Effects of beta-adrenergic receptor blockade on metabolic rate and mixed venous blood temperature during dynamic exercise.

Mixed venous blood temperature and energy exchange were measured in sixteen healthy male volunteers at rest and during bicycle exercise of 12 min duration performed before and after intravenous administration of propranolol. The rise in blood temperature induced by exercise was about 30% greater following propranolol. This effect was probably due to a reduction of skin blood flow, which impaired the conditions for heat loss from the blood. The exercise induced rise in total energy exchange was reduced after propranolol by around 20 W. In consequence, the calculated mechanical efficiency increased significantly. Since the respiratory quotient showed no increase but rather the reverse, the observed effect could not be attributed to any transient changes in body oxygen stores. The results thus indicate that propranolol caused a true deceleration of the metabolic rate. This effect might be secondary to inhibition of catecholamine-dependent oxygen consuming metabolic processes in peripheral organs. A possible hypothalamic mechanism, triggered by the blood temperature level, is also discussed.