Antiarrhythmic Quinidine Research Articles

Widened Scope of Drug Repurposing/Chiral Switches, Elements of Secondary Pharmaceuticals: The Quinine/Quinidine Case

Drug repurposing to new medical uses and chiral switches are elements of secondary pharmaceuticals. This article focuses on drug repurposing/chiral switches of the diastereomeric quasi-enantiomeric antimalarial quinine and antiarrhythmic quinidine, based on the histories of these drugs (1638–2022), applying a widened scope. Quinine, an essential medicine, changed the world. Drug repurposing is a strategy for identifying new uses for approved or investigational drugs outside the scope of the original medical indications. Potential drugs are not included in the definition of drug repurposing. Drug repurposing may be within or outside the therapeutic group, e.g., quinidine to quinine repurposing, from treatment of arrhythmia or severe malaria to uncomplicated malaria. The scope of chiral switches included racemate to single enantiomer and other switches of the status of chirality, e.g., racemate and quasi-racemate to scalemic mixtures. There are 16 quinine/quinidine stereoisomers. Given the multiple pharmacological activities of Cinchona alkaloid stereoisomers, this article calls for subjecting them to comprehensive drug repurposing/chiral switch searches for new medical uses.

4-phenylbutyric acid re-trafficking hERG/G572R channel protein by modulating the endoplasmic reticulum stress-associated chaperones and endoplasmic reticulum-associated degradation gene.

Long QT syndrome type 2 (LQT2) is caused by mutations in the KCNH2/human ether-à-go-go-related gene (hERG). Some hERG genetic mutation-associated diseases are alleviated by hERG-specific drug chaperones (glycerol, dimethyl sulfoxide, trimethylamine N-oxide, thapsigargin), delayed rectifier K+ current (IKr) blockers methanesulfonanilide E4031, the antihistamine astemizole, or the prokinetic drug cisapride, and the anti-arrhythmic drug quinidine. Meanwhile, many in vivo and in vitro studies have reported the efficacy of 4-phenylbutyric acid (4-PBA) in diseases with inherited genetic mutations. This study aims to explore potential therapeutic agents for hERG/G572R mutated ion channel. pcDNA3/hERG [wild type (WT)]-FLAG and pcDNA3/hERG (G572R)-FLAG plasmids were transfected into HEK293 cells. A western blot (WB) experiment was conducted to analyze protein expression. Quantitative real-time polymerase chain reaction (qPCR) was used to analyze the messenger RNA (mRNA) expression levels in the WT/G572R heterozygous HEK293 cell model treated with or without 4-PBA. The interaction between WT/G572R and BIP (GRP78), GRP94, and 3-hydroxy-3-methylglutaryl coenzyme A reductase degradation protein 1 (HRD1) was tested by co-immunoprecipitation (co-IP). To investigate the effect of 4-PBA on the WT/G572R channel current, we used electrophysiological assays (patch-clamp electrophysiological recordings). The results showed that WT/G572R activated the ATF6 pathway in the endoplasmic reticulum stress (ERS), the ERS response markers GRP78, GRP94, and calreticulin (CRT)/calnexin (CNX), and HRD1, which decreased after application of the ERS inhibitor 4-PBA. The results of co-IP confirmed that the ability of hERG interacted with GRP78, GRP94, and HRD1. Moreover, 4-PBA increased the current of WT/G572R and reversed the gating kinetics of the WT/G572R channel. 4-PBA corrects hERG channel transport defects by inhibiting excessive ERS and the endoplasmic reticulum-associated degradation (ERAD)-related gene E3 ubiquitin ligase HRD1. Additionally, 4-PBA improved WT/G572R channel current. 4-PBA is expected to be developed as a new treatment method for LQT2.

