Procedure for low-speed isolation of rat and human skeletal muscle sarcoplasmic reticulum

Decreased sarcoplasmic reticulum phospholipids in human skeletal muscle are associated with metabolic syndrome

Metabolic syndrome affects more than one in three adults and is associated with increased risk of diabetes, cardiovascular disease, and all-cause mortality. Muscle insulin resistance is a major contributor to the development of the metabolic syndrome. Studies in mice have linked skeletal muscle sarcoplasmic reticulum (SR) phospholipid composition to sarcoplasmic/endoplasmic reticulum Ca2+-ATPase activity and insulin sensitivity. To determine if the presence of metabolic syndrome alters specific phosphatidylcholine (PC) and phosphatidylethanolamine (PE) species in human SR, we compared SR phospholipid composition in skeletal muscle from sedentary subjects with metabolic syndrome and sedentary control subjects without metabolic syndrome. Both total PC and total PE were significantly decreased in skeletal muscle SR of sedentary metabolic syndrome patients compared with sedentary controls, particularly in female participants, but there was no difference in the PC:PE ratio between groups. Total SR PC levels, but not total SR PE levels or PC:PE ratio, were significantly negatively correlated with BMI, waist circumference, total fat, visceral adipose tissue, triglycerides, fasting insulin, and homeostatic model assessment for insulin resistance. These findings are consistent with the existence of a relationship between skeletal muscle SR PC content and insulin resistance in humans.

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.