In order to observe the membrane property in steroid myopathy, the ouabain-sensitive (Na
++K
+)-ATPase activities of skeletal muscle, kidney, and brain membrane fractions were studied with the mice administered dexamethasone intraperitoneally for 7.0 -8.5 weeks at a dosage of 5.5 mg/kg. The results obtained were as follows. 1) The body weight of mice treated with dexamethasome for 7.0-8.5 weeks decreased by approximately 5 g (ca. 20% of the control body wt.) against that of saline-treated mice. 2) The ouabain-sensitive (Na
++K
+)-ATPase activity of skeletal muscle sarcolemmal fraction from the steroid-treated mice was significantly higher than that from control by 30-50%. 3) With kidney and brain membrane fractions, no differences were observed between control and steroid-treated mice in the ouabain-sensitive (Na
++K
+)-ATPase activity. These results show that the membrane property of skeletal muscle is specifically influenced by the long-term administration of dexamethasone.
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