Kinetics of Secretion of Luteinizing Hormone Subunits and Free Alpha, and Effects of Gonadotropin Releasing Hormone

While GnRH is known to stimulate release of pituitary gonadotropins, its acute effects on the kinetics of secretion of the various hormones and subunits are not well characterized. Pulse‐chase experiments were therefore performed to compare the time course of secretion of newly synthesized LH subunits and free α from rat pituitary quarters, and to study the effects of GnRH. After a l‐h pulse labeling with [³⁵S]cysteine in the presence or absence of 10⁻⁸ M GnRH, cultures were chased with excess unlabeled cysteine for 1, 2, 4, 8, or 20 h. Tissue lysates and media were immunoprecipitated sequentially with antisera to PRL, GH, LHβ and LHα, and the products were analyzed by gel electrophoresis. Labeled LHα was completely secreted by 4 h of chase without GnRH, and by 2 h with GnRH, as shown by its appearance in media and depletion from pituitary. Newly synthesized LHβ was depleted only at 8 h with GnRH from pituitary, suggesting much slower secretion. Incorporation of ³⁵S into LHβ was approximately half that into LHα. Newly synthesized free α subunit was secreted between 4 and 20 h without GnRH, and by 8 h with GnRH. Free α incorporated a similar amount of radioactivity as LHβ. GnRH had no effect on the timing of the secretion of labeled total protein, PRL, GH, or TSH subunits. The amount of label incorporated during the pulse was greatest for PRL, approximately 15% of total protein label. GH incorporated approximately 20% as much label as PRL, with the glycoprotein subunits somewhat lower. The total amount of ³⁵S incorporated into each protein studied was not affected by GnRH. The data indicate that short term in vitro GnRH exposure during a 1‐h pulse labeling and chase acts to accelerate early secretion of LHα, LHβ and free α but does not affect overall protein synthesis. The peptide hormones PRL and GH incorporated the greatest amount of label and were the most briskly secreted hormones, while LHα led the glycoprotein subunits in the amount of incorporation and rapidity of secretion. 1989 American Society of Andrology