Early osmoregulatory stimulation of neurohypophyseal hormone secretion and thirst after gastric NaCl loads.

Cerebral osmoreceptors mediate thirst and neurohypophyseal secretion stimulated by increases in the effective osmolality of plasma (P(osmol)). The present experiments determined whether an intragastric load of hypertonic saline (ig HS; 0.5 M NaCl, 4 ml) would potentiate these responses before induced increases in P(osmol) in the general circulation could be detected by cerebral osmoreceptors. Adult rats deprived of water overnight and then given intragastric HS consumed much more water in 15-30 min than rats given either pretreatment alone, even though systemic P(osmol) had not yet increased significantly because of the gastric load. In other rats pretreated with an intravenous infusion of 1 M NaCl (2 ml/h for 2 h), plasma levels of vasopressin and oxytocin were considerably elevated 15 and 25 min after intragastric HS treatment, whereas systemic P(osmol) was not increased further. These and other findings are consistent with previous reports that hepatic portal osmoreceptors (or Na(+) receptors) stimulate thirst and neurohypophyseal hormone secretion in euhydrated rats given gastric NaCl loads and indicate that these effects are potentiated when animals are dehydrated.