Demonstration that progesterone 'blocks' uterine activity in the ewe in vivo by a direct action on the myometrium

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Abstract
Intrauterine pressure was monitored in vivo in estrogen-treated ovariectomized ewes before, during and after treatment with progesterone (50 mg s.c./day for 3 days). Progesterone reversibly reduced the frequency and amplitude of myometrial activity and abolished uterine reactivity to oxytocin (i.v.) and PGF2.alpha. (intrauterine infusion). The rate of rise of intrauterine pressure during active pressure cycles was significantly reduced. These results confirm that the action of progesterone on the ovine myometrium is comparable to the classic progesterone block. The intrauterine infusion of PGF2.alpha. (10 .mu.g/min), which elicited a marked mechanical response in the control animals, failed to stimulate the progesterone blocked uterus, suggesting that the inhibition produced by progesterone is due to a direct action of the hormone on the uterine muscle and not to an indirect mechanism operating through endometrial prostaglandin output.