Time-related effects of a progestogen on the isoforms of serum gonadotrophins in 17 beta-oestradiol treated post-menopausal women.

Abstract

It has previously been shown that 17 beta-oestradiol (E2) implants counteract the formation of more acidic isoforms of the gonadotrophins in post-menopausal women. A much lesser effect was observed on the charge of the gonadotrophin isoforms in women with chronic oral daily therapy with 2 mg E2 combined with a progestogen, 1 mg norethisterone acetate (NETA), in spite of similar serum levels of E2 and SHBG. The presence of the progestogen in the latter study may explain the difference observed. The present study investigated the effect of the progestogen NETA on the charge and concentration of serum FSH and LH in E2 implant treated women. A group of 8 post-menopausal women, mean age 65 years (range 50-80 years) treated with 20 mg E2 implants every 6 months, participated in the study. The women were given a daily oral medication of 5 mg NETA for a 4-week period starting at 4 weeks after the insertion of an E2 implant (mean serum E2 420 pmol/l). This treatment with NETA was repeated in 6 of the women starting at 18 weeks after the insertion of the E2 implant (mean serum E2 317 pmol/l). Blood samples were obtained at the start of the NETA therapy, after 2 and 4 weeks of treatment and at 4 weeks after the last NETA treatment. The effects of NETA therapy on the charge of the serum gonadotrophin isoforms was determined by electrophoresis in 0.1% agarose suspension and FSH, LH, E2, and SHBG were determined with fluoroimmunoassays. The NETA treatment decreased the serum FSH and LH levels after 2 weeks to 24 and 23% of the levels before NETA and after 4 weeks to 14.6 and 8.8%, which were 1.3 and 2.2% of the mean levels for non-treated post-menopausal women. Both FSH and LH isoforms became more acidic during the first 2 weeks of treatment. During the following 2 weeks of NETA treatment the isoforms of both FSH and LH became more basic again. Ten weeks later both the concentration and the charge of the gonadotrophins were similar to those before the NETA treatment. The changes in concentration and charge of the gonadotrophins during the second treatment period were similar to those during the first. All the changes were statistically significant (P < 0.05 - < 0.001). The mean SHBG level decreased (P < 0.01) from 84.5 to 70.6 nmol/l after 2 weeks and to 59.9 nmol/l after 4 weeks of NETA treatment and increased (P < 0.01) 10 weeks later to 77 nmol/l. In the oestradiol treated women, the effect of the progestogen norethisterone acetate on the charge of the gonadotrophin isoforms was time-related. The oestradiol effect on the charge was counteracted during the first 2 weeks of progestogen treatment and more acidic isoforms appeared in the circulation. During the following 2 weeks the isoforms became more basic again. The levels of the gonadotrophins were efficiently decreased after 2 weeks of progestogen treatment and further decreased after 4 weeks. The time-related effect of the progestogen on the gonadotrophin isoforms may be mediated via changes in the pattern of GnRH release from the hypothalamus. The observed gradual decrease in the SHBG level during the progestogen therapy may cause an increased oestradiol effect on the hypothalamus and pituitary.