scholarly journals Mechanism of Rectification in Inward-rectifier K+ Channels

2003 ◽  
Vol 121 (4) ◽  
pp. 261-276 ◽  
Author(s):  
Donglin Guo ◽  
Yajamana Ramu ◽  
Angela M. Klem ◽  
Zhe Lu
Keyword(s):  
Membrane Potential ◽  
Voltage Dependence ◽  
Inward Rectifier ◽  
Equilibrium Potential ◽  
Amine Group ◽  
Channel Block ◽  
K Channels ◽  
Alkyl Amines ◽  
Amine Molecule ◽  
The Difference

Rectification in inward-rectifier K+ channels is caused by the binding of intracellular cations to their inner pore. The extreme sharpness of this rectification reflects strong voltage dependence (apparent valence is ∼5) of channel block by long polyamines. To understand the mechanism by which polyamines cause rectification, we examined IRK1 (Kir2.1) block by a series of bis-alkyl-amines (bis-amines) and mono-alkyl-amines (mono-amines) of varying length. The apparent affinity of channel block by both types of alkylamines increases with chain length. Mutation D172N in the second transmembrane segment reduces the channel's affinity significantly for long bis-amines, but only slightly for short ones (or for mono-amines of any length), whereas a double COOH-terminal mutation (E224G and E299S) moderately reduces the affinity for all bis-amines. The apparent valence of channel block increases from ∼2 for short amines to saturate at ∼5 for long bis-amines or at ∼4 for long mono-amines. On the basis of these and other observations, we propose that to block the channel pore one amine group in all alkylamines tested binds near the same internal locus formed by the COOH terminus, while the other amine group of bis-amines, or the alkyl tail of mono-amines, “crawls” toward residue D172 and “pushes” up to 4 or 5 K+ ions outwardly across the narrow K+ selectivity filter. The strong voltage dependence of channel block therefore reflects the movement of charges carried across the transmembrane electrical field primarily by K+ ions, not by the amine molecule itself, as K+ ions and the amine blocker displace each other during block and unblock of the pore. This simple displacement model readily accounts for the classical observation that, at a given concentration of intracellular K+, rectification is apparently related to the difference between the membrane potential and the equilibrium potential for K+ ions rather than to the membrane potential itself.

scholarly journals Interaction Mechanisms between Polyamines and IRK1 Inward Rectifier K+ Channels

2003 ◽  
Vol 122 (5) ◽  
pp. 485-500 ◽  
Author(s):  
Donglin Guo ◽  
Zhe Lu
Keyword(s):  
Voltage Dependence ◽  
Divalent Cations ◽  
Inward Rectifier ◽  
Affinity Binding ◽  
Channel Block ◽  
K Channels ◽  
High Affinity ◽  
Voltage Dependent ◽  
Varying Length ◽  
Amine Groups

Rectification of macroscopic current through inward-rectifier K+ (Kir) channels reflects strong voltage dependence of channel block by intracellular cations such as polyamines. The voltage dependence results primarily from the movement of K+ ions across the transmembrane electric field, which accompanies the binding–unbinding of a blocker. Residues D172, E224, and E299 in IRK1 are critical for high-affinity binding of blockers. D172 appears to be located somewhat internal to the narrow K+ selectivity filter, whereas E224 and E299 form a ring at a more intracellular site. Using a series of alkyl-bis-amines of varying length as calibration, we investigated how the acidic residues in IRK1 interact with amine groups in the natural polyamines (putrescine, spermidine, and spermine) that cause rectification in cells. To block the pore, the leading amine of bis-amines of increasing length penetrates ever deeper into the pore toward D172, while the trailing amine in every bis-amine binds near a more intracellular site and interacts with E224 and E299. The leading amine in nonamethylene-bis-amine (bis-C9) makes the closest approach to D172, displacing the maximal number of K+ ions and exhibiting the strongest voltage dependence. Cells do not synthesize bis-amines longer than putrescine (bis-C4) but generate the polyamines spermidine and spermine by attaching an amino-propyl group to one or both ends of putrescine. Voltage dependence of channel block by the tetra-amine spermine is comparable to that of block by the bis-amines bis-C9 (shorter) or bis-C12 (equally long), but spermine binds to IRK1 with much higher affinity than either bis-amine does. Thus, counterintuitively, the multiple amines in spermine primarily confer the high affinity but not the strong voltage dependence of channel block. Tetravalent spermine achieves a stronger interaction with the pore by effectively behaving like a pair of tethered divalent cations, two amine groups in its leading half interacting primarily with D172, whereas the other two in the trailing half interact primarily with E224 and E299. Thus, nature has optimized not only the blocker but also, in a complementary manner, the channel for producing rapid, high-affinity, and strongly voltage-dependent channel block, giving rise to exceedingly sharp rectification.

