5-HD abolishes ischemic preconditioning independently of monophasic actionpotential duration in the heart

Objective: Blocking of the K-ATP channel with either glibenclamide or 5-hyd roxydecanoate (5-HD) has been shown to abolish the infarct reducing effect of ischemic preconditioning (IPC) in hearts from several species, but the r esults in rat and rabbit have been equivocal. In this study we investigated if 5-HD could abolish IPC in rat and rabbit and further if IPC or IPC + 5- HD were affecting action potential duration in the rabbit heart. Methods: The rat hearts were isolated and retrogradely perfused on a Langen dorff perfusion apparatus with Krebs-Henseleit buffer. The rabbit experimen ts were performed in an in situ model. Rat and rabbit hearts were subjected to 30 min regional ischemia by ligating a coronary artery followed by 120 min (rat) or 150 min (rabbit) of reperfusion. The preconditioning protocol was one or three cycles of 5 min ischemia plus 5 min reperfusion in the rat and one cycle of 5 min ischemia plus 10 min reperfusion in the rabbit. In the rat 5-HD was added to the reservoir before ischemic preconditioning in different concentrations, and in the rabbit 5-HD was given as a bolus 5 mg/ kg intraventricularly 2 min before the preconditioning ischemia. In the rab bit epicardial monophasic action potential duration at 50% repolarization ( MAPD(50)) was measured at 1, 2 and 5 min in each of the ischemic periods us ing a contact pressure electrode. Infarcts were measured with tetrazolium s taining and risk zone volumes with fluorescent microspheres. Results: All data are presented as infarct size in % of risk zone volume (m ean +/- SEM). In the rat 200 mu M of 5-HD abolished the protective effect o f one cycle of IPC (28.6 +/- 4.7 versus 8.4 +/- 0.8) and 500M of 5-HD aboli shed three cycles of IPC (50.7 +/- 7.8 versus 8.4 +/- 2.0). Control was 40. 9 +/- 2.8. In the rabbit 5-HD abolished IPC (41.2 +/- 7.2 versus 8.1 +/- 3.2). Control was 53.5 +/- 12.4. MAPD(50) were significantly more shortened compared to control at 1 and 2 min into the 30 min ischemia for the IPC and IPC+5-HD. Conclusions: We conclude that 5-HD abolishes ischemic preconditioning when given before the preconditioning ischemia in both rat and rabbit but does n ot abolish the ischemia induced shortening of the action potential duration in the rabbit; thus, a role for the mitochondrial Kp;yp channel and not th e sarcolemmal K-ATP channel in the protective mechanism behind IPC is proba ble.