Changes of SERCA activity have only modest effects on sarcoplasmic reticulum Ca2+ content in rat ventricular myocytes

Partial Genetic Deletion of Klotho Aggravates Cardiac Calcium Mishandling in Acute Kidney Injury

Acute kidney injury (AKI) is associated with an elevated risk of cardiovascular major events and mortality. The pathophysiological mechanisms underlying the complex cardiorenal network interaction remain unresolved. It is known that the presence of AKI and its evolution are significantly associated with an alteration in the anti-aging factor klotho expression. However, it is unknown whether a klotho deficiency might aggravate cardiac damage after AKI. We examined intracellular calcium (Ca²⁺) handling in native ventricular isolated cardiomyocytes from wild-type (+/+) and heterozygous hypomorphic mice for the klotho gene (+/kl) in which an overdose of folic acid was administered to induce AKI. Twenty-four hours after AKI induction, cardiomyocyte contraction was decreased in mice with the partial deletion of klotho expression (heterozygous hypomorphic klotho named +/kl). This was accompanied by alterations in Ca²⁺ transients during systole and an impairment of sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCA2a) function in +/kl mice after AKI induction. Moreover, Ca²⁺ spark frequency and the incidence of Ca²⁺ pro-arrhythmic events were greater in cardiomyocytes from heterozygous hypomorphic klotho compared to wild-type mice after AKI. A decrease in klotho expression plays a role in cardiorenal damage aggravating cardiac Ca²⁺ mishandling after an AKI, providing the basis for future targeted approaches directed to control klotho expression as novel therapeutic strategies to reduce the cardiac burden that affects AKI patients.

A hydroalcoholic extract of Senecio nutans SCh. Bip (Asteraceae); its effects on cardiac function and chemical characterization

ETHNOPHARMACOLOGY RELEVANCE

The plant Senecio nutans SCh. Bip. is used by Andean communities to treat altitude sickness. Recent evidence suggests it may produce vasodilation and negative cardiac inotropy, though the cellular mechanisms have not been elucidated.

PURPOSE

To determinate the mechanisms action of S. nutans on cardiovascular function in normotensive animals.

METHODS

The effect of the extract on rat blood pressure was measured with a transducer in the carotid artery and intraventricular pressure by a Langendorff system. The effects on sheep ventricular intracellular calcium handling and contractility were evaluated using photometry. Ultra-high-performance liquid-chromatography with diode array detection coupled with heated electrospray-ionization quadrupole-orbitrap mass spectrometric detection (UHPLC-DAD-ESI-Q-OT-MSn) was used for extract chemical characterization.

RESULTS

In normotensive rats, S. nutans (10 mg/kg) reduced mean arterial pressure (MAP) by 40% (p < 0.05), causing a dose-dependent coronary artery dilation and decreased left ventricular pressure. In isolated cells, S. nutans extract (1 μg/ml) rapidly reduced the [Ca²⁺]_i transient amplitude and sarcomere shorting by 40 and 49% (p < 0.001), respectively. The amplitude of the caffeine evoked [Ca²⁺]_i transient was reduced by 24% (p < 0.001), indicating reduced sarcoplasmic reticulum (SR) Ca²⁺ content. Sodium-calcium exchanger (NCX) activity increased by 17% (p < 0.05), while sarcoendoplasmic reticulum Ca²⁺-ATPase (SERCA) activity was decreased by 21% (p < 0.05). LC-MS results showed the presence of vitamin C, malic acid, and several antioxidant phenolic acids reported for the first time. Dihydroeuparin and 4-hydroxy-3-(3-methylbut-2-enyl) acetophenone were abundant in the extract.

CONCLUSION

In normotensive animals, S. nutans partially reduces MAP by decreasing heart rate and cardiac contractility. This negative inotropy is accounted for by decreased SERCA activity and increased NCX activity which reduces SR Ca²⁺ content. These results highlight the plant's potential as a source of novel cardio-active phytopharmaceuticals or nutraceuticals.

Inhibition of myocardial cathepsin-L release during reperfusion following myocardial infarction improves cardiac function and reduces infarct size

AIMS

Identifying novel mediators of lethal myocardial reperfusion injury that can be targeted during primary percutaneous coronary intervention (PPCI) is key to limiting the progression of patients with ST-elevation myocardial infarction (STEMI) to heart failure. Here, we show through parallel clinical and integrative preclinical studies the significance of the protease cathepsin-L on cardiac function during reperfusion injury.

METHODS AND RESULTS

We found that direct cardiac release of cathepsin-L in STEMI patients (n = 76) immediately post-PPCI leads to elevated serum cathepsin-L levels and that serum levels of cathepsin-L in the first 24 h post-reperfusion are associated with reduced cardiac contractile function and increased infarct size. Preclinical studies demonstrate that inhibition of cathepsin-L release following reperfusion injury with CAA0225 reduces infarct size and improves cardiac contractile function by limiting abnormal cardiomyocyte calcium handling and apoptosis.

CONCLUSION

Our findings suggest that cathepsin-L is a novel therapeutic target that could be exploited clinically to counteract the deleterious effects of acute reperfusion injury after an acute STEMI.

Hyperglycaemia-Induced Contractile Dysfunction and Apoptosis in Cardiomyocyte-Like Pulsatile Cells Derived from Mouse Embryonic Stem Cells

Hyperglycaemia, a key metabolic abnormality in diabetes mellitus, is implicated in pathological cardiogenesis during embryological development. However, the underlying mechanisms and potential therapeutic targets remain unknown. We, therefore, studied the effect of hyperglycaemia on mouse embryonic stem cell (mESC) cardiac differentiation. The mESCs were differentiated via embryoid body (EB) formation and cultured under conditions with baseline (25 mM) or high (50 mM) glucose. Time-lapse microscopy images of pulsatile mESCs and Ca²⁺ transients were recorded. Biomarkers of cellular changes were detected using immunocytochemistry, terminal deoxynucleotidyl transferase dUTP nick-end labelling (TUNEL) assay, and Western blot analyses. Differentiated, spontaneously beating mESCs stained positive for cardiac troponin T, α-actinin 2, myosin heavy chain, and connexin 43. Hyperglycaemia decreased the EB diameter and number of beating EBs as well as the cellular amplitude of contraction, the Ca²⁺ transient, and the contractile response to caffeine (1 mM), but had no effect on the expression of the sarco-endoplasmic reticulum calcium transport ATPase 2 (SERCA 2). Furthermore, hyperglycaemia decreased the expression of B cell lymphoma 2 (Bcl-2) and increased the expression of cytoplasmic cytochrome c and the number of TUNEL-positive cells, but had no effect on the expression of one of the mitochondrial fusion regulatory proteins, optic atrophy protein 1 (OPA1). Overall, hyperglycaemia suppressed the mESC cardiomyocyte-like differentiation and induced contractile dysfunction. The results are consistent with mechanisms involving abnormal Ca²⁺ handling and mitochondrial-dependent apoptosis, factors which represent potential therapeutic targets in developmental diabetic cardiac disease.

Enhanced Klotho availability protects against cardiac dysfunction induced by uraemic cardiomyopathy by regulating Ca2+ handling

BACKGROUND AND PURPOSE

Klotho is a membrane-bound or soluble protein, originally identified as an age-suppressing factor and regulator of mineral metabolism. Klotho deficiency is associated with the development of renal disease, but its role in cardiac function in the context of uraemic cardiomyopathy is unknown.

EXPERIMENTAL APPROACH

We explored the effects of Klotho on cardiac Ca2+ cycling. We analysed Ca2+ handling in adult cardiomyocytes from Klotho-deficient (kl/kl) mice and from a murine model of 5/6 nephrectomy (Nfx). We also studied the effect of exogenous Klotho supplementation, by chronic recombinant Klotho treatment, or endogenous Klotho overexpression, using transgenic mice overexpressing Klotho (Tg-Kl), on uraemic cardiomyopathy. Hearts from Nfx mice were used to study Ca2+ sensitivity of ryanodine receptors and their phosphorylation state.

KEY RESULTS

Cardiomyocytes from kl/kl mice showed decreased amplitude of intracellular Ca2+ transients and cellular shortening together with an increase in pro-arrhythmic Ca2+ events compared with cells from wild-type mice. Cardiomyocytes from Nfx mice exhibited the same impairment in Ca2+ cycling as kl/kl mice. Changes in Nfx cardiomyocytes were explained by higher sensitivity of ryanodine receptors to Ca2+ and their increased phosphorylation at the calmodulin kinase type II and protein kinase A sites. Ca2+ mishandling in Nfx-treated mice was fully prevented by chronic recombinant Klotho administration or transgenic Klotho overexpression.

CONCLUSIONS AND IMPLICATIONS

Klotho emerges as an attractive therapeutic tool to improve cardiac Ca2+ mishandling observed in uraemic cardiomyopathy. Strategies that improve Klotho availability are good candidates to protect the heart from functional cardiac alterations in renal disease.