Inward rectifier K+ currents in smooth muscle cells from rat coronary arteries: block by Mg2+, Ca2+, and Ba2+

1996 ◽  
Vol 271 (2) ◽  
pp. H696-H705 ◽  
Author(s):  
B. E. Robertson ◽  
A. D. Bonev ◽  
M. T. Nelson
Keyword(s):  
Smooth Muscle ◽  
Membrane Potential ◽  
Coronary Artery ◽  
Smooth Muscle Cells ◽  
Coronary Arteries ◽  
Inward Rectifier ◽  
Inward Rectification ◽  
K Channels ◽  
Inward Currents ◽  
K Currents

Inward rectifier K+ channels have been implicated in the control of membrane potential and external K(+)-induced dilations of small coronary arteries. To identify and characterize inward rectifier K+ currents in coronary artery smooth muscle, whole cell K+ currents in smooth muscle cells enzymatically isolated from rat coronary (septal) arteries (diameters, 100-150 microns) were measured in the conventional and perforated configurations of the patch-clamp technique. Ba(2+)-sensitive, whole cell K+ current-voltage relationships exhibited inward rectification. Blockers of Ca(2+)-activated K+ channels (1 mM tetraethylammonium ion), ATP-sensitive K+ channels (10 microM glibenclamide), and voltage-dependent K+ channels (1 mM 4-aminopyridine) in smooth muscle did not affect inward rectifier K+ currents. The nonselective K+ channel inhibitor phencyclidine (100 microM) reduced inward rectifier K+ currents by approximately 50%. External Ba2+ reduced inward currents, with membrane potential hyperpolarization increasing inhibition. The half-inhibition constant for Ba2+ was 2.1 microM at -60 mV, decreasing e-fold for a 25-mV hyperpolarization. External Cs+ also blocked inward rectifier K+ currents, with the half-inhibition constant for Cs+ of 2.9 mM at -60 mV. External Ca2+ and Mg2+ reduced inward rectifier K+ currents. At -60 mV, Ca2+ and Mg2+ (1 mM) reduced inward currents by 33 and 21%, respectively. Inward rectification was not affected by dialysis of the cell's interior with a nominally Ca(2+)- and Mg(2+)-free solution. These findings indicate that inward rectifier K+ channels exist in coronary artery smooth muscle and that Ba2+ may be a useful probe for the functional role of inward rectifier K+ channels in coronary arteries.

scholarly journals K+ channels of stomatal guard cells. Characteristics of the inward rectifier and its control by pH.

1992 ◽  
Vol 99 (4) ◽  
pp. 615-644 ◽  
Author(s):  
M R Blatt
Keyword(s):  
Intracellular Ph ◽  
Guard Cells ◽  
Inward Rectifier ◽  
Equilibrium Potential ◽  
Egg Cell ◽  
K Channels ◽  
Inward Rectifiers ◽  
Stomatal Guard Cells ◽  
K Current ◽  
Current Reversal