From the Literature

From the Literature

Structural Basis for Pore Blockade of the Human Cardiac Sodium Channel Nav1.5 by the Antiarrhythmic Drug Quinidine**

AbstractNav1.5, the primary voltage‐gated Na+ (Nav) channel in heart, is a major target for class I antiarrhythmic agents. Here we present the cryo‐EM structure of full‐length human Nav1.5 bound to quinidine, a class Ia antiarrhythmic drug, at 3.3 Å resolution. Quinidine is positioned right beneath the selectivity filter in the pore domain and coordinated by residues from repeats I, III, and IV. Pore blockade by quinidine is achieved through both direct obstruction of the ion permeation path and induced rotation of an invariant Tyr residue that tightens the intracellular gate. Structural comparison with a truncated rat Nav1.5 in the presence of flecainide, a class Ic agent, reveals distinct binding poses for the two antiarrhythmics within the pore domain. Our work reported here, along with previous studies, reveals the molecular basis for the mechanism of action of class I antiarrhythmic drugs.

Interplay of Nav1.5 Disease-Causing Mutations and Phosphorylation Revealed by Protein Semi-Synthesis and Molecular Dynamics Simulations

Activation of the voltage-gated sodium channel Nav1.5 initiates the cardiac action potential and numerous Nav1.5 mutations are known to cause severe and potentially life-threatening arrhythmias. These are typically caused by changes to channel function, such as altered inactivation. Additionally, it is becoming increasingly recognised that Nav1.5 undergoes heavy post-translational modification in vivo. However, conventional approaches are unable to reliably mimic post-translational modification, such as phosphorylation. This has prevented investigation of a potential functional interplay with patient mutations. Here, we overcome this limitation by using protein semi-synthesis of Nav1.5 in live cells in combination with molecular dynamics simulations. Indeed, we introduce stable phosphorylation mimics on both WT and two different pathogenic long-QT mutant channel backgrounds and decipher functional and pharmacological effects with unique precision. Our data demonstrate that the altered steady-state inactivation profile caused by phosphorylation of Y1495 is due to a destabilization of the inactivation particle in its binding site. Surprisingly, we find that unlike the ΔK1500 mutant, the Q1476R mutation does not itself change channel inactivation properties, but that phosphorylation of this mutant results in a greatly increased right-shift in steady-state inactivation compared to WT (20 mV vs 10 mV). Additionally, free energy calculations suggest that unbinding of the inactivation particle is energetically favored when Y1495 is phosphorylated, and this effect is again exacerbated in the presence of the Q1476R mutation. Lastly, we show that both phosphorylation and patient mutations can impact Nav1.5 sensitivity towards the clinically used anti-arrhythmic drug quinidine, but not flecainide. In short, functional effects can be exacerbated by phosphorylation in the presence of human patient mutations in Nav1.5. This has implications for the interpretation of mutational phenotypes, as well as possibly the design of future drug regimens.

Role of KCNAB2 expression in modulating hormone secretion in somatotroph pituitary adenoma.

Prior profiling of the human pituitary adenoma (PA) DNA methylome showed the potassium channel subunit-encoding gene KCNAB2 to be highly differentially methylated between nonfunctional PAs (NFPAs) and growth hormone (GH)-secreting PAs, with greater KCNAB2 methylation detected in secretory PAs. KCNAB2 encodes an aldo-keto reductase that, among other things, negatively regulates members of the voltage-gated potassium channel (Kv) family. In this study, the authors aimed to determine whether modulation of Kcnab2 expression would alter GH secretion in the GH3 mammosomatotroph rat cell line. In addition, they examined whether dosing GH3 cells with the antiarrhythmic drug quinidine, a known inhibitor of Kv and voltage-gated sodium channels, would affect hormonal secretion. Previously generated RNA-seq data were reanalyzed to compare KCNAB2 expression levels in human NFPAs and GH-secreting PAs. Kcnab2 was overexpressed in GH3 cells using plasmid transfection and knocked down using shRNA, with confirmation by quantitative polymerase chain reaction (qPCR). GH concentrations in cell culture supernatants collected 24 hours after cell seeding were measured using enzyme-linked immunosorbent assay (ELISA). Separately, quinidine was administered to GH3 cells at graduated doses. GH and prolactin concentrations in supernatants collected 48 hours after quinidine treatment were measured by fluorometric immunoassay. Modulation of expression at the transcript level in GH3 cells resulted in proportionate changes in the expression of GH mRNA and secretion of GH peptide, as confirmed by qPCR and ELISA. Specifically, partial knockdown of Kcnab2 was associated with fewer GH RNA transcripts and less GH secretion compared with controls, while augmentation of Kcnab2 expression was associated with more GH transcripts and secretion than the controls. Administration of quinidine (≥ 50 µM) reduced both GH and prolactin secretion in a dose-dependent fashion (p ≤ 0.05). GH secretion in a somatotroph cell line is partially dependent on KCNAB2 gene expression and may be mitigated in vitro by quinidine. These results collectively suggest a potential new target and pharmacological candidate to be considered in the development of clinical therapeutics for acromegaly.

