Contractility and protein phosphorylation in cardiomyocytes: effects of isoproterenol and AR-L57.

The cardiotonic drugs AR-L57 [2-(2,4-dimethoxyphenyl)-1H-imidazo(4,5b)-pyridine] and isoproterenol stimulated contractility in cultured heart cells in concentration-dependent manners; only the effects of isoproterenol were blocked by propranolol. Isoproterenol, but not AR-L57, enhanced the phosphorylation state of seven protein bands [relative molecular weights (MrS) 155,000, 96,000, 27,000, 24,000, 20,000, 16,000, 12,000] and resulted in the dephosphorylation of one protein band (Mr 21,000). Also, only isoproterenol increased the activation states of adenosine 3',5'-cyclic monophosphate (cAMP)-dependent protein kinase and glycogen phosphorylase. The eight protein bands resolved by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis and detected by autoradiography were altered by isoproterenol in time- and concentration-dependent manners. The 24,000-Mr protein substrate phosphorylated in response to isoproterenol was converted to a 12,000-Mr species by heating in the presence of SDS prior to electrophoresis, suggesting that the two substrates were in fact identical proteins. A comparison of the 2-min responses to varying concentrations of isoproterenol resulted in excellent correlations between the phosphorylation states of individual protein bands and contractility. This was true even for the 21,000-Mr species that was dephosphorylated. However, only the 27,000-, 24-12,000-, and 16,000-Mr substrates were phosphorylated rapidly enough to be associated with the onset of the inotropic response. Cultured myocytes are an important feature of these studies as they are 84% pure ventricular cells that remain 100% viable throughout an experiment. Because this system is suitable for biochemical measurements and the effects of agents on heart cell contractility can be determined, it is possible to correlate changes in biochemical parameters with alterations in physiological state.