HEAT-STRESS CAUSES EXCESSIVE DRINKING IN FED AND FOOD-DEPRIVED PREGNANT GOATS

Water intake was studied in six goats during late pregnancy to investi gate if heat stress induces primary polydipsia. The goats were fed twi ce daily or deprived of food for 24 hr to avoid postprandial drinking and induce hyponatremia. Signs of stress was evaluated by determinatio n of plasma cortisol concentration. Ambient temperature was increased from 20 degrees C to 38.0-39.5 degrees C for 5.15 hr. Water temperatur e was 35 +/- 1 degrees C during heat stress. Fed goats started to drin k 13 +/- 3 min after the heaters were turned on and water diuresis dev eloped in five of them. The body weight increased simultaneously with decreased plasma Na concentration and osmolality showing that the goat s also stored water. During food deprivation, plasma Na concentration and osmolality fell, and plasma aldosterone concentration increased. D espite hyponatremia, food deprived goats started to drink 44 +/- 15 mi n after the heaters were turned on and then they drank repeatedly. Thr ee of them underwent water diuresis. Hemodilution was observed both in fed and food-deprived goats during heat stress. These results show th at heat stress induced primary polydipsia in pregnant goats. It is sug gested that stimulating signals from warmth receptors overrode inhibit ing influences from receptors signalling hyponatremia and hypoosmolali ty at the ''thirst center'' in the hypothalamus. Heat stress did not i ncrease plasma cortisol concentration, but catching sight of food caus ed an abrupt, short-lasting elevation.