Regulation of Na+/Ca2+ exchange current in the normal and failing heart

B-type natriuretic peptide and its role in altering Ca2+-regulatory proteins in heart failure-mechanistic insights

Heart failure (HF) is a worldwide disease with high levels of morbidity and mortality. The pathogenesis of HF is complicated and involves imbalances in hormone and electrolyte. B-type natriuretic peptide (BNP) has served as a biomarker of HF severity, and in recent years, it has been used to treat the disease, thanks to its cardio-protective effects, such as diuresis, natriuresis, and vasodilatation. In stage C/D HF, symptoms are severe despite elevated BNP. Disturbances in Ca²⁺ homeostasis are often a dominating feature of the disease, causing Ca²⁺-regulatory protein dysfunction, including reduced expression and activity of sarcoplasmic reticulum Ca²⁺-ATPase2a (SERCA2a), impaired ryanodine receptors (RYRs) function, intensive Na⁺-Ca²⁺ exchanger (NCX), and downregulation of S100A1. The relationship between natriuretic peptides (NPs) and Ca²⁺-regulatory proteins has been widely studied and represents important mechanisms in the etiology of HF. In this review, we present evidence that BNP may regulate Ca²⁺-regulatory proteins, in particular, suppressing SERCA2a and S100A1 expression. However, relationships between BNP and other Ca²⁺-regulatory proteins remain vague.

Mitochondrial oxidative stress during cardiac lipid overload causes intracellular calcium leak and arrhythmia

BACKGROUND

Diabetes and obesity are associated with an increased risk of arrhythmia and sudden cardiac death. Abnormal lipid accumulation is observed in cardiomyocytes of obese and diabetic patients, which may contribute to arrhythmia, but the mechanisms are poorly understood. A transgenic mouse model of cardiac lipid overload, the peroxisome proliferator-activated receptor-γ (PPARg) cardiac overexpression mouse, has long QT and increased ventricular ectopy.

OBJECTIVE

The purpose of this study was to evaluate the hypothesis that the increase in ventricular ectopy during cardiac lipid overload is caused by abnormalities in calcium handling due to increased mitochondrial oxidative stress.

METHODS

Ventricular myocytes were isolated from adult mouse hearts to record sparks and calcium transients. Mice were implanted with heart rhythm monitors for in vivo recordings.

RESULTS

PPARg cardiomyocytes have more frequent triggered activity and increased sparks compared to control. Sparks and triggered activity are reduced by mitotempo, a mitochondrial-targeted antioxidant. This is explained by a significant increase in oxidation of RyR2. Calcium transients are increased in amplitude, and sarcoplasmic reticulum (SR) calcium stores are increased in PPARg cardiomyocytes. Computer modeling of the cardiac action potential demonstrates that long QT contributes to increased SR calcium. Mitotempo decreased ventricular ectopy in vivo.

CONCLUSION

During cardiac lipid overload, mitochondrial oxidative stress causes increased SR calcium leak by oxidizing RyR2 channels. This promotes ventricular ectopy, which is significantly reduced in vivo by a mitochondrial-targeted antioxidant. These results suggest a potential role for mitochondrial-targeted antioxidants in preventing arrhythmia and sudden cardiac death in obese and diabetic patients.

Leucine-rich repeat kinase 2-sensitive Na+/Ca2+ exchanger activity in dendritic cells

