Iwasa_1983_J.Biol.Chem_258_4571

The phosphorylation of cardiac membrane proteins has been studied in preparations of newborn chick hearts. Membranes were isolated from 32P-loaded tissue after treatment with or without the beta-adrenergic receptor agonist isoproterenol and/or the muscarinic cholinergic receptor agonist oxotremorine. The phosphorylation of a low molecular weight membrane protein was enhanced by isoproterenol as early as 10 s after adding the drug. This phosphoprotein had a molecular weight of approximately 26,000 or 14,000 depending on the conditions used to solubilize the membranes prior to electrophoresis. It is most probably phospholamban/calciductin. The apparent molecular weight of the protein observed at 26,000 increased by approximately 1,000 as phosphorylation increased. The phosphorylation of this protein was abolished by short term treatment of the isoproterenol-treated tissue with the muscarinic receptor agonist oxotremorine. Effects of oxotremorine were observed within 30 s and were maximal between 2-5 min. The oxotremorine-induced decrease in phosphorylation was accompanied by a decrease in molecular weight. This phosphoprotein was found in a membrane fraction enriched in cardiac sarcolemma as well as in another containing sarcolemma and sarcoplasmic reticulum. The phosphorylation of this membrane component may play a role in the effects of beta-adrenergic and muscarinic cholinergic agonists on cardiac contractile force.