Comparison of the Biosynthesis and Release of Luteinizing Hormone by Rat Pituitaries inVitroin Response to Gonadotropin-Releasing Hormone Analogs*

Abstract

In an attempt to determine whether gonadotropin-releasing hormone (GnRH) regulates synthesis and release of LH via independent processes, the ability of several GnRH analogs to regulate LH release was compared with their ability to regulate LH synthesis. Pituitary glands from ovariectomized rats were incubated with [³H]glucosamine in the presence of increasing doses of either GnRH or GnRH agonists: (des-Gly-NH₂¹⁰)GnRH ethylamide, (D-Ala⁶, des-Gly-NH₂¹⁰)GnRH ethylamide, or (D-Lys⁶, η-polyglutamate)GnRH. After incubation, total immunoreactive LH (IR-LH) was measured by RIA and radiolabeled LH was measured by immunoprecipitation with specific anti-LHβ serum. The dose-response curves for synthesis (medium plus tissue) of [³H]glucosamine-labeled LH ([³H]LH) were similar to those for release. However, the dose required for half-maximal stimulation of release (KmR) of [³H]LH differed from the dos required for half-maximal stimulation of synthesis (KmS) of [³H]LH for GnRH and all agonists. The ratio of KmR to KmS for [³H]LH ranged from 0.75–1.98. The ratio of KmR for IR-LH to KmS for [³H]LH ranged from 1.4–4.3. Addition of the antagonist (des-His², D-Ala⁶)GnRH to the incubation system inhibited GnRH-induced synthesis of [³H]LH and release of [³H]LH and IR-LH. The dose required for half-maximal inhibition of GnRH-induced release of either IR-LH or [³H]LH was 0.4 times that required for half-maximal inhibition of synthesis. Comparison of the medium to tissue ratios of IR-LH and [³H]LH in response to increasing doses of GnRH or agonists revealed that the dose per se affected the relative rates of release of both [³H]LH and IR-LH. Overall, these data suggest 1) that GnRH analogs have similar effects on LH synthesis and release, 2) that the doses of GnRH analogs required for half-maximal stimulation of LH synthesis are lower than those required for half-maximal stimulation of release, 3) that the dose of GnRH regulates relative rates of release vs. synthesis of LH, and 4) that the dose of GnRH regulates relative rates of release of newly synthesized vs. stored LH.