scholarly journals Dynamics of aminopyridine block of potassium channels in squid axon membrane.

1976 ◽  
Vol 68 (5) ◽  
pp. 519-535 ◽  
Author(s):  
J Z Yeh ◽  
G S Oxford ◽  
C H Wu ◽  
T Narahashi
Keyword(s):  
Time Course ◽  
Membrane Surface ◽  
K Channel ◽  
Dependent Manner ◽  
Squid Axon ◽  
Axon Membrane ◽  
K Channels ◽  
Voltage Dependent ◽  
K Current ◽  
Kinetics Of

Aminopyridines (2-AP, 3-AP, and 4-AP) selectively block K channels of squid axon membranes in a manner dependent upon the membrane potential and the duration and frequency of voltage clamp pulses. They are effective when applied to either the internal or the external membrane surface. The steady-state block of K channels by aminopyridines is more complete for low depolarizations, and is gradually relieved at higher depolarizations. The K current in the presence of aminopyridines rises more slowly than in control, the change being more conspicuous in 3-AP and 4-AP than in 2-AP. Repetitive pulsing relieves the block in a manner dependent upon the duration and interval of pulses. The recovery from block during a given test pulse is enhanced by increasing the duration of a conditioning depolarizing prepulse. The time constant for this recovery is in the range of 10-20 ms in 3-AP and 4-AP, and shorter in 2-AP. Twin pulse experiments with variable pulse intervals have revealed that the time course for re-establishment of block is much slower in 3-AP and 4-AP than in 2-AP. These results suggest that 2-AP interacts with the K channel more rapidly than 3-AP and 4-AP. The more rapid interaction of 2-AP with K channels is reflected in the kinetics of K current which is faster than that observed in 3-AP or 4-AP, and in the pattern of frequency-dependent block which is different from that in 3-AP or 4-AP. The experimental observations are not satisfactorily described by alterations of Hodgkin-Huxley n-type gating units. Rather, the data are consistent with a simple binding scheme incorporating no changes in gating kinetics which conceives of aminopyridine molecules binding to closed K channels and being released from open channels in a voltage-dependent manner.

scholarly journals Potassium channel types in arterial chemoreceptor cells and their selective modulation by oxygen.

1992 ◽  
Vol 100 (3) ◽  
pp. 401-426 ◽  
Author(s):  
M D Ganfornina ◽  
J López-Barneo
Keyword(s):  
Time Course ◽  
Single Channel ◽  
K Channel ◽  
Open Probability ◽  
K Channels ◽  
Glomus Cells ◽  
Control Value ◽  
Voltage Dependent ◽  
K Current ◽  
Chemoreceptor Cells

Single K+ channel currents were recorded in excised membrane patches from dispersed chemoreceptor cells of the rabbit carotid body under conditions that abolish current flow through Na+ and Ca2+ channels. We have found three classes of voltage-gated K+ channels that differ in their single-channel conductance (gamma), dependence on internal Ca2+ (Ca2+i), and sensitivity to changes in O2 tension (PO2). Ca(2+)-activated K+ channels (KCa channels) with gamma approximately 210 pS in symmetrical K+ solutions were observed when [Ca2+]i was greater than 0.1 microM. Small conductance channels with gamma = 16 pS were not affected by [Ca2+]i and they exhibited slow activation and inactivation time courses. In these two channel types open probability (P(open)) was unaffected when exposed to normoxic (PO2 = 140 mmHg) or hypoxic (PO2 approximately 5-10 mmHg) external solutions. A third channel type (referred to as KO2 channel), having an intermediate gamma(approximately 40 pS), was the most frequently recorded. KO2 channels are steeply voltage dependent and not affected by [Ca2+]i, they inactivate almost completely in less than 500 ms, and their P(open) reversibly decreases upon exposure to low PO2. The effect of low PO2 is voltage dependent, being more pronounced at moderately depolarized voltages. At 0 mV, for example, P(open) diminishes to approximately 40% of the control value. The time course of ensemble current averages of KO2 channels is remarkably similar to that of the O2-sensitive K+ current. In addition, ensemble average and macroscopic K+ currents are affected similarly by low PO2. These observations strongly suggest that KO2 channels are the main contributors to the macroscopic K+ current of glomus cells. The reversible inhibition of KO2 channel activity by low PO2 does not desensitize and is not related to the presence of F-, ATP, and GTP-gamma-S at the internal face of the membrane. These results indicate that KO2 channels confer upon glomus cells their unique chemoreceptor properties and that the O2-K+ channel interaction occurs either directly or through an O2 sensor intrinsic to the plasma membrane closely associated with the channel molecule.

