Extracellular Potassium Concentration Research Articles

Neuromodulation of Astrocytic K+ Clearance.

Potassium homeostasis is fundamental for brain function. Therefore, effective removal of excessive K+ from the synaptic cleft during neuronal activity is paramount. Astrocytes play a key role in K+ clearance from the extracellular milieu using various mechanisms, including uptake via Kir channels and the Na+-K+ ATPase, and spatial buffering through the astrocytic gap-junction coupled network. Recently we showed that alterations in the concentrations of extracellular potassium ([K+]o) or impairments of the astrocytic clearance mechanism affect the resonance and oscillatory behavior of both the individual and networks of neurons. These results indicate that astrocytes have the potential to modulate neuronal network activity, however, the cellular effectors that may affect the astrocytic K+ clearance process are still unknown. In this study, we have investigated the impact of neuromodulators, which are known to mediate changes in network oscillatory behavior, on the astrocytic clearance process. Our results suggest that while some neuromodulators (5-HT; NA) might affect astrocytic spatial buffering via gap-junctions, others (DA; Histamine) primarily affect the uptake mechanism via Kir channels. These results suggest that neuromodulators can affect network oscillatory activity through parallel activation of both neurons and astrocytes, establishing a synergistic mechanism to maximize the synchronous network activity.

Effects of Increased Extracellular Potassium Concentration Induced by Ischemia on the Vulnerability of Ventricular Arrhythmias and the Regularity of Related Ventricular Tachycardia

Myocardial ischemia could induce arrhythmias such as ventricular tachycardia and ventricular fibrillation, leading to sudden death and other serious consequences. This manuscript adopted the cardiac modeling and simulation method to study the activity pattern of myocardial ischemia-related ventricular tachycardia and the effect of increased extracellular potassium concentration on arrhythmia vulnerability. A whole ventricular electrophysiological model of endocardial ischemia caused by distal occlusion of left anterior descending coronary artery was established. The simulation results suggested that the relationship between the vulnerability of ventricular arrhythmias and extracellular potassium concentration was bell shaped with a peak in susceptibility at 12 mM. This result was caused by the effect of extracellular potassium concentration on the dispersion of repolarization and the effective refractory period of cardiomyocytes. The extension of the effective refractory period was due to the electrical remodeling of the ventricle. Specifically, it was because of the delayed recovery of the INa current. In addition, the regularity of endocardial/epicardial reentrant pattern during non-transmural ischemia was also analyzed. The endocardium formed micro-reentrant, while the epicardium established macro-reentrant rotating around the ischemic regions provided a new idea for the determination of clinical ablation targets.

Loss of muscular force in isolated rat diaphragms is related to changes in muscle fibre size

Objective. Passivity of the diaphragm during prolonged mechanical ventilation can lead to ventilation-induced diaphragmatic dysfunction reasoned by a reduction of diaphragmatic muscle strength. Electrical stimulation may be utilised to modulate diaphragm muscle strength. Therefore we intended to investigate diaphragmatic muscle strength based on stimulation with electric impulses. Approach. Diaphragms of Wistar rats were excised, embedded in various incubation solutions and placed in a diaphragm force measurement device. Pressure amplitudes generated by the diaphragm in dependency of the embedding solution, stimulation frequency and time (360 min) were determined. Furthermore, the diaphragms were histologically evaluated. Main results. The ex vivo diaphragms evoked no pressure if embedded in incubation solutions with high potassium concentrations and up to >20 cmH2O if embedded in incubation solutions with extracellular potassium concentrations. Although vitality was well maintained after 360 min (78%) cultivation, the diaphragm’s force dropped by 90.8% after 240 min. The decline in the diaphragm’s force progressed faster if stimulation was performed every 20 min compared to every 120 min. The size of Type I muscle fibres was largest in diaphragms stimulated every 120 min. The fibre size of Type 2b/x muscle cells was lower in diaphragms after electrical stimulation compared to non-stimulated diaphragms. Significance. The force that the diaphragm can develop in ex vivo conditions depends on the incubation solution and the conditions of activation. Activity-related changes in the diaphragm’s muscular force are accompanied by specific changes in muscle fibre size.