Human Atrial Arrhythmogenesis and Sinus Bradycardia in KCNQ1-Linked Short QT Syndrome: Insights From Computational Modelling.

Atrial fibrillation (AF) and sinus bradycardia have been reported in patients with short QT syndrome variant 2 (SQT2), which is underlain by gain-of-function mutations in KCNQ1 encoding the α subunit of channels carrying slow delayed rectifier potassium current, IKs. However, the mechanism(s) underlying the increased atrial arrhythmogenesis and impaired cardiac pacemaking activity arising from increased IKs remain unclear. Possible pharmacological interventions of AF in the SQT2 condition also remain to be elucidated. Using computational modelling, we assessed the functional impact of SQT2 mutations on human sinoatrial node (SAN) pacemaking, atrial repolarisation and arrhythmogenesis, and efficacy of the anti-arrhythmic drug quinidine. Markov chain formulations of IKs describing two KCNQ1 mutations – V141M and V307L – were developed from voltage-clamp experimental data and then incorporated into contemporary action potential (AP) models of human atrial and SAN cells, the former of which were integrated into idealised and anatomically detailed tissue models. Both mutations shortened atrial AP duration (APD) through distinct IKs ‘gain-of-function’ mechanisms, whereas SAN pacemaking rate was slowed markedly only by the V141M mutation. Differences in APD restitution steepness influenced re-entry dynamics in tissue – the V141M mutation promoted stationary and stable spiral waves whereas the V307L mutation promoted non-stationary and unstable re-entrant waves. Both mutations shortened tissue excitation wavelength through reduced effective refractory period but not conduction velocity, which served to increase the lifespan of re-entrant excitation in a 3D anatomical human atria model, as well as the dominant frequency (DF), which was higher for the V141M mutation. Quinidine was effective at terminating arrhythmic excitation waves associated with the V307L but not V141M mutation, and reduced the DF in a dose-dependent manner under both mutation conditions. This study provides mechanistic insights into different AF/bradycardia phenotypes in SQT2 and the efficacy of quinidine pharmacotherapy.

Action potential clamp characterization of the S631A hERG mutation associated with short QT syndrome.

The hERG potassium channel is critical to normal repolarization of cardiac action potentials (APs) and loss‐ and gain‐of‐function hERG mutations are associated, respectively, with long and short QT syndromes, pathological conditions that can lead to arrhythmias and sudden death. hERG current (I h ERG) exhibits uniquely fast inactivation involving conformational changes to the channel pore. The S631A hERG pore mutation was originally engineered to interrogate hERG channel inactivation, but has very recently been found in a family with short QT syndrome (SQTS). Accordingly, this study characterized the effects of the S631A mutation on I h ERG profile during ventricular, atrial, and Purkinje fiber (PF) AP waveforms, using patch clamp recording from hERG expressing HEK 293 cells at 37°C. Under conventional voltage clamp, the current–voltage (I–V) relation for I h ERG exhibited a marked right‐ward shift in the region of negative slope at positive membrane potentials. Under ventricular AP clamp, the S631A mutation resulted in augmented I h ERG, which also peaked much earlier during the AP plateau than did wild‐type (WT) I h ERG. Instantaneous I–V relations showed a marked positive shift in peak repolarizing current during the ventricular AP in the S631A setting, while the instantaneous conductance‐voltage relation showed an earlier and more sustained rise in S631A compared to WT I h ERG conductance during ventricular repolarization. Experiments with atrial and PF APs in each case also showed augmented and positively shifted I h ERG in the S631A setting, indicating that the S631A mutation is likely to accelerate repolarization in all three cardiac regions. Ventricular AP clamp experiments showed retained effectiveness of the class Ia antiarrhythmic drug quinidine (1 μmol/L) against S631A I h ERG. Quinidine is thus likely to be effective in reducing excessively fast repolarization in SQTS resulting from the S631A hERG mutation.

