Interactions of edrophonium, physostigmine and methanesulfonyl fluoride with the snake end-plate acetylcholine receptor-channel complex

Abstract

The actions of two reversible anticholinesterase agents, edrophonium and physostigmine, were compared with the irreversible agent methanesulfonyl fluoride (MSF) on miniature end-plate currents (MEPCs) and ACh-induced end-plate current fluctuations recorded from twitch fibers of costocutaneous muscles of garter snakes (Thamnophis sp.). Low concentrations of edrophonium (less than 25 microM) produced a concentration-dependent increase in both the MEPC amplitude and the time constant of MEPC decay. MEPCs recorded at all concentrations studied (25-100 microM) decayed as a single exponential function with time. As the concentration of edrophonium was increased, MEPC amplitude was initially increased and then decreased such that, at concentrations above 50 microM, MEPC amplitude was decreased below control values. Concentrations of edrophonium greater than 30 microM produced power density spectra that required the sum of two Lorentzian components--one faster and one slower than control. Single-channel conductance was not significantly altered by edrophonium. Low concentrations of physostigmine (less than 10 microM) increased MEPC amplitude and prolonged MEPC decay. Higher concentrations of physostigmine (10-100 microM) decreased mean peak MEPC amplitude and accelerated MEPC decay in a concentration-dependent manner. The relationship between MEPC decay and membrane potential was reduced, and then reversed, with increasing concentrations of physostigmine. Current fluctuation spectra recorded in physostigmine were described by a single Lorentzian function at all membrane voltages and concentrations studied. Increasing the concentration of physostigmine or membrane hyperpolarization did not alter single-channel conductance but shortened the apparent lifetime of open end-plate channels. MSF increased MEPC amplitude and increased the time constant of MEPC decay without altering the temperature and voltage dependence of both MEPC decay and channel lifetime determined from end-plate current fluctuations. Exposure of MSF-treated end-plates to either edrophonium or physostigmine produced results that were similar to those obtained before MSF treatment. These results demonstrate that edrophonium and physostigmine have direct actions on the end-plate receptor-channel complex that are unrelated to their inhibitory action on junctional acetylcholinesterase.