Single cholinergic receptor channel currents in cultured human muscle

Single cholinergic channel currents were recorded in adult human muscle tissue culture. The agonists suberyldicholine and carbamylcholine produce channels with the same conductance as channels produced by acetylcholine but with different closing kinetics. The antagonist tubocurarine, alone or mixed with suberyldicholine, activates channels which close very rapidly. For agonist-activated channels, the distribution of open state lifetimes shows deviations from the usual single exponential form. An excess of short duration openings indicates the presence of an additional faster kinetic process. The lifetime distribution data can be interpreted in terms of varying proportions of slow and fast components which are present in a ratio determined by curve-fitting the appropriate two-exponential function to observed open time distributions. This ratio shows great variability in muscle from older cultures, but the fast and slow time constants are relatively constant. The observation of double exponential open time distributions indicates that the mechanism of channel closing is more complicated than earlier evidence indicated.