Membrane Depolarization Inhibits BIMEL Upregulation but Prevents Neuronal Apoptosis Primarily by Increasing Cellular GSH Levels.

Sympathetic neurons deprived of nerve growth factor (NGF) die by apoptosis. Chronic depolarization with elevated concentrations of extracellular potassium ([K+]E) supports long-term survival of these and other types of neurons in culture. While depolarization has long been used to support neuronal cultures, little is known about the mechanism. We explored how chronic depolarization of NGF-deprived mouse sympathetic neurons in culture blocks apoptotic death. First, we determined the effects of elevated [K+]E on proapoptotic BH3-only proteins reported to be upregulated in sympathetic neurons after NGF withdrawal. Upregulation of BIMEL was blocked by depolarization while upregulation of PUMA was not. BMF levels did not increase after NGF withdrawal, and elevated [K+]E had no effect on its expression. dp5/HRK was not detectable. A large increase in production of mitochondria-derived reactive species (RS), including reactive oxygen species (ROS), occurs in NGF-deprived sympathetic neurons. Suppressing these RS prevents cytochrome c release from mitochondria and apoptosis. The addition of high [K+]E to cultures rapidly blocked increased RS and cytochrome c release. Elevated [K+]E caused an increase of the cellular antioxidant glutathione (GSH). Preventing this increase prevented the elevated [K+]E from blocking cytochrome c release and death. While suppression of BIMEL upregulation by elevated [K+]E may contribute to high [K+]E pro-survival effects, we conclude that elevated [K+]E prevents apoptotic death of NGF-deprived sympathetic neurons primarily via an antioxidant mechanism.

Cation Flux and Electrolyte Composition of Frozen-Deglycerolized Blood

Cation flux and the extracellular and erythrocyte concentrations of sodium, potassium, calcium, magnesium, chloride, and acid soluble phosphorus were measured in fresh blood collected in various anticoagulant solutions, ACD blood stored at 4 C, and in frozen-deglycerolized blood stored at -80 to -120 C. Considerable variations in concentrations of extracellular and erythrocyte electrolytes were encountered due to certain characteristics of the anticoagulants employed and to the effects of storage, temperature, pH, and extracellular electrolyte concentration on erythrocyte electrolyte concentration. In contrast to ACD blood stored at 4 C, frozen-deglycerolized blood 1 day to 32 months old had slightly elevated erythrocyte concentrations of sodium and chloride and normal erythrocyte concentrations of potassium and phosphorus. Both ACD and frozen-deglycerolized blood had normal sodium and potassium fluxes. These data show that prolonged freezing and glycerolization of erythrocytes do not significantly impair the active transport processes responsible for maintenance of cellular electrolyte concentration.

Punicalagin Damages the Membrane of Salmonella Typhimurium

Salmonella, a bacterial foodborne pathogen, can contaminate meat, milk, and vegetables. While appropriate measures are available to control Salmonella, the inhibitory phytochemicals from plants are gaining increased attention. Punicalagin, a natural antimicrobial, is one of the main active tannins isolated from Punica granatum L. To obtain a broader understanding of the effect of punicalagin on the cell membranes of Salmonella Typhimurium, the growth curves, extracellular potassium concentration, release of cell constituents, intracellular pH, membrane potential, and morphological features were characterized to elucidate the mechanisms of action. Treatment with punicalagin induced an increase in the extracellular concentrations of potassium and a release of cell constituents. A higher pH gradient, an increase in the intracellular pH, and cell membrane depolarization were observed after punicalagin treatment. Electron microscopy observations showed that the cell membrane structures of Salmonella Typhimurium were damaged by punicalagin. It is concluded that punicalagin inhibits the proliferation of Salmonella Typhimurium and destroys the integrity of the cell membrane, leading to a loss of cell homeostasis. These findings indicate that punicalagin has the potential to be developed as a future alternative to control Salmonella Typhimurium contamination in foods and reduce the risk of salmonellosis.

