Effect of intragastric and intraperitoneal water and saline loads on the pharmacologic induction of drinking in rats

Water intake, induced by subcutaneous (SC) administration of angiotensin II (AII, 200 micrograms/kg), isoproterenol (25 micrograms/kg), 5-hydroxytryptophan (25 mg/kg), and bethanechol (8 mg/kg), was reduced completely to control level by administration of either an intragastric (IG) or intraperitoneal (IP) load of distilled water (3% of body weight) prior to administration of the dipsogen. A similar load of isotonic saline given by both the IG and IP routes was either ineffective or only partially effective in reducing the dipsogenic response. The similar effectiveness of IP and IG loads of water in inhibiting the responsiveness to administration of dipsogenic agents argues against a gastrointestinal mechanism as an integral part of the feedback process limiting water intake in the rat. In addition, the lack of a significant effect of similar IP and IG loads of saline suggests that stretch of either the gastrointestinal or intraperitoneal spaces is not an important factor in the inhibition observed. A possibility exists that osmotic dilution, as a result of the water load, may play a role in the feedback inhibition of the drinking responses to the dipsogenic agents used in these studies, even though the responses were mediated via the AII receptor pathway rather than the osmoreceptor pathway.