Transmembrane calcium movement in 20,25-diazacholesterol myotonia

An abnormality in myoplasmic Ca²⁺ regulation has frequently been proposed in 20,25-diazacholesterol (20,25-D) myotonia. We report here the results of several studies of transmembrane Ca²⁺ movement in this animal model. (i) Physiologic Ca²⁺ release by intact sarcoplasmic reticulum (SR) was examined in chemically skinned single muscle fibers preloaded in EGTA-buffered Ca²⁺ solutions (pCa²⁺ 7.0 to 6.4). Isometric tension development and Ca²⁺ release thresholds in response to Cl^- or caffeine showed no differences between control and 20,25-D fibers at any pCa²⁺. (ii) The kinetics of energy-dependent Ca²⁺ accumulation in purified SR vesicles were followed spectrophotometrically using Ca²⁺-sensitive dyes. The apparent rate for ATP-dependent Ca²⁺ uptake and Ca²⁺ sequestering capacity were unchanged in SR from 20,25-D animals vs. controls. (iii) Surface membrane Ca²⁺ ATPase activity was measured in red blood cell ghosts and sarcolemma. Enzyme V_max was decreased by 25 to 50% in both membranes in the 20,25-D-treated animals with a compensatory increase in the number of Ca²⁺ ATPase molecules. In general, the SR handling of Ca²⁺ appears normal in 20,25-D myotonia, although the activity of Ca²⁺ ATPase in membranes with high sterol content may be altered in response to changes in the lipid environment in this model.