Heat stress causes excessive drinking in fed and food deprived pregnant goats.

Water intake was studied in six goats during late pregnancy to investigate if heat stress induces primary polydipsia. The goats were fed twice daily or deprived of food for 24 hr to avoid postprandial drinking and induce hyponatremia. Signs of stress was evaluated by determination of plasma cortisol concentration. Ambient temperature was increased from 20 degrees C to 38.0-39.5 degrees C for 5.15 hr. Water temperature was 35 +/- 1 degree C during heat stress. Fed goats started to drink 13 +/- 3 min after the heaters were turned on and water diuresis developed in five of them. The body weight increased simultaneously with decreased plasma Na concentration and osmolality showing that the goats also stored water. During food deprivation, plasma Na concentration and osmolality fell, and plasma aldosterone concentration increased. Despite hyponatremia, food deprived goats started to drink 44 +/- 15 min after the heaters were turned on and then they drank repeatedly. Three of them underwent water diuresis. Hemodilution was observed both in fed and food-deprived goats during heat stress. These results show that heat stress induced primary polydipsia in pregnant goats. It is suggested that stimulating signals from warmth receptors overrode inhibiting influences from receptors signalling hyponatremia and hypoosmolality at the "thirst center" in the hypothalamus. Heat stress did not increase plasma cortisol concentration, but catching sight of food caused an abrupt, short-lasting elevation.