Repolarisation and vulnerability to re-entry in the human heart with short QT syndrome arising from KCNQ1 mutation--a simulation study

Idiopathic short QT syndrome (SQTS) is a recently identified, genetically heterogeneous condition characterised by abbreviated QT intervals and an increased susceptibility to arrhythmia and sudden death. This simulation study identifies mechanisms by which cellular electrophysiological changes in the SQT2 (slow delayed rectifier, I_Ks, -linked) SQTS variant increases arrhythmia risk. The channel kinetics of the V307L mutation of the KCNQ1 subunit of the I_Ks channel were incorporated into human ventricular action potential (AP) models and into 1D and 2D transmural tissue simulations. Incorporating the V307L mutation into simulations reproduced defining features of the SQTS: abbreviation of the QT interval, and increases in T wave amplitude and T_peak–T_end duration. In the single-cell model, the V307L mutation abbreviated ventricular cell AP duration at 90% repolarisation (APD₉₀) and increased the maximal transmural voltage heterogeneity (δV) during APs; this resulted in augmented transmural heterogeneity of APD₉₀ and of the effective refractory period (ERP). In the intact tissue model, the vulnerable window for unidirectional conduction block was also increased. In 2D tissue the V307L mutation facilitated and maintained reentrant excitation. Thus, in SQT2 increases in transmural heterogeneity of APD, δV, ERP and an increased vulnerable window for unidirectional conduction block generate an electrical substrate favourable to reentrant arrhythmia.     

Differential effects of stretch and compression on membrane currents and [Na+]c in ventricular myocytes

Mechano-electrical feedback was studied in the single ventricular myocytes. A small fraction (approximately 10%) of the cell surface could be stretched or compressed by a glass stylus. Stretch depolarised, shortened the action potential and induced extra systoles. Stretch activated non-selective cation currents (I_ns) showed a linear voltage dependence, a reversal potential of 0 mV, a pure cation selectivity, and were blocked by 8 μM Gd³⁺ or 30 μM streptomycin. Stretch reduced Ca²⁺ and K⁺ (I_K) currents. Local compression of broadwise attached cells activated I_K but not I_ns. Cytochalasin D or colchicin, thought to disrupt the cytoskeleton, suppressed the mechanosensitivity of I_ns and I_K. During stretch, the cytosolic sodium concentration increased with spatial heterogeneities, local hotspots with [Na⁺]_c>24 mM appeared close to surface membrane and t-tubules (pseudoratiometric imaging using Sodium Green fluorescence). Electronprobe microanalysis confirmed this result and indicated that stretch increased total sodium [Na] in cell compartments such as mitochondria, nuclear envelope and nucleus. Our results obtained by local stretch differ from those obtained by end-to-end stretch (literature). We speculate that channels may be activated not only by axial but also by shear stress, and, that stretch can activate channels outside the deformed sarcomeres via second messenger.     

The canine virtual ventricular wall: a platform for dissecting pharmacological effects on propagation and arrhythmogenesis

We have constructed computational models of canine ventricular cells and tissues, ultimately combining detailed tissue architecture and heterogeneous transmural electrophysiology. The heterogeneity is introduced by modifying the Hund–Rudy canine cell model in order to reproduce experimentally reported electrophysiological properties of endocardial, midmyocardial (M) and epicardial cells. These models are validated against experimental data for individual ionic current and action potential characteristics, and their rate dependencies. 1D and 3D heterogeneous virtual tissues are constructed, with detailed tissue architecture (anisotropy and orthotropy, due to fibre orientation and sheet structure) of the left ventricular wall wedge extracted from a diffusion tensor imaging data set. The models are used to study the effects of tissue heterogeneity and class III drugs on transmural propagation and tissue vulnerability to re-entry.

