Cardiac Muscle Excitation Contraction Mediated From Sarcoplasmic Reticulum Biology Essay
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In cardiac muscle, excitation-contraction coupling is mediated by calcium-induced calcium release from the sarcoplasmic reticulum through ryanodine receptors that are activated by calcium entry through L-type calcium channels on the sarcolemmal membrane. Although Ca2+ induced Ca2+ release triggered by the L-typed calcium current is the primary pathway for triggering Ca2+ from the sarcoplasmic reticulum, there are many other mechanisms that can also activate Ca2 + release from the sarcoplasmic reticulum such as Calcium induced calcium release (CICR) induced by T-typed calcium current, CICR triggered by calcium influx through Na+/Ca2+ exchange, and CICR mediated by calcium through tetrodotoxin (TTX)-sensitive Ca2+ current
(ICa,TTX). As calcium is an important second messenger which is essential in regulating cardiac electrical activity as well as being the main activator of the myofilaments to which cause cardiac contraction. Mishandling of calcium is thought to lead many pathophysiological conditions. Knowledge of the mechanisms involved in regulating intracellular calcium and therefore contraction of the heart, may help to prevent and/or treat pathological conditions such as cardiac hypertrophy, arrhythmias or heart failure by using therapeutic agents targeted at modulating intracellular calcium.
LIST OF FIGURES
Figure 1: Calcium transport in ventricular myocytes 3
Figure 2: Six possible mechanism of cardiac excitation-contraction coupling 9
LIST OF ABBREVIATIONS
LTCC = L-type calcium channels
CICR = Calcium induced calcium release
ECC = Excitation-contraction coupling
NCX = Sodium-Calcium Exchange
SR = Sarcoplasmic Recticulum
ICa = Calcium current
ICa,T = T-type calcium current
ICa,L = L-type calcium current
ICa,TTX = Tetrodotoxin-sensitive calcium current
RyRs = Ryanodine Receptor
[Ca2+]i = Intracellular calcium concentration
[Ca2+]Tot = Total concentration of Calcium
PKA = Protein Kinase A
LVH = Left Ventricular Hypertrophy
HOCM = Hypertrophic obstructive cardiomyopathy
In heart muscle cell, the depolarization of action potential is due to the entering of Na+ ions via voltage gated Na+ channels and it is called fast inward current. The immediate repolarization is not possible due to rapidly inactivation of Na+ channel and initial depolarization allow the entering of calcium through voltage-grated Ca2+ channels and it is called second or the slow inward current. The rate of sodium channels inactivation is more rapid than that of calcium channels so that Ca2+ enters into the cell providing the membrane potential to close to 0mV for some part of action potential of heart muscle (Reuter, 1984).
Excitation-contraction coupling (ECC) is the process in which an action potential triggers a myocyte to contract. In excitable muscle cells, the excitation signal causes rapid depolarization that produces the physiological response of contraction. Calcium is a ubiquitous second messenger, important in both, regulating the electrical activity of the heart as well as stimulating the myofilaments directly to cause contraction (Bers, 2001). In mammalian cardiac myocytes, the process of ECC is mediated by Ca2+ influx from the extracellular space that triggers Ca2+ Calcium - induced Calcium release (CICR) from the sarcoplasmic reticulum (SR) (Bers, 1991; Stern & Lakatta, 1992).
When action potential reaches the myocyte, causing it to undergo depolarization, which causes calcium ions to enter the cell through L type calcium channel located on the sarcolemma and thereby trigger calcium release from the SR. Calcium influx and the intracellular calcium concentration trigger the contraction of heart due to binding of Ca2+ to cardiac muscle fiber protein, troponin C. For activation of SR calcium release, the L-type calcium current is the most widely accepted mechanism thought to be responsible for CICR. However, SR calcium release can also be triggered by calcium influx through sodium-calcium exchange, calcium influx via T-type Ca2+ current or through tetrodotoxin-sensitive Ca2+ current, or Inositol (1,4,5)-triphosphate (but not so much in cardiac muscle). Declining of calcium level in the cells cause the detachment of calcium from myofilament and resulting in relaxation of the heart. There are four main pathways for Ca2+ transport out of the cytosol including SR Ca2+ ATPase, sarcolemmal Ca2+-ATPase or mitochondrial Ca2+ uniport and sarcolemmal Na+/Ca2+ exchange. (Bers, 2002).
