Gonadotropin-releasing hormone binding and activation of enriched population of pituitary gonadotrophs

The properties of GnRH receptors were analyzed in isolated rat pituitary cells by binding studies with the labeled GnRH agonist, ¹²⁵I]iodo-d-Ser(tBu)⁶]des-Gly¹⁰-GnRH-N-ethylamide. The concentration of GnRH-binding sites in pituitary cells from immature female rats was twice as high as in cells from adult females. Electron-microscopic irnmunocytochemistry revealed twice as many gonadotrophs in the immature rat pituitary, indicating that gonadotrophs from immature and mature female rats contain the same number of binding sites. An enriched population of gonadotrophs prepared from immature female rat pituitaries by velocity sedimentation at unit gravity contained 77% of the total radioimmunoassayable LH and 71% of the total GnRH receptors. A second population of small, poorly granulated gonadotrophs was distributed among the cells of other fractions of the gradient and could be detected only by irnmunocytochemistry. The smaller, possibly immature, gonadotrophs did not contain a measurable number of GnRH receptors. When incubated with the cells recovered from the gradient, GnRH stimulated LH release only in the mature gonadotroph-enriched fraction and not in other regions of the gradient. These results indicate that GnRH interacts preferentially with gonadotrophs in the pituitary. The findings also suggest that functional heterogeneity exists among pituitary gonadotrophs and that GnRH binds to and activates only the mature gonadotrophs which exhibit the highest LH-storage capacity.