Dual Mechanism for Inhibition of Inwardly Rectifying Kir2.x Channels by Quinidine Involving Direct Pore Block and PIP2-interference.

Class IA antiarrhythmic drug quinidine was one of the first clinically used compounds to terminate atrial fibrillation and acts as multichannel inhibitor with well-documented inhibitory effects on several cardiac potassium channels. In the mammalian heart, heteromeric assembly of Kir2.1-2.3 channels underlies IK1 current. Although a low-affinity block of quinidine on Kir2.1 has already been described, a comparative analysis of effects on other Kir2.x channels has not been performed to date. Therefore, we analyzed the effects of quinidine on wild-type and mutant Kir2.x channels in the Xenopus oocyte expression system. Quinidine exerted differential inhibitory effects on Kir2.x channels with the highest affinity toward Kir2.3 subunits. Onset of block was slow and solely reversible in Kir2.2 subunits. Quinidine inhibited Kir2.x currents in a voltage-independent manner. By means of comparative Ala-scanning mutagenesis, we further found that residues E224, F254, D259, and E299 are essential for quinidine block in Kir2.1 subunits. Analogously, quinidine mediated Kir2.3 inhibition by binding corresponding residues E216, D247, D251, and E291. In contrast, Kir2.2 current block merely involved corresponding residue D260. Using channel mutants with altered (phosphatidylinositol 4,5-bisphosphate PIP2) affinities, we were able to demonstrate that high PIP2 affinities (i.e., Kir2.3 I214L) correlate with low quinidine sensitivity. Inversely, mutant channels interacting only weakly with PIP2 (i.e., Kir2.1 K182Q, and L221I) are prone to a higher inhibitory effect. Thus, we conclude that inhibition of Kir2.x channels by quinidine is mediated by joint modes of action involving direct cytoplasmic pore block and an impaired channel stabilization via interference with PIP2.

Role of quinidine therapy after dimethoate acute intoxication in the rat diaphragm

Background Accidental and occupational acute intoxication by dimethoate (an organophosphorous compound) has gained attention in developing countries. The antiarrhythmic drug quinidine is known to decrease the conductivity and irritability of the myocardium. Aim of the work The current work studied the role of quinidine therapy in the biochemical and ultrastructural changes on the diaphragm, at the neuromuscular junction, of male albino rats following acute dimethoate intoxication. Materials and methods Fifty adult male Wistar albino rats were divided equally into five groups. Groups I and II served as the control group; group III received 200 mg/kg body weight (BW) of quinidine; group IV received oral dimethoate (300 mg/kg BW); and in group V the dimethoate-intoxicated animals were treated with 200 mg/kg BW of quinidine. Serum creatine kinase and lactate dehydrogenase were measured and assessed statistically. The left hemidiaphragms were processed for transmission electron microscopic study. Results The study of the diaphragm after treatment of the dimethoate-intoxicated group with quinidine showed residual foci of subsarcolemmal aggregation of the mitochondria. The sarcomere banding, sarcoplasmic reticulum, T tubules, and Z lines were nearly similar to control sections. In addition, mitochondria were observed around the Z lines, compared with untreated intoxicated rats. The electron density of the postsynaptic membrane at the junctional folds had an uneven appearance. The increased serum creatine kinase and lactate dehydrogenase of the intoxicated group decreased significantly after quinidine therapy. Conclusion The current results suggest that quinidine had a partial protective effect on the acute toxicity from dimethoate on the rat diaphragmatic ultrastructure. Thus, early administration of quinidine is beneficial for guarding against the respiratory failure following acute organophosphorus insecticide poisoning.

