We recently demonstrated that protein kinase C (PKC) activities were elevated in hypertrophic cardiomyopathic (HCM) hamster hearts and that activation of PKC resulted in stimulation of cAMP-dependent phosphodiesterase (PDE) activity. In this study, we determined the composition of PKC isozymes in control and HCM hearts and identified the PKC isozyme responsible for the modulation of PDE activity in HCM hearts. Using quantitative autoradiographic techniques with PKC isozyme-specific antibodies, we found that the PKC alpha, epsilon, and zeta isozymes were expressed in both control and HCM hearts. The immunoreactive amounts of cytosolic PKC alpha and PKC epsilon and of membrane PKC zeta were significantly increased in HCM hearts. The enzymatic activity of PKC in HCM hearts was significantly elevated in both membrane (148.0 +/- 13.7 versus 78.9 +/- 1.9 pmol/mg/min in controls, four experiments) and cytosol (117.3 +/- 5.1 versus 75.7 +/- 5.1 pmol/mg/min in controls, four experiments). Contribution of individual PKC isozyme activity was assessed by the immunoprecipitable PKC activity with isozyme-specific antibodies. The membrane PKC epsilon (41.7 +/- 4.9 versus 18.7 +/- 0.3 pmol/mg/min in controls, four experiments, p < 0.05) and PKC zeta (61.5 +/- 14.0 versus 20.3 +/- 2.7 pmol/mg/min in controls, four experiments, p < 0.05) but not PKC alpha (50.9 +/- 6.8 versus 44.3 +/- 1.5 pmol/mg/min, four experiments, p = N.S.) were increased in HCM hearts. On the other hand, the cytosolic PKC alpha (47.7 +/- 4.1 versus 27.0 +/- 1.4 pmol/mg/min, four experiments, p < 0.05) and PKC epsilon (42.8 +/- 3.1 versus 19.1 +/- 3.9 pmol/mg/min, four experiments, p < 0.05) but not PKC zeta (27.2 +/- 3.0 versus 32.0 +/- 2.1, four experiments, p = N.S.) were increased in HCM hearts. Furthermore, after immunoprecipitation of PKC alpha, activation of PKC could no longer potentiate the PDE activity in HCM hearts. Removal of PKC epsilon or PKC zeta, on the other hand, did not affect the PKC-mediated PDE stimulation in HCM hearts. These results suggest that there is an increase in the quantitative expression of PKC isozymes in HCM hearts and that the cross-talk between PKC and PDE in these hearts is mediated specifically via the PKC alpha isozyme.