Ba2+, TEA+, and quinine effects on apical membrane K+ conductance and maxi K+ channels in gallbladder epithelium

1990 ◽  
Vol 259 (1) ◽  
pp. C56-C68 ◽  
Author(s):  
Y. Segal ◽  
L. Reuss
Keyword(s):  
Apical Membrane ◽  
Membrane Conductance ◽  
Membrane Resistance ◽  
K Channel ◽  
Dependent Manner ◽  
Gallbladder Epithelium ◽  
Concentration Dependent Manner ◽  
K Channels ◽  
Voltage Dependent ◽  
Channel Currents

The apical membrane of Necturus gallbladder epithelium contains a voltage-activated K+ conductance [Ga(V)]. Large-conductance (maxi) K+ channels underlie Ga(V) and account for 17% of the membrane conductance (Ga) under control conditions. We examined the Ba2+, tetraethylammonium (TEA+), and quinine sensitivities of Ga and single maxi K+ channels. Mucosal Ba2+ addition decreased resting Ga in a concentration-dependent manner (65% block at 5 mM) and decreased Ga(V) in a concentration- and voltage-dependent manner. Mucosal TEA+ addition also decreased control Ga (60% reduction at 5 mM). TEA+ block of Ga(V) was more potent and less voltage dependent that Ba2+ block. Maxi K+ channels were blocked by external Ba2+ at millimolar levels and by external TEA+ at submillimolar levels. At 0.3 mM, quinine (mucosal addition) hyperpolarized the cell membranes by 6 mV and reduced the fractional apical membrane resistance by 50%, suggesting activation of an apical membrane K+ conductance. At 1 mM, quinine both activated and blocked K(+)-conductive pathways. Quinine blocked maxi K+ channel currents at submillimolar concentrations. We conclude that 1) Ba2+ and TEA+ block maxi K+ channels and other K+ channels underlying resting Ga; 2) parallels between the Ba2+ and TEA+ sensitivities of Ga(V) and maxi K+ channels support a role for these channels in Ga(V); and 3) quinine has multiple effects on K(+)-conductive pathways in gallbladder epithelium, which are only partially explained by block of apical membrane maxi K+ channels.

scholarly journals A time- and voltage-dependent K+ current in single cardiac cells from bullfrog atrium.

1986 ◽  
Vol 88 (6) ◽  
pp. 777-798 ◽  
Author(s):  
J R Hume ◽  
W Giles ◽  
K Robinson ◽  
E F Shibata ◽  
R D Nathan ◽  
...  
Keyword(s):  
Time Course ◽  
Voltage Dependence ◽  
Outward Current ◽  
Cardiac Cells ◽  
Delayed Rectifier ◽  
Mechanical Dispersion ◽  
Atrial Cells ◽  
Voltage Dependent ◽  
K Current ◽  
Kinetics Of

