scholarly journals Kinetics of intramembrane charge movement and conductance activation of batrachotoxin-modified sodium channels in frog node of Ranvier.

1985 ◽  
Vol 86 (3) ◽  
pp. 381-394 ◽  
Author(s):  
J M Dubois ◽  
M F Schneider
Keyword(s):  
Pulse Duration ◽  
Sodium Channels ◽  
Time Course ◽  
Sodium Current ◽  
Node Of Ranvier ◽  
Charge Movement ◽  
Exponential Time ◽  
Current Activation ◽  
Time Courses ◽  
Single Exponential

Sodium current and intramembrane gating charge movement (Q) were monitored in voltage-clamped frog node of Ranvier after modification of all sodium channels by batrachotoxin (BTX). Sodium current activation followed a single-exponential time course, provided a delay was interposed between the onset of the step ON depolarization and that of the current change. The delay decreased with increased ON depolarization and, for a constant ON depolarization, increased with prehyperpolarization. ON charge movement followed a single-exponential time course with time constants tau Q,ON slightly larger than tau Na, ON. For pulses between -70 and -50 mV, tau Q,ON/tau Na,ON = 1.14 +/- 0.08. The OFF charge movement and OFF sodium current tails after a depolarizing pulse followed single-exponential time courses, with tau Q, OFF larger than tau Na, OFF. tau Q,OFF/tau Na,OFF increased with OFF voltage from 1 near -100 mV to 2 near -160 mV. At a set OFF potential (-120 mV), both tau Q,OFF and tau Na,OFF increased with ON pulse duration. The delay in INa activation and the effect of ON pulse duration on tau Q,OFF and tau Na,OFF are inconsistent with a simple two-state, single-transition model for the gating of batrachotoxin-modified sodium channels.

scholarly journals Kinetics of intramembrane charge movement and sodium current in frog node of Ranvier.

1982 ◽  
Vol 79 (4) ◽  
pp. 571-602 ◽  
Author(s):  
J M Dubois ◽  
M F Schneider
Keyword(s):  
Time Constant ◽  
Time Course ◽  
Sodium Current ◽  
Node Of Ranvier ◽  
Charge Movement ◽  
Intramembrane Charge Movement ◽  
Time Courses ◽  
Charge Movements ◽  
Kinetics Of ◽  
Microsecond Pulse

Intramembrane charge movement (Q) and sodium current (INa) were monitored in isolated voltage-clamped frog nodes of Ranvier, ON charge movements (QON) for pulses from the holding potential (-100 mV) to potentials V less than or equal to 0 mV followed single exponential time courses, whereas two exponentials were found for pulses to V greater than or equal to 20 mV. The voltage dependence of both QON and its time constant tauON indicated that the two ON components resolved at V greater than or equal to 20 mV were also present, though not resolvable, for pulses to V less than or equal to 0 mV. OFF charge movements (QOFF) monitored at various potentials were well described by single exponentials. When QOFF was monitored at -30 or -40 mV after a 200-microsecond pulse to +20 mV and QON was monitored at the same potential using pulses directly from -100 mV, tauON/tauOFF = 2.5 +/- 0.3. At a set OFF potential (-90 to -70 mV), tauOFF first increased with increasing duration tON of the preceding pulse to a given potential (0 to +30 mV) and then decreased with further increases in tON. The declining phase of tauOFF followed a time course similar to that of the decline in QOFF with tON. For the same pulse protocol, the OFF time constant tauNa for INA also first increased with tON but then remained constant over the tON interval during which tauOFF and QOFF were declining. After 200- or 300-microsecond pulses to +20, +20, or +50 mV, tauOFF/tauNa at -70 to -90 mV was 1.2 +/- 0.1. Similar tauOFF/tauNa ratios were predicted by channel models having three identical charged gating particles that can rapidly and reversibly form an immobile dimer or trimer after independently crossing the membrane from their OFF to their ON locations.

scholarly journals Voltage dependence of intramembrane charge movement and conductance activation of batrachotoxin-modified sodium channels in frog node of Ranvier.

