nNOS and eNOS modulate cGMP formation and vascular response in contracting fast-twitch skeletal muscle

Nitric oxide (NO) from Ca²⁺-dependent neuronal nitric oxide synthase (nNOS) in skeletal muscle fibers may modulate vascular tone by a cGMP-dependent pathway similar to NO derived from NOS in endothelial cells (eNOS). In isolated fast-twitch extensor digitorum longus (EDL) muscles from control mice, cGMP formation increased ∼166% with electrical stimulation (30 Hz, 15 s). cGMP levels were not altered in slow-twitch soleus muscles. The NOS inhibitor N^ω-nitro-L-arginine abolished the contraction-induced increase in cGMP content in EDL muscles, and the NO donor sodium nitroprusside (SNP) increased cGMP content ∼167% in noncontracting EDL muscles. SNP treatment but not electrical stimulation increased cGMP formation in muscles from nNOS^-/- mice. cGMP formation in control and stimulated EDL muscles from eNOS^-/- mice was less than that obtained with similarly treated muscles from control mice. Arteriolar relaxation in contracting fast-twitch mouse cremaster muscle was attenuated in muscles from mice lacking either nNOS or eNOS. These findings suggest that increases in cGMP and NO-dependent vascular relaxation in contracting fast-twitch skeletal muscle may require both nNOS and eNOS.