Thirst and solute excretion: their effectiveness in osmostatic control of body fluid

The effect of intravenous infusion of hypertonic NaCl at three different rates (series I, 4.0 mmol/min; series II, 8.0 mmol/min; series III, 16.0 mmol/min) on osmotic thirst threshold and postloading restitution of plasma osmolality (Posm) has been determined in dogs. Osmotic thirst threshold increased proportionally to the rate of delivery of the osmotic load. Relative suppression of osmotic thirst at the higher rates of infusion was temporary and largely disappeared within 10 min after the end of hypertonic infusion. During the postloading period excretion of osmotic load and water intake was proportional to the magnitude of the load administered. However, the animals with high osmotic thirst threshold stopped drinking at a higher Posm than those with a low osmotic thirst threshold. In series I and II, Posm decreased during 1 h to a level not significantly different from the preloading value. In each series renal excretion of osmotic load was more important than water intake for restitution of Posm during 1 h. Amount of water ingested during this period was significantly smaller than that theoretically required to restore resting Posm when excretion of osmotic load was not taken into account. However, total amount of water intake and osmotic load excreted during 1 h were together well adjusted to restore preloading Posm.