Regulation of the rate of synthesis of nitric oxide by Mg2+ and hypoxia. Studies in rat heart mitochondria

 In isolated rat heart mitochondria, L-arginine is oxidized by a nitric oxide synthase (mtNOS) achieving maximal rates at 1 mM L-arginine. The NOS inhibitor NG-nitro-L-arginine methyl ester (NAME) inhibits the increase in NO production. Extramitochondrial free magnesium inhibited NOS production by 59% at 3.2 mM. The mitochondrial free Mg2+ concentration increased to different extents in the presence of L-arginine (29%), the NO donor (S-nitroso-N-acetylpenicillamine) (105%) or the NOS inhibitors L-NAME (48%) or NG-nitro-L-arginine methyl ester, NG-monomethyl-L-arginine (L-NMMA) (53%). Under hypoxic conditions, mtNOS activity was inhibited by Mg2+ by up to 50% after 30 min of incubation. Reoxygenation restored the activity of the mtNOS to pre-hypoxia levels. The results suggest that in heart mitochondria there is an interaction between Mg2+ levels and mtNOS activity which in turn is modified by hypoxia and reoxygenation.