Ion transport in human skeletal muscle cells: disturbances in myotonic dystrophy and Brody's disease.

After excitation of skeletal muscle, the disturbed ion homeostasis is restored by Na+, K+ ATPase of the sarcolemma and Ca2+ ATPase of the sarcoplasmic reticulum (SR). Contrary to Na+, K+ ATPase, the concentration and isoenzyme distribution of SR Ca2+ ATPase in human skeletal muscle depend on fibre type and age. In cultured human muscle cells the concentration and activity of Na+, K+ ATPase and SR Ca2+ ATPase increase with maturation. In skeletal muscle and cultured muscle cells of patients suffering from myotonic dystrophy (MyD), the activity and the concentration of both Na+, K+ ATPase and SR Ca2+ ATPase are decreased by about 40%. In addition, we measured in cultured MyD muscle cells at rest an increased cytosolic Ca2+ concentration ([Ca2+]i) caused by active voltage-operated Ca2+ channels, which are inactive in resting control cells. However, the restoration of a stimulus-induced Ca2+ transient is unaffected. A differentiation-related disturbance of membranes or a modulation defect of membrane proteins may play a role in MyD. In skeletal muscle and cultured muscle cells of patients suffering from Brody's disease, which is characterized by impaired muscle relaxation, the SR Ca2+ ATPase activity is reduced by about 50%, but the concentrations of total SR Ca2+ ATPase and the predominant SERCA1 isoform are normal. Diseased muscle cells show a delayed restoration of [Ca2+]i after stimulation, which might be explained by structural modifications of SERCA1. Reduction of the Ca2+ release by drugs balances the excitation-relaxation cycle of the pathological cells.