Effect of saline loading during heat acclimatization on adrenocortical hormone levels.

Six male subjects were acclimatized to heat; once they were given sufficient 1% saline to prevent the occurrence of a salt deficit during acclimatization, and another time they were given no saline. Plasma aldosterone (PA), plasma cortisol (PC), plasma renin activity (PRA), and plasma electrolytes were measured before, during, and after and sweat electrolytes before and after the 11-day acclimatization program. PRA and PA were significantly increased by the acute stress of heat and exercise but were unaffected by acclimatization. These increases were attenuated, but not prevented, by drinking saline, whereas sweat [Na] and PC were reduced by acclimatization but were unaffected by saline. Thus adrenocortical activity has been shown not to be increased after heat acclimatization, and mineralocorticoid activity, although potentiated by a Na deficit, appears to be determined primarily by the acute stress of heat and of exercise. Hence, the increased Na conservation with acclimatization is likely to be a normal response to heat and exercise even in the absence of a negative Na balance.