Receptor regulation of the volume-sensitive efflux of taurine and iodide from human SH-SY5Y neuroblastoma cells:: Differential requirements for Ca2+ and protein kinase C

The basal (swelling-induced) and receptor-stimulated effluxes of I-125(-) and taurine have been monitored to determine whether these two osmolytes are released from human SH-SY5Y cells under hypotonic conditions via common or distinct mechanisms. Under basal conditions, both I-125(-) (used as a tracer for Cl-) and taurine were released from the cells in a volume-dependent manner. The addition of thrombin, mediated via the proteinase-activated receptor-1 (PAR-1) subtype, significantly enhanced the release of both I-125(-) and taurine (3-6-fold) and also increased the threshold osmolarity for efflux of these osmolytes ("set-point") from 200 to 290 mOsM. Inclusion of a variety of broad-spectrum anion channel blockers and of 4-[(2-butyl- 6,7-dichloro-2-cyclopentyl-2,3-dihydro-1-oxo-1H-inden-5- yl)oxy] butanoic acid attenuated the release of both I-125(-) and taurine under basal and receptor-stimulated conditions. Basal release of I-125(-) and taurine was independent of Ca2+ or the activity of protein kinase C (PKC). However, although PAR-1-stimulated taurine efflux was attenuated by either a depletion of intracellular Ca2+ or inhibition of PKC by chelerythrine, the enhanced release of I-125(-) was independent of both parameters. Stimulated efflux of I-125(-) after activation of muscarinic cholinergic receptors was also markedly less dependent on Ca2+ availability and PKC activity than that observed for taurine release. These results indicate that, although the osmosensitive release of these two osmolytes from SH-SY5Y cells may occur via pharmacologically similar membrane channels, the receptor-mediated release of I-125(-) and taurine is differentially regulated by PKC activity and Ca2+ availability.