CaMKII mediates myocardial ischemia/reperfusion injury­induced contracture in isolated rat heart.

Contracture or diastolic dysfunction is a primary cause of injury following ischemia/reperfusion (IR). The present study examined whether Ca2+/calmodulin­dependent kinase II (CaMKII) is involved in contracture. Isolated rat hearts were subjected to either global IR or Ca2+ paradox (CaP), which is characterized by contracture. Left ventricular end­diastolic pressure, electron microscopy and troponin I  TnI) in coronary effluent were examined to indicate the extent of contracture. Compared with control hearts, both the IR and CaP groups exhibited an increase in necrosis and apoptosis, and a marked depression in contractile function. Western blot analysis showed that IR stimulated the phosphorylation (Thr287) and oxidation (Met281/282) of CaMKII, and the phosphorylation of phospholamban (PLN), a substrate of CaMKII. By contrast, only the phosphorylation of CaMKII was increased in the CaP group. Treatment with either 3 µM KN­62, an inhibitor of CaMKII, or 5 µM KB­R7943, an inhibitor of the Na+/Ca2+ exchanger, mitigated the damage and the post­translational modification of both CaMKII and PLN. Similar to the effect of the negative inotropic agent 2,3­butanedione­monoxime, the increased cell survival after treatment with KN­62 was associated with improved diastolic function. Examination using electron microscopy and a biochemical test showed the development of contraction bands, disruption of the sarcolemmal membrane and an increase in the release of TnI in both IR and CaP hearts; these results indicated the occurrence of contracture. Furthermore, these changes were inhibited by either KN­62 or KB­R7943. Taken together, these data provided evidence that CaMKII mediates reperfusion­elicited contracture, and that the activation of CaMKII via phosphorylation is involved in this process.