Are the antioxidative effects of 17 beta-estradiol modified by concomitant administration of a progestin?

Estrogens are known to exhibit antioxidative effects. At present little information exists on the influence of co-administered progestins upon this effect. Therefore we investigated the influence of levonorgestrel, a potent antiestrogenic progestin, on the inhibition of the low-density lipoprotein (LDL) oxidation by 17 beta-estradiol or 17 beta-estradiol valerate in vitro and ex vivo.

After isolation from blood, the in vitro oxidation of LDL was induced by copper ions and measured continuously by monitoring the formation of conjugated dienes. In 21 female ovariectomized White New Zealand rabbits the antioxidative action of 17 beta-estradiol alone or in combination with levonorgestrel after subcutaneous infusion for 3 days was determined using the copper-induced LDL-oxidation as an endpoint. Eleven postmenopausal women were exposed to sequential estrogen-progestin replacement therapy (day 1-21:2 mg estradiol valerate/day, day 10-21: 0.15 mg levonorgestrel/day). Blood samples were collected at three times: on day 1, on day 10, on day 22 (after the combination phase). The lag time of ex vivo oxidation of LDL, the plasma estradiol and estrone levels were estimated.

In the chosen cell-free system, 17 beta-estradiol increased the lag time of the LDL-oxidation in a dose-dependent manner. Levonorgestrel showed neither pro-oxidative nor antioxidative effects when administered alone in different concentrations. Co-administration of different doses of levonorgestrel did not modify the antioxidative action of estrogen either. The two ex-vivo models confirmed these results. In rabbits the co-administered 19-nortestosterone derivative levonorgestrel did not impair or reverse the estradiol-dependent effect. In postmenopausal women the daily oral administration of levonorgestrel in conjunction with 17 beta-estradiol valerate did not diminish the antioxidative action of this estrogen given for the first 9 days.

The antioxidative potential of estradiol and estradiol valerate is maintained in the presence of levonorgestrel.