1983 ◽  
Vol 81 (6) ◽  
pp. 829-844 ◽  
Author(s):  
J M Dubois ◽  
M F Schneider ◽  
B I Khodorov
Keyword(s):  
Sodium Channel ◽  
Sodium Channels ◽  
Voltage Dependence ◽  
Sodium Current ◽  
Node Of Ranvier ◽  
Charge Movement ◽  
Gating Charge ◽  
Before And After ◽  
Intramembrane Charge Movement ◽  
The Individual

Sodium current and sodium channel intramembrane gating charge movement (Q) were monitored in voltage-clamped frog node of Ranvier after modification of all sodium channels by batrachotoxin (BTX). BTX caused an approximately threefold increase in steepness of the Q vs. voltage relationship and a 50-mV negative shift in its midpoint. The maximum amount of intramembrane charge was virtually identical before and after BTX treatment. BTX treatment eliminated the charge immobilization observed in untreated nodes after relatively long depolarizing pulses and slowed the rate of OFF charge movement after a pulse. After BTX treatment, the voltage dependence of charge movement was the same as the steady-state voltage dependence of sodium conductance activation. The observations are consistent with the hypothesis that BTX induces an aggregation of the charged gating particles associated with each channel and causes them to move as a unit having approximately three times the average valence of the individual particles. Movement of this single aggregated unit would open the BTX-modified sodium channel.

scholarly journals Charge Movement Associated with the Opening and Closing of the Activation Gates of the Na Channels

1974 ◽  
Vol 63 (5) ◽  
pp. 533-552 ◽  
Author(s):  
Clay M. Armstrong ◽  
Francisco Bezanilla
Keyword(s):  
Time Course ◽  
Sodium Current ◽  
Close Association ◽  
Outward Current ◽  
Transient Outward Current ◽  
Activation Process ◽  
Charge Movement ◽  
Gating Current ◽  
Dipolar Molecules ◽  
Opening And Closing

The sodium current (INa) that develops after step depolarization of a voltage clamped squid axon is preceded by a transient outward current that is closely associated with the opening of the activation gates of the Na pores. This "gating current" is best seen when permeant ions (Na and K) are replaced by relatively impermeant ones, and when the linear portion of capacitative current is eliminated by adding current from positive steps to that from exactly equal negative ones. During opening of the Na pores gating current is outward, and as the pores close there is an inward tail of current that decays with approximately the same time-course as INa recorded in Na-containing medium. Both outward and inward gating current are unaffected by tetrodotoxin (TTX). Gating current is capacitative in origin, the result of relatively slow reorientation of charged or dipolar molecules in a suddenly altered membrane field. Close association with the Na activation process is clear from the time-course of gating current, and from the fact that three procedures that reversibly block INa also block gating current: internal perfusion with Zn2+, prolonged depolarization of the membrane, and inactivation of INa with a short positive prepulse.

Voltage-clamp analysis of the ionic conductances in a leech neuron with a purely calcium-dependent action potential

1987 ◽  
Vol 58 (6) ◽  
pp. 1468-1484 ◽  
Author(s):  
J. Johansen ◽  
J. Yang ◽  
A. L. Kleinhaus
Keyword(s):  
Steady State ◽  
Action Potential ◽  
Time Course ◽  
Outward Current ◽  
Time Constants ◽  
Exponential Time ◽  
Calcium Dependent ◽  
Voltage Dependent ◽  
Ca Currents ◽  
Single Exponential

