ObjectiveSepsis is characterized by systemic vasodilation and hyporesponsiveness to constrictor agents, at a time when the pulmonary circulation exhibits varying degrees of vasoconstriction. Plasma endothelin-1 concentrations are increased, but the role of this potent vasoconstrictor peptide in modulating the vascular response to sepsis is unknown. Therefore, we assessed the effect of endothelin-A receptor antagonism in the response of pulmonary arteries from rats treated with lipopolysaccharide to endothelin-1, and determined the vasomotor role of the endothelin-B receptors that are known to be located on rat pulmonary artery smooth muscle and endothelium.DesignProspective, controlled study.SettingAnimal research laboratory.SubjectsMale Wistar rats (275 to 300 g).InterventionsAnimals were injected with either lipopolysaccharide (20 mg/kg ip) or saline (1 mL ip) 4 hrs before being killed. The main pulmonary arterles were cut into 2-mm rings, and suspended in an organ bath. In the first set of experiments, half of the rings underwent a procedure that removed the endothelium, and the contractile response to cumulative doses of endothelin-1 (10 sup -11 to 10 sup -6 M) was measured. Half of the rings were pretreated with the endothelin-A receptor antagonist, BQ123 (10 sup -5 M or 10 sup -6 M), and the other half of the rings were treated with vehicle. In a separate group of experiments, the contractile response to cumulative concentrations of the selective endothelin-B agonist, sarafotoxin S6c (10 sup -11 to 10 sup -6 M), was measured in rings at baseline tension. Second, the possible dilator effect of endothelin-B receptor activation was tested by the administration of sarafotoxin S6c (10 sup -7 to 10 sup -6 M) to rings preconstricted by 10 sup -6 M of U46619, a thromboxane receptor agonist, either in the presence or absence of the nitric oxide synthase inhibitor, Nomega-nitro-L-arginine-methylester (10 sup -4 M). Acetylcholine-induced (10 sup -4 M), endothelium-dependent vasodilation was also measured.Measurements and Main ResultsBQ123 (10 sup -5 or 10 sup -6 M) caused consecutive rightward shifts in the endothelin-1 concentration-contraction curves for all ring types, including the intact rings from endotoxemic animals. Sarafotoxin S6c failed to induce any direct constriction in rings from sham-treated or lipopolysaccharide-treated rats. However, sarafotoxin S6c induced transient vasodilation at the initial dose in rings from sham-treated rats but not lipopolysaccharide-treated rats-an effect that was attenuated by Nomega-nitro-L-arginine-methylester. Acetylcholine induced an Nomega-nitro-L-arginine-methylester-sensitive vasodilation that was reduced in rings from endotoxin-treated rats.ConclusionsEndothelin-A receptor blockade is an effective means of attenuating endothelin-1-induced contraction of isolated pulmonary artery rings, even from rats rendered endotoxemic. Endothelin-B receptors on the pulmonary artery cause vasodilation via the release of nitric oxide, and have no constrictor component. The functional effects of endothelin-B receptors on tone are lost after lipopolysaccharide treatment. The endothelium is involved in both the constrictor and dilator effects of endothelin in rat pulmonary artery, confirming a pivotal role for endothelial cells in the vascular response to sepsis. (Crit Care Med 1996; 24:2007-2013)