Characteristics of L- and T-type Ca2+ currents in canine cardiac Purkinje cells

1989 ◽  
Vol 256 (5) ◽  
pp. H1478-H1492 ◽  
Author(s):  
Y. Hirano ◽  
H. A. Fozzard ◽  
C. T. January
Keyword(s):  
Kinetic Parameters ◽  
Voltage Clamp ◽  
Purkinje Cells ◽  
Current Density ◽  
Voltage Dependence ◽  
Cell Voltage ◽  
Voltage Clamp Technique ◽  
Recovery From Inactivation ◽  
Cardiac Purkinje Cells ◽  
Voltage Dependent

Two types of Ca2+ currents were recorded in single dialyzed canine cardiac Purkinje cells using a whole cell voltage clamp technique. T-type current was easily separated from L-type current, because its voltage dependence of inactivation and activation was more negative and it decayed rapidly. L-type current was available at more depolarized holding potentials, activated at more positive voltages, and decayed slowly. In 2 mM extracellular Ca2+ concentration [( Ca]o), the average peak T- and L-type current density was 1.70 and 2.87 pA/pF, respectively. T-type current was relatively insensitive to modification by Ca2+, nifedipine, Cd2+, BAY K 8644, or isoproterenol. T-type current was more sensitive to block by Ni2+ and amiloride. Replacement of Ca2+ by Ba2+ or Sr2+ did not increase T-type current. Changes in the Ca2+ or Ba2+ concentration caused parallel shifts in the voltage dependence of several kinetic parameters for L- and T-type current. In 2 mM [Ca]o, the V1/2 (Boltzmann fit) for inactivation of T-type current was -68 mV with a slope of 3.9, and for L-type current the V1/2 was -31 mV with a slope of 5.5. Recovery from inactivation of L- and T-type current was voltage dependent, and for similar conditions L-type current recovered from inactivation more rapidly than T-type current. These findings show that T- and L-type currents are large in cardiac Purkinje cells, and they can easily be separated by their voltage, kinetic, and pharmacological differences. Both may have important physiological roles.

scholarly journals Kinetic analysis of single sodium channels from canine cardiac Purkinje cells.

1990 ◽  
Vol 95 (3) ◽  
pp. 411-437 ◽  
Author(s):  
B E Scanley ◽  
D A Hanck ◽  
T Chay ◽  
H A Fozzard
Keyword(s):  
Maximum Likelihood ◽  
Kinetic Parameters ◽  
Purkinje Cells ◽  
Voltage Dependence ◽  
Neuroblastoma Cells ◽  
Likelihood Method ◽  
Pituitary Cells ◽  
Cardiac Purkinje Cells ◽  
Channel Open Time ◽  
Near Threshold

Single sodium channel events were recorded from cell-attached patches on single canine cardiac Purkinje cells at 10-13 degrees C. Data from four patches containing two to four channels and one patch with one channel were selected for quantitative analysis. The channels showed prominent reopening behavior at voltages near threshold, and the number of reopenings declined steeply with depolarization. Mean channel open time was a biphasic function of voltage with the maximum value (1-1.5 ms) occurring between -50 and -40 mV and lower values at more and at less hyperpolarized levels. Inactivation without opening was also prominent near threshold, and this occurrence also declined with depolarization. The waiting time distributions and the probability of being open showed voltage and time dependence as expected from whole-cell current studies. The results were analyzed in terms of a five-state Markovian kinetic model using both histogram analysis and a maximum likelihood method to estimate kinetic parameters. The kinetic parameters of the model fits were similar to those of GH3 pituitary cells (Horn, R., and C. A. Vandenberg. 1984. Journal of General Physiology. 84:505-534) and N1E115 neuroblastoma cells (Aldrich, R. W., and C. F. Stevens. Journal of Neuroscience. 7:418-431). Both histogram and maximum likelihood analysis implied that much of the voltage dependence of cardiac Na current is in its activation behavior, with inactivation showing modest voltage dependence.

