Effects of melatonin on isolated pulmonary artery and vein: role of the vascular endothelium.

The influence of melatonin on tension of isolated pulmonary vascular smooth muscle rings was examined in these experiments. Melatonin caused a dose-dependent relaxation of precontracted (30 mM KCl) pulmonary artery and pulmonary vein. Removal of the endothelium did not alter the relaxant response to melatonin in pulmonary artery. In pulmonary artery with endothelium intact, preincubation with indomethacin, but not L-NG-monomethyl-L-arginine (L-NMMA) or deferoxamine, antagonized the relaxant response to melatonin. In pulmonary artery with endothelium removed, preincubation with deferoxamine, but not indomethacin, phorbol esters, or lithium chloride, antagonized the relaxant response to melatonin. In pulmonary vein, removal of the endothelium significantly increased the maximum relaxation attained in response to melatonin. In pulmonary vein with endothelium intact, preincubation with indomethacin, but not L-NMMA or deferoxamine, antagonized the relaxant response to melatonin. On the other hand, preincubation of de-endothelialized pulmonary vein with lithium chloride significantly increased the relaxation response to melatonin. These experiments demonstrate that both pulmonary artery and vein relax in response to melatonin and the mechanisms are apparently different.