Spironolactone and its main metabolite, canrenoic acid, block human ether-a-go-go-related gene channels

Spironolactone as a Potential New Treatment to Prevent Arrhythmias in Arrhythmogenic Cardiomyopathy Cell Model

Arrhythmogenic cardiomyopathy (ACM) is a rare genetic disease associated with ventricular arrhythmias in patients. The occurrence of these arrhythmias is due to direct electrophysiological remodeling of the cardiomyocytes, namely a reduction in the action potential duration (APD) and a disturbance of Ca²⁺ homeostasis. Interestingly, spironolactone (SP), a mineralocorticoid receptor antagonist, is known to block K⁺ channels and may reduce arrhythmias. Here, we assess the direct effect of SP and its metabolite canrenoic acid (CA) in cardiomyocytes derived from human-induced pluripotent stem cells (hiPSC-CMs) of a patient bearing a missense mutation (c.394C>T) in the DSC2 gene coding for desmocollin 2 and for the amino acid replacement of arginine by cysteine at position 132 (R132C). SP and CA corrected the APD in the muted cells (vs. the control) in linking to a normalization of the hERG and KCNQ1 K⁺ channel currents. In addition, SP and CA had a direct cellular effect on Ca²⁺ homeostasis. They reduced the amplitude and aberrant Ca²⁺ events. In conclusion, we show the direct beneficial effects of SP on the AP and Ca²⁺ homeostasis of DSC2-specific hiPSC-CMs. These results provide a rationale for a new therapeutical approach to tackle mechanical and electrical burdens in patients suffering from ACM.

Suxiao Jiuxin Pills Prevent Ventricular Fibrillation from Inhibiting L-type Calcium Currents CaV1.2 in vivo and in vitro

OBJECTIVE

To investigate whether Suxiao Jiuxin Pills (SJP), a Chinese herbal remedy, is an anti-ventricular fibrillation (VF) agent.

METHODS

VF was induced by isoproterenolol (ISO) intraperitoneal injection followed by electrical pacing in mice and rabbits. The effects of SJP on the L-type calcium channel current (CaV1.2), voltage-dependent sodium channel current (I_Na), rapid and slow delayed rectifier potassium channel current (I_Kr and I_Ks, respectively) were studied by whole-cell patch-clamp method. Computer simulation was implemented to incorporate the experimental data of SJP effects on the CaV1.2 current into the action potential (AP) and pseudo-electrocardiography (pseudo-ECG) models.

RESULTS

SJP prevented VF induction and reduced VF durations significantly in mice and rabbits. Patch-clamp experiments revealed that SJP decreased the peak amplitude of the CaV1.2 current with a half maximal concentration (IC₅₀) value of 16.9 mg/L (SJP-30 mg/L, -32.8 ± 6.1 pA; Verapamil, -16.2 ±1.8 pA; vs. control, -234.5 ±16.7 pA, P<0.01, respectively). The steady-state activation curve, inactivation curve, and the recovery from inactivation of the CaV1.2 current were not shifted significantly. Specifically, SJP did not altered I_Na, I_Kr, and I_Ks currents significantly (SJP vs. control, P>0.05). Computer simulation showed that SJP-reduced CaV1.2 current shortened the AP duration, transiting VF into sinus rhythm in pseudo-ECG.

CONCLUSION

SJP reduced VF via inhibiting the CaV1.2 current with in vivo, in vitro, and in silico studies, which provide experimental basis for SJP anti-VF clinical application.

Mechanisms of gefitinib-induced QT prolongation

Gefitinib, a tyrosine kinase inhibitor, was the first targeted therapy for non-small cell lung cancer (NSCLC). Gefitinib could block human Ether-à-go-go-Related Gene (hERG) channel, an important target in drug-induced long QT syndrome. However, it is unclear whether gefitinib could induce QT interval prolongation. Here, whole-cell patch-clamp technique was used for evaluating the effect of gefitinib on rapidly-activating delayed rectifier K⁺ current (I_Kr), slowly-activating delayed rectifier K⁺ current (I_Ks), transient outward potassium current (I_to), inward rectifier K⁺ current (I_K1) and on action potentials in guinea pig ventricular myocytes. The Langendorff heart perfusion technique was used to determine drug effect on the ECG. Gefitinib depressed I_Kr by binding to open and closed hERG channels in a concentration-dependent way (IC_50: 1.91 μM). The inhibitory effect of gefitinib on wildtype hERG channels was reduced at the hERG mutants Y652A, S636A, F656V and S631A (IC_50: 8.51, 13.97, 18.86, 32.99 μM), indicating that gefitinib is a pore inhibitor of hERG channels. In addition, gefitinib accelerated hERG channel inactivation and decreased channel steady-state inactivation. Gefitinib also decreased I_Ks with IC₅₀ of 23.8 μM. Moreover, gefitinib increased action potential duration (APD) in guinea pig ventricular myocytes and the corrected QT interval (QTc) in isolated perfused guinea pig hearts in a concentration-dependent way (1-30 μM). These findings indicate that gefitinib could prolong QTc interval by potently blocking hERG channel, modulating kinetic properties of hERG channel. Partial block of KCNQ1/KCNE1 could also contribute to delayed repolarization and prolonged QT interval. Thus, caution should be taken when gefitinib is used for NSCLC treatment.

Topical Treatment of Human Skin and Cultured Keratinocytes with High-Dose Spironolactone Reduces XPB Expression and Induces Toxicity

Spironolactone (SP) is used to treat a variety of disparate disease states ranging from heart failure to acne through antagonism of the mineralocorticoid and androgen receptors. Although normally taken as an oral medication, recent studies have explored the topical application of SP onto the skin. However, because SP induces the proteolytic degradation of the XPB protein, which plays critical roles in DNA repair and transcription, there may be safety concerns with the use of topical SP. In this study, we show that the topical application of a high concentration of either SP or its metabolite canrenone onto human skin ex vivo lowers XPB protein levels and induces toxic responses in the epidermis. Interestingly, although SP and canrenone both inhibit cell proliferation, induce replication stress responses, and stimulate apoptotic signaling at high concentrations in cultured keratinocytes in vitro, these effects were not correlated with XPB protein loss. Thus, high concentrations of SP and canrenone likely inhibit cell proliferation and induce toxicity through additional mechanisms to XPB proteolytic degradation. This work suggests that care may need to be taken when using high concentrations of SP directly on human skin.

Spironolactone and perioperative atrial fibrillation occurrence in cardiac surgery patients: Rationale and design of the ALDOCURE trial

BACKGROUND

After artery bypass grafting (CABG), the presence of perioperative AF (POAF) is associated with greater short- and long-term cardiovascular morbidity. Underlying POAF mechanisms are complex and include the presence of an arrhythmogenic substrate, cardiac fibrosis and electrical remodeling. Aldosterone is a key component in this process. We hypothesize that perioperative mineralocorticoid receptor (MR) blockade may decrease the POAF incidence in patients with a left ventricular ejection fraction (LVEF) ≥50% who are referred for CABG with or without aortic valve replacement (AVR).

STUDY DESIGN

The ALDOCURE trial (NCT03551548) will be a multicenter, randomized, double-blind, placebo-controlled trial testing the superiority of a low-cost MR antagonist (MRA, spironolactone) on POAF in 1500 adults referred for on-pump elective CABG surgery with or without AVR, without any history of heart failure or atrial arrhythmia. The primary efficacy end point is the occurrence of POAF from randomization to within 5 days after surgery, assessed in a standardized manner. The main secondary efficacy end points include the following: postoperative AF occurring within 5 days after cardiac surgery, perioperative myocardial injury, major cardiovascular events and death occurring within 30 days of surgery, hospital and intensive care unit length of stay, need for readmission, LVEF at discharge and significant ventricular arrhythmias within 5 days after surgery. Safety end points, including blood pressure, serum potassium levels and renal function, will be monitored regularly throughout the trial duration.

