Sequence and functional expression in Xenopus oocytes of a human insulinoma and islet potassium channel

Potassium channels as molecular targets of endocannabinoids

Endocannabinoids are a group of endogenous mediators derived from membrane lipids, which are implicated in a wide variety of physiological functions such as blood pressure regulation, immunity, pain, memory, reward, perception, reproduction, and sleep. N-Arachidonoylethanolamine (anandamide; AEA) and 2-arachidonoylglycerol (2-AG) represent two major endocannabinoids in the human body and they exert many of their cellular and organ system effects by activating the G_i/o protein-coupled, cannabinoid type 1 (CB1) and type 2 (CB2) receptors. However, not all effects of cannabinoids are ascribable to their interaction with CB1 and CB2 receptors; indeed, macromolecules like other types of receptors, ion channels, transcription factors, enzymes, transporters, and cellular structure have been suggested to mediate the functional effects of cannabinoids. Among the proposed molecular targets of endocannabinoids, potassium channels constitute an intriguing group, because these channels not only are crucial in shaping action potentials and controlling the membrane potential and cell excitability, thereby regulating a wide array of physiological processes, but also serve as potential therapeutic targets for the treatment of cancer and metabolic, neurological and cardiovascular disorders. This review sought to survey evidence pertaining to the CB1 and CB2 receptor-independent actions of endocannabinoids on ion channels, with an emphasis on AEA and potassium channels. To better understand the functional roles as well as potential medicinal uses of cannabinoids in human health and disease, further mechanistic studies to delineate interactions between various types of cannabinoids and ion channels, including members in the potassium channel superfamily, are warranted.

Bioelectric gene and reaction networks: computational modelling of genetic, biochemical and bioelectrical dynamics in pattern regulation

Gene regulatory networks (GRNs) describe interactions between gene products and transcription factors that control gene expression. In combination with reaction-diffusion models, GRNs have enhanced comprehension of biological pattern formation. However, although it is well known that biological systems exploit an interplay of genetic and physical mechanisms, instructive factors such as transmembrane potential (Vmem) have not been integrated into full GRN models. Here we extend regulatory networks to include bioelectric signalling, developing a novel synthesis: the bioelectricity-integrated gene and reaction (BIGR) network. Using in silico simulations, we highlight the capacity for Vmem to alter steady-state concentrations of key signalling molecules inside and out of cells. We characterize fundamental feedbacks where Vmem both controls, and is in turn regulated by, biochemical signals and thereby demonstrate Vmem homeostatic control, Vmem memory and Vmem controlled state switching. BIGR networks demonstrating hysteresis are identified as a mechanisms through which more complex patterns of stable Vmem spots and stripes, along with correlated concentration patterns, can spontaneously emerge. As further proof of principle, we present and analyse a BIGR network model that mechanistically explains key aspects of the remarkable regenerative powers of creatures such as planarian flatworms. The functional properties of BIGR networks generate the first testable, quantitative hypotheses for biophysical mechanisms underlying the stability and adaptive regulation of anatomical bioelectric pattern.

Rate-Dependent Role of IKur in Human Atrial Repolarization and Atrial Fibrillation Maintenance

The atrial-specific ultrarapid delayed rectifier K⁺ current (I_Kur) inactivates slowly but completely at depolarized voltages. The consequences for I_Kur rate-dependence have not been analyzed in detail and currently available mathematical action-potential (AP) models do not take into account experimentally observed I_Kur inactivation dynamics. Here, we developed an updated formulation of I_Kur inactivation that accurately reproduces time-, voltage-, and frequency-dependent inactivation. We then modified the human atrial cardiomyocyte Courtemanche AP model to incorporate realistic I_Kur inactivation properties. Despite markedly different inactivation dynamics, there was no difference in AP parameters across a wide range of stimulation frequencies between the original and updated models. Using the updated model, we showed that, under physiological stimulation conditions, I_Kur does not inactivate significantly even at high atrial rates because the transmembrane potential spends little time at voltages associated with inactivation. Thus, channel dynamics are determined principally by activation kinetics. I_Kur magnitude decreases at higher rates because of AP changes that reduce I_Kur activation. Nevertheless, the relative contribution of I_Kur to AP repolarization increases at higher frequencies because of reduced activation of the rapid delayed-rectifier current I_Kr. Consequently, I_Kur block produces dose-dependent termination of simulated atrial fibrillation (AF) in the absence of AF-induced electrical remodeling. The inclusion of AF-related ionic remodeling stabilizes simulated AF and greatly reduces the predicted antiarrhythmic efficacy of I_Kur block. Our results explain a range of experimental observations, including recently reported positive rate-dependent I_Kur-blocking effects on human atrial APs, and provide insights relevant to the potential value of I_Kur as an antiarrhythmic target for the treatment of AF.

Potassium currents of olfactory bulb juxtaglomerular cells: characterization, simulation, and implications for plateau potential firing

Odor identity is encoded by the activity of olfactory bulb glomeruli, which receive primary sensory input and transfer it to projection neurons. Juxtaglomerular cells (JGCs) may influence glomerular processing via firing of long lasting plateau potentials. Though inward currents have been investigated, little is known regarding potassium current contribution to JGC plateau potentials. We pursued study of these currents, with the overarching goal of creating components for a computational model of JGC plateau potential firing. In conditions minimizing calcium-activated potassium current (I(K(Ca))), we used whole cell voltage clamp and in vitro slice preparations to characterize three potassium currents in rat JGCs. The prominent component I(kt1) displayed rapid kinetics (τ(10%-90% rise), 0.6-2 ms; τ(inactivation), 5-10 ms) and was blocked by high concentration 4-aminopyridine (4-AP) (5 mM) and tetramethylammonium (TEA) (40 mM). It had half maximal activation at -10 mV (V(½)max) and little inactivation at rest. I(kt2), with slower kinetics (τ(10%-90% rise), 11-15 ms; τ(inactivation), 100-300 ms), was blocked by low concentration 4-AP (0.5 mM) and TEA (5 mM). The V(½)max was 0 mV and inactivation was also minimal at rest. Sustained current I(kt3) showed sensitivity to low concentration 4-AP and TEA, and had V(½)max of +10 mV. Further experiments, in conditions of physiologic calcium buffering, suggested that I(K(Ca)) contributed to I(kt3) with minimal effect on plateau potential evolution. We transformed these characterizations into Hodgkin-Huxley models that robustly mimicked experimental data. Further simulation demonstrated that I(kt1) would be most efficiently activated by plateau potential waveforms, predicting a critical role in shaping JGC firing. These studies demonstrated that JGCs possess a unique potassium current profile, with delayed rectifier (I(kt3)), atypical A-current (I(kt1)), and D-current (I(kt2)) in accordance with known expression patterns in olfactory bulb (OB) glomeruli. Our simulations also provide an initial framework for more integrative models of JGC plateau potential firing.

Kv1.3: a potential pharmacological target for diabetes

K(+) channels, which are ubiquitous membrane proteins, play a central role in regulating the resting membrane potential and the shape and duration of the action potential in pancreatic beta-cells. There are at least three types of K(+) channels (K(ATP), K(Ca), and Kv2.1 channels) that are involved in glucose-stimulated insulin secretion in pancreatic beta-cells, and one type (Kv1.3) that is associated with the regulation of insulin sensitivity in peripheral target tissues. This article reviews the function of Kv1.3 channels that contribute to mediating insulin action in insulin-sensitive tissues. Pharmacological strategies for targeting Kv1.3 are then discussed with a focus on a rationale for the potential therapeutic use of Kv1.3 blocker in diabetic treatment.