Since CICR is a positive-feedback mechanism, it has to be terminated which is essential for diastolic refilling of the heart. There are three main pathways for termination of calcium release such as local depletion of SR Ca2+, Ryanodine ( RyR) inactivation (or adaptation), and stochastic attrition. (Lukyanenko et al., 1998). Mutation in calcium channels can lead to life-threatening arrhythmias. The improper contractile function and abnormal heart rate associated with cardiac hypertrophy and heart failure is due to the mishandling of calcium in heart muscle cell (Pogwizd et al., 2001). In this dissertation, here I discuss about the key mechanism of how Ca2+ transport in cardiac ventricular myocytes. Moreover, I also discuss about how they are modulated and regulated as well as how they interact specifically. In addition, by knowing the subcellular mechanism of E-C coupling, here I discuss about how calcium is altering and getting mutated so as to cause cardiovascular diseases. The important molecular signaling pathways in contraction of heart will also be addressed.
Figure 1. Calcium transport in ventricular myocytes. (Adapted from Bers, 2002) The figure shows the time course of an action potential, Ca2+ transient and contraction in rat ventricular myocytes, NCX, and other protein involved in contraction.
Calcium channels in contraction of the heart
In cardiac muscle, calcium has a role for the ability to make the cardiac cell to contract. There are five types of calcium channels; L, T, N, P/Q and R types. Among them, L-type and T-type calcium channels are two major types of calcium channels in the cells of cardiac tissues (Bean, 1989). L-type Ca2+ channels have many subunits in the heart such as Î±1, Î±2, Î´ and Î² subunits. The Î±1 subunit is the dihydropyridine (DHP) receptors which are important for calcium entry into the cells (Liu et al., 2000). L-type calcium channels (long-lasting) can activates at more positive membrane potential (Em), at greater than -40mV and generate peak inward current at 0mV and slowly inactivated, and is sensitive to dihydropyridines (Tsien et al., 1987). Thus, the L-type Ca2+ channels are the majority of calcium channels responsible for entering of Ca2+ into the cardiac cell during phase 2 (plateau phase) of the action potential. On the other hand, T-type (tiny or transient) Ca2+ channels cause the activation and inactivation at more negative membrane potential (Em) and dihydropyridines cannot block effectively (Nowycky et al., 1985). However T-type Ca2+ channels have faster kinetics than compared to L-type Ca2+ channels. During development and hypertrophy, T - type calcium current is more prominent and the T-type current is typically small or absent in ventricular myocytes. The entering of Ca2+ into the cell by passing through I Ca,T is only responsible for smaller amount of Ca2+ than that passing through ICa,L. In most ventricular myocytes, T-type calcium current is almost negligible. It shows that the releasing and refilling is mainly provided by Ica,L. The amount of L-type calcium current and T-type calcium current is variable among cardiac myocytes. L-types calcium current is present in all cardiac myocytes whereas T-type calcium current have larger component in the canine Purkinje fiber (Zhou, 1998). Depolarization during the action potential causes activation of calcium current. During an action potential, the amount of calcium entry is limited by calcium dependent inactivation at the cytosolic side. L-type calcium channel is located at the sarcolemmal-SR junction where ryanodine receptors exist (Scriven et al., 2000). There is a negative feedback effect on Ca2+ influx and SR Ca2+ release during excitation-contraction mechanism. When there is increased Ca2+ influx or release, further release of Ca2+ is turned off.
There are many isoforms of ryanodine receptors, (RyR1, RyR2, RyR3), among them, RYR2 is the cardiac isoform. RyR2 mediated release of Ca2+ from sarcoplasmic recticulum is an important step in cardiac E-C coupling in the heart. RyR2 is a Ca2+-gated channel (Nabauer et al, 1989). RyR2 is activated by Ca2+ influx through L-type Ca2+ channel or dihydropyridine receptor (Adachi-Akahane, 1996). Cytosolic Ca2+ is increased by the RyR2 opening and bind with contractile protein (troponin C) that trigger the contraction of heart. In ventricular myocytes, there are much more ryanodine receptors than dihydropyridine receptors. Therefore, four or ten RyRs can be associated with a single L-type Ca2+ channel (Bers et al., 1991). Defection in excitation- contraction coupling can occur due to either if RyRs channels sensitivity is altered for activation/inactivation or if the SR Ca2+ is depleted. There has been demonstrated in animal model of cardiomyopathy (Gomez et al, 1997).
