Interactions between calcium and protein kinase C in the control of signaling and secretion in pituitary gonadotrophs.
Single pituitary gonadotrophs exhibit episodes of spontaneous fluctuations in cytoplasmic calcium concentration [( Ca2+]i) due to entry through voltage-sensitive calcium channels (VSCC) and show prominent agonist-induced oscillations in [Ca2+]i that are generated by periodic release of intracellular Ca2+. Gonadotropin releasing hormone (GnRH) elicited three types of Ca2+ responses: at low doses, subthreshold, with an increase in basal [Ca2+]i; at intermediate doses, oscillatory, with dose-dependent modulation of spiking frequency; and at high doses, biphasic, without oscillations. Elevation of [Ca2+]i or activation of protein kinase C (PKC) did not influence the frequency of agonist-induced [Ca2+]i spikes but caused dose-dependent reductions in amplitude for all types of Ca2+ response. Stimulation of transient Ca2+ spikes by GnRH was followed by inhibition of the spontaneous fluctuations. GnRH also reduced the ability of high extracellular K+ to promote Ca2+ influx through VSCC. Activation of PKC by phorbol esters stimulated Ca2+ influx in quiescent cells but inhibited influx when VSCC were already activated, either spontaneously or by high K+. In contrast to their biphasic actions on [Ca2+]i, phorbol esters exerted only stimulatory actions on gonadotropin release, even when Ca2+ influx was concomitantly reduced. However, pituitary cells had to be primed with an appropriate [Ca2+]i level before exocytosis could be amplified by PKC. In PKC-depleted cells, all actions of phorbol esters on Ca2+ entry and amplitude modulation, and on LH release, were abolished. GnRH-induced LH secretion was also significantly reduced, especially the plateau phase of the response. These data indicate that Ca2+ and PKC serve as interacting signals during the cascade of cellular events triggered by agonist stimulation, in which Ca2+ turns cell responses on or off, and PKC amplifies the positive and negative effects of Ca2+.
Stojilković
S S
SS
Endocrinology and Reproduction Research Branch, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland 20892.
Iida
T
T
Merelli
F
F
Torsello
A
A
Krsmanović
L Z
LZ
Catt
K J
KJ
D000470:Alkaloids / Q000494:pharmacology; D000818:Animals; D001692:Biological Transport; D015153:Blotting, Western; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000378:metabolism; D002478:Cells, Cultured; D004789:Enzyme Activation; D005260:Female; D007987:Gonadotropin-Releasing Hormone / Q000378:metabolism; D007986:Luteinizing Hormone / Q000037:antagonists & inhibitors / Q000378:metabolism; D008564:Membrane Potentials; D010903:Pituitary Gland, Anterior / Q000166:cytology / Q000378:metabolism* / Q000502:physiology; D011188:Potassium / Q000494:pharmacology; D011493:Protein Kinase C / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains; D015398:Signal Transduction; D019311:Staurosporine; D013696:Temperature; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology
D000470:Alkaloids; D015220:Calcium Channels; D007987:Gonadotropin-Releasing Hormone; D007986:Luteinizing Hormone; D011493:Protein Kinase C; D019311:Staurosporine; D013755:Tetradecanoylphorbol Acetate; D011188:Potassium; D002118:Calcium