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Results for “Potassium Channels, Inwardly Rectifying”

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PubMed1995

Inward rectifier potassium channels in plants differ from their animal counterparts in response to voltage and channel modulators.

We have investigated the electrophysiological basis of potassium inward rectification of the KAT1 gene product from Arabidopsis thaliana expressed in Xenopus oocytes and of functionally related K+ channels in the plasma membrane of guard and root cells from Vicia faba and Zea mays. The whole-cell currents passed by these channels activate, following steps to membrane potentials more negative than -100 mV, with half activation times of tens of milliseconds. This voltage dependence was unaffected by the removal of cytoplasmic magnesium. Consequently, unlike inward rectifier channels of animals, inward rectification of plant potassium channels is an intrinsic property of the channel protein itself. We also found that the activation kinetics of KAT1 were modulated by external pH. Decreasing the pH in the range 8.5 to 4.5 hastened activation and shifted the steady state activation curve by 19 mV per pH unit. This indicates that the activity of these K+ channels and the activity of the plasma membrane H(+)-ATPase may not only be coordinated by membrane potential but also by pH. The instantaneous current-voltage relationship, on the other hand, did not depend on pH, indicating that H+ do not block the channel. In addition to sensitivity towards protons, the channels showed a high affinity voltage dependent block in the presence of cesium, but were less sensitive to barium. Recordings from membrane patches of KAT1 injected oocytes in symmetric, Mg(2+)-free, 100 mM-K+, solutions allowed measurements of the current-voltage relation of single open KAT1 channels with a unitary conductance of 5 pS. We conclude that the inward rectification of the currents mediated by the KAT1 gene product, or the related endogenous channels of plant cells, results from voltage-modulated structural changes within the channel proteins. The voltage-sensing or the gating-structures appear to interact with a titratable acidic residue exposed to the extracellular medium.

Animals↗

PubMed2002

Spermine block of the strong inward rectifier potassium channel Kir2.1: dual roles of surface charge screening and pore block.

Inward rectification in strong inward rectifiers such as Kir2.1 is attributed to voltage-dependent block by intracellular polyamines and Mg(2+). Block by the polyamine spermine has a complex voltage dependence with shallow and steep components and complex concentration dependence. To understand the mechanism, we measured macroscopic Kir2.1 currents in excised inside-out giant patches from Xenopus oocytes expressing Kir2.1, and single channel currents in the inside-out patches from COS7 cells transfected with Kir2.1. We found that as spermine concentration or voltage increased, the shallow voltage-dependent component of spermine block at more negative voltages was caused by progressive reduction in the single channel current amplitude, without a decrease in open probability. We attributed this effect to spermine screening negative surface charges involving E224 and E299 near the inner vestibule of the channel, thereby reducing K ion permeation rate. This idea was further supported by experiments in which increasing ionic strength also decreased Kir2.1 single channel amplitude, and by mutagenesis experiments showing that this component of spermine block decreased when E224 and E299, but not D172, were neutralized. The steep voltage-dependent component of block at more depolarized voltages was attributed to spermine migrating deeper into the pore and causing fast open channel block. A quantitative model incorporating both features showed excellent agreement with the steady-state and kinetic data. In addition, this model accounts for previously described substate behavior induced by a variety of Kir2.1 channel blockers.

Animals↗

PubMed1994

Amiodarone blocks the inward rectifier potassium channel in isolated guinea pig ventricular cells.

