The Plasma Level of 5α-DHP After Intramuscular Injection of 5α-DHP and the Induction of Uteroglobin in Ovariectomized Rabbits

A competitive protein-binding assay for the determination of 5α-pregnan-3, 20-dione (5α-DHP) in rabbit plasma is described. It is based on the specificity of the progesterone binding plasma protein (PBP) of the pregnant guinea pig. This method was applied to plasma samples obtained from ovariectomized rabbits which had received intramuscular injections of oil vehicle or 2 mg/kg bodyweight of 5α-DHP. The intramuscular injection of 5α-DHP resulted in plasma levels of 5α-DHP about one-tenth those of progesterone after the I.M. injection of progesterone. Two mg/kg bodyweight of 5α-DHP and 0.2 mg/kg bodyweight of progesterone resulted in similar blood concentration of each steroid (3-10 ng/ml). This concentration of progesterone, but not 5α-DHP, induced uteroglobin in the uterus. Therefore, it is concluded that 5α-DHP does not mediate progesterone induction of uteroglobin in the rabbit uterus.