Biochemical characteristics of free and junctional sarcoplasmic reticulum and of transverse tubules in human skeletal muscle

The microsomal fraction of normal human skeletal muscle was subfractionated by isopycnic sucrose-density centrifugation, using the procedure originally described by Saito et al. for rabbit fast muscle, and specific markers of the junctional face membrane of terminal cisternae (TC) (ryanodine receptor, high-molecular-weight feet proteins and membrane-associated calcium-binding protein calsequestrin), of the sarcoplasmic reticulum (SR) Ca-pump membrane (chicken antibody to rabbit Ca-ATPase), and of transverse tubules (TT) (dihydropiridine receptor, membrane cholesterol), respectively. The results show that isolated TC from human skeletal muscle share extensive morphological characteristics, protein composition, as well as Ca-release properties with rabbit TC, as tested with an inhibitor (Ruthenium red) and an activator (doxorubicin) of SR Ca-release. The Ca-pump membrane of human muscle SR, in distinction to rabbit fast muscle SR, showed a relatively low specific activity of the Ca-ATPase, as expected from the mixed fiber composition of human muscles, but shared the presence of minor protein components, such as a Con A binding protein of about 57 kDa and blue-staining peptides in the 170-120 kDa range of molecular weights. Human muscle TT, as isolated from the same sucrose gradient, demonstrated a high affinity (3H)-dihydropiridine binding activity in the range of previously reported values for purified TT from rabbit skeletal muscle.