Conduction Of Electrical Signals Research Articles

Properties of cardiac conduction in a cell-based computational model.

The conduction of electrical signals through cardiac tissue is essential for maintaining the function of the heart, and conduction abnormalities are known to potentially lead to life-threatening arrhythmias. The properties of cardiac conduction have therefore been the topic of intense study for decades, but a number of questions related to the mechanisms of conduction still remain unresolved. In this paper, we demonstrate how the so-called EMI model may be used to study some of these open questions. In the EMI model, the extracellular space, the cell membrane, the intracellular space and the cell connections are all represented as separate parts of the computational domain, and the model therefore allows for study of local properties that are hard to represent in the classical homogenized bidomain or monodomain models commonly used to study cardiac conduction. We conclude that a non-uniform sodium channel distribution increases the conduction velocity and decreases the time delays over gap junctions of reduced coupling in the EMI model simulations. We also present a theoretical optimal cell length with respect to conduction velocity and consider the possibility of ephaptic coupling (i.e. cell-to-cell coupling through the extracellular potential) acting as an alternative or supporting mechanism to gap junction coupling. We conclude that for a non-uniform distribution of sodium channels and a sufficiently small intercellular distance, ephaptic coupling can influence the dynamics of the sodium channels and potentially provide cell-to-cell coupling when the gap junction connection is absent.

Preparation of an Electrically Conductive Graphene Oxide/Chitosan Scaffold for Cardiac Tissue Engineering.

Cardiac tissue engineering is of great importance for therapeutic and pharmaceutical applications. The scaffolds that can provide electrical conductivity and structural organization will be highly beneficial for cardiac tissue engineering. Here, we developed conductive scaffolds with electrical conductivity and porous structure composed of chitosan (CS) blending with graphene oxide (GO) for cardiac tissue engineering. Our results showed that the swelling, porosity, and conductive properties of GO/CS scaffolds could be modulated via adjusting the ratio of GO to CS. More importantly, GO/CS scaffolds had a swelling ratio ranging from 23.20 to 27.38 (1000%) and their conductivity (0.134S/m) fell in the range of reported conductivities for native cardiac tissue. Furthermore, we assessed their biological activity by seeding heart H9C2 cells in GO/CS scaffolds. Our data showed that these GO/CS scaffolds exhibited good cell viability, promotion of cell attachment and intercellular network formation, and upregulation of the cardiac-specific gene and protein expression involved in muscle conduction of electrical signals (Connexin-43). Overall, it is concluded that the GO/CS scaffolds promote the properties of cardiac tissue constructs. Our findings provide a new strategy and insight in developing new scaffolds for cardiac tissue engineering.

Echo-Based FSI Models to Simulate Ventricular Electrical Signal Conduction in Pig Pacemaker Models

Cardiac pacing has been an effective treatment in the management of patients with arrhythmia. Different pacemaker location may have different impact on pacemaker effectiveness. A novel image-based ventricle animal modeling approach was proposed to integrate echocardiography images, propagating dynamic electric potential on ventricle surface to perform myocardial function assessment. The models will be used to simulate ventricular electrical signal conduction and optimize pacemaker location for better cardiac outcome. One health female adult pig weight 42.5kg was used to make pacing animal model with different ventricle pacing locations. Pig health status was assessed before undergoing experimental procedures. Ventricle surface electric signal, blood pressure and echo image were acquired 15 minutes after the pacemaker was implanted. Echo-based left ventricle (LV) fluid-structure interaction (FSI) models were constructed to perform ventricle function analysis and investigate impact of pacemaker location on cardiac outcome. The nonlinear Mooney-Rivlin model was used for ventricle tissue material model. With the measured electric signal map from the pig associated with the actual pacemaker site, electric potential conduction of myocardium was modeled by material stiffening and softening in our model, with stiffening simulating contraction and softening simulating relaxation. Material stiffness parameters were adjusted in a cardiac cycle to match Echo-measured LV deformation and volume variations. Mapping between material stiffness and ventricle electric signal was quantified using data measured from the animal with different pacemaker locations. Ventricle model without pacemaker and three ventricle models with the following pacemaker locations were simulated: right ventricular apex (RVA), posterior interventricular septum (PIVS) and right ventricular outflow tract (RVOT). Data for ventricle volume change, ejection fraction, stress and strain, flow velocity and shear stress data were collected for comparisons. Our results demonstrating that PIVS pacing model had higher peak flow velocity and stress/strain. It indicated that PIVS pacemaker site may be the best location. This modeling approach could be used as “virtual surgery” to try various pacemaker locations and avoid risky and dangerous surgical experiments on real patients.

