5-HYDROXYTRYPTAMINE INDUCES ENDOTHELIUM-INDEPENDENT RELAXATIONS OF SHEEP PULMONARY VEIN - ROLE OF CYCLIC-NUCLEOTIDE

5-Hydroxytryptamine produced concentration-dependent relaxations in is olated sheep pulmonary vein, which were insensitive to removal of the endothelium. 5-Hydroxytryptamine stimulated concentration-dependent in creases of cyclic AMP levels in the pulmonary vein, and there was a si gnificant linear correlation between relaxations elicited by 5-hydroxy tryptamine and tissue cyclic AMP formations. The soluble guanylate cyc lase inhibitor methylene blue (10 mu M) failed to block 5-hydroxytrypt amine-induced relaxations. The results suggest that 5-hydroxytryptamin e-induced relaxations of sheep pulmonary vein are mediated, at least i n part, by increases of tissue cyclic AMP levels.