plural: sarcoplasmic reticula
(cell biology) The special type of smooth endoplasmic reticulum found in smooth and striated muscle fibers whose function is to store and release calcium ions
The endoplasmic reticulum (ER) is an organelle that occurs as interconnected network of flattened sacs or tubules (called cisternae) in the cytoplasm. The membranes of the ER are connected to the outer nuclear envelope. They may also extend into the cell membrane. There are two kinds of ER: the rER, or the rough endoplasmic reticulum, and the sER, or the smooth endoplasmic reticulum. The sER, as opposed to the rough endoplasmic reticulum, does not have ribosomes on its surface, thus the name smooth. A specialized type of SER occurs in muscle cells where calcium ions are stored. It is referred to as sarcoplasmic reticulum.
The sarcoplasmic reticulum (SR) is a type of smooth endoplasmic reticulum that abounds in the myocyte (muscle cell). In myocytes, it can be seen as a membrane-bound structure inside the myocyte, containing calcium ions. The network of tubules extends throughout the myocyte. It surrounds the myofibrils, i.e. the contractile units of the muscle cell. In cardiac muscle and skeletal muscle, a part of the SR is closely associated with the transverse tubules. A transverse tubule is an extension of the cell membrane (particularly called sarcolemma) that together with that parts of the SR forms a so-called triad. The parts of the SR that are closely associated with the transverse tubules are called terminal cisternae, which is the major site of calcium release.
For SR to carry out the role of absorbing calcium ions, it has ion channel pumps in its membrane. These calcium pumps in the SR are called SERCA (i.e. Sarco(endo)plasmic reticulum ATPases). The SERCA is comprised of 13 subunits: M1, M2, M3, M4, M5, M6, M7, M8, M9, M10, N, P, and A subunits. M1 to M10 subunits are located inside the membrane of the SR whereas the N, P, and A subunits are found outside the SR. The subunits in which calcium ions bind to are M1 to M10 subunits. The N, P, and A subunits are where the ATP binds to. The most abundant type of SERCA in cardiac and skeletal muscles is SERCA 2a. Inside the SR is a protein calsequestrin, which is involved in calcium storage. About 50 calcium ions can bind to this protein. Thus, this protein can mitigate the number of free calcium ions within the SR. It is chiefly found in the terminal cisternae. Calcium ion release via the SR is by way of the ryanodine receptors (RyRs). In mammals, RyR1 is found typically in skeletal muscles whereas RyR2 is in cardiac muscles. RyR3 is the most common type, and is found especially in the brain.
The SR is responsible for the release, the absorption, and the storage of calcium ions. It releases calcium ions during muscle contraction and absorb them during relaxation. The regulation of calcium ion level has to be regulated. Too much calcium inside the cell could lead to calcification and the hardening of intracellular structures, and eventually to cell death. Through SR, the level of calcium ions is kept relatively constant. It facilitates the calcium ion concentration to be several times smaller inside the cell than the calcium ion concentration outside the cell.
Common biological reactions
Common biological reactions
Through the ion channel pumps in the SR, the calcium ions are absorbed into the SR. Thus, the calcium ions are greater in number inside the SR relative to the cytoplasm. Because of this, the absorption of calcium ions by the SR is by way of active transport, requiring energy (via ATP) to move more calcium ions into the SR. Calcium absorption occurs when two calcium ions and ATP bind to the cytosolic side of the calcium pump. The phosphate group released from the ATP binds to the pump and changes its shape, resulting in the opening of the pump. When this happens, the two calcium ions enter through it. The pump on the outer (cytosolic) side closes while the inner side opens, releasing the calcium ions into the SR.1
Common biological reactions
The SR releases calcium ions at the terminal cisternae via the receptor, ryanodine receptor (RyR). When RyR is activated, it opens to release calcium ions. The release of stored calcium ions from the SR is called calcium spark. Calcium spark may either be evoked or spontaneous. One mechanism of releasing calcium ions is by calcium-induced calcium release. In this mechanism, the receptors dihydropyridine receptors in the sarcolemma is triggered by an action potential to change its shape and become an ion channel where calcium ions can enter the cell. The influx of calcium ions results in their binding to the RyRs in the SR. When four calcium ions bind to the RyR, the latter is activated and opens resulting in the release of more calcium ions (into the cytosol from the SR). This evoked calcium spark occurs in cardiac and smooth muscle. In skeletal muscle, the evoked calcium spark occurs without the prior flooding of calcium ions. Rather, RyRs open when dihydropyridine receptors change their shape. This is because the dihydropyridine receptors in the sarcolemma of the skeletal muscle touch the RyR of SR. Thus, when dihydropyridine receptors change their shape they trigger RyRs directly to undergo conformational change to open. In smooth and cardiac muscles, the dihydropyridine receptors are not in touch with the RyR but are located opposite to RyR. Thus, calcium ion flooding and binding occurs prior to calcium release.
As for the spontaneous calcium spark, the release of calcium ions from SR occurs without the initial stimulation by an action potential. Rather, it occurs when the calcium ion concentration is too high inside the SR. The calcium ions bind to the inner side of the RyR of the SR, causing the RyR to open. The detachment of calsequestrin from the RyR also leads to the opening of the RyR. (On the contrary, a low calcium ion concentration causes the calsequestrin to bind tightly to RyR and thereby prevent it from opening.)
- Greek sarx, meaning “flesh”
- Kekenes-Huskey, P.M., Metzger, V.T., Grant, B.J. and McCammon, A.J. (2012b) ‘Calcium binding and allosteric signaling mechanisms for the sarcoplasmic reticulum Ca2+ ATPase’, 21(10).
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