Calcium metabolism and sodium sensitivity in hypertensive subjects.

A pattern of negative calcium balanced with lowered levels of serum ionized calcium (Ca2+) and increased urinary excretion of calcium has been reported in hypertensive men. In the present study, ten untreated hypertensive subjects were salt loaded (20 g NaCl) for one week after a week on a low salt diet (< 3 g). The change in mean blood pressure (MBP) at the end of the high compared with the low salt diet was called salt sensitivity and was related to indexes of mineral metabolism. It was found that salt sensitivity was significantly correlated with both plasma ionized calcium (Ca2+) and serum calcium concentrations (both r = 0.64, P < 0.05) on the different diets. These relationships were strongest when sodium sensitivity was measured in the standing position during the low salt intake suggesting a role for an increased sympathetic tone. Salt loading increased the urinary excretion of calcium by 95% and also induced reductions in haemoglobin, serum albumin and serum calcium (P < 0.001). Ca2+, on the other hand, remained constant after salt loading. In conclusion, low levels of plasma ionized calcium and serum calcium were mainly found in hypertensive subjects with a low sensitivity to salt. Salt loading induced an increased calciuresis, haemodilution and possibly a shift of calcium from its protein-bound to its ionized form. The findings support the view that calcium metabolism is related to the regulation of BP.