Inhibition of the rapid component of the delayed rectifier potassium current in ventricular myocytes by angiotensin II via the AT1 receptor

Berberine attenuates sunitinib-induced cardiac dysfunction by normalizing calcium regulation disorder via SGK1 activation

Sunitinib (SNT)-induced cardiotoxicity is associated with abnormal calcium regulation caused by phosphoinositide 3 kinase inhibition in the heart. Berberine (BBR) is a natural compound that exhibits cardioprotective effects and regulates calcium homeostasis. We hypothesized that BBR ameliorates SNT-induced cardiotoxicity by normalizing the calcium regulation disorder via serum and glucocorticoid-regulated kinase 1 (SGK1) activation. Mice, neonatal rat cardiomyocytes (NRVMs), and human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) were used to study the effects of BBR-mediated SGK1 activity on the calcium regulation disorder caused by SNT as well as the underlying mechanism. BBR offered prevention against SNT-induced cardiac systolic dysfunction, QT interval prolongation, and histopathological changes in mice. After the oral administration of SNT, the Ca²⁺ transient and contraction of cardiomyocytes was significantly inhibited, whereas BBR exhibited an antagonistic effect. In NRVMs, BBR was significantly preventive against the SNT-induced reduction of calcium transient amplitude, prolongation of calcium transient recovery, and decrease in SERCA2a protein expression; however, SGK1 inhibitors resisted the preventive effects of BBR. In hiPSC-CMs, BBR pretreatment significantly prevented SNT from inhibiting the contraction, whereas coincubation with SGK1 inhibitors antagonized the effects of BBR. These findings indicate that BBR attenuates SNT-induced cardiac dysfunction by normalizing the calcium regulation disorder via SGK1 activation.

Prolonged QT predicts prognosis in COVID-19

Background

Coronavirus disease‐2019 (COVID‐19) causes severe illness and multi‐organ dysfunction. An abnormal electrocardiogram is associated with poor outcome, and QT prolongation during the illness has been linked to pharmacological effects. This study sought to investigate the effects of the COVID‐19 illness on the corrected QT interval (QTc).

Method

For 293 consecutive patients admitted to our hospital via the emergency department for COVID‐19 between 01/03/20 ‐18/05/20, demographic data, laboratory findings, admission electrocardiograph and clinical observations were compared in those who survived and those who died within 6 weeks. Hospital records were reviewed for prior electrocardiograms for comparison with those recorded on presentation with COVID‐19.

Results

Patients who died were older than survivors (82 vs 69.8 years, p < 0.001), more likely to have cancer (22.3% vs 13.1%, p = 0.034), dementia (25.6% vs 10.7%, p = 0.034) and ischemic heart disease (27.8% vs 10.7%, p < 0.001). Deceased patients exhibited higher levels of C‐reactive protein (244.6 mg/L vs 146.5 mg/L, p < 0.01), troponin (1982.4 ng/L vs 413.4 ng/L, p = 0.017), with a significantly longer QTc interval (461.1 ms vs 449.3 ms, p = 0.007). Pre‐COVID electrocardiograms were located for 172 patients; the QTc recorded on presentation with COVID‐19 was longer than the prior measurement in both groups, but was more prolonged in the deceased group (448.4 ms vs 472.9 ms, pre‐COVID vs COVID, p < 0.01). Multivariate Cox‐regression analysis revealed age, C‐reactive protein and prolonged QTc of >455 ms (males) and >465 ms (females) (p = 0.028, HR 1.49 [1.04‐2.13]), as predictors of mortality. QTc prolongation beyond these dichotomy limits was associated with increased mortality risk (p = 0.0027, HR 1.78 [1.2‐2.6]).

Conclusion

QTc prolongation occurs in COVID‐19 illness and is associated with poor outcome.

Inhibition of the INa/K and the activation of peak INa contribute to the arrhythmogenic effects of aconitine and mesaconitine in guinea pigs

Aconitine (ACO), a main active ingredient of Aconitum, is well-known for its cardiotoxicity. However, the mechanisms of toxic action of ACO remain unclear. In the current study, we investigated the cardiac effects of ACO and mesaconitine (MACO), a structurally related analog of ACO identified in Aconitum with undocumented cardiotoxicity in guinea pigs. We showed that intravenous administration of ACO or MACO (25 μg/kg) to guinea pigs caused various types of arrhythmias in electrocardiogram (ECG) recording, including ventricular premature beats (VPB), atrioventricular blockade (AVB), ventricular tachycardia (VT), and ventricular fibrillation (VF). MACO displayed more potent arrhythmogenic effect than ACO. We conducted whole-cell patch-clamp recording in isolated guinea pig ventricular myocytes, and observed that treatment with ACO (0.3, 3 μM) or MACO (0.1, 0.3 μM) depolarized the resting membrane potential (RMP) and reduced the action potential amplitude (APA) and durations (APDs) in a concentration-dependent manner. The ACO- and MACO-induced AP remodeling was largely abolished by an I_Na blocker tetrodotoxin (2 μM) and partly abolished by a specific Na⁺/K⁺ pump (NKP) blocker ouabain (0.1 μM). Furthermore, we observed that treatment with ACO or MACO attenuated NKP current (I_Na/K) and increased peak I_Na by accelerating the sodium channel activation with the EC₅₀ of 8.36 ± 1.89 and 1.33 ± 0.16 μM, respectively. Incubation of ventricular myocytes with ACO or MACO concentration-dependently increased intracellular Na⁺ and Ca²⁺ concentrations. In conclusion, the current study demonstrates strong arrhythmogenic effects of ACO and MACO resulted from increasing the peak I_Na via accelerating sodium channel activation and inhibiting the I_Na/K. These results may help to improve our understanding of cardiotoxic mechanisms of ACO and MACO, and identify potential novel therapeutic targets for Aconitum poisoning.

Iron Overload, Oxidative Stress and Calcium Mishandling in Cardiomyocytes: Role of the Mitochondrial Permeability Transition Pore

Iron (Fe) plays an essential role in many physiological processes. Hereditary hemochromatosis or frequent blood transfusions often cause iron overload (IO), which can lead to cardiomyopathy and arrhythmias; however, the underlying mechanism is not well defined. In the present study, we assess the hypothesis that IO promotes arrhythmias via reactive oxygen species (ROS) production, mitochondrial membrane potential (∆Ψm) depolarization, and disruption of cytosolic Ca dynamics. In ventricular myocytes isolated from wild type (WT) mice, both cytosolic and mitochondrial Fe levels were elevated following perfusion with the Fe3+/8-hydroxyquinoline (8-HQ) complex. IO promoted mitochondrial superoxide generation (measured using MitoSOX Red) and induced the depolarization of the ΔΨm (measured using tetramethylrhodamine methyl ester, TMRM) in a dose-dependent manner. IO significantly increased the rate of Ca wave (CaW) formation measured in isolated ventricular myocytes using Fluo-4. Furthermore, in ex-vivo Langendorff-perfused hearts, IO increased arrhythmia scores as evaluated by ECG recordings under programmed S1-S2 stimulation protocols. We also carried out similar experiments in cyclophilin D knockout (CypD KO) mice in which the mitochondrial permeability transition pore (mPTP) opening is impaired. While comparable cytosolic and mitochondrial Fe load, mitochondrial ROS production, and depolarization of the ∆Ψm were observed in ventricular myocytes isolated from both WT and CypD KO mice, the rate of CaW formation in isolated cells and the arrhythmia scores in ex-vivo hearts were significantly lower in CypD KO mice compared to those observed in WT mice under conditions of IO. The mPTP inhibitor cyclosporine A (CsA, 1 µM) also exhibited a protective effect. In conclusion, our results suggest that IO induces mitochondrial ROS generation and ∆Ψm depolarization, thus opening the mPTP, thereby promoting CaWs and cardiac arrhythmias. Conversely, the inhibition of mPTP ameliorates the proarrhythmic effects of IO.

