scholarly journals Voltage-dependent gating of veratridine-modified Na channels.

1986 ◽  
Vol 87 (1) ◽  
pp. 25-46 ◽  
Author(s):  
M D Leibowitz ◽  
J B Sutro ◽  
B Hille
Keyword(s):  
Time Course ◽  
Drug Binding ◽  
Na Channels ◽  
Current Voltage ◽  
Instantaneous Current ◽  
Activation Gating ◽  
Voltage Dependent ◽  
Ca Ions ◽  
Rate Of Recovery ◽  
Gating Process

Na channels of frog muscle fibers treated with 100 microM veratridine became transiently modified after a train of repetitive depolarizations. They open and close reversibly with a gating process whose midpoint lies 93 mV more negative than the midpoint of normal activation gating and whose time course shows no appreciable delay in the opening or closing kinetics but still requires more than two kinetic states. Like normal activation, the voltage dependence of the modified gating can be shifted by changing the bathing Ca2+ concentration. The instantaneous current-voltage relation of veratridine-modified channels is curved at potentials negative to -90 mV, as if external Ca ions produced a voltage-dependent block but also permeated. Modified channels probably carry less current than normal ones. When the concentration of veratridine is varied between 5 and 100 microM, the initial rate of modification during a pulse train is directly proportional to the concentration, while the rate of recovery from modification after the train is unaffected. These are the properties expected if drug binding and modification of channels can be equated. Hyperpolarizations that close modified channels slow unbinding. Allethrin and DDT also modify channels. They bind and unbind far faster than veratridine does, and their binding requires open channels.

L- and T-type voltage-gated Ca2+ currents in adrenal medulla endothelial cells

1999 ◽  
Vol 276 (4) ◽  
pp. H1313-H1322 ◽  
Author(s):  
Raul Vinet ◽  
Fernando F. Vargas
Keyword(s):  
Endothelial Cells ◽  
Adrenal Medulla ◽  
Time Course ◽  
Maximum Amplitude ◽  
Secretory Cells ◽  
Activation Time ◽  
Patch Clamp Technique ◽  
Activation Threshold ◽  
Current Voltage ◽  
Voltage Dependent

We investigated voltage-dependent Ca2+ channels of bovine adrenal medulla endothelial cells with the whole cell version of the patch-clamp technique. Depolarization elicited an inward current that was carried by Ca2+ and was composed of a transient (T) current, present in all the cells tested, and a sustained (L) current, present in 65% of them. We separated these currents and measured their individual kinetic and gating properties. The activation threshold for T current was approximately −50 mV, and its maximum amplitude was −49.8 ± 4.8 pA (means ± SE, n = 19) at 0 mV. The time constant was 10.2 ± 1.5 ms ( n= 4) for activation and 18.4 ± 2.8 ms ( n = 4) for inactivation. The L current activated at −40 mV, and it reached a plateau at −20.1 ± 2.3 pA ( n = 6). Its activation time course was a single exponential with an activation time contant of 26.8 ± 2.3 ms ( n = 4). Current-voltage curves, kinetics, gating, response to BAY K 8644, nifedipine, amiloride, and different selectivity for Ba2+ and Ca2+ indicated that the underlying channels for the observed currents are only of the T- and L-types that resemble those of the endocrine secretory cells.

scholarly journals Augmentation of Recovery from Inactivation by Site-3 Na Channel Toxins

1999 ◽  
Vol 113 (2) ◽  
pp. 333-346 ◽  
Author(s):  
G. Richard Benzinger ◽  
Gayle S. Tonkovich ◽  
Dorothy A. Hanck
Keyword(s):  
Time Course ◽  
Genetic Diseases ◽  
Low Frequency ◽  
Periodic Paralysis ◽  
Na Channel ◽  
Na Channels ◽  
Cell Recovery ◽  
Reverse Transcription Pcr ◽  
Recovery From Inactivation ◽  
Voltage Dependent

