Maximum oxygen consumption and catecholamines in thyroidectomized dogs

Comparisons have been made in 7 dogs between maximum oxygen consumption recorded before (N dogs) and after thyroidectomy (T dogs). The comparisons were performed under two conditions 1) during severe cold stress (CVo2 max), 2) during a short period of exhaustive work (Ex Vo2 max). Heart rate, plasma catecholamine and substrate concentrations (glucose, lactic acid, FFA) were measured under each condition. 1. Thyroidectomy induced a more substantial decrease in CVo2 max than in Ex Vo2 max. 2. At CVo2 max, average plasma epinephrine and norepinephrine concentrations rose to a higher level in T dogs than in N dogs. In T dogs, correlations were found between plasma epinephrine concentrations and CVo2 max values, and between plasma norepinephrine concentrations and CVo2 max values. At Ex Vo2 max, average plasma norepinephrine concentrations were similar in N dogs and in T dogs, and average plasma epinephrine concentrations were not significantly different from each other. 3. At Ex Vo2 max, average plasma concentrations of the various substrates were not significantly different in N dogs and T dogs. At CVo2 max, plasma FFA levels were higher in T dogs. It may be concluded that in dogs, thyroidectomy affects mechanisms which are more specifically involved in heat production than in muscular exercise. The increased catecholamine secretion in response to cold which occurred in T dogs appeared merely to limit the decrease in heat production. It seems possible that increased catecholamine secretion compensates for the decreased sensitivity of beta receptors to catecholamine but it cannot fully account for the effects of thyroidectomy.