ANTITACHYPHYLACTIC EFFECTS OF PROGESTERONE AND OXYTOCIN ON TERM HUMANMYOMETRIAL CONTRACTILE ACTIVITY IN-VITRO

Objective: To determine whether progesterone causes any changes in the action of oxytocin on the contractile activity of term human myometri um in vitro. Methods: Myometrial biopsies from 13 term pregnant women undergoing cesarean delivery were immediately immersed in Hepes buffer or buffer containing 5 mug/mL progesterone. In the laboratory under a stereomicroscope, the specimens were dissected into 1 x 7.5-mm muscle strips. These were mounted into six tissue baths and superfused with Hepes buffer and buffer containing 5 mug/mL progesterone, 5 mug/mL pro gesterone plus 10 mU/mL oxytocin, 5 mug/mL progesterone plus 100 mU/mL oxytocin, 10 mU/mL oxytocin, and 100 mU/mL oxytocin. Isometric tensio n was recorded constantly for at least 3 hours. Results: The frequency of myometrial contractions and tonus increased by superfusion with pr ogesterone, progesterone plus oxytocin, and oxytocin alone compared to buffer. In addition, a gradual decrease in frequency was observed aft er 60 minutes of contractions with oxytocin alone, whereas progesteron e plus oxytocin had no such decrease. The activity area of contraction s was greater with 10 mU/mL oxytocin and decreased with progesterone, progesterone plus both concentrations of oxytocin, and 100 mU/mL oxyto cin alone, compared to buffer. Conclusion: Progesterone stimulated the frequency of contractions and tonus of strips from the lower uterine segment of term human myometrium. A lasting increase in the frequency of contractions was observed with superfusion of progesterone plus oxy tocin, suggesting that progesterone counteracted the reaction of tachy phylaxis to oxytocin.