Effects of preloads of water and saline on thermal dehydration-induced thirst

The relative contribution of cellular and extracellular water deficits to the genesis of thirst due to thermal dehydration was studied in male Sprague-Dawley rats following exposure to a 40 degrees C environment for 4 h. Intragastric (I.G.) and intravenous (I.V.) preloads of water reduced the elevated plasma sodium and plasma osmolality of thermally dehydrated rats to control levels, but preloads of saline did not. I.G. and I.V. preloads of saline returned the hematocrit and plasma protein concentration of thermally dehydrated rats to control levels. Both the I.G. water preload and the I.G. saline preload reduced water intake, with the I.G. water preload having a greater effect. The I.V. water preload reduced water intake nearly to control levels, whereas the I.V. saline preload was without effect on water intake. These data indicate that the water intake of thermally dehydrated rats is primarily due to a cellular water deficit and that oral and gastric factors are also important in terminating water intake in thermally dehydrated rats.