Serotonin receptors subserve only contraction in canine and rat pulmonary arteries and veins.

Precise information with respect to whether serotonin (5-HT) exerts direct effects on mammalian pulmonary arteries and veins is lacking. The present studies, using isolated pulmonary arterial strips of dogs and rats, as well as isolated pulmonary veins of dogs, indicate that serotonin exerts direct contractile, but not relaxant, effects on these blood vessels. Contractile responses to 5-HT were not influenced by "specific" receptor-blocking doses of pyrilamine (H1-receptor antagonist), metiamide (H2-receptor antagonist), phentolamine (alpha-adrenoceptor antagonist), propranolol (beta-receptor antagonist), atropine (cholinergic antagonist) or morphine ("M" tryptamine antagonist). These data demonstrate the lack of interaction of 5-HT with either H1- and H2-histamine, alpha and beta-adrenergic, cholinergic, or "M" tryptamine receptors in these pulmonary blood vessels. Selective, competitive antagonism of 5-HT-induced contractions by methysergide (parallel shifts of concentration-effect curves) demonstrate a direct action of 5-HT on D-type serotonin receptors in these pulmonary blood vessels.