Mechanism of action of GnRH: the participation of calcium mobilization and activation of protein kinase C in gonadotropin secretion.
The role of protein kinase C in luteinizing hormone (LH) release was analyzed in studies on the secretory responses to gonadotropin releasing hormone (GnRH) and phorbol esters in pituitary cell cultures. 12-O-tetradecanoyl-phorbol 13-acetate (TPA), 4 beta-phorbol 12,13-bibenzoate, and 4 beta-phorbol 12,13-diacetate stimulated LH release with ED50s of 5, 10 and 1000 nM, respectively, and with about 70% of the efficacy of GnRH. Phorbol ester-stimulated LH secretion was decreased but not abolished by progressive reduction of [Ca2+] in the incubation medium, and the residual response was identical with that of GnRH in Ca2+-deficient medium. TPA increased [Ca2+]i to a peak after 30 s in normal medium but not in the absence of extracellular Ca2+, indicating that protein kinase C promotes calcium entry but can also mediate secretory responses without changes in calcium influx and [Ca2+]i. The extracellular Ca2+-dependent action of TPA on LH release was blocked by CoCl2 but not by nifedipine. The secretory actions of TPA and GnRH were additive at low doses and converged to a common maximum LH response at high concentrations of the agonists. TPA caused rapid translocation of cytosolic protein kinase C to the particulate fraction, followed by a progressive decrease in total enzyme activity to less than 10% after 6 h. Partial recovery of the cytosolic enzyme (to 20%) occurred after washing and reincubation for 15 h. Such kinase C-depleted cells showed prominent dose-dependent reductions in the actions of both GnRH and TPA on LH release in normal and Ca2+-deficient media. These observations show that the actions of kinase C on LH release include extracellular Ca2+-dependent and independent components, and support the hypothesis that protein kinase C participates in the LH secretory response to GnRH in pituitary gonadotrophs.
Stojilković
S S
SS
Endocrinology and Reproduction Research Branch, National Institute of Child Health and Human Development, Bethesda, MD 20892.
Chang
J P
JP
Ngo
D
D
Tasaka
K
K
Izumi
S
S
Catt
K J
KJ
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002478:Cells, Cultured; D004789:Enzyme Activation; D005260:Female; D007986:Luteinizing Hormone / Q000378:metabolism*; D010703:Phorbol Esters / Q000494:pharmacology; D010903:Pituitary Gland, Anterior / Q000187:drug effects / Q000378:metabolism*; D010906:Pituitary Hormone-Releasing Hormones / Q000502:physiology*; D011493:Protein Kinase C / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains
D010703:Phorbol Esters; D010906:Pituitary Hormone-Releasing Hormones; D007986:Luteinizing Hormone; D011493:Protein Kinase C; D002118:Calcium