Osmoregulation during high salt intake: relative importance of drinking and vasopressin secretion.

Studies determined the relative contribution of drinking vs. vasopressin secretion in the regulation of extracellular osmolality in response to changes of Na intake. Daily Na intake was increased from 30 to 200 meq in dogs maintained under three conditions: normal dogs with ad libitum drinking, normal dogs with "fixed drinking," and neurohypophysectomized dogs with "fixed drinking" and vasopressin replaced by continuous infusion. (Drinking was fixed to that amount consumed during the normal Na control period.) The mechanisms of osmoregulation were highly nonlinear. As daily Na intake increased from 30 to 100 meq, renal natriuretic mechanisms predominated with only small contributions from either the thirst or vasopressin systems. At high levels of Na intake (200 meq/day), both drinking and vasopressin release contributed significantly to osmoregulation. The studies also determined that, in the absence of excess vasopressin secretion and increased drinking, plasma osmolality rose to nearly twice the levels as those observed in normal dogs that increased vasopressin secretion. We conclude that vasopressin-related renal conservation of water contributes to buffering the rise of osmolality when Na intake is increased without increased drinking. The studies also confirm that with available water to drink, the thirst mechanism together with renal Na excretory mechanisms are the predominant controllers of osmolality in situations of high sodium intake.