OSMOTIC AND HORMONAL RESPONSE TO HEAT AND DEHYDRATION IN THE FOWL

Osmotic and hormonal responses to heat and dehydration in birds were studied. Heat exposure did not affect the osmotic and hormonal status of normally hydrated fowls. Dehydration for 48 h (normothermia, .apprx. 13% body weight loss) significantly (P < 0.001) increased plasma osmolality and concentrations of Na and chloride, and consequently the plasma concentrations of the antidiuretic hormone (AVT) and prolactin. Additional heat exposure of the dehydrated fowls further accelerated these changes, together with significantly (P < 0.01) increasing plasma aldosterone concentration. Upon recovery (30 min of drinking) the osmotic and hormonal responses were significantly(P < 0.001) diminished. Both temperature was not significantly affected by dehydration, yet during heat exposure the dehydrated fowls were significantly hyperthermic (P < 0.05) compared with normally hydrated heat exposed fowls. A significant overall correlation (r = 0.43, P < 0.02) between body temperature and plasma Na-to-Ca ratio suggests this ratio to be involved in the control of the hypothalamic thermoregulatory set-point. The associated thermoregulatory, osmoregulatory and hormonal changes revealed in the present study, together with literature information, are integrated into a model summarizing the operating mechanisms in dehydrated, heat-exposed birds.