We examined the effects of amiodarone (5-20 microM) on both whole-cell inward rectifier potassium current (IK1) and single IK1 channel activity in isolated guinea pig ventricular myocytes using patch-clamp techniques. In whole-cell voltage-clamp experiments (n = 8), amiodarone (10-20 microM) caused only a small reduction of outward current at -50 mV (12 +/- 6%, no significant difference, N.S.). However, inward current was significantly reduced at -120 mV (21 +/- 7%; P < .05). When CdCl2 (100 microM) and tetrodotoxin (10 microM) were used to block inward Ca++ and Na+ current, respectively, amiodarone significantly reduced IK1 in both the inward (14 +/- 5% at -120 mV; P < .02) and outward (12 +/- 5% at -50 mV; P < .05; n = 11) directions. However, block required high drug concentrations (10-20 microM) and was slow in onset. In contrast, amiodarone did not affect membrane current when IK1 had been previously blocked by Ba++ (5 mM). In inside-out patch-clamp experiments, amiodarone (5 microM) reduced single IK1 channel open probability by increasing interburst interval (from 0.6 +/- 0.03 to 3.1 +/- 0.9 sec; n = 5; P < .05) with no significant difference in the duration of mean open and closed times or the number of shut events within a burst. The net result was that there was only a small change in both burst duration and single-channel kinetics within a burst. Complete channel block occurred after the increase in interburst interval (n = 6 of six cells).(ABSTRACT TRUNCATED AT 250 WORDS)

Action Potentials↗

PubMed2002

Human inward rectifier potassium channels in chronic and postoperative atrial fibrillation.

OBJECTIVE: We showed recently that the 825T allele of the G-protein beta 3-subunit C825T polymorphism is associated with large inward rectifier K(+) currents I(K1) but low acetylcholine-activated K(+) current I(K,ACh) amplitudes. During chronic atrial fibrillation (AF), I(K1) and I(K,ACh) current densities were increased when compared to sinus rhythm (SR). It is unknown whether chronic AF and G beta 3 gene status are independent contributors to atrial K(+) current activity. We measured I(K1) and I(K,ACh) in tissue from AF patients with different G beta 3 genotypes and assessed the relation between the I(K1) and I(K,ACh) amplitudes and the incidence of postoperative AF. METHODS: We measured the amplitudes of I(K1) and I(K,ACh) in atrial myocytes from 26 patients with sinus rhythm (SR) and from 16 patients with chronic AF (>6 months). The K(+) currents were measured with standard patch-clamp techniques. The G beta 3 gene status of the patients was determined by PCR and restriction analysis. RESULTS: At -100 mV, the amplitude of I(K1) was larger in AF (10.9+/-1.0 pA/pF, n=49/16, cells/patients) than in SR (6.3+/-0.6 pA/pF, n=68/26, P<0.05), whereas the amplitude of I(K,ACh) was smaller in chronic AF (2.9+/-0.7 pA/pF, n=49/16) than in SR (6.3+/-0.7 pA/pF, n=68/26, P<0.05). These changes were independent of the patient G beta 3 gene status. Eight patients out of 26 in the SR group (31%) developed postoperative AF. When analysed based on incidence of postoperative AF, current amplitudes did not differ significantly. CONCLUSION: We provide evidence for up-regulation of I(K1) but down-regulation of I(K,ACh) in chronic AF which are independent of G beta 3 gene status. Atrial myocytes from patients who are in SR but later develop postoperative AF have no manifestation of altered I(K1) and I(K,ACh) at the time of cardiac surgery. Our results suggest that the AF-related changes of I(K1) and I(K,ACh) may be a consequence of or a contributory factor to chronic AF.

Acetylcholine↗

PubMed1998

Characterization of an inwardly rectifying potassium channel in the rabbit superior lacrimal gland.

