The expression of SR calcium transport ATPase and the Na(+)/Ca(2+)Exchanger are antithetically regulated during mouse cardiac development and in Hypo/hyperthyroidism

The mouse has been used extensively for generating transgenic animal models to study cardiovascular disease. Recently, a number of transgenic mouse models have been created to investigate the importance of sarcoplasmic reticulum (SR) Ca(2+)transport proteins in cardiac pathophysiology. However, the expression and regulation of cardiac SR Ca(2+)ATPase and other Ca(2+)transport proteins have not been studied in detail in the mouse. In this study, we used multiplex RNase mapping analysis to determine SERCA2, phospholamban (PLB), and Na(+)/Ca(2+)-exchanger (NCX-1) gene expression throughout mouse heart development and in hypo/hyperthyroid animals. Our results demonstrate that the expression of SERCA2 and PLB mRNA increase eight-fold from fetal to adult stages, indicating that SR function increases with heart development. In contrast, the expression of the Na(+)/Ca(2+)-exchanger gene is two-fold higher in fetal heart compared to adult. Our study also makes the important observation that in hypothyroidic hearts the NCX-1 mRNA and protein levels were upregulated, whereas the SERCA2 mRNA/protein levels were downregulated. In hyperthyroidic hearts, however, an opposite response was identified. These findings are important and point out that the expression of NCX-1 is regulated antithetically to that of SERCA2 during heart development and in response to alterations in thyroid hormone levels.     

Differences in Ca(2+)-handling and sarcoplasmic reticulum Ca(2+)-content in isolated rat and rabbit myocardium

We made novel measurements of the influence of rest intervals and stimulation frequency on twitch contractions and on sarcoplasmic reticulum (SR) Ca(2+)-content (using rapid cooling contractures, RCCs) in isolated ventricular muscle strips from rat and rabbit hearts at a physiological temperature of 37 degrees C. In addition, the frequency-dependent relative contribution of SR Ca(2+)-uptake and Na(+)/Ca(2+)-exchange for cytosolic Ca(2+)-removal was assessed by paired RCCs. With increasing rest intervals (1-240 s) post-rest twitch force and RCC amplitude decreased monotonically in rabbit myocardium (after 240 s by 45+/-10% and 61+/-11%, respectively P<0. 05, n=14). In contrast, rat myocardium (n=11) exhibited a parallel increase in post-rest twitch force (by 67+/-16% at 240 s P<0.05) and RCC amplitude (by 20+/-14%P<0.05). In rabbit myocardium (n=11), increasing stimulation frequency from 0.25 to 3 Hz increased twitch force by 295+/-50% (P<0.05) and RCC amplitude by 305+/-80% (P<0.05). In contrast, in rat myocardium (n=6), twitch force declined by 43+/-7% (P<0.05), while RCC amplitude decreased only insignificantly (by 16+/-7%). The SR Ca(2+)-uptake relative to Na(+)/Ca(2+)-exchange (based on paired RCCs) increased progressively with frequency in rabbit, but not in rat myocardium (;66+/-2% at all frequencies). We conclude that increased SR Ca(2+)-load contributes to the positive force-frequency relationship in rabbits and post-rest potentiation of twitch force in rats. Decreased SR Ca(2+)-load contributes to post-rest decay of twitch force in rabbits, but may play only a minor role in the negative force-frequency relationship in rats. SR Ca(2+)-release channel refractoriness may contribute importantly to the negative force-frequency relationship in rat and recovery from refractoriness may contribute to post-rest potentiation.     

Gingerol, isoproterenol and ouabain normalize impaired post-rest behavior but not force-frequency relation in failing human myocardium

