Sarcoplasmic Reticulum(SR) is a special type of smooth ER found in smooth and striated muscle. The only structural difference between this organelle and the smooth endoplasmic reticulum is the medley of protein they have, both bound to their membranes and drifting within the confines of their lumens. This fundamental difference is indicative of their functions: the smooth ER synthesizes molecules and the sarcoplasmic reticulum stores and pumps calcium ions. The sarcoplasmic reticulum contains large stores of calcium, which it sequesters and then releases when the cell is depolarized. This has the effect of triggering muscle contraction.
Sarcoplasmic Reticulum is physically separate from the sarcolemma and surrounds each myofibril. Sarcoplasmic reticulum membrane contains high levels of calcium ATPase. The sarcoplasmic reticulum functions to uptake calcium from the sarcoplasm and to release calcium into the sarcoplasm to initiate contraction and sequester it during relaxation.
it has been implicated as a major contributor to the depressed function in heart failure. The SR acts as a calcium source during contraction and a calcium sink during relaxation. Relaxation is mediated by the transport of calcium into the sarcoplasmic reticulum lumen by a Ca-ATPase, which is under reversible regulation by phospholamban, a low molecular weight phosphoprotein. Calcium then binds to calsequestrin in the lumen of the SR. For the initiation of contraction, calcium is released through the calcium channels or ryanodine receptors, which are under regulation by junctin, triadin and partially by a novel protein, the histidine rich calcium-binding protein. Thus, the SR is the major regulator of Ca2+-handling and contractility in muscle
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