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

Abstract

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.