Sarcoplasmic reticulum in heart failure: central player or bystander?

Deficiency of MicroRNA miR-1954 Promotes Cardiac Remodeling and Fibrosis

Background Cardiac fibrosis occurs because of disruption of the extracellular matrix network leading to myocardial dysfunction. Angiotensin II (AngII) has been implicated in the development of cardiac fibrosis. Recently, microRNAs have been identified as an attractive target for therapeutic intervention in cardiac pathologies; however, the underlying mechanism of microRNAs in cardiac fibrosis remains unclear. Next-generation sequencing analysis identified a novel characterized microRNA, miR-1954, that was significantly reduced in AngII-infused mice. The finding led us to hypothesize that deficiency of miR-1954 triggers cardiac fibrosis. Methods and Results A transgenic mouse was created using α-MHC (α-myosin heavy chain) promoter and was challenged with AngII infusion. AngII induced cardiac hypertrophy and remodeling. The in vivo overexpression of miR-1954 showed significant reduction in cardiac mass and blood pressure in AngII-infused mice. Further analysis showed significant reduction in cardiac fibrotic genes, hypertrophy marker genes, and an inflammatory gene and restoration of a calcium-regulated gene (Atp2a2 [ATPase sarcoplasmic/endoplasmic reticulum Ca2+ transporting 2]; also known as SERCA2), but no changes were observed in apoptotic genes. THBS1 (thrombospondin 1) is indicated as a target gene for miR-1954. Conclusions Our findings provide evidence, for the first time, that miR-1954 plays a critical role in cardiac fibrosis by targeting THBS1. We conclude that promoting the level of miR-1954 would be a promising strategy for the treatment of cardiac fibrosis.

Effect of distinct sources of Ca2+ on cardiac hypertrophy in cardiomyocytes

It is believed that intracellular calcium (Ca2+) overload can cause the cardiac hypertrophy, but it is possible that the Ca2+ entering the cytoplasm through distinct pathways will induce various effects on cardiomyocytes. The aim of the present study is to explore the effect of different sources of Ca2+ on cardiomyocyte hypertrophy. The cardiomyocytes isolated from neonatal Sprague–Dawley rats were treated with three agents (ionomycin, caffeine and angiotensin II [Ang II]) that increased the intracellular Ca2+ concentration via different pathways. Treatments with ionomycin, caffeine and Ang II for 24 h caused a significant increase in resting [Ca2+]i by 108.0 ± 7.8%, 102.0 ± 6.9% and 59.8 ± 3.3%, respectively. Caffeine and Ang II increased the cell surface area of cardiomyocytes and the mRNA level of atrial natriuretic peptide, brain natriuretic peptide and β-myosin heavy chain, but ionomycin did not. Moreover, sarcoplasmic reticulum Ca2+-ATPase (SERCA2a) activity and the amplitudes of the twitch [Ca2+]i transients were reduced in the caffeine-treated group and Ang II-treated group. Furthermore, cardiomyocyte hypertrophy induced by caffeine was inhibited by cyclosporin A (CsA) and KN93, whereas cardiomyocyte hypertrophy induced by Ang II was inhibited by KN93, but not CsA. Our results show that cardiomyocyte hypertrophy is associated with SERCA2a activity, contractile performance and signaling pathways of CaMKII and/or calcineurin, whereas the Ca2+ overload is not sufficient to cause the cardiomyocyte hypertrophy.

Protein phosphatase 2A affects myofilament contractility in non-failing but not in failing human myocardium

