Characterization of contractile adrenoceptors in the human umbilical artery.

Adrenoceptors mediating contraction in ring segments of human umbilical arteries from normal term pregnancies were investigated in vitro. Contraction was elicited by (order of potency indicated): noradrenaline = the alpha 2-adrenoceptor agonist oxymetazoline >> the alpha 1-adrenoceptor agonist phenylephrine. The alpha 1-adrenoceptor antagonist prazosin antagonized the contraction elicited by noradrenaline and phenylephrine. The alpha 2-adrenoceptor antagonist rauwolscine antagonized the contraction elicited by noradrenaline and oxymetazoline. Oxymetazoline had an efficacy 5 times higher than that of noradrenaline and the 5-hydroxytryptamine receptor antagonist methysergide antagonized the contraction elicited by oxymetazoline. It is suggested that the contractile adrenoceptors in the human umbilical artery consist of both alpha 1 and alpha 2 subtypes. Furthermore, the contractile effect of oxymetazoline seems to be mediated via both alpha 2-adrenoceptors and 5-hydroxytryptamine receptors.