Myocardial protection afforded by nicorandil and ischaemic preconditioning in a rabbit infarct model in vivo.

We previously showed that preoperative nicorandil, a hybrid potassium channel opener and nitrate compound, conferred cardioprotective effects in a hypoxia/reoxygenation model of isolated human atrial muscle by using functional recovery as an end point, and that ischaemic preconditioning surprisingly abolished the protection afforded by nicorandil. In view of this paradoxic result, this study was undertaken to assess whether ischaemic preconditioning influences any protective effect of nicorandil by using infarct size as an end point. In addition, we investigated the underlying mechanisms of the protective action of nicorandil. Rabbits underwent a midline sternotomy under anaesthesia. A left coronary branch was occluded for 30 min followed by 120 min of reperfusion. Nicorandil (100 microg/kg bolus + 10 microg/kg/min) was given intravenously 30 min before coronary occlusion and continued to the time of reperfusion (early treatment) or 5 min before reperfusion and continued throughout reperfusion (late treatment). Ischaemic preconditioning was achieved by a single episode of 5-min coronary occlusion followed by 10-min reperfusion before the 30-minute occlusion in the presence or absence of nicorandil. Risk volume and infarct volume were determined by fluorescent microspheres and tetrazolium staining, respectively. Early treatment with nicorandil conferred a significant decrease in percentage of infarct size within the risk zone (24.9 +/- 2.9%) when compared with control (39.2 +/- 4.3%; p < 0.01). Late treatment with nicorandil had no effect on infarct size (43.5 +/- 3.4%). Ischaemic preconditioning also resulted in significant reduction in infarct size (13.4 +/- 4.3%; p < 0.01 vs. control). The combination of ischaemic preconditioning with nicorandil (early treatment) showed an intermediate protective efficacy between early treatment with nicorandil alone and ischaemic preconditioning alone (18.1 +/- 4.2%; p < 0.01 vs. control). Nitroglycerin (10 microg/kg bolus + 1 microg/kg/kg/min, i.v.) given before and during ischaemia tended to reduce infarct size, but the effect was not statistically significant (28.9 +/- 2.9%; p > 0.05 vs. control). Although an adenosine triphosphate (ATP)-sensitive potassium channel blocker, 5-hydroxydecanoate (5 mg/kg, i.v.) by itself had no effect on infarct size (38.8 +/- 3.6%), the protective effect of nicorandil was abolished by 5-hydroxydecanoate (37.7 +/- 5.8%; p < 0.05 vs. early treatment of nicorandil). There were no differences in area at risk or haemodynamics between groups. Our results show that nicorandil has a protective effect against myocardial infarction in our rabbit model when infused before and during ischaemia, but not during reperfusion, and the protective effect is abolished by an ATP-sensitive potassium channel blocker. Furthermore, the addition of ischaemic preconditioning does not detrimentally influence the effect of nicorandil. This suggests that nicorandil can confer an infarct-limiting effect by opening of ATP-sensitive potassium channels with or without intermittent ischaemia, as may happen in patients with unstable angina.