HORMONAL-REGULATION OF PROTEIN-PHOSPHORYLATION IN ISOLATED RAT-HEART CELLS

A recently developed preparation of Ca-tolerant isolated rat cardiac ventricular cells was used to investigate certain aspects of hormone-mediated protein phosphorylation in heart tissue. Isoproterenol or dibutyryl cAMP promoted the phosphorylation of at least 13 proteins and promoted the dephosphorylation of a single protein of relative MW 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. When excess propranolol was added to the cells after exposure to isoproterenol, there appeared to be 2 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 Ca-dependent ATPase. The MW 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 MW 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. The role of .beta.-adrenergic catecholamines in the regulation of protein phosphorylation is discussed.