Dilated cardiomyopathy mutations in thin-filament regulatory proteins reduce contractility, suppress systolic Ca2+, and activate NFAT and Akt signaling

Dilated cardiomyopathy (DCM) is clinically characterized by dilated ventricular cavities and reduced ejection fraction, leading to heart failure and increased thromboembolic risk. Mutations in thin-filament regulatory proteins can cause DCM and have been shown in vitro to reduce contractility and myofilament Ca²⁺-affinity. In this work we have studied the functional consequences of mutations in cardiac troponin T (R131W), cardiac troponin I (K36Q) and α-tropomyosin (E40K) using adenovirally transduced isolated guinea pig left ventricular cardiomyocytes. We find significantly reduced fractional shortening with reduced systolic Ca²⁺. Contraction and Ca²⁺ reuptake times were slowed, which contrast with some findings in murine models of myofilament Ca²⁺ desensitization. We also observe increased sarcoplasmic reticulum (SR) Ca²⁺ load and smaller fractional SR Ca²⁺ release. This corresponds to a reduction in SR Ca²⁺-ATPase activity and increase in sodium-calcium exchanger activity. We also observe dephosphorylation and nuclear translocation of the nuclear factor of activated T cells (NFAT), with concordant RAC-α-serine/threonine protein kinase (Akt) phosphorylation but no change to extracellular signal-regulated kinase activation in chronically paced cardiomyocytes expressing DCM mutations. These changes in Ca²⁺ handling and signaling are common to all three mutations, indicating an analogous pathway of disease pathogenesis in thin-filament sarcomeric DCM. Previous work has shown that changes to myofilament Ca²⁺ sensitivity caused by DCM mutations are qualitatively opposite from hypertrophic cardiomyopathy (HCM) mutations in the same genes. However, we find several common pathways such as increased relaxation times and NFAT activation that are also hallmarks of HCM. This suggests more complex intracellular signaling underpinning DCM, driven by the primary mutation.NEW & NOTEWORTHY Dilated cardiomyopathy (DCM) is a frequently occurring cardiac disorder with a degree of genetic inheritance. We have found that DCM mutations in proteins that regulate the contractile machinery cause alterations to contraction, calcium-handling, and some new signaling pathways that provide stimuli for disease development. We have used guinea pig cells that recapitulate human calcium-handling and introduced the mutations using adenovirus gene transduction to look at the initial triggers of disease before remodeling.

Sacubitrilat reduces pro-arrhythmogenic sarcoplasmic reticulum Ca2+ leak in human ventricular cardiomyocytes of patients with end-stage heart failure

Aims

Inhibition of neprilysin and angiotensin II receptor by sacubitril/valsartan (Val) (LCZ696) reduces mortality in heart failure (HF) patients compared with sole inhibition of renin–angiotensin system. Beneficial effects of increased natriuretic peptide levels upon neprilysin inhibition have been proposed, whereas direct effects of sacubitrilat (Sac) (LBQ657) on myocardial Ca²⁺ cycling remain elusive.

Methods and results

Confocal microscopy (Fluo‐4 AM) was used to investigate pro‐arrhythmogenic sarcoplasmic reticulum (SR) Ca²⁺ leak in freshly isolated murine and human ventricular cardiomyocytes (CMs) upon Sac (40 μmol/L)/Val (13 μmol/L) treatment. The concentrations of Sac and Val equalled plasma concentrations of LCZ696 treatment used in PARADIGM‐HF trial. Epifluorescence microscopy measurements (Fura‐2 AM) were performed to investigate effects on systolic Ca²⁺ release, SR Ca²⁺ load, and Ca²⁺‐transient kinetics in freshly isolated murine ventricular CMs. The impact of Sac on myocardial contractility was evaluated using in toto‐isolated, isometrically twitching ventricular trabeculae from human hearts with end‐stage HF. Under basal conditions, the combination of Sac/Val did not influence diastolic Ca²⁺‐spark frequency (CaSpF) nor pro‐arrhythmogenic SR Ca² leak in isolated murine ventricular CMs (n CMs/hearts = 80/7 vs. 100/7, P = 0.91/0.99). In contrast, Sac/Val treatment reduced CaSpF by 35 ± 9% and SR Ca²⁺ leak by 45 ± 9% in CMs put under catecholaminergic stress (isoproterenol 30 nmol/L, n = 81/7 vs. 62/7, P < 0.001 each). This could be attributed to Sac, as sole Sac treatment also reduced both parameters by similar degrees (reduction of CaSpF by 57 ± 7% and SR Ca²⁺ leak by 76 ± 5%; n = 101/4 vs. 108/4, P < 0.01 each), whereas sole Val treatment did not. Systolic Ca²⁺ release, SR Ca²⁺ load, and Ca²⁺‐transient kinetics including SERCA activity (k_SERCA) were not compromised by Sac in isolated murine CMs (n = 41/6 vs. 39/6). Importantly, the combination of Sac/Val and Sac alone also reduced diastolic CaSpF and SR Ca²⁺ leak (reduction by 74 ± 7%) in human left ventricular CMs from patients with end‐stage HF (n = 71/8 vs. 78/8, P < 0.05 each). Myocardial contractility of human ventricular trabeculae was not acutely affected by Sac treatment as the developed force remained unchanged over a time course of 30 min (n trabeculae/hearts = 3/3 vs. 4/3).

Conclusion

This study demonstrates that neprilysin inhibitor Sac directly improves Ca²⁺ homeostasis in human end‐stage HF by reducing pro‐arrhythmogenic SR Ca²⁺ leak without acutely affecting systolic Ca²⁺ release and inotropy. These effects might contribute to the mortality benefits observed in the PARADIGM‐HF trial.

Region-specific mechanisms of corticosteroid-mediated inotropy in rat cardiomyocytes

Regional differences in ion channel activity in the heart control the sequence of repolarization and may contribute to differences in contraction. Corticosteroids such as aldosterone or corticosterone increase the L-type Ca²⁺ current (I_CaL) in the heart via the mineralocorticoid receptor (MR). Here, we investigate the differential impact of corticosteroid-mediated increase in I_CaL on action potentials (AP), ion currents, intracellular Ca²⁺ handling and contractility in endo- and epicardial myocytes of the rat left ventricle. Dexamethasone led to a similar increase in I_CaL in endocardial and epicardial myocytes, while the K⁺ currents I_to and I_K were unaffected. However, AP duration (APD) and AP-induced Ca²⁺ influx (Q_Ca) significantly increased exclusively in epicardial myocytes, thus abrogating the normal differences between the groups. Dexamethasone increased Ca²⁺ transients, contractility and SERCA activity in both regions, the latter possibly due to a decrease in total phospholamban (PLB) and an increase PLBpThr17. These results suggest that corticosteroids are powerful modulators of I_CaL, Ca²⁺ transients and contractility in both endo- and epicardial myocytes, while APD and Q_Ca are increased in epicardial myocytes only. This indicates that increased I_CaL and SERCA activity rather than Q_Ca are the primary drivers of contractility by adrenocorticoids.

Beneficial effects of paricalcitol on cardiac dysfunction and remodelling in a model of established heart failure

BACKGROUND AND PURPOSE

The synthetic vitamin D3 analogue paricalcitol acts as a selective activator of the vitamin D receptor (VDR). While there is evidence for cardioprotective effects of paricalcitol associated with the VDR pathway, less information is available about the structural and functional cardiac effects of paricalcitol on established heart failure (HF) and particularly its effects on associated electrophysiological or Ca²⁺ handling remodelling.

EXPERIMENTAL APPROACH

We used a murine model of transverse aortic constriction (TAC) to study the effect of paricalcitol on established HF. Treatment was initiated 4 weeks after surgery over five consecutive weeks, and mice were sacrificed 9 weeks after surgery. Cardiac MRI (CMRI) was performed 4 and 9 weeks after surgery. Hearts were used for biochemical and histological studies and to isolate ventricular myocytes for electrophysiological and calcium imaging studies.

KEY RESULTS

CMRI analysis revealed that, compared with vehicle, paricalcitol treatment prevented the progression of ventricular dilation and hypertrophy after TAC and halted the corresponding decline in ejection fraction. These beneficial effects were related to the attenuation of intracellular Ca²⁺ mishandling remodelling, antifibrotic and antihypertrophic effects and potentially antiarrhythmic effects by preventing the reduction of K⁺ current density and the long QT, JT and TpTe intervals observed in HF animals.

CONCLUSION AND IMPLICATIONS

The results suggest that paricalcitol treatment in established HF hampers disease progression and improves adverse electrophysiological and Ca²⁺ handling remodelling, attenuating the vulnerability to HF-associated ventricular arrhythmias. Paricalcitol may emerge as a potential therapeutic option in the treatment of HF.