Erratum to: Symptomatic and Palliative Care for Stroke Survivors

J Gen Intern Med Published 2012:27(7):863-60. DOI: 10.1007/s11606-011-1966-4 (c) Society of General Internal Medicine 2012 Erratum to: J Gen Intern Med DOI: 10.1007/s11606-011-1966-4 On page 857 of the original publication, the following statement contains a technical error. Recently, the FDA approved Nuedexta, a fixed-dose combination of the cough suppressant dextromethorphan hydrobromide and the antiarrhythmic quinidine sulfate, for treatment of pseudobulbar affect in patients with amyotrophic lateral sclerosis or multiple sclerosis. Correction: Although the FDA approved Nuedexta based on studies in amyotrophic lateral sclerosis (ALS) and multiple sclerosis (MS), the approved indication was for treatment of pseudobulbar affect (without restriction to a specific underlying condition).

Modification by KCNE1 variants of the hERG potassium channel response to premature stimulation and to pharmacological inhibition

human Ether-à-go-go-Related Gene (hERG) encodes the pore-forming subunit of cardiac rapid delayed rectifier K+ current (IKr) channels, which play important roles in ventricular repolarization, in protecting the myocardium from unwanted premature stimuli, and in drug-induced Long QT Syndrome (LQTS). KCNE1, a small transmembrane protein, can coassemble with hERG. However, it is not known how KCNE1 variants influence the channel's response to premature stimuli or if they influence the sensitivity of hERG to pharmacological inhibition. Accordingly, whole-cell patch-clamp measurements of hERG current (IhERG) were made at 37°C from hERG channels coexpressed with either wild-type (WT) KCNE1 or with one of three KCNE1 variants (A8V, D76N, and D85N). Under both conventional voltage clamp and ventricular action potential (AP) clamp, the amplitude of IhERG was smaller for A8V, D76N, and D85N KCNE1 + hERG than for WT KCNE1 + hERG. Using paired AP commands, with the second AP waveform applied at varying time intervals following the first to mimic premature ventricular excitation, the response of IhERG carried by each KCNE1 variant was reduced compared to that with WT KCNE1 + hERG. The IhERG blocking potency of the antiarrhythmic drug quinidine was similar between WT KCNE1 and the three KCNE1 variants. However, the IhERG inhibitory potency of the antibiotic clarithromycin and of the prokinetic drug cisapride was altered by KCNE1 variants. These results demonstrate that naturally occurring KCNE1 variants can reduce the response of hERG channels to premature excitation and also alter the sensitivity of hERG channels to inhibition by some drugs linked to acquired LQTS.

Candida glabrata Drug:H + Antiporter CgQdr2 Confers Imidazole Drug Resistance, Being Activated by Transcription Factor CgPdr1

The widespread emergence of antifungal drug resistance poses a severe clinical problem. Though predicted to play a role in this phenomenon, the drug:H(+) antiporters (DHA) of the major facilitator superfamily have largely escaped characterization in pathogenic yeasts. This work describes the first DHA from the pathogenic yeast Candida glabrata reported to be involved in antifungal drug resistance, the C. glabrata QDR2 (CgQDR2) gene (ORF CAGL0G08624g). The expression of CgQDR2 in C. glabrata was found to confer resistance to the antifungal drugs miconazole, tioconazole, clotrimazole, and ketoconazole. By use of a green fluorescent protein (GFP) fusion, the CgQdr2 protein was found to be targeted to the plasma membrane in C. glabrata. In agreement with these observations, CgQDR2 expression was found to decrease the intracellular accumulation of radiolabeled clotrimazole in C. glabrata and to play a role in the extrusion of this antifungal from preloaded cells. Interestingly, the functional heterologous expression of CgQDR2 in the model yeast Saccharomyces cerevisiae further confirmed the role of this gene as a multidrug resistance determinant: its expression was able to complement the susceptibility phenotype exhibited by its S. cerevisiae homologue, QDR2, in the presence of imidazoles and of the antimalarial and antiarrhythmic drug quinidine. In contrast to the findings reported for Qdr2, CgQdr2 expression does not contribute to the ability of yeast to grow under K(+)-limiting conditions. Interestingly, CgQDR2 transcript levels were seen to be upregulated in C. glabrata cells challenged with clotrimazole or quinidine. This upregulation was found to depend directly on the transcription factor CgPdr1, the major regulator of multidrug resistance in this pathogenic yeast, which has also been found to be a determinant of quinidine and clotrimazole resistance in C. glabrata.

