Voltage-Dependence of Extracellular Ca2+-Induced Modification in Properties of the Inward Rectifying K+ Channels in the Plasma Membrane of Mesophyll Protoplasts of Avena sativa

1996 ◽  
Vol 23 (3) ◽  
pp. 349 ◽  
Author(s):  
J Kourie
Keyword(s):  
Plasma Membrane ◽  
Time Constant ◽  
Avena Sativa ◽  
Voltage Dependence ◽  
Membrane Current ◽  
Time Dependent ◽  
Mesophyll Cells ◽  
K Channels ◽  
Current Passing ◽  
K Current

Data obtained using the whole-celi configuration of the patch-clamp technique reveal that characteristics of the inward rectifying K+ current across the plasma membrane of protoplasts isolated from mesophyll cells of leaves of oat (Avena sativa) are modified by increasing concentrations or removing the extracellular Ca2+. The whole-cell membrane current reveals two components. The first component an initial current II* which is the sum of two currents: (a) a linear ohmic leak current passing through non-gated channels, liNGC, and (b) a rectifying inward K+ current passing through inward rectifying gated K+ channels, IKi, that are instantaneously open. The second component of the membrane current at the steady state Iss is a time-dependent K+ current IKss defined as Iss-IiNGC and passes through inward rectifying gated K+ channels. The tail K+ current, IKT, is also defined as IT-IiNGC. Raising external calcium concentration, [Ca2+]o, from 0.1 mM to 10 mM blocked the inward rectifying currents IKi, IKss and IKT. The voltage-dependence of the activation time constant (τa) for time-dependent KC current IKss was not altered significantly by increasing [Ca2+]o whereas the deactivation time constant (τd) of the IKT increased from 16 ms to 30 ms at a Vm of -100 mV. Removal of [Ca2+]o increased the amplitude and altered the characteristics of the inward rectifying K+ current. Ten minutes after the removal of [Ca2+]o the increase in IKi was 3.5-fold larger than the increase in IKss. Furthermore, removing [Ca2+]o hastened the activation of IKss and the deactivation of IKT. However, the deactivation time constant (Td) remained dependent on membrane voltage (Vm). Extracellular Ca2+ may modulate the function of mesophyll cells by regulating K+ transport through the inward rectifying K+ channels and this may have significant implications for photosynthesis and cell expansion.

scholarly journals The Arabidopsis R-SNARE VAMP721 Interacts with KAT1 and KC1 K+ Channels to Moderate K+ Current at the Plasma Membrane

2015 ◽  
Vol 27 (6) ◽  
pp. 1697-1717 ◽  
Author(s):  
Ben Zhang ◽  
Rucha Karnik ◽  
Yizhou Wang ◽  
Niklas Wallmeroth ◽  
Michael R. Blatt ◽  
...  
Keyword(s):  
Plasma Membrane ◽  
K Channels ◽  
K Current

scholarly journals External pH effects on the depolarization-activated K channels in guard cell protoplasts of Vicia faba.

1994 ◽  
Vol 103 (5) ◽  
pp. 807-831 ◽  
Author(s):  
N Ilan ◽  
A Schwartz ◽  
N Moran
Keyword(s):  
Plasma Membrane ◽  
Surface Charge ◽  
Vicia Faba ◽  
Guard Cell ◽  
Voltage Dependence ◽  
Single Channel ◽  
Ph Effects ◽  
Homogeneous Surface ◽  
K Channels ◽  
Guard Cell Protoplasts

Previous studies reveal that the pH of the apoplastic solution in the guard cell walls may vary between 7.2 and 5.1 in closed and open stomata, respectively. During these aperture and pH changes, massive K+ fluxes cross the cellular plasma membrane driving the osmotic turgor and volume changes of guard cells. Therefore, we examined the effect of extracellular pH on the depolarization-activated K channels (KD channels), which constitute the K+ efflux pathway, in the plasma membrane of Vicia faba guard cell protoplasts. We used patch clamp, both in whole cells as well as in excised outside-out membrane patches. Approximately 500 KD channels, at least, could be activated by depolarization in one protoplast (density: approximately 0.6 micron-2). Acidification from ph 8.1 to 4.4 decreased markedly the whole-cell conductance, GK, of the KD channels, shifted its voltage dependence, GK-EM, to the right on the voltage axis, slowed the rate of activation and increased the rate of deactivation, whereas the single channel conductance was not affected significantly. Based on the GK-EM shifts, the estimated average negative surface charge spacing near the KD channel is 39 A. To quantify the effects of protons on the rates of transitions between the hypothesized conformational states of the channels, we fitted the experimental macroscopic steady state conductance-voltage relationship and the voltage dependence of time constants of activation and deactivation, simultaneously, with a sequential three-state model CCO. In terms of this model, protonation affects the voltage-dependent properties via a decrease in localized, rather than homogeneous, surface charge sensed by the gating moieties. In terms of either the CO or CCO model, the protonation of a site with a pKa of 4.8 decreases the voltage-independent number of channels, N, that are available for activation by depolarization.