Effect of Substrate Stiffness on Mechanical Coupling and Force Propagation at the Infarct Boundary

Heterogeneous intercellular coupling plays a significant role in mechanical and electrical signal transmission in the heart. Although many studies have investigated the electrical signal conduction between myocytes and nonmyocytes within the heart muscle tissue, there are not many that have looked into the mechanical counterpart. This study aims to investigate the effect of substrate stiffness and the presence of cardiac myofibroblasts (CMFs) on mechanical force propagation across cardiomyocytes (CMs) and CMFs in healthy and heart-attack-mimicking matrix stiffness conditions. The contractile forces generated by the CMs and their propagation across the CMFs were measured using a bio-nanoindenter integrated with fluorescence microscopy for fast calcium imaging. Our results showed that softer substrates facilitated stronger and further signal transmission. Interestingly, the presence of the CMFs attenuated the signal propagation in a stiffness-dependent manner. Stiffer substrates with CMFs present attenuated the signal ∼24–32% more compared to soft substrates with CMFs, indicating a synergistic detrimental effect of increased matrix stiffness and increased CMF numbers after myocardial infarction on myocardial function. Furthermore, the beating pattern of the CMF movement at the CM-CMF boundary also depended on the substrate stiffness, thereby influencing the waveform of the propagation of CM-generated contractile forces. We performed computer simulations to further understand the occurrence of different force transmission patterns and showed that cell-matrix focal adhesions assembled at the CM-CMF interfaces, which differs depending on the substrates stiffness, play important roles in determining the efficiency and mechanism of signal transmission. In conclusion, in addition to substrate stiffness, the degree and type of cell-cell and cell-matrix interactions, affected by the substrate stiffness, influence mechanical signal conduction between myocytes and nonmyocytes in the heart muscle tissue.

Toxins That Affect Voltage-Gated Sodium Channels.

Voltage-gated sodium channels (VGSCs) are critical in generation and conduction of electrical signals in multiple excitable tissues. Natural toxins, produced by animal, plant, and microorganisms, target VGSCs through diverse strategies developed over millions of years of evolutions. Studying of the diverse interaction between VGSC and VGSC-targeting toxins has been contributing to the increasing understanding of molecular structure and function, pharmacology, and drug development potential of VGSCs. This chapter aims to summarize some of the current views on the VGSC-toxin interaction based on the established receptor sites of VGSC for natural toxins.

Mechanisms of Drug Binding to Voltage-Gated Sodium Channels.

Voltage-gated sodium (Na+) channels are expressed in virtually all electrically excitable tissues and are essential for muscle contraction and the conduction of impulses within the peripheral and central nervous systems. Genetic disorders that disrupt the function of these channels produce an array of Na+ channelopathies resulting in neuronal impairment, chronic pain, neuromuscular pathologies, and cardiac arrhythmias. Because of their importance to the conduction of electrical signals, Na+ channels are the target of a wide variety of local anesthetic, antiarrhythmic, anticonvulsant, and antidepressant drugs. The voltage-gated family of Na+ channels is composed of α-subunits that encode for the voltage sensor domains and the Na+-selective permeation pore. In vivo, Na+ channel α-subunits are associated with one or more accessory β-subunits (β1-β4) that regulate gating properties, trafficking, and cell-surface expression of the channels. The permeation pore of Na+ channels is divided in two parts: the outer mouth of the pore is the site of the ion selectivity filter, while the inner cytoplasmic pore serves as the channel activation gate. The cytoplasmic lining of the permeation pore is formed by the S6 segments that include highly conserved aromatic amino acids important for drug binding. These residues are believed to undergo voltage-dependent conformational changes that alter drug binding as the channels cycle through the closed, open, and inactivated states. The purpose of this chapter is to broadly review the mechanisms of Na+ channel gating and the models used to describe drug binding and Na+ channel inhibition.

Unexpected central role of the androgen receptor in the spontaneous regeneration of myelin.