PURPOSE: To characterize the properties of an inwardly rectifying K+ (KIR) current in fresh, enzymatically isolated acinar cells from the rabbit superior lacrimal gland. METHODS: New Zealand White rabbits of both sexes were killed by injecting 45 mg/kg pentobarbital sodium, and the glands were excised. Single acinar cells were isolated enzymatically from these glands. Standard patch-clamp techniques were used to record ion currents. RESULTS: Hyperpolarizing voltages evoked KIR currents that had a conductance of 2.7 +/- 0.16 nS (n = 6) in the range -50 mV to -160 mV. The KIR current was activated with steep voltage dependence on hyperpolarization, and the conductance was approximately dependent on the square root of the external K+ concentration. Increasing the pipette Ca2+ concentration from 10(-9) M to 10(-6) M increased the conductance to 5.3 +/- 0.45 nS (n = 7). Internal substitution of K+ with various cations gave the following permeability sequence: K+ (1.0) > Rb+ (0.83) > Li+ (0.15). The KIR current was inhibited by Ba2+ (100 microns), tetraethylammonium (10 mM), and Cs+ (5 mM) but was insensitive to 4-aminopyridine (5 mM). The single-channel conductance was 43 +/- 2.7 pS (n = 11), and the relationship between between single-channel conductance (gamma) and external K+ concentration ([K]o) is given by: gamma = 7.04[K]o0.37 (pS, r2 = 0.99, P < 0.05). The relationship between [K]o and zero current potential (Erev) is given by: Erev = 35.5 log[K]o - 77.8 (mV; r2 = 0.99, P < 0.05). CONCLUSIONS: The KIR current identified in these lacrimal acini has a similar dependence on [K]o as other inward rectifiers of excitable tissues and exocrine glands. However, this study highlights that there are interspecies variations and similarities between KIR channels that could be related to their individual physiological roles. The authors' investigations suggest that one role of the KIR channel in the rabbit superior lacrimal gland acinar cells is to set and stabilize the resting membrane potential. However, this KIR channel may also be involved in secretion, as has been shown to occur in the sheep parotid gland.

Animals↗

PubMed2003

Expression and localization of the inwardly rectifying potassium channel Kir7.1 in native bovine retinal pigment epithelium.

PURPOSE: The purpose of this study was to identify the molecular basis of the apical membrane K(+) conductance in native bovine retinal pigment epithelium (RPE). METHODS: RT-PCR, Northern blot, and Western blot analyses were used to detect the expression of the inwardly rectifying K(+) (Kir) channel subunits Kir7.1 and Kir4.1 in native bovine RPE and neural retina. The distribution of Kir7.1 protein was determined in frozen sections of bovine retina-RPE-choroid by indirect immunofluorescence analysis. RESULTS: RT-PCR analysis revealed Kir7.1 transcript in both RPE and neural retina, but Kir4.1 transcript only in the neural retina. In Northern blot analysis, Kir4.1 probe hybridized to an appropriately sized-transcript in neural retina but not in RPE. Kir7.1 probe hybridized to a major transcript of approximately 1.5 kb in both RPE and neural retina, but with greater expression in RPE. In Western blot analysis, Kir7.1 antibody recognized a major monomer of approximately 53 kDa in RPE, whereas Kir4.1 antibody recognized a monomer of approximately 60 kDa in neural retina but not in RPE. Intense Kir7.1 immunolabeling was present on the apical surface of all RPE cells and appeared to extend over the length of the apical processes. Na(+),K(+)-ATPase expression varied among RPE cells, but in highly expressing cells, it colocalized with Kir7.1. CONCLUSIONS: These results indicate that the Kir7.1 channel subunit, but not Kir4.1, is a major component of the apical K(+) conductance in bovine RPE. Kir7.1 is distributed over the length of apical processes, where it probably functions in the regulation of K(+) transport and the electrical response of the RPE to light-evoked changes in subretinal K(+) concentration.

Animals↗

PubMed1985

Voltage-dependent activation of the inward-rectifier potassium channel in the ventricular cell membrane of guinea-pig heart.