We have determined relationships between the transmural dispersion of action potential duration (APD) and the vulnerable window in the 1D virtual ventricular wall, and demonstrated how changes in the transmural heterogeneity, and hence tissue vulnerability, can lead to generation of re-entry in the 3D ventricular wedge. Two class III drugs with opposite qualitative effects on transmural APD heterogeneity are considered: d-sotalol that increases transmural APD dispersion, and amiodarone that decreases it. Simulations with the 1D virtual ventricular wall show that under d-sotalol conditions the vulnerable window is substantially wider compared to amiodarone conditions, primarily in the epicardial region where unidirectional conduction block persists until the adjacent M cells are fully repolarised.

Further simulations with the 3D ventricular wedge have shown that ectopic stimulation of the epicardial region results in generation of sustained re-entry under d-sotalol conditions, but not under amiodarone conditions or in control. Again, APD increase in M cells was identified as the major contributor to tissue vulnerability—re-entry was initiated primarily due to ectopic excitation propagating around the unidirectional conduction block in the M cell region. This suggests an electrophysiological mechanism for the anti- and proarrhythmic effects of the class III drugs: the relative safety of amiodarone in comparison to d-sotalol can be explained by relatively low transmural APD dispersion, and hence, a narrow vulnerable window and low probability of re-entry in the tissue.     

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.     

Dynamics of intramural scroll waves in three-dimensional continuous myocardium with rotational anisotropy.

It has been suggested that reentrant activity in three-dimensional cardiac muscle may be organized as a scroll wave rotating around a singularity line called the filament. Experimental studies indicate that filaments are often concealed inside the ventricular wall and consequently, scroll waves do not manifest reentrant activity on the surface. Here we analyse how such concealed scroll waves are affected by a twisted anisotropy resulting from rotation of layers of muscle fibers inside the ventricular wall. We used a computer model of a ventricular slab (15x15x15 mm(3)) with a fiber twist of 120 degrees from endocardium to epicardium. The action potential was simulated using FitzHugh-Nagumo equations. Scroll waves with rectilinear filaments were initiated at various depths of the slab and at different angles with respect to fiber orientation. The analysis shows that independent of initial conditions, after a certain transitional period, the filament aligns with the local fiber orientation. The alignment of the filament is determined by the directional variations in cell coupling due to fiber rotation and by boundary conditions. Our findings provide a mechanistic explanation for the prevalence of intramural reentry over transmural reentry during polymorphic ventricular tachycardia and fibrillation.     

Simulation analysis of intracellular Na+ and Cl- homeostasis during beta 1-adrenergic stimulation of cardiac myocyte

To quantitatively understand intracellular Na⁺ and Cl⁻ homeostasis as well as roles of Na⁺/K⁺ pump and cystic fibrosis transmembrane conductance regulator Cl⁻ channel (I_CFTR) during the β1-adrenergic stimulation in cardiac myocyte, we constructed a computer model of β1-adrenergic signaling and implemented it into an excitation-contraction coupling model of the guinea-pig ventricular cell, which can reproduce membrane excitation, intracellular ion changes (Na⁺, K⁺, Ca²⁺ and Cl⁻), contraction, cell volume, and oxidative phosphorylation. An application of isoproterenol to the model cell resulted in the shortening of action potential duration (APD) after a transient prolongation, the increases in both Ca²⁺ transient and cell shortening, and the decreases in both Cl⁻ concentration and cell volume. These results are consistent with experimental data. Increasing the density of I_CFTR shortened APD and augmented the peak amplitudes of the L-type Ca²⁺ current (I_CaL) and the Ca²⁺ transient during the β1-adrenergic stimulation. This indirect inotropic effect was elucidated by the increase in the driving force of I_CaL via a decrease in plateau potential. Our model reproduced the experimental data demonstrating the decrease in intracellular Na⁺ during the β-adrenergic stimulation at 0 or 0.5 Hz electrical stimulation. The decrease is attributable to the increase in Na⁺ affinity of Na⁺/K⁺ pump by protein kinase A. However it was predicted that Na⁺ increases at higher beating rate because of larger Na⁺ influx through forward Na⁺/Ca²⁺ exchange. It was demonstrated that dynamic changes in Na⁺ and Cl⁻ fluxes remarkably affect the inotropic action of isoproterenol in the ventricular myocytes.     

