Influence of daily sodium intake on vasopressin secretion and drinking in dogs

Studies were carried out to determine the relationship between daily sodium intake, drinking, and vasopressin (AVP) secretion in normal conscious dogs. Chronic responses to 5-day elevations of daily sodium intake (200 meq/day) and 2-wk decreases in daily sodium intake (5 meq/day) were determined. Dogs were studied with ad libitum drinking and with water intake restricted to the amount drunk during the normal-sodium (30 meq/day) control period. Although acute elevations of plasma AVP occurred after a normal (40 meq Na) gastric load, chronic high-sodium intake resulted in no change of steady-state plasma AVP levels or daily AVP excretion (UAVP) with ad libitum drinking. Total water intake and frequency of drinking, however, increased nearly fourfold. In the absence of excess drinking, plasma AVP and UAVP both exhibited a nearly sixfold increase during the period of high-sodium intake. Despite elevations of plasma AVP, daily urine volume increased and urine osmolality rose only gradually during the 5 days of high-sodium intake. Chronic low-sodium intake also did not alter plasma AVP, but total water intake was reduced 20%. The data indicate that with water available, extracellular osmolality is controlled predominantly by drinking rather than by AVP secretion, that either osmolality or sodium concentration is the predominant controller of drinking and AVP secretion, and that daily water excretion need not be related directly to plasma AVP.