Site-3 toxins isolated from several species of scorpion and sea anemone bind to voltage-gated Na channels and prolong the time course of INa by interfering with inactivation with little or no effect on activation, effects that have similarities to those produced by genetic diseases in skeletal muscle (myotonias and periodic paralysis) and heart (long QT syndrome). Some published reports have also reported the presence of a noninactivating persistent current in site-3 toxin-treated cells. We have used the high affinity site-3 toxin Anthopleurin B to study the kinetics of this current and to evaluate kinetic differences between cardiac (in RT4-B8 cells) and neuronal (in N1E-115 cells) Na channels. By reverse transcription–PCR from N1E-115 cell RNA multiple Na channel transcripts were detected; most often isolated were sequences homologous to rBrII, although at low frequency sequences homologous to rPN1 and rBrIII were also detected. Toxin treatment induced a voltage-dependent plateau current in both isoforms for which the relative amplitude (plateau current/peak current) approached a constant value with depolarization, although the magnitude was much greater for neuronal (17%) than cardiac (5%) INa. Cell-attached patch recordings revealed distinct quantitative differences in open times and burst durations between isoforms, but for both isoforms the plateau current comprised discrete bursts separated by quiescent periods, consistent with toxin induction of an increase in the rate of recovery from inactivation rather than a modal failure of inactivation. In accord with this hypothesis, toxin increased the rate of whole-cell recovery at all tested voltages. Moreover, experimental data support a model whereby recovery at negative voltages is augmented through closed states rather than through the open state. We conclude that site-3 toxins produce qualitatively similar effects in cardiac and neuronal channels and discuss implications for channel kinetics.

scholarly journals Rapid and slow gating of veratridine-modified sodium channels in frog myelinated nerve.

1989 ◽  
Vol 93 (1) ◽  
pp. 43-65 ◽  
Author(s):  
T A Rando
Keyword(s):  
Resting State ◽  
Time Scale ◽  
Rana Pipiens ◽  
Activation Process ◽  
Na Channels ◽  
Myelinated Nerve ◽  
Qualitative And Quantitative ◽  
Related Alkaloid ◽  
Voltage Dependent ◽  
Gating Process

The properties of voltage-dependent Na channels modified by veratridine (VTD) were studied in voltage-clamped nodes of Ranvier of the frog Rana pipiens. Two modes of gating of VTD-modified channels are described. The first, occurring on a time scale of milliseconds, is shown to be the transition of channels between a modified resting state and a modified open state. There are important qualitative and quantitative differences of this gating process in nerve compared with that in muscle (Leibowitz et al., 1986). A second gating process occurring on a time scale of seconds, was originally described as a modified activation process (Ulbricht, 1969). This process is further analyzed here, and a model is presented in which the slow process represents the gating of VTD-modified channels between open and inactivated states. An expanded model is a step in the direction of unifying the known rapid and slow physiologic processes of Na channels modified by VTD and related alkaloid neurotoxins.

Inhibition of the voltage-dependent calcium current by extracellular ATP in hamster ventricular cardiomyocytes

1997 ◽  
Vol 273 (1) ◽  
pp. H250-H256 ◽  
Author(s):  
F. Von zur Muhlen ◽  
B. D. Gonska ◽  
H. Kreuzer
Keyword(s):  
Calcium Current ◽  
Time Course ◽  
Ventricular Myocytes ◽  
Purinergic Receptor ◽  
Extracellular Atp ◽  
Patch Clamp Technique ◽  
Ventricular Cardiomyocytes ◽  
Current Voltage ◽  
Voltage Dependent ◽  
High Level

