Circadian variations of urine excretion and adrenocortical function in different states of hydration in man

On 3 different days, the circadian rhythms of urinary volume and cortisol excretion were measured under different conditions of water supply in healthy male subjects. The water intake was systematically varied (100, 350, and 600 ml/3 hr of tap water) starting 6 hr before the beginning of the measurements, which were carried out under constant bed rest in a climatic chamber. Every 3 hr, a standardized low-protein meal was administered. The daily courses of cortisol excretion showed an increase of the mesor, which was proportional to the amount of water intake. The absolute amplitude was significantly higher when 600 ml/3 hr was taken. The circadian acrophase was shifted in advance, and the slope of the curves after the nightly minimum was significantly increased in the experiments with 350 and 600 ml/3 hr fluid intake. The urinary volume showed analogous modifications. These results are interpreted as a nonspecific activation of the adrenal cortex and a circadian variation of the sensitivity of renal regulation of the water balance.