Caffeine-stimulated GTH-II release involves Ca(2+) stores with novel properties.
Modulation of Ca(2+) stores with 10 mM caffeine stimulates robust secretion of gonadotropin (GTH-II) from goldfish gonadotropes. Although both endogenous forms of gonadotropin-releasing hormone (GnRH) utilize a common intracellular Ca(2+) store, sGnRH, but not cGnRH-II, uses an additional caffeine-sensitive mechanism. We examined caffeine signaling by using Ca(2+) imaging, electrophysiology, and cell-column perifusion. Although caffeine inhibited K+ channels, this action appeared to be unrelated to caffeine-induced GTH-II release, because the latter was insensitive to tetraethylammonium. The effects of caffeine also were not mediated by the cAMP/protein kinase A pathway. Instead, caffeine-evoked GTH-II responses were Ca(2+) signal dependent because they were abolished by 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid loading. Caffeine generated localized Ca(2+) signals that began near secretory granules. Surprisingly, caffeine-stimulated GTH-II release was insensitive to 100 microM ryanodine and, unlike GnRH action, was unaffected by inhibitors of voltage-gated Ca(2+) channels or sarco(endo)plasmic reticulum Ca(2+)-ATPases. Collectively, these data indicate that caffeine-stimulated GTH-II release is not mediated by typical agonist-sensitive Ca(2+) stores found in endoplasmic reticulum.
Johnson
James D
JD
Department of Biological Sciences, Biological Sciences Building, University of Alberta, Edmonton, Alberta, Canada T6G 2E9.
Wong
Calvin J H
CJ
Yunker
Warren K
WK
Chang
John P
JP
D000818:Animals; D002110:Caffeine / Q000494:pharmacology*; D002118:Calcium / Q000378:metabolism*; D020013:Calcium Signaling / Q000502:physiology*; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors; D002478:Cells, Cultured; D000697:Central Nervous System Stimulants / Q000494:pharmacology; D017868:Cyclic AMP-Dependent Protein Kinases / Q000037:antagonists & inhibitors; D003620:Dantrolene / Q000494:pharmacology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006054:Goldfish / Q000502:physiology*; D006065:Gonadotropins, Pituitary / Q000378:metabolism*; D006873:Hydroquinones / Q000494:pharmacology; D007546:Isoquinolines / Q000494:pharmacology; D009125:Muscle Relaxants, Central / Q000494:pharmacology; D010902:Pituitary Gland / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism; D026902:Potassium Channel Blockers / Q000494:pharmacology; D015221:Potassium Channels / Q000378:metabolism; D012433:Ryanodine / Q000494:pharmacology; D013449:Sulfonamides; D019789:Tetraethylammonium / Q000494:pharmacology
D000697:Central Nervous System Stimulants; D004791:Enzyme Inhibitors; D006065:Gonadotropins, Pituitary; D006873:Hydroquinones; D007546:Isoquinolines; D009125:Muscle Relaxants, Central; D026902:Potassium Channel Blockers; D015221:Potassium Channels; D013449:Sulfonamides; D012433:Ryanodine; C019359:2,5-di-tert-butylhydroquinone; D002110:Caffeine; D019789:Tetraethylammonium; D017868:Cyclic AMP-Dependent Protein Kinases; D000252:Calcium-Transporting ATPases; D003620:Dantrolene; C063509:N-(2-(4-bromocinnamylamino)ethyl)-5-isoquinolinesulfonamide; D002118:Calcium