Generation and amplification of the cytosolic calcium signal during secretory responses to gonadotropin-releasing hormone.
Gonadotropin-releasing hormone (GnRH) stimulates characteristic biphasic increases in cytosolic calcium concentration ([Ca2+]i) and in luteinizing hormone (LH) release in cultured gonadotrophs, with an early peak followed by a prolonged plateau in both responses. Analysis of [Ca2+]i by dual-wavelength fluorimetric assay and of LH release at 5-sec intervals in perifused pituitary cells revealed increases in both responses within a few seconds of exposure to GnRH. The maximum elevation of [Ca2+]i occurred within 20 sec, and the peak gonadotropin release in 35 sec; the total duration of the spike phase for both [Ca2+]i and LH release was 2.5 min. Under extracellular Ca2(+)-deficient conditions, the GnRH-induced peak in [Ca2+]i was reduced by about 20% and the plateau phase was abolished. Concomitantly, the magnitude of the acute phase of LH release was reduced by 40% and that of the second phase by about 90%. Recovery of the plateau phase of LH release occurred within 25 sec after addition of 1.25 mM Ca2+ to Ca2(+)-deficient medium. In a dose-dependent manner, the non-selective Ca2+ channel blockers Co2+ and Cd2+ reduced the Ca2+ current measured by whole-cell recording in pituitary gonadotrophs and abolished the extracellular Ca2(+)-dependent component of LH release. The selective calcium channel blocker, nifedipine, decreased the magnitude of the Ca2+ current and reduced the plateau phase of LH release by 50%; conversely, the dihydropyridine agonist methyl, 1,4,dihydro-2,6-dimethyl 3-nitro-4-(2-trifluorome) (Bay K 8644) consistently enhanced the amplitudes of both Ca2+ current and GnRH-induced LH release. These data reveal a close temporal correlation between changes in [Ca2+]i and LH release during GnRH action, with Ca2+ mobilization during the spike phase and Ca2+ influx through dihydropyridine-sensitive and insensitive sets of receptor-operated calcium channels during the spike and plateau phases. In addition, analysis of the magnitudes of the [Ca2+]i and LH responses to a wide range of GnRH concentrations in the presence and absence of extracellular Ca2+ is consistent with amplification of the [Ca2+]i signal in agonist-stimulated gonadotrops.
Stojilković
S S
SS
Endocrinology and Reproduction Research Branch, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland 20892.
Stutzin
A
A
Izumi
S
S
Dufour
S
S
Torsello
A
A
Virmani
M A
MA
Rojas
E
E
Catt
K J
KJ
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000187:drug effects / Q000378:metabolism; D002478:Cells, Cultured; D003600:Cytosol / Q000378:metabolism; D007987:Gonadotropin-Releasing Hormone / Q000494:pharmacology*; D007700:Kinetics; D007986:Luteinizing Hormone / Q000378:metabolism*; D010903:Pituitary Gland, Anterior / Q000187:drug effects / Q000378:metabolism; D015398:Signal Transduction / Q000502:physiology
D015220:Calcium Channels; D007987:Gonadotropin-Releasing Hormone; D007986:Luteinizing Hormone; D002118:Calcium