Intracellular calcium mishandling leads to cardiac dysfunction and ventricular arrhythmias in a mouse model of propionic acidemia

Propionic acidemia (PA) is a rare metabolic disease associated with mutations in genes encoding the α and β subunits of the enzyme propionyl-CoA carboxylase. The accumulation of toxic metabolites results in mitochondrial dysfunction, increased reactive oxygen species production and oxidative damage, which have been associated with the disease pathophysiology. Clinical symptoms are heterogeneous and include cardiac complications, mainly cardiac dysfunction and arrhythmias, which are recognized as one of the major life-threatening manifestations in patients. We aimed to investigate the molecular mechanisms underlying the cardiac phenotype using a hypomorphic mouse model (Pcca^-/-(A138T)) that recapitulates some biochemical and clinical characteristics of PA. We demonstrate that Pcca^-/-(A138T) mice present with depressed cardiac function along with impaired cell contractility when compared to the wild-type mice. Cardiac dysfunction in Pcca^-/-(A138T) mice was associated with lower systolic Ca²⁺ release ([Ca²⁺]_i transients), impairment in the sarcoplasmic reticulum (SR) Ca²⁺ load and decreased Ca²⁺ re-uptake by SR-Ca²⁺ ATPase (SERCA2a). These functional changes correlated well with the depressed activity of SERCA2a, the elevated ROS levels and SERCA2a oxidation rate in cardiomyocytes isolated from Pcca^-/-(A138T) mice. In addition, decreased SR-Ca²⁺ load in Pcca^-/-(A138T) cardiomyocytes was associated with increased diastolic Ca²⁺ release. The increase in Ca²⁺ sparks, Ca²⁺ waves and spontaneous [Ca²⁺]_i transients in Pcca^-/-(A138T) cardiomyocytes could be responsible for the induction of ventricular arrhythmias detected in these mice. Overall, our results uncover the role of impaired Ca²⁺ handling in arrhythmias and cardiac dysfunction in PA, and identify new targets for the development of therapeutic approaches for this devastating metabolic disease.

Impaired Activity of Ryanodine Receptors Contributes to Calcium Mishandling in Cardiomyocytes of Metabolic Syndrome Rats

Metabolic syndrome (MetS) has become a global epidemic. MetS is a serious health problem because of its related cardiovascular complications, which include hypertension and delayed heart rate recovery after exercise. The molecular bases of cardiac dysfunction in MetS are still under scrutiny and may be related to anomalies in the activity and expression of key proteins involved in the cardiac excitation-contraction coupling (ECC). The cardiac Ca²⁺ channel/ryanodine receptor (RyR2) participates in releasing Ca²⁺ from internal stores and plays a key role in the modulation of ECC. We examined alterations in expression, phosphorylation status, Ca²⁺ sensitivity, and in situ function (by measuring Ca²⁺ sparks and Ca²⁺ transients) of RyR2; alterations in these characteristics could help to explain the Ca²⁺ handling disturbances in MetS cardiomyocytes. MetS was induced in rats by adding commercially refined sugar (30% sucrose) to their drinking water for 24 weeks. Cardiomyocytes of MetS rats displayed decreased Ca²⁺ transient amplitude and cell contractility at all stimulation frequencies. Quiescent MetS cardiomyocytes showed a decrease in Ca²⁺ spark frequency, amplitude, and spark-mediated Ca²⁺ leak. The [³H]-ryanodine binding data showed that functionally active RyRs are significantly diminished in MetS heart microsomes; and exhibited rapid Ca²⁺-induced inactivation. The phosphorylation of corresponding Ser2814 (a preferential target for CaMKII) of the hRyR2 was significantly diminished. RyR2 protein expression and Ser2808 phosphorylation level were both unchanged. Further, we demonstrated that cardiomyocyte Ca²⁺ mishandling was associated with reduced SERCA pump activity due to decreased Thr17-PLN phosphorylation, suggesting a downregulation of CaMKII in MetS hearts, though the SR Ca²⁺ load remained unchanged. The reduction in the phosphorylation level of RyR2 at Ser2814 decreases RyR2 availability for activation during ECC. In conclusion, the impaired in situ activity of RyR2 may also account for the poor overall cardiac outcome reported in MetS patients; hence, the SERCA pump and RyR2 are both attractive potential targets for future therapies.

Hypertrophic cardiomyopathy mutations increase myofilament Ca2+ buffering, alter intracellular Ca2+ handling, and stimulate Ca2+-dependent signaling

Mutations in thin filament regulatory proteins that cause hypertrophic cardiomyopathy (HCM) increase myofilament Ca²⁺ sensitivity. Mouse models exhibit increased Ca²⁺ buffering and arrhythmias, and we hypothesized that these changes are primary effects of the mutations (independent of compensatory changes) and that increased Ca²⁺ buffering and altered Ca²⁺ handling contribute to HCM pathogenesis via activation of Ca²⁺-dependent signaling. Here, we determined the primary effects of HCM mutations on intracellular Ca²⁺ handling and Ca²⁺-dependent signaling in a model system possessing Ca²⁺-handling mechanisms and contractile protein isoforms closely mirroring the human environment in the absence of potentially confounding remodeling. Using adenovirus, we expressed HCM-causing variants of human troponin-T, troponin-I, and α-tropomyosin (R92Q, R145G, and D175N, respectively) in isolated guinea pig left ventricular cardiomyocytes. After 48 h, each variant had localized to the I-band and comprised ∼50% of the total protein. HCM mutations significantly lowered the K_d of Ca²⁺ binding, resulting in higher Ca²⁺ buffering of mutant cardiomyocytes. We observed increased diastolic [Ca²⁺] and slowed Ca²⁺ reuptake, coupled with a significant decrease in basal sarcomere length and slowed relaxation. HCM mutant cells had higher sodium/calcium exchanger activity, sarcoplasmic reticulum Ca²⁺ load, and sarcoplasmic/endoplasmic reticulum calcium ATPase 2 (SERCA2) activity driven by Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) phosphorylation of phospholamban. The ryanodine receptor (RyR) leak/load relationship was also increased, driven by CaMKII-mediated RyR phosphorylation. Altered Ca²⁺ homeostasis also increased signaling via both calcineurin/NFAT and extracellular signal-regulated kinase pathways. Altered myofilament Ca²⁺ buffering is the primary initiator of signaling cascades, indicating that directly targeting myofilament Ca²⁺ sensitivity provides an attractive therapeutic approach in HCM.

Cellular mechanisms of metabolic syndrome-related atrial decompensation in a rat model of HFpEF

Heart failure (HF) with preserved ejection fraction (HFpEF) is present in about 50% of HF patients. Atrial remodeling is common in HFpEF and associated with increased mortality. We postulate that atrial remodeling is associated with atrial dysfunction in vivo related to alterations in cardiomyocyte Calcium (Ca) signaling and remodeling. We examined atrial function in vivo and Ca transients (CaT) (Fluo4-AM, field stim) in atrial cardiomyocytes of ZSF-1 rats without (Ln; lean hypertensive) and with metabolic syndrome (Ob; obese, hypertensive, diabetic) and HFpEF.

RESULTS

At 21weeks Ln showed an increased left ventricular (LV) mass and left ventricular end-diastolic pressure (LVEDP), but unchanged left atrial (LA) size and preserved atrial ejection fraction vs. wild-type (WT). CaT amplitude in atrial cardiomyocytes was increased in Ln (2.9±0.2 vs. 2.3±0.2F/F₀ in WT; n=22 cells/group; p<0.05). Studying subcellular Ca release in more detail, we found that local central cytosolic CaT amplitude was increased, while subsarcolemmal CaT amplitudes remained unchanged. Moreover, Sarcoplasmic reticulum (SR) Ca content (caffeine) was preserved while Ca spark frequency and tetracaine-dependent SR Ca leak were significantly increased in Ln. Ob mice developed a HFpEF phenotype in vivo, LA area was significantly increased and atrial in vivo function was impaired, despite increased atrial CaT amplitudes in vitro (2.8±0.2; p<0.05 vs. WT). Ob cells showed alterations of the tubular network possibly contributing to the observed phenotype. CaT kinetics as well as SR Ca in Ob were not significantly different from WT, but SR Ca leak remained increased. Angiotensin II (Ang II) reduced in vitro cytosolic CaT amplitudes and let to active nuclear Ca release in Ob but not in Ln or WT.

SUMMARY

In hypertensive ZSF-1 rats, a possibly compensatory increase of cytosolic CaT amplitude and increased SR Ca leak precede atrial remodeling and HFpEF. Atrial remodeling in ZSF-1 HFpEF is associated with an altered tubular network in-vitro and atrial contractile dysfunction in vivo, indicating insufficient compensation. Atrial cardiomyocyte dysfunction in vitro is induced by the addition of angiotensin II.

