PKC-epsilon is upstream and PKC-alpha is downstream of mitoKATP channels in the signal transduction pathway of ischemic preconditioning of human myocardium.

Protein kinase C (PKC) is involved in the process of ischemic preconditioning (IPC), although the precise mechanism is still a subject of debate. Using specific PKC inhibitors, we investigated which PKC isoforms were involved in IPC of the human atrial myocardium sections and to determine their temporal relationship to the opening of mitochondrial potassium-sensitive ATP (mitoKATP) channels. Right atrial muscles obtained from patients undergoing elective cardiac surgery were equilibrated and then randomized to receive any of the following protocols: aerobic control, 90-min simulated ischemia/120-min reoxygenation, IPC using 5-min simulated ischemia/5-min reoxygenation followed by 90-min simulated ischemia/120-min reoxygenation and finally, PKC inhibitors were added 10 min before and 10 min during IPC followed by 90-min simulated ischemia/120-min reoxygenation. The PKC isoforms inhibitors investigated were V1-2 peptide, GO-6976, rottlerin, and LY-333531 for PKC-epsilon, -alpha, -delta and -beta, respectively. To investigate the relation of PKC isoforms to mitoKATP channels, PKC inhibitors found to be involved in IPC were added 10 min before and 10 min during preconditioning by diazoxide followed by 90-min simulated ischemia/120-min reoxygenation in a second experiment. Creatine kinase leakage and methylthiazoletetrazolium cell viability were measured. Phosphorylation of PKC isoforms after activation of the sample by either diazoxide or IPC was detected by using Western blot analysis and then analyzed by using Scion image software. PKC-alpha and -epsilon inhibitors blocked IPC, whereas PKC-delta and -beta inhibitors did not. The protection elicited by diazoxide, believed to be via mitoKATP channels opening, was blocked by the inhibition of PKC-alpha but not -epsilon isoforms. In addition, diazoxide caused increased phosphorylation of PKC-alpha to the same extent as IPC but did not affect the phosphorylation of PKC-epsilon, a process believed to be critical in PKC activation. The results demonstrate that PKC-alpha and -epsilon are involved in IPC of the human myocardium with PKC-epsilon being upstream and PKC-alpha being downstream of mitoKATP channels.