Metering of fluid intake and determinants of ad libitum drinking in rats

The contributions of homeostatis mechanisms to spontaneous drinking were assessed in a study of residual oral drinking under several conditions of exogenous water administration. Continuous and/or discontinuous infusiors were conducted for long periods with dry food present ad libitum. The routes of administration were intravenous (IV), intragastric (IG) through a nasopharyngeal catheter. All infusions reduced from noninfused levels, but the magnitude of that reduction was a function of both route and temporal characteristics. Continuous infusions were more effective via the IG route than IV, and a residual intake of about 10 ml/24 h presisted at even the highest infusion rates. Discontinuous meal-paired IV infusions suppressed mid- and postmeal drinking, consistent with the satisfaction of systemic needs induced by cry food intake. Meal-paired IG nasopharyngeal infusions competely suppressed drinking when the infusions exceeded base-line intake by only about 20%. In marked contrast, corresponding IG infusions through a direct fistula catheter were relatively ineffective. In all cases the decreases in drinking were not of a nonspecific nature beacuse food intake was unchanged. The discussion considers the different levels of metering involved and the time-varying (derivative) nature of the infusion if was suggested that there is a nonhomeostatic contribution to normal drinking.