Dietary ca²⁺ increases natriuretic and diuretic responses to volume loading in nacl-sensitive spontaneously hypertensive rats

The present study tests the hypothesis that dietary Ca²⁺ supplementation increases acute diuretic and natriuretic responses to volume loading in the NaCl-sensitive spontaneously hypertensive rat (SHR-S). Seven week old male SHR-S and normotensive Wistar—Kyoto rats (WKY) were fed one of the following diets for 2.5 weeks: basal (0.75% NaCl, 0.68% Ca² +); high NaCl (8% NaCl, 0.68% Ca^{2 +}); high Ca²⁺ (0.75% NaCl, 2.0% Ca^{2 +}); and high NaCl and Ca²⁺ (8% NaCl, 2.0% Ca^{2 +}). SHR-S on high Ca²⁺ and on high NaCl and Ca²⁺ diets for 2 weeks displayed significantly lower systolic arterial pressures (SAP) than SHR-S on basal and high NaCl diets, respectively. WKY displayed no diet-related change in SAP at any time during the study. After 2.5 weeks on the diets, pre-instrumented, conscious SHR-S and WKY received an intravenous infusion (5% body weight) of isotonic NaCl, and urine was collected through a bladder catheter for a period of 90 min. The infusion did not alter the heart rate or the mean arterial pressure in any group. WKY on the high NaCl diet excreted a significantly greater percentage of the volume and Na⁺ load than WKY on the basal diet. In contrast, SHR-S on the high NaCl (compared to basal) diet did not display significantly enhanced natriuresis or diuresis. SHR-S on the basal diet displayed excretion rates similar to WKY on the basal diet. Dietary Ca²⁺ supplementation significantly increased the natriuretic and diuretic responses to saline infusion in SHR-S on the high NaCl diet, but not in SHR-S on the basal diet or in WKY on either diet. These results indicate that SHR-S on a high NaCl diet have impaired diuretic and natriuretic responses to an isotonic volume load and that dietary Ca²⁺ supplementation increases the ability of SHR-S on a high NaCl diet to adjust fluid volume rapidly via diuresis and natriuresis. This alteration in renal function may contribute to the hypotensive effect of a high Ca²⁺ diet in NaCl sensitive forms of hypertension.