Block of TRPC5 channels by 2‐aminoethoxydiphenyl borate: a differential, extracellular and voltage‐dependent effect

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Abstract
1 2‐Aminoethoxydiphenyl borate (2‐APB) has been widely used to examine the roles of inositol 1,4,5‐trisphosphate receptors (IP3Rs) and store‐operated Ca2+ entry and is an emerging modulator of cationic channels encoded by transient receptor potential (TRP) genes. 2 Using Ca2+‐indicator dye and patch‐clamp recording we first examined the blocking effect of 2‐APB on human TRPC5 channels expressed in HEK‐293 cells. 3 The concentration–response curve has an IC50 of 20 μM and slope close to 1.0, suggesting one 2‐APB molecule binds per channel. The blocking effect is not shared by other Ca2+ channel blockers including methoxyverapamil, nifedipine, N‐propargylnitrendipine, or berberine. 4 In whole‐cell and excised membrane patch recordings, 2‐APB acts from the extracellular but not intracellular face of the membrane. 5 Block of TRPC5 by 2‐APB is less at positive voltages, suggesting that it enters the electric field or acts by modulating channel gating. 6 2‐APB also blocks TRPC6 and TRPM3 expressed in HEK‐293 cells, but not TRPM2. 7 Block of TRP channels by 2‐APB may be relevant to cell proliferation because 2‐APB has a greater inhibitory effect on proliferation in cells overexpressing TRPC5. 8 Our data indicate a specific and functionally important binding site on TRPC5 that enables block by 2‐APB. The site is only available via an extracellular route and the block shows mild voltage‐dependence. British Journal of Pharmacology (2005) 145, 405–414. doi:10.1038/sj.bjp.0706197