A rapidly activating delayed rectifier K+ channel in rabbit sinoatrial node cells

1995 ◽  
Vol 269 (2) ◽  
pp. H443-H452 ◽  
Author(s):  
H. Ito ◽  
K. Ono
Keyword(s):  
Steady State ◽  
Sinoatrial Node ◽  
Single Channel ◽  
Ensemble Average ◽  
Negative Slope ◽  
Delayed Rectifier ◽  
Channel Current ◽  
Single Channel Current ◽  
Sinoatrial Node Cells ◽  
Rabbit Sinoatrial Node

The single-channel current of the delayed rectifier K+ current (IK) was recorded in rabbit sinoatrial node cells. In the cell-attached patch, depolarization from -70 mV to potentials more positive than -50 mV activated the IK channel while repolarization deactivated it. The single-channel conductance was 7.8 pS for the outward current and 10.8 pS for the inward current (n = 6). The steady-state open probability (NPo) was maximum at around -30 mV and markedly decreased at more positive potentials. On repolarization from positive potentials, the channel was initially closed and then rapidly opened. The ensemble average showed an initial rise to a peak followed by the deactivation time course. Because the channel events were completely blocked by E-4031, the drug-sensitive component was examined in the whole cell current. The steady-state current-voltage relation of the drug-sensitive current showed a marked negative slope at potentials more positive than -10 mV. Upon repolarization, the drug-sensitive current initially increased (removal of inactivation) to the peak of the outward tail current, which was in agreement with the ensemble average of the single-channel current. We conclude that IK in the sinoatrial node cells is largely composed of the rapidly activating IK (IK,r) channels and that the inward rectification of IK,r, which is more marked than had been assumed in previous studies, is due to the decrease in NPo.

Effect of Prozac on whole cell ionic currents in lens and corneal epithelia

1995 ◽  
Vol 269 (1) ◽  
pp. C250-C256 ◽  
Author(s):  
J. L. Rae ◽  
A. Rich ◽  
A. C. Zamudio ◽  
O. A. Candia
Keyword(s):  
Potassium Channels ◽  
Single Channel ◽  
Ionic Currents ◽  
Current Amplitude ◽  
Human Lens ◽  
Delayed Rectifier ◽  
Open Probability ◽  
Channel Current ◽  
Single Channel Current ◽  
Ionic Conductances

Prozac (fluoxetine), a compound used therapeutically in humans to combat depression, has substantial effects on ionic conductances in rabbit corneal epithelial cells and in cultured human lens epithelium. In corneal epithelium, it reduces the current due to the large-conductance potassium channels that dominate this preparation. Its effects seem largely to decrease the open probability while leaving the single-channel current amplitude unaltered. In cultured human epithelium, currents from calcium-activated potassium channels and inward rectifiers are unaffected by Prozac. Delayed-rectifier potassium currents are reduced by Prozac in a complicated way that involves both gating and single-channel current amplitude. Fast tetrodotoxin-blockable sodium currents are also decreased by Prozac in this preparation. For all of these ion conductance effects, Prozac concentrations of 10(-5) to 10(-4) M are required. Whereas these levels are 10- to 100-fold higher than the plasma levels achieved in therapeutic use in humans, they are comparable to or less than levels needed for many other blockers of the ionic conductances studied here.

scholarly journals AMRP Peptides Modulate a Novel K+ Current in Pleural Sensory Neurons of Aplysia

2002 ◽  
Vol 88 (1) ◽  
pp. 323-332 ◽  
Author(s):  
Jonathan R. McDearmid ◽  
Vladimir Brezina ◽  
Klaudiusz R. Weiss
Keyword(s):  
Sensory Neurons ◽  
Single Channel ◽  
Outward Current ◽  
Negative Slope ◽  
Open Probability ◽  
Channel Current ◽  
Single Channel Current ◽  
Outward Rectification ◽  
Defensive Behaviors ◽  
Mechanosensory Neurons