Inhibition of 2-Aminoethoxydiphenyl Borate-induced Rat Atrial Ectopic Activity by Anti-arrhythmic Drugs

Background/Aims: 2-aminoethoxydiphenyl borate (2-APB) provokes spontaneous mechanical activity in isolated rat left atria. The present study is to characterize 2-APB-induced ectopic activity in rat atria and to investigate the inhibition of 2-APB-induced ectopic activity by anti-arrhythmic drugs. Methods: 2-APB-induced ectopic activity was measured through an isometric force transducer connected to a multichannel acquisition and analysis system. Intracellular [Ca²⁺]_i was measured with fluorescence laser scanning confocal microscopy. Voltage-dependent L- type Ca²⁺ currents were recorded by using patch-clamp technique. Results: 2-APB dose-dependently increased the ectopic activity of left atria at 1, 5, 10, 20, 50 µM. Anti-arrhythmic drugs, quinidine (10µM), lidocaine (10µM), verapamil (5µM), and amiodarone (50µM,100µM) inhibited 2-APB-induced ectopic activity. 2-APB-induced ectopic activity was inhibited by Ca²⁺-free bath, Na⁺/Ca²⁺ exchanger blockers, 3′,4′-dichlorobenzamil hydrochloride (DHC) and Ni²⁺, not by non-selective cation channel blocker Gd³⁺. 2-APB also induced ectopic contractions in ventricular tissue straps and the ectopic contractions were inhibited by quinidine, verapamil and DHC. Lidocaine, verapamil and DHC inhibited 2-APB-induced increase of intracellular Ca²⁺ concentration in cardiomyocytes. Low molecular weight heparin inhibited phenylephrine (PE)-induced but not 2-APB -induced atria ectopic activity, and the pattern of 2-APB-induced ectopic activity was continuous, distinct from the discontinuous activity induced by PE. Conclusion: 2-APB-induced atria ectopic activity was inhibited by classic anti-arrhythmic drugs quinidine, lidocaine, verapamil, amiodarone, and Na⁺/Ca²⁺ exchanger blockers. It can be used for testing agents able to affect any of Na⁺, Ca²⁺ channel, Na⁺/Ca²⁺ exchanger without selectivity.

Human and Murine Embryonic Stem Cell-Derived Cardiomyocytes Serve Together as a Valuable Model for Drug Safety Screening

Aims: Screening of drug safety is typically performed in diverse non-human healthy species with an intact repolarization reserve. Nevertheless, these drugs are later applied in diseased humans with a reduced repolarization reserve. It would be optimal to set up a preclinical screening tool to estimate the proarrhythmic potential of drugs in human cardiac tissue with a reduced repolarization reserve in vitro. Methods and Results: In our study spontaneously beating human embryonic stem cell-derived cardiomyocytes clusters (hESCM) and murine ES cell-derived cardiomyocytes (mESCMs) were plated onto micro-electrode arrays (MEAs) to record the extracelluar field potentials (FPs) as well as effects of several antiarrhythmic drugs. In line with clinical observations the class III antiarrhythmic drugs (±)-sotalol, E4031 and class I antiarrhythmic drug quinidine led to a prolongation of the cardiac repolarization phase (FP duration, FPdur) and a decrease of the FP frequency. Verapamil (a class IV antiarrhythmic drug) decreased the FP frequency and shortened FPdur. Both, quinidine and verapamil, but not (±)-sotalol or E4031 decreased conduction velocities in hESCM clusters. Moreover, (±)-sotalol exerted stronger effects on FPdur in early developmental stages of hESCMs, as proof for a reduced repolarization reserve. The EC₅₀ of the (±)-sotalol-induced prolongation of the FPdur was higher in mESCMs than in hESCMs implying species-dependent differences in cardiac repolarization. Likewise, the incidence of drug-induced early recurrent depolarization (ERDs) was higher in mESCMs than hESCMs. Conclusion: The combined measurement of drug effects on FP parameters in hESCMs and mESCMs serves as a reliable in vitro model for preclinical studies of drug safety.

