scholarly journals Ion permeation through light-activated channels in rhabdomeric photoreceptors. Role of divalent cations.

1996 ◽  
Vol 107 (6) ◽  
pp. 715-730 ◽  
Author(s):  
M D Gomez ◽  
E Nasi
Keyword(s):  
Time Course ◽  
Light Adaptation ◽  
Single Channel ◽  
Divalent Cations ◽  
Reversal Potential ◽  
Receptor Potential ◽  
Equilibrium Potential ◽  
Positive Equilibrium ◽  
Kinetics Of

The receptor potential of rhabdomeric photoreceptors is mediated primarily by a Na influx, but other ions must also permeate through light-dependent channels to account for some properties of the photoresponse. We examined ion conduction in macroscopic and single-channel light-induced currents of slug and scallop photoreceptors. In the absence of Na, a fivefold change in extracellular K shifted the reversal voltage of the photocurrent (Vrev) by approximately 27 mV. Because the dependency of Vrev on [K]o was sub-Nernstian, and Vrev in each condition was more positive than Ek, some other ion(s) with a positive equilibrium potential must be implicated, in addition to K. We assessed the participation of calcium, an important candidate because of its involvement in light adaptation. Three strategies were adopted to minimize the impairments to cytosolic Ca homeostasis and loss of responsiveness that normally result from the required ionic manipulations: (a) Internal dialysis with Na-free solutions, to prevent reverse operation of the Na/Ca exchanger. (b) Rapid solution changes, temporally limiting exposure to potentially detrimental ionic conditions. (c) Single-channel recording, exposing only the cell-attached patch of membrane to the test solutions. An inward whole-cell photocurrent could be measured with Ca as the only extracellular charge carrier. Decreasing the [Ca]o to 0.5 mM reduced the response by 43% and displaced the reversal potential by -4.3 mV; the shift was larger (delta Vrev = -44 mV) when intracellular permeant cations were also removed. In all cases, however, the current carried by Ca was < 5% of that measured with normal [Na]o. Unitary light-activated currents were reduced in a similar way when the pipette contained only divalent cations, indicating a substantial selectivity for Na over Ca. The fall kinetics of the photoresponse was slower when external Ca was replaced by Ba, or when the membrane was depolarized; however, dialysis with 10 mM BAPTA failed to antagonize this effect, suggesting that mechanisms other than the Ca influx participate in the modulation of the time course of the photocurrent.

scholarly journals Ion conduction and selectivity in acid-sensing ion channel 1

2014 ◽  
Vol 144 (3) ◽  
pp. 245-255 ◽  
Author(s):  
Lei Yang ◽  
Lawrence G. Palmer
Keyword(s):  
Single Channel ◽  
Divalent Cations ◽  
Reversal Potential ◽  
Xenopus Laevis Oocytes ◽  
Acid Sensing Ion Channels ◽  
Outer Vestibule ◽  
Na Currents ◽  
Ion Substitution ◽  
A Site

The ability of acid-sensing ion channels (ASICs) to discriminate among cations was assessed based on changes in conductance and reversal potential with ion substitution. Human ASIC1a was expressed in Xenopus laevis oocytes, and acid-induced currents were measured using two-electrode voltage clamp. Replacement of extracellular Na+ with Li+, K+, Rb+, or Cs+ altered inward conductance and shifted the reversal potentials consistent with a selectivity sequence of Li ∼ Na > K > Rb > Cs. Permeability decreased more rapidly than conductance as a function of atomic size, with PK/PNa = 0.1 and GK/GNa = 0.7 and PRb/PNa = 0.03 and GRb/GNa = 0.3. Stimulation of Cl− currents when Na+ was replaced with Ca2+, Sr2+, or Ba2+ indicated a finite permeability to divalent cations. Inward conductance increased with extracellular Na+ in a hyperbolic manner, consistent with an apparent affinity (Km) for Na+ conduction of 25 mM. Nitrogen-containing cations, including NH4+, NH3OH+, and guanidinium, were also permeant. In addition to passing through the channels, guanidinium blocked Na+ currents, implying competition for a site within the pore. The role of negative charges in an external vestibule of the pore was evaluated using the point mutation D434N. The mutant channel had a decreased single-channel conductance, measured in excised outside-out patches, and a macroscopic slope conductance that increased with hyperpolarization. It had a weakened interaction with Na+ (Km = 72 mM) and a selectivity that was shifted toward larger atomic sizes. We conclude that the selectivity of ASIC1 is based at least in part on interactions with binding sites both within and internal to the outer vestibule.

