Previous studies have demonstrated a renal Na+ conservation during repeated days of exercise in the heat. The present study was intended to describe the role of plasma aldosterone (PAC) in reducing urine Na+ losses during and after 60 min of exercise (60% VO2max) in a warm environment (30 degrees C, 50-53% relative humidity). Additional measurements were made of plasma renin activity (PRA) in an effort to demonstrate the relationship between PRA and PAC. This study shows that a single bout of exercise can significantly reduce urine Na+, Cl-, and H20 excretions for up to 48 hours. Both PRA and PAC were significantly elevated during and immediately after exercise and returned to the pre-exercise level within 6-12 hours of recovery. Subsequently, ingestion of 180 mEq of Na+ each day with ad libitum water intake results in an increased NaCl storage and an expansion of the extracellular volume.
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Costill et al. (1976) studied this question.