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 TEA(i). At the filtering cut-off frequencies used (3-12.4 kHz, -3 dB) the TEA(i)-induced block appeared as a reduction in single channel current amplitude, which was interpreted as the result of extremely fast on and off rates for the blocking reaction. Under the conditions of these experiments, the magnitude of TEA(i) 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 similar to 12-15 mM. Analysis of amplitude distributions in the absence and presence of TEA(i) 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 similar to 16,000 mM(-1) . s(-1) and an unblocking rate constant of similar to 200,000 s(-1). The distributions of open and closed interval durations were fit with a blocking scheme where TEA(i) 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.