Abstract
Nitric oxide (NO), produced by nitric oxide synthases (NOSs), is involved in many of the physiological properties in cells. So far, it is unclear whether endothelial NOS (eNOS) is expressed in skeletal muscle fiber or not, although it is known that neuronal NOS (nNOS) is involved under the sarcolemma. Here, we examined eNOS expression in mouse skeletal muscle by immunohistochemistry and immunoblot with three different eNOS antibodies. Each antibody showed the same cross striation labeling in skeletal muscle, and detected eNOS around 140 kDa molecular weight in skeletal muscle extraction. Confocal double-labeling images demonstrated that eNOS was found between striations of ryanodine receptors or dihydropyridine receptors, and that it was partly colocalized with them. α-actinin was distributed in almost the same way as eNOS. These findings suggest that eNOS is involved in Z-bands or in the sarcoplasmic reticulum (SR)-encompassing Z-bands. Immunoelectron micrographs clearly showed that eNOS was contained in SR-encompassing Z-bands in addition to terminal cisternae. The present study indicates that eNOS is constitutively expressed in SR of mouse skeletal muscle fiber, and raises the hypothesis that NO derived from eNOS may play a role in muscle contraction.
