RENAL AND ENDOCRINE RESPONSES IN HUMANS TO ISOTONIC SALINE INFUSION DURING MICROGRAVITY

It was the purpose of this study to investigate how the endocrine and renal mechanisms of fluid volume control in humans (n = 4) adapt to mi crogravity by applying an intravenous isotonic saline infusion. The ac ute ground-based supine (Sup) and seated (Seat) positions were chosen as references. During microgravity, renal sodium excretion (UNaV) was doubled during the second and third hours after infusion compared with during Seat (P < 0.05) but blunted during the first hour after infusi on compared with during Sup, leading to a reduction in cumulative UNaV (59 +/- 15 vs. 108 +/- 12 mmol/5 h; P < 0.05). Plasma norepinephrine (NE) attained the highest value 3 h after infusion during microgravity (31 +/- 5 x 10(-2) ng/ml vs. 19 +/- 1 and 13 +/- 3 x 10(-2) ng/ml for Seat and Sup, respectively; P < 0.05). Inflight levels of plasma reni n and aldosterone were very similar to levels during Seat. In conclusi on, 1) the microgravity-adapted renal responses to infusion reflected a condition in between that of groundbased Seat and Sup, respectively, and 2) the plasma levels of NE, renin, and aldosterone were elevated inflight and not related to the changes in UNaV and urinary flow rate. These observations are in contrast to results of ground-based simulat ion experiments and might partly have been caused by a prior inflight reduction in extracellular fluid volume. The high levels of NE during microgravity warrant further investigation.