Delayed K+ current and external K+ in single cardiac Purkinje cells

1989 ◽  
Vol 257 (6) ◽  
pp. C1086-C1092 ◽  
Author(s):  
F. Scamps ◽  
E. Carmeliet
Keyword(s):  
Voltage Clamp ◽  
Purkinje Cells ◽  
Cell Voltage ◽  
Inhibitory Effect ◽  
Free Solution ◽  
Cardiac Purkinje Cells ◽  
K Current ◽  
Kinetics Of ◽  
Direct Change ◽  
External K

The effect of external K+ on the delayed K+ current was investigated in rabbit single Purkinje cells. Whole cell voltage clamp and intracellular dialysis were used. At K+ concentrations less than 1 mM the kinetics of the delayed K+ current were not changed, but the conductance was markedly reduced. This effect was due to a direct change at an extracellular site and not due to secondary changes in intracellular Na+ or Ca2+ concentrations. A rise in intracellular Na+ or Ca2+ rather increased the delayed K+ current. The decrease in the delayed K+ current in low external K+ was absent when the experiments were done in Na+-free solution. It is concluded that external Na+ exerts an inhibitory effect on the conductance of the delayed K+ current.

Time-dependent changes in kinetics of Na+ current in single canine cardiac Purkinje cells

1992 ◽  
Vol 262 (4) ◽  
pp. H1197-H1207 ◽  
Author(s):  
D. A. Hanck ◽  
M. F. Sheets
Keyword(s):  
Purkinje Cells ◽  
Time Constant ◽  
Decay Time ◽  
Voltage Dependence ◽  
Na Channel ◽  
Time Constants ◽  
Voltage Range ◽  
Time To Peak ◽  
Cardiac Purkinje Cells ◽  
Voltage Dependent

The spontaneous hyperpolarizing shift in Na+ channel kinetics that occurs during a series of voltage-clamp recordings was characterized in single canine cardiac Purkinje cells at 10-13.5 degrees C. The change in the half-point of voltage-dependent availability, in the half-point of peak conductance, in the voltage dependence of deactivation and time to peak Na+ channel current (INa), and in the time constants of INa decay in response to step depolarizations were examined. The half points of availability and conductance shifted similarly, -0.41 +/- 0.13 and -0.47 +/- 0.19 mV/min, respectively (n = 14). These were directly correlated (slope 1.14 +/- 0.06, R2 = 0.81) with conductance shifting on average only -0.05 mV/min faster than availability. The deactivation time constant-voltage relationship shifted similarly to availability and conductance. Tail current decay time constants predicted the voltage dependence of the open to closed transition to be 0.9e-. Time to peak INa in response to step depolarizations changed e-fold for 25 mV but plateaued at positive potentials (531 microseconds, n = 22). INa decay was multiexponential between -40 and 80 mV. Decay time constants changed little as a function of voltage at positive potentials. The contribution of the second time constant to decay amplitude was 15-20% over the entire voltage range. Time to peak INa shifted in a curvilinear fashion, changing less late in an experiment. We conclude that the channel-voltage sensor responds to a changing fraction of the applied voltage during an experiment, producing similar rates of shift of voltage-dependent availability, conductance, and deactivation time constants.

Developmental change in fast Na channel properties in embryonic chick ventricular heart cells

1995 ◽  
Vol 73 (10) ◽  
pp. 1475-1484 ◽  
Author(s):  
Hideaki Sada ◽  
Takashi Ban ◽  
Takeshi Fujita ◽  
Yoshio Ebina ◽  
Nicholas Sperelakis
Keyword(s):  
Steady State ◽  
Voltage Clamp ◽  
Time Constant ◽  
Voltage Dependence ◽  
Cell Voltage ◽  
Developmental Changes ◽  
Heart Cells ◽  
Embryonic Chick ◽  
Whole Cell ◽  
Whole Cell Voltage Clamp

To assess developmental changes in kinetic properties of the cardiac sodium current, whole-cell voltage-clamp experiments were conducted using 3-, 10-, and 17-day-old embryonic chick ventricular heart cells. Experimental data were quantified according to the Hodgkin–Huxley model. While the Na current density, as examined by the maximal conductance, drastically increased (six- to seven-fold) with development, other current–voltage parameters remained unchanged. Whereas the activation time constant and the steady-state activation characteristics were comparable among the three age groups, the voltage dependence of the inactivation time constant and the steady-state inactivation underwent a shift in the voltage dependence toward negative potentials during embryonic development. Consequently, the steady-state (window current) conductance, which was sufficient to induce automatic activity in the young embryos, was progressively reduced with age.Key words: cardiac electrophysiology, whole-cell voltage-clamp experiments, fast Na currents, heart, development, developmental changes.

