Minimal model for human ventricular action potentials in tissue

Modeling the dynamics of wave propagation in human ventricular tissue and studying wave stability require models that reproduce realistic characteristics in tissue. We present a minimal ventricular (MV) human model that is designed to reproduce important tissue-level characteristics of epicardial, endocardial and midmyocardial cells, including action potential (AP) amplitudes and morphologies, upstroke velocities, steady-state action potential duration (APD) and conduction velocity (CV) restitution curves, minimum APD, and minimum diastolic interval. The model is then compared with three previously published human ventricular cell models, the Priebe and Beuckelmann (PB), the Ten Tusscher–Noble–Noble–Panfilov (TNNP), and the Iyer–Mazhari–Winslow (IMW). For the first time, the stability of reentrant waves for all four models is analyzed, and quantitative comparisons are made among the models in single cells and in tissue. The PB, TNNP, and IMW models exhibit quantitative differences in APD and CV rate adaptation, as well as completely different reentrant wave dynamics of quasi-breakup, stability, and breakup, respectively. All the models have dominant frequencies comparable to clinical values except for the IMW model, which has a large range of frequencies extending beyond the clinical range for both ventricular tachycardia (VT) and ventricular fibrillation (VF). The TNNP and IMW models possess a large degree of short-term memory and we show for the first time the existence of memory in CV restitution. The MV model also can be fitted to reproduce the dynamics of other models and is computationally more efficient: the times required to simulate the MV, TNNP, PB and IMW models follow the ratio 1:31:50:8084.     

Density and sub-cellular distribution of cardiac and neuronal sodium channel isoforms in rat ventricular myocytes

In cardiac ventricular myocytes, Na current is generated mainly by the cardiac NaV1.5 isoform, but the presence of "neuronal" Na channel isoforms in the heart has been demonstrated recently. In this study, we quantified the density and sub-cellular distribution of cardiac and neuronal channel isoforms in rat ventricular myocytes. INa was recorded using the patch clamp technique in control and detubulated myocytes. Detubulation reduced cell capacitance (by approximately 29%) but maximum conductance was not altered (1.94+/-0.15, 14 control vs 1.98+/-0.19 nS/pF, 17 detubulated myocytes). The kinetic properties of INa were similar in both cell types suggesting good voltage control of surface and t-tubule membranes. We calculated Na channel densities assuming the sub-cellular current localization we recently provided (neuronal isoform: approximately 11% of total sarcolemmal current, approximately 3% of cell surface, and approximately 31% of t-tubule current). Single channel conductances were assumed to be 2.2 and 2.5 pS for the cardiac and neuronal isoforms, respectively, after accounting for the use of low Na concentration. We calculated that the density of the cardiac Na channel isoform is relatively constant (in channels/microm2: approximately 11 in total sarcolemma, approximately 13 at the cell surface, approximately 10 at the t-tubules). In contrast, neuronal Na channel isoforms are concentrated at the t-tubules (in channels/microm2: approximately 1 in total sarcolemma, approximately 0.3 at the cell surface, approximately 2.5 at the t-tubules). We conclude that, in contrast to skeletal muscle in which Na channel density is higher at the cell surface than the t-tubules, in ventricular cardiac myocytes the sub-cellular distribution of Na channel density is relatively homogeneous (approximately 13 channels/microm2).     

Developmental changes of Ca(2+) handling in mouse ventricular cells from early embryo to adulthood