Electrophysiology of islet cells

Stimulus-Secretion Coupling (SSC) of pancreatic islet cells comprises electrical activity. Changes of the membrane potential (V(m)) are regulated by metabolism-dependent alterations in ion channel activity. This coupling is best explored in beta-cells. The effect of glucose is directly linked to mitochondrial metabolism as the ATP/ADP ratio determines the open probability of ATP-sensitive K(+) channels (K(ATP) channels). Nucleotide sensitivity and concentration in the direct vicinity of the channels are controlled by several factors including phospholipids, fatty acids, and kinases, e.g., creatine and adenylate kinase. Closure of K(ATP) channels leads to depolarization of beta-cells via a yet unknown depolarizing current. Ca(2+) influx during action potentials (APs) results in an increase of the cytosolic Ca(2+) concentration ([Ca(2+)](c)) that triggers exocytosis. APs are elicited by the opening of voltage-dependent Na(+) and/or Ca(2+) channels and repolarized by voltage- and/or Ca(2+)-dependent K(+) channels. At a constant stimulatory glucose concentration APs are clustered in bursts that are interrupted by hyperpolarized interburst phases. Bursting electrical activity induces parallel fluctuations in [Ca(2+)](c) and insulin secretion. Bursts are terminated by I(Kslow) consisting of currents through Ca(2+)-dependent K(+) channels and K(ATP) channels. This review focuses on structure, characteristics, physiological function, and regulation of ion channels in beta-cells. Information about pharmacological drugs acting on K(ATP) channels, K(ATP) channelopathies, and influence of oxidative stress on K(ATP) channel function is provided. One focus is the outstanding significance of L-type Ca(2+) channels for insulin secretion. The role of less well characterized beta-cell channels including voltage-dependent Na(+) channels, volume sensitive anion channels (VSACs), transient receptor potential (TRP)-related channels, and hyperpolarization-activated cyclic nucleotide-gated (HCN) channels is discussed. A model of beta-cell oscillations provides insight in the interplay of the different channels to induce and maintain electrical activity. Regulation of beta-cell electrical activity by hormones and the autonomous nervous system is discussed. alpha- and delta-cells are also equipped with K(ATP) channels, voltage-dependent Na(+), K(+), and Ca(2+) channels. Yet the SSC of these cells is less clear and is not necessarily dependent on K(ATP) channel closure. Different ion channels of alpha- and delta-cells are introduced and SSC in alpha-cells is described in special respect of paracrine effects of insulin and GABA secreted from beta-cells.

Action potentials and insulin secretion: new insights into the role of Kv channels

Coordinated electrical activity allows pancreatic beta-cells to respond to secretagogues with calcium entry followed by insulin secretion. Metabolism of glucose affects multiple membrane proteins including ion channels, transporters and pumps that collaborate in a cascade of electrical activity resulting in insulin release. Glucose induces beta-cell depolarization resulting in the firing of action potentials (APs), which are the primary electrical signal of the beta-cell. They are shaped by orchestrated activation of ion channels. Here we give an overview of the voltage-gated potassium (Kv) channels of the beta-cell, which are responsible in part for the falling phase of the AP, and how their regulation affects insulin secretion. beta cells contain several Kv channels allowing dynamic integration of multiple signals on repolarization of glucose-stimulated APs. Recent studies on Kv channel regulation by cAMP and arachidonic acid and on the Kv2.1 null mouse have greatly increased our understanding of beta-cell excitation-secretion coupling.

Innovative approaches to anti-arrhythmic drug therapy

Normal cardiac function requires an appropriate and regular beating rate (cardiac rhythm). When the heart rhythm is too fast or too slow, cardiac function can be impaired, with derangements that vary from mild symptoms to life-threatening complications. Irregularities, particularly those involving excessively fast or slow rates, constitute cardiac 'arrhythmias'. In the past, drug treatment of cardiac arrhythmias has proven difficult, both because of inadequate effectiveness and a risk of serious complications. However, a variety of recent advances have opened up exciting possibilities for the development of novel and superior approaches to arrhythmia therapy. This article will review recent progress and future prospects for treating two particularly important cardiac arrhythmias: atrial fibrillation and ventricular fibrillation.

Identification of novel Kv1.3 blockers using a fluorescent cell-based ion channel assay

A functional cell-based assay was developed using a generic proprietary assay protocol, based on a membrane-potential sensitive dye, for the identification of small-molecule antagonists against the Kv1.3 potassium ion channel. A high-throughput screen (HTS) was subsequently performed with 20,000 compounds from the Evotec library, preselected using known small molecule antagonists for both sodium and potassium ion channels. Following data analysis, the hit rate was measured at 1.72%, and subsequent dose-response analysis of selected hits showed a high hit confirmation rate yielding approximately 50 compounds with an apparent IC50 value lower than 10 microM. Subsequent electrophysiological characterization of selected hits confirmed the initial activity and potency of the identified hits on the Kv1.3 target and also selectivity toward Kv1.3 through measurements on HERG as well as Kv1.3-expressing cell lines. Follow-up structure-activity relationship analysis revealed a variety of different clusters distributed throughout the library as well as several singlicates. In comparison to known Kv1.3 blockers, new chemical entities and scaffolds showing potency and selectivity against the Kv1.3 ion channel were detected. In addition, a screening strategy for ion channel drug discovery HTS, medicinal chemistry, and electrophysiology is presented.

Delayed-rectifier (KV2.1) regulation of pancreatic beta-cell calcium responses to glucose: inhibitor specificity and modeling

The delayed-rectifier (voltage-activated) K(+) conductance (K(V)) in pancreatic islet beta-cells has been proposed to regulate plasma membrane repolarization during responses to glucose, thereby determining bursting and Ca(2+) oscillations. Here, we verified the expression of K(V)2.1 channel protein in mouse and human islets of Langerhans. We then probed the function of K(V)2.1 channels in islet glucose responses by comparing the effect of hanatoxin (HaTx), a specific blocker of K(V)2.1 channels, with a nonspecific K(+) channel blocker, tetraethylammonium (TEA). Application of HaTx (1 microM) blocked delayed-rectifier currents in mouse beta-cells, resulting in a 40-mV rightward shift in threshold of activation of the voltage-dependent outward current. In the presence of HaTx, there was negligible voltage-activated outward current below 0 mV, suggesting that K(V)2.1 channels form the predominant part of this current in the physiologically relevant range. We then employed HaTx to study the role of K(V)2.1 in the beta-cell Ca(2+) responses to elevated glucose in comparison with TEA. Only HaTx was able to induce slow intracellular Ca(2+) concentration ([Ca(2+)](i)) oscillations in cells stimulated with 20 mM glucose, whereas TEA induced an immediate rise in [Ca(2+)](i) followed by rapid oscillations. In human islets, HaTx acted in a similar fashion. The data were analyzed using a detailed mathematical model of ionic flux and Ca(2+) regulation in beta-cells. The results can be explained by a specific HaTx effect on the K(V) current, whereas TEA affects multiple K(+) conductances. The results underscore the importance of K(V)2.1 channel in repolarization of the pancreatic beta-cell plasma membrane and its role in regulating insulin secretion.