The modulation of the high-voltage-activated calcium current (ICa) by external ATP was examined in single ventricular cardiomyocytes of the hamster using the whole-cell configuration of the patch-clamp technique. Extracellular application of ATP (0.1-100 microM) was found to inhibit ICa reversibly. The inhibition followed a slow time course (half time approximately 25 s) and was accompanied by very small changes of the holding current and no shift in the current-voltage relationship. With 100 microM ATP, peak ICa was reduced by approximately 30%. This response was not blocked by the P1 inhibitor 8-cyclopentyl-1,3-dipropylxanthine. The nonhydrolyzable ATP analogs adenosine 5'-O-(3-thiotriphosphate) and AMP-adenosine 5'-[beta,gamma-imido]triphosphate also reduced ICa. The ATP analog alpha,beta-methylene-ATP was about equipotent with ATP at 50 microM. Internal guanosine 5'-O-(3-thiotriphosphate) (200 microM) rendered the ATP-mediated inhibition of ICa poorly reversible, whereas internal guanosine 5'-O-(2-thiodiphosphate) (200-500 microM) had no effect. Holding the intracellular adenosine 3',5'-cyclic monophosphate concentration at a constant high level did not alter the ATP response. We conclude that external ATP inhibits ICa via a P2 purinergic receptor in hamster ventricular myocytes. Our results suggest the involvement of a G protein not coupled to adenylate cyclase. The inhibition of ICa by extracellular ATP might have pathophysiological relevance under conditions of myocardial injury.

Gap junction channel gating at bass retinal electrical synapses

1996 ◽  
Vol 13 (6) ◽  
pp. 1049-1057 ◽  
Author(s):  
Chengbiao Lu ◽  
Douglas G. McMahon
Keyword(s):  
Time Constant ◽  
Time Course ◽  
Single Channel ◽  
Horizontal Cells ◽  
Electrical Synapses ◽  
Dependent Inactivation ◽  
Current Voltage ◽  
Voltage Dependent ◽  
Current Voltage Relationship ◽  
Voltage Relationship

AbstractTo further characterize the properties of retinal horizontal cell electrical synapses, we have studied the gating characteristics of gap junctions between cone-driven horizontal cells from the hybrid striped bass retina using double whole-cell voltage-clamp techniques. In a total of 105 cell pairs, the macroscopic conductance ranged from 0.4–100 nS with most cell pairs exhibiting junctional conductances between 10 and 30 nS. The junctional current-voltage relationship showed that peak or instantaneous currents (Iinst) were linear within the Vj range of ±100 mV and that steady-state junctional currents (Iss) exhibited rectification with increasing voltage beginning around ±30–40 mV Vj. The normalized junctional current-voltage relationship was well fit by a two-state Boltzmann distribution, with an effective gating charge of 1.9 charges/channel, a half-maximal voltage of approximately ±55 mV, and a normalized residual conductance of 0.28. The decay of junctional current followed a single exponential time course with the time constant decreasing with increasing Vj. Recovery of junctional current from voltage-dependent inactivation takes about 1 s following a pulse of 80 mV, and is about five times slower than the inactivation time course at the same Vj. Single-channel analysis showed that the unitary conductance of junctional channels is 50–70 pS. The overall open probability decreased in a voltage-dependent manner. Both the mean channel open time and the frequency of channel opening decreased, while the channel closure time increased. The ratio of closed time/total recording time significantly increased as Vj increased. Increased Vj reduced the number of events at all levels and shifted the unitary conductance to a lower level. Kinetic analysis of channel open duration showed that the distribution of channel open times was best fit by two exponentials and increased Vj significantly reduced the slower time constant. These results indicate that bass retina horizontal cells exhibit voltage-dependent inactivation of macroscopic junctional current. The inactivation occurs at the single-channel level mainly by increasing the rate of closure of voltage-sensitive channels.

scholarly journals Zn2(+)-induced subconductance events in cardiac Na+ channels prolonged by batrachotoxin. Current-voltage behavior and single-channel kinetics.

