Water ingestion by rats fed a high-salt diet may be mediated, in part, by visceral osmoreceptors

After surgical removal of all salivary secretions ("desalivation" ), rats increase their consumption of water while eating dry laboratory chow. In the present experiments, desalivated rats drank even more water while they ate "powdered" high-salt food (i.e., <15-mg food particles). The Na⁺ concentration of systemic plasma in these animals was not elevated during or immediately after the meal, which suggests that cerebral osmoreceptors were not involved in mediating the increased water intake. A presystemic osmoregulatory signal likely stimulated thirst because the Na ⁺ and water contents of the gastric chyme computed to a solution ∼150 mM NaCl. In contrast, desalivated rats drank much smaller volumes of water while eating "pulverized" high-salt food (i.e., 60-140-mg food particles), and the fluid mixture in the gastric chyme computed to ∼280 mM NaCl solution. These and other findings suggest that the NaCl ingested in the powdered high-salt diet was dissolved in the gastric fluid and that duodenal osmoreceptors (or Na⁺-receptors) detected when the concentration of fluid leaving the stomach was elevated after each feeding bout, and promptly stimulated thirst, whereupon rats drank water until the gastric fluid was diluted back to isotonicity. However, when rats ate the pulverized high-salt diet, much of the NaCl ingested may have been embedded in the gastric chyme and therefore was not accessible to visceral osmoreceptors once it emptied from the stomach. Consistent with that hypothesis, fluid intakes were increased considerably when desalivated rats drank 0.10 M NaCl instead of water while eating either powdered or pulverized high-salt food.