Protein phosphatase (PP) type 2A is a multifunctional serine/threonine phosphatase that is involved in cardiac excitation-contraction coupling. The PP2A core enzyme is a dimer, consisting of a catalytic C and a scaffolding A subunit, which is targeted to several cardiac proteins by a regulatory B subunit. At present, it is controversial whether PP2A and its subunits play a critical role in end-stage human heart failure. Here we report that the application of purified PP2AC significantly increased the Ca2+-sensitivity (ΔpCa50=0.05±0.01) of the contractile apparatus in isolated skinned myocytes of non-failing (NF) hearts. A higher phosphorylation of troponin I (cTnI) was found at protein kinase A sites (Ser23/24) in NF compared to failing myocardium. The basal Ca2+-responsiveness of myofilaments was enhanced in myocytes of ischemic (ICM, ΔpCa50=0.10±0.03) and dilated (DCM, ΔpCa50=0.06±0.04) cardiomyopathy compared to NF. However, in contrast to NF myocytes the treatment with PP2AC did not shift force-pCa relationships in failing myocytes. The higher basal Ca2+-sensitivity in failing myocytes coincided with a reduced protein expression of PP2AC in left ventricular tissue from patients suffering from ICM and DCM (by 50 and 56% compared to NF, respectively). However, PP2A activity was unchanged in failing hearts despite an increase of both total PP and PP1 activity. The expression of PP2AB56α was also decreased by 51 and 62% in ICM and DCM compared to NF, respectively. The phosphorylation of cTnI at Ser23/24 was reduced by 66 and 49% in ICM and DCM compared to NF hearts, respectively. Our results demonstrate that PP2A increases myofilament Ca2+-sensitivity in NF human hearts, most likely via cTnI dephosphorylation. This effect is not present in failing hearts, probably due to the lower baseline cTnI phosphorylation in failing compared to non-failing hearts.

Losartan reduces mortality in a genetic model of heart failure

Altered Ca(2+) homoeostasis accompanies heart failure. As a model of heart failure, transgenic mice (TG) with selective overexpression of calsequestrin (CSQ) in the heart were used. CSQ is the main Ca(2+) binding protein in the lumen of the junctional sarcoplasmic reticulum. Overexpression of CSQ leads to hypertrophy, fibrosis, heart failure, cardiac arrhythmias, and ultimately premature death compared to littermate controls (WT). In the present study, cardiac hypertrophy was noted at 2 months of age (relative heart weight 6.4 +/- 0.2 mg/g in WT and 11.2 +/- 0.3 mg/g in TG, n = 7, p < 0.05) which progressed at 5 months of age (relative heart weight 15.5 +/- 1.1 mg/g in TG, n = 11). Furthermore, an increased degree of fibrosis (from 0.29 +/- 0.04 in WT to 0.77 +/- 0.06 in TG, n = 8, p < 0.05) was quantified by sirius red staining. Cardiac function was greatly impaired in TG as exemplified by reduced pressure development and cardiac arrhythmias. It is hypothesized that losartan, an inhibitor of angiotensin II receptors, might be able to attenuate these detrimental effects. Hence, TG and WT were treated for 1 or 4 months perorally with losartan (5 mg/kg/day) or solvent alone (control conditions) starting at 4 weeks of age. Under control conditions, none of the WT died within the observation period whereas all TG died within 9 months. Losartan treatment reduced the mortality of TG: Mean life span was raised from 116 to 193 days (n = 18 end, p < 0.05). Likewise, losartan reduced relative heart weight and the degree of fibrosis. In addition, losartan improved hemodynamic parameters, like left ventricular pressure and its first derivative. However, losartan treatment did not modify overexpression of CSQ in the heart of TG. These results imply that the angiotensin II receptor (type 1) contributes to heart failure due to CSQ overexpression, as its blockade improved survival.

Calcium and cardiomyopathies

Regulation of Calcium (Ca) cycling by the sarcoplasmic reticulum (SR) underlies the control of cardiac contraction during excitation-contraction (E-C) coupling. Moreover, alterations in E-C coupling occurring in cardiac hypertrophy and heart failure are characterized by abnormal Ca-cycling through the SR network. A large body of evidence points to the central role of: a) SERCA and its regulator phospholamban (PLN) in the modulation of cardiac relaxation; b) calsequestrin in the regulation of SR Ca-load; and c) the ryanodine receptor (RyR) Ca-channel in the control of SR Ca-release. The levels or activity of these key Ca-handling proteins are altered in cardiomyopathies, and these changes have been linked to the deteriorated cardiac function and remodeling. Furthermore, genetic variants in these SR Ca-cycling proteins have been identified, which may predispose to heart failure or fatal arrhythmias. This chapter concentrates on the pivotal role of SR Ca-cycling proteins in health and disease with specific emphasis on their recently reported genetic modifiers.