Lost myelin can be replaced after injury or during demyelinating diseases in a regenerative process called remyelination. In the central nervous system (CNS), the myelin sheaths, which protect axons and allow the fast propagation of electrical impulses, are produced by oligodendrocytes. The abundance and widespread distribution of oligodendrocyte progenitors (OPs) within the adult CNS account for this remarkable regenerative potential. Here, we report a key role for the male gonad, testosterone, and androgen receptor (AR) in CNS remyelination. After lysolecithin-induced demyelination of the male mouse ventral spinal cord white matter, the recruitment of glial fibrillary acidic protein-expressing astrocytes was compromised in the absence of testes and testosterone signaling via AR. Concomitantly, the differentiation of OPs into oligodendrocytes forming myelin basic protein (MBP)+ and proteolipid protein-positive myelin was impaired. Instead, in the absence of astrocytes, axons were remyelinated by protein zero (P0)+ and peripheral myelin protein 22-kDa (PMP22)+ myelin, normally only produced by Schwann cells in the peripheral nervous system. Thus, testosterone favors astrocyte recruitment and spontaneous oligodendrocyte-mediated remyelination. This finding may have important implications for demyelinating diseases, psychiatric disorders, and cognitive aging. The testosterone dependency of CNS oligodendrocyte remyelination may have roots in the evolutionary history of the AR, because the receptor has evolved from an ancestral 3-ketosteroid receptor through gene duplication at the time when myelin appeared in jawed vertebrates.

Myelin repair in vivo is increased by targeting oligodendrocyte precursor cells with nanoparticles encapsulating leukaemia inhibitory factor (LIF)

Multiple sclerosis (MS) is a progressive demyelinating disease of the central nervous system (CNS). Many nerve axons are insulated by a myelin sheath and their demyelination not only prevents saltatory electrical signal conduction along the axons but also removes their metabolic support leading to irreversible neurodegeneration, which currently is untreatable. There is much interest in potential therapeutics that promote remyelination and here we explore use of leukaemia inhibitory factor (LIF), a cytokine known to play a key regulatory role in self-tolerant immunity and recently identified as a pro-myelination factor. In this study, we tested a nanoparticle-based strategy for targeted delivery of LIF to oligodendrocyte precursor cells (OPC) to promote their differentiation into mature oligodendrocytes able to repair myelin. Poly(lactic-co-glycolic acid)-based nanoparticles of ∼120 nm diameter were constructed with LIF as cargo (LIF-NP) with surface antibodies against NG-2 chondroitin sulfate proteoglycan, expressed on OPC. In vitro, NG2-targeted LIF-NP bound to OPCs, activated pSTAT-3 signalling and induced OPC differentiation into mature oligodendrocytes. In vivo, using a model of focal CNS demyelination, we show that NG2-targeted LIF-NP increased myelin repair, both at the level of increased number of myelinated axons, and increased thickness of myelin per axon. Potency was high: a single NP dose delivering picomolar quantities of LIF is sufficient to increase remyelination.Impact statementNanotherapy-based delivery of leukaemia inhibitory factor (LIF) directly to OPCs proved to be highly potent in promoting myelin repair in vivo: this delivery strategy introduces a novel approach to delivering drugs or biologics targeted to myelin repair in diseases such as MS.

Parallel computing simulation of electrical excitation and conduction in the 3D human heart.

A correctly beating heart is important to ensure adequate circulation of blood throughout the body. Normal heart rhythm is produced by the orchestrated conduction of electrical signals throughout the heart. Cardiac electrical activity is the resulted function of a series of complex biochemical-mechanical reactions, which involves transportation and bio-distribution of ionic flows through a variety of biological ion channels. Cardiac arrhythmias are caused by the direct alteration of ion channel activity that results in changes in the AP waveform. In this work, we developed a whole-heart simulation model with the use of massive parallel computing with GPGPU and OpenGL. The simulation algorithm was implemented under several different versions for the purpose of comparisons, including one conventional CPU version and two GPU versions based on Nvidia CUDA platform. OpenGL was utilized for the visualization / interaction platform because it is open source, light weight and universally supported by various operating systems. The experimental results show that the GPU-based simulation outperforms the conventional CPU-based approach and significantly improves the speed of simulation. By adopting modern computer architecture, this present investigation enables real-time simulation and visualization of electrical excitation and conduction in the large and complicated 3D geometry of a real-world human heart.

