We have examined the effects of chemical modification with trimethyloxonium (TMO) and changes in external pH on the properties of acetylcholine (ACh)-activated channels in BC3H-1 cells, a clonal muscle cell line. TMO reacts covalently and specifically with carboxylic acid moieties in proteins to convert them to neutral methyl esters. In BC3H-1 cells TMO modification reduces the whole-cell response to ACh measured at negative membrane potentials by approximately 60%. G omega seal patch-clamp recordings of single ACh channel currents showed that the reduction in ACh sensitivity is due to alterations in both the current-carrying and the kinetic properties of the channels. Under all our experimental conditions, i.e., in external solutions of normal or low ionic strength, with or without external divalent cations, and at external pHs between 5.5 and 8.1, TMO treatment reduced ACh single-channel conductance to 70-90% of normal. The effects of TMO on channel kinetics were dependent on the ionic conditions. In normal ionic strength solutions containing both calcium and magnesium ions TMO modification reduced the channel average open time by approximately 25%. A similar reduction in open time was seen in calcium-free solution, but was not present when both calcium and magnesium ions were absent from the external solution. Lowering the ionic strength of the solution increased the mean open time in normal channels by about threefold, but did not affect the kinetics of modified channels. In low ionic strength solutions normal ACh channel open times were maximal at approximately pH 6.7 and decreased by three- to fourfold at both acid and alkaline pH. TMO modification removed the pH dependence of channel kinetics, and average open times were short at all pHs between 5.5 and 8.1. We suggest that TMO modifies normally titratable groups on the external surface of ACh channels that help to determine both the gating and permeability properties of ACh channels.

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