Possible involvement of vascular endothelium-derived relaxing factor induced by platelet-activating factor (PAF) on human pulmonary artery after contraction in vitro

We studied the effects of platelet-activating factor (PAF) on human pulmonary vascular smooth muscle in vitro. 10(-6) M PAF elicited slight contractions in 2 of 7 endothelium-denuded arterial strips, but did not elicit distinct contractions or relaxations in the other strips. After incubation with PAF at 10(-7) M, the contractions of pulmonary vascular smooth muscle induced by histamine and prostaglandin F2 alpha were significantly decreased. Preincubation of 10(-3) M indomethacin did not affect the inhibitory effects of PAF, but these disappeared with removal of endothelium. These results suggest that the inhibitory effects of PAF on human pulmonary vascular smooth muscle contractions may appear not through the cyclooxygenase products but the endothelium-derived relaxing factor.