Calcium cycling in the aged heart

Calcium Cycling In The Aged Heart Ying-Ying

Zhou, MD, PHD, Edward G. Lakatta, MD, Rui-Ping

Xiao, MD, WD

Laboratory of Cardiovascular Science, Gerontology Research Center, National Institute on Aging, National Institutes of Health, Baltimore, MD, U.S.A.

One of the most important hallmarks of the aged heart is altered calcium homeostasis, possibly due to age-associated alterations in several major calcium cycling processes involved in cardiac excitation-contraction coupling. During ageing, the magnitude of the L-type Ca2+ channel current (ICa,L) becomes significantly increased in parallel with the enlargement of cardiac myocytes, resulting in an unaltered ICa,L density. Since the inactivation of ICa,L is slowed, and the action potential duration is prolonged, the net Ca2+ influx during each action potential is increased in cells of senescent myocardium relative to cells of adult control. While neither mRNA nor protein levels of the sarcoplasmic reticulum (SR) Ca2+ release channel (ryanodine receptor) significantly change with advancing adult age, the mRNA abundance and the density of SR Ca2+ pump decrease with ageing and are associated with a diminished SR Ca2+ sequestration rate in the aged heart. In addition, cardiac chronotropic and inotropic responses to P-adrenergic receptor stimulation are also reduced with advancing age. The multiple changes in Ca cycling that occur during ageing result in an augmented Ca2+ influx, slowed SR Ca2+ sequestration and prolonged durations of the Cai transient and contraction. These alterations which prolong electromechanical systole may be construed as an adaptation in that they prolong the force-bearing capacity of the senescent cells following excitation. This is helpful with respect to maintaining the cardiac function in the aged heart. However, they also increase the risk of Ca2+ overload and Ca2+-dependent arrhythmias during stress in the senescent heart. Although reduced P-adrenergic responses with ageing contribute to diminished contraction reserve, these may be viewed in part, as adaptive, in that they protect against Ca2+ overload during stress. (Asia Pacific Heart J 1999$X(2):88-96) Overview

the RyR-released Ca2+ back to the SR. Another SR membrane protein, phospholamban (PLB), negatively regulates SR Ca2+ pump activity in its unphosphorylated state, but upon phosphorylation of PLB, this inhibitory effect on the SR Ca2’ pump is removed. Any change in these Ca2+-transporting processes described above would be expected to affect the intracellular Ca2+ homeostasis and influence cardiac performance. For example, padrenergic receptor (PAR) stimulation increases cardiac contractility by augmenting the activity of the L-type Ca2+ channel via a CAMP-dependent protein kinase A (PKA)-mediated phosphorylation of the channel (Fig. 1). Additionally, the PKA-dependent phosphorylation of PLB and myofilament proteins also contributes to the PAR-mediated relaxant effect via removing the inhibition of SR Ca2+ pump by PLB and reducing the myofilament Ca2+ sensitivity (Fig. 1).

The cardiovascular system is one of the major targets of ageing, and heart disease is a primary factor of morbidity and mortality for old persons (for review see I-3). Alterations in cardiac excitation-contraction (E-C) coupling and calcium homeostasis are important hallmarks of the aged heart.1 The cardiac E-C coupling process is initiated during the action potential by Ca2+ influx through voltagedependent sarcolemmal Ca2+ channels, particularly, Ltype Ca2+ channels. The Ca2+ influx itself is not sufficient to produce a contraction, but induces sarcoplasmic reticulum (SR) Ca2+ release via ryanodine receptors (RyR) to activate the myofilaments and elicit a contraction. Cardiac relaxation occurs when the increase of intracellular Ca2’ (Cai) is recycled into SR by an SR Ca2+ pump and in part extruded via sarcolemma Ca2+ exchanger (Fig. 1). To avoid net shifts in steady-state Ca2+ content among different cellular compartments, there is a balance between Ca2+ influx and efflux both across the sarcolemma, and into and out of the SR. While the Ca2+ influx via L-type Ca2+ channels is balanced by Ca2+ extrusion, mainly via the Na+-Ca2+ exchanger in the sarcolemma, the Ca2+ pump in the SR sequestrates