Modulation of Aplysia mechanosensory neurons is thought to underlie plasticity of defensive behaviors that are mediated by these neurons. In the past, identification of modulators that act on the sensory neurons and characterization of their actions has been instrumental in providing insight into the functional role of the sensory neurons in the defensive behaviors. Motivated by this precedent and a recent report of the presence of Aplysia Mytilusinhibitory peptide-related (AMRP) neuropeptides in the neuropile and neurons of the pleural ganglia, we sought to determine whether and how pleural sensory neurons respond to the AMRPs. In cultured pleural sensory neurons under voltage clamp, AMRPs elicited a relatively rapidly developing, then partially desensitizing, outward current. The current exhibited outward rectification; in normal 10 mM K+, it was outward at membrane potentials more positive than −80 mV but disappeared without reversing at more negative potentials. When external K+ was elevated to 100 mM, the AMRP-elicited current reversed around −25 mV; the shift in reversal potential was as expected for a current carried primarily by K+. In the high-K+ solution, the reversed current began to decrease at potentials more negative than −60 mV, creating a region of negative slope resistance in the I-V relationship. The AMRP-elicited K+ current was blocked by extremely low concentrations of 4-aminopyridine (4-AP; IC50= 1.7 × 10−7 M) but was not very sensitive to TEA. In cell-attached patches, AMRPs applied outside the patch—thus presumably through a diffusible messenger—increased the activity of a K+ channel that very likely underlies the macroscopic current. The single-channel current exhibited outward rectification, and the open probability of the channel decreased with hyperpolarization; together, these two factors accounted for the outward rectification of the macroscopic current. Submicromolar 4-AP included in the patch pipette blocked the channel by reducing its open probability without altering the single-channel current. Based on the characteristics of the AMRP-modulated K+ current, we conclude that it is a novel current that has not been previously described in Aplysia mechanosensory neurons. In addition to this current, two other AMRP-elicited currents, a slow, 4-AP-resistant outward current and a Na+-dependent inward current, were occasionally observed in the cultured sensory neurons. Responses consistent with all three currents were observed in sensory neurons in situ in intact pleural ganglia.

scholarly journals Saturation and Microsecond Gating of Current Indicate Depletion-induced Instability of the MaxiK Selectivity Filter

2007 ◽  
Vol 130 (1) ◽  
pp. 83-97 ◽  
Author(s):  
Indra Schroeder ◽  
Ulf-Peter Hansen
Keyword(s):  
Single Channel ◽  
Cutoff Frequency ◽  
Selectivity Filter ◽  
Hek293 Cells ◽  
Negative Slope ◽  
High Temporal Resolution ◽  
Voltage Range ◽  
Channel Current ◽  
Single Channel Current ◽  
Cytosolic Side

Patch clamp experiments on single MaxiK channels expressed in HEK293 cells were performed with a high temporal resolution (50-kHz filter) in symmetrical solutions with 50, 150, or 400 mM KCl and 2.5 mM CaCl2 and 2.5 mM MgCl2. At membrane potentials >+100 mV, the single-channel current showed a negative slope resistance, concomitantly with a flickery block, which was not influenced by Ca2+ or Mg2+. The analysis of the amplitude histograms by beta distributions revealed that current in this voltage range was reduced by two effects: rate limitation at the cytosolic side of the pore and gating with rate constants 10–20-fold higher than the cutoff frequency of the filter (i.e., dwell times in the microsecond range). The data were analyzed in terms of a model that postulates a coupling between both effects; if the voltage over the selectivity filter withdraws ions from the cavity at a higher rate than that of refilling from the cytosol, the selectivity filter becomes instable because of ion depletion, and current is interrupted by the resulting flickering. The fit of the IV curves revealed a characteristic voltage of 35 mV. In contrast, the voltage dependence of the gating factor R, i.e., the ratio between true and apparent single-channel current, could be fitted by exponentials with a characteristic voltage of 60 mV, suggesting that only part of the transmembrane potential is felt by the flux through the selectivity filter.

scholarly journals Tl+-induced μs Gating of Current Indicates Instability of the MaxiK Selectivity Filter as Caused by Ion/Pore Interaction

2008 ◽  
Vol 131 (4) ◽  
pp. 365-378 ◽  
Author(s):  
Indra Schroeder ◽  
Ulf-Peter Hansen
Keyword(s):  
Single Channel ◽  
Outward Current ◽  
Selectivity Filter ◽  
Hek293 Cells ◽  
Negative Slope ◽  
High Temporal Resolution ◽  
Exponential Functions ◽  
Channel Current ◽  
Single Channel Current ◽  
Cytosolic Side

