Block of squid axon K channels by internally and externally applied barium ions.

We have studied the interactions of Ba ion with K channels. Ba2+ blocks these channels when applied either internally or externally in millimolar concentrations. Periodic depolarizations enhance block with internal Ba2+, but diminish the block caused by external Ba2+. At rest, dissociation of Ba2+ from blocked channels is very slow, as ascertained by infrequent test pulses applied after washing Ba2+ form either inside or outside. The time constant for recovery from internal and external Ba2+ is the same. Frequent pulsing greatly shortens recovery time constant after washing away both Ba2+in and Ba2+out. Block by Ba2+ applied internally or externally is voltage dependent. Internal Ba2+ block behaves like a one-step reaction governed by a dissociation constant (Kd) that decreases e-fold/12 mV increase of pulse voltage: block deepens with more positive pulse voltage. For external Ba2+, Kd decreases e-fold/18 mV as holding potential is made more negative: block deepens with increasing negativity. Millimolar external concentrations of some cations can either lessen (K+) or enhance (NH+4, Cs+) block by external Ba2+. NH+4 apparently enhances block by slowing exist of Ba ions from the channels. Rb+ and Cs+ also slow clearing of Ba ions from channels. We think that (a) internally applied Ba2+ moves all the way through the channels, entering only when activation gates are open; (b) externally applied Ba2+ moves two-thirds of the way in, entering predominantly when activation gates are closed; (c) at a given voltage, Ba2+ occupies the same position in the channels whether it entered from inside or outside.

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Activation and Two Modes of Blockade by Strontium of Ca2+-activated K+ Channels in Goldfish Saccular Hair Cells

The Journal of General Physiology ◽

10.1085/jgp.111.2.363 ◽

1998 ◽

Vol 111 (2) ◽

pp. 363-379 ◽

Cited By ~ 11

Author(s):

Izumi Sugihara

Keyword(s):

Dissociation Constant ◽

Time Constant ◽

Hair Cells ◽

Single Channel ◽

Hill Coefficient ◽

K Channels ◽

Voltage Dependent ◽

Time Courses ◽

The Hill ◽

Inside Out

Effects of internal Sr2+ on the activity of large-conductance Ca2+-activated K+ channels were studied in inside-out membrane patches from goldfish saccular hair cells. Sr2+ was approximately one-fourth as potent as Ca2+ in activating these channels. Although the Hill coefficient for Sr2+ was smaller than that for Ca2+, maximum open-state probability, voltage dependence, steady state gating kinetics, and time courses of activation and deactivation of the channel were very similar under the presence of equipotent concentrations of Ca2+ and Sr2+. This suggests that voltage-dependent activation is partially independent of the ligand. Internal Sr2+ at higher concentrations (>100 μM) produced fast and slow blockade both concentration and voltage dependently. The reduction in single-channel amplitude (fast blockade) could be fitted with a modified Woodhull equation that incorporated the Hill coefficient. The dissociation constant at 0 mV, the Hill coefficient, and zd (a product of the charge of the blocking ion and the fraction of the voltage difference at the binding site from the inside) in this equation were 58–209 mM, 0.69–0.75, 0.45–0.51, respectively (n = 4). Long shut events (slow blockade) produced by Sr2+ lasted ∼10–200 ms and could be fitted with single-exponential curves (time constant, τl−s) in shut-time histograms. Durations of burst events, periods intercalated by long shut events, could also be fitted with single exponentials (time constant, τb). A significant decrease in τb and no large changes in τl−s were observed with increased Sr2+ concentration and voltage. These findings on slow blockade could be approximated by a model in which single Sr2+ ions bind to a blocking site within the channel pore beyond the energy barrier from the inside, as proposed for Ba2+ blockade. The dissociation constant at 0 mV and zd in the Woodhull equation for this model were 36–150 mM and 1–1.8, respectively (n = 3).

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

The Journal of General Physiology ◽

10.1085/jgp.68.5.519 ◽

1976 ◽

Vol 68 (5) ◽

pp. 519-535 ◽

Cited By ~ 179

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.

