Separate taurine and chloride efflux pathways activated during regulatory volume decrease

Abstract

Organic osmolyte and halide permeability pathways activated in epithelial HeLa cells by cell swelling were studied by radiotracer efflux techniques and single-cell volume measurements. The replacement of extracellular Cl− by anions that are more permeant through the volume-activated Cl− channel, as indicated by electrophysiological measurements, significantly decreased taurine efflux. In the presence of less-permeant anions, an increase in taurine efflux was observed. Simultaneous measurement of the125I, used as a tracer for Cl−, and [3H]taurine efflux showed that the time courses for the two effluxes differed. In Cl−-rich medium the increase in I− efflux was transient, whereas that for taurine was sustained. Osmosensitive Cl− conductance, assessed by measuring changes in cell volume, increased rapidly after hypotonic shock. The influx of taurine was able to counteract Cl− conductance-dependent cell shrinkage but only ∼4 min after triggering cell swelling. This taurine-induced effect was blocked by DIDS. Differences in anion sensitivity, the time course of activation, and sensitivity to DIDS suggest that the main cell swelling-activated permeability pathways for taurine and Cl− are separate.

Volume changes and whole cell membrane currents activated during gradual osmolarity decrease in C6 glioma cells: contribution of two types of K+ channels

“…This is in marked contrast with the early activation of a Cl Ϫ current. This result strongly supports the notion of different permeability pathways for the two osmolytes (4,20) and provides further evidence against a common pathway for amino acids and Cl Ϫ efflux, which was based mainly on similarities in their pharmacological profile (17,19). The sensitivity to Cl Ϫ channel blockers, however, is suggestive of some interdependence of the two osmolyte pathways.…”

Reactive oxygen species are important mediators of taurine release from skeletal muscle cells

“…ROS stimulate cellular PLA2 (iPLA2) and 5-LO activities and, thereby, comprise an amplification system and a connection between the 2 signaling pathways. donic acid and LPC (40). However, the observation that cholesterol inhibits the LPC-induced taurine efflux ( Fig.…”

“…The cPLA 2 , which requires Ca 2ϩ for binding to the membrane substrate and phosphorylation to gain full activity (6), has been assigned a permissive role in the initiation of the signal cascade that is required for activation of the volume-sensitive osmolyte channels in Ehrlich cells (40). In NIH3T3 mouse fibroblasts, this role is governed by iPLA 2 (16).…”

“…It is noted that taurine efflux is carried out in KCl medium to avoid the regulatory volume reduction after the osmotic cell swelling and the concomitant inactivation of the volume-sensitive taurine efflux pathway (14). However, cells depolarize when exposed to KCl medium and as the volume-sensitive taurine efflux from, e.g., HeLa cells is reduced under depolarized conditions (40), it seems reasonable to as- 14 C activity remaining in the cells was estimated at 2-min time intervals under isotonic conditions (KCl medium, 300 mosM) and after a shift to hypotonic conditions (KCl medium, 210 mosM), as indicated by an arrow. B: the rate constant for the taurine efflux, estimated as the negative slope between one time point and its proceeding time point in A (see MATERIALS AND METHODS), is plotted vs. time.…”

“…The swelling-induced activation of taurine release in various cells involves PLA 2 -mediated release of arachidonic acid from phospholipids and sequential formation of a lipoxygenase product by the 5-lipoxygenase (5-LO) (17,21,40). On the other hand, the LPC-induced taurine release seems to involve a calmodulin-/ calmodulin-dependent kinase II (CaMKII)-modulated production of reactive oxygen species (ROS), (18).…”