In silico investigation of pro-arrhythmic effects of azithromycin on the human ventricle

The macrolide antibiotic azithromycin (AZM) is widely used for respiratory infections and has been suggested to be a possible treatment for the Coronavirus Disease of 2019 (COVID-19). However, AZM-associated QT interval prolongation and arrhythmias have been reported. Integrated mechanistic information on AZM actions on human ventricular excitation and conduction is lacking. Therefore, this study was undertaken to investigate the actions of AZM on ventricular cell and tissue electrical activity. The O'Hara- Virag-Varro-Rudy dynamic (ORd) model of human ventricular cells was modified to incorporate experimental data on the concentration-dependent actions of AZM on multiple ion channels, including I_Na, I_CaL, I_Kr, I_Ks, I_K1 and I_NaL in both acute and chronic exposure conditions. In the single cell model, AZM prolonged the action potential duration (APD) in a concentration-dependent manner, which was predominantly attributable to I_Kr reduction in the acute condition and potentiated I_NaL in the chronic condition. High concentrations of AZM also increased action potential (AP) triangulation (determined as an increased difference between APD₃₀ and APD₉₀) which is a marker of arrhythmia risk. In the chronic condition, the potentiated I_NaL caused a modest intracellular Na ⁺ concentration accumulation at fast pacing rates. At the 1D tissue level, the AZM-prolonged APD at the cellular level was reflected by an increased QT interval in the simulated pseudo-ECG, consistent with clinical observations. Additionally, AZM reduced the conduction velocity (CV) of APs in the acute condition due to a reduced I_Na, and it augmented the transmural APD dispersion of the ventricular tissue, which is also pro-arrhythmic. Such actions were markedly augmented when the effects of chronic exposure of AZM were also considered, or with additional I_Kr block, as may occur with concurrent use of other medications. This study provides insights into the ionic mechanisms by which high concentrations of AZM may modulate ventricular electrophysiology and susceptibility to arrhythmia.     

Transmembrane action potential heterogeneity in the canine isolated arterially perfused right atrium: effect of IKr and IKur/Ito block

The role of electrical heterogeneity in development of cardiac arrhythmias is well recognized. The extent to which transmembrane action potential (TAP) heterogeneity contributes to the normal electrophysiology of well-oxygenated atria is not well defined. The principal objective of the present study was to define regional and transmural differences in characteristics of the TAP in isolated superfused and arterially perfused canine right atrial (RA) preparations under baseline, rapidly activating delayed rectifier K(+) current (I(Kr)) block, and combined block of ultrarapid delayed rectifier and transient outward K(+) current (I(Kur)/I(to) block). Superfused preparations that survived generally displayed a triangle-shaped TAP. Exceptions included cells from the crista terminalis, where TAPs with a normal plateau could be recorded. In contrast, most TAPs recorded from throughout the perfused RA displayed a spike-and-dome and/or plateau morphology. The perfused RA displayed a heterogeneous distribution of repolarization, V(max), and spike-and-dome morphology along the epicardial and endocardial surfaces as well as transmurally, in the region of the upper crista terminalis. I(Kr) block with E-4031 prolonged repolarization homogeneously in the perfused RA, whereas I(Kur)/I(to) block using low concentrations of 4-aminopyridine abbreviated action potential duration at 90% repolarization heterogeneously, leading to a reduction in dispersion of repolarization. Our data indicate that the electrical heterogeneities, previously described for the canine ventricle, also exist within the atria and that I(Kr) block does not accentuate and I(Kur)/I(to) block reduces RA dispersion of repolarization. Our study also points to major differences in the transmembrane activity recorded using superfused vs. arterially perfused atrial preparations.     

In silico study of carvone derivatives as potential neuraminidase inhibitors

Recent outbreaks of highly pathogenic influenza strains have highlighted the need to develop new anti-influenza drugs. Here, we report an in silico study of carvone derivatives to analyze their binding modes with neuraminidase (NA) active sites. Two proposed carvone analogues, CV(A) and CV(B), with 36 designed ligands were predicted to inhibit NA (PDB ID: 3TI6) using molecular docking. The design is based on structural resemblance with the commercial inhibitor, oseltamivir (OTV), ligand polarity, and amino acid residues in the NA active sites. Docking simulations revealed that ligand A18 has the lowest energy binding (?G_bind) value of ?8.30 kcal mol^-1, comparable to OTV with ?G_bind of ?8.72 kcal mol^-1. A18 formed seven hydrogen bonds (H-bonds) at residues Arg292, Arg371, Asp151, Trp178, Glu227, and Tyr406, while eight H-bonds were formed by OTV with amino acids Arg118, Arg292, Arg371, Glu119, Asp151, and Arg152. Molecular dynamics (MD) simulation was conducted to compare the stability between ligand A18 and OTV with NA. Our simulation study showed that the A18-NA complex is as stable as the OTV-NA complex during the MD simulation of 50 ns through the analysis of RMSD, RMSF, total energy, hydrogen bonding, and MM/PBSA free energy calculations.     