CONCLUSION

The ALDOCURE trial will assess the effectiveness of spironolactone in addition to standard therapy for reducing POAF in patients undergoing CABG.

CLINICAL TRIAL REGISTRATION

NCT03551548.

Molecular Pathophysiology of Congenital Long QT Syndrome

Ion channels represent the molecular entities that give rise to the cardiac action potential, the fundamental cellular electrical event in the heart. The concerted function of these channels leads to normal cyclical excitation and resultant contraction of cardiac muscle. Research into cardiac ion channel regulation and mutations that underlie disease pathogenesis has greatly enhanced our knowledge of the causes and clinical management of cardiac arrhythmia. Here we review the molecular determinants, pathogenesis, and pharmacology of congenital Long QT Syndrome. We examine mechanisms of dysfunction associated with three critical cardiac currents that comprise the majority of congenital Long QT Syndrome cases: 1) IKs, the slow delayed rectifier current; 2) IKr, the rapid delayed rectifier current; and 3) INa, the voltage-dependent sodium current. Less common subtypes of congenital Long QT Syndrome affect other cardiac ionic currents that contribute to the dynamic nature of cardiac electrophysiology. Through the study of mutations that cause congenital Long QT Syndrome, the scientific community has advanced understanding of ion channel structure-function relationships, physiology, and pharmacological response to clinically employed and experimental pharmacological agents. Our understanding of congenital Long QT Syndrome continues to evolve rapidly and with great benefits: genotype-driven clinical management of the disease has improved patient care as precision medicine becomes even more a reality.

Influence of steroid hormones on ventricular repolarization

QT interval prolongation, corrected for heart rate (QTc), either spontaneous or drug-induced, is associated with an increased risk of torsades de pointes and sudden death. Women have longer QTc than men and are at higher risk of torsades de pointes, particularly during post-partum and the follicular phase. Men with peripheral hypogonadism have longer QTc than healthy controls. The role of the main sex steroid hormones has been extensively studied with inconsistent findings. Overall, estradiol is considered to promote QTc lengthening while progesterone and testosterone shorten QTc. New findings suggest more complex regulation of QTc by sex steroid hormones involving gonadotropins (i.e. follicle-stimulating hormone), the relative concentrations of sex steroid hormones (which depends on gender, i.e., progesterone/estradiol ratio in women). Aldosterone, another structurally related steroid hormone, can also prolong ventricular repolarization in both sex. Better understanding of pathophysiological hormonal processes which may lead to increased susceptibility of women (and possibly hypogonadic men) to drug-induced arrhythmia may foster preventive treatments (e.g. progesterone in women). Exogenous hormonal intake might offer new therapeutic opportunities or, alternatively, increase the risk of torsades de pointes. Some exogenous sex steroids may also have paradoxical effects on ventricular repolarization. Lastly, variations of QTc in women linked to the menstrual cycle and sex hormone fluctuations are generally ignored in regulatory thorough QT studies. Investigators and regulatory agencies promoting inclusion of women in thorough QT studies should be aware of this source of variability especially when studying drugs over several days of administration.

Rapid and MR-Independent IK1 Activation by Aldosterone during Ischemia-Reperfusion

In ST elevation myocardial infarction (STEMI) context, clinical studies have shown the deleterious effect of high aldosterone levels on ventricular arrhythmia occurrence and cardiac mortality. Previous in vitro reports showed that during ischemia-reperfusion, aldosterone modulates K⁺ currents involved in the holding of the resting membrane potential (RMP). The aim of this study was to assess the electrophysiological impact of aldosterone on I_K1 current during myocardial ischemia-reperfusion. We used an in vitro model of “border zone” using right rabbit ventricle and standard microelectrode technique followed by cell-attached recordings from freshly isolated rabbit ventricular cardiomyocytes. In microelectrode experiments, aldosterone (10 and 100 nmol/L, n=7 respectively) increased the action potential duration (APD) dispersion at 90% between ischemic and normoxic zones (from 95±4 ms to 116±6 ms and 127±5 ms respectively, P<0.05) and reperfusion-induced sustained premature ventricular contractions occurrence (from 2/12 to 5/7 preparations, P<0.05). Conversely, potassium canrenoate 100 nmol/L and RU 28318 1 μmol/l alone did not affect AP parameters and premature ventricular contractions occurrence (except Vmax which was decreased by potassium canrenoate during simulated-ischemia). Furthermore, aldosterone induced a RMP hyperpolarization, evoking an implication of a K⁺ current involved in the holding of the RMP. Cell-attached recordings showed that aldosterone 10 nmol/L quickly activated (within 6.2±0.4 min) a 30 pS K⁺-selective current, inward rectifier, with pharmacological and biophysical properties consistent with the I_K1 current (NPo =1.9±0.4 in control vs NPo=3.0±0.4, n=10, P<0.05). These deleterious effects persisted in presence of RU 28318, a specific MR antagonist, and were successfully prevented by potassium canrenoate, a non specific MR antagonist, in both microelectrode and patch-clamp recordings, thus indicating a MR-independent I_K1 activation. In this ischemia-reperfusion context, aldosterone induced rapid and MR-independent deleterious effects including an arrhythmia substrate (increased APD₉₀ dispersion) and triggered activities (increased premature ventricular contractions occurrence on reperfusion) possibly related to direct I_K1 activation.

Electrophysiological and antiarrhythmic properties of potassium canrenoate during myocardial ischemia-reperfusion

INTRODUCTION

Recent clinical studies have reported the potential benefit of an early mineralocorticoid receptor (MR) blockade with potassium canrenoate (PC) on ventricular arrhythmias (VAs) occurrence in patients experiencing an ST-segment elevation myocardial infarction (STEMI). However, most of the electrophysiological properties of PC demonstrated to date have been investigated in normoxic conditions, and therefore, in vitro experiments during an acute myocardial ischemia-reperfusion were lacking.

MATERIALS AND METHODS

We used rabbit in vitro models and standard microelectrode technique to assess the electrophysiological impact of PC during myocardial ischemia-reperfusion, including right ventricle mimicking the "border zone" existing between normal and ischemic/reperfused areas (1 µmol/L, 10 and 100 nmol/L), isolated right ventricle, and sinoatrial node (SAN) experiments (1 µmol/L, respectively).

RESULTS

During ischemia-reperfusion, acute superfusion of PC 100 nmol/L prevented the increase in action potential (AP) duration at 90% of repolarization (APD90) dispersion between ischemic and nonischemic areas and in VAs occurrence induced by aldosterone 10 nmol/L (86 ± 3 vs 114 ± 4 milliseconds for aldosterone alone, P < .05). Potassium canrenoate also induced conduction blocks and significantly decreased Vmax during simulated ischemia (from 25 ± 5 to 12 ± 4, 14 ± 3, and 14 ± 5 V/s, respectively, for PC 1 µmol/L, 100, and 10 nmol/L, P < .05). Potassium canrenoate 1 µmol/L demonstrated cycle length (CL)-dependent effects on APD90 and on Vmax, and it also reduced SAN beating CL (from 446 ± 28 to 529 ± 24 millisecond, P < .05).

CONCLUSION

Our experimental study highlights new evidence for an antiarrhythmic impact of PC during myocardial ischemia-reperfusion via multiple channels modulation. These results are in line with recent clinical trials suggesting that an early MR blockade in STEMI may be preventive of VAs.