Computational modeling of amylin-induced calcium dysregulation in rat ventricular cardiomyocytes

Hyperamylinemia is a condition that accompanies obesity and precedes type II diabetes, and it is characterized by above-normal blood levels of amylin, the pancreas-derived peptide. Human amylin oligomerizes easily and can deposit in the pancreas [1], brain [2], and heart [3], where they have been associated with calcium dysregulation. In the heart, accumulating evidence suggests that human amylin oligomers form moderately cation-selective [4,5] channels that embed in the cell sarcolemma (SL). The oligomers increase membrane conductance in a concentration-dependent manner [5], which is correlated with elevated cytosolic Ca²⁺. These findings motivate our core hypothesis that non-selective inward Ca²⁺ conduction afforded by human amylin oligomers increase cytosolic and sarcoplasmic reticulum (SR) Ca²⁺ load, which thereby magnifies intracellular Ca²⁺ transients. Questions remain however regarding the mechanism of amylin-induced Ca²⁺ dysregulation, including whether enhanced SL Ca²⁺ influx is sufficient to elevate cytosolic Ca²⁺ load [6], and if so, how might amplified Ca²⁺ transients perturb Ca²⁺-dependent cardiac pathways. To investigate these questions, we modified a computational model of cardiomyocytes Ca²⁺ signaling to reflect experimentally-measured changes in SL membrane permeation and decreased sarcoplasmic/endoplasmic reticulum calcium ATPase (SERCA) function stemming from acute and transgenic human amylin peptide exposure. With this model, we confirmed the hypothesis that increasing SL permeation alone was sufficient to enhance Ca²⁺ transient amplitudes. Our model indicated that amplified cytosolic transients are driven by increased Ca²⁺ loading of the SR and that greater fractional release may contribute to the Ca²⁺-dependent activation of calmodulin, which could prime the activation of myocyte remodeling pathways. Importantly, elevated Ca²⁺ in the SR and dyadic space collectively drive greater fractional SR Ca²⁺ release for human amylin expressing rats (HIP) and acute amylin-exposed rats (+Amylin) mice, which contributes to the inotropic rise in cytosolic Ca²⁺ transients. These findings suggest that increased membrane permeation induced by oligomeratization of amylin peptide in cell sarcolemma contributes to Ca²⁺ dysregulation in pre-diabetes.

Toll-like receptor 4-induced ryanodine receptor 2 oxidation and sarcoplasmic reticulum Ca2+ leakage promote cardiac contractile dysfunction in sepsis

Studies suggest the potential role of a sarcoplasmic reticulum (SR) Ca²⁺ leak in cardiac contractile dysfunction in sepsis. However, direct supporting evidence is lacking, and the mechanisms underlying this SR leak are poorly understood. Here, we investigated the changes in cardiac Ca²⁺ handling and contraction in LPS-treated rat cardiomyocytes and a mouse model of polymicrobial sepsis produced by cecal ligation and puncture (CLP). LPS decreased the systolic Ca²⁺ transient and myocyte contraction as well as SR Ca²⁺ content. Meanwhile, LPS increased Ca²⁺ spark-mediated SR Ca²⁺ leak. Preventing the SR leak with ryanodine receptor (RyR) blocker tetracaine restored SR load and increased myocyte contraction. Similar alterations in Ca²⁺ handling were observed in cardiomyocytes from CLP mice. Treatment with JTV-519, an anti-SR leak drug, restored Ca²⁺ handling and improved cardiac function. In the LPS-treated cardiomyocytes, mitochondrial reactive oxygen species and oxidative stress in RyR2 were increased, whereas the levels of the RyR2-associated FK506-binding protein 1B (FKBP12.6) were decreased. The Toll-like receptor 4 (TLR4)-specific inhibitor TAK-242 reduced the oxidative stress in LPS-treated cells, decreased the SR leak, and normalized Ca²⁺ handling and myocyte contraction. Consistently, TLR4 deletion significantly improved cardiac function and corrected abnormal Ca²⁺ handling in the CLP mice. This study provides evidence for the critical role of the SR Ca²⁺ leak in the development of septic cardiomyopathy and highlights the therapeutic potential of JTV-519 by preventing SR leak. Furthermore, it reveals that TLR4 activation-induced mitochondrial reactive oxygen species production and the resulting oxidative stress in RyR2 contribute to the SR Ca²⁺ leak.

Sensitivity analysis revealing the effect of modulating ionic mechanisms on calcium dynamics in simulated human heart failure

Abnormal intracellular Ca²⁺ handling is the major contributor to the depressed cardiac contractility observed in heart failure. The electrophysiological remodeling associated with this pathology alters both the action potential and the Ca²⁺ dynamics, leading to a defective excitation-contraction coupling that ends in mechanical dysfunction. The importance of maintaining a correct intracellular Ca²⁺ concentration requires a better understanding of its regulation by ionic mechanisms. To study the electrical activity and ionic homeostasis of failing myocytes, a modified version of the O’Hara et al. human action potential model was used, including electrophysiological remodeling. The impact of the main ionic transport mechanisms was analyzed using single-parameter sensitivity analyses, the first of which explored the modulation of electrophysiological characteristics related to Ca²⁺ exerted by the remodeled parameters. The second sensitivity analysis compared the potential consequences of modulating individual channel conductivities, as one of the main effects of potential drugs, on Ca²⁺ dynamic properties under both normal conditions and in heart failure. The first analysis revealed the important contribution of the sarcoplasmic reticulum Ca²⁺-ATPase (SERCA) dysfunction to the altered Ca²⁺ homeostasis, with the Na⁺/Ca²⁺ exchanger (NCX) and other Ca²⁺ cycling proteins also playing a significant role. Our results highlight the importance of improving the SR uptake function to increase Ca²⁺ content and restore Ca²⁺ homeostasis and contractility. The second sensitivity analysis highlights the different response of the failing myocyte versus the healthy myocyte to potential pharmacological actions on single channels. The result of modifying the conductances of the remodeled proteins such as SERCA and NCX in heart failure has less impact on Ca²⁺ modulation. These differences should be taken into account when designing drug therapies.

Effect of ovariectomy on intracellular Ca2+ regulation in guinea pig cardiomyocytes

This study addressed the hypothesis that long-term deficiency of ovarian hormones after ovariectomy (OVx) alters cellular Ca²⁺-handling mechanisms in the heart, resulting in the formation of a proarrhythmic substrate. It also tested whether estrogen supplementation to OVx animals reverses any alterations to cardiac Ca²⁺ handling and rescues proarrhythmic behavior. OVx or sham operations were performed on female guinea pigs using appropriate anesthetic and analgesic regimes. Pellets containing 17β-estradiol (1 mg, 60-day release) were placed subcutaneously in selected OVx animals (OVx + E). Cardiac myocytes were enzymatically isolated, and electrophysiological measurements were conducted with a switch-clamp system. In fluo-4-loaded cells, Ca²⁺ transients were 20% larger, and fractional sarcoplasmic reticulum (SR) Ca²⁺ release was 7% greater in the OVx group compared with the sham group. Peak L-type Ca²⁺ current was 16% larger in OVx myocytes with channel inactivation shifting to more positive membrane potentials, creating a larger "window" current. SR Ca²⁺ stores were 22% greater in the OVx group, and these cells showed a higher frequency of Ca²⁺ sparks and waves and shorter wave-free intervals. OVx myocytes showed higher frequencies of early afterdepolarizations, and a greater percentage of these cells showed delayed afterdepolarizations after exposure to isoprenaline compared with sham myocytes. The altered Ca²⁺ regulation occurring in the OVx group was not observed in the OVx + E group. These findings suggest that long-term deprivation of ovarian hormones in guinea pigs lead to changes in myocyte Ca²⁺-handling mechanisms that are considered proarrhythmogenic. 17β-Estradiol replacement prevented these adverse effects.NEW & NOTEWORTHY Ovariectomized guinea pig cardiomyocytes have higher frequencies of Ca²⁺ waves, and isoprenaline-challenged cells display more early afterdepolarizations, delayed afterdepolarizations, and extra beats compared with sham myocytes. These alterations to Ca²⁺ regulation were not observed in myocytes from ovariectomized guinea pigs supplemented with 17β-estradiol, suggesting that ovarian hormone deficiency modifies cardiac Ca²⁺ regulation, potentially creating proarrhythmic substrates.