Aging Impairs Electrical Conduction Along Endothelium of Resistance Arteries Through Enhanced Ca2+-Activated K + Channel Activation

Intercellular conduction of electrical signals underlies spreading vasodilation of resistance arteries. Small- and intermediate-conductance Ca(2+)-activated K(+) channels of endothelial cells serve a dual function by initiating hyperpolarization and modulating electrical conduction. We tested the hypothesis that regulation of electrical signaling by small- and intermediate-conductance Ca(2+)-activated K(+) channels is altered with advancing age. Intact endothelial tubes (60 µm wide; 1-3 mm long) were freshly isolated from male C57BL/6 mouse (Young: 4-6 months; Intermediate: 12-14 months; Old: 24-26 months) superior epigastric arteries. Using dual intracellular microelectrodes, current was injected (± 0.1-3 nA) at site 1 while recording membrane potential (Vm) at site 2 (separation distance: 50-2000 µm). Across age groups, greatest differences were observed between Young and Old. Resting Vm in Old (-38 ± 1 mV) was more negative (P<0.05) than Young (-30 ± 1 mV). Maximal hyperpolarization to both direct (NS309) and indirect (acetylcholine) activation of small- and intermediate-conductance Ca(2+)-activated K(+) channels was sustained (ΔVm ≈-40 mV) with age. The length constant (λ) for electrical conduction was reduced (P<0.05) from 1630 ± 80 µm (Young) to 1320 ± 80 µm (Old). Inhibiting small- and intermediate-conductance Ca(2+)-activated K(+) channels with apamin+charybdotoxin or scavenging hydrogen peroxide (H2O2) with catalase improved electrical conduction (P<0.05) in Old. Exogenous H2O2 (200 µmol/L) in Young evoked hyperpolarization and impaired electrical conduction; these effects were blocked by apamin+charybdotoxin. Enhanced current loss through Ca2+-activated K+ channel activation impairs electrical conduction along the endothelium of resistance arteries with aging. Attenuating the spatial domain of electrical signaling will restrict the spread of vasodilation and thereby contribute to blood flow limitations associated with advanced age.

ERK1/ERK2 MAPK Signaling is Required to Increase Myelin Thickness Independent of Oligodendrocyte Differentiation and Initiation of Myelination

Wrapping of the myelin sheath around axons by oligodendrocytes is critical for the rapid conduction of electrical signals required for the normal functioning of the CNS. Myelination is a multistep process where oligodendrocytes progress through a well coordinated differentiation program regulated by multiple extracellular growth and differentiation signals. The intracellular transduction of the extracellular signals that regulate myelination is poorly understood. Here we demonstrate a critical role for two important signaling molecules, extracelluar signal-regulated protein kinases 1 and 2 (ERK1/ERK2), downstream mediators of mitogen-activated protein kinases, in the control of CNS myelin thickness. We generated and analyzed two lines of mice lacking both ERK1/ERK2 function specifically in oligodendrocyte-lineage cells. In the absence of ERK1/ERK2 signaling NG2⁺ oligodendrocyte progenitor cells proliferated and differentiated on schedule. Mutant oligodendrocytes also ensheathed axons normally and made a few wraps of compact myelin. However, the subsequent increase in myelination that correlated myelin thickness in proportion to the axon caliber failed to occur. Furthermore, although the numbers of differentiated oligodendrocytes in the adult mutants were unchanged, they showed an inability to upregulate the transcription of major myelin genes that normally occurs during active myelination. Similarly, in vitro ERK1/ERK2-deficient oligodendrocytes differentiated normally but failed to form typical myelin-like membrane sheets. None of these effects were observed in single ERK1 or ERK2 mutants. These studies suggest that the predominant role of ERK1/ERK2 signaling in vivo is in promoting rapid myelin growth to increase its thickness, subsequent to oligodendrocyte differentiation and the initiation of myelination.

Electrical conduction along endothelial cell tubes from mouse feed arteries: confounding actions of glycyrrhetinic acid derivatives

Electrical conduction along endothelium of resistance vessels has not been determined independently of the influence of smooth muscle, surrounding tissue or blood. Two interrelated hypotheses were tested: (i) Intercellular conduction of electrical signals is manifest in endothelial cell (EC) tubes; and (ii) Inhibitors of gap junction channels (GJCs) have confounding actions on EC electrical and Ca(2+) signalling. Intact EC tubes were isolated from abdominal muscle feed (superior epigastric) arteries of C57BL/6 mice. Hyperpolarization was initiated with indirect (ACh) and direct (NS309) stimulation of intermediate- and small-conductance Ca(2+) -activated K(+) channels (IK(Ca) /SK(Ca) ). Remote membrane potential (V(m) ) responses to intracellular current injection defined the length constant (λ) for electrical conduction. Dye coupling was evaluated following intracellular microinjection of propidium iodide. Intracellular Ca(2+) dynamics were determined using Fura-2 photometry. Carbenoxolone (CBX) or β-glycyrrhetinic acid (βGA) was used to investigate the role of GJCs. Steady-state V(m) of ECs was -25 mV. ACh and NS309 hyperpolarized ECs by -40 and -60 mV respectively. Electrical conduction decayed monoexponentially with distance (λ∼1.4 mm). Propidium iodide injected into one EC spread into surrounding ECs. CBX or βGA inhibited dye transfer, electrical conduction and EC hyperpolarization reversibly. Both agents elevated resting Ca(2+) while βGA inhibited responses to ACh. Individual cells were effectively coupled to each other within EC tubes. Inhibiting GJCs with glycyrrhetinic acid derivatives blocked hyperpolarization mediated by IK(Ca) /SK(Ca) channels, regardless of Ca(2+) signalling, obviating use of these agents in distinguishing key determinants of electrical conduction along the endothelium.