During ageing, the average left ventricular myocyte increases approximately 50% in cell surface area 4-6 and approximately 20% in cell volume,7 compared to adult age. A significant prolongation of durations of action potential, Cai transient and contraction in senescent hearts has been observed (Fig. 2).‘-11 In attempting to understand the underlying mechanisms, the change in SR function and in the properties of various ionic channels

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Fig. 1. Cardiac excitation-contraction coupling and its modulation by P-adrenergic signalling. Abbreviations: AP, action potential; ICa, L-type Ca2+ current; @AR, P-adrenergic receptor; Gs, stimulatory G protein; AC, adenylyl cyclase; PKA, protein kinase A; SR, sarco lasmic reticulum; RyR, ryanodine receptor; NCX, Na+-Ca 2t exchanger; PLB, phospholamban; Pump, SR Ca2+ pump. (for review see ‘-3J*J3), particularly L-type Ca2+ channel, have been investigated. This review highlights the ageassociated alterations of sarcolemmal L-type Ca2+ current and SR Ca2+ cycling.

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L-Type Caz+ Current During Ageing L-type Ca2+ channels are often characterised by their sensitivity to dihydropyridines (DHPs). Most DHPs (for example, nifedipine and nitrendipine) act as Ca2+ channel blockers, while some DHPs act as agonists, such as (-)BayK8644. Purified L-type Ca2+ channels are oligomeric complexes of al-subunit, P-subunit and the disulfide-linked a26-subunit. At least six different genes for al-subunit (alS, alA-&, four for P-subunit @l-4) and one for a26-subunit have been identified by molecular cloning (for reviews see 14.15).In addition, alternative splicing may also add additional structural diversity to the multitude of calcium channel al, p, a26 gene products. For example, cardiac and smooth muscle L-type Ca2+ channel al-subunits are spliced variants (alC-a and alC-b) of the same gene. Although alC-a and alC-b have 95% homology in their primary sequence, they have distinct electrophysiological and pharmacological characteristics.16 For instance, alC-b is more sensitive to DHP type Ca2+ channel blockers than UlC-a.” The alCma isofonn is predominantly expressed both ale-a and ale-h in adult rat heart, whereas isoforms are expressed equally in foetal cardiac tissue.17 Since aged hearts recapitulate the foetal phenotype in many aspects (for example, switching of myosin heavy chain from Vl to V3),l*.19 whether L-type Ca2+ channel undergoes a similar switch back to the foetal isoform, and the functional consequences, merits further studies.

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Time (ms) Fig. 2. Typical examples of action potential (A) from young (23 month), middle-aged (8-9 month) and old (24-25 month) rat cells stimulated at a rate of 0.5 Hz at 35”C,4 Cai transient (B) measured via aequorin luminescence 8 and isometric twitch 9 (C), in right ventricular papillary muscles isolated from the hearts of young and senescent Wistar rats.

(approximately 50% in average) in cardiac myocytes isolated from old Wistar or Wistar Kyoto rat hearts (2225 months) relative to those from young rat hearts (2-4 months) (Fig. 3A).4-6 The enhancement of ICa,L amplitude could be either due to an increase in L-type Ca2+ channel number and/or a change in the channel properties. DHP binding studies have shown that the density of DHP binding sites remains constant in Fisher 344 rat hearts over a broad age range (4-30 month) (Table 1),2°,21suggesting the expression of L-type Ca2+ channels is adaptively increased in the hypertrophied, senescent myocytes. Thus the increase in cell size is perfectly balanced by the augmentation of ICa,L in aged cardiac myocytes. This conclusion is strengthened by the

L-type Ca2+ current amplitude The amplitude of ICa,L recorded by whole cell voltage-clamp technique is significantly increased