There and back again: Iterating between population-based modeling and experiments reveals surprising regulation of calcium transients in rat cardiac myocytes

While many ion channels and transporters involved in cardiac cellular physiology have been identified and described, the relative importance of each in determining emergent cellular behaviors remains unclear. Here we address this issue with a novel approach that combines population-based mathematical modeling with experimental tests to systematically quantify the relative contributions of different ion channels and transporters to the amplitude of the cellular Ca(2+) transient. Sensitivity analysis of a mathematical model of the rat ventricular cardiomyocyte quantified the response of cell behaviors to changes in the level of each ion channel and transporter, and experimental tests of these predictions were performed to validate or invalidate the predictions. The model analysis found that partial inhibition of the transient outward current in rat ventricular epicardial myocytes was predicted to have a greater impact on Ca(2+) transient amplitude than either: (1) inhibition of the same current in endocardial myocytes, or (2) comparable inhibition of the sarco/endoplasmic reticulum Ca(2+) ATPase (SERCA). Experimental tests confirmed the model predictions qualitatively but showed some quantitative disagreement. This guided us to recalibrate the model by adjusting the relative importance of several Ca(2+) fluxes, thereby improving the consistency with experimental data and producing a more predictive model. Analysis of human cardiomyocyte models suggests that the relative importance of outward currents to Ca(2+) transporters is generalizable to human atrial cardiomyocytes, but not ventricular cardiomyocytes. Overall, our novel approach of combining population-based mathematical modeling with experimental tests has yielded new insight into the relative importance of different determinants of cell behavior.

Biphasic decay of the Ca transient results from increased sarcoplasmic reticulum Ca leak

Key points

Ca leak from the sarcoplasmic reticulum through the ryanodine receptor (RyR) reduces the amplitude of the Ca transient and slows its rate of decay.

In the presence of β‐adrenergic stimulation, RyR‐mediated Ca leak produces a biphasic decay of the Ca transient with a fast early phase and a slow late phase.

Two forms of Ca leak have been studied, Ca‐sensitising (induced by caffeine) and non‐sensitising (induced by ryanodine) and both induce biphasic decay of the Ca transient.

Only Ca‐sensitising leak can be reversed by traditional RyR inhibitors such as tetracaine.

Ca leak can also induce Ca waves. At low levels of leak, waves occur. As leak is increased, first biphasic decay and then slowed monophasic decay is seen. The level of leak has major effects on the shape of the Ca transient.

Abstract

In heart failure, a reduction in Ca transient amplitude and contractile dysfunction can by caused by Ca leak through the sarcoplasmic reticulum (SR) Ca channel (ryanodine receptor, RyR) and/or decreased activity of the SR Ca ATPase (SERCA). We have characterised the effects of two forms of Ca leak (Ca‐sensitising and non‐sensitising) on calcium cycling and compared with those of SERCA inhibition. We measured [Ca²⁺]_i with fluo‐3 in voltage‐clamped rat ventricular myocytes. Increasing SR leak with either caffeine (to sensitise the RyR to Ca activation) or ryanodine (non‐sensitising) had similar effects to SERCA inhibition: decreased systolic [Ca²⁺]_i, increased diastolic [Ca²⁺]_i and slowed decay. However, in the presence of isoproterenol, leak produced a biphasic decay of the Ca transient in the majority of cells while SERCA inhibition produced monophasic decay. Tetracaine reversed the effects of caffeine but not of ryanodine. When caffeine (1 mmol l⁻¹) was added to a cell which displayed Ca waves, the wave frequency initially increased before waves disappeared and biphasic decay developed. Eventually (at higher caffeine concentrations), the biphasic decay was replaced by slow decay. We conclude that, in the presence of adrenergic stimulation, Ca leak can produce biphasic decay; the slow phase results from the leak opposing Ca uptake by SERCA. The degree of leak determines whether decay of Ca waves, biphasic or monophasic, occurs.

Ca leak from the sarcoplasmic reticulum through the ryanodine receptor (RyR) reduces the amplitude of the Ca transient and slows its rate of decay.

In the presence of β‐adrenergic stimulation, RyR‐mediated Ca leak produces a biphasic decay of the Ca transient with a fast early phase and a slow late phase.

Two forms of Ca leak have been studied, Ca‐sensitising (induced by caffeine) and non‐sensitising (induced by ryanodine) and both induce biphasic decay of the Ca transient.

Only Ca‐sensitising leak can be reversed by traditional RyR inhibitors such as tetracaine.

Ca leak can also induce Ca waves. At low levels of leak, waves occur. As leak is increased, first biphasic decay and then slowed monophasic decay is seen. The level of leak has major effects on the shape of the Ca transient.

Altered calcium handling and increased contraction force in human embryonic stem cell derived cardiomyocytes following short term dexamethasone exposure

One limitation in using human pluripotent stem cell derived cardiomyocytes (hPSC-CMs) for disease modeling and cardiac safety pharmacology is their immature functional phenotype compared with adult cardiomyocytes. Here, we report that treatment of human embryonic stem cell derived cardiomyocytes (hESC-CMs) with dexamethasone, a synthetic glucocorticoid, activated glucocorticoid signaling which in turn improved their calcium handling properties and contractility. L-type calcium current and action potential properties were not affected by dexamethasone but significantly faster calcium decay, increased forces of contraction and sarcomeric lengths, were observed in hESC-CMs after dexamethasone exposure. Activating the glucocorticoid pathway can thus contribute to mediating hPSC-CMs maturation.

Cardiac arrest: resuscitation and reperfusion

The modern treatment of cardiac arrest is an increasingly complex medical procedure with a rapidly changing array of therapeutic approaches designed to restore life to victims of sudden death. The 2 primary goals of providing artificial circulation and defibrillation to halt ventricular fibrillation remain of paramount importance for saving lives. They have undergone significant improvements in technology and dissemination into the community subsequent to their establishment 60 years ago. The evolution of artificial circulation includes efforts to optimize manual cardiopulmonary resuscitation, external mechanical cardiopulmonary resuscitation devices designed to augment circulation, and may soon advance further into the rapid deployment of specially designed internal emergency cardiopulmonary bypass devices. The development of defibrillation technologies has progressed from bulky internal defibrillators paddles applied directly to the heart, to manually controlled external defibrillators, to automatic external defibrillators that can now be obtained over-the-counter for widespread use in the community or home. But the modern treatment of cardiac arrest now involves more than merely providing circulation and defibrillation. As suggested by a 3-phase model of treatment, newer approaches targeting patients who have had a more prolonged cardiac arrest include treatment of the metabolic phase of cardiac arrest with therapeutic hypothermia, agents to treat or prevent reperfusion injury, new strategies specifically focused on pulseless electric activity, which is the presenting rhythm in at least one third of cardiac arrests, and aggressive post resuscitation care. There are discoveries at the cellular and molecular level about ischemia and reperfusion pathobiology that may be translated into future new therapies. On the near horizon is the combination of advanced cardiopulmonary bypass plus a cocktail of multiple agents targeted at restoration of normal metabolism and prevention of reperfusion injury, as this holds the promise of restoring life to many patients for whom our current therapies fail.

NOD1, a new player in cardiac function and calcium handling

AIMS

Inflammation is a significant contributor to cardiovascular disease and its complications; however, whether the myocardial inflammatory response is harmonized after cardiac injury remains to be determined. Some receptors of the innate immune system, including the nucleotide-binding oligomerization domain-like receptors (NLRs), play key roles in the host response after cardiac damage. Nucleotide-binding oligomerization domain containing 1 (NOD1), a member of the NLR family, is expressed in the heart, but its functional role has not been elucidated. We determine whether selective NOD1 activation modulates cardiac function and Ca(2+) signalling.

METHODS AND RESULTS

Mice were treated for 3 days with the selective NOD1 agonist C12-iE-DAP (iE-DAP), and cardiac function and Ca(2+) cycling were assessed. We found that iE-DAP treatment resulted in cardiac dysfunction, measured as a decrease in ejection fraction and fractional shortening. Cardiomyocytes isolated from iE-DAP-treated mice displayed a decrease in the L-type Ca(2+) current, [Ca(2+)]i transients and Ca(2+) load, and decreased expression of phospho-phospholamban, sarcoplasmic reticulum-ATPase, and Na(+)-Ca(2+) exchanger. Furthermore, iE-DAP prompted 'diastolic Ca(2+) leak' in cardiomyocytes, resulting from increased Ca(2+) spark frequency and RyR2 over-phosphorylation. Importantly, these iE-DAP-induced changes in Ca(2+) cycling were lost in NOD1(-/-) mice, indicating that iE-DAP exerts its actions through NOD1. Co-treatment of mice with iE-DAP and a selective inhibitor of NF-κB (BAY11-7082) prevented cardiac dysfunction and Ca(2+) handling impairment induced by iE-DAP.

CONCLUSION

Our data provide the first evidence that NOD1 activation induces cardiac dysfunction associated with excitation-contraction coupling impairment through NF-κB activation and uncover a new pro-inflammatory player in the regulation of cardiovascular function.

The effect of 2,5-di-(tert-butyl)-1,4-benzohydroquinone (TBQ) on intracellular Ca2+ handling in rat ventricular myocytes

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The suitability of TBQ as a specific inhibitor of SERCA was investigated.

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TBQ decreased SERCA activity in a concentration dependent manner.

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TBQ inhibited the calcium current.

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TBQ activated an outward current consistent with an ATP-dependent potassium channel.

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TBQ cannot be used as a specific inhibitor of SERCA in rat cardiac myocytes.