Effect of angiotensin/neprilysin inhibition on ventricular repolarization and clinical arrhythmogenesis

Background: The most common reason for sudden cardiac death in heart failure is malign ventricular arrhythmias. LCZ 696 improves hospitalization and sudden cardiac death outcomes in heart failure, however mechanisms in preventing sudden cardiac death are still unknown. There is little information available assessing effect of LCZ 696 on Tp-e interval and related calculations. In this study, we aimed to investigate the impact of Sacubitril/valsartan therapy on Tp-e interval, Tp-e/QT ratio and Tp-e/QTc ratio in heart failure patients and its reflections on clinical arrhythmogenesis. Methods: The study was designed as a prospective observational fashion. 265 patients with implantable cardioverter-defibrillator (ICD), who were on regular follow up at Samsun Training and Research Hospital Outpatient Heart Failure Clinic, were validated for the study. Clinical, echocardiographic, electrocardiogram and device data before initiation and six months after dose optimizing were obtained. Results: Stroke volume, cardiac output and ejection fraction significantly improved after LCZ 696 treatment. T wave related parameters, QT and QTC intervals significantly diminished. Furthermore, there was a negative correlation between baseline Tp-e interval duration and the absolute percentile increase of stroke volume(r:-0.234, p: 0.042) and cardiac output (r: -0.240, p: 0.037). Conclusion: In our study, switching Renin-Angiotensin-Aldosterone-System inhibitor with Angiotensin receptor/neprilysin inhibitor was associated with increase in left ventricle performance and decrease of sustained ventricular arrhythmias that required ICD shocks. These positive findings were accompanied by improvements in surface electrocaridogram changes such as Tp-e and related indices.

The cardiac electrophysiology effects of higenamine in guinea pig heart

BACKGROUND

Higenamine (HG) is an active compound derived from Aconiti root with a cardiotonic effect. It has been approved by the Chinese SFDA for clinical trials due to its effect as a potent inotropic and chronotropic agent in the heart. However, the direct mode of action of HG on cardiac electrophysiology is unclear.

METHODS

The experiments were performed at both cell levels and the isolated organ. The major cardiac ion currents and the action potential duration (APD) were measured using patch-clamps in single guinea-pig left ventricular myocytes. ECG was recorded in isolated guinea pig hearts.

RESULTS

In the left ventricular myocytes, HG increased I_Ca-L and I_Ks in concentration- and voltage-dependent manners in the left ventricular myocytes. It potentiated the I_Ca-L and I_Ks simultaneously for synchronization. The EC₅₀ values were 0.27 μM and 0.64 μM for the I_Ca-L and I_Ks, respectively. HG (0.1 μM, 0.5 μM and 1 μM) had no effect on the I_Kr and I_Na. HG slightly prolonged APD at lower concentrations, and shortened the APD at higher concentrations. HG can induce the delayed after depolarization (DAD), which showed some pro-arrhythmic effect. In the isolated perfused heart, HG increased the heart rate via an action on the sinoatrial node cells, but did not induce cardiac arrhythmias, even at high concentrations. The EC₅₀ value for the sinoatrial node that controls the heart rate was 0.13 μM. The sinoatrial node cells appeared to be more sensitive than ventricular myocytes to HG. The effects of HG on ventricular cells and sinoatrial node cells were both mediated through stimulation of β1-AR.

CONCLUSION

We show for the first time that HG produced a predominant action on the sinoatrial node. HG appears to control the cardiac electrophysiology through its predominant effect on the sinoarial node cells, without induction of the ectopic activity that causes cardiac arrhythmias. Thus, HG might be useful for the treatment of bradycardia.

Utility of the JT Peak Interval and the JT Area in Determining the Proarrhythmic Potential of QT-Shortening Agents

Drug-induced long QT increases the risk of ventricular tachyarrhythmia known as torsades de pointes (TdP). Many biomarkers have been used to predict TdP. At present, however, there are few biomarkers for arrhythmias induced by QT-shortening drugs. The objective of the present study was to identify the best biomarkers for predicting arrhythmias caused by the 4 potassium channel openers ICA-105574, NS-1643, R-L3, and pinacidil. Our results showed that, at higher concentrations, all 4 potassium channel openers induced ventricular tachycardia (VT) and ventricular fibrillation (VF) in Langendorff-perfused guinea pig hearts, but not in rabbit hearts. The electrocardiography parameters were measured including QT/QTc, JT peak, Tp-e interval, JT area, short-term beat-to-beat QT interval variability (STV), and index of cardiac electrophysiological balance (iCEB). We found that the potassium channel openers at test concentrations shortened the QT/QTc and the JT peak interval and increased the JT area. Nevertheless, even at proarrhythmic concentrations, they did not always change STV, Tp-e, or iCEB. Receiver operating characteristic curve analysis showed that the JT peak interval representing the early repolarization phase and the JT area reflecting the dispersion of ventricular repolarization were the best predictors of VT/VF. Action potential recordings in guinea pig papillary muscle revealed that except for pinacidil, the potassium channel openers shortened APD₃₀ in a concentration-dependent manner. They also evoked early or delayed afterdepolarizations at fast pacing rates. Patch-clamp recordings in guinea pig ventricular cardiomyocytes showed that the potassium channel openers enhanced the total outward currents during the early phase of action potential repolarization, especially at proarrhythmic concentrations. We concluded that the JT peak interval and the JT area are surrogate biomarkers identifying the risk of proarrhythmia associated with the administration of QT-shortening agents. The acceleration of early-phase repolarization and the increased dispersion of ventricular repolarization may contribute to the occurrence of arrhythmias.

Aldosterone downregulates delayed rectifier potassium currents through an angiotensin type 1 receptor-dependent mechanism

We have previously shown that aldosterone downregulates delayed rectifier potassium currents (I_Ks) via activation of the mineralocorticoid receptor (MR) in adult guinea pig cardiomyocytes. Here, we investigate whether angiotensin II/angiotensin type 1 receptor (AngII/AT1R) and intracellular calcium also play a role in these effects. Ventricular cardiomyocytes were isolated from adult guinea pigs and incubated with aldosterone (1 μmol·L^-1) either alone or in combination with enalapril (1 μmol·L^-1), losartan (1 μmol·L^-1), nimodipine (1 μmol·L^-1), or BAPTA-AM (2.5 μmol·L^-1) for 24 h. We used the conventional whole cell patch-clamp technique to record the I_Ks component. In addition, we evaluated expression of the I_Ks subunits KCNQ1 and KCNE1 using Western blotting. Our results showed that both enalapril and losartan, but not nimodipine or BAPTA-AM, completely reversed the aldosterone-induced inhibition of I_Ks and its effects on KCNQ1/KCNE1 protein levels. Furthermore, we found that AngII/AT1R mediates the inhibitory effects of aldosterone on I_Ks. Finally, the downregulation of I_Ks induced by aldosterone did not occur secondarily to a change in intracellular calcium concentrations. Taken together, our findings demonstrate that crosstalk between MR and AT1R underlies the effects of aldosterone, and provide new insights into the mechanism underlying potassium channels.

Arsenic trioxide and angiotensin II have inhibitory effects on HERG protein expression: Evidence for the role of PML SUMOylation

The human ether-a-go-go-related gene (HERG) channel is a novel target for the treatment of drug-induced long QT syndrome, which causes lethal cardiotoxicity. This study is designed to explore the possible role of PML SUMOylation and its associated nuclear bodies (NBs) in the regulation of HERG protein expression. Both arsenic trioxide (ATO) and angiotensin II (Ang II) were able to significantly reduce HERG protein expression, while also increasing PML SUMOylation and accelerating the formation of PML-NBs. Pre-exposure of cardiomyocytes to a SUMOylation chemical inhibitor, ginkgolic acid, or the silencing of UBC9 suppressed PML SUMOylation, subsequently preventing the downregulation of HERG induced by ATO or Ang II. Conversely, knockdown of RNF4 led to a remarkable increase in PML SUMOylation and the function of PML-NBs, further promoting ATO- or Ang II-induced HERG protein downregulation. Mechanistically, an increase in PML SUMOylation by ATO or Ang II dramatically enhanced the formation of PML and Pin1 complexes in PML-NBs, leading to the upregulation of TGF-β1 protein, eventually inhibiting HERG expression through activation of protein kinase A. The present work uncovered a novel molecular mechanism underlying HERG protein expression and indicated that PML SUMOylation is a critical step in the development of drug-acquired arrhythmia.