Intracellular microelectrode recordings and a two-electrode voltage clamp have been used to characterize the current carried by inward rectifying K+ channels of stomatal guard cells from the broadbean, Vicia faba L. Superficially, the current displayed many features common to inward rectifiers of neuromuscular and egg cell membranes. In millimolar external K+ concentrations (Ko+), it activated on hyperpolarization with half-times of 100-200 ms, showed no evidence of time- or voltage-dependent inactivation, and deactivated rapidly (tau approximately 10 ms) on clamping to 0 mV. Steady-state conductance-voltage characteristics indicated an apparent gating charge of 1.3-1.6. Current reversal showed a Nernstian dependence on Ko+ over the range 3-30 mM, and the inward rectifier was found to be highly selective for K+ over other monovalent cations (K+ greater than Rb+ greater than Cs+ much greater than Na+). Unlike the inward rectifiers of animal membranes, the current was blocked by charybdotoxin and alpha-dendrotoxin (Kd much less than 50 nM), as well as by tetraethylammonium chloride (K1/2 = 9.1 mM); gating of the guard cell K+ current was fixed to voltages near -120 mV, independent of Ko+, and the current activated only with supramillimolar K+ outside (EK+ greater than -120 mV). Most striking, however, was inward rectifier sensitivity to [H+] with the K+ current activated reversibly by mild acid external pH. Current through the K+ inward rectifier was found to be largely independent of intracellular pH and the current reversal (equilibrium) potential was unaffected by pHo from 7.4 to 5.5. By contrast, current through the K+ outward rectifier previously characterized in these cells (1988. J. Membr. Biol. 102:235) was largely insensitive to pHo, but was blocked reversibly by acid-going intracellular pH. The action of pHo on the K+ inward rectifier could not be mimicked by extracellular Ca2+ for which changes in activation, deactivation, and conductance were consonant with an effect on surface charge ([Ca2+] less than or equal to 1 mM). Rather, extracellular pH affected activation and deactivation kinetics disproportionately, with acid-going pHo raising the K+ conductance and shifting the conductance-voltage profile positive-going along the voltage axis and into the physiological voltage range. Voltage and pH dependencies for gating were consistent with a single, titratable group (pKa approximately 7 at -200 mV) residing deep within the membrane electric field and accessible from the outside.(ABSTRACT TRUNCATED AT 400 WORDS)

scholarly journals Human Myoblast Fusion Requires Expression of Functional Inward Rectifier Kir2.1 Channels

2001 ◽  
Vol 153 (4) ◽  
pp. 677-686 ◽  
Author(s):  
Jacqueline Fischer-Lougheed ◽  
Jian-Hui Liu ◽  
Estelle Espinos ◽  
David Mordasini ◽  
Charles R. Bader ◽  
...  
Keyword(s):  
Skeletal Muscle ◽  
Membrane Potential ◽  
Muscle Development ◽  
Myoblast Fusion ◽  
Inward Rectifier ◽  
Resting Membrane Potential ◽  
Skeletal Muscle Development ◽  
K Channels ◽  
Window Current ◽  
Human Myoblast

Myoblast fusion is essential to skeletal muscle development and repair. We have demonstrated previously that human myoblasts hyperpolarize, before fusion, through the sequential expression of two K+ channels: an ether-à-go-go and an inward rectifier. This hyperpolarization is a prerequisite for fusion, as it sets the resting membrane potential in a range at which Ca2+ can enter myoblasts and thereby trigger fusion via a window current through α1H T channels.

scholarly journals Mechanism of the Voltage Sensitivity of IRK1 Inward-rectifier K+ Channel Block by the Polyamine Spermine

2005 ◽  
Vol 125 (4) ◽  
pp. 413-426 ◽  
Author(s):  
Hyeon-Gyu Shin ◽  
Zhe Lu
Keyword(s):  
Steady State ◽  
Voltage Dependence ◽  
Ion Selectivity ◽  
Inward Rectifier ◽  
Selectivity Filter ◽  
K Channel ◽  
Voltage Sensitivity ◽  
Channel Block ◽  
Head Groups ◽  
Voltage Dependent