Changes in action potentials and intracellular ionic homeostasis in a ventricular cell model related to a persistent sodium current in SCN5A mutations underlying LQT3

In LQT3 patients, SCN5A mutations induce ultraslow inactivation of a small fraction of the hNav1.5 current, i.e. persistent Na⁺ current (I_pNa). We explored the time course of effects of such a change on the intracellular ionic homeostasis in a model of guinea-pig cardiac ventricular cell [Pasek, M., Simurda, J., Orchard, C.H., Christé, G., 2007b. A model of the guinea-pig ventricular cardiomyocyte incorporating a transverse–axial tubular system. Prog. Biophys. Mol. Biol., this issue]. Sudden addition of I_pNa prevented action potential (AP) repolarization when its conductance (g_pNa) exceeded 0.12% of the maximal conductance of fast I_Na (g_Na). With g_pNa at 0.1% g_Na, the AP duration at 90% repolarization (APD₉₀) was initially lengthened to 2.6-fold that in control. Under regular stimulation at 1 Hz it shortened progressively to 1.37-fold control APD₉₀, and intracellular [Na⁺]_i increased by 6% with a time constant of 106 s. Further increasing g_pNa to 0.2% g_Na caused an immediate increase in APD₉₀ to 5.7-fold that in control, which decreased to 2.2-fold that in control in 30 s stimulation at 1 Hz. At this time diastolic [Na⁺]_i and [Ca²⁺]_i were, respectively, 34% and 52% higher than in control and spontaneous erratic SR Ca release occurred.

In the presence of I_pNa causing 46% lengthening of APD₉₀, the model cell displayed arrhythmogenic behaviour when external [K⁺] was lowered to 5 mM from an initial value at 5.4 mM. By contrast, when K⁺ currents I_Kr and I_Ks were lowered in the model cell to produce the same lengthening of APD₉₀, no proarrhythmic behaviour was observed, even when external [K⁺] was lowered to 2.5 mM.     

Electrophysiological modelling of pulmonary artery smooth muscle cells in the rabbits--special consideration to the generation of hypoxic pulmonary vasoconstriction

In vascular smooth muscle cells, it has been suggested that membrane potential is an important component that initiates contraction. We developed a mathematical model to elucidate the quantitative contributions of major ion currents [a voltage-gated L-type Ca²⁺ current (I_CaL), a voltage-sensitive K⁺ current (I_KV), a Ca²⁺-activated K⁺ current (I_KCa) and a nonselective cation current (I_NSC)] to membrane potential. In order to typify the diverse nature of pulmonary artery smooth muscle cells (PASMCs), we introduced parameters that are not fixed (variable parameters). The population of cells with different parameters was constructed and the cells that have the electrophysiological properties of PASMCs were selected. The contributions of each membrane current were investigated by sensitivity analysis and modification of the current parameters. Consequently, I_KV and I_NSC were found to be the most important currents that affect the membrane potential. The occurrence of depolarisation in hypoxic pulmonary vasoconstriction (HPV) was also examined. In hypoxia, I_KV and I_KCa were reduced, but the consequent depolarisation in simulation was not enough to initiate contractions. If we add an increase of I_NSC (2.5-fold), the calculated membrane potential was enough to induce contraction. From the results, we conclude that the balance of various ion channel activities determines the resting membrane potential of PASMCs and our model was successful in explaining the depolarisation in HPV. Therefore, this model can be a powerful tool to investigate the various electrical properties of PASMCs in both normal and pathological conditions.     