Protein kinase C epsilon mediates the inhibition of angiotensin II on the slowly activating delayed-rectifier potassium current through channel phosphorylation

The slowly activating delayed rectifier K⁺ current (I_Ks) is one of the main repolarizing currents in the human heart. Evidence has shown that angiotensin II (Ang II) regulates I_Ks through the protein kinase C (PKC) pathway, but the related results are controversial. This study was designed to identify PKC isoenzymes involved in the regulation of I_Ks by Ang II and the underlying molecular mechanism. The whole-cell patch-clamp technique was used to record I_Ks in isolated guinea pig ventricular cardiomyocytes and in human embryonic kidney (HEK) 293 cells co-transfected with human KCNQ1/KCNE1 genes and Ang II type 1 receptor genes. Ang II inhibited I_Ks in a concentration-dependent manner in native cardiomyocytes. A broad PKC inhibitor Gö6983 (not inhibiting PKCε) and a selective cPKC inhibitor Gö6976 did not affect the inhibitory action of Ang II. In contrast, the inhibition was significantly attenuated by PKCε-selective peptide inhibitor εV1-2. However, direct activation of PKC by phorbol 12-myristate 13-acetate (PMA) increased the cloned human I_Ks in HEK293 cells. Similarly, the cPKC peptide activator significantly enhanced the current. In contrast, the PKCε peptide activator inhibited the current. Further evidence showed that PKCε knockdown by siRNA antagonized the Ang II-induced inhibition on KCNQ1/KCNE1 current, whereas knockdown of cPKCs (PKCα and PKCβ) attenuated the potentiation of the current by PMA. Moreover, deletion of four putative phosphorylation sites in the C-terminus of KCNQ1 abolished the action of PMA. Mutation of two putative phosphorylation sites in the N-terminus of KCNQ1 and one site in KCNE1 (S102) blocked the inhibition of Ang II. Our results demonstrate that PKCε isoenzyme mediates the inhibitory action of Ang II on I_Ks and by phosphorylating distinct sites in KCNQ1/KCNE1, cPKC and PKCε isoenzymes produce the contrary regulatory effects on the channel. These findings have provided new insight into the molecular mechanism underlying the modulation of the KCNQ1/KCNE1 channel.

Different protein kinase C isoenzymes mediate inhibition of cardiac rapidly activating delayed rectifier K+ current by different G-protein coupled receptors

BACKGROUND AND PURPOSE

Elevated angiotensin II (Ang II) and sympathetic activity contributes to a high risk of ventricular arrhythmias in heart disease. The rapidly activating delayed rectifier K⁺ current (I_Kr ) carried by the hERG channels plays a critical role in cardiac repolarization, and decreased I_Kr is involved in increased cardiac arrhythmogenicity. Stimulation of α_1A -adrenoreceptors or angiotensin II AT₁ receptors is known to inhibit I_Kr via PKC. Here, we have identified the PKC isoenzymes mediating the inhibition of I_Kr by activation of these two different GPCRs.

EXPERIMENTAL APPROACH

The whole-cell patch-clamp technique was used to record I_Kr in guinea pig cardiomyocytes and HEK293 cells co-transfected with hERG and α_1A -adrenoreceptor or AT₁ receptor genes.

KEY RESULTS

A broad spectrum PKC inhibitor Gö6983 (not inhibiting PKCε), a selective cPKC inhibitor Gö6976 and a PKCα-specific inhibitor peptide, blocked the inhibition of I_Kr by the α_1A -adrenoreceptor agonist A61603. However, these inhibitors did not affect the reduction of I_Kr by activation of AT₁ receptors, whereas the PKCε-selective inhibitor peptide did block the effect. The effects of angiotensin II and the PKCε activator peptide were inhibited in mutant hERG channels in which 17 of the 18 PKC phosphorylation sites were deleted, whereas a deletion of the N-terminus of the hERG channels selectively prevented the inhibition elicited by A61603 and the cPKC activator peptide.

CONCLUSIONS AND IMPLICATIONS

Our results indicated that inhibition of I_Kr by activation of α_1A -adrenoreceptors or AT₁ receptors were mediated by PKCα and PKCε isoforms respectively, through different molecular mechanisms.

Involvement of mitochondrial permeability transition pore (mPTP) in cardiac arrhythmias: Evidence from cyclophilin D knockout mice

In the present study, we have used a genetic mouse model that lacks cyclophilin D (CypD KO) to assess the cardioprotective effect of mitochondrial permeability transition pore (mPTP) inhibition on Ca2+ waves and Ca2+ alternans at the single cell level, and cardiac arrhythmias in whole-heart preparations. The protonophore carbonyl cyanide p-(trifluoromethoxy) phenylhydrazone (FCCP) caused mitochondrial membrane potential depolarization to the same extent in cardiomyocytes from both WT and CypD KO mice, however, cardiomyocytes from CypD KO mice exhibited significantly less mPTP opening than cardiomyocytes from WT mice (p<0.05). Consistent with these results, FCCP caused significant increases in CaW rate in WT cardiomyocytes (p<0.05) but not in CypD KO cardiomyocytes. Furthermore, the incidence of Ca2+ alternans after treatment with FCCP and programmed stimulation was significantly higher in WT cardiomyocytes (11 of 13), than in WT cardiomyocytes treated with CsA (2 of 8; p<0.05) or CypD KO cardiomyocytes (2 of 10; p<0.01). (Pseudo-)Lead II ECGs were recorded from ex vivo hearts. We observed ST-T-wave alternans (a precursor of lethal arrhythmias) in 5 of 7 WT hearts. ST-T-wave alternans was not seen in CypD KO hearts (n=5) and in only 1 of 6 WT hearts treated with CsA. Consistent with these results, WT hearts exhibited a significantly higher average arrhythmia score than CypD KO (p<0.01) hearts subjected to FCCP treatment or chemical ischemia-reperfusion (p<0.01). In conclusion, CypD deficiency- induced mPTP inhibition attenuates CaWs and Ca2+ alternans during mitochondrial depolarization, and thereby protects against arrhythmogenesis in the heart.

miRNAs Regulate hERG

BACKGROUND

The human ether-a-go-go-related gene (hERG) is the major molecular component of the rapidly activating delayed rectifier K⁺ current (I_kr ). Impairment of hERG function is believed to be a mechanism causing long-QT syndromes (LQTS). Growing evidences have shown that microRNAs (miRNAs) are involved in functional modulation of the hERG pathway. The purpose of this study was to screen and validate miRNAs that regulate the hERG pathway. The miRNAs identified in this study will provide new tools to assess the mechanism of LQTS.

METHODS

Six miRNAs were selected by algorithm predictions based on potential interaction with hERG. The effects of each miRNA on hERG were assessed by use of the Dual-Luciferase Reporter assay system, qRT-PCR, Western blotting, and confocal fluorescence microscopy. Furthermore, whole-cell patch clamp technique was used to validate the effect of miR-103a-1 on the electrophysiological characteristic of the I_kr of the hERG protein channel.

RESULTS

miR-134, miR-103a-1, miR-143, and miR-3619 significantly downregulated luciferase activity (P < 0.05) in a reporter test system. These 4 miRNAs significantly suppressed expression of hERG mRNA and protein in U2OS cells (P < 0.05).Corresponding AMOs rescued expression of hERG mRNA and protein. Confocal microscopy showed that all 4 miRNAs reduced the expression of both immature and mature hERG protein. miR-103a-1 decreased the maximum current and tail current amplitudes of hERG channel.

CONCLUSIONS

Expression and functions of hERG are regulated by specific miRNAs.

Single therapeutic and supratherapeutic doses of sacubitril/valsartan (LCZ696) do not affect cardiac repolarization

Purpose

Sacubitril/valsartan (LCZ696) is a first-in-class angiotensin receptor neprilysin inhibitor (ARNI) indicated to reduce the risk of cardiovascular death and hospitalization for heart failure in patients with chronic heart failure (NYHA class II–IV) and reduced ejection fraction. This study was aimed to evaluate the effect of single oral therapeutic (400 mg) and supratherapeutic (1200 mg) doses of LCZ696 on cardiac repolarization.

Method

This randomized double-blind crossover study in healthy male subjects compared the effect of therapeutic and supratherapeutic doses of LCZ696 with placebo and moxifloxacin 400 mg (open-label treatment) as positive control. The primary assessment was mean baseline- and placebo-corrected QTcF (∆∆QTcF; Fridericia correction). Additional assessments included the ∆∆QTcB (Bazett’s correction), PR interval, QRS duration, heart rate (HR), LCZ696 pharmacokinetics, pharmacokinetic/pharmacodynamic relationships, and safety.

