Electrophysiological effects of myasthenic serum factors studied in mouse muscle.

Miniature end-plate potential (mepp) amplitudes were investigated in mouse diaphragm exposed in vitro to different serum fractions from seven patients with myasthenia gravis who had elevated serum anti-acetylcholine receptor antibody levels and from controls. The mepp amplitudes were significantly reduced by whole myasthenic sera, restored by washing, and not reduced by heated (56 degrees C) myasthenic sera, which would inactivate complement but not antireceptor antibody. Immunoglobulin G (IgG)-depleted myasthenic sera also significantly reduced mepp amplitudes, while the IgG fraction alone or with normal serum did not. The results indicate that in vitro reduction of mepp amplitudes in mouse muscle by myasthenic sera is not dependent on the IgG fraction alone, and requires a heat-sensitive factor.