Transient but not oscillating component of the calcium mobilizing response to gonadotropin-releasing hormone depends on calcium influx in pituitary gonadotrophs.
Gonadotropin-releasing hormone (GnRH)-stimulated changes in the cytosolic free Ca2+ concentration ([Ca2+]i) were studied in gonadotrophs cultured from 3-week ovariectomized rat pituitaries. One animal was used per cell preparation. [Ca2+]i was monitored in individual gonadotrophs by dual emission microspectrofluorimetry, using Indo-1 as the intracellular fluorescent Ca2+ probe. A short stimulation with GnRH evoked a complex concentration-dependent Ca2+ response in individual gonadotrophs. 0.1-1 nM GnRH triggered a series of sinusoidal-like [Ca2+]i oscillations superimposed upon a modest slow [Ca2+]i rise--the oscillating response mode--while 10-100 nM GnRH caused a biphasic increase in [Ca2+]i consisting of a monophasic transient and oscillations--the transient/oscillating response mode. Despite the consistency of Ca2+ responses, an inter-preparation heterogeneity of [Ca2+]i oscillations frequency was noticed. Moreover, we observed that, within a given cell preparation, the frequency of [Ca2+]i oscillations was independent of GnRH concentration whereas both peak [Ca2+]i and area under the [Ca2+]i versus time curve were concentration-dependent. Thus, in gonadotrophs, the presence of the GnRH signal would lead to [Ca2+]i oscillations, while the amplitude of the [Ca2+]i responses would code for the concentration of agonist. Both transient and oscillating components of GnRH responses depended on releasing activity of Ca(2+)-sequestering pools in as much as GnRH responses were unaffected by brief removal of external Ca2+, but suppressed by chelating intracellular free Ca2+ with BAPTA. However, prolonged exposure to a Ca(2+)-free medium suppressed the transient component while leaving the oscillating component unaffected. We therefore propose that gonadotrophs employ Ca(2+)-sequestering pools, whose maintenance depends on a slow Ca(2+)-entry, to give an amplitude-coded Ca2+ rise in response to a short GnRH stimulation.
Guérineau
N C
NC
Laboratory of Neurophysiology, CNRS URA 1200, University of Bordeaux, France.
Bouali-Benazzouz
R
R
Corcuff
J B
JB
Audy
M C
MC
Bonnin
M
M
Mollard
P
P
D000818:Animals; D001692:Biological Transport; D002118:Calcium / Q000378:metabolism*; D002478:Cells, Cultured; D004305:Dose-Response Relationship, Drug; D004533:Egtazic Acid / Q000031:analogs & derivatives / Q000494:pharmacology; D005260:Female; D005456:Fluorescent Dyes / Q000378:metabolism; D007987:Gonadotropin-Releasing Hormone / Q000494:pharmacology*; D007211:Indoles / Q000378:metabolism; D009991:Oscillometry; D010052:Ovariectomy; D010903:Pituitary Gland, Anterior / Q000166:cytology / Q000187:drug effects*; D051381:Rats; D017208:Rats, Wistar; D013997:Time Factors
D005456:Fluorescent Dyes; D007211:Indoles; D007987:Gonadotropin-Releasing Hormone; D004533:Egtazic Acid; C025603:1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid; C048960:indo-1; D002118:Calcium