Displacement of cortisol from human heart by acute administration of a mineralocorticoid receptor antagonist

CONTEXT

Mineralocorticoid receptor (MR) antagonists have beneficial effects in patients with heart failure and myocardial infarction, often attributed to blocking aldosterone action in the myocardium. However, binding of aldosterone to MR requires local activity of the enzyme 11β-hydroxysteroid dehydrogenase type 2 (11β-HSD2), which inactivates cortisol to cortisone and thereby prevents receptor occupancy by cortisol. In vivo activity of 11β-HSD2 and potential occupancy of MR by cortisol in human heart have not been quantified.

OBJECTIVE

This study aimed to measure in vivo activity of 11β-HSD2 and to establish whether cortisol binds MR in human heart.

PARTICIPANTS AND INTERVENTIONS

Nine patients without heart failure undergoing diagnostic coronary angiography were infused to steady state with the stable isotope tracers 9,11,12,12-[(2)H]4-cortisol and 1,2-[(2)H]2-cortisone to quantify cortisol and cortisone production. Samples were obtained from the femoral artery and coronary sinus before and for 40 minutes after bolus iv administration of an MR antagonist, potassium canrenoate. Coronary sinus blood flow was measured by venography and Doppler flow wire.

RESULTS

There was no detectable production of cortisol or cortisone across the myocardium. After potassium canrenoate administration, plasma aldosterone concentrations increased substantially but aldosterone was not detectably released from the myocardium. In contrast, plasma cortisol concentrations did not change in the systemic circulation but tissue-bound cortisol was released transiently from the myocardium after potassium canrenoate administration.

CONCLUSIONS

Human cardiac 11β-HSD2 activity appears too low to inactivate cortisol to cortisone. Cortisol is displaced acutely from the myocardium by MR antagonists and may contribute to adverse MR activation in human heart.

Procaine, a state-dependent blocker, inhibits HERG channels by helix residue Y652 and F656 in the S6 transmembrane domain

The article evaluated the inhibitory action of procaine on wild-type and mutated HERG potassium channel current (I(HERG)) to determine whether mutations in the S6 region are important for the inhibition of I(HERG) by procaine. HERG channels (WT, Y652A, and F656A) were expressed in Xenopus laevis oocytes and studied using the standard two-microelectrode voltage-clamp technique. The results revealed that WT HERG is blocked in a concentration-, voltage-, and state-dependent manner by procaine ([IC₅₀] = 34.79 μM). The steady state activation curves slightly move to the negative, while inactivation parameters move to the positive in the presence of procaine. Time-dependent test reveals that voltage-dependent I(HERG) blockade occurs extremely rapidly. Furthermore, the mutation to Ala of Y652 and F656 produce about 11-fold and 18-fold increases in IC₅₀ for I(HERG) blockade, respectively. Simultaneously, for Y652A, the steady state activation and inactivation parameters are shifted to more positive values after perfusion of procaine. Conclusively, procaine state-dependently inhibits HERG channels (WT, Y652A, and F656A). The helix residues Y652 and F656 in the S6 transmembrane domain might play a role in interaction of the drug with the channel.

The calmodulin inhibitor N-(6-aminohexyl)-5-chloro-1-naphthalene sulphonamide directly blocks human ether à-go-go-related gene potassium channels stably expressed in human embryonic kidney 293 cells

BACKGROUND AND PURPOSE

N-(6-aminohexyl)-5-chloro-1-naphthalene sulphonamide (W-7) is a well-known calmodulin inhibitor used to study calmodulin regulation of intracellular Ca(2+) signalling-related process. Here, we have determined whether W-7 would inhibit human ether gene (hERG or K(v) 11.1) potassium channels, hK(v) 1.5 channels or hK(IR) 2.1 channels expressed in human embryonic kidney (HEK) 293 cells.

EXPERIMENTAL APPROACH

The hERG channel current, hK(v) 1.5 channel current or hK(IR) 2.1 channel current was recorded with a whole-cell patch clamp technique.

KEY RESULTS

It was found that the calmodulin inhibitor W-7 blocked hERG, hK(v) 1.5 and hK(IR) 2.1 channels. W-7 decreased the hERG current (I(hERG) ) in a concentration-dependent manner (IC(50) : 3.5 µM), and the inhibition was more significant at depolarization potentials between +10 and +60 mV. The hERG mutations in the S6 region Y652A and F656V, and in the pore helix S631A, had the IC(50) s of 5.5, 9.8 and 25.4 µM respectively. In addition, the compound inhibited hK(v) 1.5 and hK(IR) 2.1 channels with IC(50) s of 6.5 and 13.4 µM respectively.

CONCLUSION AND IMPLICATIONS

These results indicate that the calmodulin inhibitor W-7 exerts a direct channel-blocking effect on hERG, hK(v) 1.5 and hK(IR) 2.1 channels stably expressed in HEK 293 cells. Caution should be taken in the interpretation of calmodulin regulation of ion channels with W-7.

Functional effects of a missense mutation in HERG associated with type 2 long QT syndrome

BACKGROUND

Long QT syndrome (LQTS) is characterized by a prolonged QT interval that can lead to severe ventricular arrhythmias (torsades de pointes) and sudden death. Congenital LQTS type 2 (LQT2) is due to loss-of-function mutations in the KCNH2 gene encoding Kv11.1 channels responsible for the rapid component of the delayed rectifier current.

OBJECTIVE

The purpose of this study was to determine the functional properties of the LQT2-associated mutation p.E637G found in a Spanish family.

METHODS

Wild-type (WT) and p.E637G Kv11.1 channels were transiently transfected in Chinese hamster ovary cells, and currents were recorded using the patch-clamp technique.

RESULTS

The p.E637G channels lost inward rectification and K(+) selectivity, generating small but measurable slowly activating, noninactivating currents. These important alterations were corrected neither by cotransfection with WT channels nor by incubation at low temperatures or with pharmacological chaperones. As a consequence of its effects on channel gating, the mutation significantly reduced the outward repolarizing current during the action potential (AP), resulting in a marked lengthening of the duration of a simulated human ventricular AP.

CONCLUSION

We have identified and characterized an LQT2-associated mutation that through removal of C-type inactivation and reduction of K(+) selectivity causes the QT prolongation observed in the patients carrying the mutation. Moreover, the results obtained demonstrate the importance of the glutamic acid at position 637 for the inactivation process and K(+) selectivity of Kv11.1 channels.

Upstream effect for atrial fibrillation: still a dilemma?

Atrial fibrillation is the most common arrhythmia in clinical practice. Ion channel blocking agents are often characterized by limited long-term efficacy and several side effects. In addition, ablative invasive procedures are neither easily accessible nor always efficacious. The "upstream therapy," which includes angiotensin-converting enzyme inhibitors, aldosterone receptor antagonists, statins, glucocorticoids, and ω-3 poly-unsaturated fatty acids, targets arrhythmia substrate, influencing atrial structural and electrical remodeling that play an essential role in atrial fibrillation induction and maintenance. The mechanisms involved and the most important clinical evidence regarding the upstream therapy influence on atrial fibrillation are presented in this review. Some open questions are also proposed.

Steroidogenesis vs. steroid uptake in the heart: do corticosteroids mediate effects via cardiac mineralocorticoid receptors?

OBJECTIVE

To test whether glucocorticoids act as the endogenous agonist of cardiac mineralocorticoid receptors, we evaluated the cardiac effects of aldosterone and corticosterone and cardiac steroidogenesis vs. steroid uptake from plasma.