[Ca2+]i and Ca2+sparks
[Ca2+]i and total [Ca2+] determine the development of contraction which produces both isometric force and rapid shortening (Moss, 2001). The strength of cardiac contraction can be changed by two ways: (1) by changing the extent and amplitude of the Ca2+ transient, (2) by altering the myofilament sensitivity to Ca2+. The sensitivity of myofilament calcium is increased by contracting the myofilament when the heart fills with blood resulting the contraction to be stronger. Caffeine and certain inotropic agents can enhance the myofilament sensitivity whereas the increased concentrations of phosphate and Mg2+ and acidosis reduce myofilament Ca2+ sensitivity. Ca2+ sparks is the process of spontaneous release of SR Ca2+ and it was described by using confocal fluorescence microscopy (Cheng et al., 1993). The release of SR Ca2+ via single L-type Ca2+ channel or RyRs openings generates Ca2+ sparks (Song et al., 1997). Ca2+ spark is activated by the Ca2+ entery through ICa (Cannell et al, 1995). Ca2+ spark is triggered by the opening of single channel opening. There have been reported that spark probability can be depend on binding of two Ca2+ ions to the RyR (Santana et al., 1996). Thus, local cytosolic [Ca2+]i is important in the frequency of Ca2+ sparks and SR Ca2+ release. Moreover, the frequency of Ca2+ spark depends on the SR Ca2+ load (Cheng et al, 1993). When there is increased SR Ca2+ load, this may lead to increase the amplitude of Ca2+ spark. Therefore SR Ca2+ load is an important factor for Ca2+ release from SR.
Role of Sarcoplamic Recticulum
ECC and intracellular Ca2+ homeostasis are primarily regulated by sarcoplasmic recticulum (Bers, 1991). Once stimulation, calcium enters the cell, thereby stimulating the release of larger amount of calcium from SR resulting in activation of contractile protein and contraction of the heart. During cardiac relaxation, Ca2+ is taken up by SR by SR Ca2+ ATPase pump and Na+/Ca2+ exchange pump. The key SR Ca2+ release channel involved in cardiac contraction is RyRs and RyR2 is the cardiac isoform. The amount and fraction of Ca2+ release that depends on the level of SR Ca2+ load can release for a given ICa trigger (Shannon et al., 2000). Sensitivity of RyRs receptor to [Ca2+]i at high load of SR Ca2+ leads to increase spontaneous SR Ca2+ release. On the other hand, decrease in SR Ca2+ release (which is induced by ICa ) can be due to low SR[Ca2+] content. The lower the amount of the SR Ca2+ release, the more amount of Ca2+ enter the cells through Na+/Ca2+ exchange. When there is low concentration in SR Ca2+, Ca2+ release from SR is turned off during E-C coupling. Furthermore, SR Ca2+ content depends on the heart rate and duration of action potential. Ca2+ concentration release from SR can be increased by more mount of Ca2+ enter into the cell, by decreasing Ca2+ efflux or increasing SR Ca2+ uptake. Phospholamben, an endogenous inhibitor of SR Ca2+ ATPase, is triggered by activation of cAMP-dependent or calmodulin-dependent protein kinase. When this phospholamben becomes phosphorylated, Ca2+ uptake by SR is increased and allows faster cardiac relaxation and declining of [Ca2+]i. Targeted knockout of phospholamben leads to hyperdynamic hearts with negative effects (Brittsan & Kranias, 2000). Interestingly, lower SR Ca2+ uptake, reduced SR Ca2+ATPase gene and protein expression were seen in failing human heart (Pieske et al., 1995). On the other hand, there has been demonstrated that increased gene expression of sarcolemmal Na+/Ca2+ exchanger was seen in human failing heart (Reinecke et al., 1996).
