scholarly journals Muscarinic K+ Channel in the Heart

1998 ◽  
Vol 112 (2) ◽  
pp. 199-210 ◽  
Author(s):  
Tatyana T. Ivanova-Nikolova ◽  
Emil N. Nikolov ◽  
Carl Hansen ◽  
Janet D. Robishaw
Keyword(s):  
Muscarinic Receptor ◽  
Fold Increase ◽  
Receptor Activation ◽  
Membrane Receptors ◽  
Neonatal Rat ◽  
K Channel ◽  
Open Probability ◽  
K Channels ◽  
Open Time ◽  
The Mean

The membrane-delimited activation of muscarinic K+ channels by G protein βγ subunits plays a prominent role in the inhibitory synaptic transmission in the heart. These channels are thought to be heterotetramers comprised of two homologous subunits, GIRK1 and CIR, both members of the family of inwardly rectifying K+ channels. Here, we demonstrate that muscarinic K+ channels in neonatal rat atrial myocytes exhibit four distinct gating modes. In intact myocytes, after muscarinic receptor activation, the different gating modes were distinguished by differences in both the frequency of channel opening and the mean open time of the channel, which accounted for a 76-fold increase in channel open probability from mode 1 to mode 4. Because of the tetrameric architecture of the channel, the hypothesis that each of the four gating modes reflects binding of a different number of Gβγ subunits to the channel was tested, using recombinant Gβ1γ5. Gβ1γ5 was able to control the equilibrium between the four gating modes of the channel in a manner consistent with binding of Gβγ to four equivalent and independent sites in the protein complex. Surprisingly, however, Gβ1γ5 lacked the ability to stabilize the long open state of the channel that is responsible for the augmentation of the mean open time in modes 3 and 4 after muscarinic receptor stimulation. The modal regulation of muscarinic K+ channel gating by Gβγ provides the atrial cells with at least two major advantages: the ability to filter out small inputs from multiple membrane receptors and yet the ability to create the gradients of information necessary to control the heart rate with great precision.

scholarly journals Light-dependent K+ Channels in the Mollusc Onchidium Simple Photoreceptors Are Opened by cGMP

2002 ◽  
Vol 120 (4) ◽  
pp. 581-597 ◽  
Author(s):  
Tsukasa Gotow ◽  
Takako Nishi
Keyword(s):  
K Channel ◽  
Single Channels ◽  
Open Probability ◽  
Mean Values ◽  
K Channels ◽  
Open Time ◽  
Hyperpolarizing Response ◽  
The Mean ◽  
K Concentration ◽  
Inside Out

Light-dependent K+ channels underlying a hyperpolarizing response of one extraocular (simple) photoreceptor, Ip-2 cell, in the marine mollusc Onchidium ganglion were examined using cell-attached and inside-out patch-clamp techniques. A previous report (Gotow, T., T. Nishi, and H. Kijima. 1994. Brain Res. 662:268–272) showed that a depolarizing response of the other simple photoreceptor, A-P-1 cell, results from closing of the light-dependent K+ channels that are activated by cGMP. In the cell-attached patch recordings of Ip-2 cells, external artificial seawater (ASW) was replaced with a modified ASW containing 150 mM K+ and 200 mM Mg2+ to suppress any synaptic input and to maintain the membrane potential constant. When Ip-2 cells were equilibrated with this modified ASW, the internal K+ concentration was estimated to be 260 mM. Light-dependent single-channels in the cell-attached patch on these cells were opened by light but scarcely by voltage. After confirming the light-dependent channel activity in the cell-attached patches, an application of cGMP to the excised inside-out patches newly activated a channel that disappeared on removal of cGMP. Open and closed time distributions of this cGMP-activated channel could be described by the sum of two exponents with time constants τo1, τo2 and τc1, τc2, respectively, similar to those of the light-dependent channel. In both the channels, τo1 and τo2 in ms ranges were similar to each other, although τc2 over tens of millisecond ranges was different. τo1, τo2, and the mean open time τo were both independent of light intensity, cGMP concentration, and voltage. In both channels, the open probability increased as the membrane was depolarized, without changing any of τo2 or τo. In both, the reversal potentials using 200- and 450-mM K+-filled pipettes were close to the K+ equilibrium potentials, suggesting that both the channels are primarily K+ selective. Both the mean values of the channel conductance were estimated to be the same at 62 and 91 pS in 200- and 450-mM K+ pipettes at nearly 0 mV, respectively. Combining these findings with those in the above former report, it is concluded that cGMP is a second messenger which opens the light-dependent K+ channel of Ip-2 to cause hyperpolarization, and that the channel is the same as that of A-P-1 closed by light.

