Mechanism for changes in vasopressin during acute exposure at 3 atm abs air

Plasma arginine vasopressin (AVP) concentration is reduced in human subjects during prolonged saturation dive exposures of 4 atmospheres absolute (atm abs) and greater. The objectives of the present study were to determine if AVP would be reduced in eight male subjects during a 1-h exposure of 3 atm abs air and, if so, to determine the mechanisms responsible for the AVP response. Assessments of transmural central venous pressure (central venous pressure-esophageal pressure) and cardiac volume measurements were made to evaluate the possible role of cardiopulmonary receptors on the AVP response. Also, plasma osmolality (P(osmol)), venous blood gases, and mean corpuscular volume (MCV) were determined to evaluate potential effects of osmoreceptor and other fluid shifts on AVP release. AVP decreased (P < 0.05) by 0.5 microU/ml at 3 atm abs, whereas the transmural central venous pressure and cardiac volume remained unchanged throughout the experimental periods. A significant reduction (P < 0.05) in P(osmol) (by approximately 3 mosmol/kgH2O) was detected at 3 atm abs. Therefore, we conclude that the reduction in P(osmol) may cause the reduction in AVP during exposure to 3 atm abs pressure. The reduction in P(osmol) without water intake requires the postulation of an internal source of water. We propose that the threefold increase (P < 0.01) in venous PO2 and concomitant decrease (P < 0.05) in venous MCV suggest that the red blood cell may contribute to hypotonicity at 3 atm abs.