Contraceptive progestins and gonadotropin secretion in vitro

Progesterone modulation of gonadotropin secretion by dispersed rat pituitary cells in culture. IV. Follicle-stimulating hormone synthesis and release

Estradiol-treated, rat pituitary cells were studied to examine the effects of progesterone (P) on follicle-stimulating hormone (FSH) synthesis and secretion. Progesterone was administered prior to or concurrent with 3 h secretory challenges with either gonadotropin-releasing hormone (GnRH), the iontophore A23187, the protein kinase C activator phorbol 12,13-myristate (PMA), or no secretagogue. Medium FSH levels and cell FSH stores were quantified by radioimmunoassay and bioassay. Acute (< 6 h) exposures to P increased medium levels of immunoreactive and bioactive FSH following GnRH challenge without influencing total (cell + medium) values whereas chronic (9-24 h) treatments increased both parameters. Chronic P elevated total FSH levels even when no secretagogue was present. Studies with antiprogestins, 5 alpha-dihydroprogesterone and 5 alpha-reductase inhibitors revealed that this direct action of P depended on progestin receptor occupation but not on 5 alpha-reduction. These studies indicate that P selectively increases bioactive and immunoactive FSH levels, presumably by increasing FSH synthesis, and characterize the time course and cellular mechanisms of this response. To accommodate for P modulation of total FSH levels, FSH secretion was standardized as the percentage of cellular stores available for release. Progesterone modulation of GnRH-stimulated FSH secretion was multiphasic, i.e. increased at 0-6 h, unchanged at 9 h and suppressed at 24 h. Acute and chronic exposures to P similarly modulated A23187-stimulated FSH release, whereas both P treatments increased PMA-stimulated FSH secretion. In these experiments P modulated luteinizing hormone secretion in parallel fashion, suggesting that common cellular mechanisms underlie peptidergic and steroidal regulation of the secretion of both gonadotropins.

Modulatory actions of estradiol and progesterone on phorbol ester-stimulated LH secretion from cultured rat pituitary cells

We compared the ability of estradiol and progesterone to modulate gonadotropin-releasing hormone (GnRH) and protein kinase C (PKC)-mediated luteinizing hormone (LH) secretion. Long-term (48 h) treatment of rat pituitary cells with 1 nM estradiol enhanced GnRH and phorbol ester (TPA)-stimulated LH secretion. This positive effect was facilitated by additional short-term (4 h) treatment with progesterone (100 nM). However, long-term progesterone treatment, which inhibited GnRH-stimulated LH secretion, did not influence TPA-stimulated gonadotropin release. These steroid actions occurred without an effect on the total amount of LH in the cell cultures (total LH = LH secreted + LH remaining in the cell) and neither the secretagogues nor the steroids altered total LH. Since GnRH or TPA-induced LH secretion depends on Ca2+ influx into the gonadotroph, we also analyzed the effects of estradiol and progesterone under physiological extracellular Ca2+ concentrations and in the absence of extracellular Ca2+. The steroids were able to influence GnRH or TPA-induced LH secretion under both conditions. However, when TPA was used as stimulus in Ca(2+)-deficient medium the relative changes induced by estradiol and progesterone were more pronounced, possibly indicating that the extracellular Ca(2+)-independent component of PKC-mediated LH secretion is more important for the regulation of the steroid effects. It is concluded that estradiol and progesterone might mediate their modulatory actions on GnRH-stimulated LH secretion via an influence on PKC. This effect can occur independently from de novo synthesis of LH and Ca2+ influx into gonadotrophs.

Progestins and Antiprogestins: A Review of Their Role in Medicine and Bioassays Used in Their Development

The physiological role of ovarian progesterone in reproduction and the medical uses of synthetic progestins and antiprogestins are briefly reviewed. Although a number of progestins are in wide use, the search continues for better analogues that display fewer of the unwanted effects of the present compounds. The one antiprogestin approved for human use to date, RU486, exhibits considerable antiglucocortoid activity

The classical bioassays currently in use for the development of these steroid analogues depend upon in vivo progestin effects. However, progestins induce measurable responses in several cell culture systems that could be used as alternative bioassays. Rabbit uterine stromal cells in primary culture are especially suitable for such use. Addition of progesterone to these cultures stimulates synthesis of a 42 kilodalton (42kD) protein that appears in the medium. The 42kD protein response is specific for a progesterone receptor-mediated event and it can be blocked by known progestins. Using this culture system as a bioassay, one rabbit yields enough cells to measure the effect of 30 test doses in triplicate. Thus, a culture system is described that could substitute for current in vivo bioassays. This culture bioassay would allow large scale screening for potential progestin or antiprogestin activity.

The inhibitory effect of synthetic steroids on proestrous gonadotropin release in the rat

This study was performed to evaluate the inhibitory effect of synthetic steroids on the hypothalamo-pituitary axis. Six synthetic steroids, used for contraception or other gynecologic indications, were injected to female rats on diestrus 2 (2 mg/kg, sc), and the animals were sacrificed in the afternoon of proestrus. Serum levels of LH, FSH and progesterone, the pituitary contents of LH and FSH, and LH-RH in the median eminence were measured by radioimmunoassays. Among the synthetic steroids tested in this study, norethisterone, d-norgestrel, 3-keto-desogestrel and gestrinone completely blocked the gonadotropin surge on proestrus, showing stronger antigonadotropic activity than progesterone. Desogestrel did not affect proestrous gonadotropin release at this dose. The suppressive effect of danazol could be seen only in a larger dose (20 mg/kg). These findings reconfirmed previously reported results, which demonstrated that the hypothalamo-pituitary complex was a major site of action of these steroids.