scholarly journals The calcium-activated potassium channels of turtle hair cells.

1995 ◽  
Vol 105 (1) ◽  
pp. 49-72 ◽  
Author(s):  
J J Art ◽  
Y C Wu ◽  
R Fettiplace
Keyword(s):  
Resonant Frequency ◽  
Time Constant ◽  
Hair Cells ◽  
Time Course ◽  
Voltage Dependence ◽  
Bk Channels ◽  
Hair Bundle ◽  
Sk Channels ◽  
Channel Kinetics ◽  
Ca Current

A major factor determining the electrical resonant frequency of turtle cochlear hair cells is the time course of the Ca-activated K current (Art, J. J., and R. Fettiplace. 1987. Journal of Physiology. 385:207-242). We have examined the notion that this time course is dictated by the K channel kinetics by recording single Ca-activated K channels in inside-out patches from isolated cells. A hair cell's resonant frequency was estimated from its known correlation with the dimensions of the hair bundle. All cells possess BK channels with a similar unit conductance of approximately 320 pS but with different mean open times of 0.25-12 ms. The time constant of relaxation of the average single-channel current at -50 mV in 4 microM Ca varied between cells from 0.4 to 13 ms and was correlated with the hair bundle height. The magnitude and voltage dependence of the time constant agree with the expected behavior of the macroscopic K(Ca) current, whose speed may thus be limited by the channel kinetics. All BK channels had similar sensitivities to Ca which produced half-maximal activation for a concentration of approximately 2 microM at +50 mV and 12 microM at -50 mV. We estimate from the voltage dependence of the whole-cell K(Ca) current that the BK channels may be fully activated at -35 mV by a rise in intracellular Ca to 50 microM. BK channels were occasionally observed to switch between slow and fast gating modes which raises the possibility that the range of kinetics of BK channels observed in different hair cells reflects a common channel protein whose kinetics are regulated by an unidentified intracellular factor. Membrane patches also contained 30 pS SK channels which were approximately 5 times more Ca-sensitive than BK channels at -50 mV. The SK channels may underlie the inhibitory synaptic potential produced in hair cells by efferent stimulation.

Voltage Dependence of the Glycine Receptor–Channel Kinetics in the Zebrafish Hindbrain

1999 ◽  
Vol 82 (5) ◽  
pp. 2120-2129 ◽  
Author(s):  
Pascal Legendre
Keyword(s):  
Time Constant ◽  
Rise Time ◽  
Time Course ◽  
Voltage Dependence ◽  
Good Description ◽  
Glycine Receptor ◽  
Relative Amplitude ◽  
Time Constants ◽  
Low Concentration ◽  
Voltage Dependent

Electrophysiological recordings of outside-out patches to fast-flow applications of glycine were made on patches derived from the Mauthner cells of the 50-h-old zebrafish larva. As for glycinergic miniature inhibitory postsynaptic currents (mIPSCs), depolarizing the patch produced a broadening of the transient outside-out current evoked by short applications (1 ms) of a saturating concentration of glycine (3 mM). When the outside-out patch was depolarized from −50 to +20 mV, the peak current varied linearly with voltage. A 1-ms application of 3 mM glycine evoked currents that activated rapidly and deactivated biexponentially with time constants of ≈5 and ≈30 ms (holding potential of −50 mV). These two decay time constants were increased by depolarization. The fast deactivation time constant increased e-fold per 95 mV. The relative amplitude of the two decay components did not significantly vary with voltage. The fast component represented 64.2 ± 2.8% of the total current at −50 mV and 54.1 ± 10% at +20 mV. The 20–80% rise time of these responses did not show any voltage dependence, suggesting that the opening rate constant is insensitive to voltage. The 20–80% rise time was 0.2 ms at −70 mV and 0.22 ms at +20 mV. Responses evoked by 100–200 ms application of a low concentration of glycine (0.1 mM) had a biphasic rising phase reflecting the complex gating behavior of the glycine receptor. The time constant of these two components and their relative amplitude did not change with voltage, suggesting that modal shifts in the glycine-activated channel gating mode are not sensitive to the membrane potential. Using a Markov model to simulate glycine receptor gating behavior, we were able to mimic the voltage-dependent change in the deactivation time course of the responses evoked by 1-ms application of 3 mM glycine. This kinetics model incorporates voltage-dependent closing rate constants. It provides a good description of the time course of the onset of responses evoked by the application of a low concentration of glycine at all membrane potentials tested.

