Involvement of the persistent Na+ current in the diastolic depolarization and automaticity of the guinea pig pulmonary vein myocardium

Involvement of the Na+/Ca2+ Exchanger in the Automaticity of the Cardiomyocytes from the Guinea Pig Pulmonary Vein but Not the Sinus Node

Fluorescence imaging analysis was performed in cardiomyocytes from the sinus node, the orthotopic pacemaker, and the pulmonary vein, a potential ectopic pacemaker that may cause atrial fibrillation, focusing on the role of the Na+/Ca2+ exchanger (NCX). Isolated cardiomyocytes from the guinea pig pulmonary vein and sinus node showing automaticity were loaded with fluorescence probes for analysis. Inhibition of NCX by SEA0400 decreased the Ca2+ transient frequency in the pulmonary vein cardiomyocytes but not in the sinus node. The basal intracellular Ca2+ concentration, as well as the number of Ca2+ sparks in the subsarcolemmal region, was higher in the pulmonary vein cardiomyocytes than in the sinus node. By contrast, the intracellular Na+ concentration was not different between the pulmonary vein and sinus node cardiomyocytes. The equilibrium potential for NCX (ENCX) was estimated to be less negative in the pulmonary vein cardiomyocytes than in the sinus node. In conclusion, the forward mode NCX is involved in spontaneous activity in the pulmonary vein cardiomyocytes but not in the sinus node; this is probably because the Ca2+ supply and the driving force for the forward mode NCX are both larger in the pulmonary vein cardiomyocytes than in the sinus node.

α1-Adrenoceptor Blockade by Class I Antiarrhythmic Drugs in Guinea Pig Thoracic Aorta as Revealed by Mechanical and Fluorescence Displacement Analysis

The effects of thirteen Vaughn Williams class I antiarrhythmic drugs on the α1-adrenergic receptor-mediated contraction were examined in thoracic aorta tissue preparations isolated from the guinea pig. Cibenzoline, quinidine, aprindine, and ranolazine, as well as prazosin, inhibited the phenylephrine-induced contraction with pA2 values of 5.64, 5.59, 5.61, 5.08, and 8.50, respectively, but not prostaglandin F2α-induced. These drugs reduced the staining of the smooth muscle layer by fluorescent prazosin. Propafenone inhibited the phenylephrine-induced contraction with an apparent pA2 value of 5.31 and reduced the staining by fluorescent prazosin, but also inhibited the prostaglandin F2α-induced contraction. Other class I antiarrhythmic drugs, disopyramide, pirmenol, procainamide, lidocaine, mexiletine, flecainide, pilsicainide, and GS-458967, affected neither the contraction by phenylephrine nor the fluorescent staining by prazosin. These results indicate that among the class I antiarrhythmic drugs, cibenzoline, aprindine, and propafenone, as well as quinidine and ranolazine, have α1-adrenoceptor-blocking activity at therapeutically relevant concentrations.

Automaticity of the Pulmonary Vein Myocardium and the Effect of Class I Antiarrhythmic Drugs

The pulmonary vein wall contains a myocardial layer whose ectopic automaticity is the major cause of atrial fibrillation. This review summarizes the results obtained in isolated pulmonary vein myocardium from small experimental animals, focusing on the studies with the guinea pig. The diversity in the action potential waveform reflects the difference in the repolarizing potassium channel currents involved. The diastolic depolarization, the trigger of automatic action potentials, is caused by multiple membrane currents, including the Na+-Ca2+ exchanger current and late INa. The action potential waveform and automaticity are affected differentially by α- and β-adrenoceptor stimulation. Class I antiarrhythmic drugs block the propagation of ectopic electrical activity of the pulmonary vein myocardium through blockade of the peak INa. Some of the class I antiarrhythmic drugs block the late INa and inhibit pulmonary vein automaticity. The negative inotropic and chronotropic effects of class I antiarrhythmic drugs could be largely attributed to their blocking effect on the Ca2+ channel rather than the Na+ channel. Such a comprehensive understanding of pulmonary vein automaticity and class I antiarrhythmic drugs would lead to an improvement in pharmacotherapy and the development of novel therapeutic agents for atrial fibrillation.