Basolateral potassium channels in renal proximal tubule

1987 ◽  
Vol 253 (3) ◽  
pp. F476-F487 ◽  
Author(s):  
H. Sackin ◽  
L. G. Palmer
Keyword(s):  
Single Channel ◽  
Basolateral Membrane ◽  
K Channel ◽  
Channel Conductance ◽  
Single Channel Conductance ◽  
K Channels ◽  
Open Time ◽  
Excised Patches ◽  
The Mean ◽  
Inside Out

Potassium (K+) channels in the basolateral membrane of unperfused Necturus proximal tubules were studied in both cell-attached and excised patches, after removal of the tubule basement membrane by manual dissection without collagenase. Two different K+ channels were identified on the basis of their kinetics: a short open-time K+ channel, with a mean open time less than 1 ms, and a long open-time K+ channel with a mean open time greater than 20 ms. The short open-time channel occurred more frequently than the longer channel, especially in excised patches. For inside-out excised patches with Cl- replaced by gluconate, the current-voltage relation of the short open-time K+ channel was linear over +/- 60 mV, with a K+-Na+ selectivity of 12 +/- 2 (n = 12), as calculated from the reversal potential with oppositely directed Na+ and K+ gradients. With K-Ringer in the patch pipette and Na-Ringer in the bath, the conductance of the short open-time channel was 47 +/- 2 pS (n = 15) for cell-attached patches, 26 +/- 2 pS (n = 15) for patches excised (inside out) into Na-Ringer, and 36 +/- 6 pS (n = 3) for excised patches with K-Ringer on both sides. These different conductances can be partially explained by a dependence of single-channel conductance on the K+ concentration on the interior side of the membrane. In experiments with a constant K+ gradient across excised patches, large changes in Na+ at the interior side of the membrane produced no change in single-channel conductance, arguing against a direct block of the K+ channel by Na+. Finally, the activity of the short open-time channel was voltage gated, where the mean number of open channels decreased as a linear function of basolateral membrane depolarization for potentials between -60 and 0 mV. Depolarization from -60 to -40 mV decreased the mean number of open K+ channels by 28 +/- 8% (n = 6).

Alpha-subunit of Gk activates atrial K+ channels of chick, rat, and guinea pig

1988 ◽  
Vol 254 (6) ◽  
pp. H1200-H1205 ◽  
Author(s):  
G. E. Kirsch ◽  
A. Yatani ◽  
J. Codina ◽  
L. Birnbaumer ◽  
A. M. Brown
Keyword(s):  
Guinea Pig ◽  
Single Channel ◽  
Neonatal Rat ◽  
Alpha Subunit ◽  
K Channel ◽  
Muscarinic Agonist ◽  
Guanine Nucleotide Binding Protein ◽  
K Channels ◽  
Open Time ◽  
Guanine Nucleotide Binding

A specific guanine nucleotide-binding protein, Gk, is the link by which muscarinic receptors activate atrial potassium channels (Science Wash. DC 235: 207-211, 1987). In adult guinea pigs, the alpha-subunit at picomolar concentrations mediates the holo-G protein effect (Science Wash. DC 236: 442-445, 1987), but in chick embryo it has been reported that the beta gamma-dimer at nanomolar concentrations rather than the alpha-subunit is the effective mediator (Nature Lond. 325: 321-326, 1987). This difference might have a phylogenetic or ontogenetic basis, and the present experiments tested these possibilities. Preactivated alpha k derived from human red blood cell Gk, when applied to the intracellular surface of inside-out membrane patches from the atria of embryonic chick, neonatal rat, and adult guinea pig activated single K+ channel currents. In each case, the alpha k-activated channels had the same single-channel conductance and mean open time as the muscarinic agonist-activated channels. Half-maximal activation was achieved at alpha k-concentrations of 2.4-13.8 pM. Hence, alpha k-activation of these K+ channels is independent of differences in age or species. The detergent 3-[3-cholamidopropyl)-dimethyammoniol]-1-propanesulfonate (CHAPS), which was used by Logothetis et al. (Nature Lond. 325: 321-326, 1987) at 184 microM to suspend the hydrophobic beta gamma-dimers, activated the same currents. We conclude that the effects of the beta gamma-dimer on these K+ channels is unknown and that as we had proposed earlier (Science Wash. DC 236: 442-445, 1987) it is the alpha-subunit that mediates the Gk effect.

