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Uterine longitudinal and circular muscles from pre- and post-term rats were studied in vitro for their contractile and electrical activities under the influence of 20-100 microM progesterone, 20 microM estradiol-17 beta, or 20 microM stilbestrol. Uterine longitudinal or circular muscle strips were irrigated by Krebs solution at 37 degrees C containing one of the hormones mentioned above, dissolved by the use of ultrasonic wave. Muscle contractions were recorded by a force displacement transducer, and electrical activities of the muscle membrane were studied using an intracellular microelectrode. At diestrus and Day 14 of pregnancy, progesterone depressed the contractions of longitudinal muscle. At pre-term, progesterone potentiated the contractions and prolongated the membrane burst discharge of longitudinal muscles, while estradiol inhibited their contractions and suppressed spike activities. At the same stage of pregnancy, both progesterone and estradiol inhibited contractions of circular muscles. At post-partum, progesterone potentiated and then depressed longitudinal muscles, while estradiol inhibited their contractions. At this stage, both progesterone and estradiol inhibited contractions of circular muscles, although vigorous spike discharges were observed. Stilbestrol depressed contractions of both longitudinal muscles from diestrus rats and longitudinal ileal muscles from a guinea pig. In view of these findings, it was deduced that these ovarian hormones exert dual action on the uterine muscle in vitro, one through changes in the membrane activity and the other through dissociation of excitation-contraction coupling.

Effects in vitro of progesterone and estradiol-17.BETA. on the contractile and electrical responses in rat myometrium.