Dissecting and Recording from The C. Elegans Neuromuscular Junction

Neurotransmission is the process by which neurons transfer information via chemical signals to their post-synaptic targets, on a rapid time scale. This complex process requires the coordinated activity of many pre- and post-synaptic proteins to ensure appropriate synaptic connectivity, conduction of electrical signals, targeting and priming of secretory vesicles, calcium sensing, vesicle fusion, localization and function of postsynaptic receptors and finally, recycling mechanisms. As neuroscientists it is our goal to elucidate which proteins function in each of these steps and understand their mechanisms of action. Electrophysiological recordings from synapses provide a quantifiable read out of the underlying electrical events that occur during synaptic transmission. By combining this technique with the powerful array of molecular and genetic tools available to manipulate synaptic proteins in C. elegans, we can analyze the resulting functional changes in synaptic transmission. The C. elegans NMJs formed between motor neurons and body wall muscles control locomotion, therefore, mutants with uncoordinated locomotory phenotypes (known as unc s) often perturb synaptic transmission at these synapses 1. Since unc mutants are maintained on a rich supply of a bacterial food source, they remain viable as long as they retain some pharyngeal pumping ability to ingest food. This, together with the fact that C. elegans exist as hermaphrodites, allows them to pass on mutant progeny without the need for elaborate mating behaviors. These attributes, coupled with our recent ability to record from the worms NMJs 2,3,7 make this an excellent model organism in which to address precisely how unc mutants impact neurotransmission. The dissection method involves immobilizing adult worms using a cyanoacrylic glue in order to make an incision in the worm cuticle exposing the NMJs. Since C. elegans adults are only 1 mm in length the dissection is performed with the use of a dissecting microscope and requires excellent hand-eye coordination. NMJ recordings are made by whole-cell voltage clamping individual body wall muscle cells and neurotransmitter release can be evoked using a variety of stimulation protocols including electrical stimulation, light-activated channel-rhodopsin-mediated depolarization 4 and hyperosmotic saline, all of which will be briefly described.

Dissecting and Recording from The C. Elegans Neuromuscular Junction

Neurotransmission is the process by which neurons transfer information via chemical signals to their post-synaptic targets, on a rapid time scale. This complex process requires the coordinated activity of many pre- and post-synaptic proteins to ensure appropriate synaptic connectivity, conduction of electrical signals, targeting and priming of secretory vesicles, calcium sensing, vesicle fusion, localization and function of postsynaptic receptors and finally, recycling mechanisms. As neuroscientists it is our goal to elucidate which proteins function in each of these steps and understand their mechanisms of action. Electrophysiological recordings from synapses provide a quantifiable read out of the underlying electrical events that occur during synaptic transmission. By combining this technique with the powerful array of molecular and genetic tools available to manipulate synaptic proteins in C. elegans, we can analyze the resulting functional changes in synaptic transmission. The C. elegans NMJs formed between motor neurons and body wall muscles control locomotion, therefore, mutants with uncoordinated locomotory phenotypes (known as unc s) often perturb synaptic transmission at these synapses 1. Since unc mutants are maintained on a rich supply of a bacterial food source, they remain viable as long as they retain some pharyngeal pumping ability to ingest food. This, together with the fact that C. elegans exist as hermaphrodites, allows them to pass on mutant progeny without the need for elaborate mating behaviors. These attributes, coupled with our recent ability to record from the worms NMJs 2,3,7 make this an excellent model organism in which to address precisely how unc mutants impact neurotransmission. The dissection method involves immobilizing adult worms using a cyanoacrylic glue in order to make an incision in the worm cuticle exposing the NMJs. Since C. elegans adults are only 1 mm in length the dissection is performed with the use of a dissecting microscope and requires excellent hand-eye coordination. NMJ recordings are made by whole-cell voltage clamping individual body wall muscle cells and neurotransmitter release can be evoked using a variety of stimulation protocols including electrical stimulation, light-activated channel-rhodopsin-mediated depolarization 4 and hyperosmotic saline, all of which will be briefly described.

