Enhanced prostanoid-mediated vasorelaxation in pulmonary arteries isolated during experimental endotoxemia

Endotoxemia secondary to gram-negative sepsis has been shown to inhibit endothelium-dependent vasomotion in numerous vascular beds, including guinea pig aortae and coronary arteries. We tested the hypothesis that in vivo endotoxin impairs endothelium-dependent nitric oxide-mediated relaxation responses of pulmonary arteries isolated from guinea pigs given intraperitoneal injections of Escherichia coli endotoxin lipopolysaccharide (LPS) or saline (control) 16 h before sacrifice. Pulmonary rings from the main artery and primary branches were isolated and studied in vitro using conventional isometric techniques. Interestingly, endotoxemia resulted in enhanced pulmonary artery relaxation in response to the endothelium-dependent receptor agonists acetylcholine (10(-10) -10(-5) M) and adenosine diphosphate (ADP; 10(-9) -10(-5) M), as compared with control responses (p < .05). Nitric oxide synthase inhibitors N-monomethyl-L-arginine (300 microM) and N-nitro-L-arginine methyl ester (100 microm) reduced acetylcholine- and adenosine diphosphate-mediated relaxation in both groups (p < .05); however, vasodilation responses in arteries from LPS animals remained enhanced relative to those of control arteries. In contrast to nitric oxide synthase inhibitors, the cyclooxygenase inhibitor indomethacin markedly inhibited acetylcholine- and adenosine diphosphate-mediated relaxation responses of pulmonary arteries isolated from LPS-treated animals (p < .05) but not control arteries; indomethacin effectively reversed LPS-induced enhanced vasodilation of pulmonary arteries. Relaxation responses to the receptor-independent calcium ionophore (A23187) and to the direct smooth muscle vasodilator sodium nitroprusside (+ N-nitro-L-arginine methyl ester) were not significantly altered by LPS treatment (p > .05). These data suggest that in pulmonary arteries, unlike aortae and coronary arteries isolated from the same model, in vivo LPS enhances agonist-mediated endothelium-dependent vasodilation responses to acetylcholine and adenosine diphosphate. Underlying mechanisms appear to involve increased dependency upon vasodilator prostanoids and decreased dependency on nitric oxide synthesis/release for LPS-induced alterations in pulmonary relaxation responses.