Ion channels and ventricular arrhythmias: cellular and ionic mechanisms underlying the Brugada syndrome.

Brugada syndrome is characterized by ST segment elevation in the right precordial leads, V1-V3 (unrelated to ischemia or structural disease), normal QT intervals, apparent right bundle branch block, and sudden cardiac death, particularly in men of Asian origin. An autosomal dominant mode of inheritance with variable expression has been described. The only gene thus far linked to the Brugada syndrome is the cardiac sodium channel gene, SCN5A. The possible cellular and ionic basis for these features of the Brugada syndrome are discussed. Strong sodium channel block, among other modalities, has been shown to be capable of inducing epicardial and transmural dispersion of repolarization, thus providing the substrate for the development of phase 2 and circus movement reentry, which underlies ventricular tachycardia/ventricular fibrillation.     

Ion channels

Insulin secretion by the pancreatic Beta cell is dependent upon transmembrane ion fluxes gated by the ATP-regulated potassium channel and the voltage regulated, L-type calcium channel. This work group examined major recent advances in the structure and modulation of ion channels and how those advances may pertain to the physiology of insulin secretion and the pharmacological treatment of Type 2 (non-insulin-dependent) diabetes mellitus. Structural studies have revealed that voltage gated ion channels are related, complex, and comprised of multiple components: sodium channels consist of three distinct subunits. L-type calcium channels, crucial to the insulin secretory response are structurally related to the sodium channel but contain additional subunits. Potassium channels are less closely related and appear to function as homotetramers. Modulation of ion channel activity is similarly complex: site specific phosphorylation by multiple protein kinases under the control of several intracellular second messenger systems may increase or decrease conductance. Subunit composition and relatively stable changes in the modal state of ion channels also appear to be critical to ion channel gating properties. Functional studies of the Beta-cell ATP-regulated potassium channel suggest two distinct nucleotide binding sites which link this channel to the metabolic state of the Beta cell. The multiple paths of ion channel modulation provide multiple targets for therapeutic intervention. Where detailed characterisation of ion channel structure has been achieved, those targets are being used for specific drug design. Such complete characterisation has not yet been achieved for Beta-cell ion channels and this presents a major goal for diabetes research.     

离子通道与缺氧性肺血管收缩

氧敏感性离子通道广泛表达于肺动脉平滑肌细胞,它们在低氧时决定血管的紧张度,参与缺氧性肺血管收缩的发生.本文就急性和慢性缺氧性肺血管收缩时钾离子和钙离子发挥的作用作一简单综述.     

Ion channels gated by heat

All animals need to sense temperature to avoid hostile environments and to regulate their internal homeostasis. A particularly obvious example is that animals need to avoid damagingly hot stimuli. The mechanisms by which temperature is sensed have until recently been mysterious, but in the last couple of years, we have begun to understand how noxious thermal stimuli are detected by sensory neurons. Heat has been found to open a nonselective cation channel in primary sensory neurons, probably by a direct action. In a separate study, an ion channel gated by capsaicin, the active ingredient of chili peppers, was cloned from sensory neurons. This channel (vanilloid receptor subtype 1, VR1) is gated by heat in a manner similar to the native heat-activated channel, and our current best guess is that this channel is the molecular substrate for the detection of painful heat. Both the heat channel and VR1 are modulated in interesting ways. The response of the heat channel is potentiated by phosphorylation by protein kinase C, whereas VR1 is potentiated by externally applied protons. Protein kinase C is known to be activated by a variety of inflammatory mediators, including bradykinin, whereas extracellular acidification is characteristically produced by anoxia and inflammation. Both modulatory pathways are likely, therefore, to have important physiological correlates in terms of the enhanced pain (hyperalgesia) produced by tissue damage and inflammation. Future work should focus on establishing, in molecular terms, how a single ion channel can detect heat and how the detection threshold can be modulated by hyperalgesic stimuli.     

Ion channels in airway afferent neurons

Action potentials initiated at the peripheral terminal of an afferent nerve are conducted to the central nervous system therein causing release of neurotransmitters that excite secondary neurons in the brain stem or spinal cord. Various chemicals, extremes in osmolarity and pH as well as mechanical stimuli are sensed by primary afferent nerves that innervate the airways. The processes leading to excitation of afferent nerve endings, conduction of action potentials along axons, transmitter secretion, and neuronal excitability are regulated by ions flowing through channels in the nerve membrane. Voltage-gated ion channels selective for K+ and Na+ ions allow the generation and conduction of action potentials and along with families of ion channels selective for other ions such as Ca2+ or Cl- are thought to play distinctive roles in regulating neuronal excitability and transmitter secretion. Here we discuss, in general terms, the roles played by various classes of ion channels in the activation, neurotransmitter secretion and excitability of primary afferent neurons.     