OBJECTIVE: Rest- and stimulation frequency-dependent potentiation of contractile force is blunted in failing human myocardium. These alterations have been related to reduced sarcoplasmic reticulum (SR) Ca(2+)-reuptake and enhanced transsarcolemmal Ca(2+)-elimination by Na+/Ca(2+)-exchange. We investigated whether inotropic interventions that enhance SR Ca(2+)-uptake, or reduce Ca(2+)-elimination by Na+/Ca(2+)-exchange, normalize impaired post-rest and force-frequency behavior in left ventricular muscle strips from failing human hearts. METHODS: We tested the influence of [10]-gingerol which activates SR Ca(2+)-ATPase (10 mumol/l; n = 13), and isoproterenol which activates cAMP-dependent pathways (0.01, 0.1, 1 mumol/l; n = 40) on post-rest and force-frequency behavior. Ouabain which blocks Na+/K(+)-ATPase (0.03 mumol/l; n = 16) was used to test the effects of inhibiting Ca(2+)-elimination by Na+/Ca(2+)-exchange. For comparison, the effects of blocking SR Ca(2+)-uptake by thapsigargin (10 mumol/l; n = 14) were tested. In addition, Ca(2+)-uptake in myocardial homogenates was measured for gingerol (10 mumol/l; n = 6). RESULTS: Gingerol, isoproterenol (0.1, 1 mumol/l) and ouabain exerted significant positive inotropic effects under basal experimental conditions and normalized post-rest behavior. In contrast, force-frequency relation was only slightly improved by gingerol and isoproterenol (0.01 mumol/l). Ouabain and isoproterenol (1 mumol/l) further deteriorated force-frequency relation due to frequency-dependent significant increases in diastolic tension. Thapsigargin exerted negative inotropic effects and significantly deteriorated post-rest and force-frequency behavior. In addition, gingerol increased SR Ca(2+)-uptake significantly in myocardial homogenates. CONCLUSIONS: Inotropic interventions that stimulate SR Ca(2+)-ATPase or inhibit Na+/Ca(2+)-exchange normalize impaired post-rest behavior. Force-frequency behavior is only slightly improved by stimulation of SR Ca(2+)-ATPase but not by inhibition of Na+/Ca(2+)-exchange. This dissociation between post-rest and force-frequency behavior results from diastolic dysfunction at high stimulation rates.     

Altered regulation of cardiac contraction and relaxation by Ca2+ in heart failure

Myocardial contractile dysfunction in congestive heart failure is characterized by a decrease in force developed and retardation of relaxation. These alterations are mainly due to those in intracellular Ca(2 +) transients (CaT) . CaT are regulated by a number of functional proteins, including sarcolemmal L-type Ca(2 +) channels, Na(+)/Ca(2 +) exchanger and Ca(2 +) ATPase, sarcoplasmic reticulum Ca(2 +) ATPase (SERCA 2 ) , phospholamban and ryanodine receptors, and mitochondrial Ca(2 +) uniporter. Changes in expression and function of these regulatory proteins that occur in the course of increasing severity of heart failure are responsible for the characteristic changes in force development and relaxation observed under pathophysiological conditions in congestive heart failure.     

Extreme sarcoplasmic reticulum volume loss and compensatory T-tubule remodeling after Serca2 knockout

Cardiomyocyte contraction and relaxation are controlled by Ca(2+) handling, which can be regulated to meet demand. Indeed, major reduction in sarcoplasmic reticulum (SR) function in mice with Serca2 knockout (KO) is compensated by enhanced plasmalemmal Ca(2+) fluxes. Here we investigate whether altered Ca(2+) fluxes are facilitated by reorganization of cardiomyocyte ultrastructure. Hearts were fixed for electron microscopy and enzymatically dissociated for confocal microscopy and electrophysiology. SR relative surface area and volume densities were reduced by 63% and 76%, indicating marked loss and collapse of the free SR in KO. Although overall cardiomyocyte dimensions were unaltered, total surface area was increased. This resulted from increased T-tubule density, as revealed by confocal images. Fourier analysis indicated a maintained organization of transverse T-tubules but an increased presence of longitudinal T-tubules. This demonstrates a remarkable plasticity of the tubular system in the adult myocardium. Immunocytochemical data showed that the newly grown longitudinal T-tubules contained Na(+)/Ca(2+)-exchanger proximal to ryanodine receptors in the SR but did not contain Ca(2+)-channels. Ca(2+) measurements demonstrated a switch from SR-driven to Ca(2+) influx-driven Ca(2+) transients in KO. Still, SR Ca(2+) release constituted 20% of the Ca(2+) transient in KO. Mathematical modeling suggested that Ca(2+) influx via Na(+)/Ca(2+)-exchange in longitudinal T-tubules triggers release from apposing ryanodine receptors in KO, partially compensating for reduced SERCA by allowing for local Ca(2+) release near the myofilaments. T-tubule proliferation occurs without loss of the original ordered transverse orientation and thus constitutes the basis for compensation of the declining SR function without structural disarrangement.     