1. The purely calcium-dependent action potential of the anterior lateral giant (ALG) cell in the leech Haementeria was examined under voltage clamp. 2. Analysis with ion substitutions showed that the ALG cell action potential is generated by only two time- and voltage-dependent conductance systems, an inward Ca-dependent current (ICa) and an outward Ca-dependent K current IK(Ca). 3. The kinetic properties of the inward current were examined both in Cs-loaded neurons with Ca as the current carrier as well as in Ba-containing Ringer solutions with Ba as the current carrier, since Ba effectively blocked all time- and voltage-dependent outward current. 4. During a maintained depolarization, Ba and Ca currents activated with a time constant tau m, they then inactivated with the decay following a single exponential time course with a time constant tau h. The time constants for decay of both Ba and Ca currents were comparable, suggesting that the mechanism of inactivation of ICa in the ALG cell is largely voltage dependent. In the range of potentials from 5 to 45 mV, tau m varied from 8 to 2 ms and tau h varied from 250 to 125 ms. 5. The activation of currents carried by Ba, after correction for inactivation, could be described reasonably well by the expression I'Ba = I'Ba(infinity) [1--exp(-t/tau m)]. 6. The steady-state activation of the Ba-conductance mBa(infinity) increased sigmoidally with voltage and was approximated by the equation mBa(infinity) = (1 + exp[(Vh-6)/3])-1. The steady-state inactivation hBa(infinity) varied with holding potential and could be described by the equation hBa(infinity) = [1 + exp(Vh + 10/7)]-1. Recovery from inactivation of IBa was best described by the sum of two exponential time courses with time constants of 300 ms and 1.75 s, respectively. 7. The outward current IK(Ca) developed very slowly (0.5–1 s to half-maximal amplitude) and did not inactivate during a 20-s depolarizing command pulse. Tail current decay of IK(Ca) followed a single exponential time course with voltage-dependent time constants of between 360 and 960 ms. The steady-state activation n infinity of IK(Ca) increased sigmoidally with depolarization as described by the equation n infinity = [1 + exp(Vh-13.5)/-8)]-1. 8. The reversal potentials of IK(Ca) tail currents were close to the expected equilibrium potential for potassium and they varied linearly with log [K]o with a slope of 51 mV. These results suggest a high selectivity of the conductance for K ions.(ABSTRACT TRUNCATED AT 400 WORDS)

Modulation of high-threshold transmission between heart interneurons of the medicinal leech by FMRF-NH2

1994 ◽  
Vol 71 (2) ◽  
pp. 454-466 ◽  
Author(s):  
T. W. Simon ◽  
J. Schmidt ◽  
R. L. Calabrese
Keyword(s):  
Synaptic Transmission ◽  
Time Course ◽  
High Threshold ◽  
Presynaptic Neuron ◽  
Voltage Step ◽  
Exponential Time ◽  
Time To Peak ◽  
Postsynaptic Currents ◽  
Bath Application ◽  
Single Exponential

1. We examined high-threshold synaptic transmission between oscillatory pairs of leech heart interneurons. Inhibitory postsynaptic currents (IPSCs) could be reliably evoked by depolarizing the presynaptic neuron in voltage clamp from a holding potential of -35 mV. At this presynaptic potential, the Ca2+ currents underlying graded transmission are completely inactivated, and we conclude that a high-threshold Ca2+ current is extant in heart interneurons. Further evidence for this was that inhibitory postsynaptic currents were blocked when Co2+ replaced Ca2+ in the saline and thus high-threshold transmission was dependent on the presence of external Ca2+. 2. When IPSCs were evoked by a 200-ms duration voltage step from a holding potential of -35 mV in the presynaptic neuron, the time course of turn-on of the IPSC consisted of a fast (time-to-peak = 17.5 +/- 1.93 (SE) ms [n = 7]) and a slow (time-to-peak = 250 +/- 28.5 ms [n = 8]) component. FMRF-NH2 reduced the amplitude of the fast component but did not affect the slow component. When the presynaptic voltage step was ended the IPSC turned off with a single exponential time course. FMRF-NH2 slowed the time course of turn-off of the IPSC. 3. When IPSCs were evoked by a 1500-ms duration voltage step from a holding potential of -35 mV in the presynaptic neuron, these IPSCs peaked around 300 ms. Following the peak, the IPSC decayed with a single exponential time course. FMRF-NH2 accelerated the time course of this decay. At potentials of 0 mV and +5 mV, FMRF-NH2 produced a significant decrease in the peak current and at potentials of -5 mV and 0 mV, produced a significant decrease in the current integral. 4. High-threshold IPSCs could also be evoked by a spike in the presynaptic neuron. Bath application of 1 microM FMRF-NH2 decreased the amplitude of the spike-evoked IPSC and slowed the time course of its falling phase. 5. We examined the effect of FMRF-NH2 on the quantal synaptic transmission. Bath-application of FMRF-NH2 increased binomial p, the probability of release, and decreased binomial n, the number of units available for release. FMRF-NH2 had no effect on q, the unit size, when calculated from the distributions of PSPs, and increased the coefficient of variation (CV). 6. The lack of a change in q and the increase in CV suggested that FMRF-NH2 acted at a presynaptic location.(ABSTRACT TRUNCATED AT 400 WORDS)