Transplant of immature cardiomyocytes is recently attracting a great deal of interest as a new experimental strategy for the treatment of failing hearts. Full understanding of normal cardiomyogenesis is essential to make this regenerative therapy feasible. We analyzed the molecular and functional changes of Ca(2+) handling proteins during development of the mouse heart from early embryo at 9.5 days postcoitum (dpc) through adulthood. From the early to the late (18 dpc) embryonic stage, mRNAs estimated by the real time PCR for ryanodine receptor (type 2, RyR2), sarcoplasmic reticulum (SR) Ca(2+) pump (type 2, SERCA2) and phospholamban (PLB) increased by 3-15 fold in the values normalized to GAPDH mRNA, although Na(+)/Ca(2+) exchanger (type 1, NCX1) mRNA was unchanged. After birth, there was a further increase in the mRNAs for RyR2, SERCA2 and PLB by 18-33 fold, but a 50% decrease in NCX1 mRNA. The protein levels of RyR2, SERCA2, PLB and NCX1, which were normalized to total protein, showed qualitatively parallel developmental changes. L-type Ca(2+) channel currents (I(Ca-L)) were increased during the development (1.3-fold at 18 dpc, 2.2-fold at adult stage, vs. 9.5 dpc). At 9.5 dpc, the Ca(2+) transient was, unlike adulthood, unaffected by the SR blockers, ryanodine (5 microM) and thapsigargin (2 microM), and also by a blocker of the Ca(2+) entry via Na(+)/Ca(2+) exchanger, KB-R 7943 (1 microM). The Ca(2+) transient was abolished after application of nisoldipine (5 microM). These results indicate that activator Ca(2+) for contraction in the early embryonic stage depends almost entirely on I(Ca-L).     

Comparison of electrophysiological models for human ventricular cells and tissues

In this paper we briefly review currently published models for human ventricular cells and tissues. We discuss the Priebe–Beuckelmann (PB) model and the reduced version of this model constructed by Bernus et al. (redPB), the Ten Tusscher–Noble–Noble–Panfilov (TNNP) model and the Iyer–Mazhari–Winslow (IMW) model. We compare several characteristics of these models such as: sources of experimental data the models are based on, action potential morphology, action potential duration (APD) and conduction velocity (CV) restitution and computational efficiency. Finally, we discuss the application of a subset of these models—the redPB and the TNNP model—to study simulated spiral wave dynamics in 2D tissue sheets and in the human ventricles. We discuss the suitability of the different models for particular research questions and their limitations.     

Stretch-induced excitation and action potential changes of single cardiac cells

Mechanoelectric coupling (MEC) has been studied extensively in the heart at the tissue and organ levels, but to only a limited extent in single cells because of the technical challenges. New results are presented in which MEC was studied in 57 single frog ventricular myocytes that were held on both ends by glass holding pipettes. Axial stretch was applied either by displacement of the pipettes, or by a glass fiber around which the cell was wrapped, that was displaced in a pulsatile or sinusoidal fashion. Electrical activity of the cell was monitored either by active contraction, by intracellular action potentials, or by focal extracellular potentials. Of more than 350 stretches applied to 57 cells with amplitudes ranging from 3% to 35%, only 4 cases of mechanically induced stimulation were observed. In 252 stretches applied to 32 cells in which action potential duration (APD) was measured, no change >20% was observed, except in 3 cells in which APD increased by >100%, and in 2 cells with extended triggered activity. Thus, in contrast to studies in intact tissue, single frog ventricular myocytes are generally insensitive to direct axial stretch. However, robust mechanosensitive responses were observed in 7 of 57 (12%) cells. The results of other single cell studies are reviewed, and the significance of differences in tissue-level and single cell results is discussed.     

Simulation of spontaneous action potentials of cardiomyocytes in pulmonary veins of rabbits