IRK1 (Kir2.1) inward-rectifier K+ channels exhibit exceedingly steep rectification, which reflects strong voltage dependence of channel block by intracellular cations such as the polyamine spermine. On the basis of studies of IRK1 block by various amine blockers, it was proposed that the observed voltage dependence (valence ∼5) of IRK1 block by spermine results primarily from K+ ions, not spermine itself, traversing the transmembrane electrical field that drops mostly across the narrow ion selectivity filter, as spermine and K+ ions displace one another during channel block and unblock. If indeed spermine itself only rarely penetrates deep into the ion selectivity filter, then a long blocker with head groups much wider than the selectivity filter should exhibit comparably strong voltage dependence. We confirm here that channel block by two molecules of comparable length, decane-bis-trimethylammonium (bis-QAC10) and spermine, exhibit practically identical overall voltage dependence even though the head groups of the former are much wider (∼6 Å) than the ion selectivity filter (∼3 Å). For both blockers, the overall equilibrium dissociation constant differs from the ratio of apparent rate constants of channel unblock and block. Also, although steady-state IRK1 block by both cations is strongly voltage dependent, their apparent channel-blocking rate constant exhibits minimal voltage dependence, which suggests that the pore becomes blocked as soon as the blocker encounters the innermost K+ ion. These findings strongly suggest the existence of at least two (potentially identifiable) sequentially related blocked states with increasing numbers of K+ ions displaced. Consequently, the steady-state voltage dependence of IRK1 block by spermine or bis-QAC10 should increase with membrane depolarization, a prediction indeed observed. Further kinetic analysis identifies two blocked states, and shows that most of the observed steady-state voltage dependence is associated with the transition between blocked states, consistent with the view that the mutual displacement of blocker and K+ ions must occur mainly as the blocker travels along the long inner pore.

scholarly journals Coupled Ion Movement Underlies Rectification in an Inward-Rectifier K+ Channel

1998 ◽  
Vol 112 (2) ◽  
pp. 211-221 ◽  
Author(s):  
Maria Spassova ◽  
Zhe Lu
Keyword(s):  
Voltage Dependence ◽  
Inward Rectifier ◽  
Membrane Voltage ◽  
Equilibrium Potential ◽  
Inward Rectification ◽  
K Channel ◽  
Channel Blockade ◽  
Electrical Field ◽  
K Current ◽  
Internal Pore

We studied block of the internal pore of the ROMK1 inward-rectifier K+ channel by Mg2+ and five quaternary ammoniums (tetramethylammonium, tetraethylammonium, tetrapropylammonium, tetrabutylammonium, and tetrapentylammonium). The apparent affinity of these blockers varied as a function of membrane voltage. As a consequence, the channel conducted K+ current more efficiently in the inward than the outward direction; i.e., inward rectification. Although the size of some monovalent quaternary ammoniums is rather large, the zδ values (which measure voltage dependence of their binding to the pore) were near unity in symmetric 100 mM K+. Furthermore, we observed that not only the apparent affinities of the blockers themselves, but also their dependence on membrane voltage (or zδ), varied as a function of the concentration of extracellular K+. These results suggest that there is energetic coupling between the binding of blocking and permeating (K+) ions, and that the voltage dependence of channel blockade results, at least in part, from the movement of K+ ions in the electrical field. A further quantitative analysis of the results explains why the complex phenomenon of inward rectification depends on both membrane voltage and the equilibrium potential for K+.

scholarly journals Effects of external Rb+ on inward rectifier K+ channels of bovine pulmonary artery endothelial cells.

1994 ◽  
Vol 103 (4) ◽  
pp. 519-548 ◽  
Author(s):  
M R Silver ◽  
M S Shapiro ◽  
T E DeCoursey
Keyword(s):  
Endothelial Cells ◽  
Steady State ◽  
Pulmonary Artery ◽  
Voltage Dependence ◽  
Reversal Potential ◽  
Inward Rectifier ◽  
Partial Substitution ◽  
Pipette Solution ◽  
K Channels ◽  
Voltage Dependent