Gene variants of the leucine-rich repeat kinase 2 (LRRK2) are associated with susceptibility to Parkinson's disease (PD). Besides brain and periphery, LRRK2 is expressed in various immune cells including dendritic cells (DCs), antigen-presenting cells linking innate and adaptive immunity. However, the function of LRRK2 in the immune system is still incompletely understood. Here, Ca(2+)-signaling was analyzed in DCs isolated from gene-targeted mice lacking lrrk2 (Lrrk2(-/-)) and their wild-type littermates (Lrrk2(+/+)). According to Western blotting, Lrrk2 was expressed in Lrrk2(+/+) DCs but not in Lrrk2(-/-)DCs. Cytosolic Ca(2+) levels ([Ca(2+)]i) were determined utilizing Fura-2 fluorescence and whole cell currents to decipher electrogenic transport. The increase of [Ca(2+)]i following inhibition of sarcoendoplasmatic Ca(2+)-ATPase with thapsigargin (1 µM) in the absence of extracellular Ca(2+) (Ca(2+)-release) and the increase of [Ca(2+)]i following subsequent readdition of extracellular Ca(2+) (SOCE) were both significantly larger in Lrrk2(-/-) than in Lrrk2(+/+) DCs. The augmented increase of [Ca(2+)]i could have been due to impaired Ca(2+) extrusion by K(+)-independent (NCX) and/or K(+)-dependent (NCKX) Na(+)/Ca(2+)-exchanger activity, which was thus determined from the increase of [Ca(2+)]i, (Δ[Ca(2+)]i), and current following abrupt replacement of Na(+) containing (130 mM) and Ca(2+) free (0 mM) extracellular perfusate by Na(+) free (0 mM) and Ca(2+) containing (2 mM) extracellular perfusate. As a result, both slope and peak of Δ[Ca(2+)]i as well as Na(+)/Ca(2+) exchanger-induced current were significantly lower in Lrrk2(-/-) than in Lrrk2(+/+) DCs. A 6 or 24 hour treatment with the LRRK2 inhibitor GSK2578215A (1 µM) significantly decreased NCX1 and NCKX1 transcript levels, significantly blunted Na(+)/Ca(2+)-exchanger activity, and significantly augmented the increase of [Ca(2+)]i following Ca(2+)-release and SOCE. In conclusion, the present observations disclose a completely novel functional significance of LRRK2, i.e., the up-regulation of Na(+)/Ca(2+) exchanger transcription and activity leading to attenuation of Ca(2+)-signals in DCs.

S100A1: a major player in cardiovascular performance

Calcium cycling is a major determinant of cardiac function. S100A1 is the most abundant member of the calcium-binding S100 protein family in myocardial tissue. S100A1 interacts with a variety of calcium regulatory proteins such as SERCA2a, ryanodine receptors, L-type calcium channels and Na(+)/Ca(2+) exchangers, thus enhancing calcium cycling. Aside from this major function, S100A1 has an important role in energy balance, myofilament sliding, myofilament calcium sensibility, titin-actin interaction, apoptosis and cardiac remodeling. Apart from its properties regarding cardiomyocytes, S100A1 is also important in vessel relaxation and angiogenesis. S100A1 potentiates cardiac function thus increasing the cardiomyocytes' functional reserve; this is an important feature in heart failure. In fact, S100A1 seems to normalize cardiac function after myocardial infarction. Also, S100A1 is essential in the acute response to adrenergic stimulation. Gene therapy experiments show promising results, although further studies are still needed to reach clinical practice. In this review, we aim to describe the molecular basis and regulatory function of S100A1, exploring its interactions with a myriad of target proteins. We also explore its functional effects on systolic and diastolic function as well as its acute actions. Finally, we discuss S100A1 gene therapy and its progression so far.

Differential effects of S100 proteins A2 and A6 on cardiac Ca(2+) cycling and contractile performance

Defective intracellular calcium (Ca(2+)) handling is implicated in the pathogenesis of heart failure. Novel approaches targeting both cardiac Ca(2+) release and reuptake processes, such as S100A1, have the potential to rescue the function of failing cardiac myocytes. Here, we show that two members of the S100 Ca(2+) binding protein family, S100A2 and S100A6 that share high sequence homology, differentially influence cardiac Ca(2+) handling and contractility. Cardiac gene expression of S100A2 significantly enhanced both contractile and relaxation performance of rodent and canine cardiac myocytes, mimicking the functional effects of its cardiac homologue, S100A1. To interrogate mechanism, Ca(2+) spark frequency, a measure of the gating of the ryanodine receptor Ca(2+) release channel, was found to be significantly increased by S100A2. Therapeutic testing showed that S100A2 rescued the contractile defects of failing cardiac myocytes. In contrast, cardiac expression of S100A6 had no significant effects on contractility or Ca(2+) handling. These data reveal novel differential effects of S100 proteins on cardiac myocyte performance that may be useful in application to diseased cardiac muscle.

Ginseng-Aconite Decoction elicits a positive inotropic effect via the reverse mode Na+/Ca2+ exchanger in beating rabbit atria

ETHNOPHARMACOLOGICAL RELEVANCE

Ginseng-Aconite Decoction (GAD), a traditional oriental medicine composed of Panax ginseng C.A. Mey. (Araliaceae) and Aconitum carmichaeli Debx. (Ranunculaceae) has been used as treatment for cardiovascular diseases from Song Dynasty of China. The purpose of the present study was to elucidate the possible mechanisms of GAD-induced positive inotropic effect.