Regulation of Calcium current
Ica can be variable physiologically and pharmacologically. During physiological sympathetic stimulation of heart, catecholamine stimulate beta-adrenegic receptors, which improve the force of contraction (inotropic effects) and relaxation (lusitorpic effects) and declining of [Ca2+]i. In addition, stimulation of Î²-adrenergic receptor stimulates a GTP-binding protein that accelerates adenylyl cyclase for the cAMP production. cAMP activates PKA, which phospharylates severe protein such as phospholamban, RyR, L-types Ca2+ channels, myocin binding protein C and troponin I ( which are related to ECC). Activation and phosphorylation of L-type Ca2+ channels will cause Ca2+ release from SR causing contraction of the heart. Phosphorylation of troponin I and phospholamban stimulate the reuptake of Ca2+ release from SR and Ca2+ is dissociated from the myofilament and develops to cardiac relaxation (Lusitropic effect). The inotrophic effect of PKA (protein kinase A) activation is triggered by the combination greater availability of SR Ca2+ and increased calcium current. Open probability of RyR channels can also be modulated by protein kinase A. RyRs receptors are hyperphosphorylated in heart failure causing a diastolic leak of SR Ca2+. However, whether PKA-dependent phosphorylation will alter during excitation-contraction or not still remain controversial. Moreover, phosphorylation of L-type Ca2+ channels, phospholamban and troponin I are paralleled with activation of ß1-adrenergic receptors in ventricular myocytes that produce inotrophic and lusitropic effects. On the other hand, ß2-adrenergic receptors activation can give more restricted to the enhancement of ICa (Kushel et al., 1999). cAMP production can also be stimulated by the G-protein-coupled receptors such as prostaglandin E and histamine that will lead to little or no effect of inotropic effects (Vila Petroff et al, 2001). Other receptors will also regulate the signaling pathway. For instance, M2-muscarinic receptors activation can decrease cAMP and activation of PKA thereby decreasing Ca2+ entry and release. In addition, this pathway also enhances repolarization. The pharmacological effects of L-type Ca2+ channels are in which calcium sensitivity to dihydropyridines (nephedipine, amlodipine, nitrendine, nimodipine, nisoldipine). Ica is inhibited by most of DHPs and they are called Ca2+-channel blockers. In DHPs, there are two other types of specific L-type Ca2+ channel blockers (1) phenyalkylamines (eg. verapamil, D600) and (2) benzothiazepines (eg, diltiazem), and those agents can act together directly with the Ca2+ channel (Glossmann et al., 1985). Verapamil can inhibit the calcium channel in the open state but it require depolarization pulse) and this is called use dependent. The neutral ligands such as nitrendipine and nisoldipine inhibit ICa depend on the calcium channel whether they are in the opening state or inactivated state , and does not require depolarization pulse as they are voltage dependent than use dependent.
Figure 2. Six possible mechanism of cardiac excitation-contraction coupling. (Adapted from Bers, 1999)
The figure shows Ca2+ influx via ICa,L, Ca2+ influx via ICa,T,Ca2+influx through NCX, Ca2+ influx via IP3 ,Ca2+ influx via ICa,TTX and depolarization dependent Ca2+ influx.
Calcium induced calcium release during E-C coupling
There have been demonstrated that CICR in skinned ventricular myocytes (Fabiato and Fabiato, 1975). There was been proved that main pathway of E-C coupling in cardiac myocytes is by Ca entry through L-type Ca2+ channels and triggers SR Ca2+ release (Bers, 1991). When calcium channel becomes deactivates, before calcium channels close, calcium transient is induced by a large and short-lived ICa causing contraction. Moreover, Ca2+ channel activation in the absence of Ca2+ influx also cannot induce calcium release from the SR (Nabauer et al., 1989). There is supported that ICa activate SR Ca2+ release channel when there is a high concentration of Ca2+ buffer in the cell (Adachi-Akahane et al., 1996). Ca2+ release from SR is most commonly activated by L-type Ca2+ channels and this pathway is called Ca2+ induced Ca2+ release (CICR). There has been little doubt that E-C coupling occurs physiologically but there are other mechanisms which can exit in parallel and give rise to the functional effects.
Ca influx via ICa,T
In ventricular myocytes, T-type calcium channels is relatively small or absent but it is more prominent in the development and hypertrophy of the heart. Because of T-type calcium current is relatively small and rapidly inactivated, the total amount of calcium influx through T-type calcium current is absolutely small compared to calcium influx via ICa,L (Zhou, 1998). Moreover, T-type calcium current is negligible in most of ventricular myocytes. T-type Ca2+channels are not located at the SR junction, therefore the effectiveness of ICa,T as a trigger for Ca2+ release from SR is not effective as ICa,L. Furthermore, SR Ca2+ release by ICa,T is delayed on onset and slower than ICa,L. However, it can be significant in other cardiac cells such as some atrial cells and Purkinje fibers (Zhou and January, 1998). Since T-type calcium channel is non-functional in most of the myocytes of ventricle, it does not play a major role for ECC although it may function like ICa,L. So, ICa,T only plays a minor role in triggering Ca2+ release from SR during action potential.
