Polymyositis: Reduction of acetylcholine receptors in skeletal muscle

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Abstract
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 < 0.001). Incubation of cultured mammalian muscle cells with sera from PM patients significantly reduced (P < 0.005) the number of surface AChRs and increased their rate of degradation in 7/8 cases (P < 0.005). Similar effects were produced by purified lgG from PM patients. These results demonstrate the presence in PM patients of circulating lgG 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.