Miguel Vaquero

Cardiac Fibrillation: From Ion Channels to Rotors in the Human Heart

Recent new information on the dynamics and molecular mechanisms of electrical rotors and spiral waves has increased our understanding of both atrial fibrillation and ventricular fibrillation. In this brief review, we evaluate the available evidence for the separate roles played by individual sarcolemmal ion channels in atrial fibrillation and ventricular fibrillation, assessing the clinical relevance of such findings. Importantly, although human data support the idea that rotors are a crucial mechanism for fibrillation maintenance in both atria and ventricles, there are clear inherent differences between the 2 chamber types, particularly in regard to the role of specific ion channels in fibrillation. But there also are similarities. This knowledge, together with new information on the changes that take place during disease evolution and between structurally normal and diseased hearts, may enhance our understanding of fibrillatory processes pointing to new approaches to improve disease outcomes.

Nitric Oxide Increases Cardiac IK1 by Nitrosylation of Cysteine 76 of Kir2.1 Channels

Rationale: The cardiac inwardly rectifying K+ current (IK1) plays a critical role in modulating excitability by setting the resting membrane potential and shaping phase 3 of the cardiac action potential. Objective: This study aims to analyze the effects of nitric oxide (NO) on human atrial IK1 and on Kir2.1 channels, the major isoform of inwardly rectifying channels present in the human heart. Methods and Results: Currents were recorded in enzymatically isolated myocytes and in transiently transfected CHO cells, respectively. NO at myocardial physiological concentrations (25 to 500 nmol/L) increased inward and outward IK1 and IKir2.1. These effects were accompanied by hyperpolarization of the resting membrane potential and a shortening of the duration of phase 3 of the human atrial action potential. The IKir2.1 increase was attributable to an increase in the open probability of the channel. Site-directed mutagenesis analysis demonstrated that NO effects were mediated by the selective S-nitrosylation of Kir2.1 Cys76 residue. Single ion monitoring experiments performed by liquid chromatography/tandem mass spectrometry suggested that the primary sequence that surrounds Cys76 determines its selective S-nitrosylation. Chronic atrial fibrillation, which produces a decrease in NO bioavailability, decreased the S-nitrosylation of Kir2.1 channels in human atrial samples as demonstrated by a biotin-switch assay, followed by Western blot. Conclusions: The results demonstrated that, under physiological conditions, NO regulates human cardiac IK1 through a redox-related process.

Nitric oxide inhibits Kv4.3 and human cardiac transient outward potassium current (Ito1).

AIMS Chronic atrial fibrillation (CAF) is characterized by a shortening of the plateau phase of the action potentials (AP) and a decrease in the bioavailability of nitric oxide (NO). In this study, we analysed the effects of NO on Kv4.3 (I(Kv4.3)) and on human transient outward K(+) (I(to1)) currents as well as the signalling pathways responsible for them. We also analysed the expression of NO synthase 3 (NOS3) in patients with CAF. METHODS AND RESULTS I(Kv4.3) and I(to1) currents were recorded in Chinese hamster ovary cells and in human atrial and mouse ventricular dissociated myocytes using the whole-cell patch clamp. The expression of NOS3 was analysed by western blotting. AP were recorded using conventional microelectrode techniques in mouse atrial preparations. NO and NO donors inhibited I(Kv4.3) and human I(to1) in a concentration- and voltage-dependent manner (IC(50) for NO: 375.0 +/- 48 nM) as a consequence of the activation of adenylate cyclase and the subsequent activation of the cAMP-dependent protein kinase and the serine-threonine phosphatase 2A. The density of the I(to1) recorded in ventricular myocytes from wild-type (WT) and NOS3-deficient mice (NOS3(-/-)) was not significantly different. Furthermore, the duration of atrial AP repolarization in WT and NOS3(-/-) mice was not different. The increase in NO levels to 200 nM prolonged the plateau phase of the mouse atrial AP and lengthened the AP duration measured at 20 and 50% of repolarization of the human atrial CAF-remodelled AP as determined using a mathematical model. However, the expression of NOS3 was not modified in left atrial appendages from CAF patients. CONCLUSION Our results suggested that the increase in the atrial NO bioavailability could partially restore the duration of the plateau phase of CAF-remodelled AP by inhibiting the I(to1) as a result of the activation of non-canonical enzymatic pathways.

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 IKur, Ito1 and IKs, 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 μM) 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 μM) 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 (Vh−37.0±1.8 vs −40.8±1.6 mV, n=10, P<0.05). SP (10 μM) 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 IKur (29.2±5.5%) and the Ito1 (16.1±3.9%) recorded in mouse ventricular myocytes and the IK (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.

Effects of MiRP1 and DPP6 β‐subunits on the blockade induced by flecainide of KV4.3/KChIP2 channels

The human cardiac transient outward potassium current (Ito) is believed to be composed of the pore‐forming KV4.3 α‐subunit, coassembled with modulatory β‐subunits as KChIP2, MiRP1 and DPP6 proteins. β‐Subunits can alter the pharmacological response of Ito; therefore, we analysed the effects of flecainide on KV4.3/KChIP2 channels coassembled with MiRP1 and/or DPP6 β‐subunits.