Ion channels in pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is a hemodynamic abnormality that ultimately results in mortality due to right heart failure. Although the clinical manifestations of primary and secondary PAH are diverse, medial hypertrophy and arterial vasoconstriction are key components in the vascular remodeling leading to PAH. Abnormalities in the homeostasis of intracellular Ca(2+), transmembrane flux of ions, and membrane potential may play significant roles in the processes leading to pulmonary vascular remodeling. Decreased activity of K(+) channels causes membrane depolarization, leading to Ca(2+) influx. The elevated cytoplasmic Ca(2+) is a major trigger for pulmonary vasoconstriction and an important stimulus for vascular smooth muscle proliferation. Dysfunctional K(+) channels have also been linked to inhibition of apoptosis and contribute further to the medial hypertrophy. This review focuses on the relative role of K(+) and Ca(2+) ions and channels in human pulmonary artery smooth muscle cells in the development of PAH.     

Molecular biology of sodium channels and their role in cardiac arrhythmias

The sodium channel is an integral membrane protein that plays a central role in conduction of the cardiac impulse in working cardiac myocytes and cells of the His-Purkinje system. The channel has two fundamental properties, ion conduction and gating. Specific domains of the channel protein control each of these functions. Ion conduction describes the mechanisms of the selective movement of sodium ion across the pore in the cell membrane. The selectivity of the channel for sodium ions is at least 10 times greater than that for other monovalent cations; the channel does not normally conduct divalent cations. Gating describes the opening and closing of the sodium channel pore. Sodium channels open transiently during membrane depolarization and close by a process termed inactivation. The cardiac sodium channel protein is a multimeric complex consisting of an alpha and an auxiliary beta-subunit. The genes encoding the sodium channel have been cloned and sequenced. The alpha subunit gene, SCN5A is sufficient to express a functional channel. However, beta subunit co-expression increases the level of channel expression and alters the voltage dependence of inactivation. Mutations of the sodium channel may result in incomplete inactivation during maintained depolarization, a decrease in the level of channel expression or acceleration of inactivation. The resulting clinical phenotypes include long QT syndrome, type III (LQT III), Brugada syndrome, and heart block. LQT III and Brugada syndromes have a high case fatality rate and are best treated with an implantable defibrillator.     

Molecular biology of K(+) channels and their role in cardiac arrhythmias

The configuration of cardiac action potentials varies considerably from one region of the heart to another. These differences are caused by differential cellular expression of several types of K(+) channel genes. The channels encoded by these genes can be grouped into several classes depending on the stimulus that permits the channels to open and conduct potassium ions. K(+) channels are activated by changes in transmembrane voltage or binding of ligands. Voltage-gated channels are normally the most important players in determining the shape and duration of action potentials and include the delayed rectifiers and the transient outward potassium channels. Ligand-gated channels include those that probably have only minor roles in shaping repolarization under normal conditions but, when activated by extracellular acetylcholine or a decrease in the intracellular concentration of ATP, can substantially shorten action potential duration. Inward rectifier K(+) channels are unique in that they are basically stuck in the open state but can be blocked in a voltage-dependent manner by intracellular Mg(2+), Ca(2+), and polyamines. Other K(+) channels have been described that provide a small background leak conductance. Many of these cardiac K(+) channels have been cloned in the past decade, permitting detailed studies of the molecular basis of their function and facilitating the discovery of the molecular basis of several forms of congenital arrhythmias. Drugs that block cardiac K(+) channels and prolong action potential duration have been developed as antiarrhythmic agents. However, many of these same drugs, as well as other common medications that are structurally unrelated, can also cause long QT syndrome and induce ventricular arrhythmia.     