The activation kinetics of the inward-rectifier K+ channel were studied by single-channel recording in isolated single cells of the guinea-pig ventricle with two different extracellular concentrations of K+ ([K+]o 150 and 75 mM). When voltage pulses were applied from a holding potential more positive than the potassium equilibrium potential (EK), to potentials more negative than EK, the probability of the channel being in the open state (Po) increased with time after the onset of the command pulse. The ensemble averaged current increased in its initial phase (activation). When the command potential was more negative than EK-40 mV, the current decreased after rapid activation due to the inactivation of the channel. The averaged current could be divided into an instantaneous and a time-dependent activation component; the latter was fitted by a single exponential function. The time constant of the time-dependent component became shorter, at more negative command potentials. When compared at the same command potential, the instantaneous component became smaller, as the patch membrane was held at more depolarized potential. This indicates that the steady-state Po of the channel decreases with depolarization at potentials more positive than EK. The Po of the activation gate of the channel was estimated by dividing the steady-state Po of the channel by the Po of the inactivation gate at each potential. It was about 0.1 at EK + 20 mV and increased sigmoidally with hyperpolarization. At potentials more negative than EK-40 mV, the Po of the activation gate saturated and was almost 1. The single-channel analysis and the noise analysis of the steady-state current fluctuations revealed that the activation gate of the channel follows first-order kinetics between the open and closed states. The activation kinetics shifted along the voltage axis in a similar way as EK when different [K+]o were used. Thus, the activation of the channel depends not only on the membrane potential but also on EK, when [K+]o is changed. The macroscopic current flowing through the inward-rectifier K+ channel during the activation process was calculated, assuming that the elementary conductance of the channel is not voltage dependent. The calculated current showed a prominent inward-rectifying property in the steady state and formed a negative conductance region at potentials positive to EK. It was, therefore, concluded that the properties of the inward-rectifier time-independent background K+ current (iK1) in the whole-cell current records (Noble, 1979) mainly depend on the activation kinetics of the inward-rectifier K+ channel in the cardiac myocyte membrane.

Action Potentials↗

PubMed2000

Phosphorylation of the inward-rectifying potassium channel KAT1 by ABR kinase in Vicia guard cells.

A 48-kDa protein kinase was detected in Vicia faba guard cell protoplasts by an in-gel protein kinase assay using a recombinant peptide (KAT1C) of the carboxyl-terminus of an inward-rectifying voltage-dependent K+ channel cloned from Arabidopsis thaliana, KAT1. This protein kinase (ABR* kinase) was activated by pretreatment of guard cell protoplasts with ABA, but not by pretreatment with IAA, 2,4-D, kinetin or GA3. The activation of ABR* kinase was dependent on the time and concentration of ABA. The kinase activity was sensitive to staurosporine and K-252a, protein kinase inhibitors, and insensitive to Ca2+. No ABR* kinase activity was detected in mesophyll cell protoplasts. These characteristics of ABR* kinase are consistent with those of an ABA-responsive protein kinase (ABR kinase) reported previously [Mori and Muto (1997), Plant Physiol. 113: 833]. These results indicate that ABR* kinase phosphorylates the inward-rectifying K+ channel in response to treatment of stomatal guard cells with ABA. The data reported here provide evidence that this ABA-responsive protein kinase may promote ABA signaling by directly phosphorylating guard cell ion channels.

Abscisic Acid↗

PubMed2000

Direct block of inward rectifier potassium channels by nicotine.

Nicotine has been shown to depolarize membrane potential and to lengthen action potential duration in isolated cardiac preparations. To investigate whether this is a consequence of direct interaction of nicotine with inward rectifier K(+) channels which are a key determinant of membrane potentials, we assessed the effects of nicotine on two cloned human inward rectifier K(+) channels, Kir2.1 and Kir2.2, expressed in Xenopus oocytes and the native inward rectifier K(+) current I(K1) in canine ventricular myocytes. Nicotine suppressed Kir2.1-expressed currents at varying potentials negative to -20 mV, with more pronounced effects on the outward current between -70 and -20 mV relative to the inward current at hyperpolarized potentials (below -70 mV). The inhibition was concentration dependent. For the outward currents recorded at -50 mV, the IC50 was 165 +/- 18 microM. Similar effects of nicotine were observed for Kir2.2. A more potent effect was seen with I(K1) in canine myocytes. Significant blockade ( approximately 60%) was found at a concentration as low as 0.5 microM and the IC50 was 4.0 +/- 0.4 microM. The effects in both oocytes and myocytes were partially reversible upon washout of nicotine. Antagonists of nicotinic receptors (mecamylamine, 100 microM), muscarinic receptors (atropine, 1 microM), and beta-adrenergic receptors (propranolol, 1 microM) all failed to restore the depressed currents, suggesting that nicotine acted directly on Kir channels, independent of catecholamine release. This property of nicotine may explain its membrane-depolarizing and action potential duration-prolonging effects in cardiac cells and may contribute in part to its ability to promote propensity for cardiac arrhythmias.

