Nitric oxide synthase-1 is enriched in fast-twitch oxidative myofibers

Nitric oxide synthase-1 (NOS-1) is found in high concentrations in skeletal muscles, where its synthesis product nitric oxide (NO) is reported to be i nvolved in a number of processes, including the modulation of the oxidative metabolism of myofibers. Performing immunoblot analysis and quantification of formazan produced by its specific NADPH diaphorase activity, we found N OS-1 to be enriched in rat skeletal muscles with a high proportion of fast- twitch myofibers. Since these myofibers represent a metabolically heterogen eous subpopulation, we extended our investigation to the level of individua l myofibers. Using serial sections we combined myosin heavy chain-based fib er-typing with quantitative succinate dehydrogenase histochemistry to deter mine three groups of fiber-types, comprising fast-oxidative, fast-glycolyti c and slow-oxidative myofibers. Image analysis showed that NOS-1 diaphorase activity is significantly enriched in fast-oxidative myofibers compared wi th fast-glycolytic and slow-oxidative ones. In order to characterize potent ial biological effects of the fiber-type-specific enrichment of NOS-1, we p erformed cytochrome oxidase histochemistry in the presence of the NO donors NOC-9 and SNAP. Both NO donors reduced cytochrome oxidase activity in all myofibers investigated with almost identical semi-maximal inhibition rates, although fast-oxidative and slow-oxidative myofibers contained twice as mu ch basal catalytic activity than fast-glycolytic ones. In summary, we sugge st that the NOS-1/NO system of skeletal muscles exerts its biological role especially in fast-oxidative myofibers, since these myofibers express more NOS-1 than fast-glycolytic or slow-oxidative ones and also contain the high est concentrations of cytochrome oxidases as potential target molecules of NO.