The expression of NFATc1 in adult rat skeletal muscle fibres

Although numerous studies have recently implicated the calcineurin–nuclear factor of activated T‐cells (Cn–NFAT) signalling pathway in the regulation of activity‐dependent fibre type switching in adult mammalian skeletal muscles, little is known about the endogenous expression of NFAT proteins in the various fibre types present in these muscles. In this study, the immunolocalization of NFATc1 (also known as NFATc or NFAT2) in the extensor digitorum longus (EDL; a mainly fast‐twitch muscle) and the soleus (a predominantly slow‐twitch muscle) muscles of adult (∼90‐day‐old) Wistar rats was investigated. The results show that NFATc1 is expressed only in oxidative fibres (i.e. type I and type IIA fibres) that stain intensely for succinate dehydrogenase activity irrespective of whether they are from the fast‐ or slow‐twitch muscle. Thus, 99 ± 4% (n= 7 rats) of the muscle fibres in the soleus and 42 ± 2% (n= 7 rats) of those in the EDL expressed NFATc1. In the soleus muscle fibres, NFATc1 was localized mainly in the fibre nuclei, whereas in the EDL fibres it was localized in both the cytoplasm and the nuclei. However, no difference in its localization was observed between type I and type IIA fibres in both muscles. Western blot experiments showed that the soleus expressed more NFATc1 proteins than the EDL. From these results, we suggest that NFATc1 controls the number and distribution of both type I and type IIA fibres, as well as the oxidative capacity of adult mammalian skeletal muscles.