EPHEDRINE - EFFECTS ON NEUROMUSCULAR-TRANSMISSION

(-)-Ephedrine has been used in the treatment of patients with myasthen ia gravis. To investigate the possible effects of ephedrine on neuromu scular transmission, canine intercostal muscle endplates were studied by microelectrode techniques. At concentrations less than 10(-4) M, ep hedrine had no effect on neuromuscular transmission. At a concentratio n of 10(-4) M, ephedrine increased the quantal content of the endplate potential by 21%. The presynaptic store of acetylcholine quanta avail able for immediate release was unchanged, but the probability of quant al release was increased by 16%. At this concentration, ephedrine decr eased the amplitude of the miniature endplate potential by 38%. In the presence of 10(-3) M ephedrine, the miniature endplate potentials and currents became undetectable. The kinetic properties of the acetylcho line receptor channel were studied by analysis of acetylcholine-induce d endplate current noise. At 10(-4) M , ephedrine reduced the channel conductance by 43% but had no effect on the open time. At 5 x 10(-4) M , ephedrine reduced the channel conductance by 84% and increased the o pen time by 23 percent.