Calcium signaling and secretory responses in agonist-stimulated pituitary gonadotrophs.
In cultured pituitary gonadotrophs, gonadotropin-releasing hormone (GnRH) caused dose-dependent and biphasic increases in cytoplasmic calcium concentration ([Ca2+]i) and LH release. Both extra- and intracellular calcium pools participate in GnRH-induced elevation of [Ca2+]i and LH secretion. The spike phase of the [Ca2+]i response represents the primary signal derived predominantly from the rapid mobilization of intracellular Ca2+. In contrast, the prolonged phase of the Ca2+ signal depends exclusively on Ca2+ entry from the extracellular pool. The influx of Ca2+ occurs partially through dihydropyridine-sensitive calcium channels. Both [Ca2+]i and LH responses to increasing concentrations of GnRH occur over very similar time scales, suggesting that increasing degrees of receptor occupancy are transduced into amplitude-modulated Ca2+ responses, which in turn activate exocytosis in a linear manner. However, several lines of evidence indicated the complexity over the relationship between Ca2+ signaling and LH exocytosis. In contrast to [Ca2+]i measurements in cell suspension, single cell Ca2+ measurements revealed the existence of a more complicated pattern of Ca2+ response to GnRH, with a biphasic response to high agonist doses and prominent oscillatory responses to lower GnRH concentrations, with a log-linear correlation between GnRH dose and the frequency of Ca2+ spiking. In addition, analysis of the magnitudes of the [Ca2+]i and LH responses of gonadotrophs to a wide range of GnRH concentrations in the presence and absence of extracellular Ca2+, and to K+ and phorbol ester stimulation, showed non-linearity between these parameters with amplification of [Ca2+]i-mediated exocytosis. Studies on cell depleted of protein kinase C under conditions that did not change the LH pool suggested the participation of protein kinase C in this amplification, especially during the plateau phase of the secretory response to GnRH.
Endocrinology and Reproduction Research Branch, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892.
S S
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Stojilković
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A
Torsello
T
T
Iida
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E
Rojas
K J
KJ
Catt
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* / Q000494:pharmacology; D002121:Calcium Channel Blockers / Q000494:pharmacology; D015220:Calcium Channels / Q000378:metabolism; D002478:Cells, Cultured; D003035:Cobalt / Q000494:pharmacology; D005260:Female; D007987:Gonadotropin-Releasing Hormone / Q000494:pharmacology*; D007700:Kinetics; D007986:Luteinizing Hormone / Q000378:metabolism*; D008564:Membrane Potentials; D010052:Ovariectomy; D010477:Perfusion; D010903:Pituitary Gland, Anterior / Q000187:drug effects / Q000378:metabolism / Q000502:physiology*; D011188:Potassium / Q000494:pharmacology; D011493:Protein Kinase C / Q000378:metabolism; D051381:Rats; D011919:Rats, Inbred Strains; D012016:Reference Values; D015398:Signal Transduction / Q000187:drug effects; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology
D002121:Calcium Channel Blockers; D015220:Calcium Channels; D007987:Gonadotropin-Releasing Hormone; D003035:Cobalt; D001498:3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-(trifluoromethyl)phenyl)-, Methyl ester; D007986:Luteinizing Hormone; D011493:Protein Kinase C; D013755:Tetradecanoylphorbol Acetate; D011188:Potassium; D002118:Calcium