Atrial fibrillation is the most prevalent arrhythmia, but the mechanisms by which it develops are not clear. Recently, over 90% of paroxysmal atrial fibrillation was found to be located inside the main pulmonary veins (PVs). We found that single cardiac myocytes isolated from the main PVs of rabbits generate spontaneous action potentials (SAP). We therefore assayed the electrical characteristics of these cardiomyocytes. Among the diverse ionic currents identified were I_Na, I_Ca,L, I_K1, I_Kr, I_Ks, I_to, I_Ksus, I_ncx, I_pump, I_KH and I_Cl,Ca. In contrast, I_K1 was minimal, I_Ks could be detected only in the presence of 10 μM forskolin, and we were unable to detect I_f and I_Ca,T, the most important currents for pacemaking activity in sinoatrial node cells. To identify the main cause of SAP, we developed a model that can explain the electrical properties of these cardiomyocytes. After reconstructing the ionic currents based on experimental observations, we were able to use our model to successfully reconstruct the characteristics of the SAP of PV cardiomyocytes. The simulation showed that the major currents contributing to pacemaking depolarization were I_CaL, I_Kr, a background current and Na⁺–K⁺ pump current. Deactivation kinetics of I_Kr was one of the major determinants of the rate of pacemaking depolarization. The steady state inactivation of I_to was shifted to the negative voltage and the activity of I_to was minimal in the range of the SAP. The major currents for the repolarization were I_Kr and I_pump. The amplitude of most currents in these cardiac myocytes was small and no currents did not exceed 30 pA during the SAP, indicating that slight activation of other inward or outward currents will have profound effects on the SAP. To our knowledge, this report is the first to show the simulation of SAP of PV cardiomyocytes. This model may help to study on the electrophysiological basis of paroxysmal atrial fibrillation originating from PVs.     

Seasonal changes in intrinsic electrophysiological activity of song control neurons in wild song sparrows

Song behavior and its underlying neural substrate can change seasonally in adult songbirds. To test whether environmental cues induce seasonal changes in electrophysiological characteristics of song control neurons, we measured in vitro intrinsic neuronal activity in the song control nucleus RA of adult male song sparrows (Melospiza melodia) in both the fall non-breeding and spring breeding seasons. We found that RA neurons in spring-captured birds show a more than threefold increase in spontaneous firing rate compared to those from fall-captured birds. We conclude that environmental cues are sufficient to induce seasonal changes in electrophysiological properties of song control neurons, and that changes in these properties may underlie seasonal changes in song behavior.     

Multistability property in cardiac ionic models of mammalian and human ventricular cells

The underlying mechanisms of irregular cardiac rhythms are still poorly understood. Many experimental and modeling studies are aimed at identifying factors which cause cardiac arrhythmias. However, a lack of understanding of heart rhythm dynamical properties makes it difficult to uncover precise mechanisms of electrical instabilities, and hence to predict the onset of heart rhythm disorders. We review and compare the existing methods of studying cardiac dynamics, including restitution protocol (S1-S2), dynamic restitution protocol and multistability test protocol (S1-CI-S2). We focus on cardiac cell dynamics to elucidate regularities of heart rhythm. We demonstrate the advantages of our newly proposed systematic approach of analysis of cardiac cell dynamics using mammalian Luo Rudy 1991 and human ventricular Ten Tusscher 2006 single cell models under healthy and diseased conditions such as altered K⁺ or Ca²⁺ related currents. We investigate the role of ionic properties and the shape of an action potential on the nonlinear dynamics of electrical processes in periodically stimulated cardiac cells. We show the existence of multistability property for human ventricular cells. Moreover, the multistability is proposed to be an intrinsic property of cardiac cells, and is also suggested to be one of the mechanisms which could underlie the sudden triggering of life-threatening ventricular arrhythmias in the human heart.     

Computational models of heart pumping efficiencies based on contraction waves in spiral elastic bands

We present a framework for modeling biological pumping organs based on coupled spiral elastic band geometries and active wave-propagating excitation mechanisms. Two pumping mechanisms are considered in detail by way of example: one of a simple tube, which represents a embryonic fish heart and another more complicated structure with the potential to model the adult human heart. Through finite element modeling different elastic contractions are induced in the band. For each version the pumping efficiency is measured and the dynamics are evaluated. We show that by combining helical shapes of muscle bands with a contraction wave it is possible not only to achieve efficient pumping, but also to create desired dynamics of the structure. As a result we match the function of the model pumps and their dynamics to physiological observations.     