The ryanodine receptor channel as a molecular motif in atrial fibrillation: pathophysiological and therapeutic implications

Atrial fibrillation (AF) is the most common cardiac arrhythmia and is associated with substantial morbidity and mortality. It causes profound changes in sarcoplasmic reticulum (SR) Ca(2+) homeostasis, including ryanodine receptor channel dysfunction and diastolic SR Ca(2+) leak, which might contribute to both decreased contractile function and increased propensity to atrial arrhythmias. In this review, we will focus on the molecular basis of ryanodine receptor channel dysfunction and enhanced diastolic SR Ca(2+) leak in AF. The potential relevance of increased incidence of spontaneous SR Ca(2+) release for both AF induction and/or maintenance and the development of novel mechanism-based therapeutic approaches will be discussed.     

Aging increases pulmonary veins arrhythmogenesis and susceptibility to calcium regulation agents.

BACKGROUND: Aging and pulmonary veins (PVs) play a critical role in the pathophysiology of atrial fibrillation. Abnormal Ca(2+) regulation and ryanodine receptors are known to contribute to PV arrhythmogenesis. OBJECTIVE: The purpose of this study was to investigate whether aging alters PV electrophysiology, Ca(2+) regulation proteins, and responses to rapamycin, FK-506, ryanodine, and ouabain. METHODS: Conventional microelectrodes were used to record action potential and contractility in isolated PV tissue samples in 15 young (age 3 months) and 16 aged (age 3 years) rabbits before and after drug administration. Expression of sarcoplasmic reticulum Ca(2+) ATPase (SERCA2a), ryanodine receptor, and Na(+)/Ca(2+) exchanger was evaluated by western blot. RESULTS: Aged PVs had larger amplitude of delayed afterdepolarizations, greater depolarized resting membrane potential, longer action potential duration, and higher incidence of action potential alternans and contractile alternans with increased expression of Na(+)/Ca(2+) exchanger and ryanodine receptor and decreased expression of SERCA2a. Rapamycin (1,10,100 nM), FK-506 (0.01, 0.1, 1 microM), ryanodine (0.1, 1 microM), and ouabain (0.1, 1 microM) concentration-dependently increased PV spontaneous rates and the incidence of delayed afterdepolarizations in young and aged PVs. Compared with results in young PVs, rapamycin and FK-506 in aged PVs increased PV spontaneous rates to a greater extent and exhibited a larger delayed afterdepolarization amplitude. In PVs without spontaneous activity, rapamycin and FK-506 induced spontaneous activity only in aged PVs, but ryanodine and ouabain induced spontaneous activity in both young and aged PVs. CONCLUSION: Aging increases PV arrhythmogenesis via abnormal Ca(2+) regulation. These findings support the concept that ryanodine receptor dysfunction may result in high PV arrhythmogenesis and aging-related arrhythmogenic vulnerability.     

Pacing staircase phenomenon in the heart: from Bodwitch to the XXI century

The frequency of pacing is a fundamental physiological modulator of myocardial function. When the pacing rate increases there is normally an increase in contractility (a positive force-frequency relationship). However in small rodents, fish and end-stage failing myocardium, the force-frequency response has been found to be flat or even negative. The positive staircase is understood to be related with the increase in the intracellular Ca(2+) transient, mainly due to an enhanced sarcoplasmic reticulum Ca(2+) content at higher stimulation frequencies, resulting from an increase in Ca(2+) influx per unit time and reduced Ca(2+) efflux between beats. However, additional mechanisms, such as increased activity of Ca(2+)/calmodulin-dependent protein kinase or enhanced myofilament responsiveness to Ca(2+) may also play a role. Although an increase in contraction frequency has been shown to be associated with an increase in intracellular Na(+), several studies have shown a temporal dissociation between the increase in Na(i)(+) and the increase in force evoked by changes in pacing frequency. The way in which the Na(+)/Ca(2+) exchanger contributes to contraction frequency inotropy is still not well understood. The aim of this review is to examine the contribution of the fundamental components of cardiac excitation-contraction coupling to frequency inotropy in healthy and failing hearts.     

