Chronic low-sodium diet in rats: hormonal and physiological effects during exercise in the heat.
To elucidate the effects of sodium (Na+) deficiency on the ability to work in the heat, immature rats were fed a diet deficient in Na+ for approximately 2 mo. Rates of weight gain were severely affected (P less than 0.01) in the Na+-deficient rats (1.7 vs. 7.2 g/day in controls), although fluid consumption was unaffected. The low-Na+ diet effected no alterations in endurance or weight loss during exercise in the heat to hyperthermic exhaustion, but final core and skin temperatures were significantly reduced in the low-Na+ group (P less than 0.02) and hematocrit ratios were significantly (P less than 0.001) increased. Circulating Na+ and potassium (K+) concentrations were significantly (P less than 0.05) increased in both groups after hyperthermic exhaustion. In the Na+-deficient groups, plasma levels of both aldosterone and cortisol/corticosterone were significantly (P less than 0.05) increased, and these increments were exacerbated following exercise to hyperthermic exhaustion. Consumption of the low-Na+ diet elicited significant increments in circulating levels of lactate (P less than 0.01) and creatinine (P less than 0.01), both of which were increased further after exercise. We concluded that hormonal adaptations prevented circulatory hyponatremia and decrements in physical performance, but other clinical and metabolic effects of the low-Na+ diet were noted.