Kinetic properties and inactivation of the gating currents of sodium channels in squid axon

1975 ◽  
Vol 270 (908) ◽  
pp. 449-458 ◽  
Keyword(s):  
Sodium Channels ◽  
Sodium Current ◽  
Kinetic Properties ◽  
Gating Current ◽  
Gating Currents ◽  
Similar Time ◽  
Sodium Permeability ◽  
Small Component ◽  
Current Decay ◽  
Time Courses

Associated with the opening and closing of the sodium channels of nerve membrane is a small component of capacitative current, the gating current. After termination of a depolarizing step the gating current and sodium current decay with similar time courses. Both currents decay more rapidly at relatively negative membrane voltages than at positive ones. The gating current that flows during a depolarizing step is diminished by a pre-pulse that inactivates the sodium permeability. A pre-pulse has no effect after inactivation has been destroyed by internal perfusion with the proteolytic enzyme pronase. Gating charge (considered as positive charge) moves outward during a positive voltage step, with voltage dependent kinetics. The time constant of the outward gating current is a maximum at about —10 mV, and has a smaller value at voltages either more positive or negative than this value.

scholarly journals Kinetic analysis of pancuronium interaction with sodium channels in squid axon membranes.

1977 ◽  
Vol 69 (3) ◽  
pp. 293-323 ◽  
Author(s):  
J Z Yeh ◽  
T Narahashi
Keyword(s):  
Membrane Potential ◽  
Sodium Channel ◽  
Sodium Channels ◽  
Rate Constant ◽  
Time Course ◽  
Voltage Dependence ◽  
Sodium Current ◽  
Sodium Concentration ◽  
Internal Concentration ◽  
Sodium Inactivation

The interaction of pancuronium with sodium channels was investigated in squid axons. Sodium current turns on normally but turns off more quickly than the control with pancuronium 0.1-1mM present internally; The sodium tail current associated with repolarization exhibits an initial hook and then decays more slowly than the control. Pancuronium induces inactivation after the sodium inactivation has been removed by internal perfusion of pronase. Such pancuronium-induced sodium inactivation follows a single exponential time course, suggesting first order kinetics which represents the interaction of the pancuronium molecule with the open sodium channel. The rate constant of association k with the binding site is independent of the membrane potential ranging from 0 to 80 mV, but increases with increasing internal concentration of pancuronium. However, the rate constant of dissociation l is independent of internal concentration of pancuronium but decreases with increasing the membrane potential. The voltage dependence of l is not affected by changine external sodium concentration, suggesting a current-independent conductance block, The steady-state block depends on the membrane potential, being more pronounced with increasing depolarization, and is accounted for in terms of the voltage dependence of l. A kinetic model, based on the experimental observations and the assumption on binding kinetics of pancuronium with the open sodium channel, successfully simulates many features of sodium current in the presence of pancuronium.

scholarly journals Modification of inactivation in cardiac sodium channels: ionic current studies with Anthopleurin-A toxin.

1995 ◽  
Vol 106 (4) ◽  
pp. 601-616 ◽  
Author(s):  
D A Hanck ◽  
M F Sheets
Keyword(s):  
Steady State ◽  
Sodium Channels ◽  
Time Course ◽  
Sodium Current ◽  
Ionic Current ◽  
Na Channel ◽  
Channel Activation ◽  
Time To Peak ◽  
Cardiac Purkinje Cells ◽  
Cardiac Sodium Channels