Reduced myocardial sarcoplasmic reticulum Ca(2+)-ATPase protein expression in compensated primary and secondary human cardiac hypertrophy.

Pathological intracellular calcium handling has been proposed to underlie the alterations of contractile behavior in hypertrophied myocardium. However, the myocardial protein expression of intracellular calcium transport proteins in compensated human left ventricular hypertrophy has not yet been studied. We investigated septal myocardial specimens of patients suffering from hypertrophic obstructive cardiomyopathy (n=14) or from acquired aortic valve stenosis (n=11) undergoing myectomy or aortic valve replacement, respectively. For comparison, we studied non-hypertrophied myocardium of six non-failing hearts which could not be transplanted for technical reasons. The myocardial density of the calcium release channel of the sarcoplasmic reticulum (SR) was determined by(3)H-ryanodine binding. Myocardial contents of SR Ca(2+)-ATPase, phospholamban, calsequestrin and Na(+)/Ca(2+)-exchanger were analysed by Western blot analysis. The myocardial SR calcium release channel density was not significantly different in hypertrophied and non-failing human myocardium. In both hypertrophic obstructive cardiomyopathy and in aortic valve stenosis, SR Ca(2+)-ATPase expression was reduced by about 30% compared to non-failing myocardium (P<0.05), whereas the expression of phospholamban, calsequestrin, and the Na(+)/Ca(2+)-exchanger was unchanged. The decrease of SR Ca(2+)-ATPase expression was still observable when related to its regulatory protein phospholamban or to the myosin content of the homogenates (P<0.05). Furthermore, the SR Ca(2+)-ATPase expression was inversely correlated to the septum thickness assessed by echocardiography, but not to age, cardiac index or outflow tract gradient. In primary as well as in secondary hypertrophied human myocardium, the expression of SR Ca(2+)-ATPase is reduced and inversely related to the degree of the hypertrophy. The diminished SR Ca(2+)-ATPase expression might result in reduced Ca(2+)reuptake into the SR and might contribute to altered contractile behavior in hypertrophied human myocardium.     

Activation of Na+/H+ exchanger 1 is sufficient to generate Ca2+ signals that induce cardiac hypertrophy and heart failure

Activation of the sarcolemmal Na(+)/H(+) exchanger (NHE)1 is increasingly documented as a process involved in cardiac hypertrophy and heart failure. However, whether NHE1 activation alone is sufficient to induce such remodeling remains unknown. We generated transgenic mice that overexpress a human NHE1 with high activity in hearts. The hearts of these mice developed cardiac hypertrophy, contractile dysfunction, and heart failure. In isolated transgenic myocytes, intracellular pH was elevated in Hepes buffer but not in physiological bicarbonate buffer, yet intracellular Na(+) concentrations were higher under both conditions. In addition, both diastolic and systolic Ca(2+) levels were increased as a consequence of Na(+)-induced Ca(2+) overload; this was accompanied by enhanced sarcoplasmic reticulum Ca(2+) loading via Ca(2+)/calmodulin-dependent protein kinase (CaMK)II-dependent phosphorylation of phospholamban. Negative force-frequency dependence was observed with preservation of high Ca(2+), suggesting a decrease in myofibril Ca(2+) sensitivity. Furthermore, the Ca(2+)-dependent prohypertrophic molecules calcineurin and CaMKII were highly activated in transgenic hearts. These effects observed in vivo and in vitro were largely prevented by the NHE1 inhibitor cariporide. Interestingly, overexpression of NHE1 in neonatal rat ventricular myocytes induced cariporide-sensitive nuclear translocation of NFAT (nuclear factor of activated T cells) and nuclear export of histone deacetylase 4, suggesting that increased Na(+)/H(+) exchange activity can alter hypertrophy-associated gene expression. However, in transgenic myocytes, contrary to exclusive translocation of histone deacetylase 4, NFAT only partially translocated to nucleus, possibly because of marked activation of p38, a negative regulator of NFAT signaling. We conclude that activation of NHE1 is sufficient to initiate cardiac hypertrophy and heart failure mainly through activation of CaMKII-histone deacetylase pathway.     

