Isoenzyme-Specific Protein Kinase C and c-Jun N-terminal Kinase Activation by Electrically Stimulated Contraction of Neonatal Rat Ventricular Myocytes

Previous studies from our laboratory and others indicate that contraction-induced mechanical loading of cultured neonatal rat ventricular myocytes produces many of the phenotypic changes associated with cardiomyocyte hypertrophy in vivo, and that these changes occur via the activation of serine-threonine protein kinases. These may include the extracellular regulated protein kinases (ERK1 and ERK2), the c-Jun N-terminal kinases (JNK1, JNK2, and JNK3), and one or more isoenzymes of protein kinase C. In this study, we assessed whether one or more of these kinases are activated by stimulated contraction, and whether activation was isoenzyme-specific. Low-density, quiescent cultures of neonatal rat ventricular myocytes were maintained in serum-free medium, or electrically stimulated to contract (3 Hz) for up to 48 h. ERK and JNK activation was assessed by Western blotting with polyclonal antibodies specific for the phosphorylated forms of both kinases. PKC activation was analysed by subcellular fractionation, detergent extraction, and Western blotting using isoenzyme-specific monoclonal antibodies. Stimulated contractile activity produced myocyte hypertrophy, as indicated by increased cell size, a 15±5% increase in total protein/DNA ratio, and induction of ANF and β MHC gene transcription. Electrical pacing did not cause ERK1/2 or JNK1 activation, but increased JNK2 and JNK3 phosphorylation by two-fold. Subcellular fractionation revealed a time-dependent increase in PKC δ, and to a much lesser extent PKC ξ, in a Triton X-100-soluble membrane fraction within 5 min of the onset of stimulated contraction. PKC α was not activated by electrical pacing. These results indicate that contraction-induced mechanical loading acutely activates some but not all of the specific isoenzymes of JNKs and PKCs in cardiomyocytes.