Gating currents in the node of Ranvier: voltage and time dependence

1975 ◽  
Vol 270 (908) ◽  
pp. 483-492 ◽  
Keyword(s):  
Membrane Potential ◽  
Time Constant ◽  
Time Course ◽  
Voltage Dependence ◽  
Sudden Change ◽  
Node Of Ranvier ◽  
Ionic Currents ◽  
Gating Currents ◽  
Myelinated Nerve ◽  
Sodium Conductance

Like the axolemma of the giant nerve fibre of the squid, the nodal membrane of frog myelinated nerve fibres after blocking transmembrane ionic currents exhibits asymmetrical displacement currents during and after hyperpolarizing and depolarizing voltage clamp pulses of equal size. The steady-state distribution of charges as a function of membrane potential is consistent with Boltzmann’s law (midpoint potential —33.7 mV; saturation value 17200 charges/(μm 2 ). The time course of the asymmetry current and the voltage dependence of its time constant are consistent with the notion that due to a sudden change in membrane potential the charges undergo a first order transition between two configurations. Size and voltage dependence of the time constant are similar to those of the activation of the sodium conductance assuming m 2 h kinetics, The results suggest the presence of ten times more sodium channels (5000/μm2) in the node of Ranvier than in the squid giant axon with similar sodium conductance per channel (2-3 pS),

Effect of Temperature on Electrical Resonance in Leopard Frog Saccular Hair Cells

1998 ◽  
Vol 79 (1) ◽  
pp. 312-321 ◽  
Author(s):  
M. S. Smotherman ◽  
P. M. Narins
Keyword(s):  
Steady State ◽  
Resonant Frequency ◽  
Hair Cells ◽  
Outward Current ◽  
Leopard Frog ◽  
Effect Of Temperature ◽  
Channel Kinetics ◽  
Resonant Frequencies ◽  
Voltage Dependent ◽  
Electrical Resonance

Smotherman, M. S. and P. M. Narins. Effect of temperature on electrical resonance in leopard frog saccular hair cells. J. Neurophysiol. 79: 312–321, 1998. Leopard frog saccular hair cells exhibit an electrical resonance in response to a depolarizing stimulus that has been proposed to contribute to the tuning properties of the frog sacculus by acting as an electrical band-pass filter. With the whole cell patch-clamp technique, we have investigated the effect of temperature on electrical resonances in isolated saccular hair cells, and we have described the effects of temperature on the currents and channel kinetics underlying electrical resonance. A hair cell's onset resonant frequency in response to a constant depolarizing current pulse increases linearly with temperature at a rate of 11 Hz/1°C, exhibiting a mean Q 10 of 1.7 between 15 and 35°C. However, offset resonant frequencies continue to double every 10°C, exhibiting a mean Q 10 of 2.1. If steady-state voltage during the stimulus is held constant, all oscillatory frequencies increase with a mean Q 10 of 2.1. The average level of steady-state depolarization during a +150-pA depolarizing current pulse decreases with increasing temperature (−6 mV from 15 to 25°C). This temperature-dependent reduction of the steady-state membrane potential causes a shift in the voltage-dependent channel kinetics to slower rates, thus reducing the apparent Q 10 for onset resonant frequencies. The peak outward tail current and net steady-state outward current, which is the sum of a voltage-dependent inward calcium current ( I Ca) and an outward calcium-dependent potassium current ( I K(Ca)), increase with temperature, exhibiting a mean Q 10 of 1.7 between 15 and 25°C. The activation rate ( T 1/2) of the outward current exhibits a mean Q 10 of 2.3 between 15 and 25°C, while the deactivation rate (τrel) exhibits a mean Q 10 of 2.9 over the same temperature range. These results support previous models of the molecular determination of resonant frequency, which have proposed that a combination of I K(Ca) channel kinetics and the overall magnitude of the outward current are primarily responsible for determining the resonant frequency of an isolated hair cell. The robust temperature sensitivity of the hair cell receptor potential contrasts sharply with the temperature-insensitive tuning properties of in vivo saccular nerve fiber recordings. Possible explanations for this discrepancy are discussed.

