Calcium mobilization and influx during the biphasic cytosolic calcium and secretory responses in agonist-stimulated pituitary gonadotrophs.
Stimulation of enriched pituitary gonadotrophs by gonadotropin-releasing hormone (GnRH) elicits dose-dependent biphasic elevations of cytosolic calcium ([Ca2+]i) and luteinizing hormone (LH) release, with rapid initial peaks followed by sustained plateaus during continued exposure to the agonist. A potent GnRH-antagonist, [N-acetyl-D-p-Cl-Phe1,2,D-Trp3,D-Lys6,D-Ala10]GnRH, prevented the biphasic [Ca2+]i and LH responses when added before GnRH, and rapidly abolished both responses to GnRH when added during the plateau phase. In low Ca2+ medium the LH peak responses to GnRH were reduced and the subsequent sustained responses were almost completely abolished; reduction of extracellular Ca2+ during exposure to GnRH caused a prompt decline of LH release. The initial [Ca2+]i peak is derived largely from intracellular calcium mobilization with a partial contribution from calcium influx, while the sustained phase is dependent on the entry of extracellular Ca2+ through both L-type and dihydropyridine-insensitive channels. The presence of L-type voltage-sensitive calcium channels (VSCC) in pituitary gonadotrophs was indicated by the ability of elevated extracellular [K+] to stimulate calcium influx and LH release, and the sensitivity of these responses to dihydropyridine agonist and antagonist analogs. In cells pretreated with high [K+], the peak [Ca2+]i response to GnRH was enhanced but the subsequent plateau phase was markedly attenuated. This divergent effect of sustained membrane depolarization on the biphasic [Ca2+]i response suggests that calcium entry through VSCC initially potentiates agonist-induced mobilization of Ca2+ from intracellular storage sites. However, established Ca2+ entry through depolarization-activated VSCC cannot be further increased by agonist stimulation because both processes operate through the same channels, probably by changes in their activation-inactivation kinetics. Finally, the reciprocal potentiation by the dihydropyridine agonist, BK 8644, and GnRH of [Ca2+]i and LH responses confirms that both compounds act on the same type of channels, i.e., L-type VSCC, that participate in agonist-mediated calcium influx and gonadotropin secretion.
Izumi
S
S
Endocrinology and Reproduction Research Branch, National Institute of Child Health and Human Development, Bethesda, Maryland 20892.
Stojilković
S S
SS
Catt
K J
KJ
D001498:3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-(trifluoromethyl)phenyl)-, Methyl ester / Q000494:pharmacology; D000818:Animals; D001693:Biological Transport, Active / Q000187:drug effects; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D002478:Cells, Cultured; D003600:Cytosol / Q000187:drug effects / Q000378:metabolism; D004533:Egtazic Acid / Q000494:pharmacology; D005260:Female; D007987:Gonadotropin-Releasing Hormone / Q000031:analogs & derivatives* / Q000494:pharmacology*; D007700:Kinetics; D007986:Luteinizing Hormone / Q000378:metabolism; D009543:Nifedipine / Q000494:pharmacology; D010052:Ovariectomy; D010903:Pituitary Gland, Anterior / Q000187:drug effects / Q000378:metabolism*; D011189:Potassium Chloride / Q000494:pharmacology; D051381:Rats
D007987:Gonadotropin-Releasing Hormone; D004533:Egtazic Acid; D011189:Potassium Chloride; D001498:3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-(trifluoromethyl)phenyl)-, Methyl ester; C043124:N-Ac-(4-Cl-Phe)(1)-(4-Cl-Phe)(2)-Trp(3)-Lys(6)-AlaNH2(10)-LHRH; D007986:Luteinizing Hormone; D009543:Nifedipine; D002118:Calcium