2,5-Di-(tert-butyl)-1,4-benzohydroquinone (TBQ) is a reversible inhibitor of SERCA, potentially making it a useful tool to study the effects of SERCA inhibition in cardiac cells. However, it is unknown if TBQ also has effects on other components of ventricular Ca handling. The aim of these experiments was to characterise the effects of TBQ on Ca handling in rat ventricular myocytes and assess its suitability as a specific inhibitor of SERCA. This was achieved by voltage clamp via perforated patch and [Ca²⁺]_i measurement using Fluo-3 AM. TBQ produced a fully reversible, concentration dependent decrease in the rate of systolic Ca decay. 10 μM TBQ decreased the amplitude of the systolic Ca transient by 48 ± 5% and the rate of decay by 54 ± 6%. SR Ca content was also reduced by 62 ± 4%. However, 10 μM TBQ also decreased the peak L-type Ca current by 23 ± 7%. At higher concentrations (100 μM), TBQ also activated an outward current with a current–voltage relationship consistent with a potassium current. This outward current was abolished by Glibenclamide (100 μM). These data show that TBQ can be used to reversibly inhibit SERCA. However, at concentrations that decrease SERCA activity, TBQ also decreases the L-type Ca current and (at higher concentrations) activates an outward current which appears to be an ATP dependent potassium current. We conclude that TBQ cannot be used as a specific inhibitor of SERCA in rat ventricular myocytes.

Perturbed atrial calcium handling in an ovine model of heart failure: potential roles for reductions in the L-type calcium current

Heart failure (HF) is commonly associated with reduced cardiac output and an increased risk of atrial arrhythmias particularly during β-adrenergic stimulation. The aim of the present study was to determine how HF alters systolic Ca(2+) and the response to β-adrenergic (β-AR) stimulation in atrial myocytes. HF was induced in sheep by ventricular tachypacing and changes in intracellular Ca(2+) concentration studied in single left atrial myocytes under voltage and current clamp conditions. The following were all reduced in HF atrial myocytes; Ca(2+) transient amplitude (by 46% in current clamped and 28% in voltage clamped cells), SR dependent rate of Ca(2+) removal (kSR, by 32%), L-type Ca(2+) current density (by 36%) and action potential duration (APD90 by 22%). However, in HF SR Ca(2+) content was increased (by 19%) when measured under voltage-clamp stimulation. Inhibiting the L-type Ca(2+) current (ICa-L) in control cells reproduced both the decrease in Ca(2+) transient amplitude and increase of SR Ca(2+) content observed in voltage-clamped HF cells. During β-AR stimulation Ca(2+) transient amplitude was the same in control and HF cells. However, ICa-L remained less in HF than control cells whilst SR Ca(2+) content was highest in HF cells during β-AR stimulation. The decrease in ICa-L that occurs in HF atrial myocytes appears to underpin the decreased Ca(2+) transient amplitude and increased SR Ca(2+) content observed in voltage-clamped cells.

Acetate transiently inhibits myocardial contraction by increasing mitochondrial calcium uptake

Background

There is a close relationship between cardiovascular disease and cardiac energy metabolism, and we have previously demonstrated that palmitate inhibits myocyte contraction by increasing K_v channel activity and decreasing the action potential duration. Glucose and long chain fatty acids are the major fuel sources supporting cardiac function; however, cardiac myocytes can utilize a variety of substrates for energy generation, and previous studies demonstrate the acetate is rapidly taken up and oxidized by the heart. In this study, we tested the effects of acetate on contractile function of isolated mouse ventricular myocytes.

Results

Acute exposure of myocytes to 10 mM sodium acetate caused a marked, but transient, decrease in systolic sarcomere shortening (1.49 ± 0.20% vs. 5.58 ± 0.49% in control), accompanied by a significant increase in diastolic sarcomere length (1.81 ± 0.01 μm vs. 1.77 ± 0.01 μm in control), with a near linear dose response in the 1–10 mM range. Unlike palmitate, acetate caused no change in action potential duration; however, acetate markedly increased mitochondrial Ca²⁺ uptake. Moreover, pretreatment of cells with the mitochondrial Ca²⁺ uptake blocker, Ru-360 (10 μM), markedly suppressed the effect of acetate on contraction.

Conclusions

Lehninger and others have previously demonstrated that the anions of weak aliphatic acids such as acetate stimulate Ca²⁺ uptake in isolated mitochondria. Here we show that this effect of acetate appears to extend to isolated cardiac myocytes where it transiently modulates cell contraction.

Balanced changes in Ca buffering by SERCA and troponin contribute to Ca handling during β-adrenergic stimulation in cardiac myocytes

AIMS

During activation of cardiac myocytes, less than 1% of cytosolic Ca is free; the rest is bound to buffers, largely SERCA, and troponin C. Signalling by phosphorylation, as occurs during β-adrenergic stimulation, changes the Ca-binding affinity of these proteins and may affect the systolic Ca transient. Our aim was to determine the effects of β-adrenergic stimulation on Ca buffering and to differentiate between the roles of SERCA and troponin.

METHODS AND RESULTS

Ca buffering was studied in cardiac myocytes from mice: wild-type (WT), phospholamban-knockout (PLN-KO), and mice expressing slow skeletal troponin I (ssTnI) that is not protein kinase A phosphorylatable. WT cells showed no change in Ca buffering in response to the β-adrenoceptor agonist isoproterenol (ISO). However, ISO decreased Ca buffering in PLN-KO myocytes, presumably unmasking the role of troponin. This effect was confirmed in WT cells in which SERCA activity was blocked with the application of thapsigargin. In contrast, ISO increased Ca buffering in ssTnI cells, presumably revealing the effect of an increase in Ca binding to SERCA.

CONCLUSIONS

These data indicate the individual roles played by SERCA and troponin in Ca buffering during β-adrenergic stimulation and that these two buffers effectively counterbalance each other so that Ca buffering remains constant during β-adrenergic stimulation, a factor which may be physiologically important. This study also emphasizes the importance of taking into account Ca buffering, particularly in disease states where Ca binding to myofilaments or SERCA may be altered.

Direct measurements of SR free Ca reveal the mechanism underlying the transient effects of RyR potentiation under physiological conditions

AIMS

Most of the calcium that activates contraction is released from the sarcoplasmic reticulum (SR) through the ryanodine receptor (RyR). It is controversial whether activators of the RyR produce a maintained increase in the amplitude of the systolic Ca transient. We therefore aimed to examine the effects of activation of the RyR in large animals under conditions designed to be as physiological as possible while simultaneously measuring SR and cytoplasmic Ca.

METHODS AND RESULTS

Experiments were performed on ventricular myocytes from canine and ovine hearts. Cytoplasmic Ca was measured with fluo-3 and SR Ca with mag-fura-2. Application of caffeine resulted in a brief increase in the amplitude of the systolic Ca transient accompanied by an increase of action potential duration. These effects disappeared with a rate constant of ∼3 s(-1). Similar effects were seen in cells taken from sheep in which heart failure had been induced by rapid pacing. The decrease of Ca transient amplitude was accompanied by a decrease of SR Ca content. During this phase, the maximum (end-diastolic) SR Ca content fell while the minimum systolic increased.

CONCLUSIONS

This study shows that, under conditions designed to be as physiological as possible, potentiation of RyR opening has no maintained effect on the systolic Ca transient. This result makes it unlikely that potentiation of the RyR has a maintained role in positive inotropy.

Mechanisms underlying the impaired contractility of diabetic cardiomyopathy

Cardiac dysfunction is a well-known consequence of diabetes, with sustained hyperglycaemia leading to the development of a cardiomyopathy that is independent of cardiovascular disease or hypertension. Animal models of diabetes are commonly used to study the pathophysiology of diabetic cardiomyopathy, with the hope that increased knowledge will lead ultimately to better therapeutic strategies being developed. At physiological temperature, left ventricular trabeculae isolated from the streptozotocin rat model of type 1 diabetes showed decreased stress and prolonged relaxation, but with no evidence that decreased contractility was a result of altered myocardial Ca²⁺ handling. Although sarcoplasmic reticulum (SR) Ca²⁺ reuptake appeared slower in diabetic trabeculae, it was offset by an increase in action-potential duration, thereby maintaining SR Ca²⁺ content and favouring increased contraction force. Frequency analysis of t-tubule distribution by confocal imaging of ventricular tissue labeled with wheat germ agglutinin or ryanodine receptor antibodies showed a reduced T-power for diabetic tissue, but the differences were minor in comparison to other models of heart failure. The contractile dysfunction appeared to be the result of disrupted F-actin in conjunction with the increased type I collagen, with decreased myofilament Ca²⁺ sensitivity contributing to the slowed relaxation.

