Effects of oxytocin receptor antagonist atosiban on pregnant myometrium in vitro

OBJECTIVE: To investigate dose-dependent effects of oxytocin receptor antagonist, atosiban, on oxytocin-induced contractions of myometrial strips from healthy pregnant women. METHODS: During elective cesareans, myometrial biopsies were taken from the lower uterine segment and trimmed into 2 x 2 x 10-mm longitudinal strips within 36 hours. One hundred twenty-two myometrial strips showed regular spontaneous contractions and were used for measurement. Each myometrial strip was mounted in a physiologic organ bath, and isometric contraction activity and contractile patterns were analyzed after stimulation with oxytocin. Frequency and amplitude of contractions and area under force-time curves were determined by adding atosiban at concentrations of 1, 5, 10, 20, 50, 100, 250, and 500 microg/mL. Resulting contraction activity (at least 20 contractions per concentration) was compared with reference activity. RESULTS: Atosiban had significant inhibitory effects on myometrial contractions at concentrations as low as 1 microg/mL. Compared with reference activity (oxytocin stimulation only), resulting inhibition was higher than 50% with the lowest atosiban concentration used. This inhibition of contraction activity was dose-dependent and referred to the area under the curve, frequency, and amplitude of contractions. At high concentrations of atosiban, contraction activity was even lower than spontaneous activity without oxytocin stimulation. CONCLUSION: The oxytocin receptor antagonist atosiban showed a significant, dose-dependent inhibition of oxytocin-induced contractions of human myometrium in vitro. It might be effective in tocolysis at term.