Enhanced Late INa Induces Intracellular Ion Disturbances and Automatic Activity in the Guinea Pig Pulmonary Vein Cardiomyocytes

The effects of enhanced late INa, a persistent component of the Na+ channel current, on the intracellular ion dynamics and the automaticity of the pulmonary vein cardiomyocytes were studied with fluorescent microscopy. Anemonia viridis toxin II (ATX- II), an enhancer of late INa, caused increases in the basal Na+ and Ca2+ concentrations, increases in the number of Ca2+ sparks and Ca2+ waves, and the generation of repetitive Ca2+ transients. These phenomena were inhibited by eleclazine, a blocker of the late INa; SEA0400, an inhibitor of the Na+/Ca2+ exchanger (NCX); H89, a protein kinase A (PKA) inhibitor; and KN-93, a Ca2+/calmodulin-dependent protein kinase II (CaMKII) inhibitor. These results suggest that enhancement of late INa in the pulmonary vein cardiomyocytes causes disturbance of the intracellular ion environment through activation of the NCX and Ca2+-dependent enzymes. Such mechanisms are probably involved in the ectopic electrical activity of the pulmonary vein myocardium.

Selective Inhibitory Effect of Hesperetin on the Automaticity of the Guinea Pig Pulmonary Vein Myocardium

The effect of a citrus-derived flavonoid, hesperetin, on the automaticity and contraction of isolated guinea pig myocardium was examined. Hesperetin inhibited the rate of ectopic action potential firing of the pulmonary vein myocardium; the slope of the diastolic depolarization was decreased with minimum change in the action potential waveform. The effect was dependent on the concentration; the EC50 value for firing rate was 56.2 ± 14.2 µM and that for the diastolic depolarization slope was 41.6 ± 13.4 µM. In the right atria, hesperetin had no effect on the beating rate at concentrations up to 30 µM, but showed a negative chronotropic effect at 100 µM. In the ventricular papillary muscles, hesperetin had no effect on the contractile force at concentrations up to 30 µM, but showed a slight positive inotropic effect at 100 µM. In isolated pulmonary vein cardiomyocytes, 30 µM hesperetin decreased the firing rate of spontaneous Ca2+ transients. These results showed that hesperetin has a selective inhibitory action on the pulmonary vein automaticity with no inhibitory effect on the ventricular contractile force. Clarification of the mechanism of action of hesperetin as well as further exploration of flavonoids would provide clues for the development of pulmonary vein-selective therapeutic agents.

Animal Models of Atrial Fibrillation

Atrial fibrillation (AF) is the most common sustained arrhythmia encountered in humans and is a significant source of morbidity and mortality. Despite its prevalence, our mechanistic understanding is incomplete, the therapeutic options have limited efficacy, and are often fraught with risks. A better biological understanding of AF is needed to spearhead novel therapeutic avenues. Although "natural" AF is nearly nonexistent in most species, animal models have contributed significantly to our understanding of AF and some therapeutic options. However, the impediments of animal models are also apparent and stem largely from the differences in basic physiology as well as the complexities underlying human AF; these preclude the creation of a "perfect" animal model and have obviated the translation of animal findings. Herein, we review the vast array of AF models available, spanning the mouse heart (weighing 1/1000th of a human heart) to the horse heart (10× heavier than the human heart). We attempt to highlight the features of each model that bring value to our understanding of AF but also the shortcomings and pitfalls. Finally, we borrowed the concept of a SWOT analysis from the business community (which stands for strengths, weaknesses, opportunities, and threats) and applied this introspective type of analysis to animal models for AF. We identify unmet needs and stress that is in the context of rapidly advancing technologies, these present opportunities for the future use of animal models.

Differential effects of class I antiarrhythmic drugs on the guinea pig pulmonary vein myocardium: Inhibition of automatic activity correlates with blockade of a diastolic sodium current component

The effects of class I antiarrhythmic drugs on the automaticity of isolated guinea pig pulmonary vein myocardia were investigated using microelectrode and voltage clamp methods. All of the drugs examined reduced the maximum rate of rise of automatic action potentials. The firing frequency and rate of diastolic depolarization were decreased by aprindine, flecainide and propafenone, but not by cibenzoline, disopyramide and pilsicainide, which correlated with blockade of the sodium current component induced by ramp depolarization mimicking the diastolic depolarization. In conclusion, class I antiarrhythmic drugs which block the diastolic sodium current component inhibit the automaticity of the pulmonary vein myocardium.