Preconditioning the human myocardium by simulated ischemia:: studies on the early and delayed protection

Background: There are data supporting the existence of ischemic preconditioning in man. This study investigated the most effective preconditioning protocol for the human myocardium and whether the second window of ischemic preconditioning (24 h) is as protective as the first window (less than or equal to 2 h). Methods and results: Right atrial appendages (n=6/group) obtained during coronary bypass surgery were prepared and superfused with normoxic and normothermic Krebs-Henseleit solution. After 30 min stabilisation, muscles were subjected to various preconditioning protocols followed by 90 min ischemia and 120 min reperfusion. At the end of each protocol, the leakage of creatinine kinase (CK, U/g wet wt) and the reduction of MTT to insoluble formazan dye (OD/mg wet wt), an index of cell viability, were measured. In study 1, preconditioning was induced by 2, 3, 5 and 10 min of ischemia followed by 5 min reperfusion. In study 2, 1-4 cycles of 2 or 5 min ischemia-5 min reperfusion were applied. In study 3, preconditioning was induced by 5 min ischemia-5 min reperfusion followed by 1, 2, 3 or 4 h reperfusion before the subsequent 90 min ischemia. In study 4, preconditioning with 5 min ischemia followed by 5 min reperfusion either immediately preceded 30 or 90 min ischemia/120 min reperfusion or was applied 24 h before. In study 1 and 2, optimal protection was achieved with 5 min or two cycles of 2 min preconditioning ischemia (CK=3.06+/-0.31 and 2.89+/-0.02; MTT=0.56+/-0.05 and 0.47+/-0.09, respectively vs. CK=5.56+/-0.51 and MTT=0.18+/-0.04 in ischemia alone group; P<0.05). In study 3, protection was observed 2 h after preconditioning (CK=3.43+/-0.22 and MTT=0.46+/-0.09; P<0.01 vs. ischemia alone group) but it was lost beyond 2 h (CK=6.30+/-0.56 and MTT=0.16+/-0.02 after 3 h; P=NS vs. ischemia alone group). In study 4, protection was observed 24 h following preconditioning when the atrial specimens were exposed Co 30 min ischemia (CK=2.96+/-0.38 and MTT=0.61+/-0.01 vs. CK=4.56+/-0.26 and MTT=0.43+/-0.02 in ischemia alone group, P<0.05); however, when the period of ischemia was extended to 90 min the beneficial effect of preconditioning was lost (CK=10.28+/-0.5 and MTT=0.11+/-0.09 vs. CK=9.56+/-0.62 and MTT=0.104+/-0.05 in ischemia alone group, P=NS). Conclusions: In the isolated human myocardium maximal protection induced by preconditioning is achieved by a total 4-5 min ischemic stimulus, an effect that is lost beyond 2 h of its application. Two windows of protection were identified, the first (less than or equal to 2 h) being more potent than the second (24 h). (C) 2000 Published by Elsevier Science B.V. All rights reserved.