METHODS AND RESULTS

Both corticosterone and aldosterone increased left ventricular pressure in the rat heart. Aldosterone decreased coronary flow, whereas corticosterone increased it. All corticosterone effects were blocked by the glucocorticoid receptor antagonist, RU486, and unaltered by the mineralocorticoid receptor antagonist, canrenoate, or the 11beta-hydroxysteroid dehydrogenase (HSD11B)2 inhibitor, carbenoxolone. Unlike mineralocorticoid receptor blockade, RU486 did not ameliorate postischemia infarct size and arrhythmias. Corticosterone, when added to the perfusion buffer, rapidly accumulated at cardiac tissue sites, reaching steady-state levels that were identical to those in coronary effluent, independently of the presence of aldosterone, RU486 or canrenoate. After stopping the perfusion, cardiac corticosterone fully washed away with a half-life of less than 1 min. Measurements of steroid-synthesizing enzyme gene expression levels in human ventricular and atrial tissue pieces from heart-beating organ donors, patients with end-stage heart failure and hypertrophic cardiomyopathy patients revealed that under no condition, the human heart was capable of synthesizing aldosterone or cortisol de novo. Yet, expression of HSD11B1, HSD11B2, mineralocorticoid receptors and glucocorticoid receptors was found, and HSD11B2 and mineralocorticoid receptors were upregulated in pathological conditions. Moreover, aldosterone reduced cardiac inotropy in a Na/K/2Cl cotransporter-dependent manner.

CONCLUSION

Both cortisol/corticosterone and aldosterone accumulate in the cardiac interstitium. The presence of HSD11B2 and mineralocorticoid receptors/glucocorticoid receptors at cardiac tissue sites allows both steroids to exert their effects via separate mechanisms.

Increased expression of mineralocorticoid receptor in human atrial fibrillation and a cellular model of atrial fibrillation

OBJECTIVES

This study was designed to evaluate the status of steroidogenesis proteins and de novo synthesis of aldosterone in the atrium, and relationships of these factors to atrial fibrillation (AF).

BACKGROUND

The role of mineralocorticoid in the pathogenesis of AF is unknown.

METHODS

We studied atrial expression of steroidogenesis proteins and aldosterone level in patients with and without AF, and in HL-1 atrial myocytes. We also investigated the electrophysiologic effects and signal transduction of aldosterone on atrial myocytes.

RESULTS

We found basal expressions of mineralocorticoid receptors (MRs), glucocorticoid receptors, and 11-beta-hydroxysteroid dehydrogenase type 2 (11bHSD2) but not 11-beta-hydroxylase (CYP11B1) or aldosterone synthase (CYP11B2) in human atria and HL-1 myocytes. There was no significant difference of mean atrial aldosterone level between patients with AF and those with normal sinus rhythm. However, patients with AF had a significantly higher atrial MR expression compared with those with normal sinus rhythm (1.73 +/- 0.24-fold, p < 0.001). Using mouse HL-1 atrial myocytes as a cellular AF model, we found that rapid depolarization increased MR expression (1.97 +/- 0.72-fold, p = 0.008) through a calcium-dependent mechanism, thus augmenting the genomic effect of aldosterone signaling as evaluated by MR reporter. Aldosterone increased intracellular oxidative stress through a nongenomic pathway, which was attenuated by nicotinamide adenine dinucleotide phosphate oxidase inhibitor diphenyleneiodonium, but not by MR-blockade spironolactone. Aldosterone increased expression of the alpha-1G and -1H subunits of the T-type calcium channel and thus increased the T-type calcium current (-13.6 +/- 2.9 pA/pF vs. -4.5 +/- 1.6 pA/pF, p < 0.01) and the intracellular calcium load through a genomic pathway, which were attenuated by spironolactone, but not by diphenyleneiodonium.

CONCLUSIONS

Expression of MR increased in AF, thus augmenting the genomic effects of aldosterone. Aldosterone induced atrial ionic remodeling and calcium overload through a genomic pathway, which was attenuated by spironolactone. These results suggest that aldosterone may play a role in AF electrical remodeling and provide insight into the treatment of AF with MR blockade.

Block effect of capsaicin on hERG potassium currents is enhanced by S6 mutation at Y652

The objectives of this study were to investigate the inhibitory action of capsaicin on wild-type (WT) and mutation human ether-a-go-go-related gene (hERG) potassium channel currents (I(hERG)), and to determine whether mutations in the S6 region are significant for the inhibition of I(hERG) by capsaicin. The hERG channel (WT, Y652A and F656A) was expressed in Xenopus oocytes and studied using standard two-microelectrode voltage-clamp techniques. The results show that capsaicin blocks WT hERG in a concentration-dependent manner, with an IC(50) of 17.45microM and a negative shift in the steady-state inactivation curve. Characteristics of blockade were consistent with capsaicin causing components of block in both the closed and open channel states. However, mutating the Y652 residue to Ala enhances the blockade effect of capsaicin with an IC(50) of 4.11microM, whereas mutation of F656A does not significantly alter drug potency. Simultaneously, for Y652A, the steady-state activation parameter is shifted to a more positive value by 5mV and the inactivation parameter is shifted to a more negative value by -29mV in the presence of 25microM capsaicin. In conclusion, capsaicin blocks hERG channels by binding to both the closed and open channel states.Y652 was important as a molecular determinant of blockade. Mutation Y652A enhances the drug block, which may cause some patients to be particularly sensitive to capsaicin clinically.

The negative inotropic action of canrenone is mediated by L-type calcium current blockade and reduced intracellular calcium transients

BACKGROUND AND PURPOSE

Adding spironolactone to standard therapy in heart failure reduces morbidity and mortality, but the underlying mechanisms are not fully understood. We analysed the effect of canrenone, the major active metabolite of spironolactone, on myocardial contractility and intracellular calcium homeostasis.

EXPERIMENTAL APPROACH

Left ventricular papillary muscles and cardiomyocytes were isolated from male Wistar rats. Contractility of papillary muscles was assessed with force transducers, Ca(2+) transients by fluorescence and Ca(2+) fluxes by electrophysiological techniques.

KEY RESULTS

Canrenone (300-600 micromol L(-1)) reduced developed tension, maximum rate of tension increase and maximum rate of tension decay of papillary muscles. In cardiomyocytes, canrenone (50 micromol L(-1)) reduced cell shortening and L-type Ca(2+) channel current, whereas steady-state activation and inactivation, and reactivation curves were unchanged. Canrenone also decreased the Ca(2+) content of the sarcoplasmic reticulum, intracellular Ca(2+) transient amplitude and intracellular diastolic Ca(2+) concentration. However, the time course of [Ca(2+)](i) decline during transients evoked by caffeine was not affected by canrenone.

CONCLUSION AND IMPLICATIONS

Canrenone reduced L-type Ca(2+) channel current, amplitude of intracellular Ca(2+) transients and Ca(2+) content of sarcoplasmic reticulum in cardiomyocytes. These changes are likely to underlie the negative inotropic effect of canrenone.

Collation, assessment and analysis of literature in vitro data on hERG receptor blocking potency for subsequent modeling of drugs' cardiotoxic properties

The assessment of the torsadogenic potency of a new chemical entity is a crucial issue during lead optimization and the drug development process. It is required by the regulatory agencies during the registration process. In recent years, there has been a considerable interest in developing in silico models, which allow prediction of drug-hERG channel interaction at the early stage of a drug development process. The main mechanism underlying an acquired QT syndrome and a potentially fatal arrhythmia called torsades de pointes is the inhibition of potassium channel encoded by hERG (the human ether-a-go-go-related gene). The concentration producing half-maximal block of the hERG potassium current (IC(50)) is a surrogate marker for proarrhythmic properties of compounds and is considered a test for cardiac safety of drugs or drug candidates. The IC(50) values, obtained from data collected during electrophysiological studies, are highly dependent on experimental conditions (i.e. model, temperature, voltage protocol). For the in silico models' quality and performance, the data quality and consistency is a crucial issue. Therefore the main objective of our work was to collect and assess the hERG IC(50) data available in accessible scientific literature to provide a high-quality data set for further studies.

