The human heart Na channel (hill) was expressed by transient transfection in tsA201 cells, and we examined the block of Na current by a series of symmetrical tetra-alkylammonium cations: tetramethylammonium (TMA), tetraethylammonium (TEA), tetrapropylammonium (TPrA), tetrabutylammonium (TBA), and tetrapentylammonium (TPeA). Internal TEA and TBA reduce single-channel current amplitudes while having little effect on single channel open times. The reduction in current amplitude is greater at more depolarized membrane potentials. Analysis of the voltage-dependence of single-channel current block indicates that TEA, TPrA and TBA traverse a fraction of 0.39, 0.52, and 0.46 of the membrane electric field to reach their binding sites. Rank potency determined from single-channel experiments indicates that block increases with the lengths of the alkyl side chains (TBA > TPrA > TEA > TMA). Internal TMA, TEA, TPrA, and TBA also reduce whole-cell Na currents in a voltage-dependent fashion with increasing block at more depolarized voltages, consistent with each compound binding to a site at a fractional distance of 0.43 within the membrane electric field. The correspondence between the voltage dependence of the block of single-channel and macroscopic currents indicates that the blockers do not distinguish open from closed channels. In support of this idea TPrA has no effect on deactivation kinetics, and therefore does not interfere with the closing of the activation gates. At concentrations that substantially reduce Na channel currents, TMA, TEA, and TPrA do not alter the rate of macroscopic current inactivation over a wide range of voltages (-50 to +80 mV). Our data suggest that TMA, TEA, and TPrA bind to a common site deep within the pore and block ion transport by a fast-block mechanism without affecting either activation or inactivation. By contrast, internal TBA and TPeA increase the apparent rate of inactivation of macroscopic currents, suggestive of a block with slower kinetics.
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