Permeation and gating properties of a cloned renal K+ channel

1995 ◽  
Vol 268 (2) ◽  
pp. C389-C401 ◽  
Author(s):  
S. Chepilko ◽  
H. Zhou ◽  
H. Sackin ◽  
L. G. Palmer
Keyword(s):  
Single Channel ◽  
Reversal Potential ◽  
Cation Binding ◽  
K Channel ◽  
Patch Clamp Technique ◽  
Open Probability ◽  
Membrane Hyperpolarization ◽  
Channel Current ◽  
Inward Currents ◽  
Kinetics Of

The renal K+ channel (ROMK2) was expressed in Xenopus oocytes, and the patch-clamp technique was used to assess its conducting and gating properties. In cell-attached patches with 110 mM K+ in the bath and pipette, the reversal potential was near zero and the inward conductance (36 pS) was larger than the outward conductance (17 pS). In excised inside-out patches the channels showed rectification in the presence of 5 mM Mg2+ on the cytoplasmic side but not in Mg(2+)-free solution. Inward currents were also observed when K+ was replaced in the pipette by Rb+, NH4+, or thallium (Tl+). The reversal potentials under these conditions yielded a selectivity sequence of Tl+ > K+ > Rb+ > NH4+. On the other hand, the slope conductances for inward current gave a selectivity sequence of K+ = NH4+ > Tl+ > Rb+. The differences in the two sequences can be explained by the presence of cation binding sites within the channel, which interact with Rb+ and Tl+ more strongly and with NH4+ less strongly than with K+. Two other ions, Ba2+ and Cs+, blocked the channel from the outside. The effect of Ba2+ (1 mM) was to reduce the open probability of the channels, whereas Cs+ (10 mM) reduced the apparent single-channel current. The effects of both blockers are enhanced by membrane hyperpolarization. The kinetics of the channel were also studied in cell-attached patches. With K+ in the pipette the distribution of open times could be described by a single exponential (tau 0 = 25 ms), whereas two exponentials (tau 1 = 1 ms, tau 2 = 30 ms) were required to describe the closed-time distribution. Hyperpolarization of the oocyte membrane decreased the open probability and tau 0, and increased tau 1, tau 2, and the number of long closures. The presence of Tl+ in the pipette significantly altered the kinetics, reducing tau 0 and eliminating the long-lived closures. These results suggest that the gating of the channel may depend on the nature of the ion in the pore.

Functional identification of a sarcolemmal chloride channel from bovine tracheal smooth muscle

1996 ◽  
Vol 271 (5) ◽  
pp. C1716-C1724 ◽  
Author(s):  
D. Salvail ◽  
A. Alioua ◽  
E. Rousseau
Keyword(s):  
Smooth Muscle ◽  
Lipid Bilayers ◽  
Reversal Potential ◽  
Tracheal Smooth Muscle ◽  
Disulfonic Acid ◽  
Equilibrium Potential ◽  
Functional Identification ◽  
Cl Channels ◽  
Cl Conductance