Parvalbumin isoforms differentially accelerate cardiac myocyte relaxation kinetics in an animal model of diastolic dysfunction

The cytosolic Ca(2+)/Mg(2+)-binding protein alpha-parvalbumin (alpha-Parv) has been shown to accelerate cardiac relaxation; however, beyond an optimal concentration range, alpha-Parv can also diminish contractility. Mathematical modeling suggests that increasing Parv's Mg(2+) affinity may lower the effective concentration of Parv ([Parv]) to speed relaxation and, thus, limit Parv-mediated depressed contraction. Naturally occurring alpha/beta-Parv isoforms show divergence in amino acid primary structure (57% homology) and cation-binding affinities, with beta-Parv having an estimated 16% greater Mg(2+) affinity and approximately 200% greater Ca(2+) affinity than alpha-Parv. We tested the hypothesis that, at the same or lower estimated [Parv], mechanical relaxation rate would be more significantly accelerated by beta-Parv than by alpha-Parv. Dahl salt-sensitive (DS) rats were used as an experimental model of diastolic dysfunction. Relaxation properties were significantly slowed in adult cardiac myocytes isolated from DS rats compared with controls: time from peak contraction to 50% relaxation was 57 +/- 2 vs. 49 +/- 2 (SE) ms (P < 0.05), validating this model system. DS cardiac myocytes were subsequently transduced with alpha- or beta-Parv adenoviral vectors. Upon Parv gene transfer, beta-Parv caused significantly faster relaxation than alpha-Parv (P < 0.05), even though estimated [beta-Parv] was approximately 10% of [alpha-Parv]. This comparative analysis showing distinct functional outcomes raises the prospect of utilizing naturally occurring Parv variants to address disease-associated slowed cardiac relaxation.

Genetic manipulation of calcium-handling proteins in cardiac myocytes. I. Experimental studies

Two genetic experimental approaches, de novo expression of parvalbumin (Parv) and overexpression of sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA2a), have been shown to increase relaxation rates in myocardial tissue. However, the relative effect of Parv and SERCA2a on systolic function and on β-adrenergic responsiveness at varied pacing rates is unknown. We used gene transfer in isolated rat adult cardiac myocytes to gain a fuller understanding of Parv/SERCA2a function. As demonstrated previously, when Parv is expressed in elevated concentration (>0.1 mM), the transduced myocytes showed a reduction in sarcomere-shortening amplitude: 129 ± 17, 81 ± 8, and 149 ± 14 nm for control, Parv, and SERCA2a, respectively. At physiological temperature, shortening amplitude responses of Parv and SERCA2a myocytes to the β-adrenergic agonist isoproterenol (Iso) were not statistically different from that of control myocytes. However, in SERCA2a myocytes, in which baseline was slightly elevated and the Iso-stimulated value was slightly lower, the increase in shortening was slightly less than in Parv or control myocytes: 108 ± 14, 169 ± 39, and 34 ± 12% for control, Parv, and SERCA2a, respectively. In another test set, Parv myocytes had the strongest early postrest potentiation among all groups studied (rest time = 2–10 s), and SERCA2a myocytes were the least sensitive to variations in stimulation rhythm. To replicate the deficient Ca2+ removal observed in heart failure, we used 150 nM thapsigargin. Under these conditions, control myocytes exhibited slowed relaxation, whereas Parv myocytes retained their rapid kinetics, showing that Parv is still able to control relaxation, even when SERCA2a function is impaired.

Abnormal Ca2+ release, but normal ryanodine receptors, in canine and human heart failure

Sarcoplasmic reticulum (SR) Ca2+ transport proteins, especially ryanodine receptors (RyR) and their accessory protein FKBP12.6, have been implicated as major players in the pathogenesis of heart failure (HF), but their role remain controversial. We used the tachycardia-induced canine model of HF and human failing hearts to investigate the density and major functional properties of RyRs, SERCA2a, and phospholamban (PLB), the main proteins regulating SR Ca2+ transport. Intracellular Ca2+ is likely to play a role in the contractile dysfunction of HF because the amplitude and kinetics of the [Ca2+]i transient were reduced in HF. Ca2+ uptake assays showed 44+/-8% reduction of Vmax in canine HF, and Western blots demonstrated that this reduction was due to decreased SERCA2a and PLB levels. Human HF showed a 30+/-5% reduction in SERCA2a, but PLB was unchanged. RyRs from canine and human HF displayed no major structural or functional differences compared with control. The P(o) of RyRs was the same for control and HF over the range of pCa 7 to 4. Subconductance states, which predominate in FKBP12.6-stripped RyRs, were equally frequent in control and HF channels. An antibody that recognizes phosphorylated RyRs yields equal intensity for control and HF channels. Further, phosphorylation of RyRs by PKA did not appear to change the RyR/FKBP12.6 association, suggesting minor beta-adrenergic stimulation of Ca2+ release through this mechanism. These results support a role for SR in the pathogenesis of HF, with abnormal Ca2+ uptake, more than Ca2+ release, contributing to the depressed and slow Ca2+ transient characteristic of HF.

