Disparate effect of clomiphene and tamoxifen on pituitary gonadotropin release in vitro.

The direct effects of clomiphene citrate (Clomid), tamoxifen, and estradiol (E2) on the gonadotropin-releasing hormone (GnRH)-stimulated release of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) were studied in cultured anterior pituitary cells obtained from adult ovariectomized rats. Treatment of pituitary cells with Clomid or enclomid (10(-8) M) in vitro for 2 days resulted in a marked sensitization of the gonadotroph to GnRH as reflected by a 6.5-fold decrease in the ED50 of GnRH in terms of LH release from 2.2 x 10(-9) M in untreated cells to 3.6 x 10(-10) M. Treatment with E2 or Clomid also increased the sensitivity of the gonadotroph to GnRH in terms of FSH release by 4.3- and 3.3-fold respectively. Tamoxifen, a related antiestrogen, comparable to Clomid in terms of its ability to compete with E2 for pituitary estrogen receptors, was without effect on the GnRH-stimulated LH release at a concentration of 10(-7) M. Furthermore, tamoxifen, unlike Clomid, caused an apparent but not statistically significant inhibition of the sensitizing effect of E2 on the GnRH-stimulated release of LH. Our findings suggest that Clomid and its Enclomid isomer, unlike tamoxifen, exert a direct estrogenic rather than an antiestrogenic effect on cultured pituitary cells by enhancing the GnRH-stimulated release of gonadotropin.