Effect of the potassium-channel opener nicorandil as an adjunct to cardioplegia on myocardial preservation in isolated rabbit hearts

Mechanisms by which KATP channel openers produce acute and delayed cardioprotection

Mitochondria are being increasingly studied for their critical role in cell survival. Multiple diverse signaling pathways have been shown to converge on the K+-sensitive ATP channels as the effectors of cytoprotection against necrosis and apoptosis. The role of potassium channel openers in regulation and transformation of cell membrane excitability, action potential and electrolyte transfer has been extensively studied. Cardiac mitoK(ATP) channels are the key effectors in cardioprotection during ischemic preconditioning, as yet with an undefined mechanism. They have been hypothesized to couple myocardial metabolism with membrane electrical activity and provide an excellent target for drug therapy. A number of K(ATP) channel openers have been characterized for their beneficial effects on the myocardium against ischemic injury. This review updates recent progress in understanding the physiological role of K(ATP) channels in cardiac protection induced by preconditioning and highlights relevant questions and controversies in the light of published data.

Nicorandil ameliorates posttransplant dysfunction in cardiac allografts harvested from non-heart-beating donors

OBJECTIVE

Warm ischemia is a major cause of cardiac allograft dysfunction in non-heart-beating donors (NHBDs). We evaluated the cardioprotective effects of nicorandil, an adenosine triphosphate-sensitive potassium channel opener, on the early posttransplant left ventricular (LV) function of hearts harvested from asphyxiated canine NHBDs.

METHODS

Hypoxic cardiac arrest was induced in 12 donor dogs. In 6, nicorandil was administered intravenously at 100 micrograms/kg + 25 micrograms/kg/min after respiratory arrest and hearts were preserved with nicorandil-supplemented cardioplegic solution (nicorandil group). The remaining 6 did not receive nicorandil at any time during the experiment (control group). Hearts were orthotopically transplanted after a mean myocardial ischemic time of 4 hours.

RESULTS

All 12 recipients were weaned from cardiopulmonary bypass without inotropic support. In the control group, posttransplant cardiac indices and left ventricular end-systolic pressure (LVESP) decreased significantly, while LV max-dP/dt and Tau increased over pretransplant values. No differences were seen in parameters between pretransplant and posttransplant values in the nicorandil group. Posttransplant cardiac indices, LVESP, and LV max + dP/dt were higher in the nicorandil group than in controls, while posttransplant LV max-dP/dt in the nicorandil group was lower.

CONCLUSIONS

Our results indicate that pretreatment with nicorandil during hypoxic perfusion before cardiac arrest and subsequent preservation with nicorandil-supplemented cardioplegia ameliorates early posttransplant LV dysfunction of hearts harvested from asphyxiated NHBDs.

Ischemic preconditioning and nicorandil pretreatment improve donor heart preservation

The present study investigated the effects of ischemic preconditioning (IPC) and nicorandil pretreatment on myocardial storage in a donor heart preservation model. Isolated rat hearts were separated into groups: group 1, non-preconditioned control group; group 2, 2.5 min of normothermic ischemia followed by 15 min of normothermic Langendorff perfusion (one IPC cycle); and group 3, 2 cycles of IPC. All hearts were subsequently stored in University of Wisconsin solution at 4 degrees C for 2, 4 and 6h, and the concentrations of high-energy phosphate metabolites were measured for each time point. Heart function parameters (aortic flow, coronary flow and cardiac output) were measured when the heart was reperfused following the 2, 4 or 6 h of preservation. The effects of nicorandil, an ATP-sensitive potassium channel opener, on heart function following preservation were also evaluated. Nicorandil was injected intravenously before heart harvesting. The results showed that the energy status was well preserved in the IPC groups. The 2-cycle IPC group showed better recovery of heart function following preservation. Pretreatment with nicorandil also improved functional recovery of the heart following preservation. The present study showed that IPC of the rat heart resulted in improved myocardial energy metabolism and functional recovery after hypothermic preservation, and that nicorandil has potential for pharmacological preconditioning in heart preservation for transplantation.

Activation of mitochondrial ATP-sensitive K(+) channel for cardiac protection against ischemic injury is dependent on protein kinase C activity

Protein kinase C (PKC) is involved in the second messenger signaling cascade during ischemic and Ca(2+) preconditioning. Given that the pharmacological activation of mitochondrial ATP-sensitive K(+) (mitoK(ATP)) channels also mimics preconditioning, the mechanisms linking PKC activation and mitoK(ATP) channels remain to be established. We hypothesize that PKC activity is important for the opening of the mitoK(ATP) channel. To examine this, a specific opener of the mitoK(ATP) channel, diazoxide, was used in conjunction with subcellular distribution of PKC in a model of ischemia/reperfusion (I/R). Langendorff-perfused rat hearts were subjected to 40-minute ischemia followed by 30-minute reperfusion. Effects of activation of the mitoK(ATP) channel and other interventions on functional, biochemical, and pathological changes in ischemic hearts were assessed. In hearts treated with diazoxide, left ventricular end-diastolic pressure and coronary flow were significantly improved after I/R; lactate dehydrogenase release was also significantly decreased. The morphology was well preserved in diazoxide-treated hearts compared with nontreated ischemic control hearts. The salutary effects of diazoxide on the ischemic injury were similar to those of Ca(2+) preconditioning. Administration of sodium 5-hydroxydecanoate, an effective blocker of the mitoK(ATP) channel, or chelerythrine or calphostin C, an inhibitor of PKC, during diazoxide pretreatment or during continuous presence of diazoxide in the ischemic period, completely abolished the beneficial effects of the diazoxide on the I/R injury. Blockade of Ca(2+) entry during diazoxide treatment by inhibiting the L-type Ca(2+) channel with verapamil also completely reversed the beneficial effect of diazoxide during I/R. PKC-alpha was translocated to sarcolemma, whereas PKC-delta was translocated to the mitochondria and intercalated disc, and PKC-epsilon was translocated to the intercalated disc of the diazoxide-pretreated hearts. Colocalization studies for mitochondrial distribution with tetramethylrhodamine ethyl ester (TMRE) and PKC isoforms by immunoconfocal microscopy revealed that PKC-delta antibody specifically stained the mitochondria. ATP was significantly increased in the diazoxide-treated hearts. Moreover, the data suggest that activation and translocation of PKC to mitochondria appear to be important for the protection mediated by mitoK(ATP) channel.