The role of nitric oxide (NO) in ventillatory muscle contractile function has been
under debate for several years. Moreover little is known about NO role under hypoxic
conditions and the contribution of inducible nitric oxide synthetase (iNOS) in its
generation. The aim of this study was to investigate the effect of NO on the force
generation and fatigue resistance of the rat diaphragm muscle under acute in vitro
hypoxia and to compare these effects to those under hyperoxic conditions. The effects
of the NOS inhibitor NG-monomethyl-L-arginine (L-NMMA), the NO scavenger
hemoglobin, and the NO donor Na nitroprusside on the maximal twitch force (Ft),
submaximal tetanic force (F30), maximal tetanic force (F0) and isotonic endurance
time under hyperoxic and hypoxic conditions were evaluated. Also diaphragm iNOS
activity and nitrotyrosine level as a marker of peroxynitrite were measured. Inhibition
of NO production and its scavenging using L-NMMA and Hb respectively had no
effect on the diaphragm isometric contraction, the recruitment of its muscle fibers and
fatigue resistance under hyperoxic condition. Hypoxia significantly reduced Ft, F30 ,
F0 and fatigue resistance with increased diaphragm iNOS activity and nitrotyrosine
level. Hypoxia significantly reduced Ft and F0 in L-NMMA group compared to
hyperoxic control one. While L-NMMA significantly increased F30 and decreased
isotonic fatigability during hypoxia concomitant with reduction of iNOS activity and
nitrotyrosine level compared to hypoxic control group. Moreover Hb induced similar
results with additional significant improvement of Ft and F0. The effects of L-NMMA
were prevented by co-administration with the NOS substrate L-arginine. On the other
hand, Excessive exogenous NO production by Nanitroprusside markedly decreased
isometric contractile and fatigue properties during both hyperoxia and hypoxia. In
conclusion the results of this study showed that the iNOS is activated in the
diaphragm under hypoxia and may contribute partially to NO generation in hypoxia.
Also they indicate that NO has a more prominent role in rat diaphragm under in vitro
hypoxia compared to hyperoxic condition and that it contributes to the depression of
force generation in the hypoxic diaphragm in vitro. Inhibition of NO generation
during hypoxia may have a protective effect which could be a target in clinical
conditions.