Restoration of intravascular fluid volume following acute hypovolemia in rats.

i~~yavas~ula~ Juid vohme following acute hypovolemia in rats. Am. 3. Physiol. 218(l): 191496. 1970.-Subcutaneous injection of hyperoncotic polyethylene glycol (PC) solution produced hypovolemia in rats by gradually withdrawing isosmotic plasma fluid into the local interstitium, Significant drinking was observed for 6-8 hr, although water ingestion subsequently diminished despite persistent hypovolemia. This inhibition of thirst in the presence of substantial plasma deficits has been attributed to osmotic dilution of body fluids. When 0.5 1 M NaCl solution was additionally present, rats manifested a sodium appetite 6-9 hr after treatment and water intake progressed without evident inhibition. Apparently, osmotic dilution potentiated a sodium appetite in the hypovolemic rats, which removed the inhibition of thirst by increasing body fluid osmolality. Because of the increased fluid intakes and low urine volumes and sodium concentrations, increasing amounts of nearisotonic fluid were retained and eventually plasma deficits were repaired. These results indicate that thirst and sodium appetite are interdependent behavioral mechanisms of volume regulation which complement well-known antidiuretic and antinatriuretic processes in providing for the restoration of intravascular Auid volume in hypovolemic rats.