Animals↗

PubMed2001

Tyrosine decaging leads to substantial membrane trafficking during modulation of an inward rectifier potassium channel.

Tyrosine side chains participate in several distinct signaling pathways, including phosphorylation and membrane trafficking. A nonsense suppression procedure was used to incorporate a caged tyrosine residue in place of the natural tyrosine at position 242 of the inward rectifier channel Kir2.1 expressed in Xenopus oocytes. When tyrosine kinases were active, flash decaging led both to decreased K(+) currents and also to substantial (15-26%) decreases in capacitance, implying net membrane endocytosis. A dominant negative dynamin mutant completely blocked the decaging-induced endocytosis and partially blocked the decaging-induced K(+) channel inhibition. Thus, decaging of a single tyrosine residue in a single species of membrane protein leads to massive clathrin-mediated endocytosis; in fact, membrane area equivalent to many clathrin-coated vesicles is withdrawn from the oocyte surface for each Kir2.1 channel inhibited. Oocyte membrane proteins were also labeled with the thiol-reactive fluorophore tetramethylrhodamine-5-maleimide, and manipulations that decreased capacitance also decreased surface membrane fluorescence, confirming the net endocytosis. In single-channel studies, tyrosine kinase activation decreased the membrane density of active Kir2.1 channels per patch but did not change channel conductance or open probability, in agreement with the hypothesis that tyrosine phosphorylation results in endocytosis of Kir2.1 channels. Despite the Kir2.1 inhibition and endocytosis stimulated by tyrosine kinase activation, neither Western blotting nor (32)P labeling produced evidence for direct tyrosine phosphorylation of Kir2.1. Therefore, it is likely that tyrosine phosphorylation affects Kir2.1 function indirectly, via interactions between clathrin adaptor proteins and a tyrosine-based sorting motif on Kir2.1 that is revealed by decaging the tyrosine side chain. These interactions inhibit a fraction of the Kir2.1 channels, possibly via direct occlusion of the conduction pathway, and also lead to endocytosis, which further decreases Kir2.1 currents. These data establish that side chain decaging can provide valuable time-resolved data about intracellular signaling systems.

Animals↗

PubMed1993

Modulation by histamine of an inwardly rectifying potassium channel in human endothelial cells.

1. Whole-cell and single-channel currents were recorded together with intracellular Ca2+ in voltage clamped, single endothelial cells isolated from human umbilical vein. 2. The major current component under resting conditions in the whole-cell configuration was a strongly inwardly rectifying potassium current. 3. This current is due to activation of a K+ channel with an inward conductance of 29 +/- 3 pS (n = 7) with symmetrical 140 mM K+ on both sides of the membrane. This channel could be measured both in the cell-attached and in the inside-out configuration. At potentials below -110 mV both whole-cell and averaged single-channel currents showed a fast inactivation. 4. During stimulation of endothelial cells with histamine, whole-cell K+ currents initially increased but then substantially declined, despite the sustained increase in intracellular Ca2+ concentration ([Ca2+]i). 5. The blockade of the inwardly rectifying K+ channel by histamine could not be observed in cell-attached patches if histamine was added to the bath. 6. It is concluded that endothelial cells possess K+ channels that are directly inhibited by agonists, such as histamine. Blocking these channels may depolarize the cell membrane and thereby reduce the driving force for Ca2+ influx.

Calcium↗

PubMed1999

A steep dependence of inward-rectifying potassium channels on cytosolic free calcium concentration increase evoked by hyperpolarization in guard cells