Frequency-dependent effects of various IKr blockers on cardiac action potential duration in a human atrial model

Rapidly activating K(+) current (I(Kr)) blockers prolong action potential (AP) duration (APD) in a reverse-frequency-dependent manner and may induce arrhythmias, including torsade de pointes in the ventricle. The I(Kr) blocker dofetilide has been approved for treatment of atrial arrhythmias, including fibrillation. There are, however, a limited number of studies on the action of I(Kr) blockers on atrial AP. When we tested a mathematical model of the human atrial AP (M Courtemanche, RJ Ramirez, S Nattel. Am J Physiol Heart Circ Physiol 275: H301-H321, 1998) to examine the effects of dofetilide-type I(Kr) blockade, this model could not reproduce the reverse-frequency-dependent nature of I(Kr) blockade on atrial APD. We modified the model by introducing a slowly activating K(+) current activation parameter. As the slow time constant was increased, dofetilide-type blockade induced more prominent reverse-frequency-dependent APD prolongation. Using the modified model, we also examined the effects of two more types of I(Kr) blockade similar to those of quinidine and vesnarinone. Voltage- and time-dependent block of I(Kr) through the onset of inhibition by quinidine is much faster than by vesnarinone. When we incorporated the kinetics of the effects of these drugs on I(Kr) into the model, we found that quinidine-type blockade caused a reverse-frequency-dependent prolongation of APD that was similar to the effect of dofetilide-type blockade, whereas vesnarinone-type blockade did not. This finding coincides with experimental observations. The lack of the reverse frequency dependence in vesnarinone-type blockade was accounted for by the slow development of I(Kr) blockade at depolarized potentials. These results suggest that the voltage- and time-dependent nature of I(Kr) blockade by drugs may be critical for the phenotype of the drug effect on atrial AP.     

In silico determination of potential antisense targets for human beta-globin variants

The functional characterization of available genomic sequences is the major task of the research in the post-genome era. This complex task requires an integrative approach of high-throughput systems with in vitro and in vivo models in order to have a reliable evaluation of the biological function. The oligonucleotide antisense technology is one of the most promising approaches for the investigation of gene function; the crucial point of antisense experiments is the identification of optimal target sites for hybridisation. In this paper we have applied a bioinformatic tool for the recognition of optimal antisense targets. In order to evaluate the effect of mutational events on target selection we have tested the program on a sample of human beta-hemoglobin variants. The proposed algorithm software will be integrated in a web based tool at the site: http://www.nettab.org/agewa.     

Effects of electrical and structural remodeling on atrial fibrillation maintenance: a simulation study

Atrial fibrillation, a common cardiac arrhythmia, often progresses unfavourably: in patients with long-term atrial fibrillation, fibrillatory episodes are typically of increased duration and frequency of occurrence relative to healthy controls. This is due to electrical, structural, and contractile remodeling processes. We investigated mechanisms of how electrical and structural remodeling contribute to perpetuation of simulated atrial fibrillation, using a mathematical model of the human atrial action potential incorporated into an anatomically realistic three-dimensional structural model of the human atria. Electrical and structural remodeling both shortened the atrial wavelength--electrical remodeling primarily through a decrease in action potential duration, while structural remodeling primarily slowed conduction. The decrease in wavelength correlates with an increase in the average duration of atrial fibrillation/flutter episodes. The dependence of reentry duration on wavelength was the same for electrical vs. structural remodeling. However, the dynamics during atrial reentry varied between electrical, structural, and combined electrical and structural remodeling in several ways, including: (i) with structural remodeling there were more occurrences of fragmented wavefronts and hence more filaments than during electrical remodeling; (ii) dominant waves anchored around different anatomical obstacles in electrical vs. structural remodeling; (iii) dominant waves were often not anchored in combined electrical and structural remodeling. We conclude that, in simulated atrial fibrillation, the wavelength dependence of reentry duration is similar for electrical and structural remodeling, despite major differences in overall dynamics, including maximal number of filaments, wave fragmentation, restitution properties, and whether dominant waves are anchored to anatomical obstacles or spiralling freely.     

