Ability of ketanserin to block different receptors in human placental vessels.

5-Hydroxytryptamine (5-HT), noradrenaline (NA) and histamine induced concentration-dependent contractions in segments of human chorionic arteries and veins, whereas clonidine, an alpha 2-adrenoceptor agonist, had no effect. 5-HT and histamine induced strong contractions, while NA elicited weak contractions in some segments. The maximal response was similar for 5-HT and histamine. The order of potency (EC50 values) was: 5-HT greater than or equal to NA greater than or equal to histamine. These agonists induced tachyphylaxis, and single concentrations caused transient contractions. Contractions elicited by 5-HT were antagonized by ketanserin, a 5-HT2-receptor antagonist, which also antagonized the responses to NA and histamine, but at greater concentrations than those needed for 5-HT responses. Contractions elicited by histamine were reduced by diphenydramine. Low concentrations of 5-HT amplified contractions caused by NA and histamine. The results indicate that: (i) 5-HT is the most potent constrictor agent tested in these vessels; (ii) its effects are mediated by 5-HT2-receptors; and (iii) ketanserin at therapeutic plasma concentrations (10(-7) M) seems to block mainly 5-HT2-receptors, and alpha 1-adrenergic- and H1-receptors to a small extent only.