Cardioprotective Effect of Ischemic Preconditioning: Relationship between Intracellular Glycogen and Protein Kinase C

ABSTRACT

BACKGROUND: Recent studies suggest that the cardioprotective effect of ischemic preconditioning (IPC) is related to intracellular glycogen content in rat hearts, however, controversies still remain. METHODS: To test this hypothesis, isolated Langendorff-perfused rabbit hearts were subjected to 45 min global ischemia followed by 120 min reperfusion with IPC (n=0) or without IPC (ischemic control, n=). IPC was induced by one cycle of 5 min global ischemia and 10 min reperfusion. In the glucose (G)-free preconditioned group (n=0), G depletion-repletion was induced by perfusion with G-free Tyrode solution for 5 min and then G-containing Tyrode solution for 10 min followed by 45 min ischemia and 120 min reperfusion. For glycogen depletion or loading, hearts were treated with sodium acetate (NA, 5 mM, n=) or insulin (Ins, 1 unit/L, n=) for 15 min before 45 min ischemia. Left ventricular function and coronary flow (CF) were continuously recorded during experiments. Myocardial cytosolic and membrane protein kinase C (PKC) activities were measured by 32P-gamma-ATP incorporation into PKC-specific pepetide; glycogen content in the cardiac myocytes was determined by spectrophotometry with amyloglucosidase; expression of PKC isozymes was determined by Western blot with monoclonal antibodies. Infarct size was determined by staining with tetrazolium salt and planimetry. Data were analyzed by ANOVA and Tukey's post-hoc test. RESULTS: IPC or G-free preconditioning enhanced LV functional recovery; NA did not influence on functional recovery but Ins depressed it. Infarct size was significantly reduced by IPC, G-free preconditioning, and NA treatment (35.3+/-2.1% in the ischemic control, 18.7+/-1.2% in the IPC, 22.1+/-1.2% in the G-free preconditioned, 16.3+/-1.2% in the NA-treated group, and 32.8+/-1.6% in the Ins-treated group, p<0.05). Membrane PKC activities significantly increased by IPC, IPC and 45 min ischemia, G-free preconditioning, and G-free preconditioning and 45 min ischemia; especially, expression of membrane PKC-epsilon increased by IPC and G-free preconditioning. Glycogen content decreased by 45 min ischemia, IPC, G-free preconditioning, and by NA treatment, but increased by Ins treatment. CONCLUSION: These results suggest that in rabbit heart, intracellular glycogen may not significantly be related with the cardioprotective effect of IPC; G-free preconditioning could not improve post-ischemic contractile dysfunction but it has an infarct size-limiting effect; this cardioprotective effect may be related in part to activation of PKC, especially epsilon isozyme.