Slow inward Ca current in frog heart: theoretical evidence against a voltage-dependent inactivation

1982 ◽  
Vol 60 (9) ◽  
pp. 1185-1192 ◽  
Author(s):  
Rodolphe Fischmeister ◽  
Magda Horackova
Keyword(s):  
Time Course ◽  
Voltage Dependence ◽  
Frog Heart ◽  
Type Model ◽  
Actual Behavior ◽  
Ca Current ◽  
Recovery From Inactivation ◽  
Dependent Inactivation ◽  
Voltage Dependent ◽  
Theoretical Evidence

The validity of a Hodgkin–Huxley type voltage-dependent inactivation of slow inward Ca current (Isi) was tested in frog heart using a computer simulation. The time course of Isi, was calculated during the development of a frog atrial action potential (AP). With a time constant of inactivation (τf) of 55 ms at a membrane potential (Em) of –15 mV, the variation of Isi was biphasic; after a transient increase followed by a decrease to zero, Isi partially "reactivated" (at the beginning of the AP repolarization phase) and then fully deactivated. The "reactivation" phase of Isi developed whether τf was an increasing, decreasing, U-shaped, or bell-shaped function of Em. The addition of an independent and slower process responsible for the recovery from inactivation only partly suppressed the "reactivation" phase. However, until now there was no experimental evidence supporting such a biphasic variation of Isi during AP repolarization. Thus our results indicate that the Hodgkin–Huxley type model of the voltage-dependence of Isi-inactivation process may not correctly represent the actual behavior of frog cardiac muscle.

scholarly journals Tamoxifen inhibits BK channels in chick cochlea without alterations in voltage-dependent activation

2009 ◽  
Vol 297 (1) ◽  
pp. C75-C85 ◽  
Author(s):  
Mingjie Tong ◽  
R. Keith Duncan
Keyword(s):  
Hair Cells ◽  
Molecular Mechanisms ◽  
Single Channel ◽  
Frequency Tuning ◽  
Low Frequency ◽  
Bk Channels ◽  
Bk Channel ◽  
Basilar Papilla ◽  
Channel Kinetics ◽  
Open Probability

Large-conductance, Ca2+-activated, and voltage-gated potassium channels (BK, BKCa, or Maxi-K) play an important role in electrical tuning in nonmammalian vertebrate hair cells. Systematic changes in tuning frequency along the tonotopic axis largely result from variations in BK channel kinetics, but the molecular changes underpinning these functional variations remain unknown. Auxiliary β1 have been implicated in low-frequency tuning at the cochlear apex because these subunits dramatically slow channel kinetics. Tamoxifen (Tx), a (xeno)estrogen compound known to activate BK channels through the β-subunit, was used to test for the functional presence of β1. The hypotheses were that Tx would activate the majority of BK channels in hair cells from the cochlear apex due to the presence of β1 and that the level of activation would exhibit a tonotopic gradient following the expression profile of β1. Outside-out patches of BK channels were excised from tall hair cells along the apical half of the chicken basilar papilla. In low-density patches, single-channel conductance was reduced and the averaged open probability was unaffected by Tx. In high-density patches, the amplitude of ensemble-averaged BK current was inhibited, whereas half-activation potential and activation kinetics were unaffected by Tx. In both cases, no tonotopic Tx-dependent activation of channel activity was observed. Therefore, contrary to the hypotheses, electrophysiological assessment suggests that molecular mechanisms other than auxiliary β-subunits are involved in generating a tonotopic distribution of BK channel kinetics and electric tuning in chick basilar papilla.

