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

Abstract

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. 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 microM). 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 (Delta Psi(m)), measured with the potentiometric dye, tetra-methyl rhodium methyl ester (TMRM), in isolated mitochondria,by flow cytometry. SNAP dose-dependently attenuated infarct size, with maximal protection observed at 2 microM (17+/-4% versus controls 32+/-3%, P<0.01). At greater concentrations however, protection was lost with infarct sizes tending towards control at 20 microM (29+/-3%). These results were paralleled by changes in Delta Psi(m) in the isolated mitochondria: Delta Psi(m) depolarisation peaking with 1 microM 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 Delta Psi(m) depolarisation in the mitochondria. MPG however did not influence the cardioprotective properties of SNAP. We demonstrate that nitric oxide can mediate cardioprotection in a dose-dependent fashion by an effect that may be related to Delta Psi(m). Both cardioprotection and Delta Psi(m) changes are sensitive to 5-HD and the cardioprotection appears independent of free-radical synthesis.