DRINKING AFTER INTRAGASTRIC NACL WITHOUT INCREASE IN SYSTEMIC PLASMA OSMOLALITY IN RATS

Drinking after intragastric hypertonic solutions was examined to deter mine whether increased plasma osmolality always accompanied initiation of drinking. A 2-ml infusion through a gastric catheter was the begin ning of tests in Sprague-Dawley male rats. Latency to drink was shorte r and l-h water intake was greater for increasing concentrations of Na Cl (600, 1,200, and 1,800 mosmol/kg) compared with baseline (290 mosmo l/kg). Although 600, 900, or 1,200 mosmol/kg NaCl elicited drinking, s uch infusions failed to change systemic plasma osmolality, and 900 mos mol/kg also failed to change plasma sodium, protein, renin activity, o r packed cell volume at the initiation of drinking. Intragastric 900 m osmol/kg sodium bicarbonate, sodium isethionate, potassium chloride, l ithium chloride, and mannitol differentially increased water intake. T otal subdiaphragmatic vagotomy abolished drinking elicited by intragas tric NaCl; selective gastric or hepatic vagotomy attenuated intake und er some conditions. These results support the hypothesis of a vagally mediated, gastrointestinal and/or hepatic-portal, osmosensitive mechan ism for initiation of drinking in advance of postprandial increases in systemic osmolality.