Prolonged phorbol ester treatment down-regulates protein kinase C isozymes and increases contraction rate in neonatal cardiac myocytes

We have determined the effects of chronic exposure to the protein kinase C (PKC) activating drug 4-p phorbol 12-myristate-13-acetate (PMA) on PKC isozymes and the rate of spontaneous contraction in neonatal rat cardiac myocytes in culture. Western blot analyses revealed that a two day exposure to 0.1-1 nM PMA increased the total amount of δPKC, whereas, 100 nM PMA concentrations caused a complete down-regulation of the αPKC and an 80 kDa ζPKC-like protein. In addition, 100 nM PMA treatment for 2 days did not result in complete down-regulation of the β, 6, and εPKC isozymes in Western blot and immunocytochemical studies. We also found a PKC-mediated enhancement of the rate of contraction in these cells following prolonged exposure to PMA (1-100nM). Our studies suggest that this enhancement of contraction rate may be mediated by the β, δ, or εPKC isozymes. A better understanding of the role(s) of PKC isozymes in the modulation of cardiac functions may reveal selective targets for therapies of cardiac disorders.