sarcoplasmic reticulum

The SR is an intracellular membrane-bound network surrounding myofibrils, analogous to the smooth ER in non-muscle cells. It consists of longitudinal tubules and terminal cisternae that closely associate with the T-tubules (transverse tubules), forming triads in skeletal muscle (one T-tubule with two terminal cisternae) or dyads in cardiac muscle.

In skeletal muscle, depolarization of the T-tubule membrane causes a conformational change in the dihydropyridine receptor (DHPR, an L-type calcium channel acting as a voltage sensor), which is mechanically coupled to the ryanodine receptor (RyR1) on the SR membrane, triggering direct calcium release from the SR into the cytosol.

In cardiac muscle, depolarization opens L-type calcium channels (DHPR) allowing extracellular calcium influx, which then binds RyR2 receptors on the SR to trigger a larger calcium release from the SR—this is calcium-induced calcium release (CICR).

Released calcium binds troponin C, causing a conformational shift that moves tropomyosin off the actin binding sites, allowing myosin-actin cross-bridge formation and contraction (sliding filament model).

Relaxation occurs when calcium is pumped back into the SR via SERCA (sarco/endoplasmic reticulum calcium ATPase), an ATP-dependent pump regulated by phospholamban (which inhibits SERCA when unphosphorylated; PKA phosphorylation of phospholamban, e.g., via beta-adrenergic stimulation, relieves this inhibition and enhances calcium reuptake, contributing to increased contractility and faster relaxation—positive inotropy and lusitropy).

Clinical relevance: - Malignant hyperthermia: mutations in RYR1 (or DHPR) cause uncontrolled calcium release from the SR upon exposure to volatile anesthetics or succinylcholine, leading to sustained muscle contraction, hypermetabolism, hyperthermia, and rhabdomyolysis. Treated with dantrolene, which blocks RyR1 and inhibits calcium release. - Heart failure: altered SERCA2a expression/activity and phospholamban regulation affect calcium handling and contractility; SERCA2a gene therapy has been studied as a potential treatment. - Catecholaminergic polymorphic ventricular tachycardia (CPVT): due to RYR2 mutations causing leaky calcium release during stress/exercise, leading to arrhythmias. - Duchenne/Becker muscular dystrophy indirectly affects calcium handling due to membrane fragility affecting SR/T-tubule interactions.

Understanding SR calcium handling is essential for understanding muscle physiology, pharmacology of cardiac glycosides (indirectly affecting calcium via Na+/Ca2+ exchanger), and pathophysiology of both skeletal muscle disorders and heart failure.