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,swell), but it is not known how the physical stimulus of cell swelling is transferred to the channels underlying I(Cl,swell). We have investigated the precise relation between cell volume and I(Cl,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 (OSM(o)). This model also predicts significant changes in intracellular ionic strength (Γ(i)) when OSM(o) is altered. Under all experimental conditions I(Cl,swell) closely followed the changes in Γ(i), whereas I(Cl,swell) and cell volume were often found to change independently. These results do not support the hypothesis that Γ(i) regulates the volume set point for activation of I(Cl,swell). Instead, they are in complete agreement with a model in which a decrease of Γ(i) rather than an increase in cell volume is the initial trigger for activation of I(Cl,swell).