Reaction of tetraethylammonium with the open and closed conformations of the acetylcholine receptor ionic channel complex.

Abstract

The effect of tetraethylammonium (TEA) bromide on the neurally and iontophoretically evoked endplate current (EPC) of frog sartorius muscle was studied using voltage-clamp and noise analysis techniques and its binding to the acetylcholine (ACh) receptor ionic channel complex was determined on the electric organ of Torpedo ocellata. TEA (250-500 .mu.M) produced an initial enhancement followed by a slow decline in the amplitude of the endplate potential [EPP] and EPC but caused only depression in the amplitude of the miniature endplate potential [MEPP] and current [MEPC]. In normal ringer''s solution, the EPC current-voltage relationship was approximately linear and the decay phase varied exponentially with membrane potential. Upon addition of 50-100 .mu.M TEA the current-voltage relationship became markedly nonlinear at hyperpolarized command potentials; with 250-2000 .mu.M TEA there was an initial linear segment, an intermediate nonlinear segment and a region of negative conductance. The onset of nonlinearity was dose-dependent, undergoing a 50 mV shift for a 10-fold increase in TEA concentration. The EPC decay phase was shortened by TEA at hyperpolarized but not depolarized potentials and remained a single exponential function of time at all concentrations and membrane potentials examined. These actions of TEA were independent of the sequence of polarizations, the length of the conditioning pulse and the level of the initial holding potential. TEA shifted the power spectrum of ACh noise to higher frequencies and produced a significant depression of single channel conductance. The shortening in the mean channel lifetime agreed with the decrease in the EPC decay time constant. At the concentrations tested TEA did not alter the EPC reversal potential nor the resting membrane potential and had little effect on the action potential duration. TEA inhibited the binding of both [3H] ACh (Ki = 200 .mu.M) and [3H]perhydrohistrionicotoxin (Ki = 280 .mu.M) to receptor-rich membranes from the electric organ of T. ocellata and inhibited the carbamylcholine-activated 22Na+ efflux from these microsacs. Apparently TEA reacts with the nicotinic ACh-receptor and its ion channel; the voltage-dependent actions are associated with blockade of the ion channel. The results are compatible with a kinetic model in which TEA first binds to the closed conformation of the receptor-ionic channel complex to produce a voltage-dependent depression of endplate conductance and subsequently to its open conformation, giving rise to the shortening in the EPC decay and mean channel lifetime.