Dantrolene suppresses spontaneous Ca2+ release without altering excitation-contraction coupling in cardiomyocytes of aged mice

Cardiac dysfunction in the aged heart reflects abnormalities in cardiomyocyte Ca(2+) homeostasis including altered Ca(2+) cycling through the sarcoplasmic reticulum (SR). The ryanodine receptor antagonist dantrolene exerts antiarrhythmic effects by preventing spontaneous diastolic Ca(2+) release from the SR. We tested the hypothesis that dantrolene prevents spontaneous Ca(2+) release without altering excitation-contraction coupling in aged myocardium. Left ventricular cardiomyocytes isolated from young (3 to 4 mo) and aged (24-26 mo) C57BL/6 mice were loaded with the Ca(2+) indicator fluo-4. Amplitudes of action potential-induced Ca(2+) transients at 1-Hz pacing were similar between young and aged mice, yet cell shortening was impaired in aged mice. Isoproterenol (1 μM) increased Ca(2+) transient amplitude and cell shortening to identical levels in young and aged; dantrolene (1 μM) had no effect on Ca(2+) transients or cell shortening during pacing. Under Ca(2+) overload conditions induced with 10 mM extracellular Ca(2+) concentration, spontaneous Ca(2+) waves were of diminished amplitude and associated with lower SR Ca(2+) content in aged versus young mice. Despite no effect in young mice, dantrolene increased SR Ca(2+) content and Ca(2+) wave amplitude in aged mice. In the presence of isoproterenol following rest from 1-Hz pacing, Ca(2+) spark frequency was elevated in aged mice, yet the time to spontaneous Ca(2+) wave was similar between young and aged mice; dantrolene decreased Ca(2+) spark frequency and prolonged the time to Ca(2+) wave onset in aged mice with no effect in young mice. Thus dantrolene attenuates diastolic Ca(2+) release in the aged murine heart that may prove useful in preventing cardiac dysfunction.

SERCA2a gene therapy in heart failure: an anti-arrhythmic positive inotrope

Therapeutic options that directly enhance cardiomyocyte contractility in chronic heart failure (HF) therapy are currently limited and do not improve prognosis. In fact, most positive inotropic agents, such as β-adrenoreceptor agonists and PDE inhibitors, which have been assessed in HF patients, cause increased mortality as a result of arrhythmia and sudden cardiac death. Cardiac sarcoplasmic reticulum Ca(2)(+) -ATPase2a (SERCA2a) is a key protein involved in sequestration of Ca(2)(+) into the sarcoplasmic reticulum (SR) during diastole. There is a reduction of SERCA2a protein level and function in HF, which has been successfully targeted via viral transfection of the SERCA2a gene into cardiac tissue in vivo. This has enhanced cardiac contractility and reduced mortality in several preclinical models of HF. Theoretical concerns have been raised regarding the possibility of arrhythmogenic adverse effects of SERCA2a gene therapy due to enhanced SR Ca(2)(+) load and induction of SR Ca(2)(+) leak as a result. Contrary to these concerns, SERCA2a gene therapy in a wide variety of preclinical models, including acute ischaemia/reperfusion, chronic pressure overload and chronic myocardial infarction, has resulted in a reduction in ventricular arrhythmias. The potential mechanisms for this unexpected beneficial effect, as well as mechanisms of enhancement of cardiac contractile function, are reviewed in this article.

Complete atrial-specific knockout of sodium-calcium exchange eliminates sinoatrial node pacemaker activity

The origin of sinoatrial node (SAN) pacemaker activity in the heart is controversial. The leading candidates are diastolic depolarization by "funny" current (If) through HCN4 channels (the "Membrane Clock" hypothesis), depolarization by cardiac Na-Ca exchange (NCX1) in response to intracellular Ca cycling (the "Calcium Clock" hypothesis), and a combination of the two ("Coupled Clock"). To address this controversy, we used Cre/loxP technology to generate atrial-specific NCX1 KO mice. NCX1 protein was undetectable in KO atrial tissue, including the SAN. Surface ECG and intracardiac electrograms showed no atrial depolarization and a slow junctional escape rhythm in KO that responded appropriately to β-adrenergic and muscarinic stimulation. Although KO atria were quiescent they could be stimulated by external pacing suggesting that electrical coupling between cells remained intact. Despite normal electrophysiological properties of If in isolated patch clamped KO SAN cells, pacemaker activity was absent. Recurring Ca sparks were present in all KO SAN cells, suggesting that Ca cycling persists but is uncoupled from the sarcolemma. We conclude that NCX1 is required for normal pacemaker activity in murine SAN.

Genetic modulation of the SERCA activity does not affect the Ca(2+) leak from the cardiac sarcoplasmic reticulum

The Ca(2+) content in the sarcoplasmic reticulum (SR) determines the amount of Ca(2+) released, thereby regulating the magnitude of Ca(2+) transient and contraction in cardiac muscle. The Ca(2+) content in the SR is known to be regulated by two factors: the activity of the Ca(2+) pump (SERCA) and Ca(2+) leak through the ryanodine receptor (RyR). However, the direct relationship between the SERCA activity and Ca(2+) leak has not been fully investigated in the heart. In the present study, we evaluated the role of the SERCA activity in Ca(2+) leak from the SR using a novel saponin-skinned method combined with transgenic mouse models in which the SERCA activity was genetically modulated. In the SERCA overexpression mice, the Ca(2+) uptake in the SR was significantly increased and the Ca(2+) transient was markedly increased. However, Ca(2+) leak from the SR did not change significantly. In mice with overexpression of a negative regulator of SERCA, sarcolipin, the Ca(2+) uptake by the SR was significantly decreased and the Ca(2+) transient was markedly decreased. Again, Ca(2+) leak from the SR did not change significantly. In conclusion, the selective modulation of the SERCA activity modulates Ca(2+) uptake, although it does not change Ca(2+) leak from the SR.

The effects of tumor necrosis factor-alpha on systolic and diastolic function in rat ventricular myocytes

The proinflammatory cytokine tumor necrosis factor-alpha (TNF-α) is associated with myocardial dysfunction observed in sepsis and septic shock. There are two fundamental components to this dysfunction. (1) systolic dysfunction; and (2) diastolic dysfunction. The aim of these experiments was to determine if any aspect of whole-heart dysfunction could be explained by alterations to global intracellular calcium ([Ca²⁺]_i), contractility, and [Ca²⁺]_i handling, by TNF-α, at the level of the individual rat myocyte. We took an integrative approach to simultaneously measure [Ca²⁺]_i, contractility and sarcolemmal Ca fluxes using the Ca indicator fluo-3, video edge detection, and the perforated patch technique, respectively. All experiments were performed at 37°C. The effects of 50 ng/mL TNF-α were immediate and sustained. The amplitude of systolic [Ca²⁺]_i was reduced by 31% and systolic shortening by 19%. Diastolic [Ca²⁺]_i, myocyte length and relaxation rate were not affected, nor were the activity of the [Ca²⁺]_i removal mechanisms. The reduction in systolic [Ca²⁺]_i was associated with a 14% reduction in sarcoplasmic reticulum (SR) content and a 11% decrease in peak L-type Ca current (I_Ca-L). Ca influx was decreased by 7% associated with a more rapid I_Ca-L inactivation. These data show that at the level of the myocyte, TNF-α reduces SR Ca which underlies a reduction in systolic [Ca²⁺]_i and thence shortening. Although these findings correlate well with aspects of systolic myocardial dysfunction seen in sepsis, in this model, acutely, TNF-α does not appear to provide a cellular mechanism for sepsis-related diastolic myocardial dysfunction.

Trypanosoma brucei cathepsin-L increases arrhythmogenic sarcoplasmic reticulum-mediated calcium release in rat cardiomyocytes

Aims

African trypanosomiasis, caused by Trypanosoma brucei species, leads to both neurological and cardiac dysfunction and can be fatal if untreated. While the neurological-related pathogenesis is well studied, the cardiac pathogenesis remains unknown. The current study exposed isolated ventricular cardiomyocytes and adult rat hearts to T. brucei to test whether trypanosomes can alter cardiac function independent of a systemic inflammatory/immune response.

Methods and results

Using confocal imaging, T. brucei and T. brucei culture media (supernatant) caused an increased frequency of arrhythmogenic spontaneous diastolic sarcoplasmic reticulum (SR)-mediated Ca²⁺ release (Ca²⁺ waves) in isolated adult rat ventricular cardiomyocytes. Studies utilising inhibitors, recombinant protein and RNAi all demonstrated that this altered SR function was due to T. brucei cathepsin-L (TbCatL). Separate experiments revealed that TbCatL induced a 10–15% increase of SERCA activity but reduced SR Ca²⁺ content, suggesting a concomitant increased SR-mediated Ca²⁺ leak. This conclusion was supported by data demonstrating that TbCatL increased Ca²⁺ wave frequency. These effects were abolished by autocamtide-2-related inhibitory peptide, highlighting a role for CaMKII in the TbCatL action on SR function. Isolated Langendorff perfused whole heart experiments confirmed that supernatant caused an increased number of arrhythmic events.