Individual myocytes were isolated from bullfrog atrium by enzymatic and mechanical dispersion, and a one-microelectrode voltage clamp was used to record the slow outward K+ currents. In normal [K+]o (2.5 mM), the slow outward current tails reverse between -95 and -100 mV. This finding, and the observed 51-mV shift of Erev/10-fold change in [K+]o, strongly suggest that the "delayed rectifier" in bullfrog atrial cells is a K+ current. This current, IK, plays an important role in initiating repolarization, and it is distinct from the quasi-instantaneous, inwardly rectifying background current, IK. In atrial cells, IK does not exhibit inactivation, and very long depolarizing clamp steps (20 s) can be applied without producing extracellular K+ accumulation. The possibility of [K+]o accumulation contributing to these slow outward current changes was assessed by (a) comparing reversal potentials measured after short (2 s) and very long (15 s) activating prepulses, and (b) studying the kinetics of IK at various holding potentials and after systematically altering [K+]o. In the absence of [K+]o accumulation, the steady state activation curve (n infinity) and fully activated current-voltage (I-V) relation can be obtained directly. The threshold of the n infinity curve is near -50 mV, and it approaches a maximum at +20 mV; the half-activation point is approximately -16 mV. The fully activated I-V curve of IK is approximately linear in the range -40 to +30 mV. Semilog plots of the current tails show that each tail is a single-exponential function, which suggests that only one Hodgkin-Huxley conductance underlies this slow outward current. Quantitative analysis of the time course of onset of IK and of the corresponding envelope of tails demonstrate that the activation variable, n, must be raised to the second power to fit the sigmoid onset accurately. The voltage dependence of the kinetics of IK was studied by recording and curve-fitting activating and deactivating (tail) currents. The resulting 1/tau n curve is U-shaped and somewhat asymmetric; IK exhibits strong voltage dependence in the diastolic range of potentials. Changes in the [Ca2+]o in the superfusing Ringer's, and/or addition of La3+ to block the transmembrane Ca2+ current, show that the time course and magnitude of IK are not significantly modulated by transmembrane Ca2+ movements, i.e., by ICa. These experimentally measured voltage- and time-dependent descriptors of IK strongly suggest an important functional role for IK in atrial tissue: it initiates repolarization and can be an important determinant of rate-induced changes in action potential duration.

scholarly journals Charybdotoxin selectively blocks small Ca-activated K channels in Aplysia neurons.

1987 ◽  
Vol 90 (1) ◽  
pp. 27-47 ◽  
Author(s):  
A Hermann ◽  
C Erxleben
Keyword(s):  
Single Channel ◽  
Delayed Rectifier ◽  
Pacemaker Activity ◽  
Dependent Manner ◽  
Open Probability ◽  
Apparent Dissociation Constant ◽  
External Application ◽  
K Channels ◽  
Voltage Dependent ◽  
K Current

The action of charybdotoxin (ChTX), a peptide component isolated from the venom of the scorpion Leiurus quinquestriatus, was investigated on membrane currents of identified neurons from the marine mollusk, Aplysia californica. Macroscopic current recordings showed that the external application of ChTX blocks the Ca-activated K current in a dose- and voltage-dependent manner. The apparent dissociation constant is 30 nM at V = -30 mV and increases e-fold for a +50- to +70-mV change in membrane potential, which indicates that the toxin molecule is sensitive to approximately 35% of the transmembrane electric field. The toxin is bound to the receptor with a 1:1 stoichiometry and its effect is reversible after washout. The toxin also suppresses the membrane leakage conductance and a resting K conductance activated by internal Ca ions. The toxin has no significant effect on the inward Na or Ca currents, the transient K current, or the delayed rectifier K current. Records from Ca-activated K channels revealed a single channel conductance of 35 +/- 5 pS at V = 0 mV in asymmetrical K solution. The channel open probability increased with the internal Ca concentration and with membrane voltage. The K channels were blocked by submillimolar concentrations of tetraethylammonium ions and by nanomolar concentrations of ChTX, but were not blocked by 4-aminopyridine if applied externally on outside-out patches. From the effects of ChTX on K current and on bursting pacemaker activity, it is concluded that the termination of bursts is in part controlled by a Ca-activated K conductance.

scholarly journals Cation permeation through the voltage-dependent potassium channel in the squid axon. Characteristics and mechanisms.

