Factors Influencing Circular Muscle Activity in the Pregnant Rat Uterus1

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Abstract
Near the end of pregnancy the circular muscle of the rat uterus exhibits characteristic changes that are apparently prerequisite for normal delivery. The effects on the circular muscle of some intrinsic factors believed to regulate the myometrium during pregnancy, namely, uterine volume (stretch), ovarian steroids and the fetoplacental unit were studied. The activity of circular muscle isolated from gravid and nongravid uterine horns of unilaterally pregnant rats, normal and dislocated horns of unilaterally placental-dislocated rats, ovariectomized rats and normal pregnant rats was compared. Microelectrode and tension recordings were made on the circular muscle segments obtained from these groups of animals at the end of gestation and during parturition. Uterine volume and circulating estrogens apparently are more important than the fetoplacental unit in the evolution of circular muscle activity in the pregnant rat.

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