Effect of protein kinase C inhibitors on cardioprotection by ischemic preconditioning depends on the number of preconditioning episodes
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Objectives: This study examined the possibility that the role of PKC and PC, and thus the response to PFC inhibitors, may differ depending on how many ischemic episodes are employed to precondition the heart. Methods: In the first series of experiments, myocardial infarct was induced by 30 min of coronary occlusion and 3 h of reperfusion in the rabbit. Infarct size was determined by tetrazolium staining and expressed as a percentage of area at risk ( % IS/AR). Prior to the 30-min ischemia, rabbits were subjected to no PC, single PC (i.e., PC with an episode of 5 min ischemia/5 min reperfusion), and repetitive PC (2 cycles of 5 min ischemia/5 min reperfusion) with or without one of three treatments: polymyxin B (PolyB), staurosporine (Stauro), and 8-sulfophenylthephylline (SPT). In the second series of experiments, the rabbits received 5 min of coronary occlusion after repetitive PC with or without PolyB or Stauro treatment. Then, myocardial tissue in the ischemic region was sampled for assay of PKC activity. Untreated rabbits served as controls. Results: Single and repetitive PC limited % IS/AR to the same extent ( % IS/AR = 9.8 ± 1.9 and 10.4 ± 2.3, both p < 0.05, vs. the control value of 44.5 ± 3.4), and single PC was blocked by PolyB ( % IS/AR = 43.9 ± 2.7) and Stauro ( % IS/AR = 31.5 ± 3.2). Although the protocol of PolyB injection maintained the plasma PolyB level during sustained ischemia well above its Ki for PKC, this agent and also Stauro failed to abolish the protection by repetitive PC ( % IS/AR = 21.6 ± 3.0 and 11.4 ± 4.3, respectively). SPT, an adenosine receptor antagonist, not only blocked single PC ( % IS/AR = 44.4 ± 4.4) but also attenuated protection by repetitive PC ( % IS/AR = 28.3 ± 3.6). Infarct sizes in non-preconditioned hearts were not modified by PolyB, Stauro, or SPT. The ratio of membrane fraction PKC activity to cytosolic fraction PKC activity was elevated by repetitive PC plus 5 min ischemia, and this change in PKC was inhibited in hearts given PolyB and Stauro. Conclusions: In contrast to single PC, repetitive PC protects the heart against infarction even when PolyB and Stauro are administered to inhibit PKC during ischemic insult. This difference may be attributable to a PKC-independent mechanism, in which the adenosine receptor may be partly involved.