Ca influx via Na+/Ca2+ exchange
Although L-type Ca2+ current is a major role of CICR in contraction of the heart, some argued that the L-type Ca2+ channels could not be the only way to trigger the calcium release from SR. There is an alternative trigger of calcium release in mammalian cardiac myocytes (Chunlei Han et al., 2002). The result of Ca2+ release by Na+/Ca2+ exchanger has been proved by examination on rats (Wasserstorm and Vites, 1996), rabbit (Litwin et al., 1998) and guinea pig (Sipida et al., 1997). Immunofluorescence labeling shows that the exchanger current is present in the cardiac T-tubules system (Scriven et al, 2000). There are two ways of triggering Ca2+ release from SR by Na+/ Ca2+ exchanger. The first mechanism is Na+ current by increasing local [Na+]sm, increasing Ca2+ entry through Na+/Ca2+ exchanger and causing SR Ca2+ release (Levesque et al.,1994 ). The second one is that depolarization directly stimulates outward INa/Ca and Ca2+ release and contraction when L-type Ca2+ channel become blocked or at high positive Em (Levi et al.,1994 and Litwin et al.,1998 ). Increased intracellular sodium stimulate the Na+/Ca+ exchanger (Evans and Cannell, 1997 ) and, if INa is low ([Na+]i=10nM) or lower, the reverse current of the Na+/Ca2+ exchange could trigger Ca2+ release account for 25%. When [Na+]i=30nM, the contribution of Na+/Ca2+ exchanger increase up to 100%. Additionally, the exchanger current is more dependent on the temperature and changes in the intracellular sodium and calcium concentrations than compared to L-type calcium current. Furthermore, these changes are larger in the microdomain or subspace (interaction between RyR receptors and L-type Ca2+ channels occurs) than compared to the rest of the cytoplasm (Vornanen et al., 1994). Although LTCC are faster than the exchanger current in triggering of Ca2+ release from SR, Ca2+ entry through the exchanger into the subspace is faster in beginning than L-type Ca2+ current when there is action potential stimulations because the action potentials upstroke and sodium (inward) current is associated with rapid increase in [Na+]i. Therefore, any physiological stimulation or medication that alters the intracellular sodium becomes the regulator of calcium release from sarcoplasmic reticulum. Stimulation via hormone, such as activation of endothelin-1 (ET-1) receptor (Alvarez et al., 1999), and increasing frequency of action potential (Simor et al., 1997) will increase the intracellular sodium concentration, causing calcium release triggering via the Na+/Ca2+ exchanger by opposing to via the L-type calcium current. Ca2+ release from SR is slower via Ca2+ influx through Na+/Ca2+ exchanger than through L-type calcium channel (Spido et al., 1997) .
Ca influx via TTX sensitive-Na channels
Aggarwal and co-worker reported voltage-gated, calcium conducting sodium channel, (ICa,TTX), calcium entry via tetrodotoxin-sentive Na+ channels can also mediates CICR. This channel activates at membrane potential of -60mV and has faster kinetics than L-type Ca2+ channels. It can alter selectivity of cardiac Na+ channels triggers by either activation of agonist effects Î²-adrenergic receptor or cardioactive steroids or cardiac glycosides, resulting Na+ channel prefer Ca2+ than Na channels and it is called slip mode or altered selectivity mode. The tetrodotoxin-sensitive Ca2+ influx can also trigger the SR Ca2+ release. The inotrophic effects of cardiac glycosides and Î²-adrenergic agonists could be a novel mechanism. These effects could be triggered by SR Ca2+-pump activity and increased ICa or by Na+/K+ ATPase inhibition and also decreased Ca2+ efflux through Na+/Ca2+ exchange for cardiotonic glycosides (Borgatta et al., 1991). Moreover, one study in rat ventricular myocytes reported that Na+ current is activated by the phyosphoryation by protein kinase A or by the cardiotonic steroids. (slip mode conductance) (Santana, 1998). In addition, modified Na+currents conduct ICa,TTX which in turn triggers CICR. The relation between slip mode conductance and ICa,TTX is still controversial (Nuss, 1999). On the other hand, another study demonstrarted that Ca2+ current due to ICa,TTX or slip mode conductance is not related and identical. The reason is that the presence of cardiac steroids or activation of PKA is not a requirement for the detection of ICa(TTX). A small fraction of Na + currents can conduct ICa(TTX) even without phosphorylation of PKA (Nuss, 1999). Furthermore, TTX sensitive-Na+ channels cannot be inhibited by the blockers of T-type or L-type Ca2+ channels. Recently, one of studies showed that ICa(TTX) and T-type Ca2+channel coexit in guinea pig venricular myocytes because 10 mM mibefradil could block both ICa(TTX) and T-type Ca2+current (Heubach, 2000). Although this current is not the major current for triggering the CICR, its possible functional roles are important in normal heart cells such as promoting the sodium current activation and modulating rhythmicity of the heart.