Oxytocin actions on voltage-dependent ionic channels in pregnant rat uterine smooth muscle cells

1992 ◽  
Vol 70 (12) ◽  
pp. 1597-1603 ◽  
Author(s):  
Yoshihito Inoue ◽  
Keiichi Shimamura ◽  
Nicholas Sperelakis
Keyword(s):  
Smooth Muscle ◽  
Voltage Clamp ◽  
Calcium Current ◽  
Uterine Contraction ◽  
Cell Voltage ◽  
Ionic Channels ◽  
Pregnant Rat ◽  
Uterine Smooth Muscle ◽  
Voltage Dependent ◽  
Sodium And Potassium

The effects of oxytocin, a uterotonic polypeptide hormone, on the voltage-dependent slow calcium, fast sodium, and potassium channel currents were studied using whole-cell voltage clamp of freshly isolated cells from late pregnant (18–21 day) rat myometrium. The calcium current was rapidly inhibited by oxytocin (about 25% inhibition at 20 nM) in a dose-dependent manner, and this inhibitory effect was completely reversible by washout. However, inhibition was not observed when barium was used as the charge carrier. Sodium current and potassium current were not modified by oxytocin, thus sodium and potassium currents may not play important roles in oxytocin-induced augmentation of uterine contraction. It is concluded that oxytocin stimulates uterine contraction by mechanisms other than augmentation of the voltage-dependent calcium current, e.g., by release of Ca from sarcoplasmic reticulum (by inositol trisphosphate) or by activation of a receptor-operated Ca channel. The inhibition of the slow calcium current may be induced by the elevation of [Ca]i.Key words: oxytocin, ionic channels, uterine smooth muscle, whole-cell voltage clamp, pregnant rat myometrium.

Protein kinase C stimulates Ca2+ current in pregnant rat myometrial cells

1994 ◽  
Vol 72 (11) ◽  
pp. 1304-1307 ◽  
Author(s):  
Keiichi Shimamura ◽  
Masumi Kusaka ◽  
Nicholas Sperelakis
Keyword(s):  
Protein Kinase C ◽  
Protein Kinase ◽  
Voltage Clamp ◽  
Cell Voltage ◽  
Pregnant Rat ◽  
Whole Cell ◽  
Myometrial Cells ◽  
Kinase C ◽  
Whole Cell Voltage Clamp ◽  
Voltage Dependent

The factors that regulate the voltage-dependent Ca2+ channels in pregnant uterine smooth muscle cells have not been elucidated, including any roles for protein kinase C (PKC). Therefore, the role of PKC in the regulation of the slow (L type) Ca2+ channels was examined in myometrial cells isolated from late pregnant (18–19 day) rat uterus, using the nystatin-perforated whole-cell voltage clamp. A PKC activator, phorbol 12, 13-dibutyrate (PDB), increased the L-type Ca2+ current (ICa(L)). Bath application of PDB (0.03 and 0.3 μM) increased the peak amplitude of ICa(L) by 21 ± 14% (n = 6) and 37 ± 8% (n = 9, p < 0.01), respectively. PDB did not change the holding current or shift the current–voltage relationship for ICa(L). The PKC inhibitors, H-7 (20 μM) or staurosporine (10 nM), reversed the effect of PDB. These results indicate that PKC may play a role in regulating Ca2+ channel function in pregnant rat myometrial cells and, therefore, may be involved in control of uterine contraction.Key words: protein kinase C, phorbol ester, calcium current, myometrial cell, nystatin-perforated patch, whole-cell voltage clamp.

scholarly journals Mechanism of Ba2+ block of M-like K channels of rod photoreceptors of tiger salamanders.