The biophysical and pharmacological characteristics of unitary Cl- currents from bovine tracheal smooth muscle cells were studied after reconstitution of microsomal vesicles into planar lipid bilayers. Two types of currents were recorded simultaneously in KCl buffer: the well-defined Ca(2+)-dependent K+ conductance [GK(Ca)] and a much smaller Cl- current, indicating that the Cl- channels under scrutiny originate from the same membrane as the GK(Ca)-type channels, the plasma membrane of airway smooth muscle (ASM) cells. The GK(Ca) activities were eliminated by the use of CsCl buffer. The average unitary Cl- conductance measured in 50 mM trans-250 mM cis CsCl was 77 +/- 6 pS (n = 21), and the reversal potential measured in various CsCl gradients followed the Cl- equilibrium potential as determined from the Nernst equation. In contrast with the previous reports describing the Ca2+ sensitivity of macroscopic ASM Cl- currents, this channel was found to be insensitive to cytoplasmic and extracellular Ca2+ levels. Phosphorylation cocktails, including protein kinases A, G, or C, did not alter the activity of the channel nor did changes in pH. Among a series of Cl- channel inhibitors, 4,4'-diisothiocyanostilbene-2, 2'-disulfonic acid [50% effective concentration (EC50) = 30 microM] and 5-nitro-2-(3-phenylpropylamino) benzoic acid (EC50 = 130 microM) were the most potent blockers of the current examined. The exact role of this surface Cl- conductance remains unclear, and its involvement in cellular activity needs further investigation.

scholarly journals Two Components of the Cardiac Action Potential

1969 ◽  
Vol 54 (5) ◽  
pp. 607-635 ◽  
Author(s):  
Antonio Paes de Carvalho ◽  
Brian Francis Hoffman ◽  
Marilene de Paula Carvalho
Keyword(s):  
Action Potential ◽  
Action Potentials ◽  
Time Course ◽  
Slow Component ◽  
Slow Inward Current ◽  
Equilibrium Potential ◽  
Slow Phase ◽  
Positive Equilibrium ◽  
Cardiac Action

Transmembrane potentials recorded from the rabbit heart in vitro were displayed as voltage against time (V, t display), and dV/dt against voltage (V, V or phase-plane display). Acetylcholine was applied to the recording site by means of a hydraulic system. Results showed that (a) differences in time course of action potential upstroke can be explained in terms of the relative magnitude of fast and slow phases of depolarization; (b) acetylcholine is capable of depressing the slow phase of depolarization as well as the plateau of the action potential; and (c) action potentials from nodal (SA and AV) cells seem to lack the initial fast phase. These results were construed to support a two-component hypothesis for cardiac electrogenesis. The hypothesis states that cardiac action potentials are composed of two distinct and physiologically separable "components" which result from discrete mechanisms. An initial fast component is a sodium spike similar to that of squid nerve. The slow component, which accounts for both a slow depolarization during phase 0 and the plateau, probably is dependent on the properties of a slow inward current having a positive equilibrium potential, coupled to a decrease in the resting potassium conductance. According to the hypothesis, SA and AV nodal action potentials are due entirely or almost entirely to the slow component and can therefore be expected to exhibit unique electrophysiological and pharmacological properties.

scholarly journals MEC-2 and MEC-6 in the Caenorhabditis elegans Sensory Mechanotransduction Complex: Auxiliary Subunits that Enable Channel Activity

2008 ◽  
Vol 131 (6) ◽  
pp. 605-616 ◽  
Author(s):  
Austin L. Brown ◽  
Zhiwen Liao ◽  
Miriam B. Goodman
Keyword(s):  
Caenorhabditis Elegans ◽  
Single Channel ◽  
Divalent Cations ◽  
Surface Expression ◽  
Homologous Proteins ◽  
Auxiliary Subunits ◽  
Single Channel Currents ◽  
Cholesterol Binding