Results

Of the 84 subjects enrolled, 81 completed the study. The maximum upper bound of the two-sided 90 % confidence interval for ∆∆QTcF for LCZ696 400 mg and 1200 mg were <10 ms, and assay sensitivity was confirmed with moxifloxacin. No relevant treatment-emergent changes were observed in any of the ECG-derived parameters with LCZ696 or placebo, and the incidence of adverse events was comparable among the treatment groups.

Conclusion

Single therapeutic and supratherapeutic doses of LCZ696 did not affect cardiac repolarization as defined by the E14 ICH guidelines.

Electronic supplementary material

The online version of this article (doi:10.1007/s00228-016-2062-9) contains supplementary material, which is available to authorized users.

Effects of Ang Ⅱ perfusion on transmural heterogeneous of Cx43 in acute myocardial ischemia reperfusion

OBJECTIVE

To observe the effects of angiotensin Ⅱ(Ang Ⅱ) perfusion on transmural heterogeneity of Cx43 expression in the rabbit model with acute myocardial ischemia reperfusion (MIR), and investigate the role of rennin-angiotensin system in malignant ventricular arrhythmia induced by MIR.

METHODS

Twenty rabbits were randomly divided into MIR group (n = 10) and Ang Ⅱ group (n = 10). MIR model was produced with traditional ligation and opening of the anterior descending coronary artery in all animal. The hearts in vitro in the MIR group and the Ang Ⅱ group were perfused with simply improved Tyrode's solution and containing Ang Ⅱ Tyrode's solution respectively. 90% monophasic action potential repolarization duration, transmural dispersion of repolarization, Cx43 protein (Cx43-pro) and mRNA (Cx43-Cq) expression in subepicardial, midmyocardial and subendocardial myocardium were measured in both groups. The greatest differences of Cx43-pro and Cx43-Cq among three myocardial layers were calculated and shown with ΔCx43-pro and ΔCx43-Cq respectively.

RESULTS

After Ang Ⅱ perfusion, 90% monophasic action potential repolarization duration among three myocardial layer were significantly prolonged (P < 0.05 and P < 0.01), and transmural dispersion of repolarization also significantly increased compared with the MIR group (P < 0.05). Compare with the MIR group, three myocardial Cx43-pro and Cx43-Cq expression in the Ang Ⅱ group were significantly decreased (P < 0.05 and P < 0.01), but ΔCx43-pro and ΔCx43-Cq were significant increased.

CONCLUSIONS

Renin-angiotensin system increases transmural heterogeneity of Cx43 expression in the rabbit model with MIR by Ang Ⅱ, and enlarge transmural dispersion of repolarization among three myocardial layers of left ventricular which induces malignant ventricular arrhythmia.

Angiotensin II Type 1 Receptor-Mediated Electrical Remodeling in Mouse Cardiac Myocytes

We recently characterized an autocrine renin angiotensin system (RAS) in canine heart. Activation of Angiotensin II Type 1 Receptors (AT1Rs) induced electrical remodeling, including inhibition of the transient outward potassium current Ito, prolongation of the action potential (AP), increased calcium entry and increased contractility. Electrical properties of the mouse heart are very different from those of dog heart, but if a similar system existed in mouse, it could be uniquely studied through genetic manipulations. To investigate the presence of a RAS in mouse, we measured APs and Ito in isolated myocytes. Application of angiotensin II (A2) for 2 or more hours reduced Ito magnitude, without affecting voltage dependence, and prolonged APs in a dose-dependent manner. Based on dose-inhibition curves, the fast and slow components of Ito (Ito,fast and IK,slow) appeared to be coherently regulated by [A2], with 50% inhibition at an A2 concentration of about 400 nM. This very high K0.5 is inconsistent with systemic A2 effects, but is consistent with an autocrine RAS in mouse heart. Pre-application of the microtubule destabilizing agent colchicine eliminated A2 effects on Ito and AP duration, suggesting these effects depend on intracellular trafficking. Application of the biased agonist SII ([Sar1-Ile4-Ile8]A2), which stimulates receptor internalization without G protein activation, caused Ito reduction and AP prolongation similar to A2-induced changes. These data demonstrate AT1R mediated regulation of Ito in mouse heart. Moreover, all measured properties parallel those measured in dog heart, suggesting an autocrine RAS may be a fundamental feedback system that is present across species.

Aldosterone down-regulates the slowly activated delayed rectifier potassium current in adult guinea pig cardiomyocytes

BACKGROUND AND PURPOSE

There is emerging evidence that the mineralocorticoid hormone aldosterone is associated with arrhythmias in cardiovascular disease. However, the effect of aldosterone on the slowly activated delayed rectifier potassium current (IK s ) remains poorly understood. The present study was designed to investigate the modulation of IK s by aldosterone.

EXPERIMENTAL APPROACH

Adult guinea pigs were treated with aldosterone for 28 days via osmotic pumps. Standard glass microelectrode recordings and whole-cell patch-clamp techniques were used to record action potentials in papillary muscles and IK s in ventricular cardiomyocytes.

KEY RESULTS

The aldosterone-treated animals exhibited a prolongation of the QT interval and action potential duration with a higher incidence of early afterdepolarizations. Patch-clamp recordings showed a significant down-regulation of IK s density in the ventricular myocytes of these treated animals. These aldosterone-induced electrophysiological changes were fully prevented by a combined treatment with spironolactone, a mineralocorticoid receptor (MR) antagonist. In addition, in in vitro cultured ventricular cardiomyocytes, treatment with aldosterone (sustained exposure for 24 h) decreased the IK s density in a concentration-dependent manner. Furthermore, a significant corresponding reduction in the mRNA/protein expression of IKs channel pore and auxiliary subunits, KCNQ1 and KCNE1 was detected in ventricular tissue from the aldosterone-treated animals.

CONCLUSIONS AND IMPLICATIONS

Aldosterone down-regulates IK s by inhibiting the expression of KCNQ1 and KCNE1, thus delaying the ventricular repolarization. These results provide new insights into the mechanism underlying K(+) channel remodelling in heart disease and may explain the highly beneficial effects of MR antagonists in HF.

Upregulation of SERCA2a following short-term ACE inhibition (by enalaprilat) alters contractile performance and arrhythmogenicity of healthy myocardium in rat

Chronic angiotensin-converting enzyme inhibitor (ACEIs) treatment can suppress arrhythmogenesis. To examine whether the effect is more immediate and independent of suppression of pathological remodelling, we tested the antiarrhythmic effect of short-term ACE inhibition in healthy normotensive rats. Wistar rats were administered with enalaprilat (ENA, i.p., 5 mg/kg every 12 h) or vehicle (CON) for 2 weeks. Intraarterial blood pressure in situ was measured in A. carotis. Cellular shortening was measured in isolated, electrically paced cardiomyocytes. Standard 12-lead electrocardiography was performed, and hearts of anaesthetized open-chest rats were subjected to 6-min ischemia followed by 10-min reperfusion to examine susceptibility to ventricular arrhythmias. Expressions of calcium-regulating proteins (SERCA2a, cardiac sarco/endoplasmic reticulum Ca(2+)-ATPase; CSQ, calsequestrin; TRD, triadin; PLB, phospholamban; Thr(17)-PLB-phosphorylated PLB at threonine-17, FKBP12.6, FK506-binding protein, Cav1.2-voltage-dependent L-type calcium channel alpha 1C subunit) were measured by Western blot; mRNA levels of L-type calcium channel (Cacna1c), ryanodine receptor (Ryr2) and potassium channels Kcnh2 and Kcnq1 were measured by qRT-PCR. ENA decreased intraarterial systolic as well as diastolic blood pressure (by 20%, and by 31%, respectively, for both P < 0.05) but enhanced shortening of cardiomyocytes at basal conditions (by 34%, P < 0.05) and under beta-adrenergic stimulation (by 73%, P < 0.05). Enalaprilat shortened QTc interval duration (CON 78 ± 1 ms vs. ENA 72 ± 2 ms; P < 0.05) and significantly decreased the total duration of ventricular fibrillations (VF) and the number of VF episodes (P < 0.05). Reduction in arrhythmogenesis was associated with a pronounced upregulation of SERCA2a (CON 100 ± 20 vs. ENA 304 ± 13; P < 0.05) and complete absence of basal Ca(2+)/calmodulin-dependent phosphorylation of PLB at Thr(17). Short-term ACEI treatment can provide protection against I/R injury-induced ventricular arrhythmias in healthy myocardium, and this effect is associated with increased SERCA2a expression.