Effect of H+ on ATP-regulated K+ channels in feline ventricular myocytes

1991 ◽  
Vol 261 (3) ◽  
pp. H755-H761 ◽  
Author(s):  
J. Cuevas ◽  
A. L. Bassett ◽  
J. S. Cameron ◽  
T. Furukawa ◽  
R. J. Myerburg ◽  
...  
Keyword(s):  
Ventricular Myocytes ◽  
Reversal Potential ◽  
State Probability ◽  
K Channel ◽  
K Channels ◽  
Open Time ◽  
Current Voltage ◽  
Effects Of Ph ◽  
The Mean ◽  
Current Voltage Relationship

Using patch-clamp techniques, we examined the effects of pH on properties of ATP-regulated K+ channels in single myocytes isolated from cat left ventricles. ATP-K+ channels of inside-out patches were bilaterally exposed to 140 mM K+ solutions (22 degrees C). In the absence of ATP and Mg2+, the channels had a linear current-voltage relationship during hyperpolarizing pulses (20-100 mV negative to the reversal potential) at both intracellular pH (pHi) 7.4 and 6.5, but the slope conductance was 66 +/- 2 pS at pHi 7.4 and 46 +/- 2 pS at pHi 6.5. Lowering pHi from 7.4 to 6.5 increased the mean open time (from 15.9 +/- 4.6 to 35.9 +/- 7.9 ms, P less than 0.01) but decreased the open-state probability measured at 50 mV positive to the reversal potential (from 0.35 +/- 0.04 to 0.16 +/- 0.04, P less than 0.01). However, in the presence of both 0.2 mM ATP and 1 mM MgCl2, lowering pHi from 7.4 to 6.5 increased the mean open time (from 5.0 +/- 2.6 to 17.9 +/- 5.9 ms, P less than 0.01) and the open-state probability (from 0.025 +/- 0.010 to 0.098 +/- 0.024, P less than 0.01). These data indicate that increases in intracellular H+ concentration modulate cardiac ATP-K+ channel properties. Ischemia-associated decreases in pHi may enhance the opening of cardiac ATP-regulated K+ channels and resultant action potential shortening.

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.

Large conducting potassium channel reconstituted from airway smooth muscle

1992 ◽  
Vol 262 (3) ◽  
pp. L327-L336 ◽  
Author(s):  
D. Savaria ◽  
C. Lanoue ◽  
A. Cadieux ◽  
E. Rousseau
Keyword(s):  
Smooth Muscle ◽  
Lipid Bilayers ◽  
Airway Smooth Muscle ◽  
Single Channel ◽  
Specific Inhibitor ◽  
Physiological Role ◽  
Membrane Receptors ◽  
K Channel ◽  
Open Probability ◽  
K Channels

Microsomal fractions were prepared from canine and bovine airway smooth muscle (ASM) by differential and gradient centrifugations. Surface membrane vesicles were characterized by binding assays and incorporated into planar lipid bilayers. Single-channel activities were recorded in symmetric or asymmetric K+ buffer systems and studied under voltage and Ca2+ clamp conditions. A large-conductance K(+)-selective channel (greater than 220 pS in 150 mM K+) displaying a high Ca2+, low Ba2+, and charybdotoxin (CTX) sensitivity was identified. Time analysis of single-channel recordings revealed a complex kinetic behavior compatible with the previous schemes proposed for Ca(2+)-activated K+ channels in a variety of biological surface membranes. We now report that the open probability of the channel at low Ca2+ concentration is enhanced on in vitro phosphorylation, which is mediated via an adenosine 3',5'-cyclic monophosphate-dependent protein kinase. In addition to this characterization at the molecular level, a second series of pharmacological experiments were designed to assess the putative role of this channel in ASM strips. Our results show that 50 nM CTX, a specific inhibitor of the large conducting Ca(2+)-dependent K+ channel, prevents norepinephrine transient relaxation on carbamylcholine-precontracted ASM strips. It was also shown that CTX reversed the steady-state relaxation induced by vasoactive intestinal peptide and partially antagonized further relaxation induced by cumulative doses of this potent bronchodilatator. Thus it is proposed that the Ca(2+)-activated K+ channels have a physiological role because they are indirectly activated on stimulation of various membrane receptors via intracellular mechanisms.

scholarly journals Two novel cardiac atrial K+ channels, IK.AA and IK.PC.