scholarly journals Activation of Ca2+-Dependent K+ Channels Contributes to Rhythmic Firing of Action Potentials in Mouse Pancreatic β Cells

1999 ◽  
Vol 114 (6) ◽  
pp. 759-770 ◽  
Author(s):  
Sven O. Göpel ◽  
Takahiro Kanno ◽  
Sebastian Barg ◽  
Lena Eliasson ◽  
Juris Galvanovskis ◽  
...  
Keyword(s):  
Electrical Stimulation ◽  
Electrical Activity ◽  
Time Constant ◽  
Action Potentials ◽  
Β Cells ◽  
Β Cell ◽  
Pancreatic Β Cells ◽  
K Channels ◽  
K Current ◽  
Rhythmic Firing

We have applied the perforated patch whole-cell technique to β cells within intact pancreatic islets to identify the current underlying the glucose-induced rhythmic firing of action potentials. Trains of depolarizations (to simulate glucose-induced electrical activity) resulted in the gradual (time constant: 2.3 s) development of a small (<0.8 nS) K+ conductance. The current was dependent on Ca2+ influx but unaffected by apamin and charybdotoxin, two blockers of Ca2+-activated K+ channels, and was insensitive to tolbutamide (a blocker of ATP-regulated K+ channels) but partially (>60%) blocked by high (10–20 mM) concentrations of tetraethylammonium. Upon cessation of electrical stimulation, the current deactivated exponentially with a time constant of 6.5 s. This is similar to the interval between two successive bursts of action potentials. We propose that this Ca2+-activated K+ current plays an important role in the generation of oscillatory electrical activity in the β cell.

Voltage-gated potassium currents in acutely dissociated rat cortical neurons

1993 ◽  
Vol 70 (1) ◽  
pp. 51-63 ◽  
Author(s):  
R. C. Foehring ◽  
D. J. Surmeier
Keyword(s):  
Time Constant ◽  
Cortical Neurons ◽  
Voltage Dependence ◽  
Inactivation Kinetics ◽  
Voltage Gated ◽  
Time To Peak ◽  
Recovery From Inactivation ◽  
Rat Cortical Neurons ◽  
K Current ◽  
A Current

1. We describe three outward K+ current components in acutely dissociated neurons from rat sensorimotor cortex on the basis of inactivation kinetics and voltage dependence. 2. The fast A current (IAf) was completely inactivated at -40 mV and half-inactivated at -52 mV. It activated [time to peak (TTP) 8 ms at -10 mV] and was inactivated (tau inact = 12 ms at -10 mV) rapidly. Recovery from inactivation had a time constant of approximately 80 ms at -100 mV. It was insensitive to tetraethyl ammonium (TEA) and dendrotoxin but was blocked by 4-aminopyridine (4-AP, IC50 = 1 mM). 3. The slowly inactivating current (IKS) was the largest current seen in acutely dissociated adult neurons. It was completely inactivated at -40 mV, half-inactivated at -98 mV, and was kinetically slower (TTP = 130 ms at -10 mV; tau inact = 293 ms at -10 mV) than the fast A current. Deactivation tails were fit with the sum of two exponentials with time constants of 2-10 and 15-40 ms. IKS recovered from inactivation with a time constant of approximately 1,200 ms at -100 mV. 4. There were two components that inactivated with even slower kinetics. The very slowly inactivating current (IKSS) was operationally defined as the current remaining after a 5-s hold at -40 mV. One component inactivated with a time constant of 1,927 ms at -10 mV. The other component showed no inactivation over a 5-s test command, but in 40- to 50-s steps to -10 mV, inactivated with a tau of approximately 20 s. The very slowly inactivating current activated with similar kinetics to IKS (TTP = 121 ms at -10 mV), and two deactivation tails, with kinetics similar to those after the -100 mV prepulse, were observed after holding at -40 mV. 5. Both IKS and IKSS were sensitive to TEA. Seventy-six percent (76%) of IKSS was blocked by 30 mM TEA. Two components to the TEA block were present for IKSS, with IC50s of 88 microM (67% of blockable current) and 7 mM (33%). Seventy percent (70%) of IKS was blocked by 30 mM TEA. For the IKS current, there were also two effective concentrations, with IC50s of 8 microM (21% of blockade current) and 3 mM (79%). 6. IKS and IKSS were also sensitive to 4-AP. Seventy-six percent (76%) of IKSS was blocked by 3-5 mM 4-AP. IKSS exhibited two components of 4-AP block.(ABSTRACT TRUNCATED AT 400 WORDS)

Simulation of the currents involved in rhythmic oscillations in thalamic relay neurons

1992 ◽  
Vol 68 (4) ◽  
pp. 1373-1383 ◽  
Author(s):  
J. R. Huguenard ◽  
D. A. McCormick
Keyword(s):  
Continuous Function ◽  
Time Constant ◽  
Voltage Dependence ◽  
Constant Field ◽  
Cationic Current ◽  
Thalamic Relay ◽  
K Current ◽  
Thalamic Relay Neurons ◽  
Kinetics Of ◽  
Relay Neurons