Remodeling of Ca(2+)-handling by atrial tachycardia: evidence for a role in loss of rate-adaptation

BACKGROUND: Loss of rate-dependent action potential (AP) duration (APD) adaptation is a characteristic feature of atrial tachycardia-induced remodeling (ATR). ATR causes sarcolemmal ion-channel remodeling (ICR) and changes in Ca(2+)-handling. The present studies were designed to quantify Ca(2+)-handling changes and then to apply a mathematical AP model to assess the contributions of Ca(2+)-handling abnormalities and ICR to loss of APD rate-adaptation. METHODS: Indo-1 fluorescence was used to measure intracellular Ca(2)-transients and whole-cell patch-clamp to record APs in atrial myocytes from control dogs and dogs subjected to atrial pacing at 400/min for 6 weeks. A previously developed ionic model of the canine atrial AP was modified to reproduce measured Ca(2+)-transients of control and ATR myocytes. RESULTS: In control, APD to 95% repolarization (APD(95)) decreased by 91 ms experimentally and by 88 ms in the model over the 1-6 Hz range. In ATR myocytes, APD(95) failed to decrease over the 1-6 Hz range. Ca(2+)-handling abnormalities in ATR myocytes included slowed upstroke, decreased amplitude and strong single-beat post-rest potentiation. Unaltered Ca(2+)-handling properties included caffeine-releasable Ca(2+)-stores and Ca(2+)-transient relaxation before and after exposure to the sarcoplasmic reticulum Ca(2+)-ATPase (SERCA) inhibitor cyclopiazonic acid (CPA). Including ICR alone in the model accounted for loss of APD(50) rate-adaptation; however, KR alone reduced APD(95) rate-adaptation by only 19% to 71 ms. When both ICR and Ca(2+)-handling changes were incorporated, APD(95) rate-adaptation decreased to 6 ms, accounting for experimental observations. CONCLUSION: ICR alone does not fully account for loss of APD rate-adaptation with atrial remodeling: Ca(2+)-handling changes appear to contribute to this clinically significant phenomenon.     

The cardiac sarcoplasmic/endoplasmic reticulum calcium ATPase: a potent target for cardiovascular diseases

The cardiac isoform of the sarcoplasmic/endoplasmic reticulum calcium ATPase (SERCA2a) is a calcium ion (Ca(2+)) pump powered by ATP hydrolysis. SERCA2a transfers Ca(2+) from the cytosol of the cardiomyocyte to the lumen of the sarcoplasmic reticulum during muscle relaxation. As such, this transporter has a key role in cardiomyocyte Ca(2+) regulation. In both experimental models and human heart failure, SERCA2a expression is significantly decreased, which leads to abnormal Ca(2+) handling and a deficient contractile state. Following a long line of investigations in isolated cardiac myocytes and small and large animal models, a clinical trial is underway that is restoring SERCA2a expression in patients with heart failure by use of adeno-associated virus type 1. Beyond its role in contractile abnormalities in heart failure, SERCA2a overexpression has beneficial effects in a host of other cardiovascular diseases. Here we describe the mechanism of Ca(2+) regulation by SERCA2a, examine the beneficial effects as well as the failures, risks and complexities associated with SERCA2a overexpression, and discuss the potential of SERCA2a as a target for the treatment of cardiovascular disease.     

Altered myocardial Ca2+ cycling after left ventricular assist device support in the failing human heart

