| Abstract: | Perfusion of isolated rat hearts with isoproterenol resulted in increases in the level of protein-bound phosphate of the myofibrils. After perfusion of the hearts with 32P, followed by SDS-polyacrylamide gel electrophoresis of the purified myofibrils, four major 32P-containing protein bands were identified. Most of the increased 32P incorporation produced by isoproterenol was localized on the troponin I and myosin light chain bands, and, to lesser extent, on the M-protein band. ATPase activity was tested in the purified myofibrils. No changes in Ca2+ requirement for activation were found after isoproterenol perfusion. However, maximal ATPase activity was markedly reduced in the myofibrils obtained from isoproterenol-treated hearts. It would appear that the myofibrillar protein phosphorylation induced by isoproterenol perfusion results in a decrease in actomyosin ATPase activity. |