In silico identification of potential therapeutic targets in the human pathogen Helicobacter pylori

Availability of genome sequences of pathogens has provided a tremendous amount of information that can be useful in drug target and vaccine target identification. One of the recently adopted strategies is based on a subtractive genomics approach, in which the subtraction dataset between the host and pathogen genome provides information for a set of genes that are likely to be essential to the pathogen but absent in the host. This approach has been used successfully in recent times to identify essential genes in Pseudomonas aeruginosa. We have used the same methodology to analyse the whole genome sequence of the human gastric pathogen Helicobacter pylori. Our analysis revealed that out of the 1590 coding sequences of the pathogen, 40 represent essential genes that have no human homolog. We have further analysed these 40 genes by the protein sequence databases to list some 10 genes whose products are possibly exposed on the pathogen surface. This preliminary work reported here identifies a small subset of the Helicobacter proteome that might be investigated further for identifying potential drug and vaccine targets in this pathogen.     

Direct action of estradiol-17β on the atrial action potential

The effect of the two C-17 isomers of estradiol on the shape of the action potential of rat atrial tissue was studied by means of classical glass electrodes for different concentrations of estradiol. Resting potential and upstroke were not affected by estradiol, but the duration of the action potential was reduced. Only estradiol-17β exhibits an effect in a concentration dependent way, while estradiol-17α has no effect at all. The ionic mechanism was studied by adding specific ionic blockers to the perfusate. Since the effect was much less pronounced when a slow inward current blocker was added, it was concluded that estradiol-17β acts mainly via the slow inward current channel. Only a small part of the interaction takes place via the potassium outward channel.     

Mathematical analysis of canine atrial action potentials: rate, regional factors, and electrical remodeling

Dogs have been used extensively to study atrial arrhythmias, but there are no published mathematical models of the canine atrial action potential (AP). To obtain insights into the ionic mechanisms governing canine atrial AP properties, we incorporated formulations of K(+), Na(+), Ca(2+), and Cl(-) currents, based on measurements in canine atrial myocytes, into a mathematical model of the AP. The rate-dependent behavior of model APs corresponded to experimental measurements and pointed to a central role for L-type Ca(2+) current inactivation in rate adaptation. Incorporating previously described regional ionic current variations into the model largely reproduced AP forms characteristic of the corresponding right atrial regions (appendage, pectinate muscle, crista terminalis, and atrioventricular ring). When ionic alterations induced by tachycardia-dependent remodeling were incorporated, the model reproduced qualitatively the AP features constituting the cellular substrate for atrial fibrillation. We conclude that this ionic model of the canine atrial AP agrees well with experimental measurements and gives potential insights into mechanisms underlying functionally important electrophysiological phenomena in canine atrium.     

In silico study of action potential and QT interval shortening due to loss of inactivation of the cardiac rapid delayed rectifier potassium current

The rapid delayed rectifier K(+) current, I(Kr), plays a key role in repolarisation of cardiac ventricular action potentials (APs). In recent years, a novel clinical condition denoted the short QT syndrome (SQTS) has been identified and, very recently, gain in function mutations in the gene encoding the pore-forming sub-unit of the I(Kr) channel have been proposed to underlie SQTS in some patients. Here, computer simulations were used to investigate the effects of the selective loss of voltage-dependent inactivation of I(Kr) upon ventricular APs and on the QT interval of the electrocardiogram. I(Kr) and inactivation-deficient I(Kr) were incorporated into Luo-Rudy ventricular AP models. Inactivation-deficient I(Kr) produced AP shortening that was heterogeneous between endocardial, mid-myocardial, and epicardial ventricular cell models, irrespective of whether heterogeneity between these sub-regions was incorporated of slow delayed rectifier K(+) current (I(Ks)) alone, or of I(Ks) together with that of transient outward K(+) current. The selective loss of rectification of I(Kr) did not augment transmural dispersion of AP repolarisation, as AP shortening was greater in mid-myocardial than in endo- or epicardial cell models. Simulated conduction through a 1 D transmural ventricular strand was altered by incorporation of inactivation-deficient I(Kr) and the reconstructed QT interval was shortened. Collectively, these results substantiate the notion that selective loss of I(Kr) inactivation produces a gain in I(Kr) function that causes QT interval shortening.     