1. To perform simulations of the various modes of action potential generation in thalamic relay neurons, we developed Hodgkin-and-Huxley style mathematical equations that describe the voltage dependence and kinetics of activation and inactivation of four different currents, including the transient, low-voltage-activated Ca2+ current (IT), the rapidly inactivating transient K+ current (IA), the slowly inactivating K+ current (IK2), and the hyperpolarization-activated, mixed cationic current (Ih). The modeled currents were derived either from acutely dissociated rat thalamic relay neurons (IT, IA, IK2), or from guinea pig thalamic relay cells maintained in slices in vitro (Ih). 2. The voltage dependence of steady-state activation and inactivation of IT, IA, and IK2 and the activation of Ih could be modeled with Boltzmann-style equations. Modeling of the behavior of IT to depolarizing steps in voltage clamp required the use of the constant field equation to relate permeability to T-current amplitude. The time constant of activation of IT was described by a continuous bell-shaped function with a maximum near 15 ms at threshold for activation (-75 mV) and 23 degrees C. Mathematical description of the kinetics of inactivation and removal of inactivation of this current required two separate functions. 3. The rapidly activating and inactivating K+ current IA was modeled by assuming two components with different time constants of inactivation. The kinetics of activation was described as a continuous function of voltage with the slowest time constant, near 2.5 ms, at threshold for activation (-60 mV) and 23 degrees C. In contrast, the kinetics of inactivation of both components were described as voltage independent, consistent with experimental data. The rate or removal of inactivation of both components of IA was described as continuously increasing with the degree of hyperpolarization. 4. The slowly inactivating K+ current IK2 was also modeled by assuming two components with different rates of inactivation. The kinetics of activation were described by a bell-shaped function with a maximum time constant near 80 ms at -40 mV and 23 degrees C, whereas threshold for activation was approximately -60 mV. Inactivation of both components was modeled as relatively independent of voltage, whereas removal of inactivation was described as a continuous function of membrane potential. 5. The hyperpolarization-activation cationic current, Ih, was modeled by assuming that the current activates with a single exponential relation and does not inactivate.(ABSTRACT TRUNCATED AT 400 WORDS)

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

1998 ◽  
Vol 111 (2) ◽  
pp. 363-379 ◽  
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).

Effect of Na+ on Ca2+-Binding on the Plasma Membrane of Barley Mesophyll Cells: an Electrophoretic Study

1998 ◽  
Vol 39 (4) ◽  
pp. 452-457 ◽  
Author(s):  
Y. Murata ◽  
M. Fujita ◽  
T. Nakatani ◽  
I. Obi ◽  
T. Kakutani
Keyword(s):  
Plasma Membrane ◽  
Mesophyll Cells ◽  
Electrophoretic Study

scholarly journals Correlation between Charge Movement and Ionic Current during Slow Inactivation in Shaker K+ Channels

1997 ◽  
Vol 110 (5) ◽  
pp. 579-589 ◽  
Author(s):  
Riccardo Olcese ◽  
Ramón Latorre ◽  
Ligia Toro ◽  
Francisco Bezanilla ◽  
Enrico Stefani
Keyword(s):  
Time Course ◽  
Voltage Dependence ◽  
Ionic Current ◽  
K Channel ◽  
Charge Movement ◽  
Slow Inactivation ◽  
Gating Currents ◽  
Similar Time ◽  
K Channels ◽  
Recovery From Inactivation

Prolonged depolarization induces a slow inactivation process in some K+ channels. We have studied ionic and gating currents during long depolarizations in the mutant Shaker H4-Δ(6–46) K+ channel and in the nonconducting mutant (Shaker H4-Δ(6–46)-W434F). These channels lack the amino terminus that confers the fast (N-type) inactivation (Hoshi, T., W.N. Zagotta, and R.W. Aldrich. 1991. Neuron. 7:547–556). Channels were expressed in oocytes and currents were measured with the cut-open-oocyte and patch-clamp techniques. In both clones, the curves describing the voltage dependence of the charge movement were shifted toward more negative potentials when the holding potential was maintained at depolarized potentials. The evidences that this new voltage dependence of the charge movement in the depolarized condition is associated with the process of slow inactivation are the following: (a) the installation of both the slow inactivation of the ionic current and the inactivation of the charge in response to a sustained 1-min depolarization to 0 mV followed the same time course; and (b) the recovery from inactivation of both ionic and gating currents (induced by repolarizations to −90 mV after a 1-min inactivating pulse at 0 mV) also followed a similar time course. Although prolonged depolarizations induce inactivation of the majority of the channels, a small fraction remains non–slow inactivated. The voltage dependence of this fraction of channels remained unaltered, suggesting that their activation pathway was unmodified by prolonged depolarization. The data could be fitted to a sequential model for Shaker K+ channels (Bezanilla, F., E. Perozo, and E. Stefani. 1994. Biophys. J. 66:1011–1021), with the addition of a series of parallel nonconducting (inactivated) states that become populated during prolonged depolarization. The data suggest that prolonged depolarization modifies the conformation of the voltage sensor and that this change can be associated with the process of slow inactivation.

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