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Trapping of Organic Blockers by Closing of Voltage-dependent K+ Channels

The Journal of General Physiology ◽

10.1085/jgp.109.5.527 ◽

1997 ◽

Vol 109 (5) ◽

pp. 527-535 ◽

Cited By ~ 161

Author(s):

Miguel Holmgren ◽

Paula L. Smith ◽

Gary Yellen

Keyword(s):

Quaternary Ammonium ◽

Organic Molecules ◽

Slow Phase ◽

Channel Blockers ◽

Wild Type ◽

Squid Axon ◽

Small Organic Molecules ◽

K Channels ◽

Voltage Dependent ◽

Ammonium Compounds

Small organic molecules, like quaternary ammonium compounds, have long been used to probe both the permeation and gating of voltage-dependent K+ channels. For most K+ channels, intracellularly applied quaternary ammonium (QA) compounds such as tetraethylammonium (TEA) and decyltriethylammonium (C10) behave primarily as open channel blockers: they can enter the channel only when it is open, and they must dissociate before the channel can close. In some cases, it is possible to force the channel to close with a QA blocker still bound, with the result that the blocker is “trapped.” Armstrong (J. Gen. Physiol. 58:413–437) found that at very negative voltages, squid axon K+ channels exhibited a slow phase of recovery from QA blockade consistent with such trapping. In our studies on the cloned Shaker channel, we find that wild-type channels can trap neither TEA nor C10, but channels with a point mutation in S6 can trap either compound very efficiently. The trapping occurs with very little change in the energetics of channel gating, suggesting that in these channels the gate may function as a trap door or hinged lid that occludes access from the intracellular solution to the blocker site and to the narrow ion-selective pore.

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Barium ions enter chromaffin cells via voltage-dependent calcium channels and induce secretion by a mechanism independent of calcium

Journal of Biological Chemistry ◽

10.1016/s0021-9258(18)83130-1 ◽

1989 ◽

Vol 264 (14) ◽

pp. 7914-7920

Author(s):

E Heldman ◽

M Levine ◽

L Raveh ◽

H B Pollard

Keyword(s):

Calcium Channels ◽

Chromaffin Cells ◽

Barium Ions ◽

Voltage Dependent ◽

Voltage Dependent Calcium Channels

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Atypical antipsychotic olanzapine inhibits voltage-dependent K+ channels in coronary arterial smooth muscle cells

Pharmacological Reports ◽

10.1007/s43440-021-00299-z ◽

2021 ◽

Author(s):

Minji Kang ◽

Jin Ryeol An ◽

Mi Seon Seo ◽

Hee Seok Jung ◽

Ryeon Heo ◽

...

Keyword(s):

Smooth Muscle ◽

Smooth Muscle Cells ◽

Atypical Antipsychotic ◽

Muscle Cells ◽

Arterial Smooth Muscle ◽

K Channels ◽

Voltage Dependent ◽

Arterial Smooth Muscle Cells

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Activation-inactivation of potassium channels and development of the potassium-channel spike in internally perfused squid giant axons.

The Journal of General Physiology ◽

10.1085/jgp.78.1.43 ◽

1981 ◽

Vol 78 (1) ◽

pp. 43-61 ◽

Cited By ~ 17

Author(s):

I Inoue

Keyword(s):

Potassium Channels ◽

Potassium Channel ◽

Voltage Clamp ◽

Time Constant ◽

Equilibrium Potential ◽

Giant Axons ◽

Dependent Inactivation ◽

Voltage Dependent ◽

Time Courses ◽

Calcium Permeability

A spike that is the result of calcium permeability through potassium channels was separated from the action potential is squid giant axons internally perfused with a 30 mM NaF solution and bathed in a 100 mM CaCl2 solution by blocking sodium channels with tetrodotoxin. Currents through potassium channels were studied under voltage clamp. The records showed a clear voltage-dependent inactivation of the currents. The inactivation was composed of at least two components; one relatively fast, having a time constant of 20--30 ms, and the other very slow, having a time constant of 5--10 s. Voltage clamp was carried out with a variety of salt compositions in both the internal and external solutions. A similar voltage-dependent inactivation, also composed of the two components, was recognized in all the current through potassium channels. Although the direction and intensity of current strongly depended on the salt composition of the solutions, the time-courses of these currents at corresponding voltages were very similar. These results strongly suggest that the inactivation of the currents in attributable to an essential, dynamic property of potassium channels themselves. Thus, the generation of a potassium-channel spike can be understood as an event that occurs when the equilibrium potential across the potassium channel becomes positive.

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Altered Regulation of Potassium and Calcium Channels by GABAB and Adenosine Receptors in Hippocampal Neurons From Mice Lacking Gαo

Journal of Neurophysiology ◽

10.1152/jn.2000.83.2.1010 ◽

2000 ◽

Vol 83 (2) ◽

pp. 1010-1018 ◽

Cited By ~ 33

Author(s):

Gabriela J. Greif ◽

Deborah L. Sodickson ◽

Bruce P. Bean ◽

Eva J. Neer ◽

Ulrike Mende

Keyword(s):

