Progesterone binding in the immature rabbit and guinea pig uterus.

Comparative sucrose gradient studies in the immature rabbit and guinea pig have established the presence of a latent progesteronespecific receptor in the unprimed animal which sediments in the “7–8S” region and demonstrated a parallelism in its binding specificity in primed and unprimed animals of different species. In the rabbit, progesterone binds specifically both in the “7–8S” and “4–5S” region; in the guinea pig, in the “7–8S” region only. The highly potent synthetic progestin, R 5020, binds specifically in the “7–8S” region only whatever the species and therefore constitutes an ideal model for competition experiments. The intrinsic association constants for the binding of both progesterone and R 5020 for the “7–8S” receptor have been measured by equilibrium dialysis: identical values are obtained for the rabbit and guinea pig for each steroid, but the values for R 5020 are 4 times higher than for progesterone. These studies support the theory of a uniform uterine mechanism of action for progesterone...