1987 ◽  
Vol 90 (2) ◽  
pp. 261-290 ◽  
Author(s):  
P K Wagoner ◽  
G S Oxford
Keyword(s):  
Binding Energies ◽  
Ion Concentration ◽  
K Channel ◽  
Delayed Rectifier ◽  
Dependent Manner ◽  
K Channels ◽  
Energy Profiles ◽  
Current Voltage ◽  
Voltage Dependent ◽  
Ion Binding Sites

Characteristics of cation permeation through voltage-dependent delayed rectifier K channels in squid giant axons were examined. Axial wire voltage-clamp measurements and internal perfusion were used to determine conductance and permeability properties. These K channels exhibit conductance saturation and decline with increases in symmetrical K+ concentrations to 3 M. They also produce ion- and concentration-dependent current-voltage shapes. K channel permeability ratios obtained with substitutions of internal Rb+ or NH+4 for K+ are higher than for external substitution of these ions. Furthermore, conductance and permeability ratios of NH+4 or Rb+ to K+ are functions of ion concentration. Conductance measurements also reveal the presence of an anomalous mole fraction effect for NH+4, Rb+, or Tl+ to K+. Finally, internal Cs+ blocks these K channels in a voltage-dependent manner, with relief of block by elevations in external K+ but not external NH+4 or Cs+. Energy profiles for K+, NH+4, Rb+, Tl+, and Cs+ incorporating three barriers and two ion-binding sites are fitted to the data. The profiles are asymmetric with respect to the center of the electric field, have different binding energies and electrical positions for each ion, and (for K+) exhibit concentration-dependent barrier positions.

Halothane and Isoflurane Decrease the Open State Probability of Potassium sup + Channels in Dog Cerebral Arterial Muscle Cells 

1995 ◽  
Vol 82 (2) ◽  
pp. 479-490 ◽  
Author(s):  
Hanna Eskinder ◽  
Debebe Gebremedhin ◽  
Joseph G. Lee ◽  
Nancy J. Rusch ◽  
Franjo D. Supan ◽  
...  
Keyword(s):  
Patch Clamp ◽  
Muscle Cells ◽  
Volatile Anesthetics ◽  
State Probability ◽  
K Channel ◽  
Dependent Manner ◽  
External Application ◽  
Pipette Solution ◽  
K Channels ◽  
K Current

Background Both halothane and isoflurane evoke cerebral vasodilation. One of the potential mechanisms for arterial vasodilation is enhanced K+ efflux resulting from an increased opening frequency of membrane K+ channels. The current study was designed to determine the effects of volatile anesthetics on K+ channel current in single vascular smooth muscle cells isolated from dog cerebral arteries. Methods Patch clamp recording techniques were used to investigate the effects of volatile anesthetics on macroscopic and microscopic K+ channel currents. Results In the whole-cell patch-clamp mode, in cells dialyzed with pipette solution containing 2.5 mM EGTA and 1.8 mM CaCl2, depolarizing pulses from -60 to +60 mV elicited an outward K+ current that was blocked 65 +/- 5% by 3 mM tetraethylammonium (TEA). Halothane (0.4 and 0.9 mM) depressed the amplitude of this current by 18 +/- 4% and 34 +/- 6%, respectively. When 10 mM EGTA was used in the pipette solution to strongly buffer intracellular free Ca2+, an outward K+ current insensitive to 3 mM TEA was elicited. This K+ current, which was reduced 51 +/- 4% by 1 mM 4-aminopyridine, was also depressed by 17 +/- 5% and 29 +/- 7% with application of 0.4 and 0.9 mM halothane, respectively. In cell-attached patches using 145 mM KCl in the pipette solution and 5.2 mM KCl in the bath, the unitary conductance of the predominant channel type detected was 99 pS. External application of TEA (0.1 to 3 mM) reduced the unitary current amplitude of the 99 pS K+ channel in a concentration-dependent manner. The open state probability of this 99 pS K+ channel was increased by 1 microM Ca2+ ionophore (A23187). These findings indicate that the 99 pS channel measured in cell-attached patches was a TEA-sensitive, Ca(2+)-activated K+ channel. Halothane and isoflurane reversibly decreased the open state probability (NPo), mean open time, and frequency of opening of this 99 pS K+ channel without affecting single channel amplitude or the slope of the current-voltage relationship. Conclusions Halothane and isoflurane suppress the activity of K+ channels in canine cerebral arterial cells. These results suggest that mechanisms other than K+ channel opening likely mediate volatile anesthetic-induced vasodilation.

