Early osmoregulatory signals in the control of water intake and neurohypophyseal hormone secretion.

Dehydrated dogs inhibit secretion of vasopressin (VP) within minutes after drinking water, before plasma osmolality (pOsm) diminishes. In recent studies, we found that water ingestion by rats similarly inhibits VP and oxytocin (OT) secretion rapidly, before pOsm is diluted. Adult male rats were infused with 1 M NaCl (2 ml/h iv) for 240 min to stimulate VP and OT secretion. After 220 min of infusion, rats were given water or isotonic saline (IS) to drink for 5 min, and blood samples were taken 5 and 15 min later. Plasma levels of VP (pVP) and OT (pOT) were much lower when rats ingested water instead of IS, even though rats drank comparable amounts of both fluids ( approximately 5.5 ml) and pOsm was not significantly affected in either case. In another study, rats were infused with 1 M NaCl (2 ml/h iv) for 120 min before receiving 4-ml gastric loads of either 0.5 M NaCl (HS) or IS; blood samples taken 25 min later showed that pVP and pOT were much higher when rats were given gastric loads of HS instead of IS, even though pOsm was not significantly altered. Comparable results were obtained when gastric loads of HS or IS were given to rats that had been deprived of drinking water overnight. Other dehydrated rats treated similarly but given access to drinking water consumed much more when they had been given gastric loads of HS instead of IS. Collectively, these and other findings suggest the importance of early signals, perhaps from hepatoportal osmoreceptors or Na(+) receptors, in the control of VP and OT secretion and water intake in rats.