Effects of hyperthyroidism on delayed rectifier K+ currents in left and right murine atria

Hyperthyroidism has been associated with atrial fibrillation (AF); however, hyperthyroidism-induced ion channel changes that may predispose to AF have not been fully elucidated. To understand the electrophysiological changes that occur in left and right atria with hyperthyroidism, the patch-clamp technique was used to compare action potential duration (APD) and whole cell currents in myocytes from left and right atria from both control and hyperthyroid mice. Additionally, RNase protection assays and immunoblotting were performed to evaluate the mRNA and protein expression levels of K(+) channel alpha-subunits in left and right atria. The results showed that 1) in control mice, the APD was shorter and the ultra-rapid delayed rectifier K(+) conductance (I(Kur)) and the sustained delayed rectifier K(+) conductance (I(ss)) were larger in the left than in the right atrium; also, mRNA and protein expression levels of Kv1.5 and Kv2.1 were higher in the left atrium; 2) in hyperthyroid mice, the APD was shortened and I(Kur) and I(ss) were increased in both left and right atrial myocytes, and the protein expression levels of Kv1.5 and Kv2.1 were increased significantly in both atria; and 3) the influence of hyperthyroidism on APD and delayed rectifier K(+) currents was more prominent in right than in left atrium, which minimized the interatrial APD difference. In conclusion, hyperthyroidism resulted in more significant APD shortening and greater delayed rectifier K(+) current increases in the right vs. the left atrium, which can contribute to the propensity for atrial arrhythmia in hyperthyroid heart.

Homology Models of the Tetramerization Domain of Six Eukaryotic Voltage-gated Potassium Channels Kv1.1-Kv1.6

The homology models of the tetramerization (T1) domain of six eukaryotic potassium channels, Kv1.1-Kv1.6, were constructed based on the crystal structure of the Shaker T1 domain. The results of amino acid sequence alignment indicate that the T1 domains of these K+ channels are highly conserved, with the similarities varying from 77% between Shaker and Kv1.6 to 93% between Kv1.2 and Kv1.3. The homology models reveal that the T1 domains of these Kv channels exhibit similar folds as those of Shaker K+ channel. These models also show that each T1 monomer consists of three distinct layers, with N-terminal layer 1 and C-terminal layer 3 facing the cytoplasm and the membrane, respectively. Layer 2 exhibits the highest structural conservation because it is located around the central hydrophobic core. For each Kv channel, four identical subunits assemble into the homotetramer architecture around a four-fold axis through the hydrogen bonds and salt bridges formed by 15 highly conserved polar residues. The narrowest opening of the pore is formed by the four conserved residues corresponding to R115 of the Shaker T1 domain. The homology models of these Kv T1 domains provide particularly attractive targets for further structure-based studies.

A set of homology models of pore loop domain of six eukaryotic voltage-gated potassium channels Kv1.1-Kv1.6

Homology models of the pore loop domain of six eukaryotic potassium channels, Kv1.1-Kv1.6, were generated based on the crystallographic structure of KcsA. The results of amino acid sequence alignment indicate that these Kv channels are composed of two structurally and functionally independent domains: the N-terminal 'voltage sensor' domain and the C-terminal 'pore loop' domain. The homology models reveal that the pore loop domains of these Kv channels exhibit similar folds to those of KcsA. The structural features and specific packing of aromatic residues around the selectivity filter of these Kv channels are nearly identical to those of KcsA, whereas most of the structural variations occur in the turret as well as in the inner and outer helices. The distribution of polar and nonpolar side chains on the surfaces of the KcsA and Kv channels reveals that they exhibit a segregation of side chains common to most integral membrane proteins. As the hydrogen bond between Glu71 and Asp80 in KcsA plays an important role in stabilizing the channel, the substituted Val residue in the Kv family corresponding to Glu71 of KcsA stabilizes the channel by making hydrophobic contact with Tyr residue from the signature sequence of the selectivity filter. The homology models of these Kv channels provide particularly attractive subjects for further structure-based studies.

Expression of voltage-gated potassium channels in human and rhesus pancreatic islets

Voltage-gated potassium channels (Kv channels) are involved in repolarization of excitable cells. In pancreatic beta-cells, prolongation of the action potential by block of delayed rectifier potassium channels would be expected to increase intracellular free calcium and to promote insulin release in a glucose-dependent manner. However, the specific Kv channel subtypes responsible for repolarization in beta-cells, most importantly in humans, are not completely resolved. In this study, we have investigated the expression of 26 subtypes from Kv subfamilies in human islet mRNA. The results of the RT-PCR analysis were extended by in situ hybridization and/or immunohistochemical analysis on sections from human or Rhesus pancreas. Cell-specific markers were used to show that Kv2.1, Kv3.2, Kv6.2, and Kv9.3 are expressed in beta-cells, that Kv3.1 and Kv6.1 are expressed in alpha-cells, and that Kv2.2 is expressed in delta-cells. This study suggests that more than one Kv channel subtype might contribute to the beta-cell delayed rectifier current and that this current could be formed by heterotetramers of active and silent subunits.

Ileitis modulates potassium and sodium currents in guinea pig dorsal root ganglia sensory neurons

Intestinal inflammation induces hyperexcitability of dorsal root ganglia sensory neurons, which has been implicated in increased pain sensation. This study examined whether alteration of sodium (Na+) and/or potassium (K+) currents underlies this hyperexcitability. Ileitis was induced in guinea pig ileum with trinitrobenzene sulphonic acid (TBNS) and dorsal root ganglion neurons innervating the site of inflammation were identified by Fast Blue or DiI fluorescence labelling. Whole cell recordings were made from acutely dissociated small-sized neurons at 7-10 days. Neurons exhibited transient A-type and sustained outward rectifier K+ currents. Compared to control, both A-type and sustained K+ current densities were significantly reduced (42 and 34%, respectively; P < 0.05) in labelled neurons from the inflamed intestine but not in non-labelled neurons. A-type current voltage dependence of inactivation was negatively shifted in labelled inflamed intestine neurons. Neurons also exhibited tetrodotoxin-sensitive and resistant Na+ currents. Tetrodotoxin-resistant sodium currents were increased by 37% in labelled neurons from the inflamed intestine compared to control (P < 0.01), whereas unlabelled neurons were unaffected. The activation and inactivation curves of these currents were unchanged by inflammation. These data suggest ileitis increases excitability of intestinal sensory neurons by modulating multiple ionic channels. The lack of effect in non-labelled neurons suggests signalling originated at the nerve terminal rather than through circulating mediators and, given that Na+ currents are enhanced whereas K+ currents are suppressed, one or more signalling pathways may be involved.

Voltage-dependent K(+) channels in pancreatic beta cells: role, regulation and potential as therapeutic targets

Insulin secretion from pancreatic islet beta cells is acutely regulated by a complex interplay of metabolic and electrogenic events. The electrogenic mechanism regulating insulin secretion from beta cells is commonly referred to as the ATP-sensitive K(+) (K(ATP)) channel dependent pathway. Briefly, an increase in ATP and, perhaps more importantly, a decrease in ADP stimulated by glucose metabolism depolarises the beta cell by closing K(ATP) channels. Membrane depolarisation results in the opening of voltage-dependent Ca(2+) channels, and influx of Ca(2+) is the main trigger for insulin secretion. Repolarisation of pancreatic beta cell action potential is mediated by the activation of voltage-dependent K(+) (Kv) channels. Various Kv channel homologues have been detected in insulin secreting cells, and recent studies have shown a role for specific Kv channels as modulators of insulin secretion. Here we review the evidence supporting a role for Kv channels in the regulation of insulin secretion and discuss the potential and the limitations for beta-cell Kv channels as therapeutic targets. Furthermore, we review recent investigations of mechanisms regulating Kv channels in beta cells, which suggest that Kv channels are active participants in the regulation of beta-cell electrical activity and insulin secretion.

