Activation of ATP-dependent K+ channels enhances myocardial protection due to cold high potassium cardioplegia: A force-frequency relationship study

The hypothesis that nicorandil might enhance myocardial protection due to cold St Thomasʹ Hospital (STH) solution ([K+]o 16 mmol/1) through opening of cardiac KATP channels was assessed in isometrically contracting guinea-pig papillary muscles submitted to 120 min of cardioplegic hypoxia followed by 60 min of normothermic reoxygenation. Right ventricular papillary muscles were paced (2 ms, 4 mA) in an organ bath and superfused with oxygenated (O2 content 16 ml/l) Tyrodeʹs solution (37 °C. The force-frequency relationship in the range 1600-300 ms cycle length (CL) was studied. Preparations were randomized to receive 120 min cold (20 °C), non-oxygenated (O2 content 5 ml/1) STH solution while continuously stimulated at 1600 ms CL, with: (1) saline (No-additive, n = 12); (2) DMSO 1% (Vehicle, n = 8); (3) nicorandil 1 mmol/l (n = 8): (4) nicorandil 1 mmol/I plus glibenclamide 1 μmol/l, the latter also given, before STH solution, in Tyrodeʹs solution for 15 min (n = 8); (5) glibenclamide 1 μmol/l, also circulated, before STH solution, in Tyrodeʹs solution for 15 min (n = 8); (6) nitroglycerin 100 μmol/l (n = 4); in addition, we studied: (7) STH solution with no-additive and no-pacing (n = 4); (8) cold Tyrodeʹs in place of cold STH solution (n = 4). Isotropic state was investigated by measuring: (i) velocity of developed tension (DT), obtained by dividing DT by time to peak tension; (ii) percentage (from precardioplegia values) velocity changes of DT; (iii) log velocity of DT. Post-cardioplegic recovery of contractility (including force-frequency relationship) was assessed in all preparations: (a) 60 min after reoxygenation with Tyrodeʹs solution; (b) after further 15 min superfusion with the positive isotropic agent dobutamine (10 ymol/1). In parallel experiments, action potential duration (APD) 50% changes induced by nicorandil or glibenclamide plus nicorandil in spontaneously beating atrial (n = 4) or electrically driven (1600 ms CL) ventricular (n = 8) tissues during 10 min of STH solution were investigated. Based on force-frequency relationship, at 60 min reoxygenation, in absence of cardioplegia, the lowest recovery of myocardial contractility was seen (stunning). In STH solution, there was moderate to severe stunning, which was unaffected by removing pacing during cardioplegia, or by vehicle or nitroglycerin. In contrast, nicorandil improved recovery of contractility (F =3.01, P = 0.0106). After dobutamine, nicorandil preparations showed the highest positive isotropic response, which was completely offset by glibenclamide (F = 3.47. P = 0.0046). Multivariate statistical analysis (at 1600 ms CL only) showed correlations (0.59