Inhibition of thirst when dehydrated rats drink water or saline

Abstract

The present experiments sought to identify the physiological signals that inhibit thirst when dehydrated rats drink water or NaCl solution. Rats were deprived of drinking fluid but not food overnight. When allowed to drink again, the dehydrated animals consumed water or saline (0.05 M, 0.10 M, 0.15 M, or 0.20 M NaCl solution) almost continuously for 5-8 min before stopping. The volumes consumed were similar regardless of which fluid they ingested, but blood analyses indicated that increased plasma osmolality and decreased plasma volume, or both, still remained when drinking terminated. These results suggest that the composition of the ingested fluid is less significant than its volume in providing an early signal that inhibits thirst and fluid consumption by dehydrated rats. Analyses of the gastrointestinal tracts revealed that the cumulative volume in the stomach and small intestine correlated highly with the amount consumed regardless of which fluid was ingested. These and other results suggest that the volume of fluid ingested by dehydrated rats is sensed by stretch receptors detecting distension of the stomach and small intestine, which provide an early inhibitory stimulus of thirst. dehydration; gastrointestinal distension; visceral osmoreceptors; water deprivation WATER DEPRIVATION ELICITS thirst and secretion of neurohypophyseal vasopressin (VP), the antidiuretic hormone, in rats and other animals (31). When dehydrated animals drink water, both responses are inhibited before systemic rehydration is evident. In dogs and human subjects, the early signal that inhibits thirst and VP secretion has been related to the volume of fluid consumed, apparently detected by oropharyngeal receptors that monitor the swallowing of liquids regardless of whether water, isotonic saline, or hypertonic saline solution is consumed (2,13,24,32). However, recent experiments indicate that early inhibition of VP secretion in rats is not mediated by oropharyngeal receptors (17, 29) because plasma VP levels (pVP) were not affected either when dehydrated rats ingested isotonic saline or when water was consumed but drained from the stomach through an open gastric fistula (29). In contrast, water ingested normally by thirsty rats initiated a rapid decline in pVP that began while systemic plasma Na ϩ concentrations (pNa) still were elevated (17, 29), suggesting that the inhibitory effect was not mediated by cerebral osmoreceptors but by a potent presystemic stimulus, related to the composition of ingested fluid, which might arise from osmoreceptors or Na ϩ receptors in the viscera (4, 19). Furthermore, the signal that terminated an initial bout of water consumption occurred a few minutes later, in association with a reduction of systemic pNa but continued plasma volume deficits (29). Because the intake of isotonic saline also ceased after similar volumes were consumed, without any associated change in pNa (29), these observations allow the hypothesis that thirst in rats is inhibited by signals related to the volume of ingested fluid rath...

Regulation of NaCl solution intake and gastric emptying in adrenalectomized rats

“…thesis, 2006) that similar relations between intake and gastric emptying were observed whether adrex rats drank 0.30 M NaCl after 16 h or 40 h of saline deprivation. Further support is provided by studies of saline ingestion and gastric emptying when thirst was induced in rats by overnight water deprivation (8). Specifically, the rate of ingestion (1.7 ml/min when dehydrated rats drank 0.05 M, 0.10 M, 0.15 M, or 0.20 M NaCl) was similar to that observed when adrex rats drank those fluids, as were the relations between intestinal distance and intestinal volume (y = 21.75 ln(x) + 40.95) and between mean intestinal distension and intestinal volume (0.010 ml/cm per ml).…”

“…We propose that they drank the less concentrated solutions at a maximally high rate (in ml/min) until intake was inhibited by a presystemic visceral signal related to the volume of ingested saline, as appears to be the case in DOCA-treated rats (16). Note that thirst and vasopressin secretion in dogs and humans are inhibited by an early volume-dependent signal generated by oropharyngeal receptors that monitor the swallowing of liquids (6,24), but this control mechanism does not exist in rats (8,19).…”

“…Our goal was to determine the total number of such bins that occurred before the rats stopped drinking saline in the initial bout, so we could use that number to anticipate the end of an initial drinking bout by other adrex rats in these experiments. The analyses were limited to the intakes by the 8 adrex rats during the first 15 min of the sessions on the last day of exposure to each fluid; that is, on Days 4,8,12,16,and 17. (Note that the daily intakes of each of the first four NaCl solutions in this sequence differed by <10% during the days in which it was available. )…”

“…Briefly, in previous studies (8,16) DOCA-treated rats and water-deprived rats were fitted with gastric fistulae and adapted to cages that allowed drainage and collection of gastric fluid. On average, 12.6% more fluid drained from the rats' stomachs than was consumed.…”

“…We propose that they drank the less concentrated solutions at a maximally high rate (in ml/min) until intake was inhibited by a presystemic visceral signal related to the volume of ingested saline, as appears to be the case in DOCA-treated rats (16). Note that thirst and vasopressin secretion in dogs and humans are inhibited by an early volume-dependent signal generated by oropharyngeal receptors that monitor the swallowing of liquids (6,24), but this control mechanism does not exist in rats (8,19).Saline ingestion by adrex rats occurred at a faster rate (0.05 M, 0.15 M, and 0.30 M NaCl, 1.66 ml/min; 0.50 M NaCl, 1.06 ml/min) than did gastric emptying during the initial bout (0.05 M and 0.15 M NaCl, 1.06 ml/min; 0.30 M and 0.50 M NaCl, 0.33 ml/min). Consequently, ingested fluid accumulated in the stomach.…”