Conclusion

These data demonstrate for the first time that African trypanosomes alter cardiac function independent of a systemic immune response, via a mechanism involving extracellular cathepsin-L-mediated changes in SR function.

Enhanced Ca²+ influx through cardiac L-type Ca²+ channels maintains the systolic Ca²+ transient in early cardiac atrophy induced by mechanical unloading

Cardiac atrophy as a consequence of mechanical unloading develops following exposure to microgravity or prolonged bed rest. It also plays a central role in the reverse remodelling induced by left ventricular unloading in patients with heart failure. Surprisingly, the intracellular Ca²⁺ transients which are pivotal to electromechanical coupling and to cardiac plasticity were repeatedly found to remain unaffected in early cardiac atrophy. To elucidate the mechanisms underlying the preservation of the Ca²⁺ transients, we investigated Ca²⁺ cycling in cardiomyocytes from mechanically unloaded (heterotopic abdominal heart transplantation) and control (orthotopic) hearts in syngeneic Lewis rats. Following 2 weeks of unloading, sarcoplasmic reticulum (SR) Ca²⁺ content was reduced by ~55 %. Atrophic cardiac myocytes also showed a much lower frequency of spontaneous diastolic Ca²⁺ sparks and a diminished systolic Ca²⁺ release, even though the expression of ryanodine receptors was increased by ~30 %. In contrast, current clamp recordings revealed prolonged action potentials in endocardial as well as epicardial myocytes which were associated with a two to fourfold higher sarcolemmal Ca²⁺ influx under action potential clamp. In addition, Cav1.2 subunits which form the pore of L-type Ca²⁺ channels (LTCC) were upregulated in atrophic myocardium. These data suggest that in early cardiac atrophy induced by mechanical unloading, an augmented sarcolemmal Ca²⁺ influx through LTCC fully compensates for a reduced systolic SR Ca²⁺ release to preserve the Ca²⁺ transient. This interplay involves an electrophysiological remodelling as well as changes in the expression of cardiac ion channels.

Long-term administration of rosuvastatin prevents contractile and electrical remodelling of diabetic rat heart

In recent years, many findings have been presented about the potential benefit of statin therapy on diabetes-induced cardiovascular complications. Cardioprotective effects of statins were suggested to be mediated at least in part through inhibition of small GTPases, particularly those of the Rho family. The present study was designed to examine whether rosuvastatin can improve electrical remodeling and contractile dysfunction in type 1 diabetic rat heart via modulation of RhoA pathway. Type 1 diabetes was induced by single dose injection of STZ (50 mg/kg). One week after injection rosuvastatin (10 mg/kg/day) and sham treatment was given for 5 weeks in the diabetic rats, as well as in control groups. Shortening and Ca²⁺ transients were recorded in myocytes loaded with Fura2-AM. Membrane currents and Ca²⁺ transients were measured synchronously via whole-cell patch clamping. In untreated diabetic rats, relaxation of shortening and decay of the matched Ca²⁺ transients were prolonged. Fractional shortening and Ca²⁺ transients were also decreased. Rosuvastatin treatment reversed those changes. I(CaL) density did not change in either group but rosuvastatin recovered the loss of sarcoplasmic reticulum Ca²⁺ and Na⁺/Ca²⁺ exchange as evidenced from amplitude and decay of caffeine-induced Ca²⁺ transients, peak INCX and calculated sarcoplasmic reticulum Ca²⁺ content. Diabetes-induced attenuation of I(to) and I(sus) was also reversed, whilst I(K1) was unchanged in diabetes and unaffected by treatment. Rosuvastatin prevented the diabetes-induced increase in RhoA expression. Plasma cholesterol and triglyceride levels were higher in diabetic rats, but rosuvastatin reduced only the latter. In conclusion, HMG-CoA reductase inhibitor rosuvastatin can prevent diabetes-induced electrical and functional remodeling of heart due to inhibition of RhoA signalling rather than reduction of cholesterol level.

Calcium flux balance in the heart

This article reviews the consequences of the need for the cardiac cell to be in calcium flux balance in the steady state. We first discuss how this steady state condition affects the control of resting [Ca(2+)]i. The next section considers how sarcoplasmic reticulum (SR) Ca content is controlled by a feedback mechanism whereby changes of SR Ca affect the amplitude of the Ca transient and this, in turn, controls sarcolemmal Ca fluxes. Subsequent sections review the effects of altering the activity of individual Ca handling proteins. Increasing the activity of the SR Ca-ATPase (SERCA) increases both the amplitude and rate constant of decay of the systolic Ca transient. The Ca flux balance condition requires that this must be achieved with no change of Ca efflux placing constraints on the magnitude of change of amplitude and decay rate. We analyze the quantitative dependence of Ca transient amplitude and SR content on SERCA activity. Increasing the open probability of the RyR during systole is predicted to have no steady state effect on the amplitude of the systolic Ca transient. We discuss the effects of changing the amplitude of the L-type Ca current in the context of both triggering Ca release from the SR and loading the cell with calcium. These manoeuvres are considered in the context of the effects of β-adrenergic stimulation. Finally, we review calcium flux balance in the presence of Ca waves.

Facilitation of cytosolic calcium wave propagation by local calcium uptake into the sarcoplasmic reticulum in cardiac myocytes

The widely accepted paradigm for cytosolic Ca(2+) wave propagation postulates a 'fire-diffuse-fire' mechanism where local Ca(2+)-induced Ca(2+) release (CICR) from the sarcoplasmic reticulum (SR) via ryanodine receptor (RyR) Ca(2+) release channels diffuses towards and activates neighbouring release sites, resulting in a propagating Ca(2+) wave. A recent challenge to this paradigm proposed the requirement for an intra-SR 'sensitization' Ca(2+) wave that precedes the cytosolic Ca(2+) wave and primes RyRs from the luminal side to CICR. Here, we tested this hypothesis experimentally with direct simultaneous measurements of cytosolic ([Ca(2+)](i); rhod-2) and intra-SR ([Ca(2+)](SR); fluo-5N) calcium signals during wave propagation in rabbit ventricular myocytes, using high resolution fluorescence confocal imaging. The increase in [Ca(2+)](i) at the wave front preceded depletion of the SR at each point along the calcium wave front, while during this latency period a transient increase of [Ca(2+)](SR) was observed. This transient elevation of [Ca(2+)](SR) could be identified at individual release junctions and depended on the activity of the sarco-endoplasmic reticulum Ca(2+)-ATPase (SERCA). Increased SERCA activity (β-adrenergic stimulation with 1 μM isoproterenol (isoprenaline)) decreased the latency period and increased the amplitude of the transient elevation of [Ca(2+)](SR), whereas inhibition of SERCA (3 μM cyclopiazonic acid) had the opposite effect. In conclusion, the data provide experimental evidence that local Ca(2+) uptake by SERCA into the SR facilitates the propagation of cytosolic Ca(2+) waves via luminal sensitization of the RyR, and supports a novel paradigm of a 'fire-diffuse-uptake-fire' mechanism for Ca(2+) wave propagation in cardiac myocytes.

Isolated rabbit working heart function during progressive inhibition of myocardial SERCA activity

RATIONALE

The extent to which sarcoplasmic reticulum Ca(2+)ATPase (SERCA) activity alone determines left ventricular (LV) pump function is unknown.

OBJECTIVE

To correlate SERCA activity with hemodynamic function of rabbit LV during thapsigargin perfusion.

METHODS AND RESULTS

Isolated rabbit hearts were perfused in working heart configuration, and LV pump function was assessed using a pressure-volume catheter. Rapid and complete (>95%) inhibition of SERCA was associated with a moderate decrease in cardiac function (to 70%-85% of control). Further decrease in cardiac function to 50%-75% of control occurred over the next ≈ 30 minutes despite no detectable further inhibition of SERCA activity. Analysis of the 20 seconds prior to pump failure revealed a rapid decrease in end diastolic volume. Intermediate levels of SERCA function (≈ 50% of control) had only minor hemodynamic effects. Parallel experiments in field-stimulated isolated ventricular cardiomyocytes monitored intracellular Ca(2+) and cell shortening. On perfusion with thapsigargin, Ca(2+) transient amplitude and cell shortening fell to ≈ 70% of control followed by increased diastolic Ca(2+) concentration and diastolic cell shortening to achieve a new steady state.

CONCLUSIONS

The relationship between SERCA activity and LV function in the rabbit is highly nonlinear. In the short term, only moderate effects on LV pump function were observed despite almost complete (>95%) reduction in SERCA activity. The terminal decline of function was associated with sudden sustained increase in diastolic tone comparable to the sustained contraction observed in isolated cardiomyocytes. Secondary increases of intracellular Ca(2+) and Na(+) following complete SERCA inhibition eventually limit contractile function and precipitate LV pump failure.