The ion channel formed by the homologous proteins MEC-4 and MEC-10 forms the core of a sensory mechanotransduction channel in Caenorhabditis elegans. Although the products of other mec genes are key players in the biophysics of transduction, the mechanism by which they contribute to the properties of the channel is unknown. Here, we investigate the role of two auxiliary channel subunits, MEC-2 (stomatin-like) and MEC-6 (paraoxonase-like), by coexpressing them with constitutively active MEC-4/MEC-10 heteromeric channels in Xenopus oocytes. This work extends prior work demonstrating that MEC-2 and MEC-6 synergistically increase macroscopic current. We use single-channel recordings and biochemistry to show that these auxiliary subunits alter function by increasing the number of channels in an active state rather than by dramatically affecting either single-channel properties or surface expression. We also use two-electrode voltage clamp and outside-out macropatch recording to examine the effects of divalent cations and proteases, known regulators of channel family members. Finally, we examine the role of cholesterol binding in the mechanism of MEC-2 action by measuring whole-cell and single-channel currents in MEC-2 mutants deficient in cholesterol binding. We suggest that MEC-2 and MEC-6 play essential roles in modulating both the local membrane environment of MEC-4/MEC-10 channels and the availability of such channels to be gated by force in vivo.

scholarly journals The light-sensitive conductance of hyperpolarizing invertebrate photoreceptors: a patch-clamp study.

1994 ◽  
Vol 103 (6) ◽  
pp. 939-956 ◽  
Author(s):  
M P Gomez ◽  
E Nasi
Keyword(s):  
Single Channel ◽  
Light Stimulus ◽  
Divalent Cations ◽  
Reversal Potential ◽  
Outward Current ◽  
Membrane Conductance ◽  
Resting Potential ◽  
Light Stimulation ◽  
Positive Direction ◽  
Single Channel Currents

Tight-seal recording was employed to investigate membrane currents in hyperpolarizing ciliary photoreceptors enzymatically isolated from the eyes of the file clam (Lima scabra) and the bay scallop (Pecten irradians). These two organisms are unusual in that their double retinas also possess a layer of depolarizing rhabdomeric cells. Ciliary photoreceptors from Lima have a rounded soma, 15-20 microns diam, and display a prominent bundle of fine processes up to 30 microns long. The cell body of scallop cells is similar in size, but the ciliary appendages are modified, forming small spherical structures that protrude from the cell. In both species light stimulation at a voltage near the resting potential gives rise to a graded outward current several hundred pA in amplitude, accompanied by an increase in membrane conductance. The reversal potential of the photocurrent is approximately -80 mV, and shifts in the positive direction by approximately 39 mV when the concentration of extracellular K is increased from 10 to 50 mM, consistent with the notion that light activates K-selective channels. The light-activated conductance increases with depolarization in the physiological range of membrane voltages (-30 to -70 mV). Such outward rectification is greatly reduced after removal of divalent cations from the superfusate. In Pecten, cell-attached recordings were also obtained; in some patches outwardly directed single-channel currents could be activated by light but not by voltage. The unitary conductance of these channels was approximately 26 pS. Solitary ciliary cells also gave evidence of the post stimulus rebound, which is presumably responsible for initiating the "off" discharge of action potentials at the termination of a light stimulus: in patches containing only voltage-dependent channels, light stimulation suppressed depolarization-induced activity, and was followed by a strong burst of openings, directly related to the intensity of the preceding photostimulation.

scholarly journals Shaker potassium channel gating. I: Transitions near the open state.

1994 ◽  
Vol 103 (2) ◽  
pp. 249-278 ◽  
Author(s):  
T Hoshi ◽  
W N Zagotta ◽  
R W Aldrich
Keyword(s):  
Time Course ◽  
Single Channel ◽  
Activation Process ◽  
Time Data ◽  
Shaker Potassium Channel ◽  
Voltage Dependent ◽  
Average Current ◽  
Shaker Potassium Channels ◽  
Kinetics Of ◽  
Deactivation Process

Kinetics of single voltage-dependent Shaker potassium channels expressed in Xenopus oocytes were studied in the absence of fast N-type inactivation. Comparison of the single-channel first latency distribution and the time course of the ensemble average current showed that the activation time course and its voltage dependence are largely determined by the transitions before first opening. The open dwell time data are consistent with a single kinetically distinguishable open state. Once the channel opens, it can enter at least two closed states which are not traversed frequently during the activation process. The rate constants for the transitions among these closed states and the open state are nearly voltage-independent at depolarized voltages (> -30 mV). During the deactivation process at more negative voltages, the channel can close directly to a closed state in the activation pathway in a voltage-dependent fashion.