Molecular dissection of the contribution of negatively and positively charged residues in S2, S3, and S4 to the final membrane topology of the voltage sensor in the K+ channel, KAT1

Voltage-dependent ion channels control changes in ion permeability in response to membrane potential changes. The voltage sensor in channel proteins consists of the highly positively charged segment, S4, and the negatively charged segments, S2 and S3. The process involved in the integration of the protein into the membrane remains to be elucidated. In this study, we used in vitro translation and translocation experiments to evaluate interactions between residues in the voltage sensor of a hyperpolarization-activated potassium channel, KAT1, and their effect on the final topology in the endoplasmic reticulum (ER) membrane. A D95V mutation in S2 showed less S3-S4 integration into the membrane, whereas a D105V mutation allowed S4 to be released into the ER lumen. These results indicate that Asp(95) assists in the membrane insertion of S3-S4 and that Asp(105) helps in preventing S4 from being releasing into the ER lumen. The charge reversal mutation, R171D, in S4 rescued the D105R mutation and prevented S4 release into the ER lumen. A series of constructs containing different C-terminal truncations of S4 showed that Arg(174) was required for correct integration of S3 and S4 into the membrane. Interactions between Asp(105) and Arg(171) and between negative residues in S2 or S3 and Arg(174) may be formed transiently during membrane integration. These data clarify the role of charged residues in S2, S3, and S4 and identify posttranslational electrostatic interactions between charged residues that are required to achieve the correct voltage sensor topology in the ER membrane.

Expression of voltage-dependent potassium channels in the developing visual system of Xenopus laevis

Accumulating evidence suggests that voltage-dependent potassium (Kv) channels have important and varied roles in the development of neuronal and non-neuronal cell types. They have been implicated in processes such as proliferation, cell adhesion, migration, neurite outgrowth, and axon guidance. In this study, we used antibodies against several electrically active Kv channel alpha-subunits (Kv1-4) to describe the spatial and temporal expression patterns of Kv channel subunits in Xenopus laevis retinal ganglion cell (RGC) somata, axons, and growth cones. We found that RGCs express Kv1.3-, Kv1.5-, Kv3.4-, and Kv4.2-like subunits. Each subunit displayed unique cellular and subcellular distributions. Moreover, the expression patterns changed considerably over the major period of Xenopus retinal cell genesis and differentiation. Weak or no immunoreactivity was observed with antibodies against Kv1.1, Kv1.2, Kv1.4, Kv1.6, and Kv3.2 subunits in RGCs or other retinal cell types. In support of our previous pharmacologic evidence implicating Kv channels in RGC axon outgrowth, we found that Kv1.5-, Kv3.4-, and Kv4.2-like proteins, but not Kv1.3-like subunits, are abundantly expressed in RGC growth cones.

Cardiac ion channels

The normal electrophysiologic behavior of the heart is determined by ordered propagation of excitatory stimuli that result in rapid depolarization and slow repolarization, thereby generating action potentials in individual myocytes. Abnormalities of impulse generation, propagation, or the duration and configuration of individual cardiac action potentials form the basis of disorders of cardiac rhythm, a continuing major public health problem for which available drugs are incompletetly effective and often dangerous. The integrated activity of specific ionic currents generates action potentials, and the genes whose expression results in the molecular components underlying individual ion currents in heart have been cloned. This review discusses these new tools and how their application to the problem of arrhythmias is generating new mechanistic insights to identify patients at risk for this condition and developing improved antiarrhythmic therapies.

TRP genes: candidates for nonselective cation channels and store-operated channels in insulin-secreting cells

Nonselective cation channels may play a role in insulin secretion by regulating pancreatic beta-cell plasma membrane potential, Ca(2+) homeostasis, and thereby glucose signaling. Transient receptor potential channel (TRPC)-related genes encode nonselective cation channels, some of which are similar to those described for beta-cells. Some TRPC-like channels are activated via G-protein--coupled mechanisms, some have been reported to be calcium-store-operated channels (SOC), and others are activated by novel signaling molecules or are sensitive to pressure and osmotic strength. Here we report the cloning and expression of mSTRPC4 from a mouse insulinoma cDNA library. mSTRPC4 encoded a protein of 97 kd, expressed in both endocrine cells and the brain. Stable cell lines expressing mSTRPC4 showed abundant mSTRPC4 protein, but no reproducible currents could be detected. mSTRPC4 therefore probably functions as a heteromultimer. We also report that LTRPC2, a G-protein and adenosine 5'-diphosphoribose (ADPR)-activated nonselective cation channel, is also expressed in human islets. TRPC-like channels may provide a pathway for depolarization or Ca(2+) entry in beta-cells and may be interesting targets for manipulating beta-cell function.

Evaluation of functional interaction between K(+) channel alpha- and beta-subunits and putative inactivation gating by Co-expression in Xenopus laevis oocytes

Animal K(+) channel alpha- (pore-forming) subunits form native proteins by association with beta-subunits, which are thought to affect channel function by modifying electrophysiological parameters of currents (often by inducing fast inactivation) or by stabilizing the protein complex. We evaluated the functional association of KAT1, a plant K(+) channel alpha-subunit, and KAB1 (a putative homolog of animal K(+) channel beta-subunits) by co-expression in Xenopus laevis oocytes. Oocytes expressing KAT1 displayed inward-rectifying, non-inactivating K(+) currents that were similar in magnitude to those reported in prior studies. K(+) currents recorded from oocytes expressing both KAT1 and KAB1 had similar gating kinetics. However, co-expression resulted in greater total current, consistent with the possibility that KAB1 is a beta-subunit that stabilizes and therefore enhances surface expression of K(+) channel protein complexes formed by alpha-subunits such as KAT1. K(+) channel protein complexes formed by alpha-subunits such as KAT1 that undergo (voltage-dependent) inactivation do so by means of a "ball and chain" mechanism; the ball portion of the protein complex (which can be formed by the N terminus of either an alpha- or beta-subunit) occludes the channel pore. KAT1 was co-expressed in oocytes with an animal K(+) channel alpha-subunit (hKv1.4) known to contain the N-terminal ball and chain. Inward currents through heteromeric hKv1. 4:KAT1 channels did undergo typical voltage-dependent inactivation. These results suggest that inward currents through K(+) channel proteins formed at least in part by KAT1 polypeptides are capable of inactivation, but the structural component facilitating inactivation is not present when channel complexes are formed by either KAT1 or KAB1 in the absence of additional subunits.

