Gestrinone has been used for treatment of pelvic endometriosis, in doses of 2.5 mg twice a week. This study is designed to clarify it from the dynamics of sex steroid receptors in rabbit uterus. 2. Four different regimens were scheduled, namely daily 1 microgram estradiol-17 beta (E2, consistent with endogenous level of women) for 3 days, and together with either 30, 60 (consistent with 2.5 microgram of clinical dose) or 120 microgram(s) gestrinone, in single dose or with 20 microgram(s) gestrinone daily (divided dose in single 60 microgram(s) gestrinone administration) for 3 days. Receptors for estrogen (ER, type I and II) or progestin (PR) were determined by charcoal adsorption in cytosol and KCl extract, or sedimentation in non-KCl extractable fraction, using [3H]E2 or [3H]promegestone respectively. 3. Gestrinone decreased total ER (type I and II) levels, dose-dependently, except the case of 20 micrograms gestrinone daily in ER type II. Total ER type I level did not return to the pre-level until 3 days only after 120 microgram(s) gestrinone administration. Total ER type II level was decreasing in 3 days after 30, 60 or 120 micrograms gestrinone therapy. Total PR level was decreased in the order of strength: 120 micrograms gestrinone greater than 60 micrograms gestrinone greater than 30 microgram gestrinone greater than 20 micrograms gestrinone in group, and the recovery was not obtained until 72 hr only after 120 micrograms gestrinone therapy. The uterine weight was decreasing with the same strength in 3 days of the therapy independently upon the regimen. 4. In conclusion, gestrinone dose of 60 micrograms (2.5 mg of clinical dose) every 3 days is considered to be effective for anti-steroid action in sex steroid receptor dynamics in the rabbit uterus, contributing to the rationale for gestrinone treatment of pelvic endometriosis.
