Interactions of monovalent cations with sodium channels in squid axon: II. Modification of pharmacological inactivation gating

The time-, frequency-, and voltage-dependent blocking actions of several cationic drug molecules on open Na channels were investigated in voltage-clamped, internally perfused squid giant axons. The relative potencies and time courses of block by the agents (pancuronium [PC], octylguanidinium [C8G], QX-314, and 9-aminoacridine [9-AA]) were compared in different intracellular ionic solutions; specifically, the influences of internal Cs, tetramethylammonium (TMA), and Na ions on block were examined. TMA⁺ was found to inhibit the steady state block of open Na channels by all of the compounds. The time-dependent, inactivation-like decay of Na currents in pronase-treated axons perfused with either PC, 9-AA, or C8G was retarded by internal TMA⁺. The apparent dissociation constants (at zero voltage) for interaction between PC and 9-AA with their binding sites were increased when TMA⁺ was substituted for Cs⁺ in the internal solution. The steepness of the voltage dependence of 9-AA or PC block found with internal Cs⁺ solutions was greatly reduced by TMA⁺, resulting in estimates for the fractional electrical distance of the 9-AA binding site of 0.56 and 0.22 in Cs⁺ and TMA⁺, respectively. This change may reflect a shift from predominantly 9-AA block in the presence of Cs⁺ to predominantly TMA⁺ block. The depth, but not the rate, of frequency-dependent block by QX-314 and 9-AA is reduced by internal TMA⁺. In addition, recovery from frequency-dependent block is not altered. Elevation of internal Na produces effects on 9-AA block qualitatively similar to those seen with TMA⁺. The results are consistent with a scheme in which the open channel blocking drugs, TMA (and Na) ions, and the inactivation gate all compete for a site or for access to a site in the channel from the intracellular surface. In addition, TMA ions decrease the apparent blocking rates of other drugs in a manner analogous to their inhibition of the inactivation process.