EVIDENCE FOR INVOLVEMENT OF OPIOID RECEPTORS IN ISCHEMIC PRECONDITIONING IN RAT HEARTS

The purpose of the present study was to investigate a possible role of opioid receptors in ischemic preconditioning (PC). To test this hypot hesis, anesthetized, open-chest, male Wistar rats were subjected to fi ve different protocols. In group I, the control group was subjected to 30 min of left coronary artery occlusion and 2 h of reperfusion. In g roup II, ischemic PC was elicited by three 5-min occlusion periods int erspersed with 5 min of reperfusion. In group III, naloxone (NL, 3 mg/ kg iv), a nonselective opioid antagonist, was given to nonprecondition ed rats 10 min before the 30-min occlusion period. Finally, NL was adm inistered 10 min before preconditioning (NL + PC, group TV) or immedia tely after the last 5-min preconditioning period (PC + NL, group V). I nfarct size (IS) as a percentage of the area at risk (AAR) (IS/AAR) wa s determined by 2,3,5-triphenyltetrazolium chloride staining. PC resul ted in a marked reduction in myocardial IS from 45 +/- 5 to 8 +/- 1 (P < 0.05). NL treatment before or immediately after PC abolished this p rotective effect; however, NL had no effect on IS in non-PC rats. Thes e results are the first to support the hypothesis that activation of o pioid receptors may play an important role in ischemic PC in the rat m yocardium.