Where Are Calcium Ions Stored Within the Fiber

Sarcoplasmic reticulum

The SR/ER volume and distribution of the tubules, fenestrated sheets, and surface couplings differ in phasic and tonic SMs. The fenestrated sheets of SR (Figure 83.5) are reminiscent of the longitudinal SR encircling the A-band in striated muscles.

From: Muscle , 2012

Mechanisms of Cardiac Contraction and Relaxation

Douglas P. Zipes MD , in Braunwald's Heart Disease: A Textbook of Cardiovascular Medicine , 2019

Calcium Uptake into Sarcoplasmic Reticulum by SERCA

Ca²⁺ is transported into the SR by SERCA, which constitutes almost 90% of the SR protein. Its molecular weight is 115 kDa, with 10 transmembrane domains and large cytosolic and small SR-luminal domains. Three isoforms exist, but in cardiac myocytes the dominant form is SERCA2a. For each molecule of ATP hydrolyzed by this enzyme, two calcium ions are taken up into the SR ( Fig. 22.10 ;see also Fig. 22.9 ). SR Ca²⁺ uptake is the primary driver of cardiac myocyte relaxation, and reuptake starts as soon as [Ca²⁺]_i begins to rise. Because Ca²⁺ removal is slower than Ca²⁺ influx and release, a characteristic rise and fall in [Ca²⁺]_i called the Ca²⁺ transient, takes place. As [Ca²⁺]_i falls, Ca²⁺ dissociates from troponin C, which progressively switches off the myofilaments. A reduction in SERCA expression or function (as seen in heart failure or energetic limitations) can thus directly result in slower rates of cardiac relaxation. In addition, the strength of SR Ca²⁺ uptake directly influences the diastolic SR Ca²⁺ content and [Ca²⁺]_SR, which dictates both the sensitivity of the RyR and the flux rate of SR Ca²⁺ release. Thus, SR Ca²⁺ uptake and release are an integrated system.

Phospholamban (PLB) was so named by its discoverers Tada and Katz ²⁴ to mean "phosphate receiver." PLB is a single-transmembrane pass protein that binds directly to SERCA2a. Under basal conditions, this reduces the affinity of SERCA for cytosolic Ca²⁺, which results in weaker SR Ca²⁺ uptake at any given [Ca²⁺]_i. However, when PLB is phosphorylated by either PKA or CaMKII (at Ser16 or Thr17, respectively), the inhibitory effect is relieved, thereby resulting in increased rates of SR Ca²⁺ uptake, cardiac relaxation (lusitropic effect), and increased SR Ca²⁺ content, which drives stronger contraction (inotropic effect; Fig. 22.10 ).

The Ca²⁺ taken up into the SR is stored within the SR before further release. The highly charged, low-affinity Ca²⁺ buffer (K_d approximately 600 µM)calsequestrin is found primarily at the jSR and enhances the local availability of Ca²⁺ for release by the nearby RyR.Calreticulin is another Ca²⁺-storing protein that is similar in structure to calsequestrin and probably similar in function. There is also evidence that calsequestrin and two other proteins located in the SR membrane (junctin and triadin) may regulate the properties of the RyR and may be part of the mechanism by which higher [Ca]_SR enhances RyR opening. ¹⁹ Reuptake by SERCA occurs everywhere in the SR membrane in the network that surrounds the myofilaments. Diffusion of Ca²⁺ within the SR is relatively fast, which allows restoration of [Ca²⁺]_SR at the jSR to occur quickly because Ca²⁺ is taken back up everywhere. ²⁵ Indeed, during normal Ca²⁺ release, intra-SR Ca²⁺ diffusion is rapid enough to limit Ca²⁺ gradients between SR release sites in the jSR and the Ca²⁺ uptake sites. This diffusion also ensures that [Ca²⁺]_SR is relatively uniform throughout the myocyte, which facilitates the uniformity of SR Ca²⁺ release and myofilament activation throughout the cell.

DESIGN AND PHYSIOLOGY OF THE HEART | Cardiac Excitation–Contraction Coupling: Routes of Cellular Calcium Flux

H.A. Shiels , in Encyclopedia of Fish Physiology, 2011

Ca Storage in the SR

The SR contributes to the falling phase of the Ca transient by pumping Ca out of the cytosol and into the SR. This pumping action is achieved through the efforts of the SR Ca-pump or SERCA. Pumping Ca into the SR takes energy, which is generated by breaking down ATP.