MATERIAL AND METHODS

GAD-induced changes in atrial dynamics and cAMP efflux were determined in isolated perfused beating rabbit atria.

RESULTS

GAD significantly increased atrial dynamics such as stroke volume, pulse pressure and augmented cAMP efflux in beating rabbit atria. The inotropic effect was significantly attenuated by pre-treatment with KB-R7943, a reverse mode Na(+)/Ca(2+) exchanger blocker. The GAD-induced increase in atrial dynamics was also markedly inhibited by staurosporine, a non-selective protein kinase inhibitor, and partly blocked by KT5720, a selective PKA inhibitor. The effect of GAD on atrial dynamics was not altered by pre-treatment with propranolol, a β-adrenergic receptor inhibitor, or diltiazem, an L-type Ca(2+)channel blocker. The phosphodiesterase (PDE) inhibitor 3-isobutyl-1-methylxanthine (IBMX) failed to modulate the GAD-induced increase in atrial dynamics, but markedly attenuated cAMP efflux in the beating atria.

CONCLUSION

These results suggest that the GAD-induced positive inotropic effect in beating rabbit atria may be attributable to stimulation of the reverse mode Na(+)/Ca(2+) exchanger, while PKA activity would, at least in part, be participated in the course.

Altered regulation of cytosolic Ca²⁺ concentration in dendritic cells from klotho hypomorphic mice

The function of dendritic cells (DCs), antigen-presenting cells regulating naïve T-cells, is regulated by cytosolic Ca²⁺ concentration ([Ca²⁺]i). [Ca²⁺]i is increased by store-operated Ca²⁺ entry and decreased by K⁺-independent (NCX) and K⁺-dependent (NCKX) Na⁺/Ca²⁺ exchangers. NCKX exchangers are stimulated by immunosuppressive 1,25-dihydroxyvitamin D3 [1,25(OH)₂D₃], the biologically active form of vitamin D. Formation of 1,25(OH)₂D₃ is inhibited by the antiaging protein Klotho. Thus 1,25(OH)₂D₃ plasma levels are excessive in Klotho-deficient mice (klothohm). The present study explored whether Klotho deficiency modifies [Ca²⁺]i regulation in DCs. DCs were isolated from the bone marrow of klothohm mice and wild-type mice (klotho+/+) and cultured for 7-9 days with granulocyte-macrophage colony-stimulating factor. According to major histocompatibility complex II (MHC II) and CD86 expression, differentiation and lipopolysaccharide (LPS)-induced maturation were similar in klothohm DCs and klotho+/+ DCs. However, NCKX1 membrane abundance and NCX/NCKX-activity were significantly enhanced in klothohm DCs. The [Ca²⁺]i increase upon acute application of LPS (1 μg/ml) was significantly lower in klothohm DCs than in klotho+/+ DCs, a difference reversed by the NCKX blocker 3',4'-dichlorobenzamyl (DBZ; 10 μM). CCL21-dependent migration was significantly less in klothohm DCs than in klotho+/+ DCs but could be restored by DBZ. NCKX activity was enhanced by pretreatment of klotho+/+ DC precursors with 1,25(OH)₂D₃ the first 2 days after isolation from bone marrow. Feeding klothohm mice a vitamin D-deficient diet decreased NCKX activity, augmented LPS-induced increase of [Ca²⁺]i, and enhanced migration of klothohm DCs, thus dissipating the differences between klothohm DCs and klotho+/+ DCs. In conclusion, Klotho deficiency upregulates NCKX1 membrane abundance and Na⁺/Ca²⁺-exchange activity, thus blunting the increase of [Ca²⁺]i following LPS exposure and CCL21-mediated migration. The effects are in large part due to excessive 1,25(OH)₂D₃ formation.

