Effect of passive immunization against progesterone on its distribution in circulation and tissues of mice

Anti-progesterone monoclonal antibody, injected into mice 32 h after mating at a dose that blocks the establishment of pregnancy, produced a significant reduction in the concentration of progesterone in the ovary and uterus within 6 h after treatment. Uterine concentrations remained lower in treated compared with control animals for at least 24 h after injection. There was an associated transient increase in plasma LH and FSH concentrations, but there was no change in plasma prolactin values. The percentage of total progesterone in the circulation that was unbound was reduced after treatment, but the concentration of unbound progesterone was increased. Studies of antibody binding of steroid in the presence of uterine progesterone receptor protein showed that there was a stoichiometric relationship in the distribution of ligand between the two binders. The present findings suggest that the effects of passive immunization against progesterone are associated with perturbation of tissue concentrations of steroid in the target organ as a result of high antibody concentrations in the circulation.