Structural atrial remodeling alters the substrate and spatiotemporal organization of atrial fibrillation: a comparison in canine models of structural and electrical atrial remodeling

Several animal models of atrial fibrillation (AF) have been developed that demonstrate either atrial structural remodeling or atrial electrical remodeling, but the characteristics and spatiotemporal organization of the AF between the models have not been compared. Thirty-nine dogs were divided into five groups: rapid atrial pacing (RAP), chronic mitral regurgitation (MR), congestive heart failure (CHF), methylcholine (Meth), and control. Right and left atria (RA and LA, respectively) were simultaneously mapped during episodes of AF in each animal using high-density (240 electrodes) epicardial arrays. Multiple 30-s AF epochs were recorded in each dog. Fast Fourier transform was calculated every 1 s over a sliding 2-s window, and dominant frequency (DF) was determined. Stable, discrete, high-frequency areas were seen in none of the RAP or control dogs, four of nine MR dogs, four of six CHF dogs, and seven of nine Meth dogs in either the RA or LA or both. Average DFs in the Meth model were significantly greater than in all other models in both LA and RA except LA DFs in the RAP model. The RAP model was the only one with a consistent LA-to-RA DF gradient (9.5 +/- 0.2 vs. 8.3 +/- 0.3 Hz, P < 0.00005). The Meth model had a higher spatial and temporal variance of DFs and lower measured organization levels compared with the other AF models, and it was the only model to show a linear relationship between the highest DF and dispersion (R(2) = 0.86). These data indicate that structural remodeling of atria (models known to have predominantly altered conduction) leads to an AF characterized by a stable high-frequency area, whereas electrical remodeling of atria (models known to have predominantly shortened refractoriness without significant conduction abnormalities) leads to an AF characterized by multiple high-frequency areas and multiple wavelets.     

In silico study of human aquaporin AQP11 and AQP12 channels

AQP11 and AQP12 are the most distantly related paralogs of the aquaporin family in human. They share indeed a low sequence similarity with other aquaporins and exhibit a modified N‐terminal NPA signature motif. Furthermore, they have an anomalous subcellular localization. The AQP11 and AQP12 biological role remains to be fully clarified and their ability to allow transport of water is still debated. We have built accurate 3D‐models for AQP11 and AQP12 and comprehensively compared their sequence and structure to other known aquaporins. In order to investigate whether they appear compatible or not with water permeability, we especially focused on the amino acid composition and electrostatics of their channels, keeping the structure of the low‐water efficiency AQP0 as a reference system. Our analysis points out a possible alternative ar/R site and shows that these aquaporins feature unique residues at key pore‐lining positions that make the shape, composition and electrostatics of their channel peculiar. Such residues can represent pivotal hints to study and explain the AQP11 and AQP12 biological and molecular function.     

In silico pattern-based analysis of the human cytomegalovirus genome

More than 200 open reading frames (ORFs) from the human cytomegalovirus genome have been reported as potentially coding for proteins. We have used two pattern-based in silico approaches to analyze this set of putative viral genes. With the help of an objective annotation method that is based on the Bio-Dictionary, a comprehensive collection of amino acid patterns that describes the currently known natural sequence space of proteins, we have reannotated all of the previously reported putative genes of the human cytomegalovirus. Also, with the help of MUSCA, a pattern-based multiple sequence alignment algorithm, we have reexamined the original human cytomegalovirus gene family definitions. Our analysis of the genome shows that many of the coded proteins comprise amino acid combinations that are unique to either the human cytomegalovirus or the larger group of herpesviruses. We have confirmed that a surprisingly large portion of the analyzed ORFs encode membrane proteins, and we have discovered a significant number of previously uncharacterized proteins that are predicted to be G-protein-coupled receptor homologues. The analysis also indicates that many of the encoded proteins undergo posttranslational modifications such as hydroxylation, phosphorylation, and glycosylation. ORFs encoding proteins with similar functional behavior appear in neighboring regions of the human cytomegalovirus genome. All of the results of the present study can be found and interactively explored online (http://cbcsrv.watson.ibm.com/virus/).     

