Expression of neuronal nitric oxide synthase in fast rat skeletal muscle

The neuronal isoform of nitric oxide synthase (nNOS), the enzyme that catalyzes the formation of nitric oxide (NO), is expressed in a normal rat skeletal muscle where it is localized at the sarcolemma, reflecting the molecular association between nNOS and subsarcolemmal dystrophin complex. The expression of nNOS in fast twitch skeletal muscles such as m. extensor digitorum longus (EDL) and m. tibialis anterior (TA) is much greater than in slow-twitch muscles such as m. soleus (SOL). Muscle fibers that predominate in fast-twitch skeletal muscles are fast muscle fibers (type IIA, IIX and IIB) while slow-twitch muscles are mainly composed of slow type of fibers (type I). The aim of present study was to explore if all fast fiber types expressed nNOS. By immunohistochemical analysis of muscle cryosections of EDL and TA rat skeletal muscles we demonstrated that the staining for nNOS expression was similar in type IIA and IIX at the lower level in type I, while it was absent in IIB muscle fibers.