REDUCED NATRIURESIS DURING WEIGHTLESSNESS

The kidney response to weightlessness was measured in one volunteer du ring a 1-week space mission. Shortly after entering microgravity and l ater during the mission, consecutive urine sampling periods were monit ored, covering in total about 50% of the inflight time. Preflight refe rences were a sequence of ground-based experiments, which evaluated bo dy fluid metabolism with different degrees of standardization. Additio nal variables, such as circadian rhythms and cortisol-associated stres s, were also monitored. In contrast to current hypotheses, the volunte er showed a pronounced reduction in natriuresis and diuresis during th e entire space flight, despite a considerable weight loss. For the fir st time, the urinary excretion of the renal natriuretic peptide urodil atin was also measured. Both, during the preflight experiments and dur ing weightlessness, close correlations between urodilatin excretion an d sodium excretion were observed. However, the correlation between nat riuresis and urodilatin excretion was considerably altered during weig htlessness. We conclude that the loss of body weight during space flig ht is not related to an increased renal fluid loss and that urodilatin might counteract the decrease in renal excretion observed in weightle ssness.