Identification, cloning and expression of rabbit vascular smooth muscle Kv1.5 and comparison with native delayed rectifier K+ current

1. The molecular basis of voltage-gated, delayed rectifier K+ (KDR) channels in vascular smooth muscle cells is poorly defined. In this study we employed (i) an antibody against Kv1.5 and (ii) a cDNA clone encoding Kv1.5 derived from rabbit portal vein (RPV) to demonstrate Kv1.5 expression in RPV and to compare the properties of RPVKv1.5 expressed in mammalian cells with those of native RPV KDR current. 2. Expression of Kv1.5 channel protein in RPV was demonstrated by (i) immunocytolocalization of an antibody raised against a C-terminal epitope of mouse cardiac Kv1.5 in permeabilized, freshly isolated RPV smooth muscle cells and (ii) isolation of a cDNA clone encoding RPVKv1.5 by reverse transcription-polymerase chain reaction (RT-PCR) using mRNA derived from endothelium-denuded and adventitia-free RPV. 3. RPVKv1.5 cDNA was expressed in mammalian L cells and human embryonic kidney (HEK293) cells and the properties of the expressed channels compared with those of native KDR channels of freshly dispersed myocytes under identical conditions. 4. The kinetics and voltage dependence of activation of L cell-expressed RPVKv1.5 and native KDR current were identical, as were the kinetics of recovery from inactivation and single channel conductance. In contrast, there was little similarity between HEK293 cell-expressed RPVKv1.5 and native KDR current. 5. Inactivation occurred with the same voltage for half-maximal availability, but the kinetics and slope constant for the voltage dependence of inactivation for L cell-expressed RPVKv1.5 and the native current were different: slow time constants were 6.5 +/- 0.6 and 3.5 +/- 0.4 s and slope factors were 4.7 +/- 0.2 and 7.0 +/- 0.8 mV, respectively. 6. This study provides immunofluorescence and functional evidence that Kv1.5 alpha-subunits are a component of native KDR channels of vascular smooth muscle cells of RPV. However, the differences in kinetics and voltage sensitivity of inactivation between L cell- and HEK293 cell-expressed channels and native KDR channels provide functional evidence that vascular KDR current is not due to homomultimers of RPV Kv1.5 alone. The channel structure may be more complex, involving heteromultimers and modulatory Kvbeta-subunits, and/or native KDR current may have other components involving Kvalpha-subunits of other families.

Modulation of slow inactivation in human cardiac Kv1.5 channels by extra- and intracellular permeant cations

1. The properties and regulation of slow inactivation by intracellular and extracellular cations in the human heart K+ channel hKv1.5 have been investigated. Extensive NH2- and COOH-terminal deletions outside the central core of transmembrane domains did not affect the degree of inactivation. 2. The voltage dependence of steady-state inactivation curves of hKv1.5 channels was unchanged in Rb+ and Cs+, compared with K+, but biexponential inactivation over 10 s was reduced from approximately 100 % of peak current in Na+ to approximately 65 % in K+, approximately 50 % in Rb+ and approximately 30 % in Cs+. This occurred as a result of a decrease in both fast and slow components of inactivation, with little change in inactivation time constants. 3. Changes in extracellular cation species and concentration (5-300 mM) had only small effects on the rates of inactivation and recovery from inactivation (tau recovery approximately 1 s). Mutation of residues at a putative regulatory site at R487 in the outer pore mouth did not affect slow inactivation or recovery from inactivation of hKv1.5, although sensitivity to extracellular TEA was conferred. 4. Symmetrical reduction of both intra- and extracellular cation concentrations accelerated and augmented both components of inactivation of K+ (Kd = 34.7 mM) and Cs+ (Kd = 20.5 mM) currents. These effects could be quantitatively accounted for by unilateral reduction of intracellular K+ (K+i) (Kd = 43.4 mM) or Cs+i with constant 135 mM external ion concentrations. 5. We conclude that inactivation and recovery from inactivation in hKv1.5 were not typically C-type in nature. However, the ion species dependence of inactivation was still closely coupled to ion permeation through the pore. Intracellular ion modulatory actions were more potent than extracellular actions, although still of relatively low affinity. These results suggest the presence of ion binding sites capable of regulating inactivation located on both intracellular and extracellular sides of the pore selectivity filter.

Ultrarapid delayed rectifier current inactivation in human atrial myocytes: properties and consequences

The ultrarapid delayed rectifier current (IK,ur) plays a significant role in human atrial repolarization and is generally believed to show little rate dependence because of slow and partial inactivation. This study was designed to evaluate in detail the properties and consequences of IK,ur inactivation in isolated human atrial myocytes. IK,ur inactivated with a biexponential time course and a half-inactivation voltage of -7.5 +/- 0.6 mV (mean +/- SE), with complete inactivation during 50-s pulses to voltages positive to +10 mV (37 degreesC). Recovery from inactivation proceeded slowly, with time constants of 0.42 +/- 0.06 and 7.9 +/- 0.9 s at -80 mV (37 degreesC). Substantial frequency dependence was observed at 37 degreesC over a clinically relevant range of frequencies. Inactivation was faster and occurred at more positive voltages at 37 degreesC compared with room temperature. The voltage and time dependencies of Kv1.5 inactivation were studied in Xenopus oocytes to avoid overlapping currents and strongly resembled those of IK,ur in native myocytes. We conclude that, while IK,ur inactivation is slow, it is extensive, and slow recovery from inactivation confers important frequency dependence with significant consequences for understanding the role of IK,ur in human atrial repolarization.

Evidence for multiple open and inactivated states of the hKv1.5 delayed rectifier

The kinetic properties of hKv1.5, a Shaker-related cardiac delayed rectifier, expressed in Ltk- cells were studied. hKv1.5 currents elicited by membrane depolarizations exhibited a delay followed by biphasic activation. The biphasic activation remained after 5-s prepulses to membrane potentials between -80 and -30 mV; however, the relative amplitude of the slow component increased as the prepulse potential approached the threshold of channel activation, suggesting that the second component did not reflect activation from a hesitant state. The decay of tail currents at potentials between -80 and -30 mV was adequately described with a biexponential. The time course of deactivation slowed as the duration of the depolarizing pulse increased. This was due to a relative increase in the slowly decaying component, despite similar initial amplitudes reflecting a similar open probability after 50- and 500-ms prepulses. To further investigate transitions after the initial activated state, we examined the temperature dependence of inactivation. The time constants of slow inactivation displayed little temperature and voltage dependence, but the degree of the inactivation increased substantially with increased temperature. Recovery from inactivation proceeded with a biexponential time course, but long prepulses at depolarized potentials slowed the apparent rate of recovery from inactivation. These data strongly indicate that hKv1.5 has both multiple open states and multiple inactivated states.

Genomic organization, chromosomal localization, tissue distribution, and biophysical characterization of a novel mammalian Shaker-related voltage-gated potassium channel, Kv1.7

We report the isolation of a novel mouse voltage-gated Shaker-related K+ channel gene, Kv1.7 (Kcna7/KCNA7). Unlike other known Kv1 family genes that have intronless coding regions, the protein-coding region of Kv1.7 is interrupted by a 1.9-kilobase pair intron. The Kv1.7 gene and the related Kv3.3 (Kcnc3/KCNC3) gene map to mouse chromosome 7 and human chromosome 19q13.3, a region that has been suggested to contain a diabetic susceptibility locus. The mouse Kv1.7 channel is voltage-dependent and rapidly inactivating, exhibits cumulative inactivation, and has a single channel conductance of 21 pS. It is potently blocked by noxiustoxin and stichodactylatoxin, and is insensitive to tetraethylammonium, kaliotoxin, and charybdotoxin. Northern blot analysis reveals approximately 3-kilobase pair Kv1.7 transcripts in mouse heart and skeletal muscle. In situ hybridization demonstrates the presence of Kv1.7 in mouse pancreatic islet cells. Kv1.7 was also isolated from mouse brain and hamster insulinoma cells by polymerase chain reaction.