Defined Solution Corrects Phenotypic Response and Improves Detection Accuracy of Human Induced Pluripotent Stem Cell-Derived Cardiomyocytes to the 28 Comprehensive In Vitro Proarrhythmia Assay Standards

We reported that blockade of Ca2+ channel increased beating rate on human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) when in hiPSC-CM culture media, but decreased beating rate if extracellular potassium concentration was elevated above certain threshold. When hiPSC-CMs are used for cardiac safety assessment of drug candidates, a corrected phenotype of Ca2+ channel blockade, particularly for drug candidates possessing multiple cardiac ion channel activities, is expected to minimize exaggerated mitigation of human ether-à-go-go-related gene channel blockage and yield improved accuracy. To further demonstrate this concept, we tested the 28 Comprehensive in vitro Proarrhythmia Assay standards using 10 mM K+ defined solution, and compared the results with data from study using hiPSC-CM culture media. Identical test standard dimethyl sulfoxide stocks, experimental procedure, and data analysis method were used to reduce unrelated variation. We found that in the high-risk category, under 10 mM K+ condition, the early afterdepolarization (EAD)-causing concentrations for azimilide, D, l-sotalol, disopyramide, dofetilide, ibutilide, quinidine, and vandetanib were 14-fold, 0.7-fold, 14-fold, 1.6-fold, 0.01-fold, 3.2-fold, and 33-fold of their respective clinical free therapeutic concentrations. For intermediate-risk standards, 10 mM K+ condition still made hiPSC-CMs more sensitive to detect EADs in cases of cisapride, domperidone, droperidol, and pimozide. Also, the net effect of clozapine was shown to be due to Ca2+ channel blockade. The low-risk standards, metoprolol, mexiletine, and ranolazine no longer stopped hiPSC-CMs beating in 10 mM K+ condition. We conclude that 10 mM K+ condition improved detection accuracy for high- and intermediate-risk compounds, without overestimating torsadogenic risk of low-risk standards. Finally, we proposed an empirical proarrhythmic score system, helping to evaluate torsadogenic liability of drug candidates tested with hiPSC-CMs.

Hypoxia and hypocapnia storage of γ-irradiated red cell concentrates.

γ-irradiation is used to treat red blood cell (RBC) concentrates (RCCs) transfused to immunosuppressed patients. This treatment damages RBCs and increases storage lesions. Several studies have shown the beneficial effect of reducing O2 content during RBC storage. The present research work investigated the effect of γ-irradiation on RCCs stored under normal and hypoxia/hypocapnia conditions. O2 concentration (measured as oxyhaemoglobin fraction, sO2) and ABO-matched RCCs from whole blood donations, leukoreduced and prepared in phosphate, adenine, glucose, guanosine, saline and mannitol (PAGGSM) were pooled and split in two identical RCCs within 24 h post donation. One bag (Hx) was submitted to O2 and CO2 adsorption for 3 h on an orbital shaker at 22±2 °C and then transferred to a storage bag impermeable to gas. The other bag (Ctrl) was left as it was. The two bags were then stored at 4 °C. γ-irradiation (25 Gy) was applied at day 2 or 14, and the RCCs were stored until day 43. Different parameters (metabolites, haemolysis, morphology) were measured. Starting sO2 values were 63.7±18.4% (n=12) in Ctrl and 20.8±9.8% (n=12) in Hx bags, and reached 90.8±9.1% and 6.6±5.9% at day 43, respectively. As expected, an increase in glycolysis rate was observed after deoxygenation. Extracellular potassium concentrations were identical and reached around 70 mM at expiry with an irradiation-dependent kinetic release. No difference in haemolysis was observed after irradiation on day 2 in either group (<0.40%, p>0.9999). When irradiated at day 14, haemolysis was lower (p=0.033) in RCCs under hypoxia at the end of storage (day 28, 0.67±0.16%) compared to control (1.06±0.33%). Percentages of spherocytes were lower under hypoxia. The storage under hypoxia provided equivalent storage when RCCs were irradiated at day 2 and was advantageous when irradiated at day 14. In summary, O2-depletion of RCCs enable a better storage of RBCs, particularly when late irradiation is applied.

Neuronal gamma oscillations and activity-dependent potassium transients remain regular after depletion of microglia in postnatal cortex tissue.

