title: Evidence that IP3 and ryanodine-sensitive intra-cellular Ca2+ stores are not involved in acute hyposmotically-induced prolactin release in Tilapia.
abstract: Prolactin (PRL) cells from the euryhaline tilapia, Oreochromis mossambicus, behave like osmoreceptors by responding directly to reductions in medium osmolality with increased secretion of the osmoregulatory hormone PRL. Extracellular Ca(2+) is essential for the transduction of a hyposmotic stimulus into PRL release. In the current study, the presence and possible role of intracellular Ca(2+) stores during hyposmotic stimulation was investigated using pharmacological approaches. Changes in intracellular Ca(2+) concentration were measured with fura-2 in isolated PRL cells. Intracellular Ca(2+) stores were depleted in dispersed PRL cells with thapsigargin (1 microM) or cyclopiazonic acid (CPA, 10 microM). Pre-incubation with thapsigargin prevented the rise in [Ca(2+)](i) induced by lysophosphatidic acid (LPA, 1 microM), an activator of the IP(3) signalling cascade, but did not prevent the hyposmotically-induced rise in [Ca(2+)](i) in medium with normal [Ca(2+)] (2mM). Pre-treatment with CPA produced similar results. Prolactin release from dispersed cells followed a pattern that paralleled observed changes in [Ca(2+)](i). CPA inhibited LPA-induced prolactin release but not hyposmotically-induced release. Xestospongin C (1microM), an inhibitor of IP(3) receptors, had no effect on hyposmotically-induced PRL release. Pre-exposure to caffeine (10mM) or ryanodine (1microM) did not prevent a hyposmotically-induced rise in [Ca(2+)](i). Taken together these results indicate the presence of IP(3) and ryanodine-sensitive Ca(2+) stores in tilapia PRL cells. However, the rapid rise in intracellular [Ca(2+)] needed for acute PRL release in response to hyposmotic medium can occur independently of these intracellular Ca(2+) stores.
authors.affiliation: Hawaii Institute of Marine Biology, University of Hawaii, Kaneohe 96744, USA.
authors.forename: André P
authors.initials: AP
authors.lastname: Seale
authors.forename: Ian M
authors.initials: IM
authors.lastname: Cooke
authors.forename: Tetsuya
authors.initials: T
authors.lastname: Hirano
authors.forename: Gordon E
authors.initials: GE
authors.lastname: Grau
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002478:Cells, Cultured; D004791:Enzyme Inhibitors / Q000494:pharmacology; D007295:Inositol Phosphates / Q000378:metabolism*; D008246:Lysophospholipids / Q000378:metabolism; D009995:Osmosis; D009997:Osmotic Pressure; D010902:Pituitary Gland / Q000166:cytology / Q000378:metabolism*; D011388:Prolactin / Q000378:metabolism*; D012433:Ryanodine / Q000494:pharmacology*; D015398:Signal Transduction / Q000187:drug effects; D017210:Tilapia / Q000378:metabolism*
chemical_list: D004791:Enzyme Inhibitors; D007295:Inositol Phosphates; D008246:Lysophospholipids; D012433:Ryanodine; C092449:inositol 1,2,3-trisphosphate; D011388:Prolactin; C032881:lysophosphatidic acid; D002118:Calcium