OBJECTIVES: The objective of the present study was to determine whether improved contractility after left ventricular assist device (LVAD) support reflects altered myocyte calcium cycling and changes in calcium-handling proteins. BACKGROUND: Previous reports demonstrate that LVAD support induces sustained unloading of the heart with regression of pathologic hypertrophy and improvements in contractile performance. METHODS: In the human myocardium of subjects with heart failure (HF), with non-failing hearts (NF), and with LVAD-supported failing hearts (HF-LVAD), intracellular calcium ([Ca(2+)](i)) transients were measured in isolated myocytes at 0.5 Hz, and frequency-dependent force generation was measured in multicellular preparations (trabeculae). Abundance of sarcoplasmic reticulum Ca(2+) adenosine triphosphatase (SERCA), Na(+)/Ca(2+) exchanger (NCX), and phospholamban was assessed by Western analysis. RESULTS: Compared with NF myocytes, HF myocytes exhibited a slowed terminal decay of the Ca(2+) transient (DT(terminal), 376 +/- 18 ms vs. 270 +/- 21 ms, HF vs. NF, p < 0.0008), and HF-LVAD myocytes exhibited a DT(terminal) that was much shorter than that observed in HF myocytes (278 +/- 10 ms, HF vs. HF-LVAD, p < 0.0001). Trabeculae from HF showed a negative force-frequency relationship, compared with a positive relationship in NF, whereas a neutral relationship was observed in HF-LVAD. Although decreased SERCA abundance in HF was not altered by LVAD support, improvements in [Ca(2+)](i) transients and frequency-dependent contractile function were associated with a significant decrease in NCX abundance and activity from HF to HF-LVAD. CONCLUSIONS: Improvement in rate-dependent contractility in LVAD-supported failing human hearts is associated with a faster decay of the myocyte calcium transient. These improvements reflect decreases in NCX abundance and transport capacity without significant changes in SERCA after LVAD support. Our results suggest that reverse remodeling may involve selective, rather than global, normalization of the pathologic patterns associated with the failing heart.     

Interaction between Na+/K+-pump and Na+/Ca2+-exchanger modulates intercellular communication

Ouabain, a specific inhibitor of the Na(+)/K(+)-pump, has previously been shown to interfere with intercellular communication. Here we test the hypothesis that the communication between vascular smooth muscle cells is regulated through an interaction between the Na(+)/K(+)-pump and the Na(+)/Ca(2+)-exchanger leading to an increase in the intracellular calcium concentration ([Ca(2+)](i)) in discrete areas near the plasma membrane. [Ca(2+)](i) in smooth muscle cells was imaged in cultured rat aortic smooth muscle cell pairs (A7r5) and in rat mesenteric small artery segments simultaneously with force. In A7r5 coupling between cells was estimated by measuring membrane capacitance. Smooth muscle cells were uncoupled when the Na(+)/K(+)-pump was inhibited either by a low concentration of ouabain, which also caused a localized increase of [Ca(2+)](i) near the membrane, or by ATP depletion. Reduction of Na(+)/K(+)-pump activity by removal of extracellular potassium ([K(+)](o)) also uncoupled cells, but only after inhibition of K(ATP) channels. Inhibition of the Na(+)/Ca(2+)-exchange activity by SEA0400 or by a reduction of the equilibrium potential (making it more negative) also uncoupled the cells. Depletion of intracellular Na(+) and clamping of [Ca(2+)](i) at low concentrations prevented the uncoupling. The experiments suggest that the Na(+)/K(+)-pump may affect gap junction conductivity via localized changes in [Ca(2+)](i) through modulation of Na(+)/Ca(2+)-exchanger activity.     

Regulation of sarcoplasmic reticulum Ca2+ ATPase pump expression and its relevance to cardiac muscle physiology and pathology

Cardiac sarcoplasmic reticulum (SR) Ca(2+) ATPase (SERCA2a) plays a central role in myocardial contractility. SERCA2a actively transports Ca(2+) into the SR and regulates cytosolic Ca(2+) concentration, SR Ca(2+) load, and the rate of contraction and relaxation of the heart. In the heart, SERCA pump activity is regulated by two small molecular weight proteins: phospholamban (PLB) and sarcolipin (SLN). Decreases in the expression levels of SERCA2a have been observed in a variety of pathological conditions. In addition, altered expression of PLB and SLN has been reported in many cardiac diseases. Thus, SERCA2a is a major regulator of intracellular Ca(2+) homeostasis, and changes in the expression and activity of the SERCA pump contribute to the decreased SR Ca(2+) content and cardiac dysfunction during pathogenesis. In this review, we discuss the mechanisms controlling SERCA pump expression and activity both during normal physiology and under pathological states.     