Microglial cells (resident macrophages) feature rapid activation in CNS disease and can acquire multiple phenotypes exerting neuroprotection or neurotoxicity. The functional impact of surveying ("resting") microglia on neural excitability and neurotransmission in physiology is widely unknown, however. We addressed this issue in male rat hippocampal slice cultures (in situ) by pharmacological microglial ablation within days and by characterizing neuronal gamma-band oscillations (30-70Hz) that are highly sensitive to neuromodulators and disturbances in ion and energy regulation. Gamma oscillations support action potential timing and synaptic plasticity, associate with higher brain functions like perception and memory, and require precise communication between excitatory pyramidal cells and inhibitory (GABAergic) interneurons. The slice cultures featured well-preserved hippocampal cytoarchitecture and parvalbumin-positive interneuron networks, microglia with ramified morphology, and low basal levels of IL-6, TNF-α, and nitric oxide (NO). Stimulation of slice cultures with the pro-inflammatory cytokine IFN-γ or bacterial LPS serving as positive controls for microglial reactivity induced MHC-II expression and increased cytokine and NO release. Chronic exposure of slice cultures to liposome-encapsulated clodronate reduced the microglial cell population by about 96%, whereas neuronal structures, astrocyte GFAP expression, and basal levels of cytokines and NO were unchanged. Notably, the properties of gamma oscillations reflecting frequency, number and synchronization of synapse activity were regular after microglial depletion. Also, electrical stimulus-induced transients of the extracellular potassium concentration ([K+ ]o ) reflecting cellular K+ efflux, clearance and buffering were unchanged. This suggests that nonreactive microglia are dispensable for neuronal homeostasis and neuromodulation underlying network signaling and rhythm generation in cortical tissue.

MLKL Aggravates Ox-LDL-Induced Cell Pyroptosis via Activation of NLRP3 Inflammasome in Human Umbilical Vein Endothelial Cells.

Atherosclerosis is a progressive chronic inflammation in the arterial walls. It is believed that the deposition of low-density lipoprotein (LDL) and its damage to endothelial cells play a vital role in atherosclerosis. Oxidized LDL (Ox-LDL) was confirmed to induce endothelial cell pyroptosis which plays an important role in intima inflammation and the development of atherosclerosis, but the underlying molecular mechanism needs to be explored. Here, we showed that ox-LDL upregulated the expression of mixed lineage kinase domain-like (MLKL) protein at both the mRNA and protein levels in endothelial cells, associated with the augment of pro-caspase-1 cleavage, interleukin-1β (IL-1β) maturation, pro-IL-1β production, and lactate dehydrogenase (LDH) release. Overexpression of MLKL substantially aggravated ox-LDL-induced increasing levels of caspase-1, IL-1β, pro-IL-1β, and LDH. MLKL-induced caspase-1 activation and IL-1β maturation were abolished by NLR family, pyrin domain-containing 3 (NLRP3) specific inhibitor MCC950, or extracellular high potassium concentration. Our findings indicated that MLKL is essential for regulation of ox-LDL-induced pyroptosis and inflammation through the activation of NLRP3 inflammasome, and suggested that MLKL could act as potential therapeutic targets to ameliorate atherosclerosis-related diseases.

Dynamics of a neuron\u2013glia system: the occurrence of seizures and the influence of electroconvulsive stimuli

In this paper, we investigate the dynamics of a neuron–glia cell system and the underlying mechanism for the occurrence of seizures. For our mathematical and numerical investigation of the cell model we will use bifurcation analysis and some computational methods. It turns out that an increase of the potassium concentration in the reservoir is one trigger for seizures and is related to a torus bifurcation. In addition, we will study potassium dynamics of the model by considering a reduced version and we will show how both mechanisms are linked to each other. Moreover, the reduction of the potassium leak current will also induce seizures. Our study will show that an enhancement of the extracellular potassium concentration, which influences the Nernst potential of the potassium current, may lead to seizures. Furthermore, we will show that an external forcing term (e.g. electroshocks as unidirectional rectangular pulses also known as electroconvulsive therapy) will establish seizures similar to the unforced system with the increased extracellular potassium concentration. To this end, we describe the unidirectional rectangular pulses as an autonomous system of ordinary differential equations. These approaches will explain the appearance of seizures in the cellular model. Moreover, seizures, as they are measured by electroencephalography (EEG), spread on the macro–scale (cm). Therefore, we extend the cell model with a suitable homogenised monodomain model, propose a set of (numerical) experiment to complement the bifurcation analysis performed on the single–cell model. Based on these experiments, we introduce a bidomain model for a more realistic modelling of white and grey matter of the brain. Performing similar (numerical) experiment as for the monodomain model leads to a suitable comparison of both models. The individual cell model, with its seizures explained in terms of a torus bifurcation, extends directly to corresponding results in both the monodomain and bidomain models where the neural firing spreads almost synchronous through the domain as fast traveling waves, for physiologically relevant paramenters.

