Acidosis, as Occurs During Myocardial Ischemia, Inhibits Nitric Oxide Synthesis But Increases Superoxide Production from Nitric Oxide Synthase

Abstract

In ischemic and reperfused hearts, superoxide (·O2−) and nitric oxide (NO) production is increased. NO reacts with ·O2− at near diffusion-limited rates to form peroxynitrite, a potent oxidant, and mediator of tissue injury. Under normal conditions, NO is synthesized by the NO synthase enzyme (NOS). The inducible isoform iNOS has a high rate of NO production and can also produce ·O2− and peroxynitrite. During ischemia intracellular acidification to pH 5.5 or lower can occur. iNOS also produces ·O2− when BH4 or L-arginine are depleted, which can occur in postischemic tissue. We hypothesize that acidosis may cause uncoupling of NADPH oxidation from NO production with a shift to increased ·O2− production. Therefore, spin-trapping EPR and UV-Vis spectroscopic studies were performed to measure the effect of acidosis on NO and ·O2− production from iNOS. Under acidotic conditions as occur in the ischemic heart, NO production drastically decreased while ·O2− production greatly increased and was strongly coupled to NADPH oxidation. Initially, with moderate acidosis peroxynitrite production may occur; however, with more severe acidosis NO production is abolished and the enzyme solely functions as a ·O2− producing NADPH oxidase. Thus, acidosis inhibits NO production but stimulates ·O2− production from iNOS.