scholarly journals AKT2/3 Subunits Render Guard Cell K+ Channels Ca2+ Sensitive

2005 ◽  
Vol 125 (5) ◽  
pp. 483-492 ◽  
Author(s):  
Natalya Ivashikina ◽  
Rosalia Deeken ◽  
Susanne Fischer ◽  
Peter Ache ◽  
Rainer Hedrich
Keyword(s):  
Guard Cell ◽  
Guard Cells ◽  
Inward Rectifier ◽  
Hek293 Cells ◽  
K Channel ◽  
Dependent Manner ◽  
Wild Type ◽  
K Channels ◽  
Isolation And Characterization ◽  
Voltage Dependent

Inward-rectifying K+ channels serve as a major pathway for Ca2+-sensitive K+ influx into guard cells. Arabidopsis thaliana guard cell inward-rectifying K+ channels are assembled from multiple K+ channel subunits. Following the recent isolation and characterization of an akt2/3-1 knockout mutant, we examined whether the AKT2/3 subunit carries the Ca2+ sensitivity of the guard cell inward rectifier. Quantification of RT-PCR products showed that despite the absence of AKT2 transcripts in guard cells of the knockout plant, expression levels of the other K+ channel subunits (KAT1, KAT2, AKT1, and AtKC1) remained largely unaffected. Patch-clamp experiments with guard cell protoplasts from wild type and akt2/3-1 mutant, however, revealed pronounced differences in Ca2+ sensitivity of the K+ inward rectifier. Wild-type channels were blocked by extracellular Ca2+ in a concentration- and voltage-dependent manner. Akt2/3-1 mutants lacked the voltage-dependent Ca2+ block, characteristic for the K+ inward rectifier. To confirm the akt2/3-1 phenotype, two independent knockout mutants, akt2-1 and akt2::En-1 were tested, demonstrating that the loss of AKT2/3 indeed affects the Ca2+ dependence of guard cell inward rectifier. In contrast to AKT2 knockout plants, AKT1, AtKC1, and KAT1 loss-of-function mutants retained Ca2+ block of the guard cell inward rectifier. When expressed in HEK293 cells, AKT2 channel displayed a pronounced susceptibility toward extracellular Ca2+, while the dominant guard cell K+ channel KAT2 was Ca2+ insensitive. Thus, we conclude that the AKT2/3 subunit constitutes the Ca2+ sensitivity of the guard cell K+ uptake channel.

A novel approach allows identification of K channels in the lateral membrane of rat CCD

1994 ◽  
Vol 266 (5) ◽  
pp. F813-F822 ◽  
Author(s):  
W. H. Wang ◽  
C. M. McNicholas ◽  
A. S. Segal ◽  
G. Giebisch
Keyword(s):  
Membrane Potential ◽  
Cell Membrane ◽  
Cell Membrane Potential ◽  
K Channel ◽  
Dependent Manner ◽  
Patch Clamp Technique ◽  
K Channels ◽  
Lateral Membrane ◽  
Novel Approach ◽  
Voltage Dependent

We have developed a novel approach to study K channels in the lateral membrane of principal cells (PC) in rat cortical collecting ducts (CCD). The technique consists of 1) exposing the CCD apical membrane, 2) removing the intercalated cells adjoining a PC by gentle suction through a pipette, and 3) applying patch-clamp technique to the lateral membrane of PC. Functional viability of the PC was confirmed by three indexes: 1) maintenance of physiological cell membrane potentials (-85 +/- 3 mV); 2) depolarization of the cell membrane potential with 1 mM Ba2+; and 3) hyperpolarization of the cell potential with 0.1 mM amiloride. Two types of K channels were identified: a low-conductance K channel and an intermediate-conductance K channel. In cell-attached patches the slope conductance of the low-conductance K channel was 27 pS and that of the intermediate-conductance K channel was 45 pS. The open probability (Po) of the 27-pS K channel was 0.81 +/- 0.02 and was not voltage dependent. In contrast, the Po of the 45-pS K channel was 0.23 +/- 0.01 at the spontaneous cell membrane potential and was increased by hyperpolarization. In addition, decrease of the bath pH from 7.4 to 6.7 reduced the 27-pS K channel current amplitude in a voltage-dependent manner, but the Po was not affected. Finally, two time constants were required to fit open- and closed-time histograms of both populations of K channels. Application of 1 mM Ba2+ completely blocked these K channels. We conclude that two types of K channel are present in the basolateral membrane of PC.

