Osmoregulation and volume regulation in rats: inhibition of hypovolemic thirst by water.

STRICKER, EDWARD hf. Osmoregdation and vokdme regulation in rats: inhibition of hypovokmk thirst by water, Am. J. Physiol. 2 17( 1) : 98-l 05, 1969.-Subcutaneous injections of hyperoncotic polyethylene glycol (PG) solutions produced hypovolemia in rats by withdrawing isosmotic plasma fluid into the local interstitium. All animals showed increased thirst, but stopped drinking water well before plasma deficits were restored. Concurrent hyperosmolality (produced by injection of hypertonic NaCl solutions) increased the water intakes of PG-treated rats in proportion to both intracellular and intravascular dehydrations, yet continued hypovolemia was again evident when drinking discontinued. Similarly, water preloads almost totally eliminated drinking in PG-treated rats but had little effect on plasma volumes. Inhibition of drinking was accompanied by significant osmotic dilution in all three experiments. In contrast, ingestion or preloads of isotonic N&l solutions, which did not affect plasma osmolality, did not eliminate thirst in PG-treated rats unless plasma deficits were repaired. These findings indicate that an inhibitory mechanism, apparently stimulated by osmotic dilution of body fluids, is involved in the regulation of water intake during hypovolemia. This mechanism, perhaps represented in the supraoptic hypothalamus, may be functionally similar to the ventromedial hypothalamic 3atiety center” involved in the regulation of food intake.