scholarly journals Dynamics of Signaling between Ca2+ Sparks and Ca2+- Activated K+ Channels Studied with a Novel Image-Based Method for Direct Intracellular Measurement of Ryanodine Receptor Ca2+ Current

2000 ◽  
Vol 116 (6) ◽  
pp. 845-864 ◽  
Author(s):  
Ronghua ZhuGe ◽  
Kevin E. Fogarty ◽  
Richard A. Tuft ◽  
Lawrence M. Lifshitz ◽  
Kemal Sayar ◽  
...  
Keyword(s):  
Smooth Muscle ◽  
High Speed ◽  
Time Course ◽  
Imaging System ◽  
Release Site ◽  
Bk Channels ◽  
Bk Channel ◽  
Global Changes ◽  
Channel Kinetics ◽  
K Channels

Ca2+ sparks are highly localized cytosolic Ca2+ transients caused by a release of Ca2+ from the sarcoplasmic reticulum via ryanodine receptors (RyRs); they are the elementary events underlying global changes in Ca2+ in skeletal and cardiac muscle. In smooth muscle and some neurons, Ca2+ sparks activate large conductance Ca2+-activated K+ channels (BK channels) in the spark microdomain, causing spontaneous transient outward currents (STOCs) that regulate membrane potential and, hence, voltage-gated channels. Using the fluorescent Ca2+ indicator fluo-3 and a high speed widefield digital imaging system, it was possible to capture the total increase in fluorescence (i.e., the signal mass) during a spark in smooth muscle cells, which is the first time such a direct approach has been used in any system. The signal mass is proportional to the total quantity of Ca2+ released into the cytosol, and its rate of rise is proportional to the Ca2+ current flowing through the RyRs during a spark (ICa(spark)). Thus, Ca2+ currents through RyRs can be monitored inside the cell under physiological conditions. Since the magnitude of ICa(spark) in different sparks varies more than fivefold, Ca2+ sparks appear to be caused by the concerted opening of a number of RyRs. Sparks with the same underlying Ca2+ current cause STOCs, whose amplitudes vary more than threefold, a finding that is best explained by variability in coupling ratio (i.e., the ratio of RyRs to BK channels in the spark microdomain). The time course of STOC decay is approximated by a single exponential that is independent of the magnitude of signal mass and has a time constant close to the value of the mean open time of the BK channels, suggesting that STOC decay reflects BK channel kinetics, rather than the time course of [Ca2+] decline at the membrane. Computer simulations were carried out to determine the spatiotemporal distribution of the Ca2+ concentration resulting from the measured range of ICa(spark). At the onset of a spark, the Ca2+ concentration within 200 nm of the release site reaches a plateau or exceeds the [Ca2+]EC50 for the BK channels rapidly in comparison to the rate of rise of STOCs. These findings suggest a model in which the BK channels lie close to the release site and are exposed to a saturating [Ca2+] with the rise and fall of the STOCs determined by BK channel kinetics. The mechanism of signaling between RyRs and BK channels may provide a model for Ca2+ action on a variety of molecular targets within cellular microdomains.

scholarly journals Calcium Balance and Mechanotransduction in Rat Cochlear Hair Cells

2010 ◽  
Vol 104 (1) ◽  
pp. 18-34 ◽  
Author(s):  
Maryline Beurg ◽  
Jong-Hoon Nam ◽  
Qingguo Chen ◽  
Robert Fettiplace
Keyword(s):  
Time Constant ◽  
Hair Cells ◽  
Pump Current ◽  
Outer Hair Cells ◽  
Hair Bundle ◽  
Calcium Balance ◽  
Cochlear Hair Cells ◽  
Fluorescent Indicators ◽  
Intracellular Ca