Ca-activated K channels in apical membrane of mammalian CCT, and their role in K secretion

1987 ◽  
Vol 252 (3) ◽  
pp. F458-F467 ◽  
Author(s):  
G. Frindt ◽  
L. G. Palmer
Keyword(s):  
Apical Membrane ◽  
K Channel ◽  
Dependent Manner ◽  
Patch Clamp Technique ◽  
K Channels ◽  
Voltage Dependent ◽  
K Secretion ◽  
Excised Patches ◽  
Cytoplasmic Side ◽  
High Conductance

High conductance, Ca-activated K channels were studied in the apical membrane of the rat cortical collecting tubule (CCT) using the patch-clamp technique. In cell-attached patches the channels were found mainly in the closed state at the spontaneous apical membrane potential. They spent progressively more time in the open state as the pipette potential was made negative relative to the bath. In excised patches these channels had a high selectivity for K over Na and were activated by micromolar concentrations of Ca2+ on the cytoplasmic side of the membrane in a voltage-dependent manner. They had a low conductance to Rb and were blocked by Ba (1-100 microM) from the cytoplasmic side and tetraethylammonium (TEA) (0.2-1 mM) from the luminal side. Block by external TEA and small conductance to Rb were used to investigate the role of these channels in K transport by the isolated perfused rabbit CCT. Ba (2.5 mM), a well-studied blocker of apical K conductance in this segment, hyperpolarized the transepithelial voltage (VT) by 3.7 +/- 0.9 mV when added to the luminal solution of the perfused tubule. Addition of TEA (5 mM) to the luminal solution has no effect on VT. When Na transport was abolished by luminal amiloride, perfusion with 30 mM K (replacing Na) resulted in a lumen-negative VT (18-34 mV). Under these conditions, VT was reduced by 6.0 +/- 1.5 mV by 2.5 mM Ba, whereas TEA had no effect. Perfusion with 30 mM Rb (replacing Na) also caused a lumen-negative VT that was approximately 50% of that observed with 30 mM K. The apical K conductance of the perfused CCT appears to be insensitive to luminal TEA and only modestly selective for K over Rb. This conductance, at least under the conditions of our studies, is probably not mediated by the high conductance Ca-activated K channel.

scholarly journals K+ CHANNEL PERMEATION AND BLOCK IN THE MIDGUT EPITHELIUM OF THE TOBACCO HORNWORM MANDUCA SEXTA

1994 ◽  
Vol 197 (1) ◽  
pp. 179-200
Author(s):  
K Schirmanns ◽  
W Zeiske
Keyword(s):  
Manduca Sexta ◽  
Short Circuit ◽  
Short Circuit Current ◽  
K Channel ◽  
K Channels ◽  
Zero Current ◽  
Channel Opening ◽  
Voltage Dependent ◽  
K Current

The K+-secreting larval midgut of Manduca sexta in vitro was voltage- or current-clamped. In contrast to Tl+, NH4+ and Na+, both Rb+ and K+ generated a short-circuit current, although with different saturation kinetics. The dependence of the short-circuit current on Rb+/K+ mole fraction gave no evidence for multi-ion occupation of the basolateral K+ channels. After 'functionally' eliminating the apical membranes using the ionophore amphotericin B and the 'apical K+ pump' blockers trimethyltin chloride or Tl+, the K+ channels could be more closely investigated. By measuring zero-current potentials, permeability ratios PX/PK were estimated using an adapted version of the Goldman­Hodgkin­Katz voltage equation. Their sequence was K+ (1) = Tl+ > Rb+ (0.38) > NH4+ (~0.3) > Cs+ (0.03) > Na+ (~0). The K+ channels could not be blocked by basally applied Cs+, Na+ or tetraethylammonium. Blockade of K+ current by Ba2+ was typically voltage-dependent, but only at moderate transbasal voltages. The relative electrical distance delta of the Ba2+ binding site from the basal channel opening was determined to be 0.2. At zero transbasal voltage, the apparent inhibition constant for barium KBa* was 1.7 mmol l-1.

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