scholarly journals Potassium channel types in arterial chemoreceptor cells and their selective modulation by oxygen.

1992 ◽  
Vol 100 (3) ◽  
pp. 401-426 ◽  
Author(s):  
M D Ganfornina ◽  
J López-Barneo
Keyword(s):  
Time Course ◽  
Single Channel ◽  
K Channel ◽  
Open Probability ◽  
K Channels ◽  
Glomus Cells ◽  
Control Value ◽  
Voltage Dependent ◽  
K Current ◽  
Chemoreceptor Cells

Single K+ channel currents were recorded in excised membrane patches from dispersed chemoreceptor cells of the rabbit carotid body under conditions that abolish current flow through Na+ and Ca2+ channels. We have found three classes of voltage-gated K+ channels that differ in their single-channel conductance (gamma), dependence on internal Ca2+ (Ca2+i), and sensitivity to changes in O2 tension (PO2). Ca(2+)-activated K+ channels (KCa channels) with gamma approximately 210 pS in symmetrical K+ solutions were observed when [Ca2+]i was greater than 0.1 microM. Small conductance channels with gamma = 16 pS were not affected by [Ca2+]i and they exhibited slow activation and inactivation time courses. In these two channel types open probability (P(open)) was unaffected when exposed to normoxic (PO2 = 140 mmHg) or hypoxic (PO2 approximately 5-10 mmHg) external solutions. A third channel type (referred to as KO2 channel), having an intermediate gamma(approximately 40 pS), was the most frequently recorded. KO2 channels are steeply voltage dependent and not affected by [Ca2+]i, they inactivate almost completely in less than 500 ms, and their P(open) reversibly decreases upon exposure to low PO2. The effect of low PO2 is voltage dependent, being more pronounced at moderately depolarized voltages. At 0 mV, for example, P(open) diminishes to approximately 40% of the control value. The time course of ensemble current averages of KO2 channels is remarkably similar to that of the O2-sensitive K+ current. In addition, ensemble average and macroscopic K+ currents are affected similarly by low PO2. These observations strongly suggest that KO2 channels are the main contributors to the macroscopic K+ current of glomus cells. The reversible inhibition of KO2 channel activity by low PO2 does not desensitize and is not related to the presence of F-, ATP, and GTP-gamma-S at the internal face of the membrane. These results indicate that KO2 channels confer upon glomus cells their unique chemoreceptor properties and that the O2-K+ channel interaction occurs either directly or through an O2 sensor intrinsic to the plasma membrane closely associated with the channel molecule.

scholarly journals Gating of Recombinant Small-Conductance Ca-activated K+ Channels by Calcium

1998 ◽  
Vol 111 (4) ◽  
pp. 565-581 ◽  
Author(s):  
Birgit Hirschberg ◽  
James Maylie ◽  
John P. Adelman ◽  
Neil V. Marrion
Keyword(s):  
Time Course ◽  
Single Channel ◽  
Cell Types ◽  
K Channel ◽  
Open Probability ◽  
K Channels ◽  
Sensitive Clone ◽  
Single Channel Currents ◽  
Open States ◽  
Small Conductance

Small-conductance Ca-activated K+ channels play an important role in modulating excitability in many cell types. These channels are activated by submicromolar concentrations of intracellular Ca2+, but little is known about the gating kinetics upon activation by Ca2+. In this study, single channel currents were recorded from Xenopus oocytes expressing the apamin-sensitive clone rSK2. Channel activity was detectable in 0.2 μM Ca2+ and was maximal above 2 μM Ca2+. Analysis of stationary currents revealed two open times and three closed times, with only the longest closed time being Ca dependent, decreasing with increasing Ca2+ concentrations. In addition, elevated Ca2+ concentrations resulted in a larger percentage of long openings and short closures. Membrane voltage did not have significant effects on either open or closed times. The open probability was ∼0.6 in 1 μM free Ca2+. A lower open probability of ∼0.05 in 1 μM Ca2+ was also observed, and channels switched spontaneously between behaviors. The occurrence of these switches and the amount of time channels spent displaying high open probability behavior was Ca2+ dependent. The two behaviors shared many features including the open times and the short and intermediate closed times, but the low open probability behavior was characterized by a different, long Ca2+-dependent closed time in the range of hundreds of milliseconds to seconds. Small-conductance Ca- activated K+ channel gating was modeled by a gating scheme consisting of four closed and two open states. This model yielded a close representation of the single channel data and predicted a macroscopic activation time course similar to that observed upon fast application of Ca2+ to excised inside-out patches.

scholarly journals Charybdotoxin selectively blocks small Ca-activated K channels in Aplysia neurons.

