Protein kinase C isoform–dependent myocardial protection by ischemic preconditioning and potassium cardioplegia

Objective: Ischemic preconditioning combined with potassium cardioplegia does not always confer additive myocardial protection. This study tested the hypothesis that the efficacy of ischemic preconditioning under potassium cardioplegia is dependent on protein kinase C isoform. Methods: Isolated and crystalloid-perfused rat hearts underwent 5 cycles of 1 minute of ischemia and 5 minutes of reperfusion (low-grade ischemic preconditioning) or 3 cycles of 5 minutes of ischemia and 5 minutes of reperfusion (high-grade ischemic preconditioning) or time-matched continuous perfusion. These hearts received a further 5 minutes of infusion of normal buffer or oxygenated potassium cardioplegic solution. The isoform nonselective protein kinase C inhibitor chelerythrine (5 μmol/L) was administered throughout the preischemic period. All hearts underwent 35 minutes of normothermic global ischemia followed by 30 minutes of reperfusion. Isovolumic left ventricular function and creatine kinase release were measured as the end points of myocardial protection. Distribution of protein kinase C α, δ, and ϵ in the cytosol and the membrane fractions were analyzed by Western blotting and quantified by a densitometric assay. Results: Low-grade ischemic preconditioning was almost as beneficial as potassium cardioplegia in improving functional recovery; left ventricular developed pressure 30 minutes after reperfusion was 70 ± 15 mm Hg (P < .01) in low-grade ischemic preconditioning and 77 ± 14 mm Hg (P < .001) in potassium cardioplegia compared with values found in unprotected control hearts (39 ± 12 mm Hg). Creatine kinase release during reperfusion was also equally inhibited by low-grade ischemic preconditioning (18.2 ± 10.6 IU/g dry weight, P < .05) and potassium cardioplegia (17.6 ± 6.7 IU/g, P < .01) compared with control values. However, low-grade ischemic preconditioning in combination with potassium cardioplegia conferred no significant additional myocardial protection; left ventricular developed pressure was 80 ± 17 mm Hg, and creatine kinase release was 14.8 ± 11.0 IU/g. In contrast, high-grade ischemic preconditioning with potassium cardioplegia conferred better myocardial protection than potassium cardioplegia alone; left ventricular developed pressure was 121 ± 16 mm Hg (P < .001), and creatine kinase release was 8.3 ± 5.8 IU/g (P < .05). Chelerythrine itself had no significant effect on functional recovery and creatine kinase release in the control hearts, but it did inh