To assess a potential underlying rhythm in the sensitivity of the pregnant primate myometrium to oxytocin we infused five pregnant rhesus monkeys (gestational age, 121-142 days) with oxytocin at three different periods of the 24-h day, in the order night, afternoon, and morning, via the maternal inferior vena cava. Oxytocin was administered for 1 min every 5 min over 30 min at four doses 400, 800, 2000, and 4000 pg/minkg. The response of the myometrium, evaluated as the number of contractions per pulse of oxytocin, was greatest in the early hours of darkness. Studies in three additional monkeys in which the order of the oxytocin challenge tests was changed, again showed that the response to oxytocin was greatest in the hours of darkness. We conclude that an underlying, as yet undefined, rhythm exists in the sensitivity of the pregnant primate myometrium to oxytocin at different times of the 24-h day. This change in myometrial sensitivity may be due to variation in one or more of the many potential regulatory sites of oxytocin’s action at the cellular level. We hypothesize that this difference in myometrial sensitivity to oxytocin plays a role in the switch from myometrial contractures to contractions that occurs around the hours of darkness, e.g. postsurgery, during food withdrawal, and before delivery.
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