Atypical tryptamine receptors in sheep pulmonary vein

Both the pulmonary artery and vein of the sheep contracted dose-dependently to histamine, carbamoylcholine, prostaglandin F2a, noradrenaline and bradykinin and relaxed in the presence of isoprenaline or prostaglandin E1. 2 The effect of 5-hydroxytryptamine (5-HT) on the artery was consistently to produce dose-dependent contractions without tachyphylaxis. The effect on the vein was biphasic. 5HT 5 X 10(-10) to 5 X 10(-8) M relaxed the partially constricted vein. 5-HT 10(-7) to 10(-6) m caused brief venoconstriction followed by relaxation. 5-HT greater than 10(-6) M caused dose-related contraction of the vein. 3 Methysergide effectively blocked the contractile response of the artery to 5-HT, but only weakly inhibited the contractions of the vein (dose-ratio less than 20). 4 Each of ten antagonists tested failed to inhibit the 5-HT-induced relaxation of the vein. Sheep pulmonary vein possesses tryptamine receptors which mediate relaxation and which are not of the classicl M- or D-type. These receptors appear not to be involved directly or indirectly with responses to acetylcholine, catecholamines, histamine or prostaglandins.