血小板活化因子对豚鼠心室肌细胞动作电位和钾通道的影响

应用全细胞膜片钳技术研究了血小板活化因子(platelet activatingfactor,PAF)对豚鼠心室肌细胞动作电位和钾电流的影响.结果发现,当电极内液ATP浓度为5 mmol/L(模拟正常条件)时,1 μmol/L PAF使APD90由对照的225.8±23.3 ms延长至352.8±29.8ms(n=5,P＜0.05);使IK尾电流在指令电压 30 mV由对照的173.5±16.7 pA降至152.1±11.5 pA(P＜0.05,n=4);使Ikl在指令电压为-120 mV时由对照组的-6.1±1.3 nA降至-5.6±1.1 nA(P＜0.05,n=5);但PAF在生理膜电位范围(-90mV～ 20mV)对IK1没有影响.当电极内液ATP浓度为0mmol/L时,IK·ATP开放(模拟缺血条件),1 μmol/LPAF却显著缩短APD90,由对照的153±24.6 ms缩短至88.2±19.4 ms(n=5,P＜0.01).而用1 μmol/L格列本脲(IK·ATP的特异阻断剂)预处理后,恢复了PAF可显著延长动作电位时程的作用.结果提示,PAF可能扩大缺血心肌和正常心肌细胞动作电位时程的不均一性,是缺血/再灌注性心律失常发生的重要原因.     

兔左室壁三层心肌细胞的分离及动作电位、钙和钾电流分布的异质性

探讨兔左室壁三层心肌单个细胞的分离方法以及电生理特征,实验以胶原酶按二步消化法分离兔心肌细胞,其中用剃须刀分离左室游离壁内,中,外三层心肌,采用全细胞膜片钳记录AP和离子电流,结果显示：（1）中层细胞上的动作电位时程明显长于内膜下心肌和外膜下心肌,且存在显著的1相切迹和2相驼峰;（2）中层细胞的Ica,L和Lto较内,外膜下的大,IK,s相反,可见三层心肌细胞上多种电流存在显著差异。     

Steady-state solutions in mathematical models of atrial cell electrophysiology and their stability

The steady states of the Fenton-Karma, the Courtemanche and the Nygren cell models were studied by determining the fixed points of the dynamical system describing their cell kinetics. The linear stability of the fixed points was investigated, as well as their response to external stimuli. Symbolic calculations were carried out as far as possible in order to prove the existence of these fixed points. In the Fenton-Karma model, a unique stable fixed point was found, namely the resting state. In contrast, the Courtemanche model had an infinite number of fixed points. A bifurcation diagram was constructed by classifying these fixed points according to a conservation law. Initial conditions were identified, for which the dynamical behavior of the cell was auto-oscillatory. In its original formulation, the Nygren model had no fixed point. After having restored charge conservation, the system was found to have an infinite number of fixed points, resulting in a bifurcation diagram similar to that of the Courtemanche model. The approach proposed in this paper assists in the exploration of the high-dimensional parameter space of the cell models and the identification of the conditions leading to spontaneous pacemaker activity.     

Genetic variants in post myocardial infarction patients presenting with electrical storm of unstable ventricular tachycardia

Electrical storm (ES) is a life threatening clinical situation. Though a few clinical pointers exist, the occurrence of ES in a patient with remote myocardial infarction (MI) is generally unpredictable. Genetic markers for this entity have not been studied. In the present study, we carried out genetic screening in patients with remote myocardial infarction presenting with ES by next generation sequencing and identified 25 rare variants in 19 genes predominantly in RYR2, SCN5A, KCNJ11, KCNE1 and KCNH2, CACNA1B, CACNA1C, CACNA1D and desmosomal genes - DSP and DSG2 that could potentially be implicated in electrical storm. These genes have been previously reported to be associated with inherited syndromes of Sudden Cardiac Death. The present study suggests that the genetic architecture in patients with remote MI and ES of unstable ventricular tachycardia may be similar to that of Ion channelopathies. Identification of these variants may identify post MI patients who are predisposed to develop electrical storm and help in risk stratification.     

