Unexpected stimulatory effect of progesterone on human myometrial contractile activity in vitro.

OBJECTIVE: To investigate the effects of continuous exposure of progesterone on contractile activity of term human myometrium in vitro. METHODS: Myometrial biopsy specimens were obtained from 14 term pregnant women undergoing elective cesarean. The specimens were immediately immersed in Hepes buffer and buffer containing 0.33, 4.1, and 70 micrograms/mL progesterone. Muscle biopsies were dissected along the bundles of smooth muscle under stereomicroscope. These myometrial strips were mounted in tissue baths and superfused with Hepes buffer and buffer containing different concentrations of progesterone. RESULTS: The progesterone concentration of 0.33 microgram/mL had no significant effect on myometrial contractile activity. The concentrations of 4.1 and 70 micrograms/mL increased the frequency of contractions (P < .01 for each) and tonus (P < .01 for each), decreased the activity area of contractions (P < .01 for each), and delayed the onset of contractions (P < .01 and P < .05, respectively). CONCLUSION: When the myometrium is not deprived of progesterone, this hormone may have excitatory effects on the frequency of contractions and tonus of the lower uterine segment of term human myometrium.