A Glial-Silicon Nanowire Electrode Junction Enabling Differentiation and Noninvasive Recording of Slow Oscillations from Primary Astrocytes.

The correct human brain function is dependent on the activity of non-neuronal cells called astrocytes. The bioelectrical properties of astrocytes in vitro do not closely resemble those displayed in vivo and the former are incapable of generating action potential; thus, reliable approaches in vitro for noninvasive electrophysiological recording of astrocytes remain challenging for biomedical engineering. Here it is found that primary astrocytes grown on a device formed by a forest of randomly oriented gold coated-silicon nanowires, resembling the complex structural and functional phenotype expressed by astrocytes in vivo. The device enables noninvasive extracellular recording of the slow-frequency oscillations generated by differentiated astrocytes, while flat electrodes failed on recording signals from undifferentiated cells. Pathophysiological concentrations of extracellular potassium, occurring during epilepsy and spreading depression, modulate the power of slow oscillations generated by astrocytes. A reliable approach to study the role of astrocytes function in brain physiology and pathologies is presented.

A sensitive and specific nanosensor for monitoring extracellular potassium levels in the brain

Extracellular potassium concentration affects the membrane potential of neurons, and, thus, neuronal activity. Indeed, alterations of potassium levels can be related to neurological disorders, such as epilepsy and Alzheimer's disease, and, therefore, selectively detecting extracellular potassium would allow the monitoring of disease. However, currently available optical reporters are not capable of detecting small changes in potassium, in particular, in freely moving animals. Furthermore, they are susceptible to interference from sodium ions. Here, we report a highly sensitive and specific potassium nanosensor that can monitor potassium changes in the brain of freely moving mice undergoing epileptic seizures. An optical potassium indicator is embedded in mesoporous silica nanoparticles, which are shielded by an ultrathin layer of a potassium-permeable membrane, which prevents diffusion of other cations and allows the specific capturing of potassium ions. The shielded nanosensor enables the spatial mapping of potassium ion release in the hippocampus of freely moving mice.

A New Mechanism of Cellular and Tissue Automaticity

Autorhythmicity and synchronization in excitable cells and tissue are intensely debated and linked to distinct mechanistic pathways. Recent studies reveal that inward rectifying potassium channels, implicated in autorhythmicity, are expressed within nanodomain clefts between cardiomyocytes. We hypothesized that dynamic modulation of extracellular potassium concentrations in clefts adjacent to electrically active cells (ephaptic coupling) is a new mechanism governing autorhythmicity in cells and tissue. A fraction of the potassium current (FIK) in the Hodgkin-Huxley model was divided between membrane facing a bulk extracellular space with the other currents, and a cleft volume parameter where extracellular potassium concentration and the reversal potential were tracked for FIK facing the cleft. Bifurcation analysis reveals a range of FIK and cleft volumes that support stable spontaneous oscillations in a single cell. The spontaneous activity is driven by cleft potassium accumulation which transiently raises the potassium reversal potential above the transmembrane potential. Under these conditions, potassium current is inward at the cleft and always outward elsewhere. Under specific FIK values and cleft volumes, inward potassium current in the cleft will cyclically depolarize the cell and trigger an action potential. As FIK increases, the rate of spontaneous activation first increases and then decreases. Increasing cleft volume will dramatically decrease and then gradually increase autorhythmic rates. The addition of diffusion mediated cleft potassium depletion removes the secondary autorhythmic rate increase with larger cleft volumes. In a two-cell model, reduced and heterogeneous FIK, cleft volume, and potassium diffusion can repress or support synchronized automaticity. These data suggest that extracellular nanodomain ionic modulation can facilitate and coordinate individual and coupled cellular autorhythmicity. The results also suggest a new role for fibroblasts, macrophages and/or astrocytes in the modulation of spontaneous oscillatory activity in a variety of electrically active cell networks.

