The roles of tissue nitrate reductase activity and myoglobin in securing nitric oxide availability in deeply hypoxic crucian carp.

In mammals, treatment with low doses of nitrite have cytoprotective effects in ischemia/reperfusion events, due to nitric oxide formation and S-nitrosation of proteins. Interestingly, anoxia-tolerant lower vertebrates possess an intrinsic ability to increase intracellular [nitrite] during anoxia in tissues with high myoglobin and mitochondria contents, such as the heart. Here we test the hypothesis that red and white skeletal muscle develops different nitrite levels in crucian carp exposed to deep hypoxia and whether this correlates with myoglobin concentration. We also tested if liver, muscle, and heart tissue possess nitrate reductase activity that supply nitrite to the tissues during severe hypoxia. Crucian carp exposed to deep hypoxia (1nitrite in red musculature to more than double the value in normoxic fish, while nitrite was unchanged in white musculature. There was a highly significant positive correlation between tissue concentrations of nitrite and nitros(yl)ated compounds. Myoglobin levels were 7 times higher in red than white musculature, but there was no clear correlation between [nitrite] and [Mb]. Finally, we found a low but significant nitrate reductase activity in liver and white muscle, but not in cardiomyocytes. Nitrate reduction was inhibited by allopurinol, showing that it was partly catalyzed by xanthine oxidoreductase.