1991 ◽  
Vol 97 (1) ◽  
pp. 117-142 ◽  
Author(s):  
L Schild ◽  
A Ravindran ◽  
E Moczydlowski
Keyword(s):  
Dwell Time ◽  
Single Channel ◽  
Kinetic Scheme ◽  
Dissociation Rate ◽  
Canine Heart ◽  
Na Channels ◽  
Channel Kinetics ◽  
Current Voltage ◽  
Voltage Dependent ◽  
Site Blocking

The mechanism of voltage-dependent substate production by external Zn2+ in batrachotoxin-modified Na+ channels from canine heart was investigated by analysis of the current-voltage behavior and single-channel kinetics of substate events. At the single-channel level the addition of external Zn2+ results in an increasing frequency of substate events with a mean duration of approximately 15-25 ms for the substate dwell time observed in the range of -70 to +70 mV. Under conditions of symmetrical 0.2 M NaCl, the open state of cardiac Na+ channels displays ohmic current-voltage behavior in the range of -90 to +100 mV, with a slope conductance of 21 pS. In contrast, the Zn2(+)-induced substate exhibits significant outward rectification with a slope conductance of 3.1 pS in the range of -100 to -50 mV and 5.1 pS in the range of +50 to +100 mV. Analysis of dwell-time histograms of substate events as a function of Zn2+ concentration and voltage led to the consideration of two types of models that may explain this behavior. Using a simple one-site blocking model, the apparent association rate for Zn2+ binding is more strongly voltage dependent (decreasing e-fold per +60 mV) than the Zn2+ dissociation rate (increasing e-fold per +420 mV). However, this simple blocking model cannot account for the dependence of the apparent dissociation rate on Zn2+ concentration. To explain this result, a four-state kinetic scheme involving a Zn2(+)-induced conformational change from a high conductance conformation to a substate conformation is proposed. This model, similar to one introduced by Pietrobon et al. (1989. J. Gen. Physiol. 94:1-24) for H(+)-induced substate behavior in L-type Ca2+ channels, is able to simulate the kinetic and equilibrium behavior of the primary Zn2(+)-induced substate process in heart Na+ channels. This model implies that binding of Zn2+ greatly enhances conversion of the open, ohmic channel to a low conductance conformation with an asymmetric energy profile for Na+ permeation.

scholarly journals Voltage-dependent open-state inactivation of cardiac sodium channels: gating current studies with Anthopleurin-A toxin.

1995 ◽  
Vol 106 (4) ◽  
pp. 617-640 ◽  
Author(s):  
M F Sheets ◽  
D A Hanck
Keyword(s):  
Time Course ◽  
Voltage Dependence ◽  
Na Channel ◽  
Total Charge ◽  
Na Channels ◽  
Gating Currents ◽  
Gating Charge ◽  
Cardiac Purkinje Cells ◽  
Voltage Dependent ◽  
Cardiac Sodium Channels

The gating charge and voltage dependence of the open state to the inactivated state (O-->I) transition was measured for the voltage-dependent mammalian cardiac Na channel. Using the site 3 toxin, Anthopleurin-A (Ap-A), which selectively modifies the O-->I transition (see Hanck, D. A., and M. F. Sheets. 1995. Journal of General Physiology. 106:601-616), we studied Na channel gating currents (Ig) in voltage-clamped single canine cardiac Purkinje cells at approximately 12 degrees C. Comparison of Ig recorded in response to step depolarizations before and after modification by Ap-A toxin showed that toxin-modified gating currents decayed faster and had decreased initial amplitudes. The predominate change in the charge-voltage (Q-V) relationship was a reduction in gating charge at positive potentials such that Qmax was reduced by 33%, and the difference between charge measured in Ap-A toxin and in control represented the gating charge associated with Na channels undergoing inactivation by O-->I. By comparing the time course of channel activation (represented by the gating charge measured in Ap-A toxin) and gating charge associated with the O-->I transition (difference between control and Ap-A charge), the influence of activation on the time course of inactivation could be accounted for and the inherent voltage dependence of the O-->I transition determined. The O-->I transition for cardiac Na channels had a valence of 0.75 e-. The total charge of the cardiac voltage-gated Na channel was estimated to be 5 e-. Because charge is concentrated near the opening transition for this isoform of the channel, the time constant of the O-->I transition at 0 mV could also be estimated (0.53 ms, approximately 12 degrees C). Prediction of the mean channel open time-voltage relationship based upon the magnitude and valence of the O-->C and O-->I rate constants from INa and Ig data matched data previously reported from single Na channel studies in heart at the same temperature.

