Renal responsiveness to aldosterone during exposure to simulated microgravity

We measured renal functions and hormones associated with fluid regulation a fter a bolus injection of aldosterone (Ald) during head-down tilt (HDT) bed rest to test the hypothesis that exposure to simulated microgravity altere d renal responsiveness to Aid. Six male rhesus monkeys underwent two experi mental conditions (HDT and control, 72 h each) with each condition separate d by 9 days of ambulatory activities to produce a crossover counterbalance design. One test condition was continuous exposure to 10 degrees HDT; the s econd was a control, defined as 16 h per day of 80 degrees head-up tilt and 8 h prone. After 72 h of exposure to either test condition, monkeys were m oved to the prone position, and we measured the following parameters for 4 h after injection of l-mg dose of Ald: urine volume rate (UVR); renal Na+/K + excretion ratio; renal clearances of creatinine, Na+, osmolality, and fre e water; and circulating hormones [Ald, renin activity (PRA), vasopressin ( AVP), and atrial natriuretic peptide (ANP)I. HDT increased Na+ clearance, t otal renal Na+ excretion, urine Na+ concentration, and fractional Na+ excre tion, compared with the control condition, but did not alter plasma concent rations of Aid, PRA, and AVP. Administration of Ald did not alter UVR, crea tinine clearance, Aid, PRA, AVP, or ANP but reduced Na+ clearance, total re nal Na+ excretion, urinary Na+/K+ ratio, and osmotic clearance. Although re ductions in Na+ clearance and excretion due to Aid were greater during HDT than during control, the differential (i.e., interaction) effect was minima l between experimental conditions. Our data suggest that exposure to microg ravity increases renal excretion of Na+ by a natriuretic mechanism other th an a change in renal responsiveness to Aid.