1987 ◽  
Vol 90 (1) ◽  
pp. 27-47 ◽  
Author(s):  
A Hermann ◽  
C Erxleben
Keyword(s):  
Single Channel ◽  
Delayed Rectifier ◽  
Pacemaker Activity ◽  
Dependent Manner ◽  
Open Probability ◽  
Apparent Dissociation Constant ◽  
External Application ◽  
K Channels ◽  
Voltage Dependent ◽  
K Current

The action of charybdotoxin (ChTX), a peptide component isolated from the venom of the scorpion Leiurus quinquestriatus, was investigated on membrane currents of identified neurons from the marine mollusk, Aplysia californica. Macroscopic current recordings showed that the external application of ChTX blocks the Ca-activated K current in a dose- and voltage-dependent manner. The apparent dissociation constant is 30 nM at V = -30 mV and increases e-fold for a +50- to +70-mV change in membrane potential, which indicates that the toxin molecule is sensitive to approximately 35% of the transmembrane electric field. The toxin is bound to the receptor with a 1:1 stoichiometry and its effect is reversible after washout. The toxin also suppresses the membrane leakage conductance and a resting K conductance activated by internal Ca ions. The toxin has no significant effect on the inward Na or Ca currents, the transient K current, or the delayed rectifier K current. Records from Ca-activated K channels revealed a single channel conductance of 35 +/- 5 pS at V = 0 mV in asymmetrical K solution. The channel open probability increased with the internal Ca concentration and with membrane voltage. The K channels were blocked by submillimolar concentrations of tetraethylammonium ions and by nanomolar concentrations of ChTX, but were not blocked by 4-aminopyridine if applied externally on outside-out patches. From the effects of ChTX on K current and on bursting pacemaker activity, it is concluded that the termination of bursts is in part controlled by a Ca-activated K conductance.

scholarly journals Dynamics of aminopyridine block of potassium channels in squid axon membrane.

1976 ◽  
Vol 68 (5) ◽  
pp. 519-535 ◽  
Author(s):  
J Z Yeh ◽  
G S Oxford ◽  
C H Wu ◽  
T Narahashi
Keyword(s):  
Time Course ◽  
Membrane Surface ◽  
K Channel ◽  
Dependent Manner ◽  
Squid Axon ◽  
Axon Membrane ◽  
K Channels ◽  
Voltage Dependent ◽  
K Current ◽  
Kinetics Of

Aminopyridines (2-AP, 3-AP, and 4-AP) selectively block K channels of squid axon membranes in a manner dependent upon the membrane potential and the duration and frequency of voltage clamp pulses. They are effective when applied to either the internal or the external membrane surface. The steady-state block of K channels by aminopyridines is more complete for low depolarizations, and is gradually relieved at higher depolarizations. The K current in the presence of aminopyridines rises more slowly than in control, the change being more conspicuous in 3-AP and 4-AP than in 2-AP. Repetitive pulsing relieves the block in a manner dependent upon the duration and interval of pulses. The recovery from block during a given test pulse is enhanced by increasing the duration of a conditioning depolarizing prepulse. The time constant for this recovery is in the range of 10-20 ms in 3-AP and 4-AP, and shorter in 2-AP. Twin pulse experiments with variable pulse intervals have revealed that the time course for re-establishment of block is much slower in 3-AP and 4-AP than in 2-AP. These results suggest that 2-AP interacts with the K channel more rapidly than 3-AP and 4-AP. The more rapid interaction of 2-AP with K channels is reflected in the kinetics of K current which is faster than that observed in 3-AP or 4-AP, and in the pattern of frequency-dependent block which is different from that in 3-AP or 4-AP. The experimental observations are not satisfactorily described by alterations of Hodgkin-Huxley n-type gating units. Rather, the data are consistent with a simple binding scheme incorporating no changes in gating kinetics which conceives of aminopyridine molecules binding to closed K channels and being released from open channels in a voltage-dependent manner.

scholarly journals Slo3 K+ Channels: Voltage and pH Dependence of Macroscopic Currents

2006 ◽  
Vol 128 (3) ◽  
pp. 317-336 ◽  
Author(s):  
Xue Zhang ◽  
Xuhui Zeng ◽  
Christopher J. Lingle
Keyword(s):  
Time Course ◽  
Voltage Dependence ◽  
Ph Dependence ◽  
K Channel ◽  
Open Probability ◽  
Cytosolic Ph ◽  
Α Subunit ◽  
K Channels ◽  
C Terminus ◽  
Voltage Dependent

