Our recent experimental work demonstrated that a neutrophil-dependent inflammatory response in the lung prevented the normal up-regulation of alveolar fluid clearance by
catecholamines following
hemorrhagic shock. In this study, we tested the hypothesis that the release of NO within the airspaces of the lung was responsible for the
shock-mediated failure of the alveolar epithelium to respond to
catecholamines in rats.
Hemorrhagic shock was associated with an inducible
NO synthase (iNOS)-dependent increase in the lung production of NO and a failure of the alveolar epithelium to up-regulate vectorial fluid transport in response to
beta-adrenergic agonists. Inhibition of iNOS restored the normal
catecholamine-mediated up-regulation of alveolar liquid clearance. Airspace instillation of dibutyryl cAMP, a stable analog of cAMP, restored the normal fluid transport capacity of the alveolar epithelium after prolonged
hemorrhagic shock, whereas direct stimulation of
adenyl cyclase by
forskolin had no effect. Pretreatment with
pyrrolidine dithiocarbamate or
sulfasalazine attenuated the iNOS-dependent production of NO in the lung and restored the normal up-regulation of alveolar fluid clearance by
catecholamines after prolonged
hemorrhagic shock. Based on in vitro studies with an alveolar epithelial cell line, A549 cells, the effect of
sulfasalazine appeared to be mediated in part by inhibition of
NF-kappaB activation, and the protective effect was mediated by the inhibition of
IkappaBalpha protein degradation. In summary, these results provide the first in vivo evidence that NO, released within the airspaces of the lung probably secondary to the
NF-kappaB-dependent activation of iNOS, is a major proximal inflammatory mediator that limits the rate of alveolar epithelial transport after prolonged
hemorrhagic shock by directly impairing the function of
membrane proteins involved in the
beta-adrenergic receptor-cAMP signaling pathway in alveolar epithelium.