Evidence that IP₃ and Ryanodine-sensitive Intra-cellular Ca²⁺ Stores are not Involved in Acute Hyposmotically-Induced Prolactin Release in Tilapia

Prolactin (PRL) cells from the euryhaline tilapia, Oreochromis mossambicus, behave like osmoreceptors by responding directly to reductions in medium osmolatiy with increased secretion of the osmoregulatory hormone PRL. Extracellular Ca2+ is essential for the transduction of a hyposmotic stimulus into PRL release. In the current study, the presence and possible role of intracellular Ca2+ stores during hyposmotic stimulation was investigated using pharmacological approaches. Changes in intracellular Ca2+ concentration were measured with fura-2 in isolated PRL cells. Intracellular Ca2+ stores were depleted in dispersed PRL cells with thapsigargin (1 muM) or cyclopiazonic acid (CPA, 10 muM). Preincubation with thapsigargin prevented the rise in [Ca2+](i) induced lysophosphatidic acid (LPA, 1 muM), an activator of the IP3 signalling cascade, but did not prevent the hyposmotically-induced rise in [Ca2+](i) in medium with normal [Ca2+] (2 mM). Pre-treatment with CPA produced similar results. Prolactin release from dispersed cells followed a pattern that paralleled observed changes in [Ca2+](i). CPA inhibited LPA-induced prolactin release but not hyposmotically-induced release. Xestospongin C (1 muM), an inhibitor of IP3 receptors. had no effect on hyposmotically-induced PRL release. Pre-exposure to caffeine (10 mM) or ryanodine (1 muM) did not prevent a hyposmotically-induced rise in [Ca2+](i). Taken together these results indicate the presence of IP3 and ryanodine-sensitive Ca2+ stores in tilapia PRL cells. However, the rapid rise in intracellular [Ca2+] needed for acute PRL release in response to hyposmotic medium can occur independently of these intracellular Ca2+ stores. Copyright (C) 2004 S. Karger Ag, Basel .