Background: Ischemic preconditioning (PC) is a powerful mechanism in reducing infarct size of the heart. Protection can be performed either by an ischemic stimulus of the heart itself or by ischemia of an organ distant to the heart (remote PC). We have previously shown that remote PC by infrarenal occlusion of the aorta [IOA] in the rat is as powerful as classical ischemic PC. This protection may be transmitted by humoral factors, and protein kinase C is a mediator in the signal transduction mechanism. Focus of the present study was to address the question whether remote preconditioning is dependent on the activation of the delta1-opioid receptor and/or free radicals, the infarct size was determined after either inhibition of the delta1-opioid receptor or scavenging free radicals.
Methods and results: IOA was performed in rats by occlusion of the infrarenal aorta for 15 min followed by a 10-min reperfusion period. Infarction of the heart was induced by 30 min regional ischemia followed by 30 min of reperfusion. The area of infarct was determined by propidium iodide and the risk zone was demarcated by zinc cadmium sulfide fluorescent particles. Control hearts (30 min regional ischemia of the heart followed by 30 min of reperfusion; no IOA) had an infarct size of 54 +/- 3%, whereas classical preconditioning by three ischemia/reperfusion [I/R] cycles, 5 min each, reduced it to 12 +/- 1% of the risk zone (p<0.05). Fifteen minutes IOA with 10 min of reperfusion was highly protective and reduced the infarct size to 20 +/- 5% (p<0.05 vs. control). Inhibition of the delta1-opioid receptors by 7-benzylidenenaltrexone [BNTX] blocked the protection obtained by PC and IOA (41 +/- 4% and 44 +/- 2%, respectively; p<0.05 vs. the group without BNTX). BNTX in control hearts had no influence on infarct size (52 +/- 2%). Inhibition of endogenously released radicals by N-2-mercaptopropionyl glycine [MPG] blocked the infarct size reduction of IOA (46 +/- 3%; p<0.05 vs. IOA), but had no influence on the protection in classically preconditioned hearts protected by three cycles I/R (13 +/- 4%). Only if the number of the preconditioning stimuli was reduced to one was MPG able to overcome the protection (43 +/- 4%, p<0.05 vs. PC with one I/R cycle (21 +/- 4%)).
Conclusion: Remote preconditioning using IOA protects the rat heart from infarction. Classical and remote PC share both the delta1-opioid-receptor and free radicals as common elements in their signal transduction pathways. MPG can block protection from IOA and from one, but not from three, classical preconditioning cycles. This indicates that the protection by remote preconditioning is comparable to classical PC with one I/R cycle.