Auditory transduction occurs by opening of Ca2+-permeable mechanotransducer (MT) channels in hair cell stereociliary bundles. Ca2+ clearance from bundles was followed in rat outer hair cells (OHCs) using fast imaging of fluorescent indicators. Bundle deflection caused a rapid rise in Ca2+ that decayed after the stimulus, with a time constant of about 50 ms. The time constant was increased by blocking Ca2+ uptake into the subcuticular plate mitochondria or by inhibiting the hair bundle plasma membrane Ca2+ ATPase (PMCA) pump. Such manipulations raised intracellular Ca2+ and desensitized the MT channels. Measurement of the electrogenic PMCA pump current, which saturated at 18 pA with increasing Ca2+ loads, indicated a maximum Ca2+ extrusion rate of 3.7 fmol·s−1. The amplitude of the Ca2+ transient decreased in proportion to the Ca2+ concentration bathing the bundle and in artificial endolymph (160 mM K+, 20 μM Ca2+), Ca2+ carried 0.2% of the MT current. Nevertheless, MT currents in endolymph displayed fast adaptation with a submillisecond time constant. In endolymph, roughly 40% of the MT current was activated at rest when using 1 mM intracellular BAPTA compared with 12% with 1 mM EGTA, which enabled estimation of the in vivo Ca2+ load as 3 pA at rest. The results were reproduced by a model of hair bundle Ca2+ diffusion, showing that the measured PMCA pump density could handle Ca2+ loads incurred from resting and maximal MT currents in endolymph. The model also indicated the endogenous mobile buffer was equivalent to 1 mM BAPTA.

Choline acts as agonist and blocker for Aplysia cholinergic synapses

1984 ◽  
Vol 51 (1) ◽  
pp. 1-15 ◽  
Author(s):  
D. Gardner ◽  
R. L. Ruff ◽  
R. L. White
Keyword(s):  
Time Constant ◽  
Decay Time ◽  
Time Course ◽  
Voltage Dependence ◽  
Partial Agonist ◽  
Outward Current ◽  
Decay Time Constant ◽  
Inhibitory Synapses ◽  
Cholinesterase Inhibition ◽  
Current Component

Several identified neurons of the Aplysia buccal ganglia respond to choline. Iontophoretic applications of either choline or acetylcholine (ACh) to voltage-clamped inhibitory follower neurons produce similar currents. Peak amplitudes of choline responses were 10-100% of ACh responses on the same cell. Choline currents were curare blockable and reversed at -69 +/- 2 mV, within 1 mV of postsynaptic current (IPSC) reversal. Application of 1 mM choline to the bath produces more prolonged effects than an initial conductance change. Choline depressed IPSC amplitude by 42 +/- 5% and prolonged IPSC decay time constant by 25 +/- 7%. The slowing was reversible but the depression was not. Use of choline as a Na substitute may therefore involve unexpected partial agonist action; even where conductance changes are transient or inapparent, choline may alter synaptic responses. Bath choline had variable effects on cholinergic self-inhibitory synapses, blocking in six trials but not in three others. Voltage clamping cells BL and BR7, in which monosynaptic cholinergic PSPs are diphasic, reveals underlying early inward and late outward currents. Choline activates only the late outward current component. Correspondingly, bath choline blocks only the late outward component, as does eserine and ACh. This block is not seen with neostigmine, and so is unlikely to be related to cholinesterase inhibition. The early inward current component, revealed by block of the late component by choline or ACh, decays exponentially. Decay time constant is exponentially dependent on membrane potential over the range -20 to -100 mV, with 63-mV depolarization speeding decay e-fold. Eserine prolongs decay and steepens voltage dependence. The late outward component decays with voltage-independent time constant of 48 +/- 5 ms. Both the time integral of synaptic conductance and the ratio of synaptic charge transfer to peak synaptic current of the early inward component of the cell 7 response are reduced by depolarization. Voltage-dependent duration thus combines with reduced driving force in diminishing the excitatory effect of this component at depolarized levels, allowing the inhibitory component to predominate. In this diphasic synapse, voltage dependence of the time course of one component thus serves an easily identified function.

scholarly journals Calcium entry does not drive fast mechanotransduction adaptation in cochlear hair cells

