Reductions in total and extracellular water associated with calcium-induced natriuresis and the antihypertensive effect of calcium in blacks.

Previous data from this laboratory suggest that Ca-induced reductions in blood pressure may result in part from Ca-induced natriuresis and a possible reduction in intravascular volume. Consequently, the present study was conducted to directly determine the relative effects of dietary Ca and Na on total body water (TBW) and extracellular water (ECW) as measured by tetrapolar bioelectrical impedance and to determine the potential relationship of these changes to diet-induced changes in blood pressure. Eleven hypertensive black adults were maintained for 14 days on each of four diets that contained 356 mg Ca or 1,000 mg Ca, each at 1,000 or 4,000 mg Na in a repeated measures design. Increasing dietary Na at the low Ca intake caused significant increases in supine systolic and diastolic pressure, reduced plasma renin activity (PRA), significantly increased TBW, and caused a significant reduction in total body reactance indicating an increase in ECW. Adding supplementary Ca to the low Na diet was without significant effect on blood pressure, PRA, TBW, or electrical reactance. In contrast, adding Ca to the low Ca-high Na diet caused a significant natriuretic effect that was accompanied by significant increases in PRA and electrical reactance and significant reductions in blood pressure and TBW, to baseline (low Ca-low Na levels). Thus, these data indicate that the antihypertensive effect of calcium in salt-sensitive blacks may be attributable, in part, to Ca-induced natriuresis and an associated volume contraction.