State-dependent block of HERG potassium channels by R-roscovitine: implications for cancer therapy

Human ether-a-go-go-related gene (HERG) potassium channel acts as a delayed rectifier in cardiac myocytes and is an important target for both pro- and antiarrhythmic drugs. Many drugs have been pulled from the market for unintended HERG block causing arrhythmias. Conversely, recent evidence has shown that HERG plays a role in cell proliferation and is overexpressed both in multiple tumor cell lines and in primary tumor cells, which makes HERG an attractive target for cancer treatment. Therefore, a drug that can block HERG but that does not induce cardiac arrhythmias would have great therapeutic potential. Roscovitine is a cyclin-dependent kinase (CDK) inhibitor that is in phase II clinical trials as an anticancer agent. In the present study we show that R-roscovitine blocks HERG potassium current (human embryonic kidney-293 cells stably expressing HERG) at clinically relevant concentrations. The block (IC(50) = 27 microM) was rapid (tau = 20 ms) and reversible (tau = 25 ms) and increased with channel activation, which supports an open channel mechanism. Kinetic study of wild-type and inactivation mutant HERG channels supported block of activated channels by roscovitine with relatively little effect on either closed or inactivated channels. A HERG gating model reproduced all roscovitine effects. Our model of open channel block by roscovitine may offer an explanation of the lack of arrhythmias in clinical trials using roscovitine, which suggests the utility of a dual CDK/HERG channel block as an adjuvant cancer therapy.

The 5-HT2 antagonist ketanserin is an open channel blocker of human cardiac ether-à-go-go-related gene (hERG) potassium channels

BACKGROUND AND PURPOSE

Ketanserin, a selective 5-HT receptor antagonist, prolongs the QT interval of ECG in patients. The purpose of the present study was to determine whether ketanserin would block human cardiac ether-à-go-go-related gene (hERG) potassium channels.

EXPERIMENTAL APPROACH

Whole-cell patch voltage-clamp technique was used to record membrane currents in HEK 293 cells expressing wild type or mutant hERG channel genes.

KEY RESULTS

Ketanserin blocked hERG current (I(hERG)) in a concentration-dependent manner (IC50=0.11 microM). The drug showed an open channel blocking property, the block increasing significantly at depolarizing voltages between +10 to +60 mV. Voltage-dependence for inactivation of hERG channels was negatively shifted by 0.3 microM ketanserin. A 2.8 fold attenuation of inhibition by elevation of external K+ concentration (from 5.0 to 20 mM) was observed, whereas the inactivation-deficient mutants S620T and S631A had the IC50s of 0.84 +/- 0.2 and 1.7 +/-0.4 microM (7.6 and 15.4 fold attenuation of block). In addition, the hERG mutants in pore helix and S6 also significantly reduced the channel block (2-59 fold) by ketanserin.

CONCLUSIONS AND IMPLICATIONS

These results suggest that ketanserin binds to and blocks the open hERG channels in the pore helix and the S6 domain; channel inactivation is also involved in the blockade of hERG channels. Blockade of hERG channels most likely contributes to the prolongation of QT intervals in ECG observed clinically at therapeutic concentrations of ketanserin.

The hERG potassium channel and hERG screening for drug-induced torsades de pointes

Drug-induced torsades de pointes (TdP) arrhythmia is a major safety concern in the process of drug design and development. The incidence of TdP tends to be low, so early pre-clinical screens rely on surrogate markers of TdP to highlight potential problems with new drugs. hERG (human ether-à-go-go-related gene, alternative nomenclature KCNH2) is responsible for channels mediating the 'rapid' delayed rectifier K+ current (IKr) which plays an important role in ventricular repolarization. Pharmacological inhibition of native IKr and of recombinant hERG channels is a shared feature of diverse drugs associated with TdP. In vitro hERG assays therefore form a key element of an integrated assessment of TdP liability, with patch-clamp electrophysiology offering a 'gold standard'. However, whilst clearly necessary, hERG assays cannot be assumed automatically to provide sufficient information, when considered in isolation, to differentiate 'safe' from 'dangerous' drugs. Other relevant factors include therapeutic plasma concentration, drug metabolism and active metabolites, severity of target condition and drug effects on other cardiac ion channels that may mitigate or exacerbate effects of hERG blockade. Increased understanding of the nature of drug-hERG channel interactions may ultimately help eliminate potential hERG blockade early in the design and development process. Currently, for promising drug candidates integration of data from hERG assays with information from other pre-clinical safety screens remains essential.

Drug therapy for atrial fibrillation

Atrial fibrillation (AF) is the most frequently diagnosed arrhythmia. Prevalence increases with age, and the overall incidence is expected to increase as the population continues to age. Choice of pharmacologic therapy for atrial fibrillation depends on whether or not the goal of treatment is maintaining sinus rhythm or tolerating atrial fibrillation with adequate control of ventricular rates. New antiarrhythmic drugs are being tested in clinical trials. Drugs that target remodeling and inflammation are being tested for their use as prevention of AF or as adjunctive therapy.

The membrane permeable calcium chelator BAPTA-AM directly blocks human ether a-go-go-related gene potassium channels stably expressed in HEK 293 cells

BAPTA-AM is a well-known membrane permeable Ca(2+) chelator. The present study found that BAPTA-AM rapidly and reversibly suppressed human ether a-go-go-related gene (hERG or Kv11.1) K(+) current, human Kv1.3 and human Kv1.5 channel currents stably expressed in HEK 293 cells, and the effects were not related to Ca(2+) chelation. The externally applied BAPTA-AM inhibited hERG channels in a concentration-dependent manner (IC(50): 1.3 microM). Blockade of hERG channels was dependent on channel opening, and tonic block was minimal. Steady-state activation V(0.5) of hERG channels was negatively shifted by 8.5 mV (from -3.7+/-2.8 of control to -12.2+/-3.1 mV, P<0.01), while inactivation V(0.5) was negatively shifted by 6.1 mV (from -37.9+/-2.0 mV of control to -44.0+/-1.6 mV, P<0.05) with application of 3 microM BAPTA-AM. The S6 mutant Y652A and the pore helix mutant S631A significantly attenuated blockade by BAPTA-AM at 10 microM causing profound blockade of wild-type hERG channels. In addition, BAPTA-AM inhibited hKv1.3 and hKv1.5 channels in a concentration-dependent manner (IC(50): 1.45 and 1.23 microM, respectively), and the blockade of these two types of channels was also dependent on channel opening. Moreover, EGTA-AM was found to be an open channel blocker of hERG, hKv1.3, hKv1.5 channels, though its efficacy is weaker than that of BAPTA-AM. These results indicate that the membrane permeable Ca(2+) chelator BAPTA-AM (also EGTA-AM) exerts an open channel blocking effect on hERG, hKv1.3 and hKv1.5 channels.

