Gonadotropin-releasing hormone-induced sensitization of calcium-dependent exocytosis in pituitary gonadotrophs.
Agonist-induced increases in cytoplasmic calcium concentration ([Ca2+]i) play a pivotal role in regulated exocytosis by promoting the fusion of secretory vesicles with the plasma membrane. In permeabilized and ATP-primed pituitary cells, increases in ambient [Ca2+]i stimulated the release of LH from gonadotrophs with an EC50 of 2-3 microM. In contrast, the responses of intact gonadotrophs to agonist stimulation by GnRH were characterized by transient [Ca2+]i elevations of up to 1.5 microM, followed by a plateau of 300-400 nM. The sensitivity of the exocytotic response of permeabilized cells to [Ca2+]i was significantly increased by GnRH, which reduced the EC50 for [Ca2+]i to the submicromolar concentration range. The stimulatory action of GnRH on LH release in permeabilized cells was not a consequence of intracellular Ca2+ release, but was associated with increased cytidine diphosphate diacylglycerol production. Activation of protein kinase C by phorbol esters caused a similar increase in the Ca2+ sensitivity of LH release from permeabilized cells, and this effect was not additive to that of GnRH. Furthermore, the GnRH-induced increase in the sensitivity of the exocytotic response to Ca2+ was attenuated by inhibitors of protein kinase C. These findings indicate that although elevated [Ca2+]i per se can promote LH release from permeabilized gonadotrophs, concomitant activation of protein kinase C is necessary to support exocytosis at the physiological [Ca2+]i levels that prevail in GnRH-stimulated intact cells. Such sensitization of the Ca(2+)-dependent secretory mechanism by protein kinase C may be an important step in the agonist-induced release of LH from pituitary gonadotrophs.
Jobin
R M
RM
Endocrinology and Reproduction Research Branch, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland 20892, USA.
Tomić
M
M
Zheng
L
L
Stojilkovic
S S
SS
Catt
K J
KJ
D000255:Adenosine Triphosphate / Q000502:physiology; D000818:Animals; D002118:Calcium / Q000502:physiology*; D002463:Cell Membrane Permeability; D002478:Cells, Cultured; D005089:Exocytosis / Q000187:drug effects*; D005260:Female; D007987:Gonadotropin-Releasing Hormone / Q000494:pharmacology*; D007986:Luteinizing Hormone / Q000378:metabolism*; D010703:Phorbol Esters / Q000494:pharmacology; D010903:Pituitary Gland, Anterior / Q000166:cytology / Q000502:physiology*; D011493:Protein Kinase C / Q000037:antagonists & inhibitors / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D015398:Signal Transduction / Q000187:drug effects
D010703:Phorbol Esters; D007987:Gonadotropin-Releasing Hormone; D000255:Adenosine Triphosphate; D007986:Luteinizing Hormone; D011493:Protein Kinase C; D002118:Calcium