Down-regulation of ether-a-go-go-related gene potassium channel protein through sustained stimulation of AT1 receptor by angiotensin II

We investigated the effects of AT1 receptor stimulation by angiotensin II (Ang II) on human ether-a-go-go-related gene (hERG) potassium channel protein in a heterogeneous expression system with the human embryonic kidney (HEK) 293 cells which stably expressed hERG channel protein and were transiently transfected with the human AT1 receptors (HEK293/hERG). Western-blot analysis showed that Ang II significantly decreased the expression of mature hERG channel protein (155-kDa band) in a time- and dose-dependent manner without affecting the level of immature hERG channel protein (135-kDa band). The relative intensity of 155-kDa band was 64.7±6.8% of control (P<0.01) after treatment of Ang II at 100nM for 24h. To investigate the effect of Ang II on the degradation of mature hERG channel protein, we blocked forward trafficking from ER to Golgi with a Golgi transit inhibitor brefeldin A (10μM). Ang II significantly enhanced the time-dependent reduction of mature hERG channel protein. In addition, the proteasomal inhibitor lactacystin (5μM) inhibited Ang II-mediated the reduction of mature hERG channel protein, but the lysosomal inhibitor bafilomycin A1 (1μM) had no effect on the protein. The protein kinase C (PKC) inhibitor bisindolylmaleimide 1 (1μM) antagonized the reduction of mature hERG channel protein induced by Ang II. The results indicate that sustained stimulation of AT1 receptors by Ang II reduces the mature hERG channel protein via accelerating channel proteasomal degradation involving the PKC pathway.

Modulation of the cardiac sodium/bicarbonate cotransporter by the renin angiotensin aldosterone system: pathophysiological consequences

The sodium/bicarbonate cotransporter (NBC) is one of the major alkalinizing mechanisms in the cardiomyocytes. It has been demonstrated the existence of at least two functional isoforms, one that promotes the co-influx of 1 molecule of Na⁺ per 1 molecule of HCO⁻₃ (electroneutral isoform; NBCn1) and the other one that generates the co-influx of 1 molecule of Na⁺ per 2 molecules of HCO⁻₃ (electrogenic isoform; NBCe1). Both isoforms are important to maintain intracellular pH (pH_i) and sodium concentration ([Na⁺]_i). In addition, NBCe1 generates an anionic repolarizing current that modulates the action potential duration (APD). The renin-angiotensin-aldosterone system (RAAS) is implicated in the modulation of almost all physiological cardiac functions and is also involved in the development and progression of cardiac diseases. It was reported that angiotensin II (Ang II) exhibits an opposite effect on NBC isoforms: it activates NBCn1 and inhibits NBCe1. The activation of NBCn1 leads to an increase in pH_i and [Na⁺]_i, which indirectly, due to the stimulation of reverse mode of the Na⁺/Ca²⁺ exchanger (NCX), conduces to an increase in the intracellular Ca²⁺ concentration. On the other hand, the inhibition of NBCe1 generates an APD prolongation, potentially representing a risk of arrhythmias. In the last years, the potentially altered NBC function in pathological scenarios, as cardiac hypertrophy and ischemia-reperfusion, has raised increasing interest among investigators. This review attempts to draw the attention on the relevant regulation of NBC activity by RAAS, since it modulates pH_i and [Na⁺]_i, which are involved in the development of cardiac hypertrophy, the damage produced by ischemia-reperfusion and the generation of arrhythmic events, suggesting a potential role of NBC in cardiac diseases.

Myocardial repolarization dispersion and autonomic nerve activity in a canine experimental acute myocardial infarction model

BACKGROUND

Evidence from a canine experimental acute myocardial infarction (MI) model shows that until the seventh week after MI, the relationship between stellate ganglion nerve activity (SGNA) and vagal nerve activity (VNA) progressively increases.

OBJECTIVE

The purpose of this study was to evaluate how autonomic nervous system activity influences temporal myocardial repolarization dispersion at this period.

METHODS

We analyzed autonomic nerve activity as well as QT and RR variability from recordings previously obtained in nine dogs. From a total of 48 short-term ECG segments, 24 recorded before and 24 recorded 7 weeks after experimentally-induced MI, we obtained three indices of temporal myocardial repolarization dispersion: QTe (from Q-wave to T-wave end), QTp (from Q-wave to T-wave peak), and Te (from T-wave peak to T-wave end) variability index (QTeVI, QTpVI, TeVI). We also performed heart rate variability power spectral analysis on the same segments.

RESULTS

After MI, all the QT variables increased QTeVI (median [interquartile range]) (from -1.76[0.82] to -1.32[0.68]), QTeVI (from -1.90[1.01] to -1.45[0.78]), and TeVI (from -0.72[0.67] to -0.22[1.00]), whereas all RR spectral indices decreased (P <.001 for all). Distinct circadian rhythms in QTeVI (P <.05,) QTpVI (P <.001) and TeVI (P <.05) appeared after MI with circadian variations resembling that of SGNA/VNA. The morning QTpVI and TeVI acrophases approached the SGNA/VNA acrophase. Conversely, the evening QTeVI acrophase coincided with another SGNA/VNA peak. After MI, regression analysis detected a positive relationship between SGNA/VNA and TeVI (R(2): 0.077; β: 0.278; p< 0.001).

CONCLUSION

Temporal myocardial repolarization dispersion shows a circadian variation after MI reaching its peak at a time when sympathetic is highest and vagal activity lowest.

Compound ICA-105574 prevents arrhythmias induced by cardiac delayed repolarization

Impaired ventricular repolarization can lead to long QT syndrome (LQT), a proarrhythmic disease with high risk of developing lethal ventricular tachyarrhythmias. The compound ICA-105574 is a recently developed hERG activator and it enhances IKr current with very high potency by removing the channel inactivation. The present study was designed to investigate antiarrhythmic properties of ICA-105574. For comparison, the effects of another compound NS1643 was in-parallel assessed, which also acts primarily to attenuate channel inactivation with moderate potency. We found that both ICA-105574 and NS1643 concentration-dependently shortened action potential duration (APD) in ventricular myocytes, and QT/QTc intervals in isolated guinea-pig hearts. ICA-105574, but not NS1643, completely prevented ventricular arrhythmias in intact guinea-pig hearts caused by IKr and IKs inhibitors, although both ICA-105574 and NS1643 could reverse the drug-induced prolongation of APD in ventricular myocytes. Reversing prolongation of QT/QTc intervals and antagonizing the increases in transmural dispersion of repolarization and instability of the QT interval induced by IKr and IKs inhibitors contributed to antiarrhythmic effect of ICA-105574. Meanwhile, ICA-105574 at higher concentrations showed a potential proarrhythmic risk in normal hearts. Our results suggest that ICA-105574 has more efficient antiarrhythmic activity than NS1643. However, its potential proarrhythmic risk implies that benefits and risks should be seriously taken into consideration for further developing this type of hERG activators.

Regulation of the cardiac sodium/bicarbonate cotransporter by angiotensin II: potential Contribution to structural, ionic and electrophysiological myocardial remodelling

The sodium/ bicarbonate cotransporter (NBC) is, with the Na⁺/H⁺ exchanger (NHE), an important alkalinizing mechanism that maintains cellular intracellular pH (pHi). In the heart exists at least three isoforms of NBC, one that promotes the co-influx of 1 molecule of Na⁺ per 1molecule of HCO₃^-(electroneutral isoform; nNBC) and two others that generates the co-influx of 1 molecule of Na⁺ per 2 molecules of HCO₃^- (electrogenic isoforms; eNBC). In addition, the eNBC generates an anionic repolarizing current that modulate the cardiac action potential (CAP), adding to such isoforms the relevance to modulate the electrophysiological function of the heart. Angiotensin II (Ang II) is one of the main hormones that regulate cardiac physiology. The alkalinizing mechanisms (NHE and NBC) are stimulated by Ang II, increasing pHi and intracellular Na⁺ concentration, which indirectly, due to the stimulation of the Na⁺/Ca²⁺ exchanger (NCX) operating in the reverse form, leads to an increase in the intracellular Ca²⁺ concentration. Interestingly, it has been shown that Ang II exhibits an opposite effect on NBC isoforms: it activates the nNBC and inhibits the eNBC. This inhibition generates a CAP prolongation, which could directly increase the intracellular Ca²⁺ concentration. The regulation of the intracellular Na⁺ and Ca²⁺ concentrations is crucial for the cardiac cellular physiology, but these ions are also involved in the development of cardiac hypertrophy and the damage produced by ischemia-reperfusion, suggesting a potential role of NBC in cardiac diseases.