In silico and In vitro evaluation of the anti-inflammatory potential of Centratherum punctatum Cass-A

Centratherum punctatum Cass., a herb belonging to the family Asteraceae has been traditionally used as a curative against diverse disorders like inflammation, tumor, depression, and hypertension. Though the medicinal properties of this plant have been attributed to the presence of flavonoids, glucosides, alkaloids, Vitamin C, etc., the molecular constituents of this plant and of the flavonoids that contribute to its medicinal activity have not been explored yet. This work attempts to evaluate the potential of Centratherum punctatum extract as an anti-inflammatory agent. Ethanolic extracts of Centratherum punctatum analyzed by High Performance Thin Layer Chromatography (HPTLC) and Liquid Chromatography–Mass Spectrometry (LC–MS/MS) identified the presence of the flavones kaempferol, glycoside Isorhamnetin-3-O-rutinoside, and kaempferol-3-glucoside. The plant extract exhibited anti-oxidant property as confirmed by DPPH assay and IC₅₀ value of 271.6 μg/mL during inhibition of protein denaturation, 186.8 μg/mL during RBC membrane stabilization, and 278.2 μg/mL for proteinase inhibition. Membrane stabilizing functions of flavones and flavones glycosides validated the anti-inflammatory potential of the extract. In silico evaluation using a rigorous molecular docking protocol with receptors of Cox2, TNF-α, Interleukin 1β convertase, and Histamine H1 predicted high binding affinity of the isoflavones and isoflavone glycosides of Centratherum punctatum Cass. The interactions have also been shown to compare well with that of known drugs valdecoxib through Gln178, His342, and Gly340, desloratadine (through Lys191 and Thr194) and belnacasin (through Asp288 and Gly287) proven to function through the anti-inflammatory pathway. This work establishes the anti-inflammatory potential of Centratherum punctatum Cass. extract as an alternative to existing therapeutic approach to inflammation through a systematic in silico approach supplementing the findings.     

Influence of an electrical potential on the charge transfer kinetics of bacteriorhodopsin

The adsorption of bacteriorhodopsin(bR)-containing purple membranes (PM) to black lipid membranes (BLM) was used to study the charge translocation kinetics of bR upon flash excitation.

The discharge of the PM-BLM system after charging upon illumination is found to proceed quite slowly (discharge time up to several minutes) but is considerably accelerated by addition of the protonophore FCCP.

Therefore, the dependence of the proton transfer kinetics in bR on electrical potentials generated by preceding flashes of varying repetition rate and intensity was investigated. The kinetics are slowed down with increasing flash intensity as well as repetition rate. This effect is partly abolished by small amounts of FCCP.

A new model is introduced which takes into account the instantaneous feedback of the electrical potential on the kinetics of the pump current. It explains the observed deviations from first-order kinetics and renders an approach with “distributed kinetics” unnecessary.

     

Modeling the effect of Kv1.5 block on the canine action potential

A wide range of ion channels have been considered as potential targets for pharmacological treatment of atrial fibrillation. The Kv1.5 channel, carrying the I_Kur current, has received special attention because it contributes to repolarization in the atria but is absent or weakly expressed in ventricular tissue. The dog serves as an important animal model for electrophysiological studies of the heart and mathematical models of the canine atrial action potential (CAAP) have been developed to study the interplay between ionic currents. To enable more-realistic studies on the effects of Kv1.5 blockers on the CAAP in silico, two continuous-time Markov models of the guarded receptor type were formulated for Kv1.5 and subsequently inserted into the Ramirez-Nattel-Courtemanche model of the CAAP. The main findings were: 1), time- and state-dependent Markov models of open-channel Kv1.5 block gave significantly different results compared to a time- and state-independent model with a downscaled conductance; 2), the outcome of Kv1.5 block on the macroscopic system variable APD₉₀ was dependent on the precise mechanism of block; and 3), open-channel block produced a reverse use-dependent prolongation of APD₉₀. This study suggests that more-complex ion-channel models are a prerequisite for quantitative modeling of drug effects.     

In silico study of structural determinants modulating the redox potential of Rigidoporus lignosus and other fungal laccases

