Muscle fiber type differences in nitrate and nitrite storage and nitric oxide signaling in rats

Recent studies have emphasized the importance of the nitric oxide synthase (NOS)-independent, nitrate (NO 3 − ) → nitrite (NO 2 − ) → nitric oxide (NO) pathway in skeletal muscle. In particular, it has been hypothesized that this pathway is especially active in type II, or fast-twitch, muscle fibers, necessitating greater NO 3 − and NO 2 − storage. We therefore measured NO 3 − and NO 2 − concentrations in the predominantly fast-twitch vastus lateralis and predominantly slow-twitch soleus muscles of rats. Contrary to the above hypothesis, we found that NO 3 − and NO 2 − concentrations were 3.4-fold and 1.8-fold higher, respectively, in the soleus. On the other hand, NO signaling (i.e., cyclic guanosine monophosphate (cGMP) level) was comparable in the two muscles. Although the physiological significance of these observations remains to be determined, we speculate that NO production via the NO 3 − → NO 2 − → NO pathway is normally higher in slow-twitch muscles, thus helping compensate for their inherently lower NOS activity.