ATYPICAL TRYPTAMINE RECEPTORS IN SHEEP PULMONARY VEIN

Abstract

1 Both the pulmonary artery and vein of the sheep contracted dose-dependently to histamine, carbamoylcholine, prostaglandin F_2α, 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. 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. 3 Methysergide effectively blocked the contractile response of the artery to 5-HT, but only weakly inhibited the contractions of the vein (dose-ratio <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 classical M- or D-type. These receptors appear not to be involved directly or indirectly with responses to acetylcholine, catecholamines, histamine or prostaglandins.