Laccases are multicopper oxidases in which substrate oxidation takes place at the type-1 (T1) copper site. The redox potential (E (0)) significantly varies amongst members of the family and is a key parameter for substrate specificity. Despite sharing highly conserved features at the T1 copper site, laccases span a large range of E (0), suggesting that the influence of the metal secondary coordination sphere is important. In silico analysis of structural determinants modulating the E (0) of Rigidoporus lignosus and other fungal laccases indicated that different factors can be considered. First, the length of the T1 copper coordinating histidine bond is observed to be longer in high E (0) laccases than in low E (0) ones. The hydrophobic environment around the T1 copper site appeared as another important structural determinant in modulating the E (0), with a stronger hydrophobic environment correlating with higher E (0). The analysis of hydrogen bonding network (HBN) around the T1-binding pocket revealed that the amino acids building up the metal binding site strongly interact with neighbouring residues and contribute to the stabilization of the protein folds. Changes in these HBNs that modified the Cu1 preferred coordination geometry lead to an increase of E (0). The presence of axial ligands modulates the E (0) of T1 to different extent. Stacking interactions between aromatic residues located in the second coordination shell and the metal ion coordination histidine imidazole ring have also been identified as a factor that modulates the E (0). The electrostatic interactions between the T1 copper site and backbone carbonyl oxygen indicate that Cu1-CO=NH distance is longer in the high E (0) laccases. In short, the in silico study reported herein identifies several structural factors that may influence the E (0) of the examined laccases. Some of these are dependent on the nature of the coordination ligands at the T1 site, but others can be ascribed to the hydrophobic effects, HBNs, axial ligations, stacking and electrostatic interactions, not necessary directly interacting with the copper metal.     

乙酰胆碱对豚鼠心房肌和心室肌的动作电位和收缩力的不同作用

实验采用标准玻璃微电极细胞内记录技术记录心肌细胞动作电位（action potential,AP）、肌力换能器记录心肌收缩力（force contraction,Fc）,研究乙酰胆碱（acetylcholine,ACh）对离体豚鼠心房肌、心室肌的作用。结果表明,10μmol/L ACh可缩短心房肌、心室肌动作电位的时程（action potential duration,APD）。心房肌APD在给药前后分别为208.57±36.05ms及101.78±14.41ms（n=6,P<0.01）,心室肌APD在给药前后分别为286.73±36.11ms及265.16±30.06 ms（n=6,P<0.01）。心房肌动作电位的幅度（action potential amplitude,APA）也降低,给药前后分别为88.00±9.35 mV及62.62±20.50 mV（n=6,P<0.01）,而心室肌APA无明显变化。ACh还降低心房肌、心室肌的收缩力,心房肌、心室肌Fc的抑制率分别为100％（n=6,P<0.01）和37.57±2.58％（n=6,P<0.01）。ACh对心房肌、心室肌APD和Fc的抑制作用在一定范围内（1nmol/L～100μmol/L）随ACh浓度的增高而增强。用Scott法求出ACh对心房肌、心室肌APD缩短作用的KD值,分别为0.275和0.575μmol/L,对Fc抑制作用的KD值分别为0.135和0.676μmol/L。各浓度下ACh对心房肌效应与心室肌效应作组间t检验,从10nmol/L到0.1mmol/L均有显著的统计学差异。此外,10μmol/L阿托品及20mmol/L     

Impact of sarcoplasmic reticulum calcium release on calcium dynamics and action potential morphology in human atrial myocytes: a computational study

Electrophysiological studies of the human heart face the fundamental challenge that experimental data can be acquired only from patients with underlying heart disease. Regarding human atria, there exist sizable gaps in the understanding of the functional role of cellular Ca2+ dynamics, which differ crucially from that of ventricular cells, in the modulation of excitation-contraction coupling. Accordingly, the objective of this study was to develop a mathematical model of the human atrial myocyte that, in addition to the sarcolemmal (SL) ion currents, accounts for the heterogeneity of intracellular Ca2+ dynamics emerging from a structurally detailed sarcoplasmic reticulum (SR). Based on the simulation results, our model convincingly reproduces the principal characteristics of Ca2+ dynamics: 1) the biphasic increment during the upstroke of the Ca2+ transient resulting from the delay between the peripheral and central SR Ca2+ release, and 2) the relative contribution of SL Ca2+ current and SR Ca2+ release to the Ca2+ transient. In line with experimental findings, the model also replicates the strong impact of intracellular Ca2+ dynamics on the shape of the action potential. The simulation results suggest that the peripheral SR Ca2+ release sites define the interface between Ca2+ and AP, whereas the central release sites are important for the fire-diffuse-fire propagation of Ca2+ diffusion. Furthermore, our analysis predicts that the modulation of the action potential duration due to increasing heart rate is largely mediated by changes in the intracellular Na+ concentration. Finally, the results indicate that the SR Ca2+ release is a strong modulator of AP duration and, consequently, myocyte refractoriness/excitability. We conclude that the developed model is robust and reproduces many fundamental aspects of the tight coupling between SL ion currents and intracellular Ca2+ signaling. Thus, the model provides a useful framework for future studies of excitation-contraction coupling in human atrial myocytes.