The mouse Slo3 gene (KCNMA3) encodes a K+ channel that is regulated by changes in cytosolic pH. Like Slo1 subunits responsible for the Ca2+ and voltage-activated BK-type channel, the Slo3 α subunit contains a pore module with homology to voltage-gated K+ channels and also an extensive cytosolic C terminus thought to be responsible for ligand dependence. For the Slo3 K+ channel, increases in cytosolic pH promote channel activation, but very little is known about many fundamental properties of Slo3 currents. Here we define the dependence of macroscopic conductance on voltage and pH and, in particular, examine Slo3 conductance activated at negative potentials. Using this information, the ability of a Horrigan-Aldrich–type of general allosteric model to account for Slo3 gating is examined. Finally, the pH and voltage dependence of Slo3 activation and deactivation kinetics is reported. The results indicate that Slo3 differs from Slo1 in several important ways. The limiting conductance activated at the most positive potentials exhibits a pH-dependent maximum, suggesting differences in the limiting open probability at different pH. Furthermore, over a 600 mV range of voltages (−300 to +300 mV), Slo3 conductance shifts only about two to three orders of magnitude, and the limiting conductance at negative potentials is relatively voltage independent compared to Slo1. Within the context of the Horrigan-Aldrich model, these results indicate that the intrinsic voltage dependence (zL) of the Slo3 closed–open equilibrium and the coupling (D) between voltage sensor movement are less than in Slo1. The kinetic behavior of Slo3 currents also differs markedly from Slo1. Both activation and deactivation are best described by two exponential components, both of which are only weakly voltage dependent. Qualitatively, the properties of the two kinetic components in the activation time course suggest that increases in pH increase the fraction of more rapidly opening channels.

scholarly journals A Quantitative Description of KcsA Gating I: Macroscopic Currents

2007 ◽  
Vol 130 (5) ◽  
pp. 465-478 ◽  
Author(s):  
Sudha Chakrapani ◽  
Julio F Cordero-Morales ◽  
Eduardo Perozo
Keyword(s):  
Time Course ◽  
Single Channel ◽  
Ph Dependence ◽  
Quantitative Description ◽  
Hill Coefficient ◽  
K Channel ◽  
Open Probability ◽  
Transmembrane Voltage ◽  
Voltage Dependent ◽  
Inactivation Process

The prokaryotic K+ channel KcsA is activated by intracellular protons and its gating is modulated by transmembrane voltage. Typically, KcsA functions have been studied under steady-state conditions, using macroscopic Rb+-flux experiments and single-channel current measurements. These studies have provided limited insights into the gating kinetics of KcsA due to its low open probability, uncertainties in the number of channels in the patch, and a very strong intrinsic kinetic variability. In this work, we have carried out a detailed analysis of KcsA gating under nonstationary conditions by examining the influence of pH and voltage on the activation, deactivation, and slow-inactivation gating events. We find that activation and deactivation gating of KcsA are predominantly modulated by pH without a significant effect of voltage. Activation gating showed sigmoidal pH dependence with a pKa of ∼4.2 and a Hill coefficient of ∼2. In the sustained presence of proton, KcsA undergoes a time-dependent decay of conductance. This inactivation process is pH independent but is modulated by voltage and the nature of permeant ion. Recovery from inactivation occurs via deactivation and also appears to be voltage dependent. We further find that inactivation in KcsA is not entirely a property of the open-conducting channel but can also occur from partially “activated” closed states. The time course of onset and recovery of the inactivation process from these pre-open closed states appears to be different from the open-state inactivation, suggesting the presence of multiple inactivated states with diverse kinetic pathways. This information has been analyzed together with a detailed study of KcsA single-channel behavior (in the accompanying paper) in the framework of a kinetic model. Taken together our data constitutes the first quantitative description of KcsA gating.

scholarly journals Membrane-delimited Inhibition of Maxi-K Channel Activity by the Intermediate Conductance Ca2+-activated K Channel

2006 ◽  
Vol 127 (2) ◽  
pp. 159-169 ◽  
Author(s):  
Jill Thompson ◽  
Ted Begenisich
Keyword(s):  
Single Channel ◽  
Expression System ◽  
Chemical Activation ◽  
K Channel ◽  
Channel Activity ◽  
Single Family ◽  
Open Probability ◽  
K Channels ◽  
Channel Proteins ◽  
K Activity

