ATYPICAL TRYPTAMINE RECEPTORS IN SHEEP PULMONARY VEIN

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Abstract
Both the pulmonary artery and vein of the sheep contracted dose‐dependently to histamine, carbamoylcholine, prostaglandin F2α, noradrenaline and bradykinin and relaxed in the presence of isoprenaline or prostaglandin E1. 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. 5‐HT 5 × 10−10 to 5 × 10−8 M relaxed the partially constricted vein. 5‐HT 10−7 to 10−6 M caused brief venoconstriction followed by relaxation. 5‐HT > 10−6 M caused dose‐related contraction of the vein. Methysergide effectively blocked the contractile response of the artery to 5‐HT, but only weakly inhibited the contractions of the vein (dose‐ratio <20). 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 classical M‐ or D‐type. These receptors appear not to be involved directly or indirectly with responses to acetylcholine, catecholamines, histamine or prostaglandins.