Ca(2+)-activated chloride channel activity during Ca(2+) alternans in ventricular myocytes

Calcium- and voltage-driven atrial alternans: Insight from [Ca]i and Vm asynchrony

Cardiac alternans is defined as beat-to-beat alternations in contraction strength, action potential duration (APD), and Ca transient (CaT) amplitude. Cardiac excitation-contraction coupling relies on the activity of two bidirectionally coupled excitable systems, membrane voltage (Vm ) and Ca release. Alternans has been classified as Vm - or Ca-driven, depending whether a disturbance of Vm or [Ca]i regulation drives the alternans. We determined the primary driver of pacing induced alternans in rabbit atrial myocytes, using combined patch clamp and fluorescence [Ca]i and Vm measurements. APD and CaT alternans are typically synchronized; however, uncoupling between APD and CaT regulation can lead to CaT alternans in the absence of APD alternans, and APD alternans can fail to precipitate CaT alternans, suggesting a considerable degree of independence of CaT and APD alternans. Using alternans AP voltage clamp protocols with extra APs showed that most frequently the pre-existing CaT alternans pattern prevailed after the extra-beat, indicating that alternans is Ca-driven. In electrically coupled cell pairs, dyssynchrony of APD and CaT alternans points to autonomous regulation of CaT alternans. Thus, with three novel experimental protocols, we collected evidence for Ca-driven alternans; however, the intimately intertwined regulation of Vm and [Ca]i precludes entirely independent development of CaT and APD alternans.

L-type Ca2+ channel recovery from inactivation in rabbit atrial myocytes

Adaptation of the myocardium to varying workloads critically depends on the recovery from inactivation (RFI) of L-type Ca²⁺ channels (LCCs) which provide the trigger for cardiac contraction. The goal of the present study was a comprehensive investigation of LCC RFI in atrial myocytes. The study was performed on voltage-clamped rabbit atrial myocytes using a double pulse protocol with variable diastolic intervals in cells held at physiological holding potentials, with intact intracellular Ca²⁺ release, and preserved Na⁺ current and Na⁺ /Ca²⁺ exchanger (NCX) activity. We demonstrate that the kinetics of RFI of LCCs are co-regulated by several factors including resting membrane potential, [Ca²⁺ ]_i , Na⁺ influx, and activity of CaMKII. In addition, activation of CaMKII resulted in increased I_Ca amplitude at higher pacing rates. Pharmacological inhibition of NCX failed to have any significant effect on RFI, indicating that impaired removal of Ca²⁺ by NCX has little effect on LCC recovery. Finally, RFI of intracellular Ca²⁺ release was substantially slower than LCC RFI, suggesting that inactivation kinetics of LCC do not significantly contribute to the beat-to-beat refractoriness of SR Ca²⁺ release. The study demonstrates that CaMKII and intracellular Ca²⁺ dynamics play a central role in modulation of LCC activity in atrial myocytes during increased workloads that could have important consequences under pathological conditions such as atrial fibrillations, where Ca²⁺ cycling and CaMKII activity are altered.

Cardiac Alternans Occurs through the Synergy of Voltage- and Calcium-Dependent Mechanisms

Cardiac alternans is characterized by alternating weak and strong beats of the heart. This signaling at the cellular level may appear as alternating long and short action potentials (APs) that occur in synchrony with alternating large and small calcium transients, respectively. Previous studies have suggested that alternans manifests itself through either a voltage dependent mechanism based upon action potential restitution or as a calcium dependent mechanism based on refractoriness of calcium release. We use a novel model of cardiac excitation-contraction (EC) coupling in the rat ventricular myocyte that includes 20,000 calcium release units (CRU) each with 49 ryanodine receptors (RyR2s) and 7 L-type calcium channels that are all stochastically gated. The model suggests that at the cellular level in the case of alternans produced by rapid pacing, the mechanism requires a synergy of voltage- and calcium-dependent mechanisms. The rapid pacing reduces AP duration and magnitude reducing the number of L-type calcium channels activating individual CRUs during each AP and thus increases the population of CRUs that can be recruited stochastically. Elevated myoplasmic and sarcoplasmic reticulum (SR) calcium, [Ca2+]myo and [Ca2+]SR respectively, increases ryanodine receptor open probability (Po) according to our model used in this simulation and this increased the probability of activating additional CRUs. A CRU that opens in one beat is less likely to open the subsequent beat due to refractoriness caused by incomplete refilling of the junctional sarcoplasmic reticulum (jSR). Furthermore, the model includes estimates of changes in Na+ fluxes and [Na+]i and thus provides insight into how changes in electrical activity, [Na+]i and sodium-calcium exchanger activity can modulate alternans. The model thus tracks critical elements that can account for rate-dependent changes in [Na+]i and [Ca2+]myo and how they contribute to the generation of Ca2+ signaling alternans in the heart.

