Acetylcholine Receptor Activation by a Site-selective Ligand: Nature of Brief Open and Closed States in BC3H-1 Cells

Single-channel currents were recorded through acetylcholine receptor channels of clonal BC3H-1 muscle cells activated by the curare-like compound, DMT binds selectively to the two alpha-neurotoxin-binding sites on these receptors, with apparent dissociation constants differing by about 1000-fold (Sine & Taylor, 1981). Receptor channels do not open with DMT bound only to the high-affinity site, but only at DMT concentrations at which both high- and low-affinity sites are occupied. Open-duration histograms are not single exponentials, but are described by the sums of two (or three) exponentials. Both brief- and long-duration openings are observed in the presence of 3 microM-DMT, and are seen at the same relative frequency up to 80 microM-DMT. Long-duration openings are interrupted by brief closures with a mean duration of 50 microseconds and which occur at a frequency of 50-60 per second of open time. These temporal characteristics closely parallel those of the brief closures observed with the full agonists, acetylcholine, carbamylcholine, and suberyldicholine. Raised concentrations of DMT apparently block open channels in a voltage-dependent fashion. It is concluded that both brief- and long-duration openings arise from receptors with two molecules of DMT bound. Furthermore, brief closures in general do not appear to reflect receptor activation processes. Instead, they seem to arise through entry to a closed state with properties independent of the agonist, but characteristic of open channels.