Inactivation of inward-rectifying K+ channels (IK,in) by a rise in cytosolic free [Ca2+] ([Ca2+]i) is a key event leading to solute loss from guard cells and stomatal closure. However, [Ca2+]i action on IK,in has never been quantified, nor are its origins well understood. We used membrane voltage to manipulate [Ca2+]i (A. Grabov and M.R. Blatt [1998] Proc Natl Acad Sci USA 95: 4778-4783) while recording IK,in under a voltage clamp and [Ca2+]i by Fura-2 fluorescence ratiophotometry. IK,in inactivation correlated positively with [Ca2+]i and indicated a Ki of 329 +/- 31 nM with cooperative binding of four Ca2+ ions per channel. IK,in was promoted by the Ca2+ channel antagonists Gd3+ and calcicludine, both of which suppressed the [Ca2+]i rise, but the [Ca2+]i rise was unaffected by the K+ channel blocker Cs+. We also found that ryanodine, an antagonist of intracellular Ca2+ channels that mediate Ca2+-induced Ca2+ release, blocked the [Ca2+]i rise, and Mn2+ quenching of Fura-2 fluorescence showed that membrane hyperpolarization triggered divalent release from intracellular stores. These and additional results point to a high signal gain in [Ca2+]i control of IK,in and to roles for discrete Ca2+ flux pathways in feedback control of the K+ channels by membrane voltage.

Journal Article↗

PubMed1995

Activation of inwardly rectifying potassium channels (GIRK1) by co-expressed rat brain cannabinoid receptors in Xenopus oocytes.

The neuronal cannabinoid receptor clone was expressed of saturable [3H]WIN 55,212-2 binding sites. Co-expression of the cannabinoid receptor with cRNA coding for the G-protein-gated inwardly rectifying K+ channel (GIRK1) resulted in oocytes exhibiting large inward K+ currents in response to the cannabinoid agonist WIN 55,212-2. The activation of the potassium current by WIN 55,212-2 was dose-dependent with an EC50 of 630 nM. These results suggest that activation of inwardly rectifying K+ channels may be an additional effector mechanism for brain cannabinoid receptors.

Analgesics↗

PubMed1995

Cloning and functional expression of the cDNA encoding an inwardly-rectifying potassium channel expressed in pancreatic beta-cells and in the brain.

A cDNA clone encoding an inwardly-rectifying K-channel (BIR1) was isolated from insulinoma cells. The predicted amino acid sequence shares 72% identity with the cardiac ATP-sensitive K-channel rcKATP (KATP-1;[6]). The mRNA is expressed in the brain and insulinoma cells. Heterologous expression in Xenopus oocytes produced currents which were K(+)-selective, time-independent and showed inward rectification. The currents were blocked by external barium and caesium, but insensitive to tolbutamide and diazoxide. In inside-out patches, channel activity was not blocked by 1 mM internal ATP. The sequence homology with KATP-1 suggests that BIR1 is a subunit of a brain and beta-cell KATP channel. However, pharmacological differences and the lack of ATP-sensitivity, suggest that if, this is the case, heterologous subunits must exert strong modulatory influences on the native channel.

Amino Acid Sequence↗

PubMed1999

Identification by differential display of a hypertonicity-inducible inward rectifier potassium channel highly expressed in chloride cells.

By using differential mRNA display to monitor the molecular alterations associated with adaptation of euryhaline eels to different salinities, we identified a cDNA fragment strongly induced in seawater eel gills. Cloning of a full-length cDNA and its expression in COS-7 cells indicated that the clone codes for an inward rectifier K+ channel (eKir) of 372 amino acid residues, which has two transmembrane segments and a typical pore-forming region (H5). Only low sequence similarities are present, except the H5 region, compared with other members of the inward rectifier K+ channel family (Kir). Consistent with this divergence in the amino acid sequence, a phylogenetic analysis indicated early divergence and independent evolution of eKir from other members; it is only distantly related to the Kir5.0 subfamily members. RNase protection analysis showed that eKir is highly expressed in the seawater eel gill, kidney, and posterior intestine but very weakly in freshwater eels. Immunohistochemistry of gill sections revealed dense localization of eKir in the chloride cells. Immunoelectron microscopy indicated that eKir is mainly present in the microtubular system in the chloride cell. This location and its salt-inducible nature suggest that the eKir channel cloned here is a novel member of the Kir5.0 subfamily of the Kir family and is implicated in osmoregulation.

Adaptation, Physiological↗

PubMed1988

Open-state substructure of inwardly rectifying potassium channels revealed by magnesium block in guinea-pig heart cells.