Altered function in atrium of transgenic mice overexpressing triadin 1

Triadin 1 is a protein in the cardiac junctional sarcoplasmic reticulum (SR) that interacts with the ryanodine receptor, junctin, and calsequestrin, proteins that are important for Ca(2+) release. To better understand the role of triadin 1 in SR-Ca(2+) release, we studied the time-dependent expression of SR proteins and contractility in atria of 3-, 6-, and 18-wk-old transgenic mice overexpressing canine cardiac triadin 1 under control of the alpha-myosin heavy chain (MHC) promoter. Three-week-old transgenic atria exhibited mild hypertrophy. Finally, atrial weight was increased by 110% in 18-wk-old transgenic mice. Triadin 1 overexpression was accompanied by time-dependent changes in the protein expression of the ryanodine receptor, junctin, and cardiac/slow-twitch muscle SR Ca(2+)-ATPase isoform. Force of contraction was already decreased in 3-wk-old transgenic atria. The application of caffeine led to a positive inotropic effect in transgenic atria of 3-wk-old mice. Rest pauses resulted in an increased potentiation of force of contraction after restimulation in 3- and 6-wk-old mice and a reduced potentiation of force of contraction in 18-wk-old transgenic mice. Hence, triadin 1 overexpression triggered time-dependent alterations in SR protein expression, Ca(2+) homeostasis, and contractility, indicating for the first time an inhibitory function of triadin 1 on SR-Ca(2+) release in vivo.

Defective intracellular Ca(2+) signaling contributes to cardiomyopathy in Type 1 diabetic rats

The goal of the study was to determine whether defects in intracellular Ca(2+) signaling contribute to cardiomyopathy in streptozotocin (STZ)-induced diabetic rats. Depression in cardiac systolic and diastolic function was traced from live diabetic rats to isolated individual myocytes. The depression in contraction and relaxation in myocytes was found in parallel with depression in the rise and decline of intracellular free Ca(2+) concentration ([Ca(2+)](i)). The sarcoplasmic reticulum (SR) Ca(2+) store and rates of Ca(2+) release and resequestration into SR were depressed in diabetic rat myocytes. The rate of Ca(2+) efflux via sarcolemmal Na(+)/Ca(2+) exchanger was also depressed. However, there was no change in the voltage-dependent L-type Ca(2+) channel current that triggers Ca(2+) release from the SR. The depression in SR function was associated with decreased SR Ca(2+)-ATPase and ryanodine receptor proteins and increased total and nonphosphorylated phospholamban proteins. The depression of Na(+)/Ca(2+) exchanger activity was associated with a decrease in its protein level. Thus it is concluded that defects in intracellular Ca(2+) signaling caused by alteration of expression and function of the proteins that regulate [Ca(2+)](i) contribute to cardiomyopathy in STZ-induced diabetic rats. The increase in phospholamban, decrease in Na(+)/Ca(2+) exchanger, and unchanged L-type Ca(2+) channel activity in this model of diabetic cardiomyopathy are distinct from other types of cardiomyopathy.

Mechanical alternans and restitution in failing SHHF rat left ventricles

We examined mechanical alternans and electromechanical restitution in normal and failing rat hearts. Alternans occurred at 5 Hz in failing versus 9 Hz in control hearts and was reversed by 300 nM isoproterenol, 6 mM extracellular Ca(2+), 300 nM -BAY K 8644, or 50 nM ryanodine. Restitution curves comprised phase I, which was completed before relaxation of the steady-state beat, and phase II, which occurred later. Phase I action potential area and developed pressure ratios were significantly reduced in the failing versus control hearts. Phase II was a monoexponential increase in relative developed pressure as the extrasystolic interval was increased. The plateau of phase II was significantly elevated in failing hearts. Thapsigargin (3 microM) plus ryanodine (200 nM) potentiated phase I to a significantly greater extent in control versus failing hearts and abolished phase II in both groups. The results suggest that both regulation of Ca(2+) influx across the sarcolemma and Ca(2+) release by the sarcoplasmic reticulum may contribute to altered excitation-contraction coupling in the failing spontaneously hypertensive heart failure prone rat heart.

