Mitochondrial K_ATP channel as an end effector of cardioprotection during late preconditioning: Triggering role of nitric oxide

Nitric oxide (NO) has been implicated in the "second-window" of ischemic preconditioning (PC). However, the identity of the end effector after initiation of preconditioning by NO is not known. It is likely that NO is involved in opening of mitochondrial ATP-sensitive potassium (mitoK_ATP) channels. We hypothesized that NO is an important trigger for the opening of mitoK_ATP channels in the late phase of preconditioning and inducible nitric oxide synthase (iNOS) up-regulation via NFκB plays a critical role in diazoxide-induced cardioprotection. To examine this, diazoxide (7 mg/kg) was administered to wild-type (WT) mice and mice lacking the gene 24 hours before 40 minutes of global ischemia. Hearts were perfused in a Langendorff mode and effects of activation of mitoK_ATP channel and other interventions on functional, biochemical and pathological changes in ischemic hearts were assessed. In hearts from WT mice treated diazoxide, left-ventricular-developed pressure, end-diastolic pressure and coronary flow were significantly improved after ischemia/reperfusion (I/R); lactate dehydrogenase (LDH) release was also significantly decreased, while ATP contents were significantly higher. Administration of 5-HD, a specific blocker of mitoK_ATP channel or L-NAME, an inhibitor of iNOS before I/R, during diazoxide-pretreatment completely blocked the late cardioprotection against ischemia. Late cardioprotection was also blocked by inhibition of either PKC-δ by rottlerin or NFκB by DDTC before diazoxide pretreatment. Diazoxide pretreatment significantly increased nuclear translocation of p65 which was blocked by protein kinase C (PKC) or nitric oxide synthase (NOS) inhibition. Diazoxide was totally ineffective in iNOS knockout mice. These results suggest that diazoxide activates NFκB via PKC signaling pathway and that leads to iNOS up-regulation after 24 hours. NO which is generated upon ischemic stress triggers the opening of mitoK_ATP channel as an end effector of cardioprotection during late PC.