Intramural Purkinje Cell Network of Sheep Ventricles as the Terminal Pathway of Conduction System

To identify the anatomical basis for cardiac electrical signal conduction, particularly seeking the intramural terminals of conduction pathway within the ventricles, sheep hearts were examined compared with human hearts utilizing the characteristic morphology of Purkinje cells as a histological marker. In 15 sheep and five human autopsies of noncardiac death, prevalence of Purkinje or Purkinje-type cells were histologically examined in the atrioventricular node, its distal conduction pathway, the interventricular septum, and the right- and left-ventricular free walls. Myocardial tissue cleavages were examined in the transmural sections (along cardiac base-to-apex axis) obtained from the septum and ventricular free walls. Serial histological sections through virtually the entirety of the septum in selected sheep were used as the basis of a three-dimensional reconstruction of the conduction pathway, particularly of the intramural Purkinje cell network. Purkinje cells were found within the mural myocardium of sheep ventricles whereas no intramural Purkinje-type cell was detected within the human ventricles. In the sheep septum, every intramural Purkinje cell composed a three-dimensional network throughout the mural myocardium, which proximally connected to the subendocardial extension of the bundle branches and distally formed an occasional junction with ordinary working myocytes. The Purkinje-cell network may participate in the ventricular excitation as the terminal conduction pathway. Individual connections among the Purkinje cells contain the links of through-wall orientation which would benefit the signal conduction crossing the architectural barriers by cleavages in sheep hearts. The myocardial architectural changes found in diseased hearts could disrupt the network links including those with transmural orientation. Anat Rec, 2009. (c) 2008 Wiley-Liss, Inc.

Highlights inDD

Similar yet different (Dev Dyn 236:2493–2501) A mouse could never be mistaken for a chicken—unless their neural crests are being compared. Neural crest cells migrate from the neural tube to various tissues, where they differentiate into several cell types. Chick neural crest migration is under the control of bone morphogenetic proteins (BMPs). Seemingly in agreement, Bmp2 null mice lack migratory crest. But, is the mouse phenotype due to defects in migration or induction of crest fate? Correia et al. put this question to rest. Consistent with a role in migration, they show that early crest markers are induced in Bmp2 null embryos, but the cells remain associated with the neural tube. However, when they dig deeper, they find direct comparisons between mouse and chick neural crest falter. Whereas chick Cad6B is only expressed in premigratory crest, it is also in migratory crest in mouse. Additionally, unlike in chick, mouse neural crest Cad6B, Id2, and Wnt1 expression is not regulated by BMPs. The data illustrate that, although the top of the neural crest migration hierarchy is the same, downstream mechanisms differ. Not-so-“bad” cholesterol (Dev Dyn 236:2708–2712) Ever get a craving for fried food? You are not the only one. Work by Zhao and colleagues suggests that, at times, oligodendrocytes cannot get enough “bad” cholesterol either. Differentiated oligodendrocytes form a cholesterol-prich myelin sheath that enhances conduction of electrical signals along axons. One way that central nervous system glia procure cholesterol is by synthesizing their own. Here, the authors find evidence that the cells also uptake cholesterol from the surrounding environment. Mature oligodendrocytes express very low-density lipoprotein receptors (VLDLRs) and LDLRs in the spinal cord and brain, with some regional differences between the two. Highest expression of both receptors coincides with the peak of myelination in mouse spinal cords. These findings suggest that oligodendrocytes endocytose LDL and VLDL, and use them to build the myelin sheath. Apparently oligodendrocytes cannot get too much of a bad, er good, thing. Feast or famine? (Dev Dyn 236:2792–2799) In the face of an unpredictable food supply, the body's ability to adjust energy usage and storage is integral to survival. Energy homeostasis is a product of crosstalk between specific neurons in the hypothalamus and brainstem, and cues from the periphery, such as adipose tissue. Simon et al. show that, in the absence of the homeobox gene Sax2, these signals get crossed. Despite sufficient intake of food, energy stores in 2-week-old Sax2 null pups are depleted. They fail to incorporate lipids into white and brown adipose tissues, and liver glycogen stores and blood glucose levels are decreased. Consistent with a state of fasting, mRNA levels of neuropeptide Y in the hindbrain are low, whereas pro-opiomelanocortin levels are high. By contrast, serotonin mRNA levels are high, indicative of satiation. Because Sax2 is expressed in the brainstem close to serotonergic neurons, and not in affected peripheral tissues, the authors propose that the imbalance is due to dysregulation of central nervous system energy homeostasis signals by Sax2. Future studies promise to determine specifically where Sax2 fits into the energy homeostasis pathway.

