Nitric oxide regulates oxygen uptake and hydrogen peroxide release by the isolated beating rat heart

Abstract

Isolated rat heart perfused with 1.5–7.5 μM NO solutions or bradykinin, which activates endothelial NO synthase, showed a dose-dependent decrease in myocardial O₂uptake from 3.2 ± 0.3 to 1.6 ± 0.1 (7.5 μM NO, n = 18,P < 0.05) and to 1.2 ± 0.1 μM O₂ ⋅ min⁻¹ ⋅ g tissue⁻¹ (10 μM bradykinin, n = 10,P < 0.05). Perfused NO concentrations correlated with an induced release of hydrogen peroxide (H₂O₂) in the effluent (r = 0.99, P < 0.01). NO markedly decreased the O₂ uptake of isolated rat heart mitochondria (50% inhibition at 0.4 μM NO, r = 0.99,P < 0.001). Cytochrome spectra in NO-treated submitochondrial particles showed a double inhibition of electron transfer at cytochrome oxidase and between cytochrome b and cytochrome c, which accounts for the effects in O₂uptake and H₂O₂ release. Most NO was bound to myoglobin; this fact is consistent with NO steady-state concentrations of 0.1–0.3 μM, which affect mitochondria. In the intact heart, finely adjusted NO concentrations regulate mitochondrial O₂uptake and superoxide anion production (reflected by H₂O₂), which in turn contributes to the physiological clearance of NO through peroxynitrite formation.