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The present study tests the hypothesis that dietary Ca2+ supplementation increases acute diuretic and natriuretic responses to volume loading in the NaCI-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% NaCI, 0.68% Ca2+ ); high NaCI (8% NaCI, 0.68% Ca2+ ); high Ca2+ (0.75% NaCI, 2.0% Ca2+ ); and high NaCI and Ca2+ (8% NaCI, 2.0% Ca2+ ). SHR-S on high Ca2+ and on high NaCI and Ca2+ diets for 2 weeks displayed significantly lower systolic arterial pressures (SAP) than SHR-S on basal and high NaCI 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 NaCI, 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 NaCI 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 NaCI (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 NaCI 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 NaCI 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 NaCI 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 NaCI sensitive forms of hypertension.