Volume--activated chloride currents in HeLa cells are blocked by tamoxifen but not by a membrane impermeant quaternary analogue.
OBJECTIVE
Tamoxifen has been shown to inhibit volume activated chloride currents in many cell types. Tamoxifen has also been reported to inhibit a number of cation channels as well as cytosolic proteins such as calmodulin. The mechanism of channel block by tamoxifen is not known but three hypotheses can be proposed: i) a direct effect following binding to the channel protein from the aqueous environment or ii) a direct effect on the channel protein after partitioning into the lipid membrane or iii) an indirect mechanism via binding to intracellular regulatory proteins after diffusion across the lipid membrane. The aim of these experiments was to distinguish between these hypotheses using membrane permeant and impermeant antioestrogens.


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 microM inhibited volume activated chloride currents in HeLa cells whereas EBT had no effect up to 10 microM when applied either to the extracellular bathing solution or the intracellular solution via the patch pipette.


CONCLUSIONS
Eliminating the ability of tamoxifen to cross the plasma membrane abolishes its channel blocking activity against volume activated chloride channels in HeLa cells.
Department of Pharmacy and Biomolecular Sciences, University of Brighton, Brighton, UK.
M
M
Sahebgharani
S P
SP
Hardy
A W
AW
Lloyd
A C
AC
Hunter
M C
MC
Allen
D018118:Chloride Channels / Q000037:antagonists & inhibitors / Q000187:drug effects*; D005110:Extracellular Space; D006367:HeLa Cells; D006801:Humans; D007038:Hypotonic Solutions / Q000378:metabolism; D007424:Intracellular Fluid; D015640:Ion Channel Gating / Q000187:drug effects; D009997:Osmotic Pressure; D000644:Quaternary Ammonium Compounds / Q000494:pharmacology*; D013629:Tamoxifen / Q000031:analogs & derivatives* / Q000494:pharmacology*
D018118:Chloride Channels; D007038:Hypotonic Solutions; D000644:Quaternary Ammonium Compounds; D013629:Tamoxifen