Myasthenic serum selectively blocks acetylcholine receptors with long channel open times at developing rat endplates

We have examined the physiological effects of antibodies from a highly specific myasthenic serum on acetylcholine receptors at developing rat endplates. The antibodies reduced the amplitude of miniature endplate potentials by 60% in 3- to 6-day-old animals but had no effect after day 14. Between days 7 and 12 the antibodies had an intermediate effect. This is the same period during which acetylcholine receptors with long channel open times (slow channels) disappear and receptors with short open times (fast channels) increase in number. Therefore, we examined the effect of the antibodies at endplates with a mixture of channel types more carefully. At all times tested, both noise analysis and analysis of miniature endplate currents showed that the antibodies reduced slow channel activity selectively. Single-channel recordings indicated that acetylcholine receptors that remained active after antibody treatment had normal gating properties. Thus, the antibodies appeared to silence slow channels selectively.