The effect of hyperthermia and localized heating of the anterior hypothalamus on the sympatho-adrenal system of the ox (Bos taurus)

Abstract

Arterial plasma catecholamine concentrations, rectal temperatures and respiratory rates were measured in the un-anaesthetlzed ax in a cool environment (15/11.5[degree] C dry bulb/wet bulb (D.B./W.B.) in a hot, dry environment (40/22[degree] C), in a hot, humid environment (40/39.1[degree] C) and during localized heating of the hypothalamus. There was no significant difference in the plasma catecholamine levels of animals exposed to a cool environment (15/11.5[degree] C) and to a hot, dry (40/22[degree] C) environment. When the animals became hyperthermic in the hot, humid environment, considerable increases in the plasma levels of adrenaline [epinephrlne] and smaller increases in noradrenaline [norepinephrine] concentration occurred. Bilateral adreno-medullary denervation abolished the changes in plasma catecholamine levels associated with hyperthermia. Localized heating of the anterior hypothalamus in either a cool or a warm environment did not result in any change in plasma catecholamine levels. It was concluded that the source of the increased levels of plasma catechola-mines during hyperthermia was the adrenal medulla and that the stimulus for their release was a nervous one rather than direct chemical or thermal stimulation of the adrenal medulla. The cause of the increased sympatho-adrenal activity was not apparent, but neither thermal stimulation of cutaneous and hypothalamic receptors nor the associated changes in respiratory activity seemed to be implicated in the response.