Gonadotropin-releasing hormone-induced rise in cytosolic free Ca2+ levels: mobilization of cellular and extracellular Ca2+ pools and relationship to gonadotropin secretion.
Addition of GnRH to pituitary gonadotrophs preloaded with Quin 2 resulted in a rapid (approximately 8 s) mobilization of an ionomycin-sensitive intracellular Ca2+ pool. A second component of Ca2+ entry via voltage dependent channels contributed about 45% of the peak cytosolic free Ca2+ concentration ([Ca2+]i). Thereafter, influx of Ca2+ via voltage-sensitive and -insensitive channels is responsible for maintenance of elevated [Ca2+]i during the second phase of GnRH action. Addition of inositol 1,4,5-trisphosphate (IP3) to permeabilized pituitary cells resulted in a Ca2+ transient, released from a nonmitochondrial pool, which maintained ambient free Ca2+ concentration around 170 nM in an ATP-dependent mechanism. Successive stimulations of the cells with IP3 produced an attenuated response. Elevation of the gonadotroph [Ca2+]i by ionomycin, to levels equivalent to that induced by GnRH, resulted in LH release amounting to only 45% of the response to the neurohormone. Activation of the voltage-dependent Ca2+ channels by the dihydropyridine Ca2+-agonist [methyl 1,4-dihydro-2,6-dimethyl-3-nitro-4-(2-trifluoromethylphenyl)-pyridine- 5-carboxylate (BAYK8644)] stimulated LH release, 36% of the GnRH (100 nM) response being reached by 10(-8) M of the drug, both [Ca2+]i elevation and GnRH-induced LH release were inhibited similarly (40-50%) by the dihydropyridine Ca2+-antagonist nifedipine. The results indicate that peak [Ca2+]i induced by GnRH in pituitary gonadotrophs is derived mainly from ionomycin-sensitive cellular stores most likely via IP3 formation.(ABSTRACT TRUNCATED AT 250 WORDS)
Naor
Z
Z
Department of Biochemistry, George S. Wise Faculty of Life Sciences, Tel Aviv University, Israel.
Capponi
A M
AM
Rossier
M F
MF
Ayalon
D
D
Limor
R
R
D001498:3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-(trifluoromethyl)phenyl)-, Methyl ester / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D003600:Cytosol / Q000378:metabolism; D004987:Ethers / Q000494:pharmacology; D005089:Exocytosis; D005260:Female; D007987:Gonadotropin-Releasing Hormone / Q000494:pharmacology*; D015544:Inositol 1,4,5-Trisphosphate; D007295:Inositol Phosphates / Q000494:pharmacology; D015759:Ionomycin; D007700:Kinetics; D007986:Luteinizing Hormone / Q000378:metabolism*; D009543:Nifedipine / Q000494:pharmacology; D010903:Pituitary Gland, Anterior / Q000187:drug effects / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains
D004987:Ethers; D007295:Inositol Phosphates; D007987:Gonadotropin-Releasing Hormone; D015759:Ionomycin; D001498:3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-(trifluoromethyl)phenyl)-, Methyl ester; D015544:Inositol 1,4,5-Trisphosphate; D007986:Luteinizing Hormone; D009543:Nifedipine; D002118:Calcium