A surprising finding is that despite its limited role in e–c coupling, the fish SR has a massive ability to store Ca. The steady-state SR Ca content of the rainbow trout atrial myocyte is > 1000 μmoll⁻¹ (measured via application of caffeine, which opens ryanodine receptors and lets Ca out of the SR and into the cytoplasm). Mammalian SR Ca content is in the range of ∼50–250 μmoll⁻¹. Furthermore, the mammalian SR spontaneously releases Ca when the content reaches much above 150 μmoll⁻¹. Thus, fish SR can hold much greater amounts of Ca without spontaneously releasing it. How the fish SR is able to store this level of Ca is unclear. Differences in intra-SR buffering via calsequestrin may be involved. Other Ca buffers inside the SR or accessory proteins (triadin and junctin) that regulate ryanodine receptor function may be involved but this awaits future study. Certainly, for the rainbow trout at least, the SR contains more than enough Ca to support contraction. In fact, if the SR released all of its Ca into the cytosol in one go, levels could approach those associated with Ca toxicity.

A separate and fascinating question is: Why is the fish SR capable of accumulating large amounts of Ca? Again, the reasons are unclear. Because SR Ca content is only high in active fish species (or cold fish species), it has been speculated that the SR may act as a Ca reservoir that can be tapped into if physiology demands it. For example, if a faster or greater Ca transient is required (because the fish is chasing or being chased), the SR may provide a Ca reserve that can be added to SL Ca influx to bolster the size and rate of the Ca transient. However, so far this hypothesis has been difficult to prove.

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Pathophysiology of Heart Failure

Douglas P. Zipes MD , in Braunwald's Heart Disease: A Textbook of Cardiovascular Medicine , 2019

Sarcoplasmic Reticulum Ca²⁺ Reuptake and Sarcolemmal Ca²⁺ Elimination

Relaxation of the contractile proteins occurs after dissociation of Ca²⁺ from troponin C and Ca²⁺ elimination from the cytosol. In the human heart, there are two main mechanisms responsible for elimination of Ca²⁺ from the cytosol: SR uptake of Ca²⁺ by the SERCA2a Ca²⁺ pump and transsarcolemmal Ca²⁺ elimination through NCX. Under normal conditions, approximately 75% of Ca²⁺ is taken up by the SR and 25% extruded from the cell through NCX. In HF there is decreased uptake of Ca²⁺ by the SR secondary to decreased SERCA2a protein levels and SERCA2a function. In addition, phosphorylation of phospholamban (PLB) is reduced in the failing heart, resulting in increased PLB-dependent inhibition of the SR Ca²⁺ pump. ²⁴ The decrease of SR Ca²⁺ uptake in the failing heart results in a relative increase of transsarcolemmal Ca²⁺ elimination by the NCX, which is most likely secondary to increased expression of NCX protein.

Restoring deficient SERCA2a by gene transfer has been shown to improve contractile function and restore electrical stability experimentally. However, the recent CUPID trial failed to show clinical benefit of SERCA2a gene transfer in patients with HF, whereas the gene transfer procedure itself seems to be safe. ²⁶ Although the increase in NCX activity may result in increased Ca²⁺ elimination from the myocyte, thereby preserving diastolic calcium levels and preventing diastolic dysfunction when SR calcium uptake is reduced, increased NCX activity may further reduce SR Ca²⁺ accumulation/content and may therefore reduce Ca²⁺ activation of contractile proteins. ²⁴ As noted, electrogenic NCX activity induces DADs and arrhythmias.

Anesthesia for Neonates and Premature Infants

Claire M. Brett , ... George Bikhazi , in Smith's Anesthesia for Infants and Children (Seventh Edition), 2006

Sarcoplasmic Reticulum

The sarcoplasmic reticulum is the major intracellular organelle for controlling the cytosolic calcium during contraction. In the adult, only a small amount of extracellular calcium enters the myocyte and elicits the release of a larger amount of calcium from the sarcoplasmic reticulum. The immature heart has an underdeveloped sarcoplasmic reticulum. That is, the volume of sarcoplasmic reticulum within the cell and the ability to pump calcium increase with age. An increase in Ca ² ⁺-ATPase activity has been linked to the increased function of the sarcoplasmic reticulum (Mahony and Jones, 1986; Mahony, 1988). A significant finding is that the newborn heart requires a higher extracellular calcium concentration to achieve maximal contractility (Jarmakani et al., 1982). Caffeine, which increases the release of calcium from the sarcoplasmic reticulum, has little effect on neonatal contractility, also suggesting that extracellular calcium, rather than intracellular calcium, has a primary role in control of contractility in the neonatal myocardium. With maturation, the quantity of sarcoplasmic reticulum, the number of specialized connections of the sarcoplasmic reticulum with other membranes, and the activity of Ca² ⁺-ATPase increases, as does the efficiency of pumping calcium.

