The role of adrenoceptors in norepinephrine-stimulated VO2 in muscle.

The purpose of this study was to characterize the norepinephrine (NE)-evoked calorigenic response of an isolated perfused rat muscle preparation into alpha-and/or beta-adrenergic components. Epinephrine was as effective as NE in evoking the increase in oxygen consumption. The alpha-agonist, phenylephrine, was also as effective as NE in eliciting an increase in oxygen consumption by the muscle. Isoproterenol (beta-agonist) had no effect on the VO2 of the preparation. Phentolamine, an alpha-blocker, completely blocked the NE-stimulated VO2. Propranolol (beta-antagonist) could also completely block the NE induced VO2 but this drug was found to be less potent. It was concluded that the NE-induced calorigenic response of muscle cannot be classified either strictly as an alpha-or beta-response, but unlike brown adipose tissue, the alpha-response seems to predominate in skeletal muscle. Also, it appears as though different mechanisms of action may be involved in the NE-stimulated VO2 in skeletal muscle and brown fat.