Stealth dendrimers for antiarrhythmic quinidine delivery

Dendrimers have been attracting growing attention because of their unique well-defined globular nanoscale architecture and numerous functional groups on the surface. Attachment of PEG to the dendrimer generates stealth dendrimers, which have promising structural advantages for drug delivery. In this study, synthetic methods were explored to deliver antiarrhythmic quinidine by stealth dendrimers. In particular, quinidine was covalently attached to anionic G2.5 and cationic G3.0 polyamidoamine (PAMAM) dendrimers via a glycine spacer, respectively. The resulting quinidine-PAMAM-PEG conjugates were characterized and confirmed by FT-IR and (1)H-NMR. In vitro hydrolysis was carried out in pH 7.4 PBS buffer at 37 degrees C to confirm the bioavailability of the conjugated quinidine.

Effects of quinidine and verapamil on human cardiovascular alpha1-adrenoceptors.

The antiarrhythmic drugs quinidine and verapamil are known to block alpha1-adrenoceptors (alpha1ARs). Alpha1ARs are a heterogeneous family of three subtypes (alpha1A, alpha1B, and alpha1D), and little is known about the effects of quinidine and verapamil on the different human alpha1AR subtypes. Reverse transcriptase-polymerase chain reaction showed that all alpha1AR subtypes are expressed in both human heart (atrium and ventricle) and the mesenteric artery. Pharmacological profiles of quinidine and verapamil actions on the alpha1AR subtypes were characterized with Chinese hamster ovary cells stably expressing cloned human alpha1AR subtypes. Radioligand binding studies showed that quinidine and verapamil had high affinities for all alpha1AR subtypes. Also, both drugs synergistically inhibited alpha1AR-mediated inositol 1,4,5-triphosphate production at the clinical effective concentration range (1 micromol/L quinidine and 0.1 micromol/L verapamil). The results show that all alpha1AR subtypes are expressed in the human cardiovascular system and that quinidine and verapamil may have a potent, synergistic inhibitory effect on the alpha1ARs. Clinically observed hypotension after quinidine plus verapamil can be explained by their synergistic inhibitory effects on human alpha1ARs.

Electrophysiologic Effects of the New Class III Antiarrhythmic Drug Dofetilide in an Experimental Canine Model of Pacing-induced Atrial Fibrillation.