Excitation-contraction coupling and calcium release in atrial muscle

In cardiac muscle, the process of excitation-contraction coupling (ECC) describes the chain of events that links action potential induced myocyte membrane depolarization, surface membrane ion channel activation, triggering of Ca2+ induced Ca2+ release from the sarcoplasmic reticulum (SR) Ca2+ store to activation of the contractile machinery that is ultimately responsible for the pump function of the heart. Here we review similarities and differences of structural and functional attributes of ECC between atrial and ventricular tissue. We explore a novel "fire-diffuse-uptake-fire" paradigm of atrial ECC and Ca2+ release that assigns a novel role to the SR SERCA pump and involves a concerted "tandem" activation of the ryanodine receptor Ca2+ release channel by cytosolic and luminal Ca2+. We discuss the contribution of the inositol 1,4,5-trisphosphate (IP3) receptor Ca2+ release channel as an auxiliary pathway to Ca2+ signaling, and we review IP3 receptor-induced Ca2+ release involvement in beat-to-beat ECC, nuclear Ca2+ signaling, and arrhythmogenesis. Finally, we explore the topic of electromechanical and Ca2+ alternans and its ramifications for atrial arrhythmia.

Alternans in atria: Mechanisms and clinical relevance

Atrial fibrillation is the most common sustained arrhythmia and its prevalence is rapidly rising with the aging of the population. Cardiac alternans, defined as cyclic beat-to-beat alternations in contraction force, action potential (AP) duration and intracellular Ca²⁺ release at constant stimulation rate, has been associated with the development of ventricular arrhythmias. Recent clinical data also provide strong evidence that alternans plays a central role in arrhythmogenesis in atria. The aim of this article is to review the mechanisms that are responsible for repolarization alternans and contribute to the transition from spatially concordant alternans to the more arrhythmogenic spatially discordant alternans in atria.

Membrane potential determines calcium alternans through modulation of SR Ca2+ load and L-type Ca2+ current

Alternans is a risk factor for cardiac arrhythmia, including atrial fibrillation. At the cellular level alternans is observed as beat-to-beat alternations in contraction, action potential (AP) morphology and magnitude of the Ca²⁺ transient (CaT). It is widely accepted that the bi-directional interplay between membrane voltage and Ca²⁺ is crucial for the development of alternans, however recently the attention has shifted to instabilities in cellular Ca²⁺ handling, while the role of AP alternation remains poorly understood. This study provides new insights how beat- to-beat alternation in AP morphology affects occurrence of CaT alternans in atrial myocytes. Pacing-induced AP and CaT alternans were studied in rabbit atrial myocytes using combined Ca²⁺ imaging and electrophysiological measurements. To determine the role of AP morphology for the generation of CaT alternans, trains of two voltage commands in form of APs recorded during large and small alternans CaTs were applied to voltage-clamped cells. APs of longer duration (as observed during small amplitude alternans CaT) and especially beat-to-beat alternations in AP morphology (AP alternans) reduced the pacing frequency threshold and increased the degree of CaT alternans. AP morphology contributes to the development of CaT alternans by two mechanisms. First, the AP waveform observed during small alternans CaTs coincided with higher end-diastolic sarcoplasmic reticulum Ca²⁺ levels ([Ca²⁺]_SR), and AP alternans resulted in beat-to-beat alternations in end-diastolic [Ca²⁺]_SR. Second, L-type Ca²⁺ current was significantly affected by AP morphology, where the AP waveform observed during large CaT elicited L-type Ca²⁺ currents of higher magnitude and faster kinetics, resulting in more efficient triggering of SR Ca²⁺ release. In conclusion, alternation in AP morphology plays a significant role in the development and stabilization of atrial alternans. The demonstration that CaT alternans can be controlled or even prevented by modulating AP morphology has important ramifications for arrhythmia prevention and therapy strategies.