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The Heart as a Pump

Walter F. Boron MD, PhD , in Medical Physiology , 2017

The entry of Ca²⁺ from the outside triggers Ca²⁺-induced Ca²⁺ release from the sarcoplasmic reticulum

Excitation-contraction (EC) coupling in cardiac ventricular myocytes (seepp. 242 -243) is similar to EC coupling in skeletal muscle (seepp. 229–230). One major difference is that, in the case of skeletal muscle, the initiating event is the arrival of an action potential at the neuromuscular junction, the release of acetylcholine, and the initiation of an end-plate potential. In the ventricular myocyte, action potentials in adjacent myocytes depolarize the target cell through gap junctions (seep. 483) and thereby generate an action potential.

As in a skeletal muscle fiber (seepp. 229–230), the depolarization of the plasma membrane in the ventricular myocyte invades T tubules that run radially to the long axis of the myocyte. Unlike skeletal muscle cells, cardiac myocytes also haveaxial T tubules that run parallel to the long axis of the cell and interconnect adjacent radial T tubules.

Another major difference in EC coupling between cardiac and skeletal muscle is the way that the L-type Ca²⁺ channels (Cav1.2, dihydropyridine receptors) in the T-tubule membrane activate the Ca²⁺ -release channels made up of four RYR2 molecules in the sarcoplasmic reticulum (SR) membrane. In skeletal muscle, the linkage is mechanical and does not require Ca ²⁺ entry per se. If you place skeletal muscle in a Ca²⁺-free solution, the muscle can continue contracting until its intracellular Ca²⁺ stores become depleted. In contrast, cardiac muscle quickly stops beating. Why?

In cardiac muscle, Ca²⁺ entry through the L-type Ca²⁺ channel Cav1.2 (Fig. 22-11, black arrow No. 1) is essential for raising [Ca²⁺]_i in the vicinity of the RYR2 on the SR. A subset of Cav1.2 channels may be part of caveolae. This trigger Ca²⁺ activates an adjacent cluster of RYRs in concert, causing them to release Ca²⁺ locally into the cytoplasm byCa²⁺-induced Ca²⁺ release (CICR; seeFig. 22-11, black arrow No. 2). In the CICR coupling mechanism, the action of this Ca²⁺ is analogous to that of a neurotransmitter or chemical messenger that diffuses across a synapse to activate an agonist-gated channel, but in this case the synapse is the intracellular diffusion gap of ~15 nm between plasma-membrane Cav channels and RYR channels on the SR membrane. The CICR mechanism is a robust amplification system whereby the local influx of Ca²⁺ from small clusters of L-type Cav channels in the plasma membrane triggers the coordinated release of Ca²⁺ from the high-capacity Ca²⁺ stores of the SR. Such single CICR events can raise [Ca²⁺]_i to as high as 10 µM in microdomains of ~1 µm in diameter. These localized increases in [Ca²⁺]_i appear ascalcium sparks

N9-3 when they are monitored with a Ca²⁺-sensitive dye by confocal microscopy. If many L-type Ca²⁺ channels open simultaneously in ventricular myocytes, the spatial and temporal summation of many elementary Ca²⁺ sparks leads to a global increase in [Ca²⁺]_i. The time course of this global [Ca²⁺]_i increase in ventricular myocytes lasts longer than that of the action potential (compare blue and black curves in inset inFig. 22-11) because the RYR Ca²⁺-release channels remain open for a longer time than L-type Ca²⁺ channels.

Skeletal Muscle

Clara Franzini-Armstrong , Andrew G. Engel , in Muscle, 2012

Muscle Relaxation: Free SR and the Calcium Pump

All SR surfaces that do not face directly towards T-tubules/plasmalemma are part of the free SR. This includes the lateral surface of triads as well as all the "longitudinal SR", a highly convoluted system of membranes with a large surface area that completely envelops the myofibrils. The free SR surface is at least 10-fold larger that the jSR surface facing T-tubules.

In skeletal muscle calcium is very effectively cycled within the muscle fiber, while exchanges between the cytoplasm and the extracellular spaces are functionally significant, but relatively small in magnitude (see Chapter 57). Essentially all the calcium that is released returns, in time, to the lumen of the SR through the function of the calcium ATPase or calcium pump, which constitutes 90% or more of the intrinsic membrane proteins in the free SR membrane.

