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 maint ained water intake may be necessary for the maintenance of water balan ce 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 subje cts. 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 ab s, and at 1 atm abs during the postdive control period. Urine was coll ected 30 min before and 3 h after the drink. Plasma vasopressin (VP) s howed a circadian rhythm only at 1 atm abs, higher during the daytime. Because of this, and slightly lower VP levels at hyperbaria, a decrea se in VP in response to the water load was significantly detectable on ly at 1 atm abs during the daytime. At 60 min after all water loads, t here were no differences in plasma VP or plasma or urinary osmolality. Variability in the length of time of similarly reduced urine osmolali ty and increased free water excretion accounted for the increased urin e flow during day compared to night (P < 0.05) at 120 and 150 min afte r the water load. The ability to excrete a water load both day (free w ater 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 w as enhanced. It is concluded that maintained water intake at hyperbari a is necessary to maintain water balance because there is a reduced ab ility of the body through renal mechanisms to retain a water load.