[Factors influencing the pattern-change of spontaneous activity in the pregnant rat myometrium]

Near the end of pregnancy, both the longitudinal and circular muscles of the rat uterus exhibit characteristic changes, and especially in circular muscle, the plateau-type action potential observed in mid-pregnancy changes into the burst discharge which is apparently prerequisite for normal delivery. This study concerned the effects of the ovarian hormones, the stretch of the uterine wall, and the fetoplacental unit on the longitudinal muscle of the pregnant rats, and the results were compared with the activity of the circular muscle in discussion. 1. Spontaneous electrical and mechanical activities of the longitudinal muscle of the unilaterally pregnant rats showed the marked difference between the gravid and nongravid uterine horns. 2. Normal and dislocated horns of unilaterally placental-dislocated rats showed the same spontaneous activities in the longitudinal muscles. 3. In ovariectomized rats, estrogen-treated group delivered normally and the spontaneous activities of the longitudinal muscle showed normal pattern. On the contrary, estrogen-untreated group delivered abnormally, however the spontaneous activities of the longitudinal muscle were normal. The results showed that the important factor for the characteristic change of the longitudinal muscle during pregnancy is the stretch of the uterine wall, and that the roles of longitudinal and circular muscles for the parturition may be different.