Endurance training does not alter the level of neuronal nitric oxide synthase in human skeletal muscle

The effect of endurance training on neuronal nitric oxide synthase (nNOS) c ontent and distribution in muscle was investigated. Seven male subjects per formed 6 wk of one-legged knee-extensor endurance training (protocol A). Mu scle biopsies, obtained from vastus lateralis muscle in the untrained and t he trained leg, were analyzed for nNOS protein and activity as well as immu nohistochemical distribution of nNOS and endothelial nitric oxide synthase (eNOS). Muscle biopsies were also obtained from another seven male subjects before and after 6 wk of training by endurance running (protocol B) and an alyzed for nNOS protein. No difference was found in the amount of nNOS prot ein in the untrained and the trained muscle either with protocol A or proto col B (P > 0.05). In protocol A, the activity of nNOS was 4.76 +/- 0.56 pmo l.mg protein(-1) min(-1) in the control leg, and the level was not differen t in the trained leg (P > 0.05). nNOS was present in the sarcolemma and cyt osol of type I and type II muscle fibers, and the qualitative distribution was similar in untrained and trained muscle. The number of eNOS immunoreact ive structures and the number of capillaries per muscle fiber were higher ( P < 0.05) after training than before. The present findings demonstrate that , in contrast to findings on animals, nNOS levels remain unaltered with end urance training in humans. Evidence is also provided that endurance trainin g may increase the amount of eNOS, in parallel with an increase in capillar ies in human muscle.