Involvement of the Na+/Ca2+ Exchanger in the Automaticity of Guinea-Pig Pulmonary Vein Myocardium as Revealed by SEA0400

Abstract

Abstract. We examined the involvement of the Na + /Ca 2+ exchanger in the automaticity of the pulmonary vein myocardium with a specific inhibitor, SEA0400. Action potentials were recorded from the myocardial layer of isolated guinea-pig pulmonary vein preparations, and Ca 2+transients were recorded from the cardiomyocytes. Spontaneous electrical activity was observed in 17.7% of the preparations, which was inhibited by either SEA0400 or ryanodine. In quiescent preparations, ouabain induced electrical activity and spontaneous Ca 2+ transients, which were inhibited by SEA0400, as well as ryanodine. These results provide pharmacological evidence that the Na + /Ca 2+ exchanger underlies the automaticity of the pulmonary vein myocardium.Keywords: pulmonary vein myocardium, NaPulmonary veins are considered to be involved in the initiation and maintenance of atrial fibrillation, one of the most frequent arrhythmias in clinical practice (1). Pulmonary veins contain a myocardial layer, whose electrical activity is considered to underlie their arrhythmogenic activity (2). The pulmonary vein myocardium has different electrophysiological properties from those of the working myocardium, including lower density of I K1 and a less negative resting membrane potential (3). The precise mechanisms of the pulmonary vein electrical activity as well as its pharmacological properties are now receiving attention as the basis to develop an effective therapeutic strategy against atrial fibrillation.The Na + /Ca 2+ exchanger (NCX) is involved in the physiological and pathophysiological regulation of Ca 2+ concentration in the myocardium. It functions both in the forward (Ca 2+ extrusion) and reverse (Ca 2+ influx) modes, and its functional role may vary with the region and the condition of the myocardium (4, 5). The forward mode NCX activity (inward current) is the major pathway for Ca 2+ extrusion from the cytoplasm and is also considered to be involved in the normal pacemaking of the rabbit sinoatrial node (6). It was postulated that the Ca 2+ released from the sarcoplasmic reticulum (SR) during the diastolic period is pumped out of the cell through the forward mode NCX, which generates an inward current that contributes to the diastolic depolarization of the pacemaker. Although it is possible that such a mechanism is involved in the automaticity of other myocardial regions including the pulmonary vein myocardium, pharmacological evidence is limited because of the lack of an NCX inhibitor with sufficient specificity. SEA0400 {2-[4-[(2,5-difluorophenyl) methoxy] phenoxy]-5-ethoxyaniline} is a potent and selective inhibitor of NCX in cultured neurons, astrocytes, microglia, dog sarcolemmal vesicles, and cultured rat myocytes with negligible affinities towards other transporters, ion channels, and receptors (7). We have previously shown that SEA0400 is a specific inhibitor of NCX in the myocardium (8, 9). SEA0400 (1 μM), which inhibited the NCX current by more than 80%, had no effect on the Na + current, L-type Ca 2+ current, delayed recti...

Electrophysiological and Pharmacological Properties of the Pulmonary Vein Myocardium

“…The spontaneous as well as pacing-induced electrical activity was completely inhibited by carbachol, which activates the acetylcholine-activated K + current and increases the repolarizing K + current density. In ouabain-treated preparations, generation of action potentials was preceded by oscillation of the resting membrane potential; ouabain induced an increase in intracellular diastolic Ca Results obtained in the rabbit, 30) rat 21,26) and guinea pig 20) suggest the involvement of intracellular Ca 2+ in the generation of spontaneous action potentials in the pulmonary-vein myocardium (Fig. 2) , the effectiveness of ryanodine in the rabbit and guinea pig suggests the involvement of ryanodine receptor channels.…”

“…About a half of the isolated pulmonary vein tissue preparations showed automatic electrical activity, and the average frequency of continuous firing was about 1 Hz. 20) Most of the quiescent preparations showed automatic activity after noradrenaline treatment (Fig. 1A).…”

“…The pulmonary vein myocardium had a less negative resting membrane potential, smaller maximum upstroke velocity and amplitude of the action potential, and shorter action potential duration than the atrial myocardium. 3,20) The difference from the atria were more prominent in pulmonary myocardium regions distal to the atria than in the proximal. The difference in action potential upstroke velocity of the myocardial regions within the pulmonary vein correlated with the action potential conduction velocity.…”

“…In all three waveform types, the frequency of the repetitive firing was higher than those reported in other experimental animal species such as the guinea-pig, rabbit and canine in which the delayed rectifier K + current serves as the major repolarizing current. 16,20,24) The mouse myocardium relies on I to as the major repolarizing current, which is involved in the constitution of an extremely rapid early repolarization phase of the action potential. As the activation-inactivation kinetics of the Na + and Ca 2+ currents were similar among these animal species, 14,16,25,26) the highfrequency firing in the mouse could be attributed to the rapid kinetics of I to .…”

“…In the isolated guinea-pig pulmonary vein myocardium, microelectrode experiments revealed the presence of spontaneous electrical activity in about half of the tissue preparations. 20,21) Interventions which increase intracellular Ca 2+ load such as ouabain 20) or high-frequency pacing 33) induced electrical activity in quiescent preparations. The spontaneous as well as pacing-induced electrical activity was completely inhibited by carbachol, which activates the acetylcholine-activated K + current and increases the repolarizing K + current density.…”