A SERCA2 pump with an increased Ca2+ affinity can lead to severe cardiac hypertrophy, stress intolerance and reduced life span

Abnormal Ca(2+) cycling in the failing heart might be corrected by enhancing the activity of the cardiac Ca(2+) pump, the sarco(endo)plasmic reticulum Ca(2+)-ATPase 2a (SERCA2a) isoform. This can be obtained by increasing the pump's affinity for Ca(2+) by suppressing phospholamban (PLB) activity, the in vivo inhibitor of SERCA2a. In SKO mice, gene-targeted replacement of SERCA2a by SERCA2b, a pump with a higher Ca(2+) affinity, results in cardiac hypertrophy and dysfunction. The stronger PLB inhibition on cardiac morphology and performance observed in SKO was investigated here in DKO mice, which were obtained by crossing SKO with PLB(-/-) mice. The affinity for Ca(2+) of SERCA2 was found to be further increased in these DKO mice. Relative to wild-type and SKO mice, DKO mice were much less spontaneously active and showed a reduced life span. The DKO mice also displayed a severe cardiac phenotype characterized by a more pronounced concentric hypertrophy, diastolic dysfunction and increased ventricular stiffness. Strikingly, beta-adrenergic or forced exercise stress induced acute heart failure and death in DKO mice. Therefore, the increased PLB inhibition represents a compensation for the imposed high Ca(2+)-affinity of SERCA2b in the SKO heart. Limiting SERCA2's affinity for Ca(2+) is physiologically important for normal cardiac function. An improved Ca(2+) transport in the sarcoplasmic reticulum may correct Ca(2+) mishandling in heart failure, but a SERCA pump with a much higher Ca(2+) affinity may be detrimental.     

Abnormalities of calcium cycling in the hypertrophied and failing heart

Progressive deterioration of cardiac contractility is a central feature of congestive heart failure (CHF) in humans. In this report we review those studies that have addressed the idea that alterations of intracellular calcium (Ca(2+)) regulation is primarily responsible for the depressed contractility of the failing heart. The review points out that Ca(2+)transients and contraction are similar in non-failing and failing myocytes at very slow frequencies of stimulation (and other low stress environments). Faster pacing rates, high Ca(2+)and beta-adrenergic stimulation reveal large reductions in contractile reserve in failing myocytes. The underlying cellular basis of these defects is then considered. Studies showing changes in the abundance of L-type Ca(2+)channels, Ca(2+)transport proteins [sarcoplasmic reticulum Ca(2+)ATPase (SERCA2), phospholamban (PLB), Na(+)/Ca(2+) exchanger (NCX)] and Ca(2+) release channels (RYR) in excitation-contraction coupling and Ca(2+)release and uptake by the sarcoplasmic reticulum (SR) are reviewed. These observations support our hypotheses that (i) defective Ca(2+)regulation involves multiple molecules and processes, not one molecule, (ii) the initiation and progression of CHF inolves defective Ca(2+)regulation, and (iii) prevention or correction of Ca(2+)regulatory defects in the early stages of cardiac diseases can delay or prevent the onset of CHF.     

Cardiac-specific elevations in thyroid hormone enhance contractility and prevent pressure overload-induced cardiac dysfunction

Thyroid hormone (TH) is critical for cardiac development and heart function. In heart disease, TH metabolism is abnormal, and many biochemical and functional alterations mirror hypothyroidism. Although TH therapy has been advocated for treating heart disease, a clear benefit of TH has yet to be established, possibly because of peripheral actions of TH. To assess the potential efficacy of TH in treating heart disease, type 2 deiodinase (D2), which converts the prohormone thyroxine to active triiodothyronine (T3), was expressed transiently in mouse hearts by using the tetracycline transactivator system. Increased cardiac D2 activity led to elevated cardiac T3 levels and to enhanced myocardial contractility, accompanied by increased Ca(2+) transients and sarcoplasmic reticulum (SR) Ca(2+) uptake. These phenotypic changes were associated with up-regulation of sarco(endo)plasmic reticulum calcium ATPase (SERCA) 2a expression as well as decreased Na(+)/Ca(2+) exchanger, beta-myosin heavy chain, and sarcolipin (SLN) expression. In pressure overload, targeted increases in D2 activity could not block hypertrophy but could completely prevent impaired contractility and SR Ca(2+) cycling as well as altered expression patterns of SERCA2a, SLN, and other markers of pathological hypertrophy. Our results establish that elevated D2 activity in the heart increases T3 levels and enhances cardiac contractile function while preventing deterioration of cardiac function and altered gene expression after pressure overload.