Diurnal renal responses in man to water loading at sea level and 31 atm abs

The hyperbaric environment causes a sustained diuresis accompanied by normal water intake and a decrease in insensible water loss. The maintained water intake may be necessary for the maintenance of water balance because of a reduced ability of the kidney to retain water, or may be causal in the diuresis. This problem was studied in four male subjects. Each ingested 1 liter of water (15 degrees C) at 0800 and 2000 h on different days, at 1 atm abs during a predive control, at 31 atm abs, and at 1 atm abs during the postdive control period. Urine was collected 30 min before and 3 h after the drink. Plasma vasopressin (VP) showed a circadian rhythm only at 1 atm abs, higher during the daytime. Because of this, and slightly lower VP levels at hyperbaria, a decrease in VP in response to the water load was significantly detectable only at 1 atm abs during the daytime. At 60 min after all water loads, there were no differences in plasma VP or plasma or urinary osmolality. Variability in the length of time of similarly reduced urine osmolality and increased free water excretion accounted for the increased urine flow during day compared to night (P < 0.05) at 120 and 150 min after the water load. The ability to excrete a water load both day (free water clear-ance, P < 0.05 at 60 min post-drink) and night (free water clearance, P < 0.05 at 60, 90, and 120 min post-drink) at 31 atm abs was enhanced. It is concluded that maintained water intake at hyperbaria is necessary to maintain water balance because there is a reduced ability of the body through renal mechanisms to retain a water load.