The density of calcium ATPase in the SR is equal to that achieved in planar crystalline arrays. In freeze-fracture images of the SR cytoplasmic leaflet (Figure 53.6) the ATPase forms an uninterrupted carpet covering all visible SR membranes. Small protein-free lipid patches are only visible in the SR of slow twitch fibers. The freeze-fracture particles shown in Figure 53.6 actually represent small variable-size clusters of 2–6 molecules as shown by comparison of fractured SR membranes showing the intramembrane particles and freeze-dried SR tubules showing the ATPase tails on the cytoplasmic surface of the SR. On the basis of this type of image, it is estimated that the density of ATPase in the free SR membrane is ~30,000/µm². Combining this information with the morphometric measurements of SR surface area per fiber volume, the ATPase content in the myofibrillar areas of the fast toadfish swimbladder muscle is estimated to be ~290,000/µm³, giving a ratio of ATPase to RyR of 1200–1800. Thus although calcium release at each twitch is not maximal in this sound-producing muscle, a large number of slow acting ATPase molecules are necessary to eventually mop up all the calcium that exits the SR via the highly conductive RyR channels.

A stunning series of crystal structures for the Ca-ATPase pump in nine different configurations of the pump cycle has been obtained (see 32,33 for reviews). From these images, a highly visual definition of the large molecular motions that transform the pumps' high calcium affinity cytoplasmic-facing site to a low affinity luminal through an occluded period has been derived, providing a compelling explanation of this complex pumping mechanism.

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Structural and Molecular Bases of Sarcoplasmic Reticulum Ion Channel Function

Bin Liu , ... Przemysław B. Radwański , in Cardiac Electrophysiology: From Cell to Bedside (Seventh Edition), 2018

Structural Arrangement of the Sarcoplasmic Reticulum

The sarcoplasmic reticulum (SR) is a membrane-delimited intracellular organelle that spans the sarcomere and wraps up the contractile myofilaments in striated muscle of almost all species. ¹ The SR is not continuous with the external membrane, but recent data indicate that it is continuous with the nuclear envelope. ² In striated muscle, the main function of the SR is to provide the majority of Ca²⁺ ions required to activate the myofilaments and to resequester Ca²⁺ from the myoplasm to allow for relaxation. The compartmentalization of muscle fibers into small (∼2 μm) structural–functional units (sarcomeres) and the enveloping of sarcomeres by SR both ensure that Ca²⁺ diffusion from and reuptake into the SR is not a limiting step for the muscle contractile cycle.

The SR is composed of two regions: junctional SR (jSR), which directly faces invaginations of the surface membrane, called transverse tubules (T-tubules), and extrajunctional free SR (fSR), which is situated near the myofibrils. jSR forms extended, flattened cisternae with an average diameter of around 0.6 μm. ³ Each cisterna carries sets of closely grouped structures ("feet") that represent the cardiac SR Ca²⁺ release channels, also known as ryanodine receptors (RyR2s), and contains dense material, which is formed by the Ca²⁺-binding protein, calsequestrin-2 (CASQ2). ⁴ On the other hand, the fSR is devoid of CASQ2, and its external surface exhibits densely distributed particles corresponding to Ca²⁺ adenosine triphosphatase (ATPase).

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Anatomic Considerations and Examination of Cardiovascular Specimens (Excluding Devices)

J.J. Maleszewski , ... J.P. Veinot , in Cardiovascular Pathology (Fourth Edition), 2016

Sarcoplasmic Reticulum

The sarcoplasmic reticulum is a complex network of specialized smooth endoplasmic reticulum that is important in transmitting the electrical impulse as well as in the storage of calcium ions. These longitudinal tubules form a membrane-bound system of tubules and cisterns that surround the myocytes. The sarcoplasmic reticulum is not as well developed in cardiac muscle as in skeletal muscle.

At the Z line, a T tubule, formed by an invagination of the sarcolemma, extends into the myocytes and makes contact with the sarcoplasmic reticulum. The area is termed a diad. The T tubules spread a received electrical impulse through the myofiber, whereas, the sarcoplasmic reticulum releases calcium ions for excitation-contraction coupling. This overall configuration of the sarcoplasmic reticulum is similar to that of skeletal muscle, except it has a less well-developed organization and the T tubules are larger in the heart [161].