Functional consequences of methionine oxidation of hERG potassium channels

Reactive species oxidatively modify numerous proteins including ion channels. Oxidative sensitivity of ion channels is often conferred by amino acids containing sulfur atoms, such as cysteine and methionine. Functional consequences of oxidative modification of methionine in human ether à go-go related gene 1 (hERG1), which encodes cardiac I(Kr) channels, are unknown. Here we used chloramine-T (ChT), which preferentially oxidizes methionine, to examine the functional consequences of methionine oxidation of hERG channels stably expressed in a human embryonic kidney cell line (HEK 293) and native hERG channels in a human neuroblastoma cell line (SH-SY5Y). ChT (300 microM) significantly decreased whole-cell hERG current in both HEK 293 and SH-SY5Y cells. In HEK 293 cells, the effects of ChT on hERG current were time- and concentration-dependent, and were markedly attenuated in the presence of enzyme methionine sulfoxide reductase A that specifically repairs oxidized methionine. After treatment with ChT, the channel deactivation upon repolarization to -60 or -100 mV was significantly accelerated. The effect of ChT on channel activation kinetics was voltage-dependent; activation slowed during depolarization to +30 mV but accelerated during depolarization to 0 or -10mV. In contrast, the reversal potential, inactivation kinetics, and voltage-dependence of steady-state inactivation remained unaltered. Our results demonstrate that the redox status of methionine is an important modulator of hERG channel.

QT interval is prolonged but QT dispersion is maintained in patients with primary aldosteronism

The relationship between QT duration and its dispersion in patients with primary hyperaldosteronism is not clearly known. We studied 26 patients (nine males and 17 females) with primary hyperaldosteronism. The serum potassium levels were low (2.32 +/- 0.52 mmol/l), did not correlate with serum renin or aldosterone levels, or aldosterone/renin ratio (ARR). The maximum QT intervals (QTmax) were prolonged (502 +/- 62 ms), correlated well with ARRs (p = 0.005) and aldosterone levels (p = 0.019), but not to renin (p = 0.517) or potassium levels (p = 0.196). The QT dispersions (QTd) were small (60 +/- 28.8 ms) and did not correlate with potassium, renin or aldosterone levels. QTmax but not QTd correlate with aldosterone levels in patients with primary aldosteronism. The maintenance of repolarisation homogeneity with relatively unchanged QT dispersion may contribute to our understanding of the clinical observation that ventricular tachydysrhythmia is rare among patients with primary aldosteronism.

Why are mineralocorticoid receptor antagonists cardioprotective?

Two clinical trials, the Randomized ALdosterone Evaluation Study (RALES) and the EPlerenone HEart failure and SUrvival Study (EPHESUS), have recently shown that mineralocorticoid receptor (MR) antagonists reduce mortality in patients with heart failure on top of ACE inhibition. This effect could not be attributed solely to blockade of the renal MR-mediated effects on blood pressure, and it has therefore been proposed that aldosterone, the endogenous MR agonist, also acts extrarenally, in particular in the heart. Indeed, MR are present in cardiac tissue, and possibly aldosterone synthesis occurs in the heart. This review critically addresses the following questions: (1) is aldosterone synthesized at cardiac tissue sites, (2) what agonist stimulates cardiac MR normally, and (3) what effects are mediated by aldosterone/MR in the heart that could explain the beneficial effects of MR blockade in heart failure? Conclusions are that most, if not all, of cardiac aldosterone originates in the circulation (i.e., is of adrenal origin), and that glucocorticoids, in addition to aldosterone, may serve as the endogenous agonist of cardiac MR. MR-mediated effects in the heart include effects on endothelial function, cardiac fibrosis and hypertrophy, oxidative stress, cardiac inotropy, coronary flow, and arrhythmias. Some of these effects occur via or in synergy with angiotensin II, and involve a non-MR-mediated mechanism. This raises the possibility that aldosterone synthase inhibitors might exert beneficial effects on top of MR blockade.

The action of the novel gastrointestinal prokinetic prucalopride on the HERG K+ channel and the common T897 polymorph

The human ether-à-go-go related gene (HERG) encodes the alpha-subunit of a delayed rectifier potassium channel important in the repolarisation of the cardiac action potential. Excessive action potential prolongation through HERG channel inhibition is associated with a risk of torsade de pointes arrhythmias and is a major challenge for drug development. The acute effects of the novel prokinetic prucalopride were examined on heterologously expressed HERG channels in human embryonic kidney (HEK) 293 cells using the whole-cell patch-clamp technique. Prucalopride inhibited HERG channels in a concentration-dependent manner with an IC(50) of 4.1 microM. Prucalopride significantly slowed channel deactivation and recovery from inactivation, accelerated and altered the extent of inactivation. Similar concentration-dependency and kinetic changes were observed with the minor T897 polymorphic HERG variant. Prucalopride block was frequency-independent due to rapid state-dependent block, with binding occurring in the open and inactivated states. Though prucalopride blocks HERG channels this is unlikely to be significant at clinically relevant concentrations.

Future directions in antiarrhythmic drug therapy for atrial fibrillation

Atrial fibrillation is the most commonly sustained cardiac arrhythmia. Drugs currently approved by the US FDA for the treatment of this arrhythmia are imperfect owing to either side effects or limited efficacy. Drug development strategies have focused on two areas: the modification of existing agents – such as Class III drugs aimed at improving their safety and efficacy profile – and targeting newly postulated mechanisms of atrial fibrillation. In this article, we review new drugs currently in development and promising drug strategies for atrial fibrillation prevention and treatment.

Association of 14-3-3 proteins to beta1-adrenergic receptors modulates Kv11.1 K+ channel activity in recombinant systems

We identify a new mechanism for the beta(1)-adrenergic receptor (beta(1)AR)-mediated regulation of human ether-a-go-go-related gene (HERG) potassium channel (Kv11.1). We find that the previously reported modulatory interaction between Kv11.1 channels and 14-3-3epsilon proteins is competed by wild type beta(1)AR by means of a novel interaction between this receptor and 14-3-3epsilon. The association between beta(1)AR and 14-3-3epsilon is increased by agonist stimulation in both transfected cells and heart tissue and requires cAMP-dependent protein kinase (PKA) activity. The beta(1)AR/14-3-3epsilon association is direct, since it can be recapitulated using purified 14-3-3epsilon and beta(1)AR fusion proteins and is abolished in cells expressing beta(1)AR phosphorylation-deficient mutants. Biochemical and electrophysiological studies of the effects of isoproterenol on Kv11.1 currents recorded using the whole-cell patch clamp demonstrated that beta(1)AR phosphorylation-deficient mutants do not recruit 14-3-3epsilon away from Kv11.1 and display a markedly altered agonist-mediated modulation of Kv11.1 currents compared with wild-type beta(1)AR, increasing instead of inhibiting current amplitudes. Interestingly, such differential modulation is not observed in the presence of 14-3-3 inhibitors. Our results suggest that the dynamic association of 14-3-3 proteins to both beta(1)AR and Kv11.1 channels is involved in the adrenergic modulation of this critical regulator of cardiac repolarization and refractoriness.

Selective Aldosterone Blockade Suppresses Atrial Tachyarrhythmias in Heart Failure

INTRODUCTION

Renin-angiotensin-aldosterone system activation may be involved in the pathogenesis of atrial arrhythmias in congestive heart failure (CHF). The effects of aldosterone blockade on atrial tachyarrhythmias have not been evaluated. This study's aim was to determine whether selective aldosterone blockade suppresses atrial tachyarrhythmia inducibility and modifies atrial electrical and/or structural remodeling in a canine model of rapid ventricular pacing (RVP)-induced CHF.

