Different inhibition patterns of tedisamil for fast and slowly inactivating transient outward current in rat ventricular myocytes

A novel modular modeling approach for understanding different electromechanics between left and right heart in rat

While ion channels and transporters involved in excitation-contraction coupling have been linked and constructed as comprehensive computational models, validation of whether each individual component of a model can be reused has not been previously attempted. Here we address this issue while using a novel modular modeling approach to investigate the underlying mechanism for the differences between left ventricle (LV) and right ventricle (RV). Our model was developed from modules constructed using the module assembly principles of the CellML model markup language. The components of three existing separate models of cardiac function were disassembled as to create smaller modules, validated individually, and then the component parts were combined into a new integrative model of a rat ventricular myocyte. The model was implemented in OpenCOR using the CellML standard in order to ensure reproducibility. Simulated action potential (AP), Ca²⁺ transient, and tension were in close agreement with our experimental measurements: LV AP showed a prolonged duration and a more prominent plateau compared with RV AP; Ca²⁺ transient showed prolonged duration and slow decay in LV compared to RV; the peak value and relaxation of tension were larger and slower, respectively, in LV compared to RV. Our novel approach of module-based mathematical modeling has established that the ionic mechanisms underlying the APs and Ca²⁺ handling play a role in the variation in force production between ventricles. This simulation process also provides a useful way to reuse and elaborate upon existing models in order to develop a new model.

Effect of ethanol on action potential and ionic membrane currents in rat ventricular myocytes

AIM

Even though alcohol intoxication is often linked to arrhythmias, data describing ethanol effect on cardiac ionic channels are rare. In addition, ethanol is used as a solvent of hydrophobic compounds in experimental studies. We investigated changes of the action potential (AP) configuration and main ionic membrane currents in rat cardiomyocytes under 20-1500 m(M) ethanol.

METHODS

  Experiments were performed on enzymatically isolated rat right ventricular myocytes using the whole cell patch-clamp technique at room temperature.

RESULTS

  Ethanol reversibly decelerated the upstroke velocity and decreased AP amplitude and duration at 0.2 and 3 Hz. The fast sodium current I(Na) , l-type calcium current I(Ca) and transient outward potassium current I(to) were reversibly inhibited in a concentration-dependent manner (50% inhibition at 446 ± 12, 553 ± 49 and 1954 ± 234 m(M), respectively, with corresponding Hill coefficients 3.1 ± 0.3, 1.1 ± 0.2 and 0.9 ± 0.1). Suppression of I(Na) and I(Ca) magnitude was slightly voltage dependent. The effect on I(Ca) and I(to) was manifested mainly as an acceleration of their apparent inactivations with a decreased slow and fast time constant respectively. As a consequence of marked differences in n(H) , sensitivity of the currents to ethanol at 10% inhibition decreases in the following order: I(Ca) (75 mm, 3.5‰), I(to) (170 m(M), 7.8‰) and I(Na) (220 m(M), 10.1‰).

CONCLUSION

  Our results suggest a slight inhibition of all the currents at ethanol concentrations relevant to deep alcohol intoxication. The concentration dependence measured over a wide range may serve as a guideline when using ethanol as a solvent.

Tedisamil: a new novel antiarrhythmic

Solvay Pharmaceuticals is currently developing tedisamil (KC-8857), a novel antiarrhythmic with additional anti-ischaemic properties, which acts via potassium channel blockade. This drug can be categorised as a class III antiarrhythmic agent due to its effects of action potential and QT interval prolongation in these patients. This agent was initially developed for its anti-ischaemic properties and Phase I trials have shown tedisamil to be an effective bradycardic agent, as well as causing a reverse rate-dependent QT interval prolongation. Subsequent Phase II results have confirmed that in patients with ischaemic heart disease, tedisamil had beneficial haemodynamic and anti-ischaemic effects. Phase III studies in patients with ischaemic heart disease indicated that tedisamil is an effective agent for the treatment of angina, resulting in a dose-dependent increase in anginal threshold (with a decrease in anginal attacks, increased exercise capacity during treadmill exercise and decreased electrocardiographic signs of exercise induced ischaemia) in comparison to placebo. Although tedisamil has been shown to be an effective anti-ischaemic agent, with Phase III trials for angina pectoris now completed, the company are now pursuing the use of tedisamil for the treatment of atrial fibrillation, for which tedisamil is still in Phase II/III clinical trials. Launch data are not yet known.