Inward rectifier (IR) K+ channels of bovine pulmonary artery endothelial cells were studied using the whole-cell, cell-attached, and outside-out patch-clamp configurations. The effects of Rb+ on the voltage dependence and kinetics of IR gating were explored, with [Rb+]o + [K+]o = 160 mM. Partial substitution of Rb+ for K+ resulted in voltage-dependent reduction of inward currents, consistent with Rb+ being a weakly permeant blocker of the IR. In cells studied with a K(+)-free pipette solution, external Rb+ reduced inward IR currents to a similar extent at large negative potentials but block at more positive potentials was enhanced. In outside-out patches, the single-channel i-V relationship was approximately linear in symmetrical K+, but rectified strongly outwardly in high [Rb+]o due to a reduced conductance for inward current. The permeability of Rb+ based on reversal potential, Vrev, was 0.45 that of K+, whereas the Rb+ conductance was much lower, 0.034 that of K+, measured at Vrev-80 mV. The steady state voltage-dependence of IR gating was determined in Rb(+)-containing solutions by applying variable prepulses, followed by a test pulse to a potential at which outward current deactivation was observed. As [Rb+]o was increased, the half-activation potential, V1/2, changed less than Vrev. In high [K+]o solutions V1/2 was Vrev-6 mV, while in high [Rb+]o V1/2 was Vrev + 7 mV. This behavior contrasts with the classical parallel shift of V1/2 with Vrev in K+ solutions. Steady state IR gating was less steeply voltage-dependent in high [Rb+]o than in K+ solutions, with Boltzmann slope factors of 6.4 and 4.4 mV, respectively. Rb+ decreased (slowed) both activation and deactivation rate constants defined at V1/2, and decreased the steepness of the voltage dependence of the activation rate constant by 42%. Deactivation of IR channels in outside-out patches was also slowed by Rb+. In summary, Rb+ can replace K+ in setting the voltage-dependence of IR gating, but in doing so alters the kinetics.

Screening Technologies for Inward Rectifier Potassium Channels: Discovery of New Blockers and Activators

2020 ◽  
Vol 25 (5) ◽  
pp. 420-433 ◽  
Author(s):  
Kenneth B. Walsh
Keyword(s):  
Membrane Potential ◽  
G Protein ◽  
Critical Role ◽  
Inward Rectifier ◽  
Resting Potential ◽  
Equilibrium Potential ◽  
Inward Rectification ◽  
Resting Membrane Potential ◽  
Kir Channels ◽  
Inward Rectifier Potassium Channels

K+ channels play a critical role in maintaining the normal electrical activity of excitable cells by setting the cell resting membrane potential and by determining the shape and duration of the action potential. In nonexcitable cells, K+ channels establish electrochemical gradients necessary for maintaining salt and volume homeostasis of body fluids. Inward rectifier K+ (Kir) channels typically conduct larger inward currents than outward currents, resulting in an inwardly rectifying current versus voltage relationship. This property of inward rectification results from the voltage-dependent block of the channels by intracellular polyvalent cations and makes these channels uniquely designed for maintaining the resting potential near the K+ equilibrium potential (EK). The Kir family of channels consist of seven subfamilies of channels (Kir1.x through Kir7.x) that include the classic inward rectifier (Kir2.x) channel, the G-protein-gated inward rectifier K+ (GIRK) (Kir3.x), and the adenosine triphosphate (ATP)-sensitive (KATP) (Kir 6.x) channels as well as the renal Kir1.1 (ROMK), Kir4.1, and Kir7.1 channels. These channels not only function to regulate electrical/electrolyte transport activity, but also serve as effector molecules for G-protein-coupled receptors (GPCRs) and as molecular sensors for cell metabolism. Of significance, Kir channels represent promising pharmacological targets for treating a number of clinical conditions, including cardiac arrhythmias, anxiety, chronic pain, and hypertension. This review provides a brief background on the structure, function, and pharmacology of Kir channels and then focuses on describing and evaluating current high-throughput screening (HTS) technologies, such as membrane potential-sensitive fluorescent dye assays, ion flux measurements, and automated patch clamp systems used for Kir channel drug discovery.

scholarly journals Ca2+-activated K+ Channels in Murine Endothelial Cells: Block by Intracellular Calcium and Magnesium

2008 ◽  
Vol 131 (2) ◽  
pp. 125-135 ◽  
Author(s):  
Jonathan Ledoux ◽  
Adrian D. Bonev ◽  
Mark T. Nelson
Keyword(s):  
Endothelial Cells ◽  
Membrane Potential ◽  
Intracellular Calcium ◽  
Membrane Current ◽  
Mesenteric Arteries ◽  
Equilibrium Potential ◽  
Inward Rectification ◽  
Resting Membrane Potential ◽  
Additional Increase ◽  
K Channels

