calcium uptake by the brush border to in vivo transport. We measured calcium uptake by brush border membrane vesicles from the two species. In the rat, our prior in vivo studies had shown that (a) calcium transport was mediated, (b) no nonmediated component was detectable, and (c) Vmax was 2.5 times greater in proximal than distal small intestine. In brush border membrane vesicles from the rat, Vmax for the saturable component of calcium uptake was again 2.5 times greater in proximal than distal intestine. Contrasting with in vivo studies, a major nonsaturable component was present in vesicles from proximal and distal small intestine. In the hamster, our previous in vivo studies had shown (1) both mediated and nonmediated components of calcium transport, (2) greater nonmediated transport in proximal than distal small intestines, and (3) Vmax for calcium transport twice as great in distal as in proximal small intestine. In the present study with brush border membrane vesicles from hamster, Vmax for saturable calcium transport was again twice as great in distal as in proximal small intestine. However, nonsaturable calcium transport rates relative to saturable rates were much greater with vesicles than in in vivo studies, and were greater in vesicles from distal than proximal small intestine. Since rates of saturable calcium uptake by brush border membrane vesicles parallel corresponding in vivo mediated transport rates, we conclude that the segmental rates of calcium transport in rat and hamster could be determined by brush border function.

Blood pressure development of the spontaneously hypertensive rat after concurrent manipulations of dietary Ca2+ and Na+. Relation to intestinal Ca2+ fluxes

The blood pressure of the spontaneously hypertensive rat (SHR) is influenced by the Ca2+ content of its diet. As the SHR's greater dependence on dietary calcium may reflect a defect in intestinal calcium absorption, we measured in vitro unidirectional Ca2+ flux (J) in the duodenum-jejunum (four segments each) of the SHR (n = 6) and the normotensive Wistar-Kyoto rat (WKY; n = 6) by a modified Ussing apparatus. Because of the known and postulated interactions between Ca2+ and Na+ in both intestinal and vascular tissue, we assessed in vivo the influence of a concurrent manipulation of Na+ intake (three levels: 0.25%, 0.45%, and 1.0%) on the blood pressure development of SHRs (n = 35) and WKYs (n = 35), between 6 and 20 wk of age, exposed to three levels of dietary calcium (0.1, 1.0, and 2%). Net calcium flux (Jnet) (mean +/- SEM) was significantly (P less than 0.01) lower in the SHR (-2.8 +/- 6.3 nmol/cm2 X h) than in the WKY (34.6 +/- 8.8 nmol/cm2 X h). The SHR's decreased Jnet resulted from a significantly (P less than 0.03) lower mucosa-to-serosa flux (Jm-s) in the SHR (41.0 +/- 5.6 nmol/cm2 X h) compared with the Jm-s of the WKY (70.1 +/- 9.1 nmol/cm2 X h). Serosa-to-mucosa flux for calcium did not differ between the SHR (43.8 +/- 6.6 nmol/cm2 X h) and the WKY (35.5 +/- 8.0 nmol/cm2 X h). The SHR's decreased (P less than 0.002) Jm-s was confirmed by additional measurements in SHRs and WKYs. Jm-s was 36.2 +/- 3.7 nmol/cm2 X h in the SHRs (n = 11) and 64.4 +/- 6.7 nmol/cm2 X h in the WKYs (n = 9). The provision of an increased dietary Ca2+ (2% by weight) and increased Na+ (1%) to the SHR prevented the emergence of hypertension (P less than 0.001) (mean +/- SEM systolic blood pressure at 20 wk of age; 135 +/- 5 mmHg for the 2% Ca2+, 1% Na+ SHR vs. 164 +/- 2 mmHg for the control diet SHR). Ca2+ (0.1%) and Na+ (0.25%) restriction accelerated the SHR's hypertension (192 +/- 2 mmHg) (P less than 0.001) and was associated with higher pressures in the WKY (146 +/- 4 mmHg in the restricted WKY vs. 134 +/- 4 mmHg in the control WKY). In a parallel group of 24 SHRs and 24 WKYs fed one of three diets (2% Ca2+/1% Na+; 1% Ca2+/0.45% Na+; or 0.1% Ca2+/0.25% Na+), the heart (P < 0.05) and kidney (P = 0.08) weight of the SHRs varied depending on the diet at 20 wk of age. Low Ca2+ and Na+ intake was associated with increased heart weight (1.6+/-0.9 g) compared with the normal diet for SHR (1.51+/-0.07 g). Increased Ca2+ and Na+ intake was associated with a significantly (P = 0.05) lower heart weight in the SHR (1.37+/-0.03 g) and in the WKY (1.35+/-0.06 g) compared with their normal diet controls. These findings show one mechanism for the SHR's depressor response to supplemental dietary Ca2+ and, in part, explain the sodium dependence of calcium's cardiovascular protective effect.