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DESIGN AND PHYSIOLOGY OF THE HEART | Cellular Ultrastructure of Cardiac Cells in Fishes

G.L.J. Galli , in Encyclopedia of Fish Physiology, 2011

The Sarcoplasmic Reticulum

The sarcoplasmic reticulum (SR) is a specialized form of endoplasmic reticulum (ER) found in most cells, which forms a system of intracellular membranes. The SR is separate from the sarcolemma and T-tubules. Unlike the ER, which is primarily involved in protein synthesis, the SR lacks ribosomes (the machinery required to make proteins) and is primarily involved in controlling Ca ²⁺ concentrations inside the cardiomyocyte. There are two proteins embedded in the SR membrane that permit this function – a Ca²⁺ pump (termed the SR Ca²⁺ ATPase, or SERCA) and a Ca²⁺-release channels (termed ryanodine receptors, or RyRs). Ca²⁺ is stored in the SR, moving into the cytoplasm via the RyR and being removed from the cytoplasm by SERCA. Therefore, Ca²⁺ is moved back and forth from the inside of the cell to the SR as part of E–C coupling.

There are two types of SR in fish cardiomyocytes: junctional SR (jSR) and free SR (fSR) (see Figure 6 ). jSR forms peripheral couplings with either the sarcolemmal or t-tubular network ( Figure 6 (a), double arrows). In these peripheral couplings where the jSR profile aligns with the sarcolemma or t-tubule, rows of proteins known as 'feet' are visible between the two membranes ( Figure 6 (b), inset). These proteins are the Ca²⁺-release ryanodine receptor channels. fSR is not closely associated with any other membrane system, and can be found throughout the cardiomyocyte ( Figure 6 (a), arrows).

Quantitative measurements of SR volume (relative to total volume) have only been made for two fish species: the perch (Perca fluviatilis; 4.5–6.5%) and the T. orientalis (2–3%). In mammals, SR density is on average much greater, but ranges between 1% and 12%. The highly active T. orientalis possesses both fSR and jSR that contribute significantly to overall Ca²⁺ regulation. In this species, atrial cardiomyocytes have a greater proportion of SR (3.2%) in comparision to ventricular cardiomyocytes (2.2%). Lastly, fish species that inhabit cold environments and those that acclimate to the cold seasonally enhance SR density. The warm-acclimated P. fluviatilis, for example, increase SR density from 4.5% to 6.5% when acclimated to the cold. Presumably, this adaptation allows Ca²⁺ levels within the cell to remain normal in the cold, which would usually slow down the rates of protein and ion channel activities.

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Immunologicalization and Structural Configuration of Membrane and Cytoskeletal Proteins Involved in Excitation-Contraction Coupling of Cardiac Muscle

Joy S. Frank , Alan Garfinkel , in The Myocardium (Second Edition), 1997

B CORBULAR SARCOPLASMIC RETICULUM

The corbular SR is the mammalian homologue of the extended jSR first described in birds (Jewett et al., 1971). It is now well recognized that cardiac myocytes contain, in addition to jSR, vesicular SR with all the anatomic features of jSR including the feet/CRCs, but their location in the cell is removed from either the peripheral or the T tubular SL (Fig. 5) . Typically the corbular SR has a full array of feet that have been demonstrated in rabbit atrial cells to contain the CRC/RR and the protein triadin (Jorgensen et al., 1993). In 3 − 4-day-old neonatal rabbit myocytes, which do not develop T tubules until ~ 12 days of age, the CRC/RRs are localized within the cytoplasm in transverse bands at the Z lines (Fig. 6). In the developing mammalian myocytes, while some of the CRC/RRs are in contact with the SL in the form of peripheral couplings, most of the CRC/RRs appear to be without a structural association to the SL and are functioning as extended jSR. As the cell ages and the T tubules develop, this relationship changes, and most of the CRC/RR will form junctions with the T tubules. The lumen of the corbular SR stores Ca^{2 +}, which if released could be a significant percentage of the contractile calcium (see Chapter 5). Evaluation of the relative amount of calsequestrin within the lumen of jSR and corbular SR suggested that there could be as much as 40% of the SR calcium in the corbular SR in papillary muscle (Jorgensen et al., 1985). This represents a significant amount of the total Ca^{2 +} storage and release sites in mammalian cardiac cells. Sommer (Jewett and Sommer, 1971; Sommer and Waugh, 1976) first proposed that the corbular/extended jSR played a significant role in E−C coupling, and the new structural and functional data suggest that any model of E−C coupling in mammalian hearts must factor in a role for corbular SR. This is especially true in the developing cells before the presence of the T tubular system.

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Where Are Calcium Ions Stored Within the Fiber

Source: https://www.sciencedirect.com/topics/medicine-and-dentistry/sarcoplasmic-reticulum

Where Are Calcium Ions Stored Within the Fiber

Sarcoplasmic reticulum

Mechanisms of Cardiac Contraction and Relaxation

Calcium Uptake into Sarcoplasmic Reticulum by SERCA