Chronic β-blockade increases skeletal muscle β-adrenergicreceptor density and enhances contractile force
Murphy, René J. L., Phillip F. Gardiner, Guy Rousseau, Michel Bouvier, and Louise Béliveau. Chronic β-blockade increases skeletal muscle β-adrenergic-receptor density and enhances contractile force. J. Appl. Physiol.83(2): 459–465, 1997.—The effects of a chronic 14-day administration of a selective β2-adrenergic-receptor antagonist (ICI-118551) on skeletal muscle were evaluated in female Sprague-Dawley rats. Chronic ICI-118551 treatment did not modify muscle mass, oxidative potential, or protein concentration of the medial gastrocnemius muscle, suggesting that maintenance of these skeletal muscle characteristics is not dependent on β2-adrenergic-receptor stimulation. However, the drug treatment increased β-adrenergic-receptor density of the lateral gastrocnemius (42%) and caused an increase in specific (g/g) isometric in situ contractile forces of the medial gastrocnemius [twitch, 56%; tetanic (200 Hz), 28%]. The elevated contractile forces observed after a chronic treatment with ICI-118551 were completely abolished when the β2-adrenergic antagonist was also administered acutely before measurement of contractile forces, suggesting that this response is β2-adrenergic-receptor dependent. Possible mechanisms for the increased forces were studied. Caffeine administration potentiated twitch forces but had little effect on tetanic force in control animals. Administration of dibutyryl adenosine 3′,5′-cyclic monophosphate in control animals also resulted in small increases of twitch force but did not modify tetanic forces. We conclude that increases in β-adrenergic-receptor density and the stimulation of the receptors by endogenous catecholamines appear to be responsible for increased contractile forces but that the mechanism remains to be demonstrated.