R ecently, it has been reported that the protein kinase C (PKC) beta isoform plays a critical role in the development of hypertrophy and heart failure. The purpose of the present study was to clarify the mechanism by which activation of PKCbeta led to depressed cardiac function. Thus, we used a PKCbeta2 overexpressing mouse, an animal model of heart failure, to examine mechanical properties and Ca2+ signals of isolated left ventricular cardiomyocytes. The percentage of shortening, rate of shortening, and rate of relengthening of cardiomyocytes were markedly reduced in PKCbeta2 overexpression mice compared to wild-type control mice, although the baseline level and amplitude of Ca2+ signals were similar. These findings suggested a decreased myofilament responsiveness to Ca2+ in transgenic hearts. Therefore, the incorporation of [32P] inorganic phosphate into cardiac myofibrillar proteins was studied in Langendorff-perfused hearts. There was a significant increase in the degree of phosphorylation of troponin I in PKCbeta2-overexpressing transgenic mice. The depressed cardiomyocyte function improved after the superfusion of a PKCbeta selective inhibitor. These findings indicate that in vivo PKCbeta2-mediated phosphorylation of troponin I may decrease myofilament Ca2+ responsiveness, and thus causes cardiomyocyte dysfunction. Since chronic and excess activation of PKCbeta2 plays a direct and contributory role in the progression of cardiac dysfunction, the PKCbeta selective inhibitor may provide a new therapeutic modality in the setting of heart failure.