METHODS AND RESULTS

Dogs were assigned randomly to treatment with oral placebo or eplerenone (50 mg/day) and divided into four groups: two sham-operated (no RVP) and two RVP groups. After 5 weeks of no RVP or RVP at 230 beats/min along with concurrent placebo or eplerenone treatment, dogs underwent electrophysiologic and echocardiographic studies. Sustained atrial tachyarrhythmia inducibility (>10-minute duration), atrial effective refractory periods (ERPs), systolic and diastolic function, and left atrial and left ventricular (LV) chamber sizes were assessed. Placebo-treated RVP dogs developed CHF with LV systolic and diastolic dysfunction, left atrial and LV enlargement, increased atrial ERPs, and inducible sustained atrial tachyarrhythmias. Eplerenone treatment in RVP dogs significantly suppressed sustained atrial tachyarrhythmia inducibility, nonuniformly prolonged atrial ERPs and attenuated LV diastolic dysfunction without modifying left atrial or LV dilation or ejection fractions in CHF. Isoproterenol (2-4 microg/min) reversed eplerenone's atrial antiarrhythmic and ERP prolonging effects in CHF. Eplerenone did not alter atrial ERPs in sham (no RVP) dogs without CHF.

CONCLUSIONS

Eplerenone suppresses inducibility of sustained atrial tachyarrhythmias, selectively prolongs atrial ERPs, and attenuates LV diastolic remodeling in RVP-induced CHF. Aldosterone blockade may be a promising new approach for atrial tachyarrhythmia prevention in CHF.

Cardioprotective Effects of Eplerenone in the Rat Heart

Mineralocorticoid receptor antagonism with eplerenone reduces mortality in heart failure, possibly because of blockade of the deleterious effects of aldosterone. To investigate these effects, rat Langendorff hearts were exposed to aldosterone and/or eplerenone. Under normal conditions, aldosterone increased left ventricular pressure and decreased coronary flow. Eplerenone did not block these effects. Eplerenone reduced infarct size (from 68±2% to 53±4%; P <0.05) and increased left ventricular pressure recovery (from 44±2% to 60±5%; P <0.05) after 45 minutes of coronary artery occlusion and 3 hours of reperfusion, whereas aldosterone did not affect these parameters. To verify the origin of cardiac aldosterone, hearts were perfused with 3 to 30 nmol/L aldosterone and either frozen immediately or exposed to washout. Without washout, cardiac aldosterone was 1.5 times aldosterone in coronary effluent (CE), that is, too high to be explained on the basis of its presence in extracellular fluid. The cardiac levels of aldosterone correlated with its CE levels ( r =0.81; P <0.01), and both were unaffected by eplerenone. During washout, tissue aldosterone disappeared monophasically (half life, 9±1 minutes), and CE aldosterone disappeared biphasically (half life 1±0 and 8±1 minutes, respectively). During buffer perfusion, cardiac aldosterone was at or below the detection limit. In conclusion, eplerenone improves the condition of the heart after ischemia and reperfusion. This does not relate to interference with the inotropic and vasoconstrictor effects of aldosterone. The majority of cardiac aldosterone, if not all, is derived from the circulation. The rapid, mineralocorticoid receptor-independent kinetics of aldosterone suggest that its accumulation in the heart involves cell surface binding rather than internalization.

Quantitative Structure−Activity Relationship Studies on Inhibition of HERG Potassium Channels

The human ether-a-go-go-related gene (HERG) protein forms the ion channel responsible for the rapidly acting delayed rectifier potassium current, I(Kr), and its blockade is a significant contributor to prolongation of the QT interval. Using descriptors which have clear physicochemical meanings and are familiar to medicinal chemists, we have carried out 2D-quantitative structure-activity relationship (2D-QSAR) studies on 104 HERG channel blockers with diverse structures collected from the literature, and we have formulated interpretable models to guide chemical-modification studies and virtual screening. Statistically significant descriptors were selected by a genetic algorithm, and the final model included the octanol/water partition coefficient, topological polar surface area, diameter, summed surface area of atoms with partial charges from -0.25 to -0.20, and an indicator variable representing the experimental conditions. The statistics were r = 0.839, r2 = 0.704, q2 = 0.671, s = 0.763, and F = 46.6. The correspondence of the molecular determinants derived from the 2D-QSAR models with the 3D structural characteristics of the putative binding site in a homology-modeled HERG channel is also discussed.

The functional HERG variant 897T is associated with Conn's adenoma

OBJECTIVE

Aldosterone secreting adenomas (aldosteronomas) have an unknown molecular origin. Ion channel currents are involved in signal transduction leading to aldosterone synthesis and secretion. HERG (human-ether-a-go-go-related gene) encodes for a potassium channel responsible for the outward rectifying delayed current and it is mutation prone. When mutated it causes most of the familial forms of both long QT and short QT syndromes. Abnormal repolarization in glomerulosa cells might increase aldosterone secretion or induce a proliferative advantage. The aims of this study were to: (1) evaluate HERG expression in aldosteronomas; (2) search for HERG somatic mutations; and (3) determine whether there is any relationship between the common HERG functional variant (A2690C, leading from lysine 897 to threonine, K897T) and aldosteronoma.

DESIGN AND METHODS

Aldosteronoma and blood samples from 17 patients were studied to evaluate HERG expression, full-length HERG complementary DNA sequencing, and genotyping for K897T alleles. The prevalence of HERG 897 alleles was also tested in a control population and a population consisting entirely of hypertensive individuals.

RESULTS

HERG was expressed in all aldosteronomas analysed. HERG somatic mutations were not detected. The 897T variant of HERG was significantly more common among patients with aldosteronoma (897T allele 41%) than in patients with moderate-severe essential hypertension (897T allele 20%, P = 0.007) or in the control population (897T allele 12%, P < 0.0001). The 897T/T genotype was present in 24% of the aldosteronoma patients versus 7% (P = 0.040) and 3% (P = 0.001) in essential hypertension and in the control population, respectively. When the chi test was performed considering the three groups together, the significance was similar (for alleles P < 0.0001 and for genotypes P = 0.004).

CONCLUSION

The common functional HERG variant 897T may predispose to the development of aldosteronoma.

Structural determinants of potassium channel blockade and drug-induced arrhythmias

Cardiac K+ channels play an important role in the regulation of the shape and duration of the action potential. They have been recognized as targets for the actions of neurotransmitters, hormones, and anti-arrhythmic drugs that prolong the action potential duration (APD) and increase refractoriness. However, pharmacological therapy, often for the purpose of treating syndromes unrelated to cardiac disease, can also increase the vul- nerability of some patients to life-threatening rhythm disturbances. This may be due to an underlying propensity stemming from inherited mutations or polymorphisms, or structural abnormalities that provide a substrate allowing for the initiation of arrhythmic triggers. A number of pharmacological agents that have proved useful in the treatment of allergic reactions, gastrointestinal disorders, and psychotic disorders, among others, have been shown to reduce repolarizing K+ currents and prolong the Q-T interval on the electrocardiogram. Understanding the structural determinants of K+ channel blockade might provide new insights into the mechanism and rate-dependent effects of drugs on cellular physiology. Drug-induced disruption of cellular repolarization underlies electrocardiographic abnormalities that are diagnostic indicators of arrhythmia susceptibility.

