Suppression of ADH during water immersion in normal man

Since previous studies have demonstrated that the redistribution of blood volume and concomitant relative central hypervolemia induced by water immersion to the neck causes a profound natriuresis and a suppression of the renin aldosterone system, it was of interest to assess whether the diuresis induced by immersion was mediated by an analogous inhibition of antidiuretic hormone (ADH). The effects of water immersion on renal water handling and urinary ADH excretion were assessed in 10 normal male subjects studied following 14 hr of overnight dehydration on 2 occasions, control and immersion. The conditions of seated posture and time of day were identical. During control ADH persisted at or above prestudy values. Immersion resulted in a progressive decrease in ADH excretion from 80.1 ± 7 (SEM) to 37.3 ± 6.3 μU/min (P&0.025). Cessation of immersion was associated with a marked increase in ADH from 37.3 ± 6.3 μU/min to 176.6 ± 72.6 μU/min during the recovery hour (P<0.05). Concomitant with these changes urine osmolality decreased significantly beginning as early as the initial hour of immersion from 1044 ± 36 to 542 ± 66 mosmol/kg H₂O during the final hour of immersion (P<0.001). Recovery was associated with a significant mean increase in U(osm) of 190 ± 40 mosmol/kg H₂O over the final hour of immersion (P<0.001). The suppression of ADH occurred without concomitant changes in plasma tonicity. These studies are consistent with the suggestion that in hydrated subjects undergoing immersion suppression of ADH release contributes to the enhanced free water clearance, which has been previously documented.