scholarly journals Ionotropic GABA Receptors and Distal Retinal ON and OFF Responses

Scientifica ◽  
2014 ◽  
Vol 2014 ◽  
pp. 1-23 ◽  
Author(s):  
E. Popova
Keyword(s):  
Gaba Receptors ◽  
Time Course ◽  
Light Adaptation ◽  
Current Knowledge ◽  
Relative Sensitivity ◽  
Bipolar Cells ◽  
Visual Signals ◽  
Gamma Aminobutyric Acid ◽  
Large Populations

In the vertebrate retina, visual signals are segregated into parallel ON and OFF pathways, which provide information for light increments and decrements. The segregation is first evident at the level of the ON and OFF bipolar cells in distal retina. The activity of large populations of ON and OFF bipolar cells is reflected in the b- and d-waves of the diffuse electroretinogram (ERG). The role of gamma-aminobutyric acid (GABA), acting through ionotropic GABA receptors in shaping the ON and OFF responses in distal retina, is a matter of debate. This review summarized current knowledge about the types of the GABAergic neurons and ionotropic GABA receptors in the retina as well as the effects of GABA and specific GABAAand GABACreceptor antagonists on the activity of the ON and OFF bipolar cells in both nonmammalian and mammalian retina. Special emphasis is put on the effects on b- and d-waves of the ERG as a useful tool for assessment of the overall function of distal retinal ON and OFF channels. The role of GABAergic system in establishing the ON-OFF asymmetry concerning the time course and absolute and relative sensitivity of the ERG responses under different conditions of light adaptation in amphibian retina is also discussed.

scholarly journals The ionic basis of the receptor potential of frog taste cells induced by water stimuli.

1993 ◽  
Vol 174 (1) ◽  
pp. 1-17
Author(s):  
Y Okada ◽  
T Miyamoto ◽  
T Sato
Keyword(s):  
Basolateral Membrane ◽  
Reversal Potential ◽  
Receptor Potential ◽  
Input Resistance ◽  
Deionized Water ◽  
Glossopharyngeal Nerve ◽  
Equilibrium Potential ◽  
Induced Response ◽  
Control Value ◽  
Taste Cells

The ionic mechanism underlying the receptor potential induced by a deionized water stimulus was studied in frog taste cells with conventional microelectrodes. The taste cells located in the proximal portion of the tongue generated a depolarizing receptor potential which averaged 10mV in response to stimulation with deionized water. The cell membrane of the water-sensitive taste cell could be divided into the taste-receptive (apical) and basolateral membranes and the cells were classified into two types: Cl(-)-dependent and Cl(-)-independent. In Cl(-)-dependent cells whose input resistance was decreased or unchanged by deionized water, the magnitude of the water-induced depolarization decreased with an increase in concentration of superficial Cl- in contact with the receptive membrane and with addition of blockers of anion channels (0.1 mmol l-1 SITS and 0.1 mmol l-1 DIDS) to deionized water. The reversal potential for the depolarization in this type shifted according to the concentration of superficial Cl-. These properties of the responses were consistent with those of the glossopharyngeal nerve which innervates the taste disc. In Cl(-)-independent cells whose input resistance was increased by deionized water, the reversal potential was approximately equal to the equilibrium potential for K+ at the basolateral membrane. The water-induced response of the glossopharyngeal nerve was decreased to about 60% of the control value by addition of interstitial 2 mmol l-1 Ba2+. It is concluded that the water-induced receptor potential is produced by Cl- secretion through the taste-receptive membrane in about 70% of water-sensitive frog taste cells, while it is generated by an inhibition of the resting K+ conductance of the basolateral membrane in the remaining 30% of the cells.

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