Astrocyte-Selective Volume Increase in Elevated Extracellular Potassium Conditions Is Mediated by the Na+/K+ ATPase and Occurs Independently of Aquaporin 4.

Astrocytes and neurons have been shown to swell across a variety of different conditions, including increases in extracellular potassium concentration (^[K+]o). The mechanisms involved in the coupling of K+ influx to water movement into cells leading to cell swelling are not well understood and remain controversial. Here, we set out to determine the effects of ^[K+]o on rapid volume responses of hippocampal CA1 pyramidal neurons and stratum radiatum astrocytes using real-time confocal volume imaging. First, we found that elevating [K+]o within a physiological range (to 6.5 mM and 10.5 mM from a baseline of 2.5 mM), and even up to pathological levels (26 mM), produced dose-dependent increases in astrocyte volume, with absolutely no effect on neuronal volume. In the absence of compensating for addition of KCl by removal of an equal amount of NaCl, neurons actually shrank in ^[K+]o, while astrocytes continued to exhibit rapid volume increases. Astrocyte swelling in ^[K+]o was not dependent on neuronal firing, aquaporin 4, the inwardly rectifying potassium channel Kir 4.1, the sodium bicarbonate cotransporter NBCe1, , or the electroneutral cotransporter, sodium-potassium-chloride cotransporter type 1 (NKCC1), but was significantly attenuated in 1 mM barium chloride (BaCl2) and by the Na+/K+ ATPase inhibitor ouabain. Effects of 1 mM BaCl2 and ouabain applied together were not additive and, together with reports that BaCl2 can inhibit the NKA at high concentrations, suggests a prominent role for the astrocyte NKA in rapid astrocyte volume increases occurring in ^[K+]o. These findings carry important implications for understanding mechanisms of cellular edema, regulation of the brain extracellular space, and brain tissue excitability.

Commentary on Zhao et al Manuscript Entitled: "Quantitative Association Between Serum/Dietary Magnesium and Cardiovascular Disease/Coronary Heart Disease Risk: A Dose-Response Meta-analysis of Prospective Cohort Studies": Magnesium and Cardiovascular Disease.

Commentary on Zhao et al Manuscript Entitled: "Quantitative Association Between Serum/Dietary Magnesium and Cardiovascular Disease/Coronary Heart Disease Risk: A Dose-Response Meta-analysis of Prospective Cohort Studies": Magnesium and Cardiovascular Disease.

Mathematical model of epileptic discharge propagation

Mathematical modelling of epileptic activity of a nervous tissue is an open problem. The previously developed biophysical model Epileptor-2 which describes the excitability of the neural network and the ionic dynamics reproduces ictal (convulsive) and interictal (usually pre-convulsive) discharges as a spatially homogeneous activity. To describe the spatial propagation of the discharges across the cerebral cortex a generalization of the model is proposed, based on an elliptical equation that takes into account the propagation of neuronal impulses via axodendritic trees. Simulated spatio-temporal patterns of extracellular potassium concentration as of the main characteristic of spatial excitability reflect the wave-like nature of the discharge propagation and are comparable with experimental observations. The propagation of neural impulses via axo-dendritic trees may play the main role in the mechanism of propagation of ictal discharges.

