Influence of head-down bed rest on the circadian rhythms of hormones and electrolytes involved in hydroelectrolytic regulation

We investigated in six men the impact of a 17-day head-down bed rest (HDBR) on the circadian rhythms of the hormones and electrolytes involved in hydr oelectrolytic regulation. This HDBR study was designed to mimic an actual s paceflight. Urine samples were collected at each voiding before, during and after HDBR. Urinary excretion of aldosterone, arginine vasopressin (AV-P), cyclic guanosine monophosphate (cGMP), cortisol, electrolytes (Na+ and K+) and creatinine were determined. HDBR resulted in a significant reduction o f body mass (P<0.01) and of caloric intake [mean (SEM) 2,778 (37) kcal.24 h (-1) to 2,450 (36) kcal.24 h(-1), where, 1 kcal.h(-1) = 1.163 J.s(-1) P<0.0 1]. There was a significant increase in diastolic blood pressure [71.8 (0.7 ) mmHg vs 75.6 (0.91) mmHg], with no significant changes in either systolic blood pressure or heart rate. The nocturnal hormonal decrease of aldostero ne was clearly evident only before and after HDBR, but the day/night differ ence did not appear during HDBR. The rhythm of K+ excretion was unchanged d uring HDBR, whereas for Na excretion, a large decrease was shown during the night as compared to the day. The circadian rhythm of cortisol. persisted. These data suggest that exposure to a 17-day HDBR could induce an exaggera tion of the amplitude of the Na+ rhythm and abolition of the aldosterone rh ythm.