Enhanced Ca²⁺ entry and Na+/Ca²⁺ exchanger activity in dendritic cells from AMP-activated protein kinase-deficient mice

In dendritic cells (DCs), chemotactic chemokines, such as CXCL12, rapidly increase cytosolic Ca(2+)concentrations ([Ca(2+)](i)) by triggering Ca(2+) release from intracellular stores followed by store-operated Ca(2+) (SOC) entry. Increase of [Ca(2+)](i) is blunted and terminated by Ca(2+) extrusion, accomplished by K(+)-independent Na(+)/Ca(2+) exchangers (NCXs) and K(+)-dependent Na(+)/Ca(2+) exchangers (NCKXs). Increased [Ca(2+)](i) activates energy-sensing AMP-activated protein kinase (AMPK), which suppresses proinflammatory responses of DCs and macrophages. The present study explored whether AMPK participates in the regulation of DC [Ca(2+)](i) and migration. DCs were isolated from AMPKα1-deficient (ampk(-/-)) mice and, as control, from their wild-type (ampk(+/+)) littermates. AMPKα1, Orai1-2, STIM1-2, and mitochondrial calcium uniporter protein expression was determined by Western blotting, [Ca(2+)](i) by Fura-2 fluorescence, SOC entry by inhibition of endosomal Ca(2+) ATPase with thapsigargin (1 μM), Na(+)/Ca(2+) exchanger activity from increase of [Ca(2+)](i), and respective whole-cell current in patch clamp following removal of extracellular Na(+). Migration was quantified utilizing transwell chambers. AMPKα1 protein is expressed in ampk(+/+) DCs but not in ampk(-/-) DCs. CXCL12 (300 ng/ml)-induced increase of [Ca(2+)](i), SOC entry, Orai 1 protein abundance, NCX, and NCKX were all significantly higher in ampk(-/-) DCs than in ampk(+/+) DCs. NCX and NCKX currents were similarly increased in ampk(-/-) DCs. Moreover, CXCL12 (50 ng/ml)-induced DC migration was enhanced in ampk(-/-) DCs. AMPK thus inhibits SOC entry, Na(+)/Ca(2+) exchangers, and migration of DCs.

Stimulation of Na+-Ca2+ exchange by purified antibody against alpha-2 repeat of Na+-Ca2+ exchanger in rat cardiomyocytes

AIM

The aim of the present study was to investigate the effect of the antibody against alpha-2 repeat on Na+-Ca2+ exchanger (NCX) current (I(Na/Ca)). To evaluate the functional specificity of this antibody, its effects on L-type Ca2+ current (I(Ca,L)), voltage-gated Na+ current (I(Na)) and delayed rectifier K+ current (I(K)) were also observed.

METHODS

The whole-cell patch-clamp technique was used in this study.

RESULTS

The antibody against alpha-2 repeat augmented both the outward and inward Na+-Ca2+ exchanger current concentration-dependently, with EC(50) values of 27.9 nmol/L and 24.7 nmol/L, respectively. Meanwhile, the antibody could also increase I(Ca,L) in a concentration-dependent manner with the EC(50) of 33.6 nmol/L. Effects of the antibody on I(Na) and I(K) were not observed in the present study.

CONCLUSION

The present results suggest that antibody against alpha-2 repeat is a stimulating antibody to NCX and could also increase I(Ca,L) in a concentration-dependent manner, but did not have an obvious effect on I(Na) and I(K).

Effects of SO2 derivatives on sodium currents in acutely isolated rat hippocampal lead-exposed neurons

In this study, the effects of acute SO2 derivatives and chronic lead exposure together on sodium currents (I(Na)) were investigated in acutely isolated rat hippocampal neurons by using the whole-cell patch clamp techniques. We found that chronic lead exposure hardly reduced the amplitudes of I(Na). In the normal condition, sodium current started to appear at around -70 mV, and reached the peak current at around -40 mV. After chronic lead exposure, the data changed to -70 and -30 mV. After adding SO2 derivatives, the data changed to -80 and -40 mV, respectively. SO2 derivatives caused a significant increase of I(Na) in hippocampal chronic-lead exposed neurons. Chronic lead exposure induced a right shift of the activation curve and a left shift of the inactivation curve of sodium channels. SO2 derivatives caused negative shifts of the activation and inactivation curves of I(Na) in hippocampal chronic-lead exposed neurons. Lead exposure put off the time reaching the peak of I(Na) activation. SO2 derivatives increased the time constants of inactivation after lead exposure. The interaction of lead and SO2 derivatives with voltage-dependent sodium channels may lead to changes in electrical activity and contribute to worsening the neurotoxicological damage.