Abstract
The relative contribution of intracellular Ca 2+ stores to basal and agonist-stimulated hormone release in pituitary cells is still not well understood, especially in non-mammalian vertebrates. Using ratiometric Ca 2+ imaging of single identified goldfish somatotropes, along with time-resolved measurements of growth hormone (GH) secretion, we investigated the Ca 2+-dependent signal transduction of two endogenous regulators of GH release from the goldfish pituitary. Two gonadotropin-releasing hormones (sGnRH and cGnRH-II) initiated GH release in nominally Ca 2+ free conditions. GnRH-evoked GH release was additive to KCl-stimulated GH responses. Ca 2+ signals and GH release elicited by both GnRHs were abolished by pretreatment with TMB-8, which blocks the release of Ca 2+ from intracellular stores. GnRH-stimulated GH secretion is mediated by caffeine-sensitive intracellular Ca 2+ stores that are functionally independent from those sensitive to thapsigargin and other inhibitors of SERCA-type Ca 2+/ATPases. The caffeine/TMB-8-sensitive Ca 2+ stores are also involved in spontaneous Ca 2+ signalling and the maintenance of prolonged GH release.
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