Abstract 582: Cardiac Fibroblast Bridges Conduct Electrical Signals between Myocyte Islands.

In this study, the electrophysiological properties of fibroblasts cocultured with cardiomyocytes were characterized using a novel cell micropatterning system. In the infarct area of a heart, the cardiac fibroblasts are known to increase collagen deposition leading to fibrosis. The fibrous tissue acts as an insulator in the damaged area and thus modulates local electrical signal conduction. This is fibroblast’s “passive role” in myocardium signal conduction. However, studies have shown that when placed in physical contact with viable myocytes, fibroblasts can form functional gap junctions and conduct electrical signal. This coupling phenomenon has been suggested as playing a role in re-entrant arrhythmias that occur in the infarct to healthy tissue border zone but there is still little known about the actual involvement fibroblasts have in the heart’s electrical communication. Our interest is in designing an in vitro model using cell micropatterning to study and characterize the electrophysiological properties of fibroblasts cocultured with myocytes. In this study we evaluate the hypothesis that fibroblast will conduct electrical signal between myocytes but this conduction will be restricted by the number of fibroblast involved. A combination of photolithography and laser micropatterning is used to achieve cell micropatterning. Polydimethyl siloxane (PDMS) stencils, created by photolithography, are used to form the myocyte islands and laser micropatterning is used to form the fibroblast cell bridge connecting the myocyte islands. Various PDMS stencils which varied the distance between the myocyte islands were made enabling us to form fibroblast bridges of various lengths (50 um to 500 um). Dual patch clamp experiments were performed to evaluate action potential propagations along the fibroblast bridge and characterize its electrical capabilities. With our laser micropatterning we were able to form fibroblast conducting bridges up to 390 um. In conclusion, fibroblast bridges can conduct electrical signals over short distances however with the increase of the fibroblast bridge length, signal propagation delay also increased. This conduction delay phenomenon observed in vitro could explain the cause of reentrant arrhythmias in vivo.

Integration of blood flow control to skeletal muscle: key role of feed arteries.

Blood flow control reflects dynamic, integrated changes in the diameter of vessels that comprise resistance networks. Vasoconstriction and vasodilation can travel rapidly along the vessel wall via the conduction of electrical signals between endothelial and/or smooth muscle cells through gap junctions. Within the hamster cheek pouch, these conducted responses reflect complementary mechanisms for co-ordinating both increases and decreases in arteriolar diameter. In the hamster retractor muscle, vasodilation also conducts along arterioles and into feed arteries, yet vasoconstriction appears constrained to the site(s) of smooth muscle activation. Thus, mechanisms for co-ordinating vasomotor control in resistance networks can vary between tissues that differ in structure and function. The resistance vessels of the retractor (and other skeletal) muscle are richly innervated by sympathetic nerves, which are absent from the cheek pouch. Propagation along sympathetic nerves rapidly co-ordinates smooth muscle cell contraction throughout the resistance network by releasing noradrenaline along the innervation pathway. Passive extension of the retractor muscle activates periarteriolar sympathetic nerves. This activity propagates antidromically into feed arteries and may complement the central (autonomic) vasoconstrictor response to exercise. In a reciprocal manner, muscle contraction evokes arteriolar dilation that is conducted (i.e. 'ascends') into feed arteries and may thereby counteract sympathetic vasoconstriction. With feed arteries anatomically positioned to control blood flow into skeletal muscle, the integration of dilator and constrictor stimuli in these vessels is a key determinant of muscle blood flow during exercise.

Combination therapy with carvedilol and amiodarone in patients with severe heart failure.

