scholarly journals Voltage Dependence of Slow Inactivation in Shaker Potassium Channels Results from Changes in Relative K+ and Na+ Permeabilities

2000 ◽  
Vol 115 (2) ◽  
pp. 107-122 ◽  
Author(s):  
John G. Starkus ◽  
Stefan H. Heinemann ◽  
Martin D. Rayner
Keyword(s):  
Potassium Channels ◽  
Outward Current ◽  
Negative Slope ◽  
Voltage Sensitivity ◽  
Slow Inactivation ◽  
Shaker Channels ◽  
Outward Currents ◽  
Voltage Dependent ◽  
Na Ions ◽  
Shaker Potassium Channels

Time constants of slow inactivation were investigated in NH2-terminal deleted Shaker potassium channels using macro-patch recordings from Xenopus oocytes. Slow inactivation is voltage insensitive in physiological solutions or in simple experimental solutions such as K+o//K+i or Na+o//K+i. However, when [Na+]i is increased while [K+]i is reduced, voltage sensitivity appears in the slow inactivation rates at positive potentials. In such solutions, the I-V curves show a region of negative slope conductance between ∼0 and +60 mV, with strongly increased outward current at more positive voltages, yielding an N-shaped curvature. These changes in peak outward currents are associated with marked changes in the dominant slow inactivation time constant from ∼1.5 s at potentials less than approximately +60 mV to ∼30 ms at more than +150 mV. Since slow inactivation in Shaker channels is extremely sensitive to the concentrations and species of permeant ions, more rapid entry into slow inactivated state(s) might indicate decreased K+ permeation and increased Na+ permeation at positive potentials. However, the N-shaped I-V curve becomes fully developed before the onset of significant slow inactivation, indicating that this N-shaped I-V does not arise from permeability changes associated with entry into slow inactivated states. Thus, changes in the relative contributions of K+ and Na+ ions to outward currents could arise either: (a) from depletions of [K+]i sufficient to permit increased Na+ permeation, or (b) from voltage-dependent changes in K+ and Na+ permeabilities. Our results rule out the first of these mechanisms. Furthermore, effects of changing [K+]i and [K+]o on ramp I-V waveforms suggest that applied potential directly affects relative permeation by K+ and Na+ ions. Therefore, we conclude that the voltage sensitivity of slow inactivation rates arises indirectly as a result of voltage-dependent changes in the ion occupancy of these channels, and demonstrate that simple barrier models can predict such voltage-dependent changes in relative permeabilities.

scholarly journals Hidden Markov Model Analysis of Intermediate Gating Steps Associated with the Pore Gate of Shaker Potassium Channels

2001 ◽  
Vol 118 (5) ◽  
pp. 547-564 ◽  
Author(s):  
Jie Zheng ◽  
Lalitha Vankataramanan ◽  
Fred J. Sigworth
Keyword(s):  
Potassium Channels ◽  
Conformational Changes ◽  
Time Course ◽  
Single Channel ◽  
Hidden Markov ◽  
Voltage Sensitivity ◽  
Open Probability ◽  
Shaker Channels ◽  
Voltage Dependent ◽  
Shaker Potassium Channels

Cooperativity among the four subunits helps give rise to the remarkable voltage sensitivity of Shaker potassium channels, whose open probability changes tenfold for a 5-mV change in membrane potential. The cooperativity in these channels is thought to arise from a concerted structural transition as the final step in opening the channel. Recordings of single-channel ionic currents from certain other channel types, as well as our previous recordings from T442S mutant Shaker channels, however, display intermediate conductance levels in addition to the fully open and closed states. These sublevels might represent stepwise, rather than concerted, transitions in the final steps of channel activation. Here, we report a similar fine structure in the closing transitions of Shaker channels lacking the mutation. Describing the deactivation time course with hidden Markov models, we find that two subconductance levels are rapidly traversed during most closing transitions of chimeric, high conductance Shaker channels. The lifetimes of these levels are voltage-dependent, with maximal values of 52 and 22 μs at −100 mV, and the voltage dependences of transitions among these states suggest that they arise from equivalent conformational changes occurring in individual subunits. At least one subconductance level is found to be traversed in normal conductance Shaker channels. We speculate that voltage-dependent conformational changes in the subunits give rise to changes in a “pore gate” associated with the selectivity filter region of the channel, producing the subconductance states. As a control for the hidden Markov analysis, we applied the same procedures to recordings of the recovery from N-type inactivation in Shaker channels. These transitions are found to be instantaneous in comparison.