Mitochondrial reactive oxygen species (ROS) as signaling molecules of intracellular pathways triggered by the cardiac renin-angiotensin II-aldosterone system (RAAS)

Mitochondria represent major sources of basal reactive oxygen species (ROS) production of the cardiomyocyte. The role of ROS as signaling molecules that mediate different intracellular pathways has gained increasing interest among physiologists in the last years. In our lab, we have been studying the participation of mitochondrial ROS in the intracellular pathways triggered by the renin-angiotensin II-aldosterone system (RAAS) in the myocardium during the past few years. We have demonstrated that acute activation of cardiac RAAS induces mitochondrial ATP-dependent potassium channel (mitoK_ATP) opening with the consequent enhanced production of mitochondrial ROS. These oxidant molecules, in turn, activate membrane transporters, as sodium/hydrogen exchanger (NHE-1) and sodium/bicarbonate cotransporter (NBC) via the stimulation of the ROS-sensitive MAPK cascade. The stimulation of such effectors leads to an increase in cardiac contractility. In addition, it is feasible to suggest that a sustained enhanced production of mitochondrial ROS induced by chronic cardiac RAAS, and hence, chronic NHE-1 and NBC stimulation, would also result in the development of cardiac hypertrophy.

AMP-activated protein kinase regulates hERG potassium channel

Besides their role in cardiac repolarization, human ether-a-go-go-related gene potassium (hERG) channels are expressed in several tumor cells including rhabdomyosarcoma cells. The channels foster cell proliferation. Ubiquitously expressed AMP-dependent protein kinase (AMPK) is a serine-/threonine kinase, stimulating energy-generating and inhibiting energy-consuming processes thereby helping cells survive periods of energy depletion. AMPK has previously been shown to regulate Na⁺/K⁺ ATPase, Na⁺/Ca²⁺ exchangers, Ca²⁺ channels and K⁺ channels. The present study tested whether AMPK regulates hERG channel activity. Wild type AMPK (α1β1γ1), constitutively active (γR70Q)AMPK (α1β1γ1(R70Q)), or catalytically inactive (αK45R)AMPK (α1(K45R)β1γ1) were expressed in Xenopus oocytes with hERG. Tail currents were determined as a measure of hERG channel activity by two-electrode-voltage clamp. hERG membrane abundance was quantified by chemiluminescence and visualized by immunocytochemistry and confocal microscopy. Moreover, hERG currents were measured in RD rhabdomyosarcoma cells after pharmacological modification of AMPK activity using the patch clamp technique. Coexpression of wild-type AMPK and of constitutively active (γR70Q)AMPK significantly downregulated the tail currents in hERG-expressing Xenopus oocytes. Pharmacological activation of AMPK with AICAR or with phenformin inhibited hERG currents in Xenopus oocytes, an effect abrogated by AMPK inhibitor compound C. (γR70Q)AMPK enhanced the Nedd4-2-dependent downregulation of hERG currents. Coexpression of constitutively active (γR70Q)AMPK decreased membrane expression of hERG in Xenopus oocytes. Compound C significantly enhanced whereas AICAR tended to inhibit hERG currents in RD rhabdomyosarcoma cells. AMPK is a powerful regulator of hERG-mediated currents in both, Xenopus oocytes and RD rhabdomyosarcoma cells. AMPK-dependent regulation of hERG may be particularly relevant in cardiac hypertrophy and tumor growth.

The KCNH2 genetic polymorphism (1956, C>T) is a novel biomarker that is associated with CCB and α,β-ADR blocker response in EH patients in China

BACKGROUND

KCNH2 (hERG) potassium channels have an integral role in regulating the excitability of smooth muscle cells. Some pathways driven by angiotensin II, nitric oxide and adrenergic receptors blocker are involved in modulating the properties of KCNH2 potassium channels. And these pathways are closely related to blood pressure regulation. Therefore, we hypothesized that KCNH2 genetic polymorphisms may affect blood pressure response to the antihypertensive drug therapies.

MATERIALS AND METHODS

To evaluate the interactions between KCNH2 genetic polymorphisms and individual blood pressure response to antihypertensive drugs, 370 subjects with essential hypertension (EH) were studied. In evaluating the interactions between KCNH2 genetic polymorphisms and drug response to blood pressure, multivariable ANOVA analysis followed by Bonferroni correction were carried out.

RESULTS

There were statistically significant interactions between KCNH2 (1956, C>T) polymorphism and DBP change (P = 0.010), MAP change (P = 0.014) on azelnidipine or nitrendipine therapy patients at the end of 6 weeks. We found that the KCNH2 (1956,C>T) polymorphism was associated with the hypotensive effects of α,β-ADR blockers of DBP change at the end of 4 and 6 weeks' treatment in an age- and gender-dependent manner (P = 0.007 and 0.019, respectively). Similar results were also observed for changes in MAP at the end of 4 and 6 weeks (P-values were 0.035 and 0.078, respectively). While patients who received imidapril, candesartan and irbesartan therapy, no significant difference in drug response among KCNH2(1956,C>T) genotype was observed.

CONCLUSION

We have reported for the first time that KCNH2 (1956, C>T) polymorphism is associated with efficacy of antihypertensive drugs CCBs and ADR blockers, and may serve as a novel biomarker for individualized therapy for certain antihypertensive drugs.

Klotho sensitivity of the hERG channel

Klotho, a hormone and enzyme, is a powerful regulator of ageing and life span. Klotho deficiency leads to cardiac arrythmia and sudden cardiac death. We thus explored whether klotho modifies cardiac K(+)-channel hERG. Current was determined utilizing dual electrode voltage clamp and hERG protein abundance utilizing immunohistochemistry and chemiluminescence in Xenopus oocytes expressing hERG with or without klotho. Coexpression of klotho increased cell membrane hERG-protein abundance and hERG current at any given voltage without significantly modifying the voltage required to activate the channel. The effect of klotho coexpression was mimicked by recombinant klotho protein and reversed by β-glucuronidase-inhibitor D-saccharic acid-1,4-lactone.

Current pharmacogenomic studies on hERG potassium channels

Genetic polymorphisms in human ether-a-go-go-related gene (hERG) potassium channels are associated with many complex diseases and sensitivity to channel-related drugs. Genotypes may underlie different sensitivities to the same drug, and different drugs selectively repair the functional deficits caused by individual mutations. In fact, not all drugs that block hERG function have adverse effects as previously thought. This suggests that the severe adverse reactions observed clinically may only occur in subjects with a particular genotype, but to others may be safe. Similarly, a drug that is ineffective in one population may be both safe and effective in another. Therefore, detecting polymorphisms in KCNH2 encoding hERG1 is of great significance in guiding the prevention and treatment of related diseases, re-evaluating drug safety, and individualizing treatment. This article reviews current pharmacogenomic studies on hERG potassium channels to provide a reference for developing individualized treatments and evaluating their safety.

Stimulation of HERG channel activity by β-catenin

The multifunctional protein ß-catenin governs as transcription factor the expression of a wide variety of genes relevant for cell proliferation and cell survival. In addition, ß-catenin is localized at the cell membrane and may influence the function of channels. The present study explored the possibility that ß-catenin participates in the regulation of the HERG K⁺ channel. To this end, HERG was expressed in Xenopus oocytes with or without ß-catenin and the voltage-gated current determined utilizing the dual electrode voltage clamp. As a result, expression of ß-catenin markedly upregulated HERG channel activity, an effect not sensitive to inhibition of transcription with actinomycin D (10 µM). According to chemiluminescence, ß-catenin may increase HERG channel abundance within the oocyte cell membrane. Following inhibition of channel insertion into the cell membrane by brefeldin A (5 µM) the decay of current was similar in oocytes expressing HERG together with ß-catenin to oocytes expressing HERG alone. The experiments uncover a novel function of APC/ß-catenin, i.e. the regulation of HERG channels.

hERG K+ Channels: Structure, Function, and Clinical Significance

The human ether-a-go-go related gene (hERG) encodes the pore-forming subunit of the rapid component of the delayed rectifier K+ channel, Kv11.1, which are expressed in the heart, various brain regions, smooth muscle cells, endocrine cells, and a wide range of tumor cell lines. However, it is the role that Kv11.1 channels play in the heart that has been best characterized, for two main reasons. First, it is the gene product involved in chromosome 7-associated long QT syndrome (LQTS), an inherited disorder associated with a markedly increased risk of ventricular arrhythmias and sudden cardiac death. Second, blockade of Kv11.1, by a wide range of prescription medications, causes drug-induced QT prolongation with an increase in risk of sudden cardiac arrest. In the first part of this review, the properties of Kv11.1 channels, including biogenesis, trafficking, gating, and pharmacology are discussed, while the second part focuses on the pathophysiology of Kv11.1 channels.