Ca influx via IP3 pathway
Inositol (1, 4, 5) - triphosphate could trigger Ca2+ release from SR and endoplamic reticulum in different cell types, they are called IP3 receptors. In ventricular myocytes, the major form of InsP3 is isoform 2 (Lipp et al., 2000). There are more InP3 receptors in atrial cells in ventricular myoctyes. Stimulation of IP3 signal transduction pathway can trigger the release of Ca2+ from SR via IP3 receptors which is located on SR. Even high concentration of InP3 in cardiac myocytes could trigger Ca2+ release from the SR, the extent of Ca2+ release from the SR are so much lower than CICR triggered by LTCC. Moreover, action potential cannot stimulate the InP3 production (Kentish et al., 1990). The production of InP3 contractile force is increased by cardiac alpha-adrenergic and muscarinic agonists (Poggioli et al., 1986). In addition, InP3 pathway only plays a very little minor role in cardiac EC coupling. To conclude for triggering Ca2+ release from SR, CICR in cardiac contraction is mainly through L-type Ca2+channel.Other mechanisms that mentioned above show minor role in SR calcium release.
During an action potential, calcium entry into the cell is slow at the end of phase 2 and there is lowering of the cytosolic calcium concentration because calcium is taken back by the SR and removing of calcium from the troponin C and finally initial sarcomere length is restored. For relaxation and cardiac ventricular filling, Ca2+ have be removed from the cytosol to lower [Ca2+]i , causing relaxation. Cardiac relaxation to occur, Ca2+ must be dissociate from troponin C and it requires Ca2+ transport out of the cytosol primarily by four main pathways involving, sarcolemmal Na+/Ca2+ exchange, SR Ca2+-ATPase, sarcolemmal Ca2+-ATPase or mitochondrial Ca2+ uniport. There are selective inhibition for each transporter during cardiac myocyte relaxation and [Ca2+]i decline (Puglisi et al., 1996). SR Ca2+ uptake can be prevented by either thapsigargin or caffeine, complete removal of extracellular Na+ and Ca2+ can prevent sodium calcium exchange. Either carboxyeosin or elevated [Ca2+]i inhibit sarcolemmel Ca2+-ATPase, and mitochondrial Ca2+ uptake can be inhibited by rapid dissipation of the electrochemical driving force for SR Ca2+ uptake by using protonophore FCCP. In rabbit ventricular myocytes, 70% of the activated Ca2+ removed by the SR Ca2+-ATPase from the cytosol, whereas 28% was removed by NCX, only 1% for sarcolemmal Ca2+-ATPase as well as mitochondrial Ca2+ uniporter remove 1% of calcium from SR ( the last two pathways are called slow systems). In rat ventricular myocytes, SR Ca2+-ATPase activity is higher due to more pump molecules in unit cell volume (Hove-Madsen & Bers, 1993). On the other hand, Ca2+ removal via Na+/Ca2+ exchange is lower, 92% with SR Ca2+-ATPase, 7% with NCX, the slow systems with 1 % respectively. In mouse ventricular myocytes, the uptake mechanism is quite similar to rat, (Li et al., 1998) while the mechanisms of Ca2+ fluxes in human ventricular myocytes, guinea pig and ferret are more similar to rabbit myocytes (Pieske et al., 1999). In contraction and relaxation of myocytes, the amount of calcium removed from the cell during relaxation must be the same as the amount of calcium entry during contraction in each heart beat, if not, the cell may gain or lose the calcium. Defects in Ca2+ removal also can cause impair relaxation
Termination of calcium release
Although CICR is a positive-feedback mechanism, termination or turning off of the Ca2+is important for diastolic refilling of the heart. There are three major ways for terminations of Ca2+release include local SR depletion, RyR inactivation or adaptation and stochastic attriction (Sham et al.,1998; Lukyanenko & Gyorke,1998). Stochastic attriction means L-type Ca2+ channels and all RyRs are closed simultaneously, then local [Ca2+]i will drop quickly to the sub-threshold level and disturbing the release from SR . However, this is only used for 1DHPR and 1-2 RyRs whereas they all will not close at once for other types of channels. In addition, local depletion of SR Ca2+ also may terminate SR Ca2+ but it cannot completely turn-off of release, because very long lasting Ca2+ sparks are found that will not decline with time (Satoh & Bers, 1997). However other regions of SR can also limit local SR Ca2+ depletion. During a global Ca2+ transient, the whole SR Ca2+ declines. During a relaxation, SR Ca2+ depletion could lead to the turning -off global SR Ca2+ release. There are two types of RyR inactivation both of which depend on [ Ca2+ ]i .One of them is absorbing inactivation ( for example like Na+ channels), in which the ryanodine receptor cannot reopen until it recovers (Sham et al., 1998; Lukyanenko & Gyorke, 1998). The another one is called RyR adaptation in which ryanodine after activation leads to a reduced open probability, but it can be reactivated by higher [Ca2+]i (Valdivia et al.,1995). RyRs inactivation could be important in reducing SR Ca2+ release events between each heart beats. To summarize, Ca2+ release during ECC is terminated mainly by a local RyRs inactivation and partial SR luminal Ca2+ depletion which leads to reduce RyR openings and variant of stochastic attrition also contributes.