scholarly journals Ion Conduction through C-Type Inactivated Shaker Channels

1997 ◽  
Vol 110 (5) ◽  
pp. 539-550 ◽  
Author(s):  
John G. Starkus ◽  
Lioba Kuschel ◽  
Martin D. Rayner ◽  
Stefan H. Heinemann
Keyword(s):  
Potassium Channels ◽  
Xenopus Oocytes ◽  
Ion Selectivity ◽  
Complete Removal ◽  
Ion Conduction ◽  
Internal Solution ◽  
Shaker Channels ◽  
Na Ions ◽  
Shaker Potassium Channels ◽  
Inside Out

C-type inactivation of Shaker potassium channels involves entry into a state (or states) in which the inactivated channels appear nonconducting in physiological solutions. However, when Shaker channels, from which fast N-type inactivation has been removed by NH2-terminal deletions, are expressed in Xenopus oocytes and evaluated in inside-out patches, complete removal of K+ ions from the internal solution exposes conduction of Na+ and Li+ in C-type inactivated conformational states. The present paper uses this observation to investigate the properties of ion conduction through C-type inactivated channel states, and demonstrates that both activation and deactivation can occur in C-type states, although with slower than normal kinetics. Channels in the C-type states appear “inactivated” (i.e., nonconducting) in physiological solutions due to the summation of two separate effects: first, internal K+ ions prevent Na+ ions from permeating through the channel; second, C-type inactivation greatly reduces the permeability of K+ relative to the permeability of Na+, thus altering the ion selectivity of the channel.

scholarly journals Probing the Cavity of the Slow Inactivated Conformation of Shaker Potassium Channels

2007 ◽  
Vol 129 (5) ◽  
pp. 403-418 ◽  
Author(s):  
Gyorgy Panyi ◽  
Carol Deutsch
Keyword(s):  
Potassium Channels ◽  
Binding Site ◽  
Conformational Change ◽  
Marked Decrease ◽  
Selectivity Filter ◽  
Slow Inactivation ◽  
Voltage Gated ◽  
Activation Gate ◽  
Shaker Potassium Channels

Slow inactivation involves a local rearrangement of the outer mouth of voltage-gated potassium channels, but nothing is known regarding rearrangements in the cavity between the activation gate and the selectivity filter. We now report that the cavity undergoes a conformational change in the slow-inactivated state. This change is manifest as altered accessibility of residues facing the aqueous cavity and as a marked decrease in the affinity of tetraethylammonium for its internal binding site. These findings have implications for global alterations of the channel during slow inactivation and putative coupling between activation and slow-inactivation gates.

Ca-Induced K+-Outward Current in Paramecium Tetraurelia

1980 ◽  
Vol 88 (1) ◽  
pp. 293-304 ◽  
Author(s):  
YOUKO SATOW ◽  
CHING KUNG
Keyword(s):  
Steady State ◽  
Voltage Clamp ◽  
Outward Current ◽  
Paramecium Tetraurelia ◽  
Ca Channels ◽  
Outward Currents ◽  
Ciliary Reversal ◽  
Voltage Dependent ◽  
K Current ◽  
K Outward Current