The intermediate (IKCa) and small (SKCa) conductance Ca2+-sensitive K+ channels in endothelial cells (ECs) modulate vascular diameter through regulation of EC membrane potential. However, contribution of IKCa and SKCa channels to membrane current and potential in native endothelial cells remains unclear. In freshly isolated endothelial cells from mouse aorta dialyzed with 3 μM free [Ca2+]i and 1 mM free [Mg2+]i, membrane currents reversed at the potassium equilibrium potential and exhibited an inward rectification at positive membrane potentials. Blockers of large-conductance, Ca2+-sensitive potassium (BKCa) and strong inward rectifier potassium (Kir) channels did not affect the membrane current. However, blockers of IKCa channels, charybdotoxin (ChTX), and of SKCa channels, apamin (Ap), significantly reduced the whole-cell current. Although IKCa and SKCa channels are intrinsically voltage independent, ChTX- and Ap-sensitive currents decreased steeply with membrane potential depolarization. Removal of intracellular Mg2+ significantly increased these currents. Moreover, concomitant reduction of the [Ca2+]i to 1 μM caused an additional increase in ChTX- and Ap-sensitive currents so that the currents exhibited theoretical outward rectification. Block of IKCa and SKCa channels caused a significant endothelial membrane potential depolarization (≈11 mV) and decrease in [Ca2+]i in mesenteric arteries in the absence of an agonist. These results indicate that [Ca2+]i can both activate and block IKCa and SKCa channels in endothelial cells, and that these channels regulate the resting membrane potential and intracellular calcium in native endothelium.

Descending vasa recta endothelia express inward rectifier potassium channels

2007 ◽  
Vol 293 (4) ◽  
pp. F1248-F1255 ◽  
Author(s):  
Chunhua Cao ◽  
Whaseon Lee-Kwon ◽  
Kristie Payne ◽  
Aurélie Edwards ◽  
Thomas L. Pallone
Keyword(s):  
Endothelial Cells ◽  
Membrane Potential ◽  
Potassium Channels ◽  
Inward Rectifier ◽  
Equilibrium Potential ◽  
Dependent Manner ◽  
Membrane Hyperpolarization ◽  
Inward Rectifier Potassium Channels ◽  
Vasa Recta ◽  
Inwardly Rectifying

Descending vasa recta (DVR) are capillary-sized microvessels that supply blood flow to the renal medulla. They are composed of contractile pericytes and endothelial cells. In this study, we used the whole cell patch-clamp method to determine whether inward rectifier potassium channels (KIR) exist in the endothelia, affect membrane potential, and modulate intracellular Ca2+ concentration ([Ca2+]cyt). The endothelium was accessed for electrophysiology by removing abluminal pericytes from collagenase-digested vessels. KIR currents were recorded using symmetrical 140 mM K+ solutions that served to maximize currents and eliminate cell-to-cell coupling by closing gap junctions. Large, inwardly rectifying currents were observed at membrane potentials below the equilibrium potential for K+. Ba2+ potently inhibited those currents in a voltage-dependent manner, with affinity k = 0.18, 0.33, 0.60, and 1.20 μM at −160, −120, −80, and −40 mV, respectively. Cs+ also blocked those currents with k = 20, 48, 253, and 1,856 μM at −160, −120, −80, and −40 mV, respectively. In the presence of 1 mM ouabain, increasing extracellular K+ concentration from 5 to 10 mM hyperpolarized endothelial membrane potential by 15 mV and raised endothelial [Ca2+]cyt. Both the K+-induced membrane hyperpolarization and the [Ca2+]cyt elevation were reversed by Ba2+. Immunochemical staining verified that both pericytes and endothelial cells of DVR express KIR2.1, KIR2.2, and KIR2.3 subunits. We conclude that strong, inwardly rectifying KIR2.x isoforms are expressed in DVR and mediate K+-induced hyperpolarization of the endothelium.

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