Responses of salt- and water-regulating hormones during a saturation dive to 31 ATA (SEADRAGON IV).

Four Japanese male subjects were studied during 3 days at 1 ATA, 3 days of compression to 31 ATA (1000 fsw), 14 days at 31 ATA, 12 days of decompression, and 3 days of postdive control at 1 ATA. The chamber was maintained at thermoneutral temperatures. During the 31-ATA exposure, urine flow increased about 500 ml/day (P less than 0.05) accompanied by an approximate 500-ml/day increase in osmotic clearance (Cosm) (P less than 0.05). Both urine flow and Cosm increases were primarily attributable to an increase in the overnight flow rates (P less than 0.01). The negative free water (-CH2O)/Cosm decreased during exposure to 31 ATA, indicating that a reduction in tubular water reabsorption may also contribute to the diuresis. Urine flow, Cosm, and -CH2O/Cosm all returned to predive values during decompression to 1 ATA. The urinary excretion rate of aldosterone increased from 2.7 +/- 0.3 micrograms/day at 1 ATA to 4.3 +/- 0.0 micrograms/day (P less than 0.01) at 31 ATA, remaining at about 3.8 micrograms/day until decompression. Urinary antidiuretic hormone (ADH) decreased from 50 +/- 7 to 33 +/- 3 mU/day (P less than 0.01) upon compression to 31 ATA and continued to decrease throughout the decompression phase. Plasma renin activity was increased by twofold (P less than 0.01) and plasma aldosterone by about 37% (NS) during exposure to 31 ATA. It is concluded that the reduction in ADH does not contribute significantly to the hyperbaric diuresis, and that the increased activity of the renin-angiotensin-aldosterone is a result of other postulated mechanisms resulting in an increased Cosm.