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 concentration increased after 5-6 days of microgravity compared with to those of the acute supine position prior to flight. Furthermore, on the Russian space station Mir, we have observed that renal fluid excretion in two astronauts following an oral water load of 600 ml was attenuated compared with that of the acute supine position on the ground. Since it was surprising that the renal responses to isotonic saline loading and to an oral water load were 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 of head-down bed rest. The results indicate that the urinary flow rate following the water load of 600 ml is the same as in the acute supine position when the subjects have not been subjected to bed rest. This is in contrast to the results of the astronauts in space. The attenuated renal responses during space-flight could be explained by an increase in renal sympathetic nervous activity and in the elevated level of the renin-angiotensin-aldosterone axis observed during the space shuttle flight. This space-induced activation of antinatriuretic mechanisms could have been caused by early in-flight reduction in total and central blood volume. Based on our unexpected results from space, in future studies we will focus on the mechanisms of renal fluid excretion during space-flight. 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.