兔界嵴细胞离子通道特性研究

目的研究生理状态下及异丙肾上腺素灌流对兔界嵴(CT)与梳状肌(PM)细胞动作电位(AP)及钠电流(INa)、短暂外向钾电流(Ito)、L型钙电流(ICa-L)、延迟整流钾电流(IK)及内向整流性钾电流(IK1)的影响,探讨CT与房性心律失常的关系。方法酶解法分离兔CT及PM细胞,利用全细胞膜片钳技术,记录生理状态下及异丙肾上腺素灌流后CT与PM细胞AP及INa、Ito、ICa-L、IK及IK1的变化。结果①生理状态下,CT细胞动作电位时程(APD)较长,可见明显的平台期;PM细胞AP形态与普通心房肌细胞相似,1期复极迅速,平台期短,类似三角形。②生理状态下,CT细胞Ito电流密度比PM细胞明显降低(7.13±0.38 pA/pF vs 10.70±0.62 pA/pF,n=9,P<0.01),而INa、Ito、ICa-L、IK及IK1则无明显差别。③异丙肾上腺素灌流时CT与PM细胞APD20、APD50、APD90均延长(n=8,P<0.01);指令电位+50 mV时,CT与PM细胞Ito电流密度均减少(n=9,P<0.01)而IK均增加(n=8,P<0.05);指令电位+10 mV时,CT与PM细胞ICa-L电流密度均增加(n=9,P<0.01);IK1在两种心肌细胞均无明显差异。结论 CT与PM细胞AP差异与Ito有关。异丙肾上腺素灌流时ICa-L与IK增强,Ito抑制使CT与PM细胞APD延长,触发机制可能是CT参与房性心律失常的机制之一。     

Ion channels in the metabolic regulation of respiratory cells

This article has focused on the characteristics of ion channels in cells of the respiratory system. Ion channels and their role in transepithelial fluid movement are best understood in tracheal epithelial cells. The bulk of the evidence indicates that a Cl- channel abnormality is etiologically involved in CF. In other cell types, such as isolated type II alveolar epithelial and vascular endothelial cells, ion channels have been described, but their functional significance is only incompletely understood. Finally, the majority of cells in the lungs has yet to be studied electrophysiologically. It is hoped that eventually studies of channel properties may enable investigators to determine how the channels affect cell function.     

Ion sensing in the RCK1 domain of BK channels

BK-type K(+) channels are activated by voltage and intracellular Ca(2+), which is important in modulating muscle contraction, neural transmission, and circadian pacemaker output. Previous studies suggest that the cytosolic domain of BK channels contains two different Ca(2+) binding sites, but the molecular composition of one of the sites is not completely known. Here we report, by systematic mutagenesis studies, the identification of E535 as part of this Ca(2+) binding site. This site is specific for binding to Ca(2+) but not Cd(2+). Experimental results and molecular modeling based on the X-ray crystallographic structures of the BK channel cytosolic domain suggest that the binding of Ca(2+) by the side chains of E535 and the previously identified D367 changes the conformation around the binding site and turns the side chain of M513 into a hydrophobic core, providing a basis to understand how Ca(2+) binding at this site opens the activation gate of the channel that is remotely located in the membrane.     

Ion channels in synaptic vesicles from Torpedo electric organ.

A simple method has been developed for fusing synaptic vesicles into spherical structures 20-50 micron in diameter. The method has been applied to purified cholinergic synaptic vesicles from Torpedo electric organ, and the membrane properties of these fused structures have been studied by the "cell"-attached version of the patch clamp technique. A large conductance potassium-preferring channel, termed the P channel, was consistently observed in preparations of fused synaptic vesicles. The selectivity of the channel for potassium over sodium was approximately equal to 2.8-fold. Two major conductance levels were observed during P-channel activity, and their relative proportion was dependent on the voltage applied to the membrane through the patch pipette. P channels were not seen in fused preparations of purified Torpedo lipids, nor was the frequency of their occurrence increased in preparations enriched with plasma membrane or nonvesicular membranes. We suggest, therefore, that the P channels are components of the synaptic vesicle membrane. Their function in synaptic transmission physiology is still unknown.     

Ion channels in transit: voltage-gated Na and K channels in axoplasmic organelles of the squid Loligo pealei.

Ion channels that give rise to the excitable properties of the neuronal plasma membrane are synthesized, transported, and degraded in cytoplasmic organelles. To determine whether plasma membrane ion channels from these organelles could be physiologically activated, we extruded axoplasm from squid giant axons, dissociated organelles from the cytoskeletal matrix, and fused the free organelles with planar lipid bilayers. Three classes of ion channels normally associated with the plasma membrane were identified based on conductance, selectivity, and gating properties determined from steady-state single-channel recordings: (i) voltage-dependent Na channels, (ii) voltage-dependent delayed rectifier K channels, and (iii) large, voltage-independent K channels. The identity of the delayed rectifier channels was confirmed by reconstructing the time course of activation from single-channel responses to depolarizing voltage steps applied across the bilayer. These observations suggest that several classes of plasma membrane ion channels are transported in cytoplasmic organelles in physiologically active forms.