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Fig. 3. Comparison of L-type Ca2+ current (Ica,L) and its response to PIAR stimulation by norepinephrine (NE) in rat ventricular myocytes of different age groups. A. Representative raw current tracings from a young (2-3 month) and an old (2425 month) rat cell at room temperature.4Holding potential at 40mV was depolarised to test potential from -45 to +60 mV with 200 ms duration in 5 mV increment at a rate of 0.5 Hz. B. Ica current-voltage relation before and during exposure to NE 10-T M.5 The peak Ica (PA) was normalised to cell membrane capacitance(pF).

Fig. 4. The occurrence of aftercontractions (AC, A) after

cessation of pacing and spontaneous ventricular fibrillation (VF, B) 28during pacing at 2.0 Hz in young (6-8 month, open symbol) and old (24-26 month, filled symbol) rat hearts as a function of extracellular Ca2+.

absence of an age-dependent difference in ICa,L density, i.e., after the ICa,L amplitude is normalised by cell capacitance (Fig. 3B).4-6

decay are significantly prolonged, and this produces a larger Ca2+ current integral and thus a greater Ca2’ influx via ICa,L in senescent v adult myocytes (Table 1).4

Information regarding the properties of single L-type Ca2+ channels in senescent cardiomyocytes, for example, single channel conductance, availability, open probability, channel gating mode or homogeneity of openings (time to first latencies) following depolarisation, is presently lacking.

This age-associated difference in ICa,L decay can be eliminated by high concentrations of EGTA (10 mM), a Ca2’ chelator, dialysed into the cell through a pipette.4 This suggests that the Ca2+-induced inactivation of ICa,L might be impaired in senescent cardiac myocytes. The fact that EGTA can blunt the difference in action potential duration between young and old rat cardiac myocytes further confirms that an impairment in Ca2+ induced inactivation of ICa,L is, at least in part (in addition to alterations in transient outward current and Na+-Ca2+ exchanger current), responsible for the prolongation of action potential and contraction.4

L-type Ca2+ current decay time In addition to its amplitude, the ICa,L decay time is another important factor which affects the total amount of Ca2+ influx during cell excitation. Most studies have shown that the decay time of ICa,L is Ca2’ and voltagedependent and can be best fitted by a biexponential function with fast and slow time constants.22-*4 Interestingly, during ageing of Wistar rat cardiac myocytes, both the fast and slow time constants for ICa,L

Modulation of L-type-Ca2+ current by P-adrenergic stimulation during ageing The DHP-sensitive L-type Ca2+ channel is one of the key targets of P-adrenergic receptor (AR) signalling (Fig.

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Table 1. General change in Ca cycling mechanisms in aged heart. mRNA

Protein

Density

Function

L-type Ca channel

unknown

unknown

unaltered

current amplitude increased but density not changed; decay time prolonged; response to P-adrenergic stimulation diminished.

RYR SR Ca pump

unaltered decreased

unaltered decreased or unaltered

slightly decreased decreased

PLB

unknown

unaltered

unaltered

unknown pump rate slowed and maximal velocity of pump activity lowered. P-adrenergic stimulated or Ca, calmodulin protein kinase mediated phosphorylation reduced controversial activity increased

Na-Ca exchanger increased or unaltered increased unknown Sarcolemmal Ca unknown unknown unknown pump Abbreviations: RyR, ryanodine receptor; SR, sarcoplasmic reticulum; PLB, phospholamban.