Molecular mechanisms of reduced sarcoplasmic reticulum Ca(2+) uptake in human failing left ventricular myocardium

BACKGROUND

Human failing heart due to idiopathic dilated cardiomyopathy is associated with decreased sarcoplasmic reticulum Ca(2+) uptake. However, it is unknown as to which mechanism leads to this abnormality.

METHODS

Immunodetectable sarcoplasmic reticulum proteins (phospholamban [PLB], phosphorylated PLB at serine-16 or threonine-17, calsequestrin and Ca(2+)-ATPase levels), the activities of Ca(2+)-calmodulin-dependent protein kinase and protein phosphatase and Ca(2+) uptake at varying Ca(2+) concentrations were determined in left ventricular specimens from the same 7 failing hearts (ejection fraction 20 +/- 2%) due to idiopathic dilated cardiomyopathy and 5 non-failing explanted control donor hearts.

RESULTS

In failing hearts, compared with control donors, decreased maximal velocity and affinity of Ca(2+) uptake for Ca(2+) were found to be associated with reduced expression levels of Ca(2+)-adenosine triphosphatase (ATPase), PLB and phosphorylated PLB at serine-16, but not of calsequestrin and phosphorylated PLB at threonine-17. In contrast, protein phosphatase activity increased significantly and the activity and protein expression level of the delta isoform of Ca(2+)-calmodulin-dependent protein kinase remained unchanged in failing hearts compared with control donors.

CONCLUSIONS

The impaired maximal velocity of sarcoplasmic reticulum Ca(2+) uptake may be due in part to reduced protein expression level of Ca(2+)-ATPase, whereas the reduced affinity may be due in part to the reduced ratio of Ca(2+)-ATPase to PLB and reduced PLB phosphorylation at serine-16 in failing hearts. The latter abnormality may be due in part to increased protein phosphatase activity. These results suggest that selective enhancement of Ca(2+) uptake into the sarcoplasmic reticulum by pharmaceutical agents, or by molecular tools that inhibit phosphatase activity, would be a valuable therapeutic approach for treating, or at least retarding, the process of heart failure.

From the sarcomere to the nucleus: role of genetics and signaling in structural heart disease

The identification of genetic mutations underlying familial structural heart disease has provided exciting new insights into how alterations in structural components of the cardiomyocyte lead to different forms of cardiomyopathy. Specifically, mutations in components of the sarcomere are frequently associated with hypertrophic cardiomyopathy, whereas mutations in cytoskeletal proteins lead to dilated cardiomyopathy. In addition, extrinsic stresses such as hypertension and valvular disease can produce myocardial remodeling that is very similar to that observed in genetic cardiomyopathy. For myocardial remodeling to occur, changes in gene expression must occur; therefore, changes in contractile function or wall stress must be communicated to the nucleus via signal transduction pathways. The identity of these signaling pathways has become a key question in molecular biology. Numerous signaling molecules have been implicated in the development of hypertrophy and failure, including the beta-adrenergic receptor, G alpha(q) and downstream effectors, mitogen-activated protein kinase pathways, and the Ca(2+)-regulated phosphatase, calcineurin. In the past it has been difficult to discern which signaling molecules actually contributed to disease progression in vivo; however, the development of numerous transgenic and knockout mouse models of cardiomyopathy is now allowing the direct testing of stimulatory and inhibitory molecules in the mouse heart. From this work it has been possible to identify signaling molecules and pathways that are required for different aspects of disease progression in vivo. In particular, a number of signaling pathways have now been identified that may be key regulators of changes in myocardial structure and function in response to mutations in structural components of the cardiomyocyte. Myocardial structure and signal transduction are now merging into a common field of research that will lead to a more complete understanding of the molecular mechanisms that underly heart disease.

