The effect of isoprenaline on adrenoceptors in human saphenous vein

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Abstract
1 The order of potency of sympathomimetic amines in causing contraction of strips of human saphenous vein was (−)-adrenaline>(−)-noradrenaline>(−)-phenylephrine>(±)-isoprenaline. 2 The competitive α-adrenoceptor blocking drugs tolazoline, phentolamine, and thymoxamine and the irreversible blocking drug phenoxybenzamine all blocked noradrenaline and isoprenaline contractions. 3 Contractions produced by 5-hydroxytryptamine were also blocked by phentolamine and thymoxamine. Fenfluramine-induced contractions were not blocked by thymoxamine or phenoxybenzamine. 4 ED50 contracting doses of isoprenaline did not cause consistent relaxation of noradrenaline-contracted strips. 5 It is concluded that human saphenous vein contains a dominant population of α-adrenoceptors which can be stimulated by high doses of isoprenaline, but the occurrence of β-adrenoceptors mediating relaxation is rare.