Late K-outward currents upon membrane depolarization were recorded in Paramecium tetraurelia under a voltage clamp. A Ca-induced K-outward component is demonstrated by subtracting the value of the outward current in a pawn A mutant lacking functional Ca-channels (pwA500). The Ca-induced K-outward current activates slowly, reaching a peak after 100 to 1000 ms. The current then remains steady or reaches the steady state after a decline of several seconds. EGTA2- injection experiments show that the Ca-induced K-outward current is dependent on the internal Ca2+ concentration. The current is shown to depend on the voltage-dependent Ca conductance, by study of the leaky pawn A mutant (pwA132), which has a lowered Ca conductance as well as a lowered Ca-induced K-current. The Ca-induced GK is thus indirectly dependent on the voltage. The maximal GK is about 40 nmho/cell at + 7 mV in 4 mM-K+. The Ca-induced K current is sustained throughout the prolonged depolarization and the prolonged ciliary reversal.

scholarly journals Cross Talk between Activation and Slow Inactivation Gates of Shaker Potassium Channels

2006 ◽  
Vol 128 (5) ◽  
pp. 547-559 ◽  
Author(s):  
Gyorgy Panyi ◽  
Carol Deutsch
Keyword(s):  
Potassium Channels ◽  
Cross Talk ◽  
Cysteine Residue ◽  
Slow Inactivation ◽  
Fundamental Factor ◽  
Functional Assays ◽  
Activation Gate ◽  
The Status ◽  
Shaker Potassium Channels ◽  
Rate Of Activation

This study addresses the energetic coupling between the activation and slow inactivation gates of Shaker potassium channels. To track the status of the activation gate in inactivated channels that are nonconducting, we used two functional assays: the accessibility of a cysteine residue engineered into the protein lining the pore cavity (V474C) and the liberation by depolarization of a Cs+ ion trapped behind the closed activation gate. We determined that the rate of activation gate movement depends on the state of the inactivation gate. A closed inactivation gate favors faster opening and slower closing of the activation gate. We also show that hyperpolarization closes the activation gate long before a channel recovers from inactivation. Because activation and slow inactivation are ubiquitous gating processes in potassium channels, the cross talk between them is likely to be a fundamental factor in controlling ion flux across membranes.

Segment-specific effects of FMRFamide on membrane properties of heart interneurons in the leech

1995 ◽  
Vol 74 (4) ◽  
pp. 1485-1497 ◽  
Author(s):  
J. Schmidt ◽  
S. Gramoll ◽  
R. L. Calabrese
Keyword(s):  
Outward Current ◽  
Membrane Conductance ◽  
Membrane Properties ◽  
Inward Current ◽  
Specific Effects ◽  
Outward Currents ◽  
Voltage Dependent ◽  
Inward Currents ◽  
K Current ◽  
Conductance Increase

1. The effects of Phe-Met-Arg-Phe (FMRF)amide (10(-6) M) on membrane properties of heart interneurons in the third, fourth, and fifth segmental ganglia [HN(3), HN(4), and HN(5) cells, respectively] of the leech were studied using discontinuous current-clamp and single-electrode voltage-clamp techniques. FMRFamide was focally applied onto the soma of the cell under investigation. 2. Application of FMRFamide depolarized HN(3) and HN(4) cells by evoking an inward current. These responses were subject to pronounced desensitization. The inward currents evoked by application of FMRFamide were associated with an increase in membrane conductance and appeared to be voltage dependent. Currents were enhanced at more depolarized potentials. 3. The responsiveness of the HN(3) and HN(4) cells was not affected when the Ca2+ concentration in the bath saline was reduced from normal (1.8 mM) to 0.1 mM. The depolarizing response on application of FMRFamide was blocked when Co2+ was substituted for Ca2+. 4. HN(3) and HN(4) cells did not respond to FMRFamide application in Na(+)-free solution. Inward currents were largely reduced when bath saline with 30% of the normal Na+ concentration was used. When Li+ was substituted for Na+ in the saline, application of FMRFamide still evoked depolarizing responses in HN(3) and HN(4) cells. 5. We conclude that focal application of FMRFamide onto the somata of HN(3) and HN(4) cells evokes a voltage-dependent inward current, carried largely by Na+. 6. Focal application of FMRFamide onto somata of HN(5) cells hyperpolarized these cells by activating a voltage-dependent outward current. 7. HN(5) cells were loaded with Cl- until inhibitory postsynaptic potentials carried by Cl- reversed. Cl(-)-loaded cells still responded with a hyperpolarization when FMRFamide was applied onto their somata. Therefore the outward current evoked by FMRFamide appears to be mediated by a K+ conductance increase. 8. Application of FMRFamide onto the somata of HN(5) cells enhanced outward currents that were evoked by depolarizing voltage steps from a holding potential of -45 mV. 9. We conclude that the hyperpolarizing response of HN(5) cells to focal application of FMRFamide onto their somata is the result of an up-regulation of a voltage-dependent K+ current.

scholarly journals Voltage-dependent conductances in Limulus ventral photoreceptors.

