AJP - Regulatory, Integrative and Comparative Physiology, Vol 251, Issue 5 878-R886, Copyright © 1986 by American Physiological Society
Osmoregulation during high salt intake: relative importance of drinking and vasopressin secretion
A. W. Cowley Jr, M. M. Skelton and D. C. Merrill
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.