Prevention by dexrazoxane of down-regulation of ryanodine receptor gene expression in anthracycline cardiomyopathy in the rat

Anthracyclines can cause cumulative dose-related cardiotoxicity characterized by changes in Ca(2+) metabolism, including dysfunction of the sacroplasmic reticulum (SR) and decreased expression of Ca(2+)-handling proteins, such as the ryanodine receptor (RyR2). In this study, we examined the effect of dexrazoxane (ICRF-187), an iron chelator which prevents anthracycline cardiotoxicity, on RyR2 gene expression in rats treated chronically with daunorubicin. Daunorubicin (2.5 mg kg(-1) i.v. weekly for 6 weeks) produced cardiotoxicity as demonstrated by histopathologic changes. The ryanodine receptor/glyceraldehyde phosphate dehydrogenase (GAPDH) mRNA ratio was decreased by 38+/-3% (P<0.02) compared to values in control rats. Dexrazoxane pre-treatment (50 mg kg(-1); 1 h prior to each daunorubicin injection) prevented the decrease in RyR2/GAPDH mRNA ratio and histopathologic lesions in daunorubicin-treated rats. This is the first report that a protective agent such as dexrazoxane can ameliorate the decreased expression of a specific gene involved in anthracycline-induced cardiotoxicity.

Mechanisms of ischemic preconditioning effects on Ca(2+) paradox-induced changes in heart

The effects of ischemic preconditioning (IP) on changes in cardiac performance and sarcoplasmic reticulum (SR) function due to Ca(2+) paradox were investigated. Isolated perfused hearts were subjected to IP (three cycles of 3-min ischemia and 3-min reperfusion) followed by Ca(2+)-free perfusion and reperfusion (Ca(2+) paradox). Perfusion of hearts with Ca(2+)-free medium for 5 min followed by reperfusion with Ca(2+)-containing medium for 30 min resulted in a dramatic decrease in the left ventricular (LV) developed pressure and a marked increase in LV end-diastolic pressure. Alterations in cardiac contractile activity due to Ca(2+) paradox were associated with depressed SR Ca(2+)-uptake, Ca(2+)-pump ATPase, and Ca(2+)-release activities as well as decreased SR protein contents for Ca(2+)-pump and Ca(2+) channels. All these changes due to Ca(2+) paradox were significantly prevented in hearts subjected to IP. The protective effects of IP on Ca(2+) paradox changes in cardiac contractile activity as well as SR Ca(2+)-pump and Ca(2+)-release activities were lost when the hearts were treated with 8-(p-sulfophenyl)-theophylline, an adenosine receptor antagonist; KN-93, a specific Ca(2+)/calmodulin-dependent protein kinase II (CaMK II) inhibitor; or chelerythrine chloride, a protein kinase C (PKC) inhibitor. These results indicate that IP rendered cardioprotection by preventing a depression in SR function in Ca(2+) paradox hearts. Furthermore, these beneficial effects of IP may partly be mediated by adenosine receptors, PKC, and CaMK II.

Defective beta-adrenergic receptor signaling precedes the development of dilated cardiomyopathy in transgenic mice with calsequestrin overexpression

Calsequestrin is a high capacity Ca(2+)-binding protein in the junctional sarcoplasmic reticulum that forms a quaternary complex with junctin, triadin, and the ryanodine receptor. Transgenic mice with cardiac-targeted calsequestrin overexpression show marked suppression of Ca(2+)-induced Ca(2+) release, myocyte hypertrophy, and premature death by 16 weeks of age (Jones, L. R., Suzuki, Y. J., Wang, W., Kobayashi, Y. M., Ramesh, V., Franzini-Armstrong, C., Cleemann, L., and Morad, M. (1998) J. Clin. Invest. 101, 1385-1393). To investigate whether alterations in intracellular Ca(2+) trigger changes in the beta-adrenergic receptor pathway, we studied calsequestrin overexpressing transgenic mice at 7 and 14 weeks of age. As assessed by echocardiography, calsequestrin mice at 7 weeks showed mild left ventricular enlargement, mild decreased fractional shortening with increased wall thickness. By 14 weeks, the phenotype progressed to marked left ventricular enlargement and severely depressed systolic function. Cardiac catheterization in calsequestrin mice revealed markedly impaired beta-adrenergic receptor responsiveness in both 7- and 14- week mice. Biochemical analysis in 7- and 14-week mice showed a significant decrease in total beta-adrenergic receptor density, adenylyl cyclase activity, and the percent high affinity agonist binding, which was associated with increased beta-adrenergic receptor kinase 1 levels. Taken together, these data indicate that alterations in beta-adrenergic receptor signaling precede the development of overt heart failure in this mouse model of progressive cardiomyopathy.