1982 ◽  
Vol 79 (2) ◽  
pp. 187-209 ◽  
Author(s):  
J E Lisman ◽  
G L Fain ◽  
P M O'Day
Keyword(s):  
Outward Current ◽  
Delayed Rectifier ◽  
Molluscan Neurons ◽  
Inward Current ◽  
Transient Component ◽  
Outward Currents ◽  
Voltage Dependent ◽  
Inward Currents ◽  
K Current ◽  
A Current

The voltage-dependent conductances of Limulus ventral photoreceptors have been investigated using a voltage-clamp technique. Depolarization in the dark induces inward and outward currents. The inward current is reduced by removing Na+ or Ca2+ and is abolished by removing both ions. These results suggest that both Na+ and Ca2+ carry voltage-dependent inward current. Inward current is insensitive to tetrodotoxin but is blocked by external Ni2+. The outward current has a large transient component that is followed by a smaller maintained component. Intracellular tetraethylammonium preferentially reduces the maintained component, and extracellular 4-amino pyridine preferentially reduces the transient component. Neither component is strongly affected by removal of extracellular Ca2+ or by intracellular injection of EGTA. It is concluded that the photoreceptors contain at least three separate voltage-dependent conductances: 1) a conductance giving rise to inward currents; 2) a delayed rectifier giving rise to maintained outward K+ current; and 3) a rapidly inactivating K+ conductance similar to the A current of molluscan neurons.

scholarly journals Voltage-dependent block of cardiac inward-rectifying potassium current by monovalent cations.

1989 ◽  
Vol 94 (2) ◽  
pp. 349-361 ◽  
Author(s):  
R D Harvey ◽  
R E Ten Eick
Keyword(s):  
Ventricular Myocytes ◽  
Negative Slope ◽  
Voltage Sensitivity ◽  
Dependent Manner ◽  
Inward Current ◽  
Current Voltage ◽  
Steady State Current ◽  
Voltage Dependent ◽  
Current Block ◽  
K Current

The inward-rectifying K+ current (IK1) in cat ventricular myocytes, like inward-rectifying K+ currents in many other preparations, exhibited a negative slope conductance region at hyperpolarized membrane potentials that was time-dependent. This was evident as an inactivation of inward current elicited by hyperpolarizing voltage-clamp pulses resulting in a negative slope region of the steady-state current-voltage relationship at potentials negative to -140 mV. Removing extracellular Na+ prevented the development of the negative slope in this voltage region, suggesting that Na+ can block IK1 channels in a time- and voltage-dependent manner. The time and voltage dependence of Cs+-induced block of IK1 was also examined. Cs+ blocked inward current in a manner similar to that of Na+, but the former was much more potent. The fraction of current blocked by Cs+ in the presence of Na+ was reduced in a time- and voltage-dependent manner, which suggested that these blocking ions compete for a common or at least similar site of action. In the absence of Na+, inactivation of IK1 could also be induced by both Cs+ and Li+. However, Li+ was less potent than Na+ in this respect. Calculation of the voltage sensitivity of current block by each of these ions suggests that the mechanism of block by each is similar.

