Generation and amplification of the cytosolic calcium signal during secretory responses to gonadotropin-releasing hormone.

SS Stojilković, A Stutzin, S Izumi, S Dufour… - The New …, 1990 - europepmc.org
SS Stojilković, A Stutzin, S Izumi, S Dufour, A Torsello, MA Virmani, E Rojas, KJ Catt
The New Biologist, 1990europepmc.org
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 …
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.
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