Volume-activated chloride currents in HeLa cells are blocked by tamoxifen but not by a membrane impermeant quaternary analogue

Background/Aims: Tamoxifen has been shown to inhibit volume activated chlor ide currents in many cell types, Tamoxifen has also been reported to inhibi t a number of cation channels as well as cytosolic proteins such as calmodu lin, The mechanism of channel block by tamoxifen is not known but three hyp otheses can be proposed: i) a direct effect following binding to the channe l protein from the aqueous environment or ii) a direct effect on the channe l protein after partitioning into the lipid membrane or iii) an indirect me chanism via binding to intracellular regulatory proteins after diffusion ac ross the lipid membrane, The aim of these experiments was to distinguish be tween these hypotheses using membrane permeant and impermeant antioestrogen s, Methods: Volume activated chloride currents were recorded from single HeLa cells using whole cell patch clamp technique, The ability of tamoxifen and its membrane impermeant quaternary derivative ethyl-bromide tamoxifen (EBT) to inhibit these currents was examined, Results: Extracellular tamoxifen at 3 muM inhibited volume activated chlori de currents in HeLa cells whereas EBT had no effect up to 10 muM when appli ed either to the extracellular bathing solution or the intracellular soluti on via the patch pipette. Conclusion: Eliminating the ability of tamoxifen to cross the plasma membra ne abolishes its channel blocking activity against volume activated chlorid e channels in HeLa cells, Copyright (C) 2001 S. Karger AG, Basel.