Patch clamp experiments on single MaxiK channels expressed in HEK293 cells were performed at high temporal resolution (50-kHz filter) in asymmetrical solutions containing 0, 25, 50, or 150 mM Tl+ on the luminal or cytosolic side with [K+] + [Tl+] = 150 mM and 150 mM K+ on the other side. Outward current in the presence of cytosolic Tl+ did not show fast gating behavior that was significantly different from that in the absence of Tl+. With luminal Tl+ and at membrane potentials more negative than −40 mV, the single-channel current showed a negative slope resistance concomitantly with a flickery block, resulting in an artificially reduced apparent single-channel current Iapp. The analysis of the amplitude histograms by β distributions enabled the estimation of the true single-channel current and the determination of the rate constants of a simple two-state O-C Markov model for the gating in the bursts. The voltage dependence of the gating ratio R = Itrue/Iapp = (kCO + kOC)/kCO could be described by exponential functions with different characteristic voltages above or below 50 mM Tl+. The true single-channel current Itrue decreased with Tl+ concentrations up to 50 mM and stayed constant thereafter. Different models were considered. The most likely ones related the exponential increase of the gating ratio to ion depletion at the luminal side of the selectivity filter, whereas the influence of [Tl+] on the characteristic voltage of these exponential functions and of the value of Itrue were determined by [Tl+] at the inner side of the selectivity filter or in the cavity.

scholarly journals Ca2+ block and flickering both contribute to the negative slope of the IV curve in BK channels

2013 ◽  
Vol 141 (4) ◽  
pp. 499-505 ◽  
Author(s):  
Indra Schroeder ◽  
Gerhard Thiel ◽  
Ulf-Peter Hansen
Keyword(s):  
Open Channel ◽  
Single Channel ◽  
Bk Channels ◽  
Negative Slope ◽  
Channel Noise ◽  
Channel Current ◽  
Single Channel Current ◽  
Current Voltage ◽  
Per Se ◽  
Iv Curves

Single-channel current–voltage (IV) curves of human large-conductance, voltage- and Ca2+-activated K+ (BK) channels are quite linear in 150 mM KCl. In the presence of Ca2+ and/or Mg2+, they show a negative slope conductance at high positive potentials. This is generally explained by a Ca2+/Mg2+ block as by Geng et al. (2013. J. Gen. Physiol. http://dx.doi.org/10.1085/jgp.201210955) in this issue. Here, we basically support this finding but add a refinement: the analysis of the open-channel noise by means of β distributions reveals what would be found if measurements were done with an amplifier of sufficient temporal resolution (10 MHz), namely that the block by 2.5 mM Ca2+ and 2.5 mM Mg2+ per se would only cause a saturating curve up to +160 mV. Further bending down requires the involvement of a second process related to flickering in the microsecond range. This flickering is hardly affected by the presence or absence of Ca2+/Mg2+. In contrast to the experiments reported here, previous experiments in BK channels (Schroeder and Hansen. 2007. J. Gen. Physiol. http://dx.doi.org/10.1085/jgp.200709802) showed saturating IV curves already in the absence of Ca2+/Mg2+. The reason for this discrepancy could not be identified so far. However, the flickering component was very similar in the old and new experiments, regardless of the occurrence of noncanonical IV curves.

scholarly journals Electrophysiological analysis of the negative chronotropic effect of endothelin‐1 in rabbit sinoatrial node cells

2001 ◽  
Vol 537 (2) ◽  
pp. 467-488 ◽  
Author(s):  
Kageyoshi Ono ◽  
Haruko Masumiya ◽  
Aiji Sakamoto ◽  
Georges Christé ◽  
Toshinori Shijuku ◽  
...  
Keyword(s):  
Sinoatrial Node ◽  
Chronotropic Effect ◽  
Endothelin 1 ◽  
Negative Chronotropic Effect ◽  
Electrophysiological Analysis ◽  
Sinoatrial Node Cells ◽  
Rabbit Sinoatrial Node
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