1991 ◽  
Vol 98 (5) ◽  
pp. 921-939 ◽  
Author(s):  
M A Wallert ◽  
M J Ackerman ◽  
D Kim ◽  
D E Clapham
Keyword(s):  
Fatty Acids ◽  
Arachidonic Acid ◽  
Neonatal Rat ◽  
K Channel ◽  
Specific Mechanism ◽  
Lipophilic Compounds ◽  
K Channels ◽  
Open Time ◽  
Inside Out ◽  
Rat Atrial Cells

Two K(+)-selective channels in neonatal rat atrial cells activated by lipophilic compounds have been characterized in detail. The arachidonic acid-stimulated channel (IK.AA) had a slope conductance of 124 +/- 17 pS at +30 mV in symmetrical 140 mM potassium and a mean open time of approximately 1 ms, and was relatively voltage independent. IK.AA activity was reversibly increased by lowering pH to 6.0. Arachidonic acid was most effective in activating this channel, although a number of lipophilic compounds resulted in activation. Surprisingly, choline, a polar molecule, also activated the channel. A second K+ channel was activated by 10 microM phosphatidylcholine applied to the intracellular surface of inside-out atrial patches. This channel (IK.PC) had a slope conductance of 60 +/- 6 pS at +40 mV and a mean open time of approximately 0.6 ms, and was also relatively voltage independent. Fatty acids are probably monomeric in the membrane under the conditions of our recording; thus detergent effects are unlikely. Since a number of compounds including fatty acids and prostaglandins activated these two channels, an indirect, channel-specific mechanism may account for activation of these two cardiac K+ channels.

Potent inhibition of the aortic smooth muscle maxi-K channel by clinical doses of ethanol

2000 ◽  
Vol 279 (4) ◽  
pp. C1107-C1115 ◽  
Author(s):  
F. S. Walters ◽  
M. Covarrubias ◽  
J. S. Ellingson
Keyword(s):  
Smooth Muscle ◽  
Lipid Bilayers ◽  
Single Channel ◽  
K Channel ◽  
Open Probability ◽  
K Channels ◽  
Aortic Smooth Muscle ◽  
Slope Factor ◽  
The Mean ◽  
Relationship Change

We investigated the effects of clinically relevant ethanol concentrations (5–20 mM) on the single-channel kinetics of bovine aortic smooth muscle maxi-K channels reconstituted in lipid bilayers (1:1 palmitoyl-oleoyl-phosphatidylethanolamine: palmitoyl-oleoyl-phosphatidylcholine). Ethanol at 10 and 20 mM decreased the channel open probability ( P o) by 75 ± 20.3% mainly by increasing the mean closed time (+82 to +960%, n = 7). In some instances, ethanol also decreased the mean open time (−40.8 ± 22.5%). The P o-voltage relation in the presence of 20 mM ethanol exhibited a rightward shift in the midpoint of voltage activation (Δ V ½ ≅ 17 mV), a slightly steeper relationship (change in slope factor, Δ k, ≅ −2.5 mV), and a decreased maximum P o (from ∼0.82 to ∼0.47). Interestingly, channels inhibited by ethanol at low Ca2+ concentrations (2.5 μM) were very resistant to ethanol in the presence of increased Ca2+ (≥ 20 μM). Alcohol consumption in clinically relevant amounts may alter the contribution of maxi-K channels to the regulation of arterial tone.

scholarly journals Proton modulation of a Ca(2+)-activated K+ channel from rat skeletal muscle incorporated into planar bilayers.

