Irregular costameres represent nitric oxide synthase-1-positive sarcolemmainvaginations enriched in contracted skeletal muscle fibres

NADPH diaphorase histochemistry and NOS-1 immunohistochemistry on 60 mum th ick frozen sections of rat extensor digitorum longus muscles led to the det ection of prominent rings clearly encompassing the surface of the muscle fi bres. These so far unknown costameres were usually found as doublets flanki ng a space of about 2 mum width. Because these costameric doublets did not appear in regular periods, we designate them irregular costameres to discri minate them from regular ones with a 1 mum periodicity overlying Z-discs an d M-lines. Irregular costameres were thicker than the regular ones and free of intercostameres. Immunohistochemistry demonstrated that NOS-1 was co-lo calized with integral beta -dystroglycan, alpha -sarcoglycan) and periphera l (caveolin-3, dystrophin) members of the enlarged dystrophin complex in th e irregular costameres but not with non-sarcolemmal organized proteins (myo sin heavy chain, alpha -actinin, desmin and sarcoplasmic reticulum-located Ca2+-dependent ATPase-1). Invaginations of the sarcolemma to form irregular costameres were observed. In teased myofibres the sarcolemma between two f ollowing irregular costameres was ballooned, while the irregular costameres themselves clamped the fibres together. Finally, the number of detectable irregular costameres was significantly increased in maximally contracted ex tensor digitorum longus muscles generated by electric stimulation but decre ased in mechanically stretched ones. Combining these observations, we hypot hesize that irregular costameres belong to a reserve zone for the sarcolemm a necessary for the contraction/relaxation cycle in myofibres.