Role of Nitric Oxide and Its Interaction With Superoxide in the Suppression of Cardiac Muscle Mitochondrial Respiration

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Abstract
Background Nitric oxide (NO); superoxide anion (O 2 . d − ); the reaction product of NO with O 2 . d − , peroxynitrite (ONOO − ); and ischemia/reperfusion have all been reported to inhibit respiration in isolated mitochondria. However, the specific species involved in the inhibition of respiration in intact tissues are poorly understood. Methods and Results O 2 consumption in isolated cardiac muscle from bovine calf hearts was quantified by use of a Clark-type electrode. Exogenous and endogenous sources of NO, from S -nitroso- N -acetylpenicillamine (SNAP) and bradykinin or carbachol, reversibly inhibited respiration, whereas the O 2 .− releasing agent, pyrogallol (PG), inhibited respiration in a manner that was only partially reversed when examined 15 minutes after the removal of PG. The generation of ONOO − with SNAP+PG caused a potentiation of the O 2 .− -elicited inhibition of respiration when examined 15 minutes after the removal of the ONOO − generating system. Tiron (a scavenger of O 2 .− ) did not alter the actions of SNAP, but it attenuated the direct inhibitory effects of PG±SNAP and essentially eliminated the suppression of respiration observed 15 minutes after removal of the O 2 .− or ONOO − generating system. Urate (a scavenger of ONOO − ) antagonized only the actions of PG+SNAP. After exposure of muscle slices to a model of hypoxia (15 minutes) and reoxygenation (10 minutes), respiratory inhibition was observed. This reoxygenation-induced inhibition was potentiated by l -arginine, the substrate for NO biosynthesis, and was markedly blocked by nitro- l -arginine (an NO synthase inhibitor), Tiron, or urate. Conclusions The potentially physiological reversible regulation of respiration in cardiac muscle by NO is converted to an effect that does not show rapid reversibility under conditions in which ONOO − forms, and this could contribute to cardiac dysfunction in situations such as hypoxia/reoxygenation.

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