1). While basal ICa,L density is largely preserved in healthy ageing, the response of L-type Ca2’ channels to PAR stimulation is markedly reduced with ageing.5JQs The augmentation of ICa,L by Pl AR agonist norepinephrine (NE) in senescent Wistar rat ventricular myocytes is attenuated at all test potentials (Fig. 3B).5 In addition, stimulation of Pl AR by NE shifted leftward the inactivation curve in young but not in old hearts, resulting in a relatively enhanced window current (the current generated by the crossover of the steady-state activation and inactivation curves) of ICa,L in the ageing heart.5

effect of PAR activation to L-type Ca2+ channels has been the focus of many studies. It is widely accepted that occupation of PAR by an agonist leads to the activation of the G,-adenylyl cyclase (AC)-CAMP-PKA signalling cascade (Fig. 1). The phosphorylation of L-type Ca2+ channels by catalytic subunits of PKA modulates the channel activity.32-34 In addition, a more rapid, direct coupling of G, to L-type Ca2+ channels may exist.35 The CAMP-independent regulation of L-type Ca2+ channels is more profound in smooth muscle cells,36 and in neonatal cardiac myocytes,” than that in adult cardiac myocytes. This may be due to the fact that the cardiac isoform of L-type Ca2+ channel is predominantly expressed in the adult heart, while both cardiac and smooth muscle isoforms are equally expressed in premature cardiac tissue.‘7 Furthermore, it has recently been demonstrated that P2ARs in cardiomyocytes couple to Gi proteins in addition to Gs protein,38 and that the coupling to Gi proteins functionally compartmentalises the effect of PZAR-Gs-mediated CAMP signalling pathway.39 Therefore, any alteration affecting the PAR, Gs/Gi, AC, CAMP content or compartmentation, and/or responsiveness of the Ca2+ channel to PKA phosphorylation could be candidate mechanisms for the age-associated reduction in the regulation of ICa,L by PAR stimulation.

The physiological importance of ICa,L window current has not been fully understood. In ventricular cells, the window current may delay the termination of the action potential plateau and possibly trigger afterdepolarisations under some conditions.26J7 Thus, the ageassociated increase in window currents may contribute to the increased likelihood of Ca2+ dependent arrhythmias in senescent hearts 28 during stress or PAR stimulation (also see Fig. 4). The decreased response of ICa,L is associated with proportional reductions in the augmentation of the Cai transient amplitude and contractile response to PlAR stimulation by NE. More recent studies have indicated that the cardiac contractile responses to both PlAR and P2AR stimulation are markedly attenuated in single ventricular myocytes isolated from old Wistar rats as compared to those of young rats,*9 suggesting the decreased cardiac response to PAR stimulation by the mixed PAR agonist, isoproterenol, as reported in many studies both of human and animal hearts,s,Q5JO reflects deficits in both PAR subtype systems. Since it has been reported that the P2AR stimulated increase in ICa,L remains similar in young and old rat ventricular myocytes,6 the diminished P2AR contractile response suggests that the SR amplification function may be reduced in the aged heart, as is the case in hypertrophied or failing hearts.”

With respect to the total PAR density in the context of ageing, controversial results have been reported. In rat,Q9 guinea-pig,40 as well as human hearts,25 some studies indicate that the total PAR density is reduced by 30-40% with increasing age; however, other studies have reported no change in PAR density with ageing in human atria 41 and in rat hearts.42-44While a selective down-regulation of myocardial PAR subtypes (pl v p2) has been observed in human and rat hear&Q-5 recent studies in both cardiac tissues and single isolated ventricular myocytes have demonstrated a non-selective down-regulation of both PAR subtypes in ageing rat hearts.29 In addition to PAR density, the effectiveness of the coupling between the

The signalling pathway that couples the stimulatory

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receptor and Gs also declines with ageing, as shown by a decrease in the high-affinity state and an increase in the low-affinity state of PAR.25,45-4s

The absence of changes in mRNA or protein levels of RyR does not exclude a possibility that altered function of the normally expressed RyR may be involved in disturbed E-C coupling in the senescent heart. While the affinity of cardiac RyR is not altered, the number of cardiac RyR binding sites is slightly decreased in cardiac SR from old hamsters (Table 1).61 In addition, it has been demonstrated that in ageing rat fast twitch muscle, RyR sensitivity to Ca2+ and caffeine stimulation is reduced.62 Whether a similar defect of RyR exists in senescent myocardium requires further studies. Since RyR is known to be modulated by phosphorylation,63-65 alterations of Ca2+ release via RyR may occur in the aged cardiomyocyte, especially during stress, due to the dysfunction of the PAR system (see above) and/or downregulation of Ca2+ calmodulin-dependent protein kinase (CaMKII).GO Furthermore, as the amount of SR Ca2+ release is highly dependent on SR Ca2+ load 56 an alteration in PAR-induced SR Ca2+ load change kight result in an age-related defect in cardiac E-C coupling.

