Beta-adrenergic Receptor and Adenylate Cyclase in Transverse Tubules of Skeletal Muscle

Experiments were carried out to clarify the sites of action of beta-adrenergic agonists in skeletal muscle microsomes. Microsomes were fractionated into longitudinal reticulum, terminal cisternae, and isolated transverse tubules. Transverse tubules were selectively labeled and tracked with [3H]ouabain. beta-adrenergic receptor was identified by [3H]dihydroalprenolol binding. Assays of beta-adrenergic receptor, adenylate cyclase, and protein kinase-stimulated phosphorylation showed: 1) beta-adrenergic receptor was detected in transverse tubules with a receptor density of 0.61 pmol/mg of protein. No significant binding was detected in longitudinal reticulum or in terminal cisternae. 2) Isoproterenol-stimulated adenylate cyclase was present in microsomes but was similarly confined to the transverse tubular fraction. The activity of F- stimulated cyclase in transverse tubules was 2.3 nmol/mg of protein/min. 3) No phosphorylation of microsomes by cyclic AMP and protein kinase could be detected. We conclude that the action of epinephrine on skeletal muscle is mediated through receptors and adenylate cyclase in the external membrane.