Functional and transmural modulation of M cell behavior in canine ventricular wall

Previous studies have demonstrated a discrete population of midmyocardial (M) cells in the ventricular myocardium having excessive action potential duration (APD) prolongation during long activation cycle lengths (CL) and under the influence of APD-prolonging agents. However, M cells have not been found in other studies. Existing explanations for the discrepancies appear inadequate. We hypothesized that instead of being a discrete group, M cell behavior is functional and conditionally expressed. We mapped APDs on the cut-exposed transmural surfaces of arterially perfused ventricular wedges from 26 dogs during Na+ current modification with anemone toxin II (ATX-II). Compared with the endocardium, APDs were not statistically different in the parallel layer having the longest mean APD (APDL) and were significantly shorter in the epicardium in the 26 wedges before ATX-II. ATX-II (> or =5 nmol/l) prolonged APD heterogeneously (midmyocardium > endocardium > epicardium). The differences increased at longer CLs. ATX-II (20.0 nmol/l) shifted the APD(L) layer to 32 +/- 6.2% (6 wedges, CL: 4,000 ms) of the transmural thickness from the (sub)endocardium (8.6 +/- 7.2%, 26 wedges, ATX-II free). We detected the presence of M cell behavior (significantly longer APDs in the APDL layer than in the endocardium and epicardium, P < or = 0.04, CL: 4,000 ms) in the 18 wedges having > or =5 nmol/l ATX-II but not (P >0.36) in the other 18 wedges having < or =2.5 nmol/l ATX-II. Both the position of the APDL layer and presence of M cell-like behavior were modulated by ATX-II. The dynamic spatial modulation indicates that M cell behavior is functional and only becomes manifest under suitable conditions.     

Scroll-wave dynamics in human cardiac tissue: lessons from a mathematical model with inhomogeneities and fiber architecture

Cardiac arrhythmias, such as ventricular tachycardia (VT) and ventricular fibrillation (VF), are among the leading causes of death in the industrialized world. These are associated with the formation of spiral and scroll waves of electrical activation in cardiac tissue; single spiral and scroll waves are believed to be associated with VT whereas their turbulent analogs are associated with VF. Thus, the study of these waves is an important biophysical problem. We present a systematic study of the combined effects of muscle-fiber rotation and inhomogeneities on scroll-wave dynamics in the TNNP (ten Tusscher Noble Noble Panfilov) model for human cardiac tissue. In particular, we use the three-dimensional TNNP model with fiber rotation and consider both conduction and ionic inhomogeneities. We find that, in addition to displaying a sensitive dependence on the positions, sizes, and types of inhomogeneities, scroll-wave dynamics also depends delicately upon the degree of fiber rotation. We find that the tendency of scroll waves to anchor to cylindrical conduction inhomogeneities increases with the radius of the inhomogeneity. Furthermore, the filament of the scroll wave can exhibit drift or meandering, transmural bending, twisting, and break-up. If the scroll-wave filament exhibits weak meandering, then there is a fine balance between the anchoring of this wave at the inhomogeneity and a disruption of wave-pinning by fiber rotation. If this filament displays strong meandering, then again the anchoring is suppressed by fiber rotation; also, the scroll wave can be eliminated from most of the layers only to be regenerated by a seed wave. Ionic inhomogeneities can also lead to an anchoring of the scroll wave; scroll waves can now enter the region inside an ionic inhomogeneity and can display a coexistence of spatiotemporal chaos and quasi-periodic behavior in different parts of the simulation domain. We discuss the experimental implications of our study.     

Effect of activation sequence on transmural patterns of repolarization and action potential duration in rabbit ventricular myocardium

