Effect of tyramine on human umbilical artery in vitro

Fifteen of twenty-one perfused human umbilical artery preparations constricted in response to tyramine. These responses were unaffected by atropine, mepyramine or bromolysergic acid diethylamide but were blocked by phentolamine or dihydroergotamine. Cocaine augmented the constriction responses to tyramine or noradrenaline but not those to adrenaline, acetylcholine or 5-hydroxytryptamine. Infusion of noradrenaline restored the responses of preparations which had become tachyphylactic to tyramine.