Carvedilol and at least in some studies, amiodarone have been shown to improve symptoms and prognosis of patients with heart failure. There are no reports on the outcome of combined treatment with both drugs on top of angiotensin-converting enzyme inhibitors (ACEI), diuretics and digitalis. In 109 patients with severe heart failure submitted for heart transplantation at one single center between the years 1996 and 1998 [left ventricular ejection fraction (LVEF) 24.6+/-11%, 85% males, 52% idiopathic dilated cardiomyopathy (DCM), mean observation time 1. 9+/-0.4 years] a therapy with low-dose amiodarone (1000 mg/week) plus titrated doses of carvedilol (target 50 mg/day) was instituted. In addition, patients received a prophylactic dual chamber pacemaker (PM) in order to protect from bradycardia and for continuous holter monitoring. The devices were programmed in back-up mode with a basal rate of 40 i.p.m. with a hysteresis of 25%. Significantly, more patients were in sinus rhythm after 1 year than at study entry (85% vs. 63%, P<0.01). In 47 patients, under therapy over at least 1 year, the resting heart rate fell from 90+/-19 to 59+/-5 b.p.m. (P<0.001). Ventricular premature contractions in 24-h holter ECGs were suppressed from 1.0+/-3 to 0.1+/-0.3%/24 h (P167 b.p.m. detected by the pacemaker (1.2+/-2.8 episodes/patient/3 months vs. 0.3+/-0.8 episodes/patient/3 months after 1 year (P<0.01). The LVEF increased from 26+/-10 to 39+/-13% (P<0.001). NYHA class improved from 3. 17+/-0.3 to 1.8+/-0.6 (P<0.001) as well as right heart catheterization data. From the total cohort, seven patients (6%) developed symptomatic documented bradycardic rhythm disturbances requiring reprogramming of their pacemakers to DDD(R)/VVI(R) mode with higher basic rates. Two of these patients developed AV block, four sinu-atrial blocks or sinus bradycardia and one patient had bradycardic atrial fibrillation. During the observation period five patients died (3 sudden, 1 due to heart failure and 1 due to mesenteric infarction). Two patients had undergone heart transplants. The 1-year survival rate (Kaplan-Meier) without transplantation was 89%. Compared to historic control patients with amiodarone only (n=154) or without either agent (n=283) this rate was 64 and 57% (P<0.01). Heart failure patients benefit from a combined therapy with carvedilol and amiodarone resulting in a markedly improved NYHA stage, an increase in LV ejection fraction, a stabilization of sinus rhythm, a significant reduction in heart rate, a delay of electrical signal conduction and a suppression of ventricular ectopies. Approximately 6% of patients under such a regime became pacemaker-dependent in the first year. Compared to historic controls prognosis was better and the need for heart transplantation was lower. The exact role of either agent in combination or alone should be clarified in larger randomized studies.

Spread of vasodilatation and vasoconstriction along feed arteries and arterioles of hamster skeletal muscle.

1. In arterioles of the hamster cheek pouch, vasodilatation and vasoconstriction can spread via the conduction of electrical signals through gap junctions between cells that comprise the vessel wall. However, conduction in resistance networks supplying other tissues has received relatively little attention. In anaesthetized hamsters, we have investigated the spread of dilatation and constriction along feed arteries and arterioles of the retractor muscle, which is contiguous with the cheek pouch. 2. When released from a micropipette, acetylcholine (ACh) triggered vasodilatation that spread rapidly along feed arteries external to the muscle and arterioles within the muscle. Responses were independent of changes in wall shear rate, perivascular nerve activity, or release of nitric oxide, indicating cell-to-cell conduction. 3. Vasodilatation conducted without decrement along unbranched feed arteries, yet decayed markedly in arteriolar networks. Thus, branching of the conduction pathway dissipated the vasodilatation. 4. Noradrenaline (NA) or a depolarizing KCl stimulus evoked constriction of arterioles and feed arteries of the retractor muscle that was constrained to the vicinity of the micropipette. This behaviour contrasts sharply with the conduction of vasodilatation in these microvessels and with the conduction of vasoconstriction elicited by NA and KCl in cheek pouch arterioles. 5. Focal electrical stimulation produced constriction that spread rapidly along feed arteries and arterioles. These responses were inhibited by tetrodotoxin or prazosin, confirming the release of NA along perivascular sympathetic nerves, which are absent from arterioles studied in the cheek pouch. Thus, sympathetic nerve activity co-ordinated the contraction of smooth muscle cells as effectively as the conduction of vasodilatation co-ordinated their relaxation. 6. In the light of previous findings in the cheek pouch, the properties of vasoconstriction and vasodilatation in feed arteries and arterioles of the retractor muscle indicate that substantive differences can exist in the nature of signal transmission along microvessels of tissues that differ in structure and function.