Although transmural heterogeneity of action potential duration (APD) is established in single cells isolated from different tissue layers, the extent to which it produces transmural gradients of repolarization in electrotonically coupled ventricular myocardium remains controversial. The purpose of this study was to examine the relative contribution of intrinsic cellular gradients of APD and electrotonic influences to transmural repolarization in rabbit ventricular myocardium. Transmural optical mapping was performed in left ventricular wedge preparations from eight rabbits. Transmural patterns of activation, repolarization, and APD were recorded during endocardial and epicardial stimulation. Experimental results were compared with modeled data during variations in electrotonic coupling. A transmural gradient of APD was evident during endocardial stimulation, which reflected differences previously seen in isolated cells, with the longest APD at the endocardium and the shortest at the epicardium (endo: 165 ± 5 vs. epi: 147 ± 4 ms; P < 0.05). During epicardial stimulation, this gradient reversed (epi: 162 ± 4 vs. endo: 148 ± 6 ms; P < 0.05). In both activation sequences, transmural repolarization followed activation and APD shortened along the activation path such that significant transmural gradients of repolarization did not occur. This correlation between transmural activation time and APD was recapitulated in simulations and varied with changes in intercellular coupling, confirming that it is mediated by electrotonic current flow between cells. These data suggest that electrotonic influences are important in determining the transmural repolarization sequence in rabbit ventricular myocardium and that they are sufficient to overcome intrinsic differences in the electrophysiological properties of the cells across the ventricular wall.     

Evolution of spiral and scroll waves of excitation in a mathematical model of ischaemic border zone

Abnormal electrical activity from the boundaries of ischemic cardiac tissue is recognized as one of the major causes in generation of ischemia-reperfusion arrhythmias. Here we present theoretical analysis of the waves of electrical activity that can rise on the boundary of cardiac cell network upon its recovery from ischaemia-like conditions. The main factors included in our analysis are macroscopic gradients of the cell-to-cell coupling and cell excitability and microscopic heterogeneity of individual cells. The interplay between these factors allows one to explain how spirals form, drift together with the moving boundary, get transiently pinned to local inhomogeneities, and finally penetrate into the bulk of the well-coupled tissue where they reach macroscopic scale. The asymptotic theory of the drift of spiral and scroll waves based on response functions provides explanation of the drifts involved in this mechanism, with the exception of effects due to the discreteness of cardiac tissue. In particular, this asymptotic theory allows an extrapolation of 2D events into 3D, which has shown that cells within the border zone can give rise to 3D analogues of spirals, the scroll waves. When and if such scroll waves escape into a better coupled tissue, they are likely to collapse due to the positive filament tension. However, our simulations have shown that such collapse of newly generated scrolls is not inevitable and that under certain conditions filament tension becomes negative, leading to scroll filaments to expand and multiply leading to a fibrillation-like state within small areas of cardiac tissue.     

Effects of galanin on short circuit current and electrolyte transport in rabbit ileum

Galanin decreased short circuit current (I_sc) and increased

Abstract

Galanin decreased short circuit current (I_sc) and increased active Na⁺ and Cl⁻ absorption in rabbit ileum. In the absence of calcium, the galanin-induced decrease in I_sc was inhibited by approximately 60%. Tetrodotoxin significantly reduced the effect of galanin on I_sc, and tetrodotoxin and EGTA totally blocked the effect, indicating that the nonneuronal mediator of the effect is Ca²⁺ dependent. Galanin binding to basolateral membranes prepared from ileal epithelial cells was specific and of high affinity. These results suggest the involvement of this peptide in the regulation of intestinal epithelial cell function.     

Intermediates in the refolding of urea-denatured dimeric arginine kinase from Stichopus japonicus

The refolding of urea-denatured dimeric AK was investigated by both equilibrium and kinetic measurements. Both studies indicated that the refolding of dimeric AK is a multiphasic process. The equilibrium studies, monitored by enzyme activity, intrinsic protein fluorescence, circular dichroism (CD), 1-anilinonaphtalene-8-sulfonate (ANS) binding, size-exclusion chromatography and glutaraldehyde cross-linking showed that there were at least two intermediates involved in this process: I₁ (existing in 1.8–1.4 M urea) and I₂ (existing in 0.8–0.4 M urea). I₁ was a monomeric intermediate and possessed characteristic similar to the globular folding intermediates described in the literature. I₂ was an active native-like intermediate. The kinetic studies suggested that the refolding of AK possessed a burst phase, fast phase and slow phase, which involved at least the burst phase intermediates (I_B). Comparison of the properties of these intermediates suggested that I_B in the kinetic process corresponded to I₁ in the equilibrium process. Based on these results, a scheme for refolding of urea-denatured AK was proposed.     