The striking similarities between the decreased effectiveness of PAR stimulation with ageing and PAR desensitisation (for review see 49.50)suggests that the ageassociated decline in the response to PAR stimulation may be mediated by a desensitisation mechanism. It has been shown that phosphorylation of agonist-activated PARS by PKA and G-protein coupled receptor kinases (GRKs), such as PARK1 and GRKS, induces receptor desensitisation.54,55 However, recent studies have demonstrated that the abundance and activity of PARK1 or GRKS are not changed with ageing.29 Nevertheless, whether PKA-mediated PAR desensitisation is enhanced with ageing requires further studies.

Sarcoplasmic phospholamban

Sarcoplasmic reticulum (SR) is an intracellular membrane bound compartment which serves as the major intracellular Ca2+ store for excitation-induced Ca2+ release in the cardiac myocyte. SR Ca2+ cycling is extremely important for intracellular Ca2+ homeostasis since it accounts for releasing and sequestrating 70-90% of Ca2+ that comprises the Cai transient during each action potential in mammalian ventricular myocytes.56 calcium

release

reticulum

calcium

pump

and

Diminished SR Ca2’ sequestration has been observed in ageing hearts, and may play a critical role in the agerelated prolongation of Ca2+ transient and contraction duration. The SR Ca2+ pump, a member of P-type ion pumps, couples the hydrolysis of the terminal phosphate of ATP with the reduction of Ca2+ from the cytosol into the SR lumen. This process is not only important for the cardiac relaxation effect, but also crucial for SR Ca2+ loading, and thus for SR Ca2+ release through RyR following each excitation.66 Three different SR Ca2+ pump genes have been identified, which encode at least 5 isoforms (SERCA 1a, SERCAl b, SERCA2a, SERCA2b, SERCA3) of this important protein,67 in which SERCA2a is considered as both cardiac and slow skeletal isoforms.68 During ageing, the rate of ATP-dependent Ca2+ uptake into SR is approximately 40% slower and the maximal velocity of Ca2+ pump activity is approximately 30% lower.69-74 The abundance of the Ca2+ pump at the level of mRNA has been consistently shown to be reduced in the senescent heart as compared to the young heart.59,75,76

Sarcoplasmic Reticulum Calcium Release and Uptake

reticulum

Xiao

rat,60 or in the hamster (Table 1).61

Other components of the PAR signalling cascade are also affected by ageing (for review see 49250).While Gi activity has been reported to either increase 40-42@or remain unchanged,25 G, activity has been consistently shown to decrease,25,@,51,52 resulting in an age-associated reduction in the ratio of GzJGi. Moreover, the activation of AC activity by PAR stimulation, as well as by forskolin or NaF, is also dramatically reduced in the aged heart.25J9,@However, the contractile responses to L-type Ca2+ channel agonist, BayK8644, increasing extracellular Ca2+ concentration and CAMP analogue, CPT-CAMP are similar in old and young adult hearts.53 These results suggest that multiple alterations in the proximal part of PAR signalling cascade occur during ageing, but the downstream of PAR signal transduction remains intact.

