Caffeine-stimulated GTH-II release involves Ca stores with novel properties

Johnson, James D., Calvin J. H. Wong, Warren K. Yunker, and John P. Chang. Caffeine-stimulated GTH-II release involves Ca2 stores with novel properties. Am J Physiol Cell Physiol 282: C635–C645, 2002. First published November 7, 2001; 10.1152/ajpcell.00044.2001.—Modulation of Ca2 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 Ca2 store, sGnRH, but not cGnRH-II, uses an additional caffeine-sensitive mechanism. We examined caffeine signaling by using Ca2 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 Ca2 signal dependent because they were abolished by 1,2-bis(2-aminophenoxy)ethane-N,N,N ,N -tetraacetic acid loading. Caffeine generated localized Ca2 signals that began near secretory granules. Surprisingly, caffeine-stimulated GTH-II release was insensitive to 100 M ryanodine and, unlike GnRH action, was unaffected by inhibitors of voltage-gated Ca2 channels or sarco(endo)plasmic reticulum Ca2 -ATPases. Collectively, these data indicate that caffeine-stimulated GTH-II release is not mediated by typical agonist-sensitive Ca2 stores found in endoplasmic reticulum.