Protective effects of Ca ²⁺ handling drugs against abnormal Ca ²⁺ homeostasis and cell damage in myopathic skeletal muscle cells

Deficiency of δ-sarcoglycan (δ-SG), a component of the dystrophin-glycoprotein complex (DGC), causes skeletal muscular dystrophy and cardiomyopathy in BIO14.6 hamsters. Here, we studied the involvement of abnormal Ca ²⁺ homeostasis in muscle degeneration and the protective effect of drugs against Ca ²⁺ handling proteins in vivo as well as in vitro. First, we characterized the properties of cultured myotubes from muscles of normal and BIO14.6 hamsters (30-60 days old). While there were no apparent differences in the levels of expression of various Ca ²⁺ handling proteins (L-type Ca ²⁺ channel, ryanodine receptor, SR-Ca ²⁺ ATPase, and Na ⁺/Ca ²⁺ exchanger), muscle-specific proteins (contractile actin and acetylcholine receptor), or DGC member proteins except SGs, BIO14.6 myotubes showed a high degree of susceptibility to mechanical stressors, such as cyclic stretching and hypo-osmotic stress as compared to normal myotubes, as evidenced by marked increases in creatine phosphokinase (CK) release and bleb formation. BIO14.6 myotubes showed abnormal Ca ²⁺ homeostasis characterized by elevated cytosolic Ca ²⁺ concentration, frequent Ca ²⁺ oscillation, and increased ⁴⁵Ca ²⁺ uptake. These abnormal Ca ²⁺ events and CK release were significantly prevented by Ca ²⁺ handling drugs, tranilast, diltiazem, and FK506. The calpain inhibitor E64 prevented CK release, but not ⁴⁵Ca ²⁺ uptake. Some of these drugs (tranilast, diltiazem, and FK506) also exerted a significant protective effect for muscle degeneration in BIO14.6 hamsters and mdx mice in vivo. These observations suggest that elevated Ca ²⁺ entry through sarcolemmal Ca ²⁺ channels predominantly contributes to muscle degeneration and that the drugs tested here may have novel therapeutic potential against muscular dystrophy.