BLOCK OF NEURONAL FAST CHLORIDE CHANNELS BY INTERNAL TETRAETHYLAMMONIUM IONS

The classical potassium-selective ion channel blocker tetraethylammoni um ion (TEA) was shown to block chloride-selective ion channels from e xcised surface membranes of acutely dissociated rat cortical neurons w hen applied to the formerly intracellular membrane surface. The patch voltage clamp method was used to record single channel currents from f ast Cl channels in the presence of TEA(i). At the filtering cut-off fr equencies used (3-12.4 kHz, -3 dB) the TEA(i)-induced block appeared a s a reduction in single channel current amplitude, which was interpret ed as the result of extremely fast on and off rates for the blocking r eaction. Under the conditions of these experiments, the magnitude of T EA(i) block was independent of membrane potential. Analysis of dose-re sponse experimental results suggests that TEA binding resulted in a pa rtial block of these channels with an equilibrium dissociation constan t of similar to 12-15 mM. Analysis of amplitude distributions in the a bsence and presence of TEA(i) using the method of Yellen (1994. Journa l of General Physiology. 84:157-186.) produced a similar equilibrium d issociation 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 duration s were fit with a blocking scheme where TEA(i) binds to the open kinet ic state with the constraint that the channel must reenter the open st ate before TEA can dissociate. The increase in the mean lifetime of th e open state could be well fit by this model, but the distribution of closed interval durations could not, suggesting a more complex underly ing blocking mechanism.