Expression of nitric oxide synthase isoforms in normal ventilatory and limb muscles

Nitric oxide (NO), an important messenger molecule with widespread actions, is synthesized by NO synthases (NOS). In this study, we investigated the correlation between fiber type and NOS activity among ventilatory and limb muscles of various species. We also assessed the presence of the three NOS isoforms in normal skeletal muscles and how various NOS inhibitors influence muscle NOS activity. NOS activity was detected in various muscles; however, NOS activity in rabbits and rats varied significantly among different muscles. Immunoblotting of muscle samples indicated the presence of both the neuronal NOS and the endothelial NOS isoforms but not the cytokine-inducible NOS isoform. However, these isoforms were expressed to different degrees in various muscles. Although the neuronal NOS isoform was detectable in the canine diaphragm, very weak expression was detected in rabbit, rat, and mouse diaphragms. The endothelial NOS isoform was detected in the rat and mouse diaphragms but not in the canine and rabbit diaphragms. We also found that N-G-nitro-L-arginine methyl ester, 7-nitroindazole, and S-methylisothiourea were stronger inhibitors of muscle NOS activity than was aminoguanidine. These results indicate the presence of different degrees of constitutive NOS expression in normal ventilatory and limb muscles of various species. Our data also indicate that muscle NOS activity is not determined by fiber type distribution but by other not yet identified factors. The functional significance of this expression remains to be assessed.