Function-specific calcium stores selectively regulate growth hormone secretion, storage, and mRNA level

Ca+ stores may regulate multiple components of the secretory pathway. We examined the roles of biochemically independent intracellular Ca2+ stores on acute and long-term growth hormone (GH) release, storage, and mRNA levels in goldfish somatotropes. Thapsigargin-evoked intracellular Ca2+ concentration ([Ca2+](i)) signal amplitude was similar to the Ca2+-mobilizing agonist gonadotropin-releasing hormone, but thapsigargin (2 muM) did not acutely increase GH release, suggesting uncoupling between [Ca2+](i) and exocytosis. However, 2 muM thapsigargin affected long-term secretory function. Thapsigargin-treated cells displayed a steady secretion of GH (2, 12, and 24 h), which decreased GH content (12 and 24 h), but not GH mRNA/production (24 h). In contrast to the results with thapsigargin, activating the ryanodine (Ry) receptor (RyR) with 1 nM Ry transiently increased GH release (2 h). Prolonged activation of RyR (24 h) reduced GH release, contents and apparent production, without changing GH mRNA levels. Inhibiting RyR with 10 muM Ry increased GH mRNA levels, production, and storage (2 h). Increasing [Ca2+](i) independently of Ca2+ stores with the use of 30 mM KCl decreased GH mRNA. Collectively, these results suggest that parts of the secretory pathway can be controlled independently by function-specific Ca2+ stores.