The role of the adrenal gland in the metabolic response of young lambs to cold.

Adrenalectomy and various replacement regimens with corticosteroids and catecholamines were used to assess the role of the adrenal secretions in cold thermogenesis in young lambs. A dose of cortisol of 0.25 mg kg-1 d-1 was sufficient to increase summit metabolism significantly in lambs that had been adrenalectomized for at least 1 week, but even the massive dose of 16 mg kg-1 d-1 restored the response to only 85--90% of that in intact lambs. However, the remaining 10-15% deficit was largely restored by additional infusion of catecholamines, principally adrenaline, and there was no significant interaction between the responses to cortisol dose and to catecholamine infusion. Heart rate, arterial blood pressure and cardiac output during summit metabolism all increased with increasing dose of cortisol. Hyperglycaemic responses to cold exposure were low in adrenalectomized lambs receiving less than 0.25 mg kg-1 d-1 of cortisol, and the normal hyperlactacidaemia in intact lambs during summit metabolism was apparent only in lambs receiving more than 1 mg kg-1 d-1, unless adrenaline was also given. The increase in summit metabolism due to adrenaline was accompanied by increased shivering. There was no effect on heart rate and blood pressure, but adrenaline increased cardiac output through an apparent increase in cardiac stroke volume and a decrease in total peripheral resistance. Strong correlations of the increase in summit metabolism with the increase in cardiac output and with adrenaline-mediated increases in plasma glucose and lactate, indicate that a role of the medullary catecholamines is to maintain maximal thermogenesis by their effects on the supplies of oxygen and energy substrates to the thermogenic tissues.