Swelling-activated chloride channels in cardiac physiology and pathophysiology

Characteristics and functions of the cardiac swelling-activated Cl current (I_Cl,swell) are considered in physiologic and pathophysiologic settings. I_Cl,swell is broadly distributed throughout the heart and is stimulated not only by osmotic and hydrostatic increases in cell volume, but also by agents that alter membrane tension and direct mechanical stretch. The current is outwardly rectifying, reverses between the plateau and resting potentials (E_m), and is time-independent over the physiologic voltage range. Consequently, I_Cl,swell shortens action potential duration, depolarizes E_m, and acts to decrease cell volume. Because it is activated by stimuli that also activate cation stretch-activated channels, I_Cl,swell should be considered as a potential effector of mechanoelectrical feedback. I_Cl,swell is activated in ischemic and non-ischemic dilated cardiomyopathies and perhaps during ischemia and reperfusion. I_Cl,swell plays a role in arrhythmogenesis, myocardial injury, preconditioning, and apoptosis of myocytes. As a result, I_Cl,swell potentially is a novel therapeutic target.     

Increased vulnerability of human ventricle to re-entrant excitation in hERG-linked variant 1 short QT syndrome

The short QT syndrome (SQTS) is a genetically heterogeneous condition characterized by abbreviated QT intervals and an increased susceptibility to arrhythmia and sudden death. This simulation study identifies arrhythmogenic mechanisms in the rapid-delayed rectifier K(+) current (I(Kr))-linked SQT1 variant of the SQTS. Markov chain (MC) models were found to be superior to Hodgkin-Huxley (HH) models in reproducing experimental data regarding effects of the N588K mutation on KCNH2-encoded hERG. These ionic channel models were then incorporated into human ventricular action potential (AP) models and into 1D and 2D idealised and realistic transmural ventricular tissue simulations and into a 3D anatomical model. In single cell models, the N588K mutation abbreviated ventricular cell AP duration at 90% repolarization (APD(90)) and decreased the maximal transmural voltage heterogeneity (δV) during APs. This resulted in decreased transmural heterogeneity of APD(90) and of the effective refractory period (ERP): effects that are anticipated to be anti-arrhythmic rather than pro-arrhythmic. However, with consideration of transmural heterogeneity of I(Kr) density in the intact tissue model based on the ten Tusscher-Noble-Noble-Panfilov ventricular model, not only did the N588K mutation lead to QT-shortening and increases in T-wave amplitude, but δV was found to be augmented in some local regions of ventricle tissue, resulting in increased tissue vulnerability for uni-directional conduction block and predisposing to formation of re-entrant excitation waves. In 2D and 3D tissue models, the N588K mutation facilitated and maintained re-entrant excitation waves due to the reduced substrate size necessary for sustaining re-entry. Thus, in SQT1 the N588K-hERG mutation facilitates initiation and maintenance of ventricular re-entry, increasing the lifespan of re-entrant spiral waves and the stability of scroll waves in 3D tissue.     

Phase singularities and filaments: simplifying complexity in computational models of ventricular fibrillation

In the whole heart, millions of cardiac cells are involved in ventricular fibrillation (VF). Experimental studies indicate that VF is sustained by re-entrant activity, and that each re-entrant wave rotates around a filament of phase singularity. Filaments act as organising centres, and offer a way to simplify and quantify the complex spatio-temporal behaviour observed in VF. Where a filament touches the surface of fibrillating myocardium re-entrant activity can be observed, however the behaviour of filaments within bulk ventricular myocardium is difficult to observe directly using present experimental techniques. Large scale computational simulations of VF in three-dimensional (3D) tissue offer a tool to investigate the properties and behaviour of filaments, and the aim of this paper is to review recent advances in this area as well as to compare recent computational studies of fibrillation in whole ventricle geometries.     

