Block of Neuronal Fast Chloride Channels by Internal Tetraethylammonium Ions

The classical potassium-selective ion channel blocker tetraethylammonium ion (TEA) was shown to block chloride-selective ion channels from excised surface membranes of acutely dissociated rat cortical neurons when applied to the formerly intracellular membrane surface. The patch voltage clamp method was used to record single channel currents from fast Cl channels in the presence of TEAi. At the filtering cut-off frequencies used (3-12.4 kHz, -3 dB) the TEAi-induced block appeared as a reduction in single channel current amplitude, which was interpreted as the result of extremely fast on the off rates for the blocking reaction. Under the conditions of these experiments, the magnitude of TEAi block was independent of membrane potential. Analysis of dose-response experimental results suggests that TEA binding resulted in a partial block of these channels with an equilibrium dissociation constant of approximately 12-15 mM. Analysis of amplitude distributions in the absence and presence of TEAi using the method of Yellen (1994. Journal of General Physiology. 84:157-186.) produced a similar equilibrium dissociation constant and provided a blocking rate constant of approximately 16,000 mM-1.s-1 and an unblocking rate constant of approximately 200,000 s-1. The distributions of open and closed interval durations were fit with a blocking scheme where TEAi binds to the open kinetic state with the constraint that the channel must reenter the open state before TEA can dissociate. The increase in the mean lifetime of the open state could be well fit by this model, but the distribution of closed interval durations could not, suggesting a more complex underlying blocking mechanism.