Spironolactone and its main metabolite canrenoic acid block hKv1.5, Kv4.3 and Kv7.1 + minK channels

Both spironolactone (SP) and its main metabolite, canrenoic acid (CA), prolong cardiac action potential duration and decrease the Kv11.1 (HERG) current. We examined the effects of SP and CA on cardiac hKv1.5, Kv4.3 and Kv7.1+minK channels that generate the human I(Kur), I(to1) and I(Ks), which contribute to the control of human cardiac action potential duration.hKv1.5 currents were recorded in stably transfected mouse fibroblasts and Kv4.3 and Kv7.1 + minK in transiently transfected Chinese hamster ovary cells using the whole-cell patch clamp. SP (1 microM) and CA (1 nM) inhibited hKv1.5 currents by 23.2 +/- 3.2 and 18.9 +/- 2.7%, respectively, shifted the midpoint of the activation curve to more negative potentials and delayed the time course of tail deactivation.SP (1 microM) and CA (1 nM) inhibited the total charge crossing the membrane through Kv4.3 channels at +50 mV by 27.1 +/- 6.4 and 27.4 +/- 5.7%, respectively, and accelerated the time course of current decay. CA, but not SP, shifted the inactivation curve to more hyperpolarised potentials (V(h)-37.0 +/- 1.8 vs -40.8 +/- 1.6 mV, n = 10, P < 0.05).SP (10 microM) and CA (1 nM) also inhibited Kv7.1 + minK currents by 38.6 +/- 2.3 and 22.1 +/- 1.4%, respectively, without modifying the voltage dependence of channel activation. SP, but not CA, slowed the time course of tail current decay.CA (1 nM) inhibited the I(Kur) (29.2 +/- 5.5%) and the I(to1) (16.1 +/- 3.9%) recorded in mouse ventricular myocytes and the I(K) (21.8 +/- 6.9%) recorded in guinea-pig ventricular myocytes.A mathematical model of human atrial action potentials demonstrated that K(+) blocking effects of CA resulted in a lengthening of action potential duration, both in normal and atrial fibrillation simulated conditions. The results demonstrated that both SP and CA directly block hKv1.5, Kv4.3 and Kv7.1 + minK channels, CA being more potent for these effects. Since peak free plasma concentrations of CA ranged between 3 and 16 nM, these results indicated that blockade of these human cardiac K(+) channels can be observed after administration of therapeutic doses of SP. Blockade of these cardiac K(+) currents, together with the antagonism of the aldosterone proarrhythmic effects produced by SP, might be highly desirable for the treatment of supraventricular arrhythmias.

New Antiarrhythmic Drugs for Prevention of Atrial Fibrillation

Enhanced understanding of the mechanisms underlying atrial fibrillation (AF) and advent of catheter-based therapy for AF has altered the approach to patients with this most common arrhythmia. However, despite the success of aggressive procedural techniques, pharmacologic therapy remains the first-line and mainstay approach in the treatment of AF. This review of new antiarrhythmic drug (AAD) therapy for AF provides an in-depth overview of recently available classic and new investigational drugs being considered for AF treatment. Currently available AADs are associated with less than satisfactory efficacy in preventing AF and a significant side effect profile, including ventricular proarrhythmia. Recent investigations have focused on development of new AADs that, hopefully, will be more effective and safer even in patients with structural heart disease. These new AADs include selective multi-ion channel and atrial specific blockers and agents that target the underlying etiologies and substrate alterations that lead to AF. Included among the latter new category are agents that suppress activation of the renin-angiotensin-aldosterone system or inflammation, which represent novel targets for drug therapy for AF. Finally, new selective A1 adenosine receptor agonists may offer the possibility of more specific and successful ventricular rate control during AF. There is considerable hope and interest that improved understanding of AF mechanisms ultimately will result in more effective and less dangerous pharmacologic therapy becoming available in the future.

Genomic and nongenomic effects of aldosterone in the rat heart: why is spironolactone cardioprotective?

1. Mineralocorticoid receptor (MR) antagonism with spironolactone reduces mortality in heart failure on top of ACE inhibition. To investigate the underlying mechanism, we compared the actions of both aldosterone and spironolactone to those of angiotensin (Ang) II in the rat heart. 2. Hearts of male Wistar rats were perfused according to Langendorff. Ang II and aldosterone increased left ventricular pressure (LVP) by maximally 11+/-4 and 9+/-2%, and decreased coronary flow (CF) by maximally 36+/-7 and 20+/-4%, respectively. Spironolactone did not significantly affect LVP or CF. 3. In hearts that were exposed to a 45-min coronary artery occlusion and 3 h of reperfusion, a 15-min exposure to spironolactone prior to occlusion reduced infarct size (% of risk area) from 68+/-2 to 45+/-3%, similar to the reduction (34+/-2%) observed following 'preconditioning' (15 min occlusion followed by 10 min reperfusion) prior to the 45-min occlusion. Aldosterone exposure did not affect infarct size (71+/-5%). 4. In cardiomyocytes, aldosterone decreased [(3)H]thymidine incorporation maximally by 73+/-3%, whereas in cardiac fibroblasts it decreased [(3)H]proline incorporation by 33+/-7%. Spironolactone inhibited both effects. Ang II increased DNA and collagen synthesis, and these effects were reversed by aldosterone. 5. In conclusion, aldosterone induces positive inotropic and vasoconstrictor effects in a nongenomic manner, and these effects are comparable to those of Ang II. Aldosterone reduces DNA and collagen synthesis via MR activation, and counteracts the Ang II-induced increases in these parameters. MR blockade reduces infarct size and increases LVP recovery following coronary artery occlusion. The MR-related phenomena may underlie, at least in part, the beneficial actions of spironolactone in heart failure.

HERG-Lite®: A novel comprehensive high-throughput screen for drug-induced hERG risk

INTRODUCTION

Direct block of I(Kr) by non-antiarrhythmic drugs (NARDs) is a major cause of QT prolongation and torsades de pointes (TdP), and has made the hERG potassium channel a major target of drug safety programs in cardiotoxicity. Block of hERG currents is not the only way that drugs can adversely impact the repolarizing current I(Kr), however. We have shown recently that two drugs in clinical use do not block hERG but produce long QT syndrome (LQTS) and TdP by inhibiting trafficking of hERG to the cell surface. To address the need for an inexpensive, rapid, and comprehensive assay to predict both types of hERG risk early in the drug development process, we have developed a novel antibody-based chemiluminescent assay called HERG-Lite.

METHODS

HERG-Lite monitors the expression of hERG at the cell surface in two different stable mammalian cell lines. One cell line acts as a biosensor for drugs that inhibit hERG trafficking, while the other predicts hERG blockers based on their ability to act as pharmacological chaperones. In this study, we have validated the HERG-Lite assay using a panel of 100 drugs: 50 hERG blockers and 50 nonblockers.

RESULTS

HERG-Lite correctly predicted hERG risk for all 100 test compounds with no false positives or negatives. All 50 hERG blockers were detected as drugs with hERG risk in the HERG-Lite assay, and fell into two classes: B (for blocker) and C (for complex; block and trafficking inhibition).

DISCUSSION

HERG-Lite is the most comprehensive assay available for predicting drug-induced hERG risk. It accurately predicts both channel blockers and trafficking inhibitors in a rapid, cost-effective manner and is a valuable non-clinical assay for drug safety testing.

Angiotensin II, angiotensin II antagonists and spironolactone and their modulation of cardiac repolarization

Angiotensin II and aldosterone produce pro-arrhythmic effects by several mechanisms, including the modulation of voltage-dependent K(+) channels involved in human cardiac repolarization. Drugs that inhibit the renin-angiotensin-aldosterone system exert anti-arrhythmic actions that are related to the blockade of the pro-arrhythmic actions of angiotensin II and aldosterone. These anti-arrhythmic actions include inhibition of electrical and structural cardiac remodeling, inhibition of neurohumoral activation, reduction of blood pressure and stabilization of electrolyte disturbances. In this article, several angiotensin II AT(1) receptor antagonists (candesartan, E3174, eprosartan, irbesartan and losartan) and aldosterone receptor antagonists (canrenoic acid and spironolactone) that directly modulate the activity of the voltage-dependent K(+) channels are reviewed; the effects of these antagonists might be useful in the prevention and treatment of cardiac arrhythmias.