Evidence for modulation of cell-to-cell electrical coupling by cAMP in mouse islets of Langerhans

The effects of forskolin on electrical coupling among pancreatic β-cells were studied. Two microelectrodes were used to measure membrane potentials simultaneously in pairs of islet β-cells. Intracellular injection of a current pulse (ΔI) elicited a membrane response ΔV₁ in the injected cell and also a response ΔV₂ in a nearby β-cell confirming the existence of cell-to-cell electrical coupling among islet β-cells. In the presence of glucose (7 mM), application of forskolin evoked a transient depolarization of the membrane and electrical activity suggesting that the drug induced a partial inhibition of the β-cell membrane K⁺ conductance. Concomitant with this depolarization of the membrane there was a marked decrease in β-cell input resistance (ΔV₂/ΔI) suggesting that exposure to forskolin enhanced intercellular coupling. Direct measurements of the coupling ratio ΔV₂/ΔV₁ provided further support to the idea that forskolin enhances electrical coupling among islet cells. Indeed, application of forskolin reversibly increased the coupling ratio. These results suggest that cAMP might be involved in the modulation of electrical coupling among islet β-cells.     

Repolarization gradients in the intact heart: transmural or apico-basal?

Controversies regarding the genesis of the T wave in the electrocardiogram and the role of midmural M cells in the intact heart include: In normal, intact canine and human hearts there is no significant transmural gradient in repolarization times. The T wave results primarily from apico-basal differences in repolarization times. Also, in the intact heart there is no midmural region of prolonged action potential duration. This contrasts with isolated preparations, such as the wedge preparation or myocardial slices or disaggregated myocytes in which M cells, with action potentials longer than those of endocardial and epicardial myocardium, can be found. This disparity in action potential duration probably results from partial uncoupling of myocardial cells in the regions where measurements are made, e.g., the cut surface of a wedge preparation. In regions of a wedge where cellular coupling is normal, or in isolated myocardial bundles or sheets, no evidence for M cells is detected. In some wedge preparations, a drug-induced large transmural repolarization gradient, involving M cells, can lead to Torsade de Pointes, possibly caused by so-called phase two reentry. In contrast, when a gradient of repolarization times of more than 100?ms was created in intact hearts, no evidence for reentry was found and no spontaneous arrhythmias occurred. In conclusion, in the intact heart, M cells appear not to contribute to repolarization gradients and arrhythmias. Furthermore, no significant repolarization gradients between endocardium and epicardium exist. The T wave in the body surface electrocardiogram is caused by apico-basal and anterior-posterior differences in repolarization times.     

Role of pacemaking current in cardiac nodes: insights from a comparative study of sinoatrial node and atrioventricular node

Cardiac pacemaking in the sinoatrial (SA) node and atrioventricular (AV) node is generated by an interplay of many ionic currents, one of which is the funny pacemaker current (I_f). To understand the functional role of I_f in two different pacemakers, comparative studies of spontaneous activity and expression of the HCN channel in mouse SA node and AV node were performed. The intrinsic cycle length (CL) is 179±2.7 ms (n=5) in SA node and 258±18.7 ms (n=5) in AV node. Blocking of I_f current by 1 μmol/L ZD7288 increased the CL to 258±18.7 ms (n=5) and 447±92.4 ms (n=5) in SA node and AV node, respectively. However, the major HCN channel, HCN4 expressed at low level in the AV node compared to the SA node. To clarify the discrepancy between the functional importance of I_f and expression level of HCN4 channel, a SA node cell model was used. Increasing the I_f conductance resulted in decreasing in the CL in the model, which explains the high pacemaking rate and high expression of HCN channel in the SA node. Resistance to the blocking of I_f in the SA node might result from compensating effects from other currents (especially voltage sensitive currents) involved in pacemaking. The computer simulation shows that the difference in the intrinsic CL could explain the difference in response to I_f blocking in these two cardiac nodes.