Permeation properties and modulation of volume‐activated Cl−‐currents in human endothelial cells

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Abstract
We have studied the permeation and pharmacological properties of a recently described volume‐activated, calcium‐insensitive, small‐conductance Cl−‐channel in endothelial cells from human umbilical vein. The relative permeability for various anions was I− > Cl− ∼ Br− > F− > gluconate− (1.63 ± 0.36: 1:0.95 ± 0.16:0.46 ± 0.04:0.19 ± 0.07, n = 10). 5‐Nitro‐2‐(3‐phenylpropylamino)‐benzoic acid (NPPB) induced a fast and reversible block of the current (K1 = 29 μmol l−1). Extracellular ATP induced a low‐affinity block of the current, that showed a small voltage‐dependence (K1 = 4.9 mmol l−1 at + 80 mV and K1 = 8.2 mmol l−1 at − 80 mV). Extracellularly applied arachidonic acid (10 μmol l−1) irreversibly blocked the current in 5 out of 9 cells. This block seems to be non‐specific, because other ionic currents, e.g. inwardly rectifying K+ currents, were blocked as well. Tamoxifen induced a high affinity block of the current (K1 = 2.9 μmol l−1). Block and reversal of block were however much slower than with NPPB. Cytotoxic compounds, which are substrates of the P‐glycoprotein multidrug transporter, loaded into endothelial cells via the patch pipette, exerted only minor effects on the volume‐activated current. Vinblastine and colcemid did not affect the volume‐activated current, whereas daunomycin and vincristine induced a slow ‘run‐down’ of the current. The similarity between permeation and pharmacological properties of volume‐activated Cl−‐currents in endothelial cells and those in many other cell types may suggest that they all belong to the same family of volume‐activated small‐conductance Cl−‐channels. Evidence that they belong to the class of P‐glycoprotein associated Cl−‐channels is however only marginal, whereas their biophysical characteristics differ significantly from those of the CIC‐2 volume‐activated Cl−‐channels.