Chronic low-sodium diet in rats: responses to severe heat exposure

To determine the effects of sodium (Na+) deficiency on the responses to severe heat stress (35.5 degrees C), immature (mean wt 150.4 g) male rats (n = 21) were fed a low-Na+ diet for 71 days. Rates of weight gain and food consumption were significantly (P less than 0.001) reduced in the low-Na+ group, whereas water consumption was unaffected. Prior to heat exposure circulating Na+ levels were unaffected by dietary Na+ restriction, but both circulating potassium (K+) and hematocrit levels were significantly (P less than 0.001) increased. After 24-h exposure to severe heat stress, circulating Na+ levels did manifest a significant (P less than 0.001) decrement in the low-Na+ group. K+ levels increased significantly (P less than 0.01) in the control group after 6 h of heat exposure but remained depressed in comparison with the low-Na+ group after 48 and 72 h. Although plasma renin activity (PRA) was not increased by chronic consumption of the low-Na+ diet or by severe heat exposure in the control group, severe heat stress in the low-Na+ group did elicit significant (P less than 0.005) increments in PRA after 24 h of exposure. Alternatively, plasma aldosterone levels were significantly (P less than 0.001) elevated by both the low-Na+ diet and heat stress. We concluded from these studies that chronic consumption of the low-Na+ diet had severe effects on hematologic, endocrinological, and thermoregulatory variables as well as thermal sensitivity to prolonged and sedentary exposure to severe heat stress.