Characterization of macroscopic outward currents of canine colonic myocytes

1989 ◽  
Vol 257 (3) ◽  
pp. C461-C469 ◽  
Author(s):  
W. C. Cole ◽  
K. M. Sanders
Keyword(s):  
Outward Current ◽  
Cell Voltage ◽  
Muscle Cells ◽  
Time Dependent ◽  
Delayed Rectifier ◽  
Delayed Rectifier Current ◽  
Outward Currents ◽  
Voltage Dependent ◽  
K Current ◽  
Time Courses

Outward currents of colonic smooth muscle cells were characterized by the whole cell voltage-clamp method. Four components of outward current were identified: a time-independent and three time-dependent components. The time-dependent current showed strong outward rectification positive to -25 mV and was blocked by tetraethylammonium. The time-dependent components were separated on the basis of their time courses, voltage dependence, and pharmacological sensitivities. They are as follows. 1) A Ca2+-activated K current sensitive to external Ca2+ and Ca2+ influx was blocked by ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (0.1 X 10(-3) M) and nifedipine (1 X 10(-6) and was increased by elevated Ca2+ (8 X 10(-6) M) and BAY K 8644 (1 X 10(-6) M). 2) A "delayed rectifier" current was observed that decayed slowly with time and showed no voltage-dependent inactivation. 3) Spontaneous transient outward currents that were blocked by ryanodine (2 X 10(-6) M) were also recorded. The possible contributions of these currents to the electrical activity of colonic muscle cells in situ are discussed. Ca2+-activated K current may contribute a significant conductance to the repolarizing phase of electrical slow waves.

Sodium-dependent plateau potentials in cultured Retzius cells of the medicinal leech

1996 ◽  
Vol 76 (3) ◽  
pp. 1491-1502 ◽  
Author(s):  
J. D. Angstadt ◽  
J. J. Choo
Keyword(s):  
Outward Current ◽  
Channel Blockers ◽  
Plateau Potentials ◽  
Calcium Dependent ◽  
Current Pulses ◽  
Outward Currents ◽  
Voltage Dependent ◽  
Prolonged Duration ◽  
Input Conductance ◽  
Retzius Cells

1. Individual leech Retzius (Rz) cells were removed from mid-body ganglia and plated in cell culture on concanavalin A or polylysine. Experiments on the majority of cells were performed after 6-11 days in culture. Isolated Rz cells were superfused with normal leech saline (NS), cobalt saline (Ca2+ replaced with Co2+), or one of a variety of other modified salines. 2. Prolonged plateau potentials (PPs) with durations ranging from several seconds to nearly 2 min were evoked in isolated Rz cells in response to 1-s depolarizing current pulses delivered under discontinuous current clamp. Some PPs terminated spontaneously while others were terminated with hyperpolarizing current pulses. PPs were associated with a dramatic increase in the input conductance of the neuron. The PP decayed slightly over time, and this decay was accompanied by a small decrease in the input conductance. 3. PP duration was enhanced by penetrating cells with electrodes containing tetraethylammonium (TEA) and by bathing cells in Co2+ saline, but PPs were evoked also in NS and using electrodes without TEA. The effects of TEA and Co2+ saline suggest that voltage-dependent and especially calcium-dependent outward currents normally suppress plateau formation. 4. PPs occurred most reliably in neurons with extensive neurite sprouting. Isolated somata with few or no neurites usually failed to express PP, although there were several exceptions to this trend. 5. PPs persisted when Ca2+ was replaced with either of the calcium channel blockers Co2+, Ni2+, or Mn2+, when 200 microM Cd2+ was added to normal saline, or when Na+ was replaced with Li+. In contrast, PPs were eliminated rapidly when Na+ was replaced with N-methyl-D-glucamine. 6. Isolated Rz cells also expressed repetitive PPs either spontaneously or in response to injection of sustained depolarizing current. Spontaneous repetitive PPs were suppressed by hyperpolarizing current. Repetitive PPs in isolated Rz cells are similar in many respects to the bursting electrical activity induced by Co2+ saline in Rz and other neurons in intact ganglia. 7. The ionic dependence and prolonged duration of PPs suggest that these responses are generated by a persistent voltage-dependent Na+ current. A quantitative computer simulation of PPs was achieved using a depolarization-activated Na+ conductance with very slow inactivation. Repetitive PPs were simulated by addition of a slow outward current in the form of an electrogenic pump.

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