Junctional membrane Ca dynamics in human muscle fibers are altered by malignant hyperthermia causative RyR mutation

We used the nanometer-wide tubules of the transverse tubular (t)-system of human skeletal muscle fibers as sensitive sensors for the quantitative monitoring of the Ca-handling properties in the narrow junctional cytoplasmic space sandwiched between the tubular membrane and the sarcoplasmic reticulum cisternae in single muscle fibers. The t-system sealed with a Ca-sensitive dye trapped in it is sensitive to changes in ryanodine receptor (RyR) Ca leak, the store operated calcium entry flux, plasma membrane Ca pump, and sodium-calcium exchanger activities, thus making the sealed t-system a nanodomain Ca sensor of Ca dynamics in the junctional space. The sensor was used to assess the basal Ca-handling properties of human muscle fibers obtained by needle biopsy from control subjects and from people with a malignant hyperthermia (MH) causative RyR variant. Using this approach we show that the muscle fibers from MH-susceptible individuals display leakier RyRs and a greater capacity to extrude Ca across the t-system membrane compared with fibers from controls. This study provides a quantitative way to assess the effect of RyR variants on junctional membrane Ca handling under defined ionic conditions