Interference of diltiazem with serotonin- and potassium-induced contractions in human placental veins

1. In segments of human placental veins precontracted with K+ (25, 40 or 75 mM), diltiazem produced concentration-dependent relaxations with similar EC50 values. 2. Diltiazem (20 min preincubations) also caused concentration-dependent inhibition of the contractions induced by K+ (40 or 75 mM) and 5-HT (serotonin, 3 x 10(-7) M). IC50 values were similar, in both cases. 3. The time-course of the vasocontractions induced by 75 mM K+ and 3 x 10(-7) M 5-HT in the absence and in the presence of diltiazem were different. The contractions caused by K+ were sustained and transient those elicited by 5-HT. 4. These data suggest that (1) this vasculature is sensitive to diltiazem, and (2) the Ca influx through voltage-operated Ca channels (activated by K+) and through receptor-operated Ca channels (activated by 5-HT) is very similar.