Renal and sympathoadrenal responses in space

According to a classic hypothesis, weightlessness should promote the renal excretion rate of sodium and water nd lead to a fluid- and electrolyte-depl eted state. This hypothesis is based on experiments in which weightlessness has been simulated in humans by head-down bed rest and water immersion. Ho wever, after 5 to 6 days of space mission, the diuretic and natriuretic res ponses to an intravenous isotonic saline load were attenuated and plasma no repinephrine and renin concentrations increased compared with those of the acute supine position before flight. Renal fluid excretion after an oral wa ter load was also attenuated in space. Similar decreases were not observed during head-down bed rest. Sympathetic activity is of major importance in r egulating blood volume and renal function. Studies in space have indicated that, compared with that while in a supine position on Earth, sympathoadren al activity is increased during space flights as measured using plasma conc entration and urinary excretion of norepinephrine and epinephrine. The spac e-induced activation of antinatriuretic mechanisms and sympathoadrenal acti vity could have been caused by early in-flight reduction in total and centr al blood volume. The decreased plasma volume may be explained by such facto rs as redistribution of plasma from the lower to the upper body (thin legs and puffy face), reduced food intake, and decreased muscle activity. The de crease in plasma volume and the subsequent increase in sympathetic activity is due, at least in part, to the abrupt cessation of activity in large mus cle groups during microgravity, which normally counteracts the effects of g ravity in the upright posture. This would lead to accumulation of albumin a nd fluid in the interstitial space. (C) 2001 by the National Kidney Foundat ion, Inc.