Functional changes in potassium conductances of the human neuroblastoma cell line SH-SY5Y during in vitro differentiation

The electrophysiological properties of voltage-dependent outward currents were investigated under voltage-clamp conditions in the human neuroblastoma cell line SH-SY5Y before and after in vitro differentiation with retinoic acid, by using the whole cell variant of the patch-clamp technique. Voltage steps to depolarizing potentials from a holding level of -90 mV elicited, in both undifferentiated and differentiated cells, outward potassium currents that were blocked by tetraethylammonium, but were unaffected by 4-aminopyridine, cadmium, and by shifts of the holding potentials to -40 mV. These currents activated rapidly and inactivated slowly in a voltage-dependent manner. In undifferentiated cells the threshold for current activation was about -30 mV, with a steady-state half activation potential of 19.5 mV. Maximum conductance was 4.3 nS and mean conductance density was 0.34 mS/cm2. Steady-state half inactivation potential was -13.8 mV and approximately 10% of the current was resistant to inactivation. Both activation and inactivation kinetics were voltage dependent. In differentiated cells the threshold for current activation was about -20 mV, with a half potential for steady-state activation of 37.0 mV. Maximum conductance was 15.2 nS and mean conductance density was 0. 78 mS/cm2. Steady-state half inactivation potential was -9.7 mV and approximately 37% of the current was resistant to inactivation. Both activation and inactivation kinetics were voltage dependent. This diversity in potassium channel properties observed between undifferentiated and differentiated cells was related to differences in cell excitability. Under current-clamp conditions, the action potential repolarization rate in differentiated cells was about threefold faster than that of the abortive action potentials elicitable in undifferentiated cells. Furthermore, during prolonged stimulation, trains of spikes could be generated in some differentiated cells but not in undifferentiated cells.

The 1997 Stevenson Award Lecture. Cardiac K+channel gating: cloned delayed rectifier mechanisms and drug modulation

K+ channels are ubiquitous membrane proteins, which have a central role in the control of cell excitability. In the heart, voltage-gated delayed rectifier K+ channels, like Kv1.5, determine repolarization and the cardiac action potential plateau duration. Here we review the broader properties of cloned voltage-gated K+ channels with specific reference to the hKv1.5 channel in heart. We discuss the basic structural components of K+ channels such as the pore, voltage sensor, and fast inactivation, all of which have been extensively studied. Slow, or C-type, inactivation and the structural features that control pore opening are less well understood, although recent studies have given new insight into these problems. Information about channel transitions that occur prior to opening is provided by gating currents, which reflect charge-carrying transitions between kinetic closed states. By studying modulation of the gating properties of K+ channels by cations and with drugs, we can make a more complete interpretation of the state dependence of drug and ion interactions with the channel. In this way we can uncover the detailed mechanisms of action of K+ channel blockers such as tetraethylammonium ions and 4-aminopyridine, and antiarrhythmic agents such as nifedipine and quinidine.Key words: potassium channel, Kv1.5, channel gating, inactivation, pore region, gating currents.

Tex261, a novel gene presumably related but distinct from steroidogenic acute regulatory (StAR) gene, is regulated during the development of germ cells

Tex261 is a new gene cloned from a subtractive cDNA library from 10-day postnatal mouse testis. Tex261 transcribes three mRNAs of 3.5, 1.6 and 1.4 kb. The 3.5 kb and 1.4 kb transcripts are expressed in different gonadal and somatic tissues analyzed. However, the 1.6 Kb transcript is only detected in testis and differentially regulated during development. This 1.6 kb mRNA is highly expressed in adult testis, with detection beginning at 15 days of postnatal life, which coincides with the presence of pachytene cells in prepuberal mouse. This expression was confirmed in pachytene cells by run-off transcription assay and by in situ hybridization. A region of 86 amino acids from the predicted Tex261 was recently reported as a part of the steroidogenic acute regulatory protein StAR gene by its sequence identity to a rat StAR cloned cDNA. We demonstrate her that, in the mouse, StAR and Tex261 are two different genes with different expected functions, yet, a high identity (43%) at amino acid level is detected in a region of 153 amino acids corresponding to a transmembrane protein.

Cloning of human pancreatic islet large conductance Ca(2+)-activated K+ channel (hSlo) cDNAs: evidence for high levels of expression in pancreatic islets and identification of a flanking genetic marker

Insulin secretion from pancreatic beta cells is dependent on membrane potential changes that result from the concerted regulation of multiple ion channels. Among the distinct K+ channels known to be expressed in beta cells, large conductance Ca(2+)-activated K+ channels have been suggested to play an important role in stimulus-secretion coupling. In the course of a strategy to identify transcripts that are enriched in human pancreatic islet cells, we isolated a partial cDNA encoding a human large conductance Ca(2+)-activated K+ channel mRNA (hSlo). Northern analysis of mRNA showed that among a panel of human tissues hSlo is expressed at its highest levels in pancreatic islets. Screening of human insulinoma and islet cDNA libraries with the partial cDNA resulted in the isolation of 19 hSlo cDNAs. These comprised three splice variants: one shared the common underlying structure of previously reported Slo cDNAs, another variant encoded a novel 60-amino acid insertion in the putative Ca(2+)-sensing domain of hSlo, while the third group of clones had an alternate exon encoding eight amino acids in the predicted COOH-terminal end. Analysis of somatic-cell hybrids containing different portions of chromosome 10 indicated that hSlo maps to chromosome 10q22.2-q23.1. Furthermore, high resolution localization was obtained by analysis of genome-wide radiation hybrids and the CEPH "B" mega-YAC library, both of which identified for the first time a highly polymorphic genetic marker (D10S195) linked to hSlo. These studies provide tools with which to explore the physiological role of Ca(2+)-activated K+ channel proteins in pancreatic islets, and also to investigate the contribution of this locus to the inherited susceptibility to non-insulin-dependent diabetes mellitus.

Functional differences in Kv1.5 currents expressed in mammalian cell lines are due to the presence of endogenous Kv beta 2.1 subunits

The voltage-sensitive currents observed following hKv1.5 alpha subunit expression in HEK 293 and mouse L-cells differ in the kinetics and voltage dependence of activation and slow inactivation. Molecular cloning, immunopurification, and Western blot analysis demonstrated that an endogenous L-cell Kv beta 2.1 subunit assembled with transfected hKv 1.5 protein. In contrast, both mRNA and protein analysis failed to detect a beta subunit in the HEK 293 cells, suggesting that functional differences observed between these two systems are due to endogenous L-cell Kv beta 2.1 expression. In the absence of Kv beta 2.1, midpoints for activation and inactivation of hKv1.5 in HEK 293 cells were -0.2 +/- 2.0 and -9.6 +/- 1.8 mV, respectively. In the presence of Kv beta 2.1 these values were -14.1 +/- 1.8 and -22.1 +/- 3.7 mV, respectively. The beta subunit also caused a 1.5-fold increase in the extent of slow inactivation at 50 mV, thus completely reconstituting the L-cell current phenotype in the HEK 293 cells. These results indicate that 1) the Kv beta 2.1 subunit can alter Kv1.5 alpha subunit function, 2) beta subunits are not required for alpha subunit expression, and 3) endogenous beta subunits are expressed in heterologous expression systems used to study K+ channel function.