The complexity of mammalian physiology requires a diverse array of ion channel proteins. This diversity extends even to a single family of channels. For example, the family of Ca2+-activated K channels contains three structural subfamilies characterized by small, intermediate, and large single channel conductances. Many cells and tissues, including neurons, vascular smooth muscle, endothelial cells, macrophages, and salivary glands express more than a single class of these channels, raising questions about their specific physiological roles. We demonstrate here a novel interaction between two types of Ca2+-activated K channels: maxi-K channels, encoded by the KCa1.1 gene, and IK1 channels (KCa3.1). In both native parotid acinar cells and in a heterologous expression system, activation of IK1 channels inhibits maxi-K activity. This interaction was independent of the mode of activation of the IK1 channels: direct application of Ca2+, muscarinic receptor stimulation, or by direct chemical activation of the IK1 channels. The IK1-induced inhibition of maxi-K activity occurred in small, cell-free membrane patches and was due to a reduction in the maxi-K channel open probability and not to a change in the single channel current level. These data suggest that IK1 channels inhibit maxi-K channel activity via a direct, membrane-delimited interaction between the channel proteins. A quantitative analysis indicates that each maxi-K channel may be surrounded by four IK1 channels and will be inhibited if any one of these IK1 channels opens. This novel, regulated inhibition of maxi-K channels by activation of IK1 adds to the complexity of the properties of these Ca2+-activated K channels and likely contributes to the diversity of their functional roles.

Internal and external TEA block in single cloned K+ channels

1991 ◽  
Vol 261 (4) ◽  
pp. C583-C590 ◽  
Author(s):  
G. E. Kirsch ◽  
M. Taglialatela ◽  
A. M. Brown
Keyword(s):  
Single Channel ◽  
Channel Structure ◽  
K Channel ◽  
Cdna Clones ◽  
K Channels ◽  
Open Time ◽  
Voltage Dependent ◽  
Internal Site ◽  
Channel Open Time ◽  
Single Channel Currents

Tetraethylammonium (TEA) has been used recently to probe natural and mutational variants of voltage-dependent K+ channels encoded by cDNA clones. Its usefulness as a probe of channel structure prompted us to examine the molecular mechanism by which TEA blocks single-channel currents in Xenopus oocytes expressing the rat brain K+ channel, RCK2. TEA at the intracellular surface of membrane patches decreased channel open time and increased the duration of closed intervals. Tetrapentylammonium had similar but more potent effects. Extracellular application of TEA caused an apparent reduction of single-channel amplitude. Block was slower at the high-affinity internal site than at the low-affinity external site. Internal TEA selectively blocks open K+ channels, and the voltage dependence of the block indicates that the binding site lies within the membrane electric field at a point 25% of the distance from the cytoplasmic margin. External TEA also interacts with the open channel but is less sensitive to membrane potential. The results indicate that the internal and external TEA binding sites define the inner and outer margins of the aqueous pore.

Different properties of the atrial G protein-gated K+ channels activated by extracellular ATP and adenosine

1995 ◽  
Vol 269 (4) ◽  
pp. H1349-H1358 ◽  
Author(s):  
C. Fu ◽  
A. Pleumsamran ◽  
U. Oh ◽  
D. Kim
Keyword(s):  
G Protein ◽  
Single Channel ◽  
Extracellular Atp ◽  
K Channel ◽  
Affinity Constant ◽  
Channel Activity ◽  
K Channels ◽  
Atrial Cells ◽  
K Current ◽  
Inside Out

Extracellular ATP (ATPo) and adenosine activate G protein-gated inwardly rectifying K+ currents in atrial cells. Earlier studies have suggested that the two agonists may use separate pathways to activate the K+ current. Therefore, we examined whether the K+ channels activated by the two agonists have different properties under identical ionic conditions. In cell-attached patches, K+ channels activated by 100 microM ATP in the pipette had a single-channel conductance and mean open time of 32.0 +/- 0.2 pS and 0.5 +/- 0.1 ms, respectively, compared with 31.3 +/- 0.3 pS and 0.9 +/- 0.1 ms for the K+ channels activated by adenosine (140 mM KCl). With ATPo as the agonist, the K+ channel activity in cell-attached patches was approximately threefold lower than that in inside-out patches with 100 microM GTP in the bath. Applying ATP to the cytoplasmic side of the membrane (ATPi) produced a biphasic concentration-dependent effect on channel activity: an increase at low [mean affinity constant (K0.5) = 190 microM] and a decrease at high (K0.5 = 1.3 mM) concentrations. In contrast, with adenosine as the agonist, K+ channel activity in cell-attached patches was approximately fourfold greater than that in inside-out patches with 100 microM GTP in the bath. In inside-out patches, ATPi only augmented the K+ channel activity (K0.5 = 32 microM). These results show that although both ATPo and adenosine activate kinetically similar K+ channels in atrial cells, the channels are regulated differently by intracellular nucleotides.

scholarly journals Gating of maxi K+ channels studied by Ca2+ concentration jumps in excised inside-out multi-channel patches (myocytes from guinea pig urinary bladder).

