Hormonal regulation of protein phosphorylation in isolated rat heart cells

We used a recently developed preparation of calcium-tolerant isolated rat cardiac ventricular cells to investigate certain aspects of hormone-mediated protein phosphorylation in heart tissue. Isoproterenol or dibutyryl adenosine 3',5'-cyclic monophosphate (cAMP) promoted the phosphorylation of at least 13 proteins and promoted the dephosphorylation of a single protein of relative molecular weight (Mr) 21,000, whose phosphorylation appeared to be stimulated by insulin. The isoproterenol-induced protein phosphorylations reached maximum levels for most proteins within 5 min at slightly different rates. However, when excess propranolol was added to the cells after exposure to isoproterenol, there appeared to be two major patterns of dephosphorylation: proteins that remained fully phosphorylated after propranolol addition, exemplified by proteins tentatively identified as troponin I and C-protein, and proteins that were rapidly dephosphorylated after propranolol, exemplified by phospholamban, the modulator of the sarcoplasmic reticulum calcium-dependent ATPase. The Mr 21,000 protein was rapidly dephosphorylated in response to isoproterenol and was rephosphorylated after addition of propranolol. This protein remains unidentified; it is not the Mr 19,000 myosin light chain whose phosphorylation state was unaffected by isoproterenol. This preparation of isolated heart cells provides a convenient way to investigate the biochemical effects resulting from exposure of the heart to hormones and can separate direct hormonal effects from those resulting from changes in contractility or heart rate.