Antiarrhythmic drugs in atrial fibrillation: an overview of new agents, their mechanisms of action and potential clinical utility

Despite recent advances in our understanding of the mechanism of atrial fibrillation (AF), effective treatment remains difficult in many patients. Pharmacotherapy remains the mainstay of treatment and includes control of ventricular rate as well as restoration and maintenance of sinus rhythm. The currently available antiarrhythmic drugs are particularly effective in converting paroxysmal AF to sinus rhythm and in enhancing the positive effect of electrical cardioversion, but are limited in their efficacy in maintaining sinus rhythm. Moreover, there are limited options in the setting of co-existing ischaemic heart disease, left ventricular dysfunction and structural heart diseases. New drugs added to our clinical armamentarium have been, or are being, developed to combine better efficacy and lack of pro-arrhythmic effects. These developments have gained more interest particularly with the recent debate over rate control versus rhythm control for AF. Although some of these agents are promising, their uptake in clinical practice will not only depend on their efficacy as antiarrhythmic agents but also on their safety in acutely terminating AF and in long-term maintenance of sinus rhythm or rate control in the community.

Tedisamil and lidocaine enhance each other's antiarrhythmic activity against ischaemia-induced arrhythmias in rats

1. Combinations of the action potential-widening drug tedisamil (Class III antiarrhythmic activity), and the inactivated state sodium channel blocker lidocaine (Class Ib antiarrhythmic activity) were assessed for antiarrhythmic actions in a rat model of ischaemia-induced arrhythmias and for electrophysiological actions in normal rat myocardial tissue. 2. Both tedisamil and lidocaine dose-dependently suppressed ischaemia-induced arrhythmias. The ED(50) values were 3.0+/-1.3 and 4.9+/-0.6 micro mol kg(-1) min(-1), respectively. 3. Combinations of the two drugs acted synergistically such that the ED(50) for tedisamil was reduced to 0.8+/-0.2 micro mol kg(-1) min(-1) in the presence of 2 micro mol kg(-1) min(-1) lidocaine. Similarly, the ED(50) for lidocaine was reduced to 0.7+/-0.2 micro mol kg(-1) min(-1) in the presence of 2 micro mol kg(-1) min(-1) tedisamil (both P<0.05). 4. In a separate series of experiments in which normal ventricular tissue was electrically stimulated, 2 micro mol kg(-1) min(-1) lidocaine produced a leftward shift in the dose-response curve for tedisamil's effect on effective refractory period (P<0.05). This dose of lidocaine had no effect on its own. These data indicate that the synergistic actions of combinations of tedisamil and lidocaine were mediated, at least in part, by extension of effective refractory period in normal myocardial tissue. 5. In contrast to the strategy of developing drugs that are selective for a single electrophysiological mechanism, the results of the present study suggest that effective antiarrhythmic drugs might be developed by optimising the combination of two complimentary electrophysiological mechanisms (i.e., action potential-prolonging activity and inactivated state sodium channel blockade).

Antiarrhythmic drug therapy of atrial fibrillation: focus on new agents

The precise mechanisms of clinical effect of antiarrhythmic agents and the ideal "molecular targets" against arrhythmias, in particular atrial fibrillation, are poorly understood. Current antiarrhythmic drug development, particularly for drugs expected to be active against atrial fibrillation, has focused on drugs with multiple ionic mechanisms of action, in particular on those that block multiple potassium channels. Investigation of antiarrhythmic agents is complicated by the diversity of animal-disease models studied, by the potential multiple mechanisms of arrhythmias, and by the incompletely understood relationships between risks and benefits of antiarrhythmic drug therapy. Furthermore, rhythm control strategies in large groups of patients with atrial fibrillation have failed to show substantial clinical benefit. Nevertheless, drugs that block multiple potassium channels and appear to have relatively little organ toxicity, such as tedisamil, may represent an important new avenue in the therapeutic approach to highly symptomatic arrhythmias such as atrial fibrillation.

Inhibition of Kv2.1 voltage-dependent K+ channels in pancreatic beta-cells enhances glucose-dependent insulin secretion

Voltage-dependent (Kv) outward K(+) currents repolarize beta-cell action potentials during a glucose stimulus to limit Ca(2+) entry and insulin secretion. Dominant-negative "knockout" of Kv2 family channels enhances glucose-stimulated insulin secretion. Here we show that a putative Kv2.1 antagonist (C-1) stimulates insulin secretion from MIN6 insulinoma cells in a glucose- and dose-dependent manner while blocking voltage-dependent outward K(+) currents. C-1-blocked recombinant Kv2.1-mediated currents more specifically than currents mediated by Kv1, -3, and -4 family channels (Kv1.4, 3.1, 4.2). Additionally, C-1 had little effect on currents recorded from MIN6 cells expressing a dominant-negative Kv2.1 alpha-subunit. The insulinotropic effect of acute Kv2.1 inhibition resulted from enhanced membrane depolarization and augmented intracellular Ca(2+) responses to glucose. Immunohistochemical staining of mouse pancreas sections showed that expression of Kv2.1 correlated highly with insulin-containing beta-cells, consistent with the ability of C-1 to block voltage-dependent outward K(+) currents in isolated mouse beta-cells. Antagonism of Kv2.1 in an ex vivo perfused mouse pancreas model enhanced first- and second-phase insulin secretion, whereas glucagon secretion was unaffected. The present study demonstrates that Kv2.1 is an important component of beta-cell stimulus-secretion coupling, and a compound that enhances, but does not initiate, beta-cell electrical activity by acting on Kv2.1 would be a useful antidiabetic agent.