Sarcoplasmic

Lakatta,

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channel

The SR Ca2+ release channel is a homotetramer which has a high affinity for the neutral plant alkaloid, ryanodine, and thus is also known as ryanodine receptor (RyR). So far, three different isoforms of RyR (RyRl, RyR2, RyR3) have been identified.57 RyR2, the so-called cardiac RyR isoform, is detectable in all stages of heart development and is universally present in all kinds of cardiac cells, while RyR3 is abundant only in adult Purkinje myocytes.58 The mRNA levels of cardiac RyR do not vary from the 4-months to 24-months Wistar rat.59 Similarly, no significant difference is found in the abundance of cardiac RyR protein with ageing in the

Although the affinities for either Ca2+ or ATP of the pump are unaltered,@ the density of SR pump sites is significantly decreased in senescent hearts (Table 1).59J5J6Whether there is an age-associated reduction in the pump protein level remains controversial. Some studies in animals 69374 and humans 77 reveal an inverse correlation between SERCA2a protein levels and age, and in vivo gene transfer of SERCA2a gene in rat senescent heart improves both systolic and diastolic function of the left ventricle.78 However, other studies fail to show an age-associated change in cardiac SR Ca2+ -ATPase abundance.‘8,60,72,73,79

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Therefore, the slower SR Ca2+ uptake may be attributed to some qualitative but not quantitative change in SR pump under the latter condition, for example, a depressed coupling of ATP hydrolysis to Ca2+ transport in SR,72 as is the case of aged skeletal muscle.80 It is noteworthy that, in the hearts of exercise-trained old rats, the decay of Cai transient is accelerated 81982and the rate of SR Ca2+ uptake is enhanced, along with increases in both mRNA and protein level of SERCA2 as compared to non-training old rat control.74 These results further confirm that the diminished SR Ca2+ pump function is mainly responsible for the age-related prolongation of Cai, and also indicate that a sedentary lifestyle may be a factor for age-associated alterations in Ca2+ cycling.’

in aged heart

approximately 50% higher in senescent (24-month) as compared to adult (6-month) Wistar rat heart.93 In addition, it has been shown that fi-adrenergic stimulation modulates the activity of Na+-Ca2+ exchanger via CAMPdependent protein phosphorylation,94%95 and the downregulation of /3-adrenergic signalling system during ageing (see above) may affect the function of Na+-Ca2+ exchanger in senescent hearts under exercise or stress. Sarcolemmal Ca2 + pump While the sarcolemmal Ca2’ pump has a high affinity for Ca2+, it does not play an important role in Ca2+ extrusion under normal physiological conditions due to its slow transport rate. However, it may contribute to Ca2+ extrusion when the relaxation is slowed as in the case of ageing. Interestingly, it has been shown that, in contrast to the SR Ca2+ pump, the sarcolemmal Ca2+ pump activity is significantly increased in ageing rats (24 months) as compared to adult rats (3 months).72,73 Whether this change contributes to the alterations in Ca2+ homoeostasis in aged myocardium requires more investigation.

PLB is Ca2+-ATPase-regulatory protein, which tonically inhibits the SR Ca2+ pump activity. Phosphorylation of PLB at serine (16) and threonine (17) by PKA and CaMKII, respectively, relieves the inhibition of SR Ca2+ pump by PLB. There is only a single PLB gene in both cardiac and slow twitch skeletal muscle.83 Although no significant age-related difference in the amount of PLB has been reported,60 the response of PLB phosphorylation to PAR stimulation 79 or CaMKII activation 60 is markedly reduced with ageing (Table 1). These may contribute to the diminished response to stress in the senescent heart. Since the response of PLB phosphorylation to exogenous PKA is not altered in the ageing heart,7’*79 the diminished PAR mediated PLB phosphorylation may be largely due to an age-associated defect of PAR signalling per se, as discussed above. In addition, a reduction in CaMKII amount or activity may also contribute to the alterations in PLB function in the senescent heart.60