Pancreatic islet cells express a family of inwardly rectifying K+ channel subunits which interact to form G-protein-activated channels

Insulin secretion is associated with changes in pancreatic beta-cell K+ permeability. A degenerate polymerase chain reaction strategy based on the conserved features of known inwardly rectifying K+ (KIR) channel genes was used to identify members of this family expressed in human pancreatic islets and insulinoma. Three related human KIR transcript sequences were found: CIR (also known as cardiac KATP-1), GIRK1, and GIRK2 (KATP-2). The pancreatic islet CIR and GIRK2 full-length cDNAs were cloned, and their genes were localized to human chromosomes 11q23-ter and 21, respectively. Northern blot analysis detected CIR mRNA at similar levels in human islets and exocrine pancreas, while the abundance of GIRK2 mRNA in the two tissues was insufficient for detection by this method. Using competitive reverse-transcription polymerase chain reaction, CIR was found to be present at higher levels than GIRK2 mRNA in native purified beta-cells. Xenopus oocytes injected with M2 muscarinic receptor (M2) plus either GIRK2 or CIR cRNA expressed only very small carbachol-induced currents, while co-injection of CIR plus GIRK2 along with M2 resulted in expression of carbachol-activated strong inwardly rectifying currents. Activators of KATP channels failed to elicit currents in the presence or absence of co-expressed sulfonylurea receptor. These results show that two components of islet cell KIR channels, CIR and GIRK2, may interact to form heteromeric G-protein-activated inwardly rectifying K+ channels that do not possess the typical properties of KATP channels.

Stereoselective block of a human cardiac potassium channel (Kv1.5) by bupivacaine enantiomers

Stereoselective drug-channel interactions may help to elucidate the molecular basis of voltage-gated potassium channel block by local anesthetic drugs. We studied the effects of the enantiomers of bupivacaine on a cloned human cardiac potassium channel (hKv1.5). This channel was stably expressed in a mouse Ltk- cell line and studied using the whole-cell configuration of the patch-clamp technique. Both enantiomers modified the time course of this delayed rectifier current. Exposure to 20 microM of either S(-)-bupivacaine or R(+)-bupivacaine did not modify the activation time constant of the current, but reduced the peak outward current and induced a subsequent exponential decline of current with time constants of 18.7 +/- 1.1 and 10.0 +/- 0.9 ms, respectively. Steady-state levels of block (assessed with 250-ms depolarizing pulses to +60 mV) averaged 30.8 +/- 2.5% (n = 6) and 79.5 +/- 3.2% (n = 6) (p < 0.001), for S(-)- and R(+)-bupivacaine, respectively. The concentration dependence of hKv1.5 inhibition revealed apparent KD values of 27.3 +/- 2.8 and 4.1 +/- 0.7 microM for S(-)-bupivacaine and R(+)-bupivacaine, respectively, with Hill coefficients close to unity, suggesting that binding of one enantiomer molecule per channel was sufficient to block potassium permeation. Analysis of the rate constants of association (k) and dissociation (l) yielded similar values for l (24.9 s-1 vs. 23.6 s-1 for S(-)- and R(+)-bupivacaine, respectively) but different association rate constants (1.0 x 10(6) vs. 4.7 x 10(6) M-1 s-1 for S(-)- and R(+)-bupivacaine, respectively). Block induced by either enantiomer displayed a shallow voltage dependence in the voltage range positive to 0 mV, i.e., where the channel is fully open, consistent with an equivalent electrical distance delta of 0.16 +/- 0.01. This suggested that at the binding site, both enantiomers of bupivacaine experienced 16% of the applied transmembrane electrical field, referenced to the inner surface. Both bupivacaine enantiomers reduced the tail current amplitude recorded on return to -40 mV and slowed their time course relative to control, resulting in a "crossover" phenomenon. These data indicate 1) the charged form of both bupivacaine enantiomers block the hKv1.5 channel after it opens, 2) binding occurs within the transmembrane electrical field, 3) unbinding is required before the channel can close, 4) block of hKv1.5 channels by bupivacaine is markedly stereoselective, with the R(+)-enantiomer being the more potent one, 5) this stereoselective block was associated with a 1.11 -kcal/mol difference in binding energy between both enantiomers, and 6) the stereoselectivity derives mainly from a difference in the association rate constants, suggesting that the S(-)-enantiomer is less likely to access the binding site in an optimal configuration.

Functional expression of an epitope-tagged G protein-coupled K+ channel (GIRK1)

An epitope-tagged form of an inwardly rectifying and G protein-coupled K+ channel (GIRK1-cp) was expressed at high levels in transfected mammalian cells. Immunoblot analysis of transfected human embryonic kidney cells (HEK293) and mouse insulinoma cells (beta TC3) revealed several GIRK1-cp polypeptides, including the major 59-kDa band, corresponding to the predicted mass of the GIRK1 polypeptide plus the epitope tag. Immunohistochemical staining using two anti-tag antibodies showed abundant immunoreactive material, which was predominantly concentrated in the perinuclear area in both transfected cell types. While functional GIRK1-cp message was present in poly(A)+ RNA prepared from HEK293 cells expressing GIRK1-cp protein, appropriate K+ currents could not be detected. In contrast, whole cell recordings made directly from transfected beta TC3 cells expressing GIRK1-cp revealed inwardly rectifying, pertussis toxin-sensitive currents activated by norepinephrine and galanin. Single channel recordings in excised patches of beta TC3 cells expressing GIRK1-cp showed rectifying K+ currents when activated by 50 microM guanosine 5'-O-(thiotriphosphate), with a slope conductance of 39.1 +/- 1.0 picosiemens. This is the first report of stable heterologous expression of a functional G protein-coupled K+ channel in mammalian cells. The activity of an epitope-tagged channel in insulinoma cells demonstrates the utility of this system for further biochemical and biophysical analyses of G protein-K+ channel interactions.

A delayed rectifier potassium channel cloned from bovine adrenal medulla. Functional analysis after expression in Xenopus oocytes and in a neuroblastoma cell line

Using a cDNA library from bovine adrenal medulla, and, subsequently, a bovine genomic library, we have isolated the gene coding for a non inactivating potassium channel. This gene encodes a 597-amino acid protein which we have called BAK5 as its sequence is very similar to members of Kv1.5 potassium channel family. Neuroblastoma cells (Neuro-2a cell line) were stably transfected with BAK5 DNA. Protein expression was under the control of a heat-shock promoter. Transfected cells showed a current highly selective for potassium, insensitive to tetraethylammonium but reversibly blocked by 4-aminopyridine. Oocytes injected with BAK5 mRNA also expressed a potassium current with the same characteristics.

Structural determinant for assembly of mammalian K+ channels

K+ channel function is regulated through the assembly of channel subunit isoforms into either homo- or heterotetrameric structures each characterized by distinct pharmacologic and kinetic properties. In studying the molecular basis of subunit association in mammalian Shaker-like K+ channels, we constructed deletion mutants of the inactivating K+ channel hKv1.4 alone and in tandem with hKv1.5 and examined the functional properties electrophysiologically in Xenopus oocytes. Deletion of 255 amino acids in the amino-terminal domain of hKv1.4 prevented the formation of hybrid channels within the subfamily but had no effect on homomultimerization or voltage-dependent gating. The amino-terminal deletion mutant of Kv2.1, a noninactivating K+ channel from a distantly related subfamily also forms functional homomultimeric channels. Although members of different K+ channel subfamilies do not coassemble, coexpression of the amino-terminal deletion mutants of hKv1.4 and Kv2.1 resulted in the formation of functional hybrid channels. These results demonstrate that the amino-terminal region of mammalian K+ channels subserves two functions. It provides a recognition site necessary for hetero- but not homomultimeric channel assembly within a subfamily and prevents coassembly between subfamilies.