A mathematical model of action potential heterogeneity in adult rat left ventricular myocytes

Mathematical models were developed to reconstruct the action potentials (AP) recorded in epicardial and endocardial myocytes isolated from the adult rat left ventricle. The main goal was to obtain additional insight into the ionic mechanisms responsible for the transmural AP heterogeneity. The simulation results support the hypothesis that the smaller density and the slower reactivation kinetics of the Ca(2+)-independent transient outward K(+) current (I(t)) in the endocardial myocytes can account for the longer action potential duration (APD), and more prominent rate dependence in that cell type. The larger density of the Na(+) current (I(Na)) in the endocardial myocytes results in a faster upstroke (dV/dt(max)). This, in addition to the smaller magnitude of I(t), is responsible for the larger peak overshoot of the simulated endocardial AP. The prolonged APD in the endocardial cell also leads to an enhanced amplitude of the sustained K(+) current (I(ss)), and a larger influx of Ca(2+) ions via the L-type Ca(2+) current (I(CaL)). The latter results in an increased sarcoplasmic reticulum (SR) load, which is mainly responsible for the higher peak systolic value of the Ca(2+) transient [Ca(2+)](i), and the resultant increase in the Na(+)-Ca(2+) exchanger (I(NaCa)) activity, associated with the simulated endocardial AP. In combination, these calculations provide novel, quantitative insights into the repolarization process and its naturally occurring transmural variations in the rat left ventricle.

Effects of tedisamil on cardiovascular tissues isolated from normo- and hypertensive rats

BACKGROUND

This study was undertaken to characterize the effects of tedisamil on isolated rat cardiovascular tissues, and identify actions that could be beneficial or detrimental in the treatment of cardiac disease.

RESULTS

Tedisamil prolonged the Wistar Kyoto normotensive rat (WKY) left ventricular action potential and augmented the force of contraction of left ventricle strips. On the 12-month-old SHR model of cardiac hypertrophy, the augmenting effects of tedisamil at 10(-6) and 3 x 10(-6) M were reduced. On the 21-month-old SHR model of heart failure, the augmenting effects of tedisamil at 10(-6) and 3 x 10(-6) M were further reduced. The augmenting effect of tedisamil at 10(-5) M was reduced to 47%. The rate of the right atrium of 16- to 17-month-old WKY was reduced by tedisamil at 10(-5) and 10(-4) M, and tedisamil had a similar effect on the SHR right atrium. Tedisamil at 10(-6)--3 x 10(-5) M contracted the portal veins of WKY and aortae of 12-month-old WKY and SHR.

CONCLUSIONS

The positive inotropic and negative chronotropic effects of tedisamil in the rat, which are partially or fully maintained in hypertrophied or failing myocardium would be beneficial in the treatment of heart failure. In contrast, the vasoconstrictor action of tedisamil will be detrimental in heart failure.

Tedisamil: master switch of nature?

Decreasing heart rate is potentially useful in ischaemic heart disease. Tedisamil is a bradycardic agent resulting from its ability to inhibit transient outward current (I(to)) in atria. Tedisamil inhibits I(to), potassium current (IK), K(ATP) and the protein kinase A-activated chloride channel in ventricles as well as vascular IK and Ca(2+)-activated IK (IK((Ca))). Tedisamil prolongs cardiac action potentials and the corrected QT (QTc) of the ECG and also increases cardiac refractoriness. Tedisamil is anti-arrhythmic in animal models of ventricular arrhythmias and atrial flutter. The bradycardic effect of tedisamil is associated with a reduction in myocardial oxygen demand. On isolated rat ventricle, tedisamil is a positive inotrope and on isolated rabbit atria, tedisamil reverses the negative inotropic effect of pinacidil. Tedisamil contracts the isolated rat portal vein and aorta, reduces cromakalim-induced relaxations of contracted rat aorta and increases blood pressure in animals and humans. Tedisamil is 96% bound to plasma proteins, has a plasma half-life of about 10 h and is cleared from the kidney unchanged. Clinical trials have shown that the electrophysiology of tedisamil is that of a class III anti-arrhythmic. In coronary artery disease, tedisamil has no effect on inotropism and increases the threshold for angina. Potassium channel blockade with tedisamil may have advantages over calcium channel blockers or K(ATP) channel openers as an anti-ischaemic mechanism in coronary artery disease. In exercise-induced myocardial ischaemia, beta-blockers are probably favourable to tedisamil, as they will limit the increase in heart rate, contractility and blood pressure caused by sympathetic stimulation, whereas tedisamil will not. In heart failure patients, tedisamil reduces heart rate, but increases blood pressure. The usefulness of tedisamil as a bradycardic agent is limited by the increase in blood pressure. A drug that is bradycardic without increasing blood pressure would be an improvement on tedisamil as the master switch of nature for ischaemic heart disease.