Comparison of Aged Heart with Cardiac Hypertrophy and Failure The aged heart shares many features with cardiac hypertrophy and failure, for example (for most recent reviews see 13,96-98): (1) ICa,L increases concomitant with cell growth of the cardiomyocyte; (2) the magnitude of transient outward potassium current is dramatically decreased; (3) action potential duration is significantly prolonged; (4) the mRNA and protein levels of SR Ca2+ pump decreases. These are associated with a diminished SR Ca2+ sequestration rate, prolonged Cai transient and contraction duration;

Other Calcium Processing Mechanisms Sarcolernma sodium-calcium exchanger The cardiac Na+-Ca2+ exchanger (NCX) serves as the main transsarcolemmal Ca2+ extrusion mechanism. Three genes (NCXI, NCX2 and NCX3) encode the Na+-Ca2+ exchanger, 84-86in which NCXl is primarily present in the heart.*4,87 It has been suggested that the Na+-Ca2+ exchanger is more active in ejecting Ca2+ from cells of older versus younger heart during diastole,72x88 and an increased exchanger expression compensates for a reduced SR pump function. However, some experiments have shown that Ca2+ transport by Na+-Ca2+ exchanger is attenuated in aged Sprague Dawley rat or hamster myocardium *9,90and the activity of Na+-Ca2+ exchanger is reduced in Fischer 344 rat heart,91 while other studies fail to detect any alterations in the mRNA level 5y,9?or the activity 87of Na+-Ca2+ exchanger in aged rat as well as bovine hearts (Table 1).

(5) p myosin heavy chain is upregulated while a myosin heavy chain is downregulated; (6) the mRNA abundance as well as the function Na+/Ca2+ exchanger are increased;

of

(7) cardiac responses to PAR stimulation are blunted. It has recently been shown that the distance between adjacent DHP-sensitive L-type Ca2+ channel and the SR Ca*+ release channel (RyR) molecules is augmented in hypertrophied and failing ventricular myocytes.jl The authors 31 have proposed that such an alteration may result in an inefficient EC coupling in these enlarged myocytes. Further studies are necessary to test this hypothesis regarding physical dislocation between DHPRs and the neighbouring RyRs in enlarged myocytes of aged hearts. Despite many similarities between aged and hypertrophied hearts, part of the age-related changes may reflect adaptive or protective processes, for example,

Recently, using the more sensitive RNase protection assay, cardiac Na+-Ca2+ exchanger mRNA are shown to decline after birth and rise at the late stage of the life span.93 The abundance of NCX transcripts is

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the prolongation of the electromechanical systole may help maintain cardiac function in senescent cardiac myocytes; the reduced P-adrenergic responses with ageing may protect the heart from Ca2+ overloading during stress. However, the synergistic effects of ageing and heart disease make aged hypertrophied hearts less tolerant to ischaemic damage than mature hearts,ssJm and the morbidity of heart failure increases exponentially with ageing.iai

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Conclusion In summary, cardiac ageing is not accompanied by a general degeneration of cardiomyocytes, but by specific alterations in gene expression, protein abundance and function. With respect to sarcolemmal Ca2+ processing during cardiac E-C coupling, the number of L-type Ca2’ channels increases commensurate with the enlargement of myocytes in the ageing heart. Even though the density of ICa,L is not altered, its inactivation kinetics are slowed with ageing. Thus, together with a defect in SR Ca2+ pump function and a prolonged action potential, the net result is a prolonged integrated Cai transient during each heart beat in the senescent heart. This prolongation of Cai transient may permit the cells of the older heart to maintain normal function under rest conditions. On the other hand, sluggish Ca2+ kinetics may be implicated in the lower threshold of the aged cardiac myocytes to manifest Ca2’ overload and Ca2+ dependent arrhythmias under certain circumstances (Fig. 4). Another hallmark of the aged heart is the severely impaired reserve function, as manifested by markedly reduced chronotropic, inotropic and lusitropic responses to PAR stimulation. Further mechanistic studies are required to fully understand the age-associated alterations in cardiac Ca2+ regulation.

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Acknowledgments

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The authors thank Dr Heping Cheng for his critical and constructive comments.

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