Modulating Influence ofd,l-Propranolol on Triiodothyronine-Induced Skeletal Muscle Protein Degradation*

Abstract

Thyrotoxicosis may increase .beta.-adrenoreceptor density on some target tissues. The in vivo effect of propranolol (a nonselective .beta.-adrenoreceptor blocking agent) on [triiodothyroine] T3-induced enhancement of in situ proteolysis in fast twitch muscle fibers of the rat has been studied. Chronic treatment with T3 as opposed to saline resulted in a 76% enhancement of the rate of in situ muscle proteolysis [0.79 .+-. 0.04 (n = 8) compared with 0.45 .+-. 0.01 (n = 8) nmol tyrosine/mg muscle .cntdot. 2 h]. Treatment of rats with both T3 and 2 mg propranolol resulted in a 62% reduction in the T3-induced increment in situ muscle proteolysis [0.58 .+-. 0.02 (n = 8) vs. 0.79 .+-. 0.04]. This significant inhibition by propranolol of T3-induced enhanced proteolytic rates in vitro suggests that this may comprise 1 component of the observed beneficial clinical effects of .beta.-blockade in thyrotoxic myopathy.