Resolving the reversed rate effect of calcium channel blockers on human induced pluripotent stem cell-derived cardiomyocytes and the impact on in vitro cardiac safety evaluation

Calcium channel blockers (CCBs), such as diltiazem, nifedipine, and verapamil, cause tachycardia effects on several commercially available human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs), when tested in culture media provided by suppliers, rather than bradycardia effects, as seen in vivo. We found that in test conditions where Na+ current of hiPSC-CMs was reduced to certain threshold by either specific Na+ channel blocker tetrodotoxin (TTX), or by voltage-dependent inactivation using elevated extracellular potassium concentrations, CCBs produced bradycardia effects on hiPSC-CMs. However, elevated extracellular potassium concentrations or the presence of TTX did not change other pharmacological responses of hiPSC-CMs, including CCBs' effects on contraction intensity and duration; beating rate change by calcium channel opener FPL64176, HCN blocker ivabradine, and β-adrenergic agonist isoproterenol; and action potential duration prolongation by hERG channel blocker dofetilide. We concluded that action potentials of hiPSC-CMs, with regards to the CCB phenotype, were Na+ current driven. When Na+ channel availability was reduced to a critical level, their action potentials became Ca2+ current driven, and their responses to CCBs correlated well to those seen in vivo. Importantly, the corrected bradycardia effect of calcium channel block with our defined conditions will provide more reliable results in cardiac safety readouts of test compounds that integrate multiple effects including calcium channel inhibition.

Hypocalcemia-Induced Slowing of Human Sinus Node Pacemaking

Each heartbeat is initiated by cyclic spontaneous depolarization of cardiomyocytes in the sinus node forming the primary natural pacemaker. In patients with end-stage renal disease undergoing hemodialysis, it was recently shown that the heart rate drops to very low values before they suffer from sudden cardiac death with an unexplained high incidence. We hypothesize that the electrolyte changes commonly occurring in these patients affect sinus node beating rate and could be responsible for severe bradycardia. To test this hypothesis, we extended the Fabbri et al. computational model of human sinus node cells to account for the dynamic intracellular balance of ion concentrations. Using this model, we systematically tested the effect of altered extracellular potassium, calcium, and sodium concentrations. Although sodium changes had negligible (0.15 bpm/mM) and potassium changes mild effects (8 bpm/mM), calcium changes markedly affected the beating rate (46 bpm/mM ionized calcium without autonomic control). This pronounced bradycardic effect of hypocalcemia was mediated primarily by ICaL attenuation due to reduced driving force, particularly during late depolarization. This, in turn, caused secondary reduction of calcium concentration in the intracellular compartments and subsequent attenuation of inward INaCa and reduction of intracellular sodium. Our in silico findings are complemented and substantiated by an empirical database study comprising 22,501 pairs of blood samples and in vivo heart rate measurements in hemodialysis patients and healthy individuals. A reduction of extracellular calcium was correlated with a decrease of heartrate by 9.9 bpm/mM total serum calcium (p < 0.001) with intact autonomic control in the cross-sectional population. In conclusion, we present mechanistic in silico and empirical in vivo data supporting the so far neglected but experimentally testable and potentially important mechanism of hypocalcemia-induced bradycardia and asystole, potentially responsible for the highly increased and so far unexplained risk of sudden cardiac death in the hemodialysis patient population.

Simulation Study of Intermittent Responses of Neuronal Populations to Axonal High-Frequency Stimulation.

Deep brain stimulation (DBS) have shown a promising future for treating various brain disorders. Studies have indicated that the high frequency stimulation (HFS) used in DBS could cause a partial block in axons thereby attenuating the responses of axon fibers to the pulses of HFS. The attenuated response of axons might play a desynchronization role in modulating activity of neuronal populations. To investigate the detail behavior of individual axons under HFS, we created a computational model of neuronal populations including 1250 neurons. Each neuron consisted of a myelinated axon, an axonal initial segment, a soma and dendrites. A 10-s HFS sequence with 100 Hz pulses was applied to the axon layer by a bipolar stimulation electrode. The membrane potentials and the extracellular potassium concentration [K+]o at axons and at somata during the stimulation were investigated. The results showed that the simulation model with a mechanism of potassium accumulation could reproduce the attenuated responses of neuronal populations to persistent axonal HFS in rat experiments. The elevation of [K+]o during HFS resulted in an increase of basic membrane potentials and then generated a depolarization block in the axonal membrane thereby attenuating the responses of neuronal populations. The depolarization block in axons included both complete block (~26%) and intermittent block (~74%), which generated desynchronized firing among axons in fibers and travelled to the cell bodies to induce desynchronized firing in somata. The simulation results may provide important information for revealing the modulation mechanisms of axonal HFS in the therapy of brain stimulation.