Summary
Moderate elevations of nitrite and nitric oxide (NO) protect mammalian tissues against ischemia (anoxia)-reperfusion damage by inhibiting mitochondrial electron transport complexes and reducing the formation of reactive oxygen species (ROS) upon reoxygenation. Crucian carp appears to exploit this mechanism by up-regulating nitrite and other nitrite/NO metabolites (S-nitroso and iron-nitrosyl compounds) in several tissues when exposed to anoxia. We investigated whether this is a common strategy amongst anoxia-tolerant vertebrates by evaluating NO metabolites in red-eared slider turtles during long-term (9 days) anoxia and subsequent reoxygenation at low temperature, a situation naturally encountered by turtles in ice-covered ponds. We also measured glutathione in selected tissues and assessed the impact of anoxia on electrolyte status. Anoxia induced major increases in [nitrite] in the heart, pectoral muscle and red blood cells, while [nitrite] was maintained unaltered in brain and liver. Concomitantly, the concentrations of S-nitroso and iron-nitrosyl compounds increased, showing that nitrite was used to produce NO and to S-nitrosate cellular molecules during anoxia. The changes were gradually reversed during reoxygenation (1h and 24h), testifying that the processes were reversible. The increased NO bioavailability occurred in the absence of nitric oxide synthase activity (due to global anoxia) and may involve mobilization of internal/external nitrite reservoirs. Our data supports that anoxic up-regulation of nitrite and other NO metabolites could be a general cytoprotective strategy amongst anoxia-tolerant vertebrates. The possible mechanisms of nitrite-derived NO and S-nitrosation in protecting cells from destructive Ca2+ influx during anoxia and in limiting ROS formation during reoxygenation are discussed.