1992 ◽  
Vol 99 (6) ◽  
pp. 841-862 ◽  
Author(s):  
F Markwardt ◽  
G Isenberg
Keyword(s):  
Steady State ◽  
Time Course ◽  
Boltzmann Distribution ◽  
Hill Coefficient ◽  
K Channel ◽  
Open Probability ◽  
Apparent Dissociation Constant ◽  
K Channels ◽  
The Mean ◽  
Inside Out

Currents through maxi K+ channels were recorded in inside-out macro-patches. Using a liquid filament switch (Franke, C., H. Hatt, and J. Dudel. 1987. Neurosci, Lett. 77:199-204) the Ca2+ concentration at the tip of the patch electrode ([Ca2+]i) was changed in less than 1 ms. Elevation of [Ca2+]i from less than 10 nM to 3, 6, 20, 50, 320, or 1,000 microM activated several maxi K+ channels in the patch, whereas return to less than 10 nM deactivated them. The time course of Ca(2+)-dependent activation and deactivation was evaluated from the mean of 10-50 sweeps. The mean currents started a approximately 10-ms delay that was attributed to diffusion of Ca2+ from the tip to the K+ channel protein. The activation and deactivation time courses were fitted with the third power of exponential terms. The rate of activation increased with higher [Ca2+]i and with more positive potentials. The rate of deactivation was independent of preceding [Ca2+]i and was reduced at more positive potentials. The rate of deactivation was measured at five temperatures between 16 and 37 degrees C; fitting the results with the Arrhenius equation yielded an energy barrier of 16 kcal/mol for the Ca2+ dissociation at 0 mV. After 200 ms, the time-dependent processes were in a steady state, i.e., there was no sign of inactivation. In the steady state (200 ms), the dependence of channel openness, N.P(o), on [Ca2+]i yielded a Hill coefficient of approximately 3. The apparent dissociation constant, KD, decreased from 13 microM at -50 mV to 0.5 microM at +70 mV. The dependence of N.P(o) on voltage followed a Boltzmann distribution with a maximal P(o) of 0.8 and a slope factor of approximately 39 mV. The results were summarized by a model describing Ca2+- and voltage-dependent activation and deactivation, as well as steady-state open probability by the binding of Ca2+ to three equal and independent sites within the electrical field of the membrane at an electrical distance of 0.31 from the cytoplasmic side.

Delayed activation of single mechanosensitive channels in Lymnaea neurons

1994 ◽  
Vol 267 (2) ◽  
pp. C598-C606 ◽  
Author(s):  
D. L. Small ◽  
C. E. Morris
Keyword(s):  
Dynamic Behavior ◽  
Abrupt Onset ◽  
Fundamental Difference ◽  
K Channel ◽  
Mechanosensitive Channels ◽  
Stretch Activation ◽  
Open Probability ◽  
K Channels ◽  
Voltage Dependent ◽  
Lymnaea Neurons

Some stretch-activated (SA) channels challenged with suction jumps exhibit adaptation, a dynamic behavior that can be overlooked because of its mechanical fragility. In previous studies of neuronal SA K channels, we detected no adaptation, but the protocols used were not designed to detect dynamics. Here, we reproduce the adaptation seen by others in Xenopus SA cationic (Cat) channels but show that, with the same protocol, no adaptation occurs with SA K channels. Instead, SA K channels exhibit a different dynamic behavior, delayed activation. Lymnaea SA K channels subjected to pressure jumps responded after a 1- to 4-s delay with a gradual, rather than abrupt, onset of activation. The delay was pressure dependent and was longer for patches from older cultured neurons. Delayed responses were fragile like SA Cat channel adaptation; they disappeared with repeated stimuli. Cytochalasin D decreased the delay and increased the stretch activation of SA K channels. Unlike SA Cat channel adaptation, which occurs only at hyperpolarized potentials, SA K channel delay was not voltage dependent. We note that once SA Cat and SA K channels are "stripped" of their fragile (cytoskeleton-dependent?) dynamics, however, their gating behaviors show little fundamental difference; both are stretch activatable and have a higher open probability at depolarized potentials.

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