Relationship between absolute body-fluid deficits and fluid intake in the rat.

Acute absolute body-fluid deficits were induced in rats by injection of the diuretic drug furosemide, which caused up to 20% reduction of extracellular fluid volume and up to 2% reduction of intracellular fluid volume. Water and .3 M NaCl were subsequently made available to allow the rats to replace their body fluids by drinking. The rats increased their intake of both fluids, but replaced less than half of the total deficit, thereby tolerating larger and larger voluntary body-fluid deficits as the size of the diuretic fluid loss increased. Plasma measures showed that the rats sustained hypovolemia after drinking, while intracellular fluid volume was apparently restored. Fluid-depleted rats drank normally in response to intracellular dehydration induced by a sodium chloride load. Incomplete restoration of body-fluid balance after body-fluid depletion is due to a failure to drink in response to extracellular dehydration.