Dietary calcium, blood pressure and cell membrane cation transport systems in males

A double-blind, placebo-controlled parallel-group study was conducted on the effect of a high level of daily oral calcium supplementation (1 g elemental calcium given twice a day for 16 weeks) in normal male subjects on blood pressure, intracellular cationic concentrations and transmembrane cation transport systems, plasma total and ionized calcium, and calciotropic hormones.

After a 1-month run-in period with a limited intake of dairy products, the 32 subjects were allocated to a placebo or a calcium group. Placebo or 1 g elemental calcium was administered twice a day, in the morning and evening, for 16 weeks. All subjects were investigated at baseline and after 1, 2, 4, 8 and 16 weeks of placebo or calcium administration.

Compared with the placebo group, standing systolic blood pressure was decreased in the calcium group, whereas the standing diastolic blood pressure tended to decrease. The changes in supine systolic and diastolic blood pressure did not differ between the placebo and calcium groups. Decreased intra-erythrocyte and intraplatelet sodium and calcium concentrations, an increased activity of platelet and erythrocyte sodium-pump activity and a reduced membrane cholesterol content were observed in the calcium-treated subjects. Erythrocyte membrane surface and core microviscosity, however, did not change during calcium supplementation. Oral calcium supplementation in these men was accompanied by a reduction in the plasma concentrations of intact parathormone and 1,25-dihydroxyvitamin D3, and an increase in 24 h urinary calcium excretion, but no change in the plasma total calcium concentration, serum ionized calcium level, or plasma phosphate or 25-hydroxyvitamin D3. The intra-erythrocyte and intraplatelet potassium and magnesium concentrations as well as the activities of the erythrocyte Na,Li-countertransporter and Na,K-cotransporter, and sodium and potassium leakage did not change during calcium administration.

The lowering of standing blood pressure seen in men with a high calcium intake is accompanied by a decrease in cytosolic free platelet calcium and total erythrocyte calcium, by a reduction in intraplatelet and intra-erythrocyte sodium concentration and erythrocyte membrane cholesterol and by an increase in the activity of the erythrocyte and platelet sodium-pump.