Effects of 5‐hydroxytryptamine on human isolated placental chorionic arteries and veins

1 Effects of 5‐hydroxytrypamine (5‐HT) on cylindrical segments of human chorionic arteries and veins were investigated. Concentrations of 5‐HT (up to 3 × 10 −6 m ) produced concentration‐dependent contractions; higher concentrations induced a reduction of the maximal response. These responses were antagonized by methysergide and ketanserin in a non‐competitive manner. The contractions elicited by low 5‐HT concentrations were more affected by methysergide (10 −7 m ) than by ketanserin (10 −7 m ). Ketanserin apparently increased the responses to high 5‐HT concentrations in veins. Arteries appeared to be more sensitive to both drugs than veins. Single concentrations of 5‐HT elicited transient contractions in both kinds of vessel. 2 Marked tachyphylaxis was seen in segments exposed to high concentrations of 5‐HT or in which a concentration‐response curve was determined. 3 Contractions induced by 5‐HT were reduced in a Ca 2+ ‐free medium. Veins were more affected by the Ca 2+ antagonists, nifedipine (10 −7 m ), nicardipine (10 −5 m ) and diltiazem (10 −5 m ) than arteries. 4 5‐HT (10 −6 m ) enhanced 45 Ca 2+ uptake in those vessels in which a concentration‐response curve had not been previously determined. In veins, this increase was reduced by the three Ca 2+ antagonists. 5 The results indicate that 5‐HT responses in these vessels were greatly dependent on extracellular Ca 2+ . A type of 5‐HT 1 ‐receptor may mediate responses to low 5‐HT concentrations, while higher concentrations may activate 5‐HT 2 ‐receptors. 5‐HT may desensitize the latter by interconversion between a high affinity and low affinity state, as suggested by others, a change prevented in part by ketanserin.