EXPRESSION OF NITRIC-OXIDE SYNTHASE IN SKELETAL-MUSCLE - A NOVEL ROLEFOR NITRIC-OXIDE AS A MODULATOR OF INSULIN ACTION

Previous studies have shown that nitric oxide synthase (NOS), the enzy me that catalyzes the formation of nitric oxide (NO), is expressed in skeletal muscle, The aim of the present study was to test the hypothes is that NO can modulate glucose metabolism in slow-and fast-twitch ske letal muscles, Calcium-dependent NOS was detected in skeletal muscle, and the enzyme activity was greater in fast-type extensor digitorum lo ngus (EDL) muscles than in slow-type soleus muscles, Both the neuronal -type (nNOS) and endothelial-type (eNOS) enzymes are expressed in rest ing skeletal muscles, However, nNOS protein was only detected in EDL m uscles, whereas eNOS protein contents were comparable in soleus and ED L muscles, NOS expression in muscle cryosections (diaphorase histochem istry) was located in vascular endothelium and in muscle fibers, and t he staining was greater in type IIb than in type I and IIa fibers, The macrophage-type inducible NOS (iNOS) was not detected in resting musc le, but endotoxin treatment induced its expression, concomitant with e levated NO production, iNOS induction was associated with impaired ins ulin-stimulated glucose uptake in isolated rat muscles, In vitro, NOS blockade with specific inhibitors did not affect basal or insulin-stim ulated glucose transport in EDL or soleus muscles, In contrast, the NO donors GEA 5024 and sodium nitroprusside induced dose-dependent inhib ition (up to 50%) of maximal insulin-stimulated glucose transport in b oth muscles with minor effects on basal uptake values, GEA 5024 also b lunted insulin-stimulated glucose transport and amino acid uptake in c ultured L6 muscle cells without affecting insulin binding to its recep tor, On the other hand, the permeable cGMP analogue dibutyryl cGMP did not affect muscle glucose transport, These results strongly suggest t hat NO modulates insulin action in both slow-and fast-type skeletal mu scles, This novel autocrine action of NO in muscle appears to be media ted by cGMP-independent pathways.