Reduced intracellular ionic strength as the initial trigger for activation of endothelial volume-regulated anion channels

Most mammalian cell types, including endothelial cells, respond to cell swelling by activating a Cl- current termed I-Cl,I-swell, but it is not known how the physical stimulus of cell swelling is transferred to the channels under lying I-Cl,I-swell. We have investigated the precise relation between cell volume and I-Cl,I-swell in endothelial cells by performing whole-cell current recordings while continuously monitoring cell thickness (T-c) as a measure for cell volume. The time course of T-c was accurately predicted by a theoretical model that describes volume changes of patch-clamped cells in response to changes in the extracellular osmolality (OSMo). This model also predicts significant changes in intracellular ionic strength (Gamma(i)) when OSMo is altered. Under all experimental conditions I-Cl,I-swell closely followed the changes in Gamma(i), whereas I-Cl,I-swell and cell volume were often found to change independently, These results do not support the hypothesis that Gamma(i) regulates the volume set point for activation Of I-Cl,I-swell Instead, they are in complete agreement with a model in which a decrease of Gamma(i) rather than an increase in cell volume is the initial trigger for activation of I-Cl,I-swell.