Submitted on March 26, 2006
Accepted on December 20, 2006
Nitric Oxide Production in the Ventilatory Muscles in Response to Acute Resistive Loading
Theodoros Vassilakopoulos1, Karuthapillai Govindaraju2, Dimitri Parthenis3, David H. Eidelman4, Yasu Watanabe5, and Sabah N. A. Hussain6*
1 Critical Care, McGill, Montreal, Canada
2 Medicine, McGillUniversity, Montreal, Canada
3 Vascular Surgery, University of Athens, Athens, Greece
4 Meakins-Christie Labs, McGill University, Montreal, Canada
5 Pharmacology, Showa Pharmaceutical University, Tokyo, Japan
6 Royal Victoria Hospital Critical Care Division, Room L303, McGill University, Montreal, Canada
* To whom correspondence should be addressed. E-mail: sabah.hussain{at}muhc.mcgill.ca.
The effect of muscle activation on muscle NO production remains controversial. While NO release increases in in vitro activated muscles or in in vivo limb muscles, diaphragmatic nitric oxide synthase (NOS) activity declines after 3 h of inspiratory resistive loading (IRL). We tested in this study the hypotheses that acute IRL decreases diaphragmatic NO derivatives levels and reduces protein expression of neuronal (nNOS), endothelial (eNOS) and inducible (iNOS) nitric oxide synthases as well as 3-nitrotyrosine formation. Anesthetized, tracheostomized, spontaneously breathing adult rats were subjected to IRL (50% of the maximum inspiratory pressure) for 1, 3 or 6 h. Quietly breathing rats served as controls. After 3 h of IRL, muscle eNOS and nNOS protein levels rose by 80 and 60% of control values, respectively. While eNOS expression did not change any further, nNOS expression reached 550% of control values after 6 h of IRL. Strong iNOS protein expression was detected in the diaphragms after 6 h of IRL. Total NO derivatives levels in the diaphragm declined during IRL as a result of reduction in nitrate, nitrite and nitrosothiols. Diaphragmatic protein tyrosine nitration decreased in response to IRL and this reduction was mainly due to reduced tyrosine nitration of enolase and aldolase. We conclude that diaphragmatic NO derivatives levels decline in response to IRL and that the rise in diaphragmatic NOS protein expression may be a compensatory response designed to counterbalance the decline in NOS activity.
