NITRIC-OXIDE BIOSYNTHESIS IN AN EXOTOXIN-INDUCED SEPTIC LUNG MODEL - ROLE OF CNOS AND IMPACT ON PULMONARY HEMODYNAMICS

Nitric oxide (NO) is an important vasodilator that is produced by cons titutive (cNOS) as well as inducible (iNOS) isoforms of nitric oxide s ynthase. The pore-forming hemolysin of Escherichia coli (HlyA), an imp ortant virulence factor in extraintestinal E, coli infections, was fou nd to be a potent stimulator of NO liberation in isolated endothelial cells, and that it also causes thromboxane generation and related vaso constriction in rabbit lungs. We investigated the effect of different concentrations of HlyA on pulmonary NO synthesis in buffer-perfused ra bbit lungs. NO release into the alveolar as well as the intravascular compartment was monitored on-line by chemiluminescence detection of ex pired NO and by measurement of (peroxy-)nitrite/nitrate release into t he perfusate. HlyA induced a presser response and an immediate dose-de pendent increase of exhalative and intravascular NO liberation, furthe r enhanced by the addition of the NOS substrate L-arginine. The nonspe cific NOS inhibitor N-G-monomethyl-L-arginine (L-NMMA), but not the iN OS selective inhibitors aminoguanidine and 2-(2-aminoethyl)-2-thiopseu dourea-dihydrobromide, blocked the HlyA-evoked NO liberation into both the alveolar and the intravascular compartments. Enhancement of NO fo rmation (L-arginine) slightly reduced, and inhibition of NO synthesis (L-NMMA) amplified greatly, the HlyA-elicited vasoconstrictor response . Inhibition of the presser response by a thromboxane receptor antagon ist did not interfere with the exotoxin-elicited NO formation. We conc lude (1) that marked NO biosynthesis occurs in this model of the septi c lung, (2) that the signal transduction in response to HlyA proceeds via activation of cNOS directly related to exotoxin activity and not t o secondary changes in shear stress, and (3) that this vasodilator rel ease mitigates the HlyA-induced pulmonary vasoconstriction. These find ings may have important implications for therapeutic approaches using NOS inhibitors in sepsis.