2019 ◽  
Author(s):  
Giusy A. Caprara ◽  
Andrew A. Mecca ◽  
Yanli Wang ◽  
Anthony J. Ricci ◽  
Anthony W. Peng
Keyword(s):  
Hair Cells ◽  
Time Course ◽  
Dynamic Range ◽  
Calcium Entry ◽  
Hair Bundle ◽  
Adaptation Mechanism ◽  
Calcium Dependent ◽  
Sensory Hair Cells ◽  
Adaptation Response ◽  
Fast Adaptation

AbstractSound detection in auditory sensory hair cells depends on the deflection of the stereocilia hair bundle, which opens mechano-electric transduction (MET) channels. Adaptation is hypothesized to be a critical property of MET that contributes to the wide dynamic range and sharp frequency selectivity of the auditory system. Historically, adaptation was hypothesized to have multiple mechanisms, all of which require calcium entry through MET channels. Our recent work using a stiff probe to displace hair bundles showed that the fastest adaptation mechanism (fast adaptation) does not require calcium entry. Using a fluid-jet stimulus, others obtained data showing only a calcium-dependent fast adaptation response. Here, we identified the source of this discrepancy. Because the hair cell response to a hair bundle stimulus depends critically on the magnitude and time course of the hair bundle deflection, we developed a high-speed imaging technique to quantify this deflection. The fluid jet delivers a force stimulus, and step-like force stimuli lead to a complex time course of hair bundle displacement (mechanical creep), which affects the hair cell’s macroscopic MET current response by masking the time course of the fast adaptation response. Modifying the fluid-jet stimulus to generate a step-like hair bundle displacement produced rapidly adapting currents that did not depend on membrane potential. This indicated that fast adaptation does not depend on calcium entry. We also confirmed the presence of a calcium-dependent slow adaptation process. These results confirm the existence of multiple adaptation processes: a fast adaptation that is not driven by calcium entry and a slower calcium-dependent process.Significance StatementMechanotransduction by sensory hair cells represents a key first step for the sound sensing ability in vertebrates. The sharp frequency tuning and wide dynamic range of sound sensation are hypothesized to require a mechanotransduction adaptation mechanism. For decades, it had been accepted that all adaptation mechanisms require calcium entry into hair cells. However, more recent work indicated that the apparent calcium dependence of the fastest adaptation differs with the method of cochlear hair cell stimulation. Here, we reconcile existing data and show that calcium entry does not drive the fastest adaptation process, independent of the stimulation method.

scholarly journals Decades-old model of slow adaptation in sensory hair cells is not supported in mammals

2020 ◽  
Vol 6 (33) ◽  
pp. eabb4922
Author(s):  
Giusy A. Caprara ◽  
Andrew A. Mecca ◽  
Anthony W. Peng
Keyword(s):  
Hair Cells ◽  
Time Course ◽  
Dynamic Range ◽  
Hair Bundle ◽  
Current Decay ◽  
Vestibular Hair Cells ◽  
Slow Adaptation ◽  
Sensory Hair Cells ◽  
Myosin Motors ◽  
Motor Model

Hair cells detect sound and motion through a mechano-electric transduction (MET) process mediated by tip links connecting shorter stereocilia to adjacent taller stereocilia. Adaptation is a key feature of MET that regulates a cell’s dynamic range and frequency selectivity. A decades-old hypothesis proposes that slow adaptation requires myosin motors to modulate the tip-link position on taller stereocilia. This “motor model” depended on data suggesting that the receptor current decay had a time course similar to that of hair-bundle creep (a continued movement in the direction of a step-like force stimulus). Using cochlear and vestibular hair cells of mice, rats, and gerbils, we assessed how modulating adaptation affected hair-bundle creep. Our results are consistent with slow adaptation requiring myosin motors. However, the hair-bundle creep and slow adaptation were uncorrelated, challenging a critical piece of evidence upholding the motor model. Considering these data, we propose a revised model of hair cell adaptation.

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