Renal adjustments to microgravity

During a 10-day shuttle mission, the diuretic and natriuretic responses to an i.v. isotonic saline load were attenuated and plasma noradrenaline conce ntration increased after 5-6 days of microgravity compared with to those of the acute supine position prior to flight. Furthermore, on the Russian spa ce station Mir, we have observed that renal fluid excretion in two astronau ts following an oral water load of 600 mi was attenuated compared with that of the acute supine position on the ground. Since it was surprising that t he renal responses to isotonic saline loading and to an oral water load wer e attenuated during space-flight, we carried out a study in Japan employing the same water load protocol as in space in eight subjects after 19 days o f head-down bed rest. The results indicate that the urinary flow rate follo wing the water load of 600 mi is the same as in the acute supine position w hen the subjects have not been subjected to bed rest. This is in contrast t o the results of the astronauts in space. The attenuated renal responses du ring space-flight could be explained by an increase in renal sympathetic ne rvous activity and in the elevated level of the renin-angiotensin-aldostero ne axis observed during the space shuttle flight. This space-induced activa tion of antinatriuretic mechanisms could have been caused by early in-fligh t reduction in total and central blood volume. Based on our unexpected resu lts from space, in future studies we will focus on the mechanisms of renal fluid excretion during spaceflight. An experiment with the participation of American, European, Russian, and Japanese researchers has been selected in this regard for the early phase of the International Space Station.