The cardioprotective and mitochondrial depolarising properties of exogenous nitric oxide in mouse heart

Objective: Nitric oxide (NO) is reported to be both protective and detrimental in models of myocardial ischaemia/reperfusion injury, which may be concentration dependent. Our objective was to characterise this dichotomy using the nitric oxide donor, S-nitroso N-acetyl penicillamine (SNAP) in isolated perfused mouse heart and isolated mouse cardiac mitochondria. Methods: To determine the effect of nitric oxide concentration on myocardial viability, isolated mouse hearts were subjected to 35 min global ischaemia and 30 min reperfusion in the presence of SNAP (0.02-20 muM). To determine whether NO mediated protection was via opening of the putative mitochondrial K-ATP channel and/or free radical synthesis, SNAP perfused hearts were also treated with the mitochondrial K-ATP channel blocker, 5-hydroxy decanoate (5-HD) and the free-radical scavenger, N-(2-mercaptopropionyl)-glycine (MPG). This data was correlated with mitochondrial membrane potential measured with the potentiometric dye, tetra-methyl rhodium methyl ester (TMRM), in isolated mitochondria,by flow cytometry. Results: SNAP dose-dependently attenuated infarct size, with maximal protection observed at 2 muM (17+/-4% versus controls 32+/-3%, P<0.01). At greater concentrations however, protection was lost with infarct sizes tending towards control at 20 muM (29+/-3%). These results were paralleled by changes in DeltaPsi(m) in the isolated mitochondria: DeltaPsi(m) depolarisation peaking with 1 muM SNAP (26+/-4% shift in TMRM fluorescence, P<0.01); at greater concentrations, this relationship was lost. The mitochondrial K-ATP channel blocker, 5-HD, resulted in both abrogation of SNAP infarct size reduction and concomitant loss of AT depolarisation in the mitochondria. MPG however did not influence the cardioprotective properties of SNAP. Conclusion: We demonstrate that nitric oxide can mediate cardioprotection in a dose-dependent fashion by an effect that may be related to DeltaPsi(m). Both cardioprotection and DeltaPsi(m). changes are sensitive to 5-HD and the cardioprotection appears independent of free-radical synthesis. (C) 2003 European Society of Cardiology. Published by Elsevier Science B.V. All rights reserved.