scholarly journals Kinetics of veratridine action on Na channels of skeletal muscle.

1986 ◽  
Vol 87 (1) ◽  
pp. 1-24 ◽  
Author(s):  
J B Sutro
Keyword(s):  
Time Constant ◽  
Time Course ◽  
Voltage Dependence ◽  
Na Channels ◽  
H Infinity ◽  
Tail Current ◽  
Time Integral ◽  
Activation Gating ◽  
Chloramine T ◽  
Kinetics Of

Veratridine bath-applied to frog muscle makes inactivation of INa incomplete during a depolarizing voltage-clamp pulse and leads to a persistent veratridine-induced Na tail current. During repetitive depolarizations, the size of successive tail currents grows to a plateau and then gradually decreases. When pulsing is stopped, the tail current declines to zero with a time constant of approximately 3 s. Higher rates of stimulation result in a faster build-up of the tail current and a larger maximum value. I propose that veratridine binds only to open channels and, when bound, prevents normal fast inactivation and rapid shutting of the channel on return to rest. Veratridine-modified channels are also subject to a "slow" inactivation during long depolarizations or extended pulse trains. At rest, veratridine unbinds with a time constant of approximately 3 s. Three tests confirm these hypotheses: (a) the time course of the development of veratridine-induced tail currents parallels a running time integral of gNa during the pulse; (b) inactivating prepulses reduce the ability to evoke tails, and the voltage dependence of this reduction parallels the voltage dependence of h infinity; (c) chloramine-T, N-bromoacetamide, and scorpion toxin, agents that decrease inactivation in Na channels, each greatly enhance the tail currents and alter the time course of the appearance of the tails as predicted by the hypothesis. Veratridine-modified channels shut during hyperpolarizations from -90 mV and reopen on repolarization to -90 mV, a process that resembles normal activation gating. Veratridine appears to bind more rapidly during larger depolarizations.

Analysis of the Bias Dependent Spectral Response of a-SiC:H p-i-n Photodiode

2002 ◽  
Vol 715 ◽  
Author(s):  
P. Louro ◽  
A. Fantoni ◽  
Yu. Vygranenko ◽  
M. Fernandes ◽  
M. Vieira
Keyword(s):  
Bias Voltage ◽  
Voltage Dependence ◽  
Spectral Response ◽  
Surface Recombination ◽  
Electrical Field ◽  
Field Reversal ◽  
Current Voltage ◽  
Voltage Dependent ◽  
And Inversion ◽  
Device Operation

AbstractThe bias voltage dependent spectral response (with and without steady state bias light) and the current voltage dependence has been simulated and compared to experimentally obtained values. Results show that in the heterostructures the bias voltage influences differently the field and the diffusion part of the photocurrent. The interchange between primary and secondary photocurrent (i. e. between generator and load device operation) is explained by the interaction of the field and the diffusion components of the photocurrent. A field reversal that depends on the light bias conditions (wavelength and intensity) explains the photocurrent reversal. The field reversal leads to the collapse of the diode regime (primary photocurrent) launches surface recombination at the p-i and i-n interfaces which is responsible for a double-injection regime (secondary photocurrent). Considerations about conduction band offsets, electrical field profiles and inversion layers will be taken into account to explain the optical and voltage bias dependence of the spectral response.

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