Differences in the Expression of Protein Kinase C Isoforms and Its Translocation After Stimulation With Phorbol Ester Between Young-Adult and Middle-Aged Ventricular Cardiomyocytes Isolated From Fischer 344 Rats

It is known that the tolerance against ischemia-reperfusion and the effects of preconditioning decrease in aged hearts, but the mechanisms responsible for this diminished ischemic tolerance and reduced efficacy of preconditioning remain unknown. To determine the age-related changes in these mechanisms, protein kinase C(PKC)isoform expression and its translocation by phorbol ester were analyzed because PKC is believed to be involved in preconditioning. Immunoblotting and immunostaining analysis were performed with isoform-specific PKC antibodies using cardiomyocytes isolated from young-adult(12-week-old: 12W)and middle-aged(50-week-old: 50W)Fischer 344 rats. There was significantly greater PKC-δ expression in both the cytosolic and membrane fractions of 12W cardiomyocytes than in 50W ones. Exposure of cardiomyocytes to 100 nmol/L 4-β-phorbol 12-myristate 13-acetate(PMA)caused translocation of PKC-δ from the cytosol to the membrane in the 12W group, whereas in the 50W group, the translocation was attenuated. Immunostaining confirmed the PKC-δ translocation in the 12W cardiomyocytes. Oil pellet examination showed that the translocation of PKC-δ induced by preconditioning was associated with cell protection from ischemic injury in the 12W group only. Age-related changes in PKC isoform expression and activation in cardiomyocytes might be responsible for the reduced ischemic tolerance and less efficient preconditioning that accompanies aging.