Influence of cloned voltage-gated K+ channel expression on alanine transport, Rb+ uptake, and cell volume

Voltage-gated K+ channels are involved in regulation of action potential duration and in setting the resting membrane potential in nerve and muscle. To determine the effects of voltage-gated K+ channel expression on processes not associated with electrically excitable cells, we studied cell volume, membrane potential, Na(+)-K(+)-ATPase activity, and alanine transport after the stable expression of the Kv1.4 and Kv1.5 human K+ channels in Ltk- mouse fibroblasts (L-cells). The fast-activating noninactivating Kv1.5 channel, but not the rapidly inactivating Kv1.4 channel, prevented dexamethasone-induced increases in intracellular volume and inhibited Na(+)-K(+)-ATPase activity by 25%, as measured by 86Rb+ uptake. Alanine transport, measured separately by systems A and ASC, was lower in Kv1.5-expressing cells, indicating that the expression of this channel modified the Na(+)-dependent amino acid transport of both systems. Expression of the Kv1.4 channel did not alter alanine transport relative to wild-type or sham-transfected cells. The changes specific to Kv1.5 expression may be related to the resting membrane potential induced by this channel (-30 mV) in contrast to that measured in wild-type sham-transfected, or Kv1.4-transfected cells (-2 to 0 mV). Blocking of the Kv1.5 channel by 60 microM quinidine negated the effects of Kv1.5 expression on intracellular volume, Na(+)-K(+)-ATPase, and Na(+)-dependent alanine transport. These results indicate that delayed rectifier channels such as Kv1.5 can play a key role in the control of cell membrane potential, cell volume, Na(+)-K(+)-ATPase activity, and electrogenic alanine transport across the plasma membrane of electrically unexcitable cells.

A rapidly activating and slowly inactivating potassium channel cloned from human heart. Functional analysis after stable mammalian cell culture expression

The electrophysiological properties of HK2 (Kv1.5), a K+ channel cloned from human ventricle, were investigated after stable expression in a mouse Ltk- cell line. Cell lines that expressed HK2 mRNA displayed a current with delayed rectifier properties at 23 degrees C, while sham transfected cell lines showed neither specific HK2 mRNA hybridization nor voltage-activated currents under whole cell conditions. The expression of the HK2 current has been stable for over two years. The dependence of the reversal potential of this current on the external K+ concentration (55 mV/decade) confirmed K+ selectivity, and the tail envelope test was satisfied, indicating expression of a single population of K+ channels. The activation time course was fast and sigmoidal (time constants declined from 10 ms to < 2 ms between 0 and +60 mV). The midpoint and slope factor of the activation curve were Eh = -14 +/- 5 mV and k = 5.9 +/- 0.9 (n = 31), respectively. Slow partial inactivation was observed especially at large depolarizations (20 +/- 2% after 250 ms at +60 mV, n = 32), and was incomplete in 5 s (69 +/- 3%, n = 14). This slow inactivation appeared to be a genuine gating process and not due to K+ accumulation, because it was present regardless of the size of the current and was observed even with 140 mM external K+ concentration. Slow inactivation had a biexponential time course with largely voltage-independent time constants of approximately 240 and 2,700 ms between -10 and +60 mV. The voltage dependence of slow inactivation overlapped with that of activation: Eh = -25 +/- 4 mV and k = 3.7 +/- 0.7 (n = 14). The fully activated current-voltage relationship displayed outward rectification in 4 mM external K+ concentration, but was more linear at higher external K+ concentrations, changes that could be explained in part on the basis of constant field (Goldman-Hodgkin-Katz) rectification. Activation and inactivation kinetics displayed a marked temperature dependence, resulting in faster activation and enhanced inactivation at higher temperature. The current was sensitive to low concentrations of 4-aminopyridine, but relatively insensitive to external TEA and to high concentrations of dendrotoxin. The expressed current did not resemble either the rapid or the slow components of delayed rectification described in guinea pig myocytes. However, this channel has many similarities to the rapidly activating delayed rectifying currents described in adult rat atrial and neonatal canine epicardial myocytes.(ABSTRACT TRUNCATED AT 400 WORDS)

Molecular biology of the voltage-gated potassium channels of the cardiovascular system

K+ channels represent the most diverse class of voltage-gated ion channels in terms of function and structure. Voltage-gated K+ channels in the heart establish the resting membrane K+ permeability, modulate the frequency and duration of action potentials, and are targets of several antiarrhythmic drugs. Consequently, an understanding of K+ channel structure-function relationships and pharmacology is of great practical interest. However, the presence of multiple overlapping currents in native cardiac myocytes complicates the study of basic K+ channel function and drug-channel interactions in these cells. The application of molecular cloning technology to cardiovascular K+ channels has identified the primary structure of these proteins, and heterologous expression systems have allowed a detailed analysis of channel function and pharmacology without contaminating currents. To date six different K+ channels have been cloned from rat and human heart, and all have been functionally characterized in either Xenopus oocytes or mammalian tissue culture systems. This initial research is an important step toward understanding the molecular basis of the action potential in the heart. An important challenge for the future is to determine the cell-specific expression and relative contribution of these cloned channels to cardiac excitability.

Specification of subunit assembly by the hydrophilic amino-terminal domain of the Shaker potassium channel

The functional heterogeneity of potassium channels in eukaryotic cells arises not only from the multiple potassium channel genes and splice variants but also from the combinatorial mixing of different potassium channel polypeptides to form heteromultimeric channels with distinct properties. One structural element that determines the compatibility of different potassium channel polypeptides in subunit assembly has now been localized to the hydrophilic amino-terminal domain. A Drosophila Shaker B (ShB) potassium channel truncated polypeptide that contains only the hydrophilic amino-terminal domain can form a homomultimer; the minimal requirement for the homophilic interaction has been localized to a fragment of 114 amino acids. Substitution of the amino-terminal domain of a distantly related mammalian potassium channel polypeptide (DRK1) with that of ShB permits the chimeric DRK1 polypeptide to coassemble with ShB.

The role of ion channels in insulin secretion

Ion channels in beta cells regulate electrical and secretory activity in response to metabolic, pharmacologic, or neural signals by controlling the permeability to K+ and Ca2+. The ATP-sensitive K+ channels act as a switch that responds to fuel secretagogues or sulfonylureas to initiate depolarization. This depolarization opens voltage-dependent calcium channels (VDCC) to increase the amplitude of free cytosolic Ca2+ levels ([Ca2+]i), which triggers exocytosis. Acetyl choline and vasopressin (VP) both potentiate the acute effects of glucose on insulin secretion by generating inositol 1,4,5-trisphosphate to release intracellular Ca2+; VP also potentiates sustained insulin secretion by effects on depolarization. In contrast, inhibitors of insulin secretion decrease [Ca2+]i by either hyperpolarizing the beta cell or by receptor-mediated, G-protein-coupled effects to decrease VDCC activity. Repolarization is initiated by voltage- and Ca(2+)-activated K+ channels. A human insulinoma voltage-dependent K+ channel cDNA was recently cloned and two types of alpha 1 subunits of the VDCC have been identified in insulin-secreting cell lines. Determining how ion channels regulate insulin secretion in normal and diabetic beta cells should provide pathophysiologic insight into the beta cell signal transduction defect characteristic of non-insulin dependent diabetes (NIDDM).

The molecular biology of K+ channels

It is now clear that voltage-gated K+ channels are encoded by a set of multigene subfamilies. Expression of different members of these subfamilies, coupled with mutational analysis, has advanced our knowledge of the structure and function of voltage-dependent K+ channels.