Time Constant ◽

Hippocampal Neurons ◽

Ionic Currents ◽

Absolute Requirement ◽

Voltage Dependent ◽

Hippocampal Ca3 ◽

Ca3 Neurons ◽

Inwardly Rectifying ◽

G Protein Coupled

To examine the role of Go in modulation of ion channels by neurotransmitter receptors, we characterized modulation of ionic currents in hippocampal CA3 neurons from mice lacking both isoforms of Gαo. In CA3 neurons from Gαo −/− mice, 2-chloro-adenosine and the GABAB-receptor agonist baclofen activated inwardly rectifying K+ currents and inhibited voltage-dependent Ca2+ currents just as effectively as in Gαo +/+ littermates. However, the kinetics of transmitter action were dramatically altered in Gαo −/− mice in that recovery on washout of agonist was much slower. For example, recovery from 2-chloro-adenosine inhibition of calcium current was more than fourfold slower in neurons from Gαo −/− mice [time constant of 12.0 ± 0.8 (SE) s] than in neurons from Gαo +/+ mice (time constant of 2.6 ± 0.2 s). Recovery from baclofen effects was affected similarly. In neurons from control mice, effects of both baclofen and 2-chloro-adenosine on Ca2+ currents and K+currents were abolished by brief exposure to external N-ethyl-maleimide (NEM). In neurons lacking Gαo, some inhibition of Ca2+ currents by baclofen remained after NEM treatment, whereas baclofen activation of K+ currents and both effects of 2-chloro-adenosine were abolished. These results show that modulation of Ca2+ and K+ currents by G protein-coupled receptors in hippocampal neurons does not have an absolute requirement for Gαo. However, modulation is changed in the absence of Gαo in having much slower recovery kinetics. A likely possibility is that the very abundant Gαo is normally used but, when absent, can readily be replaced by G proteins with different properties.

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Voltage-gated transient currents in bovine adrenal fasciculata cells. I. T-type Ca2+ current.

The Journal of General Physiology ◽

10.1085/jgp.102.2.217 ◽

1993 ◽

Vol 102 (2) ◽

pp. 217-237 ◽

Cited By ~ 30

Author(s):

B Mlinar ◽

B A Biagi ◽

J J Enyeart

Keyword(s):

Time Constant ◽

Low Voltage ◽

Zona Fasciculata ◽

Patch Clamp Technique ◽

Time Constants ◽

Voltage Gated ◽

Wide Range ◽

Boltzmann Function ◽

Voltage Dependent ◽

Ca2 Channels

The whole cell version of the patch clamp technique was used to identify and characterize voltage-gated Ca2+ channels in enzymatically dissociated bovine adrenal zona fasciculata (AZF) cells. The great majority of cells (84 of 86) expressed only low voltage-activated, rapidly inactivating Ca2+ current with properties of T-type Ca2+ current described in other cells. Voltage-dependent activation of this current was fit by a Boltzmann function raised to an integer power of 4 with a midpoint at -17 mV. Independent estimates of the single channel gating charge obtained from the activation curve and using the "limiting logarithmic potential sensitivity" were 8.1 and 6.8 elementary charges, respectively. Inactivation was a steep function of voltage with a v1/2 of -49.9 mV and a slope factor K of 3.73 mV. The expression of a single Ca2+ channel subtype by AZF cells allowed the voltage-dependent gating and kinetic properties of T current to be studied over a wide range of potentials. Analysis of the gating kinetics of this Ca2+ current indicate that T channel activation, inactivation, deactivation (closing), and reactivation (recovery from inactivation) each include voltage-independent transitions that become rate limiting at extreme voltages. Ca2+ current activated with voltage-dependent sigmoidal kinetics that were described by an m4 model. The activation time constant varied exponentially at test potentials between -30 and +10 mV, approaching a voltage-independent minimum of 1.6 ms. The inactivation time constant (tau i) also decreased exponentially to a minimum of 18.3 ms at potentials positive to 0 mV. T channel closing (deactivation) was faster at more negative voltages; the deactivation time constant (tau d) decreased from 8.14 +/- 0.7 to 0.48 +/- 0.1 ms at potentials between -40 and -150 mV. T channels inactivated by depolarization returned to the closed state along pathways that included two voltage-dependent time constants. tau rec-s ranged from 8.11 to 4.80 s when the recovery potential was varied from -50 to -90 mV, while tau rec-f decreased from 1.01 to 0.372 s. At potentials negative to -70 mV, both time constants approached minimum values. The low voltage-activated Ca2+ current in AZF cells was blocked by the T channel selective antagonist Ni2+ with an IC50 of 20 microM. At similar concentrations, Ni2+ also blocked cortisol secretion stimulated by adrenocorticotropic hormone. Our results indicate that bovine AZF cells are distinctive among secretory cells in expressing primarily or exclusively T-type Ca2+ channels.(ABSTRACT TRUNCATED AT 400 WORDS)

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