1991 ◽  
Vol 98 (5) ◽  
pp. 1025-1042 ◽  
Author(s):  
C Laurido ◽  
S Candia ◽  
D Wolff ◽  
R Latorre
Keyword(s):  
Skeletal Muscle ◽  
Linear Function ◽  
K Channel ◽  
Ca Channel ◽  
Rat Skeletal Muscle ◽  
Open Probability ◽  
Planar Bilayers ◽  
Effect Of Ph ◽  
Open Time ◽  
The Mean

The effect of pH on the activation of a Ca-activated K+ [K(Ca)] channel from rat skeletal muscle incorporated into planar lipid bilayers was studied. Experiments were done at different intracellular Ca2+ and proton concentrations. Changes in pH modified channel kinetics only from the Ca-sensitive face of the channel. At constant Ca2+ concentration, intracellular acidification induced a decrease in the open probability (Po) and a shift of the channel activation curves toward the right along the voltage axis. The displacement was 23.5 mV per pH unit. This displacement was due to a change in the half saturation voltage (Vo) and not to a change in channel voltage dependence. The shifts in Vo induced by protons appeared to be independent of Ca2+ concentration. The slope of the Hill plot of the open-closed equilibrium vs. pH was close to one, suggesting that a minimum of one proton is involved in the proton-driven channel closing reaction. The change in Po with variations in pH was due to both a decrease in the mean open time (To) and an increase in the mean closed time (Tc). At constant voltage, the mean open time of the channel was a linear function of [Ca2+] and the mean closed time was a linear function of 1/[Ca2+]2. Changes in the internal pH modified the slope, but not the intercept of the linear relations To vs. [Ca2+] and Tc vs. 1/[Ca2+]2. On the basis of these results an economical kinetic model of the effect of pH on this channel is proposed. It is concluded that protons do not affect the open-closed reaction, but rather weaken Ca2+ binding to all the conformational states of the channel. Moreover, competitive models in which Ca2+ and H+ cannot bind to the same open or closed state are inconsistent with the data.

scholarly journals Phosphoinositides Decrease Atp Sensitivity of the Cardiac Atp-Sensitive K+ Channel

1999 ◽  
Vol 114 (2) ◽  
pp. 251-270 ◽  
Author(s):  
Zheng Fan ◽  
Jonathan C. Makielski
Keyword(s):  
Binding Site ◽  
Single Channel ◽  
K Channel ◽  
Atp Binding ◽  
Patch Clamp Technique ◽  
Open Probability ◽  
Open Time ◽  
Atp Inhibition ◽  
The Mean ◽  
Inwardly Rectifying

Anionic phospholipids modulate the activity of inwardly rectifying potassium channels (Fan, Z., and J.C. Makielski. 1997. J. Biol. Chem. 272:5388–5395). The effect of phosphoinositides on adenosine triphosphate (ATP) inhibition of ATP-sensitive potassium channel (KATP) currents was investigated using the inside-out patch clamp technique in cardiac myocytes and in COS-1 cells in which the cardiac isoform of the sulfonylurea receptor, SUR2, was coexpressed with the inwardly rectifying channel Kir6.2. Phosphoinositides (1 mg/ml) increased the open probability of KATP in low [ATP] (1 μM) within 30 s. Phosphoinositides desensitized ATP inhibition with a longer onset period (>3 min), activating channels inhibited by ATP (1 mM). Phosphoinositides treatment for 10 min shifted the half-inhibitory [ATP] (Ki) from 35 μM to 16 mM. At the single-channel level, increased [ATP] caused a shorter mean open time and a longer mean closed time. Phosphoinositides prolonged the mean open time, shortened the mean closed time, and weakened the [ATP] dependence of these parameters resulting in a higher open probability at any given [ATP]. The apparent rate constants for ATP binding were estimated to be 0.8 and 0.02 mM−1 ms−1 before and after 5-min treatment with phosphoinositides, which corresponds to a Ki of 35 μM and 5.8 mM, respectively. Phosphoinositides failed to desensitize adenosine inhibition of KATP. In the presence of SUR2, phosphoinositides attenuated MgATP antagonism of ATP inhibition. Kir6.2ΔC35, a truncated Kir6.2 that functions without SUR2, also exhibited phosphoinositide desensitization of ATP inhibition. These data suggest that (a) phosphoinositides strongly compete with ATP at a binding site residing on Kir6.2; (b) electrostatic interaction is a characteristic property of this competition; and (c) in conjunction with SUR2, phosphoinositides render additional, complex effects on ATP inhibition. We propose a model of the ATP binding site involving positively charged residues on the COOH-terminus of Kir6.2, with which phosphoinositides interact to desensitize ATP inhibition.

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