Plasma levels of progesterone in pseudopregnant rabbits actively immunized with a progesterone-protein conjugate.

The effects of active immunization with a progesterone protein conjugate on plasma progesterone levels in rabbits injected with a luteinizing dose of human chorionic gonadotropin (HCG) were determined. Mature female New Zealand rabbits were used. As antigen, a hemisuccinate of llalpha-hydroxyprogesterone was linked with bovine serum ablumin using the acetic anhydride method. Nuclear magnetic resonance analysis revealed that 24 moles of the hemisuccinate were linked with each mole of the protein. The conjugate was lyophilizied and stored at 4 degrees C. For immunization, an emulsion was prepared using equal volumes of Freund's complete adjuvant and .15 m NaCl solution of the immunogen. 7 rabbits received intradermal injections and 7 received weekly sc injections for 6 weeks. Sera were collected weekly for 6 weeks. Animals with high levels of antibody production were given iv injections of 1 mg of conjugate to boost antibody titers. Pseudopregnancy was induced in 14 rabbits by iv injection of 100 IU HCG. 5 weeks after the immunization schedule was finished animals were made pseudopregnant again and daily plasma progesterone was determined. Increased levels of antibodies to progesterone were found in all immunized animals but not in controls. Plasma progesterone levels in treated animals increased to a peak on Day 9 of pseudopregnancy. On Days 0 through 10, 17, and 18 progesterone values in immunized animals were significantly (p less than .05) higher than those of controls. It is concluded that immunization of rabbits by this method lead to abnormally high levels of peripheral progesterone in pseudopregnancy. I n a subsequent study it was found that despite the high plasma progesterone concentration produced in immunized animals, these females could be successfully mated.