Down-regulation of UDP-glucuronic acid biosynthesis leads to swollen plant cell walls and severe developmental defects associated with changes in pectic polysaccharides

UDP-glucose dehydrogenase (UGD) plays a key role in the nucleotide sugar biosynthetic pathway, as its product UDP-glucuronic acid is the common precursor for arabinose, xylose, galacturonic acid, and apiose residues found in the cell wall. In this study we characterize an Arabidopsis thaliana double mutant ugd2,3 that lacks two of the four UGD isoforms. This mutant was obtained from a cross of ugd2 and ugd3 single mutants, which do not show phenotypical differences compared with the WT. In contrast, ugd2,3 has a strong dwarfed phenotype and often develops seedlings with severe root defects suggesting that the UGD2 and UGD3 isoforms act in concert. Differences in its cell wall composition in comparison to the WT were determined using biochemical methods indicating a significant reduction in arabinose, xylose, apiose, and galacturonic acid residues. Xyloglucan is less substituted with xylose, and pectins have a reduced amount of arabinan side chains. In particular, the amount of the apiose containing side chains A and B of rhamnogalacturonan II is strongly reduced, resulting in a swollen cell wall. The alternative pathway to UDP-glucuronic acid with the key enzyme myo-inositol oxygenase is not up-regulated in ugd2,3. The pathway also does not complement the ugd2,3 mutation, likely because the supply of myo-inositol is limited. Taken together, the presented data underline the importance of UDP GlcA for plant primary cell wall formation.     

Propagation of normal beats and re-entry in a computational model of ventricular cardiac tissue with regional differences in action potential shape and duration

There is substantial experimental evidence from studies using both intact tissue and isolated single cells to support the existence of different cell types within the ventricular wall of the heart, each possessing different electrical properties. However other studies have failed to find these differences, and instead support the idea that electrical coupling in vivo between regions with different cell types smoothes out differences in action potential shape and duration. In this study we have used a computational model of electrical activation in heterogenous 2D and 3D cardiac tissue to investigate the propagation of both normal beats and arrhythmias. We used the Luo–Rudy dynamic model for guinea pig ventricular cells, with simplified Ca²⁺ handling and transmural heterogeneity in I_Ks and I_to. With normal cell-to-cell coupling, a layer of M cells was not necessary for the formation of an upright T wave in the simulated electrocardiogram, and the amplitude and configuration of the T wave was not greatly affected by the thickness and configuration of the M cell layer. Transmural gradients in repolarisation pushed re-entrant waves with an intramural filament towards either the base or the apex of the ventricles, and caused transient break up of re-entry with a transmural filament.     

Formative and proliferative cell divisions,cell differentiation,and developmental changes in the meristem of Azolla roots

The root of the water fern Azolla is a compact higher-plant organ, advantageous for studies of cell division, cell differentiation, and morphogenesis. The cell complement of A. filiculoides Lam. and A. pinnata R.Br. roots is described, and the lineages of the cell types, all derived ultimately from a tetrahedral apical cell, are characterised in terms of sites and planes of cell division within the formative zone, where the initial cells of the cell files are generated. Subsequent proliferation of the initial cells is highly specific, each cell type having its own programme of divisions prior to terminal differentiation. Both formative and proliferative divisions (but especially the former) occur in regular sequences. Two enantiomorphic forms of root develop, with the dispositions of certain types of cell correlating with the direction, dextrorse or sinistrorse, of the cell-division sequence in the apical cells. Root growth is determinate, the apical cell dividing about 55 times, and its cell-cycle duration decreasing from an initial 10 h to about 4 h during the major phase of root development. Sites of proliferation progress acropetally during aging, but do not penetrate into the zone of formative divisions. The detailed portrait of root development that was obtained is discussed with respect to genetic and epigenetic influences; quantal and non-quantal cell cycles; variation in cell-cycle durations; relationships between cell expansion and cell division: the role of the apical cell; and the limitation of the total number of mitotic cycles during root formation.     

