Ca²⁺ as a mediator of ischemic preconditioning

We tested the hypothesis that elevation of [Ca²⁺](i) during ischemic preconditioning (IPC) stimulates protein kinase C (PKC), which confers the protection against the ischemic injury. Langendorff-perfused rat hearts were subjected to 40-minute global ischemia followed by 30-minute reperfusion (I/R). In preconditioned groups, hearts were subjected to either IPC, consisting of 5-minute global ischemia and 10-minute reperfusion, or high- Ca²⁺ preconditioning (HCPC), ie, the 5-minute perfusion of higher Ca²⁺ perfusate (2.3 mmol/L Ca²⁺) followed by 10-minute perfusion of normal perfusate (1.8 mmol/L Ca²⁺), and then were subjected to I/R. A significant functional recovery and decreased lactate dehydrogenase release were observed in HCPC and IPC hearts compared with ischemic control hearts. ATP contents of preconditioned hearts were significantly higher than those of the ischemic control hearts. The cell structure in preconditioned hearts was preserved better than that in the ischemic control hearts. Furthermore, the activation and translocation of PKC from cytoplasm to sarcolemma were observed in the preconditioned hearts. Verapamil administered during IPC significantly attenuated the salutary effects of IPC. Administration of chelerythrine a specific PKC inhibitor, completely abolished the HCPC- and IPC-induced cardioprotection. The translocation of PKC by IPC was blocked by verapamil or chelerythrine. Immunohistochemical study using rabbit polyclonal antibody against PKC isoforms indicated that stress induced by IPC or HCPC evoked the translocation of PKCα and PKCδ to the cell membrane. Translocation of PKC isoforms was attenuated by the treatment with verapamil or chelerythrine. These results demonstrate that (I) a transient increase in [Ca²⁺](i) during IPC is an important trigger for the activation of PKC, which is responsible for cardioprotection; (2) the elevation of [Ca²⁺](i) during IPC, at least partly, resulted from Ca²⁺ entry via voltage-dependent Ca²⁺ channel; and (3) activation and translocation of PKCα and PKCδ occur during IPC and HCPC and may be important in preconditioning.