Genetically engineered mice as a model for studying cardiac arrhythmias.

Sudden cardiac death due to ventricular tachyarrhythmias remains an unresolved problem, probably because the mechanisms responsible for the progression of cardiac disease to electrophysiological failure are poorly understood. Genetically engineered mice, the principal mammalian model for studying cardiac electrophysiology, have contributed to the understanding of the genetic, molecular and systemic mechanisms involved in the initiation and/or maintenance of cardiac arrhythmias leading to cardiac death, e.g. cardiac excitability, conduction velocity and refractoriness. Several murine models harbouring human gene mutations leading to electrical and structural cardiac disorders have been developed, including channelopathies (long QT syndrome), familial conduction disorders, cardiomyopathies and other inherited cardiac disorders. This article reviews the results of the main genetically modified mice addressing the genesis of cardiac arrhythmias and sudden cardiac death.

Efectos del óxido nítrico sobre la función cardíaca

El oxido nitrico (NO) liberado por practicamente todas las celulas del corazon ejerce multiples efectos sobre la funcion cardiaca. Modula las respuestas inotropicas y cronotropicas, el flujo de entrada de Ca ++ y el ciclo del Ca ++ en el reticulo sarcoplasmico, la transmision autonomica, la frecuencia cardiaca, la respiracion mitocondrial, el consumo miocardico de O 2 y la eficiencia mecanica. El NO regula la contractilidad cardiaca en respuesta a la distension e inhibe la relacion fuerza-frecuencia y las respuesta a la estimulacion β-adrenergica. Tambien mejora la distensibilidad ventricular y aumenta el trabajo latido en pacientes con miocardiopatia dilatada, y desempena un importante papel en la fase tardia del precondicionamiento isquemico. Por ultimo, el NO puede modular la actividad de los canales cardiacos, la arritmogenesis, la apoptosis y la funcion cardiaca en el miocardio insuficiente. Para realizar todas estas funciones, las NO sintasas (NOS) se localizan en microdominios de los cardiomiocitos en intima vecindad con las vias de senalizacion que modulan. Sin embargo, es necesario conocer mejor los mecanismos implicados en la regulacion y la localizacion celular de las NOS, asi como las vias no enzimaticas de sintesis del NO, su localizacion y su inactivacion en diversas situaciones fisipatologicas antes de que podamos trasladar las multiples acciones del NO en una alternative terapeutica.

Farmacología de los ácidos grasos omega-3

El consumo de acidos grasos omega-3, como el acido eicosapentanoico (EPA) y el acido docosahexanoico (DHA), derivados de alimentos marinos y de plantas ha demostrado, en estudios epidemiologicos y clinicos, que reduce la incidencia de mortalidad coronaria y la muerte por arritmias. Recientemente, un suplemento que contiene una concentracion del 90% de acidos omega-3 (EPA y DHA) en forma de etil esteres (Omacor ® ) ha sido autorizado como tratamiento adjunto a la dieta para reducir la hipertrigliceridemia en pacientes adultos y, tambien, como tratamiento adjunto a la dieta y a otros tratamientos en la prevencion secundaria del infarto de miocardio. En este articulo, en primer lugar, revisamos la estructura quimica, las acciones farmacologicas y los mecanismos por los cuales los acidos grasos n-3 y, en particular, el Omacor ® , pueden reducir el riesgo de muerte cardiovascular. A continuacion, se analizan las propiedades farmacocineticas, la seguridad y las recomendaciones de diversos organismos para administrar suplementos de EPA+DHA u Omacor ® en los pacientes con enfermedades cardiovasculares.

Ivabradina, un bloqueador selectivo de la corriente If. Aspectos farmacológicos y tolerabilidad

La frecuencia cardiaca es el principal determinante de las demandas miocardicas de O2 y del flujo sanguineo coronario. La frecuencia cardiaca depende de la actividad electrica espontanea de las celulas marcapasos del nodulo sinoauricular. Estas celulas presentan una fase de despolarizacion diastolica que desplaza el potencial de membrana hacia su valor umbral y se inicia un nuevo potencial de accion que se propaga a traves del miocardio y produce una respuesta contractil. La corriente If de entrada de iones Na+ y K+ a traves de canales activados por la hiperpolarizacion y modulados por nucleotidos ciclicos (HCN) es la principal determinante de la inclinacion de la fase de lenta despolarizacion diastolica. Los canales se abren cuando el potencial de membrana se hiperpolariza y se modulan por la concentracion celular de adenosinmonofosfato ciclico. La ivabradina es un bloqueador especifico de la If. Para ello debe atravesar la membrana y alcanzar su receptor, que se encuentra en la boca intracelular del poro del canal. Como consecuencia, produce una reduccion dependiente de la dosis de la frecuencia cardiaca, que reduce las demandas miocardicas de O2 y aumenta el flujo sanguineo coronario. Sin embargo, a concentraciones terapeuticas no inhibe otras corrientes ionicas cardiacas, razon por la que no modifica la presion arterial, la contractilidad o las propiedades electrofisiologicas cardiacas. En este articulo se revisa el mecanismo de accion, las propiedades farmacocineticas y farmacodinamicas y las reacciones adversas y contraindicaciones de la ivabradina.