Antibacterial activities of beta-lactamase inhibitors associated with morphological changes of cell wall in Helicobacter pylori

Background. Recent study has demonstrated that β‐lactamase inhibitors including clavulanate, sulbactam and tazobactam have an vitro antibacterial effect on Helicobacter pylori. Here we describe the relationship between viability and cell profiles of H. pylori exposed to β‐lactamase inhibitors and some antibiotics in a short‐time course. Materials and methods. The antibacterial effects of β‐lactamase inhibitors including clavulanate, sulbactam and tazobactam on the bacterial viability of and morphological changes in H. pylori ATCC43504 were examined. Results. The β‐lactamase inhibitors such as clavulanate and sulbactam alone decreased the viable counts of H. pylori, depending on the antibiotic concentrations. Exposure to these β‐lactamase inhibitors resulted in morphological changes of cell shape, cell‐wall disintegration and cell lysis. Among these β‐lactamase inhibitors, clavulanate was the most active, causing a decrease in viable counts and morphological changes such as short filamentous to sphaeroplast formation and lysis. One × minimum inhibitory concentration (MIC) of amoxicillin plus 1 × MIC of clavulanate decreased viable counts effectively compared with 1 × MIC of amoxicillin or 1 × MIC of clavulanate alone, and induced morphological changes of cell shape and cell wall. Conclusion. Our results suggest that the β‐lactamase inhibitors alone have concentration‐dependent antibacterial activities against H. pylori and affect the morphology of the cell shape and the cell wall in vitro.     

Interactions of mast cell degranulating peptides with model membranes: A comparative biophysical study

In the last decade, there has been renewed interest in biologically active peptides in fields like allergy, autoimmune diseases and antibiotic therapy. Mast cell degranulating peptides mimic G-protein receptors, showing different activity levels even among homologous peptides. Another important feature is their ability to interact directly with membrane phospholipids, in a fast and concentration-dependent way. The mechanism of action of peptide HR1 on model membranes was investigated comparatively to other mast cell degranulating peptides (Mastoparan, Eumenitin and Anoplin) to evidence the features that modulate their selectivity. Using vesicle leakage, single-channel recordings and zeta-potential measurements, we demonstrated that HR1 preferentially binds to anionic bilayers, accumulates, folds, and at very low concentrations, is able to insert and create membrane spanning ion-selective pores. We discuss the ion selectivity character of the pores based on the neutralization or screening of the peptides charges by the bilayer head group charges or dipoles.     

Changes in intracellular pH following egg activation and during the early cell cycle of the amphibian Pleurodeles waltlii, coincide with changes in MPF activity.

Previous work on Xenopus laevis suggests a temporal coincidence between inactivation of the M-phase promoting factor (MPF) and intracellular pH (pHi) increase during egg activation. In addition, we recently showed that during the early cell cycle of Xenopus eggs, MPF activity cycling and pHi oscillations were temporally and functionally related. In the present work, using eggs of another amphibian, Pleurodeles waltlii, which has a natural cell cycle considerably longer than that of Xenopus laevis, we show a temporal coincidence between MPF activity and pHi changes, both at the time of egg activation and at each of the following cell cycles. Egg activation-induced pHi changes in Pleurodeles did not involve classical plasma membrane ion exchangers, and were not due to the activation of a H+ conductance. On the other hand, the pHi oscillations intervening at each cell cycle were suppressed by inhibitors of protein synthesis or phosphorylation, as were their counterparts in Xenopus eggs. We propose that physiological pHi changes in Pleurodeles and Xenopus eggs might have a metabolic origin, in direct relation with the cascade of phosphorylations-dephosphorylations of proteins implicated in the control of the cell cycle.