Polymyositis: reduction of acetylcholine receptors in skeletal muscle.

In polymyositis (PM), neither the precise target nor the mechanism of the attack against skeletal muscle have yet been defined. In this study, we report evidence of a pathologic process involving a membrane component of muscle, acetylcholine receptors (AChRs). Our results show that PM patients have significantly reduced AChRs at neuromuscular junctions, averaging 55% below control values (P less than 0.01). Incubation of cultured mammalian muscle cells with sera from PM patients significantly reduced (P less than 0.005) the number of surface AChRs and increased their rate of degradation in 7/8 cases (P less than 0.005). Similar effects were produced by purified IgG from PM patients. These results demonstrate the presence in PM patients of circulating IgG with functional effects on a component of the surface membrane of skeletal muscle. They suggest that PM and myasthenia gravis may have important features in common.