Ca2+ handling and sarcoplasmic reticulum Ca2+ content in isolated failing and nonfailing human myocardium

Disturbed sarcoplasmic reticulum (SR) Ca2+ content may underlie the altered force-frequency and postrest contractile behavior in failing human myocardium. We used rapid cooling contractures (RCCs) to assess SR Ca2+ content in ventricular muscle strips isolated from nonfailing and end-stage failing human hearts. With an increase in rest intervals (1 to 240 s; 37 degrees C), nonfailing human myocardium (n=7) exhibited a parallel increase in postrest twitch force (at 240 s by 121+/-44%; P<0.05) and RCC amplitude (by 69+/-53%; P<0.05). In contrast, in failing myocardium (n=30), postrest twitch force decreased at long rest intervals and RCC amplitude declined monotonically with rest (by 25+/-9% and 53+/-9%, respectively; P<0.05). With an increase in stimulation frequencies (0.25 to 3 Hz), twitch force increased continuously in nonfailing human myocardium (n=7) by 71+/-17% (at 3 Hz; P<0.05) and RCC amplitude increased in parallel by 247+/-55% (P<0.05). In contrast, in failing myocardium (n=26), twitch force declined by 29+/-7% (P<0. 05) and RCC amplitude increased only slightly by 36+/-14% (P<0.05). Paired RCCs were evoked to investigate the relative contribution of SR Ca2+ uptake and Na+/Ca2+ exchange to cytosolic Ca2+ removal during relaxation. SR Ca2+ uptake (relative to the Na+/Ca2+ exchange) increased significantly in nonfailing but not in failing human myocardium as stimulation rates increased. We conclude that the negative force-frequency relation in failing human myocardium is due to an inability of SR Ca2+ content to increase sufficiently at high frequencies and thus cannot overcome the frequency-dependent refractoriness of SR Ca2+ release. The rest-dependent decay in twitch force in failing myocardium is due to rest-dependent decline in SR Ca2+ content. These alterations could be secondary to depressed SR Ca2+-ATPase combined with enhanced cytosolic Ca2+ extrusion via Na+/Ca2+ exchange.

Subcellular creatine kinase alterations. Implications in heart failure

We have tested the hypothesis that decreased functioning of creatine kinase (CK) at sites of energy production and utilization may contribute to alterations in energy fluxes and calcium homeostasis in congestive heart failure (CHF). Heart failure was induced by aortic banding in 3-week-old rats. Myofilaments, sarcoplasmic reticulum (SR), mitochondrial functions, and CK compartmentation were studied in situ using selective membrane permeabilization of left ventricular fibers with detergents (saponin for mitochondria and SR and Triton X-100 for myofibrils). Seven months after surgery, animals were in CHF. A decrease in total CK activity could be accounted for by a 4-fold decrease in activity and content (Western blots) of mitochondrial CK and a 30% decrease in M isoform of CK (MM-CK) activity. In myofibrils, maximal force, crossbridge kinetics, and alpha-myosin heavy-chain expression decreased, whereas calcium sensitivity of tension development remained unaltered. Myofibrillar CK efficacy was unchanged. Calcium uptake capacities of SR were estimated from the surface of caffeine-induced tension transient (SCa) after loading with different substrates. In CHF, SCa decreased by 23%, and phosphocreatine was 2 times less efficient in enhancing calcium uptake. Oxidative capacities of the failing myocardium measured as oxygen consumption per gram of fiber dry weight decreased by 28%. Moreover, the control of respiration by creatine, ADP, and AMP was severely impaired. Our observations provide evidence that alterations in CK compartmentation may contribute to alterations of energy fluxes and calcium homeostasis in CHF.

Inotropy, calcium, and heart failure: a personal perspective

The medical treatment of heart failure has improved significantly in the last decade, yet despite the reduction of morbidity and mortality, heart attacks and stroke cause 15 million deaths each year. Cardiovascular disease (CVD) constitutes 50% of all deaths worldwide. Indeed, heart disease represents a continuing global burden, despite all the advances. When considering the major objective of medical treatment of heart failure, the paradigm has shifted from increasing cardiac output of the failing heart by augmenting contractility to a decrease in afterload and inhibition of neurohumors affecting heart muscle. Nevertheless, a direct increase in contractility is still appropriate in some conditions of heart failure for a short-term beneficial effect. This article reviews certain aspects of cellular and molecular mechanisms to provide a framework for problem solving in this vital field.