Beta-adrenergic stimulation of sarcolemmal phosphorylation in the perfused rabbit heart

To study the mechanism of catcholamine-induced positive inotropy, sarcolemmal membrane proteins were isolated from freeze-clamped rabbit hearts perfused with [32P] and proteins were separated by sodium dodecyl sulfate polyacrylamide gel electrophoresis. The major phosphorylated proteins were of apparent molecular mass 27,000 and 14,000 Da. Bolus injections of epinephrine increased phosphorylation of the 27,000 Da protein 2- and to 3-fold coincidentally with increases in developed tension. With epinephrine washout contractility returned to control levels in 2 minutes but dephosphorylation was not complete by 10 minutes. Phosphorylation of the 27,000 Da protein induced by epinephrine was dose dependent and could also be stimulated by isoproterenol and blocked by propranolol. Phenylephrine and ouabain did not stimulate phosphorylation. Forskolin, the beta-adrenergic acting drug, stimulated contractility and also increased phosphorylation. The results suggest that phosphorylation of the 27,000 Da protein may be involved in mediating increases in tension, though the lack of coincidence between dephosphorylation and decline in tension with time suggests a complicated relationship possibly involving other factors.