Is calcium more important than sodium in the pathogenesis of essential hypertension?

The hypothesis that abnormalities of calcium homeostasis at both an organ and cellular level are a primary factor in the pathogenesis of human and experimental hypertension forms the basis of this review. The rapidly expanding data base relating disordered calcium metabolism to altered vascular smooth muscle function and increased peripheral vascular resistance is summarized and integrated with the observations that reduced dietary calcium intake is the most consistent nutritional correlate of hypertension in the United States. The role of sodium and sodium chloride in pathogenesis of hypertension is reassessed in the light of new data from epidemiological clinical research, experimental models, and cell physiology investigations. The data supporting the thesis that the effects of sodium or chloride or both on blood pressure may represent, in selected situations, secondary influences mediated through induced changes in calcium homeostasis are presented. The interface between these nutritional factors and the normal regulation of vascular smooth muscle is discussed, providing a theoretical framework in which to assess the current information and to formulate the necessary future research.

Erythrocyte membrane Ca2+-pumping ATPase of hypertensive humans: reduced stimulation by calmodulin

The Ca2+-pumping ATPase of erythrocyte plasma membranes of hypertensive humans (HTN) show, in the absence of calmodulin, a low Vmax comparable to that of the enzyme of the erythrocyte membranes of normotensive humans (NTN). Although the addition of calmodulin (1.5 micrograms per ml) increased the maximum activity of the calcium pump of membranes of HTN and NTN individuals by at least 2-fold and 4-fold, respectively, the activator protein partially purified from the erythrocytes of HTN individuals enhanced the activity of the enzyme in a fashion similar to that of the protein obtained from the haemolysate of NTN individuals. A determination of the dependence of the activity of the pump on concentration of ATP revealed that the Km (ATP) of the enzyme of membranes of HTN individuals is 52% higher than that of the enzyme of membranes of NTN individuals, while the Vmax (1.75 +/- 0.28 mumol ATP mg protein-1 h-1) of the pump is 46% lower in the membranes of HTN humans than that of the enzyme of membranes of normal individuals (3.25 +/- 0.42 mumol ATP mg protein-1 h-1). It seems likely from these results that elevated erythrocyte Ca2+ concentration associated with essential hypertension may be due to a defective interaction between the Ca2+-pumping ATPase and the calmodulin Ca2+ complex.

Calcium in the pathogenesis and therapy of human hypertension

The contribution of calcium to normal regulation of blood pressure is reviewed. Epidemiologic data suggest that abnormal calcium homeostasis may play a role in dietary and environmental contributions to the risk of hypertension and cardiovascular disease. Recent studies in human and experimental hypertension point to one or more defects in cellular handling of calcium as contributing to the initiation and maintenance of increased vascular tone and, thereby, to abnormally elevated arterial blood pressure. By "correcting" some of these defects, calcium channel blockers may represent relatively specific therapy for hypertension. Data from human trials are still limited, but it is evident that, following oral administration of nifedipine and related compounds, an immediate decrease in peripheral vascular resistance and mean arterial pressure is produced. Cardiac output is not altered, and renal perfusion is maintained. In contrast to many currently used antihypertensive agents--including thiazide diuretics and beta blockers--which frequently induce alterations in the patient's serum chemical values, long-term administration of calcium channel blockers does not produce significant adverse metabolic effects or substantive changes in biochemical parameters.