The site 3 toxin, Anthopleurin-A (Ap-A), was used to modify inactivation of sodium channels in voltage-clamped single canine cardiac Purkinje cells at approximately 12 degrees C. Although Ap-A toxin markedly prolonged decay of sodium current (INa) in response to step depolarizations, there was only a minor hyperpolarizing shift by 2.5 +/- 1.7 mV (n = 13) of the half-point of the peak conductance-voltage relationship with a slight steepening of the relationship from -8.2 +/- 0.8 mV to -7.2 +/- 0.8 mV (n = 13). Increases in Gmax were dependent on the choice of cation used as a Na substitute intracellularly and ranged between 26 +/- 15% (Cs, n = 5) to 77 +/- 19% (TMA, n = 8). Associated with Ap-A toxin modification time to peak INa occurred later, but analysis of the time course INa at multiple potentials showed that the largest effects were on inactivation with only a small effect on activation. Consistent with little change in Na channel activation by Ap-A toxin, INa tail current relaxations at very negative potentials, where the dominant process of current relaxation is deactivation, were similar in control and after toxin modification. The time course of the development of inactivation after Ap-A toxin modification was dramatically prolonged at positive potentials where Na channels open. However, it was not prolonged after Ap-A toxin at negative potentials, where channels predominately inactivate directly from closed states. Steady state voltage-dependent availability (h infinity or steady state inactivation), which predominately reflects the voltage dependence of closed-closed transitions equilibrating with closed-inactivated transitions was shifted in the depolarizing direction by only 1.9 +/- 0.8 mV (n = 8) after toxin modification. The slope factor changed from 7.2 +/- 0.8 to 9.9 +/- 0.9 mV (n = 8), consistent with a prolongation of inactivation from the open state of Ap-A toxin modified channels at more depolarized potentials. We conclude that Ap-A selectively modifies Na channel inactivation from the open state with little effect on channel activation or on inactivation from closed state(s).

Kinetic properties of a transient outward current in rat neocortical neurons

1992 ◽  
Vol 68 (4) ◽  
pp. 1133-1142 ◽  
Author(s):  
M. Andreasen ◽  
J. J. Hablitz
Keyword(s):  
Time Course ◽  
Outward Current ◽  
Transient Outward Current ◽  
Late Component ◽  
Exponential Time ◽  
Neocortical Neurons ◽  
Voltage Dependent ◽  
Steady State Inactivation ◽  
Slope Factor ◽  
Single Exponential

1. Whole-cell patch-clamp techniques were used to record outward currents in embryonic rat neocortical neurons maintained in culture. In the presence of tetrodotoxin and cadmium, depolarization evoked an outward current with a complex waveform. This outward current consisted of an initial fast transient component and a late, slowly inactivating component. 2. The two outward current components could be separated pharmacologically with the use of tetraethylammonium (TEA) and 4-aminopyridine (4-AP). TEA (20 mM) applied extracellularly completely blocked the late component, unmasking a fast transient outward current (TOC). 4-AP (5 mM) applied extracellularly blocked the early component while reducing the late component by 27.8 +/- 9.7% (mean +/- SE). 3. The TOC activated after a short delay and rose rapidly to a peak. The time to peak was voltage dependent and decreased with depolarization. In the presence of 200 microM extracellular cadmium, activation threshold was around -25 mV, and current amplitude increased with depolarization. The voltage-conductance relationship was well fitted by the use of the Boltzmann equation with a Vm of +19 mV for half activation and a slope factor of +6 mV. 4. On sustained depolarization the TOC rapidly inactivated and decayed to baseline within 500-600 ms. The decay phase followed a single exponential time course with a time constant of 55-65 ms. The decay time was most rapid at potentials from +5 to +20 mV and increased slightly with further depolarization. 5. Steady-state inactivation of the TOC, in the presence of cadmium, was complete near -10 mV and was totally relieved at potentials more negative than -75 mV. With the use of the Boltzmann equation, a Vm of -34 mV for half inactivation and a slope factor of -8.6 mV were found. 6. Recovery of the TOC from steady-state inactivation followed a single exponential time course and was voltage dependent. When the membrane potential was held at -84 mV during the conditioning pulse, the time constant of recovery was 17 ms, increasing to 45.2 and 58.1 ms at holding potentials of -64 and -44 mV, respectively. Holding at potentials more negative than -84 mV produced no further change in the recovery time course. 7. The presence of 200 microM external cadmium altered the TOC activation and inactivation curves. Removal of cadmium produced a -16-mV shift in the Vm for half activation and a -25-mV shift in the inactivation curve. This sensitivity to cadmium is higher than that reported in other systems.(ABSTRACT TRUNCATED AT 400 WORDS)

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