1994 ◽  
Vol 103 (1) ◽  
pp. 45-66 ◽  
Author(s):  
L P Wollmuth
Keyword(s):  
Time Course ◽  
Voltage Dependence ◽  
Cell Voltage ◽  
Voltage Sensitivity ◽  
Rod Photoreceptors ◽  
Voltage Range ◽  
Tiger Salamanders ◽  
High K ◽  
Voltage Dependent ◽  
Time Courses

IKx is a voltage-dependent K+ current in the inner segment of rod photoreceptors that shows many similarities to M-current. The depression of IKx by external Ba2+ was studied with whole-cell voltage clamp. Ba2+ reduced the conductance and voltage sensitivity of IKx tail currents and shifted the voltage range over which they appeared to more positive potentials. These effects showed different sensitivities to Ba2+: conductance was the least sensitive (K0.5 = 7.6 mM), voltage dependence intermediate (K0.5 = 2.4 mM) and voltage sensitivity the most sensitive (K0.5 = 0.2 mM). Ca2+, Co2+, Mn2+, Sr2+, and Zn2+ did not have actions comparable to Ba2+ on the voltage dependence or the voltage sensitivity of IKx tail currents. In high K+ (100 mM), the voltage range of activation of IKx was shifted 20 mV negative, as was the tau-voltage relation. High K+ did not prevent the effect of Ba2+ on conductance, but abolished its ability to affect voltage dependence and voltage sensitivity. Ba2+ also altered the apparent time-course of activation and deactivation of IKx. Low Ba2+ (0.2 mM) slowed both deactivation and activation, with most effect on deactivation; at higher concentrations (1-25 mM), deactivation and activation time courses were equally affected, and at the highest concentrations, 5 and 25 mM Ba2+, the time course became faster than control. Rapid application of 5 mM Ba2+ suggested that the time dependent currents in Ba2+ reflect in part the slow voltage-dependent block and unblock of IKx channels by Ba2+. This blocking action of Ba2+ was steeply voltage-dependent with an apparent electrical distance of 1.07. Ba2+ appears to interact with IKx channels at multiple sites. A model which assumes that Ba2+ has a voltage-independent and a voltage-dependent blocking action on open or closed IKx channels reproduced many aspects of the data; the voltage-dependent component could account for both the Ba(2+)-induced shift in voltage dependence and reduction in voltage sensitivity of IKx tail currents.

Slow inward Ca current in frog heart: theoretical evidence against a voltage-dependent inactivation

1982 ◽  
Vol 60 (9) ◽  
pp. 1185-1192 ◽  
Author(s):  
Rodolphe Fischmeister ◽  
Magda Horackova
Keyword(s):  
Time Course ◽  
Voltage Dependence ◽  
Frog Heart ◽  
Type Model ◽  
Actual Behavior ◽  
Ca Current ◽  
Recovery From Inactivation ◽  
Dependent Inactivation ◽  
Voltage Dependent ◽  
Theoretical Evidence

The validity of a Hodgkin–Huxley type voltage-dependent inactivation of slow inward Ca current (Isi) was tested in frog heart using a computer simulation. The time course of Isi, was calculated during the development of a frog atrial action potential (AP). With a time constant of inactivation (τf) of 55 ms at a membrane potential (Em) of –15 mV, the variation of Isi was biphasic; after a transient increase followed by a decrease to zero, Isi partially "reactivated" (at the beginning of the AP repolarization phase) and then fully deactivated. The "reactivation" phase of Isi developed whether τf was an increasing, decreasing, U-shaped, or bell-shaped function of Em. The addition of an independent and slower process responsible for the recovery from inactivation only partly suppressed the "reactivation" phase. However, until now there was no experimental evidence supporting such a biphasic variation of Isi during AP repolarization. Thus our results indicate that the Hodgkin–Huxley type model of the voltage-dependence of Isi-inactivation process may not correctly represent the actual behavior of frog cardiac muscle.

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