BACKGROUND AND OBJECTIVES: Protein kinase C (PKC) has been known to play a central role in mediating ischemic preconditioning. The isoform of the PKC changes during the development of the heart in rats. Therefore, the protective effects of PKC activation may vary between neonatal and adult hearts. MATERIALS AND METHODS: To test this hypothesis, primary cultures of neonatal and adult rat ventricular myocytes (NRVM and ARVM, respectively) were subjected to ischemic condition, which consisted of a deoxygenated air supply and glucose deprivation in the media. The survival was evaluated by counting trypan blue excluding cells. The effect of PKC activation was analyzed by the addition of a PKC agonist (12-o-tetradecanoylphorbol 13-acetate, TPA), or an antagonist (staurosporin) to cultured myocytes. RESULTS: Under ischemic condition, ARVMs were more susceptible than NRVM. The survival of the ARVMs were 63.1+/-8.3%, 42.8+/-6.1%, 10.1+/-5.8% after 3, 6, 12 hours of ischemia, respectively, while those of the NRVMs were 68.9+/-6.4%, 60.3+/-7.3%, 34.3+/-7.5%, and 8.2+/-6.6% after 6, 12, 24, 36 hours of ischemia, respectively (p=0.031). However, the activation of the PKC following the addition of 100 nM TPA to the media significantly enhanced the survival of the ARVM, from 38.5+/-8.3% to 62.1+/-7.3%, after 6 hours of ischemia, which was similar to that of the controls (65.4+/-6.2%). In contrast, the activation of the PKC by the addition of TPA did not change the survival of the NRVM, from 31.8+/-5.8% to 28.5+/-7.3%, after less than 24 hours of ischemia. CONCLUSION: These findings demonstrate that the protective effect of PKC activation in adult hearts differs from that in neonatal hearts, indicating that PKC isoform variance between two tissues may affect the biologic consequence of its activation.