Dysfunction of the sarcoplasmic reticulum in polymyositis.

The transport of calcium in vesicles of sarcoplasmic reticulum isolated from muscle specimens from 6 patients with early, active polymyositis and from 11 controls was examined. The time courses of calcium uptake and calcium-dependent ATPase activity were measured simultaneously. Calcium uptake by sarcoplasmic reticulum vesicles from patients with polymyositis was 50% less than that by vesicles from controls (P less than 0.001). In contrast, no difference in calcium-dependent ATPase activity was noted between vesicles from patients with polymyositis and controls. The demonstrated defect may be important in the pathogenesis of muscle weakness in polymyositis.