Wittels, Kathleen A., Elise M. Hubert, Mark W. Musch, and Leon Goldstein. Osmolyte channel regulation by ionic strength in skate RBC. Am J Physiol Regulatory Integrative Comp Physiol 279: R69-R76, 2000. -The aim of this study was to determine whether the opening of the osmolyte channel in skate red blood cells (RBC) is regulated by intracellular electrolyte concentration and conductivity. Consistent with previous studies, experiments with hyperosmotic preincubation before cell swelling or

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... ng with an isosmotic electrolyte (e.g., ammonium chloride) showed that an increase in ionic strength inhibits the opening of the taurine channel. However, a decrease in intracellular ionic strength did not always stimulate taurine efflux to the same degree. Whereas hyposmotic swelling caused a large increase in taurine efflux, swelling induced by treatment with isosmotic nonelectrolytes produced much smaller stimulation. Results with assays for band 3 phosphorylating enzymes were consistent with those from the taurine efflux studies; stimulation of enzyme activity was lower in cells that were swollen with isosmotic nonelectrolyte media than in cells swollen in hyposmotic media. These results indicate that a decrease in ionic strength is not the only signal for the opening of the taurine channel in skate RBC. Ionic strength does affect channel activity, but there must also be some other regulator. cell volume regulation; taurine; protein kinases REGULATION OF CELLULAR VOLUME is important for cells to function properly (12). Cells that undergo regulatory volume decrease (RVD) after exposure to hyposmotic medium adjust their volume through the release of intracellular electrolytes and osmolytes (organic solutes such as amino acids, polyols, and trimethylamines) accompanied by a net efflux of water (3, 14) . Osmolyte release is thought to occur via osmolyte channels located in the membranes of the cells. In many cells, the osmolyte channels show the properties of a swelling-activated anion channel (10, 18, 20) . In fish erythrocytes, however, the osmolyte channel is proposed to be the anion exchanger band 3 or an associated channel (4, 6, 8, 9, 11, 15) . Previous studies have examined the role that ionic strength plays in cell volume regulation (2, 5, 11, 13, 15, 17) . With the use of trout red blood cells (RBC), Motais and collaborators (11, 15) tested the idea that ionic strength regulates the activation of osmolyte channels by measuring changes in electrolyte levels in the erythrocytes and correlating these changes with those of osmolyte channel activity following cell swelling in different media. Changes in levels of electrolytes were taken to reflect changes in ionic strength. The authors concluded that the change in cellular ionic strength induced by swelling is the stimulus for hyposmotic volume regulation; explaining that when ionic strength drops, the cells adopt hyposmotic swelling patterns (allowing release of taurine and electrolytes), but when swelling is accomplished by an increase in ionic strength, the cells follow isotonic swelling patterns resulting in less activation of the taurine channel and activation of ion channels (15). The results obtained by Cannon et al. (2) in studies with mammalian C6 glioma and Chinese hamster ovary (CHO) cells were consistent with Motais' findings regarding the role of ionic strength in hyposmotic cell volume regulation. The former showed that hyposmotic cell swelling activates an anion channel, volumesensitive organic osmolyte/anion channel (VSOAC), and that this channel's activity is inhibited by elevated intracellular electrolytes in the C6 glioma cells. In addition, an increase of intracellular electrolyte (CsCl) concentration in CHO cells resulted in a concentrationdependent decrease in channel activation. They concluded that both the volume set point and rate of swelling-induced activation of VSOAC are specifically modulated by cytoplasmic ionic strength (2). Nilius et al. (17) have studied the effect of intracellular ionic strength on a volume-activated Cl Ϫ current in cultured calf pulmonary endothelial cells. They compared the anion currents measured in whole cell patch clamp of cells dialysed with a pipette solution of normal ionic strength with anion currents measured in cells dialysed with a pipette solution of decreased ionic