Role of calcium channels in cardiac hypertrophy, heart failure and arrhythmia
Intracellular calcium is the major regulator of cardiac contraction. Therefore, altered cardiomyocyte regulation is important in arrhythmogenesis, cardiac mechanical dysfunction and cardiac hypertrophy associated with heart failure. Alteration in signal transduction pathways can also lead to loss of inotropic effects in heart failure. Defects in ECC have been reported in animal models of cardiomyopathy (Gomez et al., 1997). There is no E-C coupling depression was seen in pressure overload of cardiac hypertrophy with less sign of heart failure. (Rios et al., 1992). Cardiac hypertrophy is the enlargement and thickening of the heart muscle resulting in decreasing size of the chamber of the heart. Cardiac hypertrophy is the main cause of cardiac morbidity and mortality in cardiovascular system. It is associated with heart failure without myocardial infarction. Cardiac hypertrophy is associated with significant changes in myocardial contraction. These contractile dysfunctions are followed by changing in the whole-cell intracellular calcium transient. The pathogenesis and etiology of cardiac hypertrophy and heart failure related with the role of Ca2+ channels still remains controversial. of Î² subunits of L-type Ca2+ channels (LTCCÎ²) enhances the probability of channels opening as well as also favours the trafficking of the Ca2+ channels to the surface membrane leading to increase calcium current (Chen Y.H, 2004). Interestingly, there has been reported upregulation of LTCCÎ² in failing human cardiomyocytes (Hullin et al, 2003). In aortic banding, L-type Ca2+ channels concentration is remain unchange in rats myocytes with hypertrophy ( Scamps et al.,1990), cats with pulmonary artery banding (Kleinman, 1988) cardiomyopathy in Syrian hamsters (Sen, 1994) ,and ventricular myocytes in human with heart failure patients (Beuckelmann et al., 1992). On the other hand, L-type Ca2+ channel concentration is increased in guinea pigs hypertrophic myocytes with aorta banding( Ryder, 1993), and renal artery banding in rats (Keung, 1989),while it decreased in cats ventricular myocytes with aortic banding (Nuss, 1993). Additionally, the mechanism of cardiac hypertrophy on L-type Ca2+ currents also depends on the duration of the disease. There is also increased in dihydropyridine binding sites in hamster's myocardium with hereditary cardiomyopathy. Then, there is decreasing in binding sites in rat myocardium (Dixon, 1990). Surprisingly, there is no change in human heart with heart failure. The SR Ca2+ release in E-C coupling is not only determined by the amount of Ca2+ enter into the cell via the L-type Ca2+ channels but also depend on the channel properties. Therefore, the release of SR Ca depend on L-type Ca2+ channel properties can trigger alteration in hypertrophy, causing the abnormalities in contraction. This evident is supported by the observation that L-type Ca2+ current in all animal models of myocytes in cardiac hypertrophy and failure (Xiao, 1994) although no increase in duration was observed in humans ventricular myocytes with heart failure (Beuckelmann, 1991) . Moreover, Ca2+ could enter the cell via ion channels and transporters, and could also lead to altered E-C coupling causing the cardiomyopathy of the heart. Increases expression of T-type Ca2+ channels have been demonstrated in hypertrophied cardiac myocytes in rats with growth hormone secreting tumors (Xu, 1990). Surprisingly, a selective T-type Ca2+ channels blocker, mibefradil, increases survival in heart failure with coronary ligation in rat (Mulder P, 1997). There is no experiment reported with the role of the Na+/Ca2+ exchanger or ICa,TTX in E-C coupling associated with cardiac hypertrophy or heart failure. Furthermore, mis-sense mutation in L-type Ca2+ channels Cav1.2 cause long QT syndrome in patients with Timothy syndrome (Buraei et al., 2007). In Timothy syndrome, by using Ca2+ imaging studies and electrophysiological studies showed that there is prolonged action potential, excess Ca2+ influx, irregular cardiac contraction, irregular electrical activity of the heart and abnormal calcium concentration in ventricular myocytes (Masayuki Yazawa, 2011).