Regulation of cardiac excitability by protein kinase C isozymes

Cardiac excitability and electrical activity are determined by the sum of individual ion channels, gap junctions and exchanger activities. Electrophysiological remodeling during heart disease involves changes in membrane properties of cardiomyocytes and is related to higher prevalence of arrhythmia-associated morbidity and mortality. Pharmacological and genetic manipulation of cardiac cells as well as animal models of cardiovascular diseases are used to identity changes in electrophysiological properties and the molecular mechanisms associated with the disease. Protein kinase C (PKC) and several other kinases play a pivotal role in cardiac electrophysiological remodeling. Therefore, identifying specific therapies that regulate these kinases is the main focus of current research. PKC, a family of serine/threonine kinases, has been implicated as potential signaling nodes associated with biochemical and biophysical stress in cardiovascular diseases. In this review, we describe the role of PKC isozymes that are involved in cardiac excitability and discuss both genetic and pharmacological tools that were used, their attributes and limitations. Selective and effective pharmacological interventions to normalize cardiac electrical activities and correct cardiac arrhythmias will be of great clinical benefit.

Regulation of cardiac excitability by protein kinase C isozymes

Cardiac excitability and electrical activity are determined by the sum of individual ion channels, gap junctions and exchanger activities. Electrophysiological remodeling during heart disease involves changes in membrane properties of cardiomyocytes and is related to higher prevalence of arrhythmia-associated morbidity and mortality. Pharmacological and genetic manipulation of cardiac cells as well as animal models of cardiovascular diseases are used to identity changes in electrophysiological properties and the molecular mechanisms associated with the disease. Protein kinase C (PKC) and several other kinases play a pivotal role in cardiac electrophysiological remodeling. Therefore, identifying specific therapies that regulate these kinases is the main focus of current research. PKC, a family of serine/threonine kinases, has been implicated as potential signaling nodes associated with biochemical and biophysical stress in cardiovascular diseases. In this review, we describe the role of PKC isozymes that are involved in cardiac excitability and discuss both genetic and pharmacological tools that were used, their attributes and limitations. Selective and effective pharmacological interventions to normalize cardiac electrical activities and correct cardiac arrhythmias will be of great clinical benefit.

Regulation of the instantaneous inward rectifier and the delayed outward rectifier potassium channels by Captopril and Angiotensin II via the Phosphoinositide-3 kinase pathway in volume-overload-induced hypertrophied cardiac myocytes

Background

Early development of cardiac hypertrophy may be beneficial but sustained hypertrophic activation leads to myocardial dysfunction. Regulation of the repolarizing currents can be modulated by the activation of humoral factors, such as angiotensin II (ANG II) through protein kinases. The aim of this work is to assess the regulation of I_K and I_K1 by ANG II through the PI3-K pathway in hypertrophied ventricular myocytes.

Material/Methods

Cardiac eccentric hypertrophy was induced through volume-overload in adult male rats by aorto-caval shunt (3 weeks). After one week half of the rats were given captopril (2 weeks; 0.5 g/l/day) and the other half served as control. The voltage-clamp and western blot techniques were used to measure the delayed outward rectifier potassium current (I_K) and the instantaneous inward rectifier potassium current (I_K1) and Akt activity, respectively.

Results

Hypertrophied cardiomyocytes showed reduction in I_K and I_K1. Treatment with captopril alleviated this difference seen between sham and shunt cardiomyocytes. Acute administration of ANG II (10⁻⁶M) to cardiocytes treated with captopril reduced I_K and I_K1 in shunts, but not in sham. Captopril treatment reversed ANG II effects on I_K and I_K1 in a PI3-K-independent manner. However in the absence of angiotensin converting enzyme inhibition, ANG II increased both I_K and I_K1 in a PI3-K-dependent manner in hypertrophied cardiomyocytes.

Conclusions

Thus, captopril treatment reveals a negative effect of ANG II on I_K and I_K1, which is PI3-K independent, whereas in the absence of angiotensin converting enzyme inhibition I_K and I_K1 regulation is dependent upon PI3-K.

Intracrine action of angiotensin II in the intact ventricle of the failing heart: angiotensin II changes cardiac excitability from within

The influence of intracellular injection of angiotensin II (Ang II) on electrical properties of single right ventricular fibers from the failing heart of cardiomyopathic hamsters (TO2) was investigated in the intact ventricle of 8-month-old animals. Intracellular injection was performed using pressure pulses (40-70 psi) for short periods of time (20 ms) while recoding the action potential simultaneously from the same fiber. The results indicated that intracellular Ang II caused a hyperpolarization of 7.7 mV ± 4.3 mV (n = 39) (4 animals) (P < 0.05) followed by a small fall in membrane potential. The action potential duration was significantly increased at 50% and at 90% repolarization, and the refractoriness was significantly enhanced. The effect of intracellular Ang II on action potential duration was related to the inhibition of potassium conductance through PKC activation because Bis-1 (360 nM), a selective PKC inhibitor, abolished the effect of the peptide. Injections performed in different fibers of the same ventricle showed a variable effect of Ang II on action potential duration and generated spontaneous rhythmicity. The effect of intracellular Ang II on action potential duration and cardiac refractoriness remains for more than 1 h after interruption of the intracellular injection of the peptide.

Circulating KCNH2 current-activating factor in patients with heart failure and ventricular tachyarrhythmia

BACKGROUND

It is estimated that approximately half of the deaths in patients with HF are sudden and that the most likely causes of sudden death are lethal ventricular tachyarrhythmias such as ventricular tachycardia (VT) or fibrillation (VF). However, the precise mechanism of ventricular tachyarrhythmias remains unknown. The KCNH2 channel conducting the delayed rectifier K(+) current (I(Kr)) is recognized as the most susceptible channel in acquired long QT syndrome. Recent findings have revealed that not only suppression but also enhancement of I(Kr) increase vulnerability to major arrhythmic events, as seen in short QT syndrome. Therefore, we investigated the existence of a circulating KCNH2 current-modifying factor in patients with HF.

METHODOLOGY/PRINCIPAL FINDINGS

We examined the effects of serum of HF patients on recombinant I(Kr) recorded from HEK 293 cells stably expressing KCNH2 by using the whole-cell patch-clamp technique. Study subjects were 14 patients with non-ischemic HF and 6 normal controls. Seven patients had a history of documented ventricular tachyarrhythmias (VT: 7 and VF: 1). Overnight treatment with 2% serum obtained from HF patients with ventricular arrhythmia resulted in a significant enhancement in the peaks of I(Kr) tail currents compared to the serum from normal controls and HF patients without ventricular arrhythmia.

CONCLUSIONS/SIGNIFICANCE

Here we provide the first evidence for the presence of a circulating KCNH2 channel activator in patients with HF and ventricular tachyarrhythmias. This factor may be responsible for arhythmogenesis in patients with HF.