BACKGROUND: Dofetilide is a new class III antiarrhythmic drug currently under investigation for the treatment of supraventricular arrhythmias in humans. Dofetilide have been previously shown to be highly effective in terminating and suppressing reentrant atrial flutter in the experimental canine crush-injury model, in which its antiarrhythmic efficacy was correlated with prolongation of wavelength and reduction in dispersion of refractoriness, effects not produced by the class IA antiarrhythmic drug quinidine. The purpose of this study was to evaluate the antiarrhythmic efficacy and mechanisms of action of dofetilide in an experimental model of atrial fibrillation. METHODS AND RESULTS: Dofetilide was administered intravenously to seven open-chest dogs with acute sustained atrial fibrillation induced by rapid atrial pacing for up to 4 hours. Mean atrial effective refractory period (ERP), dispersion of ERP, conduction velocity and wave-length were determined by multipoint right atrial programmed stimulation and activation mappin gusing a 56-electrode mapping plaque on the right atrial free wall. Dofetilide prolonged average ERP by 22% from 104 +/- 13 to 127 +/- 15 ms (P <.001), prolonged maximum ERP by 11% from 129 +/- 7 to 143 +/- 10 (P <.003), had no effect on conduction velocity at 200 ms pacing cycle length, slowed conduction velocity by 16% from 0.89 +/- 12 to 0.75 +/-.17 ms at 150 ms pacing cycle length, slowed conduction velocity by 16% from 0.89 +/- 12 to 0.75 +/-.17 ms at 150 ms pacing cycle length (P <.001), increased wavelength by 20% from 93 +/- 7 to 112 +/- 9 mm (P <.01), reduced dispersion of ERP by 24% from 11.4 +/- 2.9 to 8.7 +/- 2.3 (P =.016), and reduced the number of adjacent electrodes with ERP difference >/=20 ms by 67% from 18.4 +/- 7.1 to 6.1 +/- 4.2 (P <.001). Dofetilide reduced the number of excitation wavelets (total over three beats) entering the region of the mapping plaque by 38% from 5.0 +/-.8 to 3.1 +/-.4 (P <.002). Dofetilide terminated atrial fibrillation in all seven dogs at a mean of 3.4 +/- 2.2 minutes into the loading infusion and prevented reinduction of atrial fibrillation in all seven dogs after completion of the loading infusion, while on maintenance infusion. Time to termination of atrial fibrillation correlated closely with change in ERP (r = 0.78, P =.036). CONCLUSIONS: Dofetilide was highly effective in terminating and suppressing sustained pacing induced atrial fibrillation in this canine model. Time to termination of atrial fibrillation correlated with the degree of change in ERP produced by dofetilide. The mechanism of termination of atrial fibrillation by dofetilide appeared to be a progressive reduction and eventual extinction of re-entrant wavelets. The predominant electrophysiologic effects of dofetilide were prolongation of ERP and wavelength and a reduction in dispersion of refractoriness. Dofetilide had little effect on conduction velocity in this model, except at very short pacing cycle lengths.

Pharmacology of the high-affinity apamin receptor in rabbit heart

Apamin is a potent blocker of calcium-activated small conductance potassium (SK) channels in neurons, liver, skeletal muscle and ileum smooth muscle, but not in cardiac muscle. Cardiac muscle is devoid of SK channels; however, in isolated, single ventricular myocytes apamin is an extremely potent blocker of the L-type calcium current, and the anti-arrhythmic drug quinidine reverses apamin block. To characterize the receptor binding properties and pharmacology of the apamin receptor in heart. The binding properties of the apamin receptor were determined by rapid filtration of purified rabbit heart membranes. Monoiodinated apamin binds to a labile, membrane-bound protein in heart membranes at a single, high-affinity site (KD = 8.07 +/- 2.14 pM and Bmax = 686 +/- 167 fmoles/mg protein, significant run test at P = 0.05 for a one site fit). 125I-apamin binding is dose-dependently inhibited by apamin, scyllatoxin, quinidine, amiloride, as well as a variety of di- and trivalent cations that are classical blockers of L-type calcium channels (e.g. Co2+, Cd2+, Mn2+, La3+, Gd3+). The cardiac apamin receptor is also critically dependent upon pH, temperature and KCl, and co-purifies in the same membrane fraction as L-type cardiac Ca2+ channels. The apamin receptor in rabbit heart P2 membranes has pharmacological and biochemical properties in common with both an SK channel and an L-type Ca2+ channel.

Effect of antiarrhythmics on the release of adenosine in rat hearts with coronary occlusion and reperfusion

In isolated perfused rat hearts the left coronary artery was occluded for 5 min, with subsequent reperfusion for 20 min. During the reperfusion severe tachyarrhythmias were observed, with ventricular fibrillation occurring in all hearts. Simultaneously, large amounts of adenosine and its degradation products inosine, hypoxanthine, xanthine and uric acid were released into the coronary perfusate. The antiarrhythmics quinidine, lidocaine and gallopamil significantly decreased the release. The effect of quinidine and lidocaine was linked with the antifibrillatory action of these drugs. Also the interruption of fibrillation immediately after its appearance by potassium chloride decreased the release of adenosine and its metabolites in a highly significant way. The effect of gallopamil on the release was independent of an antifibrillatory action. The findings indicate that different kinds of antiarrhythmic drugs can affect the release of nucleosides and oxypurines in hearts with ischaemia and reperfusion.