Loss of endothelium-dependent vasodilation in the pulmonary vessels of sheep after prolonged endotoxin

We examined the hemodynamic response of awake sheep to prolonged endotoxin infusion (10 ng · kg^-1 · min^-1 for 12 h) and the in vitro endothelium- dependent relaxation of pulmonary arterial vessels excised 12 h after the end of endotoxin infusion to determine whether the development of pulmonary hypertension after endotoxin is associated with loss of endothelium-dependent relaxation. In seven of nine sheep, there was a maintained increase (4-68% of baseline) in pulmonary arterial pressure 24 h after the beginning of endotoxin infusion. The greater the increase in pulmonary arterial pressure in vivo, the greater was the in vitro deficit in endothelium-dependent relaxation of the pulmonary vessels. The maximum in vitro vessel dilation was 59% for pulmonary artery rings isolated from sheep without a sustained increase in pulmonary arterial pressure 24 h after endotoxin. Prolonged endotoxin infusion did not alter the in vitro response of pulmonary arterial vessels to KCl or 10^-5 M norepinephrine. Force development, response to 10^-5 M norepinephrine, and vasodilation in response to acetylcholine were also not altered in pulmonary vessels taken from control sheep and exposed in vitro to tumor necrosis factor-α (400 U/ml). Our results suggest that loss of endothelium-dependent relaxation in pulmonary vessels supports the sustained pulmonary hypertension that develops after prolonged exposure to endotoxin.