Angiotensin II decreases spontaneous firing rate of guinea-pig sino-atrial node cells

Angiotensin II (Ang II) plays an important role in the regulation of cardiac function, but its electrophysiological effects on sino-atrial (SA) node are not well understood. In this study, the immediate effect of Ang II on action potentials and ionic currents were investigated by using whole-cell patch-clamps in single guinea-pig SA node pacemaker cells. We demonstrated that Ang II exerted a negative effect on spontaneous firing rate, with a concomitant reduction in the slope of diastolic depolarization. The inhibitory effect of Ang II on spontaneous activity displayed a concentration-dependent manner in the range of 1-1000 nM, with IC50 of 8.34 nM. Ang II type 1 (AT1) receptor antagonist valsartan (1 μM) abolished the inhibitory effect. In contrast, Ang II type 2 (AT2) receptor antagonist, PD123319 (1 μM) didn't affect the action of Ang II. Ang II had no significant effect on hyperpolarization-activated current (If) in SA node cells. However, it significantly slowed the deactivation of the slowly activated delayed rectifier K+ current (Iks) and increased the tail current density. Furthermore, Ang II decreased the current density of L-type Ca2+ current in SA node cells. Our data demonstrate that Ang II reduces the auto rhythm of SA node cells via enhancing Iks and reducing ICaL. The result suggests a potential mechanism by which elevated levels of Ang II may be involved in the occurrence of SA node dysfunction in cardiac pathophysiology.

L-type calcium current (ICa,L) and inward rectifier potassium current (IK1) are involved in QT prolongation induced by arsenic trioxide in rat

The present study was designed to study the effects of As(2)O(3) on QT interval prolongation and to explore the potential ionic mechanisms in isolated rat ventricular cardiomyocytes. The rats of As(2)O(3) group were treated with 0.8 mg·kg(-1)·d(-1) As(2)O(3) intravenously for 7 days consecutively and the control group with saline. The ECG was recorded to calculate heart rate-corrected QT interval (QTc). Single cardiomyocytes were isolated by using collagenase II, and the action potential duration (APD) and ion currents were recorded by whole-cell patch clamp. [Ca(2+)](i) was examined by confocal laser scanning microscopy. Our data showed that both QTc and APD were prolonged significantly after As(2)O(3)treatment. Meanwhile, As(2)O(3) suppressed I(K1) and shifted the reversal potential to more positive direction. Moreover, the density of I(Ca,L) was augmented significantly, and the steady-state activation curve became more negative, whereas, the inactivation and reactivation of I(Ca,L) were not changed notably after As(2)O(3) administration. Furthermore, the maximal [Ca(2+)](i) was enhanced obviously by either KCl or caffeine stimulation in As(2)O(3)-treated cardiomyocytes. Our results show that the potential mechanism of As(2)O(3)-induced QT interval prolongation in rat might be relative to disturbing the fine balance of transmembrane currents (increasing I(Ca,L) and decreasing I(K1)) and causing APD prolongation.

Angiotensin II inhibits the electrogenic Na+/HCO3- cotransport of cat cardiac myocytes

The Na(+)/HCO(3)(-) cotransporter (NBC) plays an important role in intracellular pH (pH(i)) regulation in the heart. In the myocardium co-exist the electrogenic (eNBC) and electroneutral (nNBC) isoforms of NBC. We have recently reported that angiotensin II (Ang II) stimulated total NBC activity during the recovery from intracellular acidosis through a reactive oxygen species (ROS) and ERK-dependent pathway. In the present work we focus our attention on eNBC. In order to study the activity of the eNBC in isolation, we induced a membrane potential depolarization by increasing extracellular K(+) [K(+)](o) from 4.5 to 45 mM (K(+) pulse). This experimental protocol enhanced eNBC driving force leading to intracellular alkalization (0.19 ± 0.008, n=6; data expressed as an increase of pH(i) units after 14 min of applying the K(+) pulse). This alkalization was completely abrogated by the NBC blocker S0859 (-0.004 ± 0.016*, n=5; * indicates p<0.05 vs control) but not by the Na(+)/H(+) exchanger blocker HOE642 (0.185 ± 0.04, n=4), indicating that we are exclusively measuring eNBC. The K(+) pulse induced alkalization was canceled by 100 nM Ang II (-0.008 ± 0.018*; n=5). This inhibitory effect was prevented when the myocytes were incubated with losartan (AT(1) receptor blocker, 0.18 ± 0.02; n=4) or SB202190 (p38 MAP kinase inhibitor, 0.25 ± 0.06; n=5). Neither chelerythrine (PKC inhibitor, -0.06 ± 0.04*; n=4), nor U0126 (ERK inhibitor, -0.07 ± 0.04*; n=4) nor MPG (ROS scavenger, -0.02 ± 0.05*; n=8) affected the Ang II-induced inhibition of eNBC. The inhibitory action of Ang II on eNBC was corroborated with perforated patch-clamp experiments, since no impact of the current produced by eNBC on action potential repolarization was observed in the presence of Ang II. In conclusion, we propose that Ang II, binding to AT(1) receptors, exerts an inhibitory effect on eNBC activity in a p38 kinase-dependent manner.

Cardioprotection by omega-3 fatty acids: involvement of PKCs?

It has been known since the 1970s that an increased consumption of n-3 long chain polyunsaturated fatty acids such as eicosapentaenoic acid and docosahexaenoic acid has cardioprotective effects. Epidemiological studies have reported that this effect is due to the prevention of the arrhythmias responsible for sudden cardiac death. Mechanistically, different hypotheses have been put forward to give an explanation. Among them, there are a direct effect of the polyunsaturated fatty acids on ion channels and/or a modification of the regulation of ion channels by protein kinase C's.

Regulation of HERG (KCNH2) potassium channel surface expression by diacylglycerol

The HERG (KCNH2) channel is a voltage-sensitive potassium channel mainly expressed in cardiac tissue, but has also been identified in other tissues like neuronal and smooth muscle tissue, and in various tumours and tumour cell lines. The function of HERG has been extensively studied, but it is still not clear what mechanisms regulate the surface expression of the channel. In the present report, using human embryonic kidney cells stably expressing HERG, we show that diacylglycerol potently inhibits the HERG current. This is mediated by a protein kinase C-evoked endocytosis of the channel protein, and is dependent on the dynein-dynamin complex. The HERG protein was found to be located only in early endosomes and not lysosomes. Thus, diacylglycerol is an important lipid participating in the regulation of HERG surface expression and function.

Neurohormonal regulation of cardiac ion channels in chronic heart failure

Alteration of neurohormonal homeostasis is a hallmark of the pathophysiology of chronic heart failure (CHF). In particular, overactivation of the renin-angiotensin-aldosterone system and the sympathetic catecholaminergic system is consistently observed. Chronic overactivation of these hormonal pathways leads to a detrimental arrhythmogenic remodeling of cardiac tissue due to dysregulation of cardiac ion channels. Sudden cardiac death resulting from ventricular arrhythmias is a major cause of mortality in patients with CHF. All the drug classes known to reduce mortality in patients with CHF are neurohormonal blockers. The aim of this review was to provide an overview of how cardiac ion channels are regulated by hormones known to play a central role in the pathogenesis of CHF.

Long-term blockade of L/N-type Ca(2+) channels by cilnidipine ameliorates repolarization abnormality of the canine hypertrophied heart

BACKGROUND AND PURPOSE

The heart of the canine model of chronic atrioventricular block is known to have a ventricular electrical remodelling, which mimics the pathophysiology of long QT syndrome. Using this model, we explored a new pharmacological therapeutic strategy for the prevention of cardiac sudden death.

EXPERIMENTAL APPROACH

The L-type Ca(2+) channel blocker amlodipine (2.5 mg.day(-1)), L/N-type Ca(2+) channel blocker cilnidipine (5 mg.day(-1)), or the angiotensin II receptor blocker candesartan (12 mg.day(-1)) was administered orally to the dogs with chronic atrioventricular block for 4 weeks. Electropharmacological assessments with the monophasic action potential (MAP) recordings and blood sample analyses were performed before and 4 weeks after the start of drug administration.

KEY RESULTS

Amlodipine and cilnidipine decreased the blood pressure, while candesartan hardly affected it. The QT interval, MAP duration and beat-to-beat variability of the ventricular repolarization period were shortened only in the cilnidipine group, but such effects were not observed in the amlodipine or candesartan group. Plasma concentrations of adrenaline, angiotensin II and aldosterone decreased in the cilnidipine group. In contrast, plasma concentrations of angiotensin II and aldosterone were elevated in the amlodipine group, whereas in the candesartan group an increase in plasma levels of angiotensin II and a decrease in noradrenaline and adrenaline concentrations were observed.

CONCLUSIONS AND IMPLICATIONS

Long-term blockade of L/N-type Ca(2+) channels ameliorated the ventricular electrical remodelling in the hypertrophied heart which causes the prolongation of the QT interval. This could provide a novel therapeutic strategy for the treatment of cardiovascular diseases.