Prostaglandin-induced myometrial activity inhibited by progesterone

PROSTAGLANDINS (PGs) are being used increasingly as abortifacients although their precise mechanism of action is uncertain. One of the compounds frequently used is PGF2α, which has been shown to stimulate the myometrium in vitro1 and in vivo2. Whether or not the direct action of this agent on the uterine muscle alone is responsible for the interruption of pregnancy is not clear, but the long interval between infusion and abortion3 suggests that other mechanisms might be involved; for example, a luteolytic effect4–6. The prompt increase in uterine activity after treatment with prostaglandin, however, argues that the direct effect plays some part in the abortifacient action. Little is known of the influence of other myometrial regulatory agents such as the ovarian steroids on the uterine response to PGs. Accordingly we examined the influence of progesterone on myometrial responses to prostaglandin F2α in the rabbit, a species in which progesterone is known to have a blocking action on the uterine muscle7.