1. Outward single-channel currents through inwardly rectifying K+ channels of cardiac myocytes were studied in the open cell-attached configuration to clarify the mechanism of the rectification. The outward currents, which were not recorded in the cell-attached configuration, appeared after the inner surface of the patch was exposed to low-Mg2+ solution by rupturing a part of the cell membrane. 2. The single-channel current-voltage (I-V) relation was linear in the absence of Mg2+ and crossed the voltage axis near the equilibrium potential for K+ (EK). The channel conductance was 22 and 16 pS (15-16 degrees C) at external K+ concentrations of 150 and 40 mM, respectively. 3. The channel rapidly closed on stepping the membrane potential of the patch to values more positive than EK. Decay of the average current during depolarization was fitted with a single-exponential function. The time constant appeared voltage dependent, but also tended to increase slowly with time after opening the cell to the bath solution. 4. Mg2+ on the cytoplasmic side blocked the outward currents without affecting the inward currents. The half-saturation concentration of the Mg2+ block was 1.7 microM as examined by measuring the mean patch current at +70 mV. 5. In the presence of internal Mg2+ at a micromolar level (2-10 microM), the outward single-channel current fluctuated between four levels including two intermediate levels (sublevels) in addition to the fully open channel current and the zero-current levels. The I-V relations of each sublevel were equally spaced with an interval of about 7 pS. Corresponding sublevels were found spontaneously in the inward direction. 6. Occupancy at each level was estimated from reconstructed traces at various Mg2+ concentrations and voltages, and compared with the value predicted from the binomial theorem. At different probabilities for the blocked state, the distribution of the current levels showed reasonable agreement with the binomial theorem. These findings suggest that the inwardly rectifying K+ channel of cardiac cells is composed of three identical conducting subunits and each subunit is blocked by Mg2+ independently. 7. Dwell times in each substate were distributed exponentially. On the assumption of the above model, the blocking (mu) and unblocking (lambda) rates were calculated. The value of mu increased with higher Mg2+ concentrations or larger depolarizations, while lambda ranged between 50 and 90 s-1 and seemed independent of Mg2+. 8. Owing to the voltage-dependent block by Mg2+, the average current decayed exponentially on depolarization beyond EK.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1996

Effect of N-feruloyl tyramine (an analogue of tyramine) on inwardly rectifying potassium channel in frog ventricular myocytes.

Electrophysiological effects of N-feruloyl tyramine (NFT), an analogue of tyramine, on potassium currents in frog ventricular myocytes were examined using single-channel recording and whole-cell voltage clamp technique. Extracellular application of NFT induced a concentration-dependent decrease of macroscopic inward rectifier potassium current (iK1) with ID50 of 198 microM, while tyramine (100 microM) was ineffective in producing an inhibitory effect on iK1. NFT reduced the mean open time of iK1 to 1.3 ms from 3.1 ms in control without affecting the amplitude of single-channel conductance. It is indicated that boi containing NFT produces a prolongation of the plateau phase caused by the suppression of inwardly rectifying K channel. Thus, this prolongation may induce an increase in the inflow of Ca ions, which in turn leads to a positive inotropic effect.

Animals↗

PubMed1996

Cloning of a G protein-activated inwardly rectifying potassium channel from human cerebellum.

Based on sequence homology with the rat atrial G protein-coupled muscarinic potassium channel (GIRK1 or KGA1/KGB1), a human cDNA encoding a G protein-activated inwardly rectifying K+ channel (HGIRK1) was isolated. The cDNA encodes a protein of 501 amino acids and shares 99% identity to rat GIRK1 in its total amino acid sequence. Southern blot analysis of genomic DNA indicates a high degree of conservation among various species. In the human population a useful NlaIII restriction fragment length polymorphism was found in the coding sequence of HGIRK1. Co-expression of HGIRK1 and the 5-HT1A receptor in Xenopus oocytes resulted in opening of the channel upon treatment with serotonin. HGIRK1 currents showed strong inward rectification and could be blocked by extracellular Ba2+. Northern blot analysis shows that HGIRK1 expression in human is most abundant in the brain, while lower levels are round in kidney and heart.

Amino Acid Sequence↗