Pre-systemic controls of fluid intake and vasopressin secretion.

In dehydrated rats, an early postingestive signal associated with the concentration of ingested fluid provides a signal that can inhibit neurohypophyseal vasopressin secretion. That inhibitory signal is not generated by the act of swallowing, as it is in dogs and many other animals, but by visceral osmoreceptors that presumably send a vagal signal to the nucleus of the solitary tract and the area postrema in the brain stem. In further contrast to dogs, the act of water ingestion does not provide an early signal inhibiting thirst in rats, but gastric emptying is so rapid that thirst can be satiated relatively quickly. When saline is consumed instead of water, thirst is inhibited by a different signal that results from the volume consumed and apparently is associated with gastrointestinal distension. These and other results emphasize the need to include gastric emptying of ingested fluid in considerations of water and Na(+) balance in rats.