In vitro inhibition of rat uterine contractility induced by 5 alpha and 5 beta progestins.

Ten natural progestins were evaluated for their capacity to inhibit the in vitro motility of rat's uterus. Progestins with their ring A reduced in the 5 beta position were significantly more potent than delta 4-3 keto and 5 alpha reduced progestins. These last progestins were ineffective to inhibit uterine motility excepting 3 alpha-hydroxy-5 alpha-pregnan-20-one which was slightly less effective than progesterone. The potency of the progestins to inhibit uterine motility was related to their capacity to induce membrane stabilization. The data indicates that 5 beta, but not 5 alpha reduction of progesterone, may be important for regulating myometrial activity.