In normal ECC, calcium influx through the L-type Ca2+ channel plays an important role and abnormal calcium handling has recently been reported in heart disease (Bers, 2002). The accessory subunit of Î²-subunit plays a significant role in modulation of calcium current (Eugenio Cingolani et al., 2007). So, LTCC becomes significant for implications of therapy in treatment of left ventricular hypertrophy (LVH) by treating with calcium channel blockers, which has been reported in animal models (Feron et al, 1996). In clinical practice, calcium channel blockers reduce blood pressure causing regression of LVH but they have not been significant for prolonged survival. In the formation of LVH, signals regulated by calcium play in a significant role (Hill, 2000). If calcium-regulated signaling pathways are inhibited, LVH due to pressure overload can be diminished without interfering the function of the systole. A partial reduction or down regulation of LTCCÎ²expression that is sufficient to prevent the activation of calcium regulated signaling pathways and prevents the development of LVH, without impairing normal excitation contraction coupling. Furthermore, there has been demonstrated that the modulation of the expression of LTCC by RNA interference followed by lentiviral vector-based shRNA expression result in reduction of ICa,L and attenuating of cardiac hypertrophic response in vitro and vivo. Moreover, it can also preserved cardiac systolic function (Eugenio angolani, 2007). Regulation of the LTCC expression has become a significant treatment for LVH and other cardiac diseases associated with calcium abnormalities such as hypertrophic obstructive cardiomyopathy (HOCM). HOCM is treated with calcium channel blockers, surgical treatment in some of the case, and also by nonsurgical septal reduction techniques (Chang et al., 2003) but they have adverse effects such as inflammation, fibrosis as well as arrhythmogenesis. Focally regulated LTCC by a vector which can reduce gene expression is also the alternative treatment for HOCM. LTCC as a potentially novel therapeutic target for calcium mishandling with associated diverse cardiac disease. The role of RNA interference in modulating the expression of LTCC is regulation of calcium influx and prevention of LVH. In addition, further more studies are required to evaluate antihypertrophic effects of LTCCÎ².Timothy syndrome is a rare autosomal dominant disorder caused by the mis-sense mutation of Ca2+ channels affecting neurological and developmental defects including long QT syndrome, a life-threatening arrhymias and heart defects and autism. (Curtis and Richard, 2007). Roscovitine is a 2, 6, 9-trisustituted purine, also known as a selective blocker of cyclin dependent kinase (Meijer L, Raymond E., 2003). In addition, it is also using currently as an anticancer drug for phase II clinical trials (Benson et al., 2007). Roscovitine has two major effects on L-type Ca2+ channels, rapid onset agonist effect and slow onset antagonist effect (Buraei Z., et al, 2007). The agonist effect of roscovitine is that it binds to the activated Cav1.2 so as to slow the closing of the channels which enhance the calcium influx during action potential (Buraei et al., 2004). The antagonist effect is that it enhances the resting inactivated state to inhibit the activity of the channels (Buraei & Elmslie, 2008). In cardiovascular disease, roscovitine is used to inhibit L-type Ca2+ channels by slowing activation and enhancing inactivation. Moreover, roscovitine that increases voltage dependent inactivation of Cav1.2 and restore the impairing of electrical activity of the heart and calcium signaling pathways resulting from timothy syndrome.
Excitation-contraction coupling mechanism is the process in which a small amount of Ca2+ enters the cell through L-type Ca2+ channels, thereby stimulation a much larger amount of Ca2+ from the sarcoplasmic recticulum called CICR. By knowing various mechanisms of this study provides many new opportunities for understanding the molecular and cellular process of calcium mishandling associated with cardiac hypertrophy, heart failure and cardiac arrhythmias in humans. In addition, it also provides a robust essay to develop new therapies for treating diseases.
To conclude, many other experiments and clinical trials are still needed to be conducted to exploit more and more about detail mechanisms of E-C coupling triggered by CICR and other signaling pathways. Thus, there are still challenging to find out and understand this key signaling molecule (calcium) as well as the path physiological processes so as to implicate in therapeutic strategies in the future.
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