The cation selectivity and voltage dependence of the light-activated potassium conductance in scallop distal photoreceptor.

Single collecting tubules of Amphiuma kidneys were perfused in vitro to characterize their electrophysiological properties. The lumen-negative potential (-24 mV) was abolished by amiloride in the lumen and by ouabain in the bath. Ion substitution experiments in the lumen demonstrated the presence of a large sodium conductance in the apical cell membrane, but no evidence was obtained for a significant potassium or chloride conductance. Ion substitutions in the bath solution and the depolarizing effect of barium on the basolateral membrane potential demonstrated the presence of a large potassium conductance in the basolateral cell membrane. Measurements of dilution potentials in amiloride-treated tubules revealed a modest cation selectivity of the paracellular pathway. These results support a cell model in which sodium reabsorption occurs by electrodiffusion across the apical cell membrane and active transport across the basolateral cell membrane. The absence of a detectable potassium conductance in the apical cell membrane suggests that secretion of this ion cannot take place by diffusion from cell to lumen.

Download Full-text

β-HELICAL TRANSMEMBRANE CHANNELS: COMMENTS ON ENERGETICS, STRUCTURE, MONOVALENT AND DIVALENT CATION SELECTIVITY AND VOLTAGE DEPENDENCE

Frontiers of Biological Energetics ◽

10.1016/b978-0-12-225402-4.50057-7 ◽

1978 ◽

pp. 1227-1234 ◽

Cited By ~ 5

Author(s):

Dan W. Urry

Keyword(s):

Divalent Cation ◽

Voltage Dependence ◽

Cation Selectivity ◽

Transmembrane Channels

Download Full-text

Phosphorylation modulates potassium conductance and gating current of perfused giant axons of squid.

The Journal of General Physiology ◽

10.1085/jgp.95.2.245 ◽

1990 ◽

Vol 95 (2) ◽

pp. 245-271 ◽

Cited By ~ 26

Author(s):

C K Augustine ◽

F Bezanilla

Keyword(s):

Steady State ◽

Voltage Dependence ◽

Potassium Conductance ◽

Charge Movement ◽

Gating Current ◽

Gating Currents ◽

Internal Solution ◽

Giant Axons ◽

Gating Charge ◽

Phosphorylation Reaction

The presence of internal Mg-ATP produced a number of changes in the K conductance of perfused giant axons of squid. For holding potentials between -40 and -50 mV, steady-state K conductance increased for depolarizations to potentials more positive than approximately -15 mV and decreased for smaller depolarizations. The voltage dependencies of both steady-state activation and inactivation also appears shifted toward more positive potentials. Gating kinetics were affected by internal ATP, with the activation time constant slowed and the characteristic delay in K conductance markedly enhanced. The rate of deactivation also was hastened during perfusion with ATP. Internal ATP affected potassium channel gating currents in similar ways. The voltage dependence of gating charge movement was shifted toward more positive potentials and the time constants of ON and OFF gating current also were slowed and hastened, respectively, in the presence of ATP. These effects of ATP on the K conductance occurred when no exogenous protein kinases were added to the internal solution and persisted even after removing ATP from the internal perfusate. Perfusion with a solution containing exogenous alkaline phosphatase reversed the effects of ATP. These results provide further evidence that the effects of ATP on the K conductance are a consequence of a phosphorylation reaction mediated by a kinase present and active in perfused axons. Phosphorylation appears to alter the K conductance of squid giant axons via a minimum of two mechanisms. First, the voltage dependence of gating parameters are shifted toward positive potentials. Second, there is an increase in the number of functional closed states and/or a decrease in the rates of transition between these states of the K channels.

Download Full-text

Ion channels based on bis-macrocyclic bolaamphiphiles: effects of hydrophobic substitutions

Canadian Journal of Chemistry ◽

10.1139/v98-097 ◽

1998 ◽

Vol 76 (7) ◽

pp. 1015-1026 ◽

Cited By ~ 10

Author(s):

T M Fyles ◽

D Loock ◽

X Zhou

Keyword(s):

Electrostatic Interactions ◽

Voltage Dependence ◽

Bilayer Membrane ◽

Time Signal ◽

Head Group ◽

Signal Shape ◽

Current Time ◽

Cation Selectivity ◽

Rapid Current ◽

New Compounds

Four new bis-macrocyclic bolaamphiphiles were prepared to explore the effects of hydrophobic substitutions on ion transport. In bilayer vesicles the new compounds were remarkably similar to more hydrophilic derivatives prepared previously. Planar bilayer conductance experiments showed the new compounds induced an unique current-time signal consisting of a rapid rise time, followed by a slower decay time. Signal shape was cation dependent and was related to a modest selectivity between cations. Cation-anion selectivity was very high, approaching an ideal cation selectivity. One compound also showed voltage dependence of the signal shape and duration. Qualitative changes in signal shape, duration, and voltage dependence were provoked by variation in the electrolyte pH and by masking the head-group electrostatic interactions with low levels of barium ions. A model for the signal shape is proposed, involving a rapid current rise due to aggregate restructuring, followed by slower decay due to development of the local Donnan potential that results from the high cation-anion selectivity.Key words: ion channel, synthesis, bilayer membrane, bilayer clamp, mechanism.

Download Full-text

Locating the Anion-selectivity Filter of the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) Chloride Channel

The Journal of General Physiology ◽

10.1085/jgp.109.3.289 ◽

1997 ◽

Vol 109 (3) ◽

pp. 289-299 ◽

Cited By ~ 77

Author(s):

Min Cheung ◽

Myles H. Akabas

Keyword(s):

Cystic Fibrosis ◽

Voltage Dependence ◽

Reaction Rates ◽

Selectivity Filter ◽

Transmembrane Conductance Regulator ◽

Transmembrane Conductance ◽

Cation Selectivity ◽

Anion Selectivity ◽

Charge Selectivity ◽

Cystic Fibrosis Transmembrane

The cystic fibrosis transmembrane conductance regulator forms an anion-selective channel; the site and mechanism of charge selectivity is unknown. We previously reported that cysteines substituted, one at a time, for Ile331, Leu333, Arg334, Lys335, Phe337, Ser341, Ile344, Arg347, Thr351, Arg352, and Gln353, in and flanking the sixth membrane-spanning segment (M6), reacted with charged, sulfhydryl-specific, methanethiosulfonate (MTS) reagents. We inferred that these residues are on the water-accessible surface of the protein and may line the ion channel. We have now measured the voltage-dependence of the reaction rates of the MTS reagents with the accessible, engineered cysteines. By comparing the reaction rates of negatively and positively charged MTS reagents with these cysteines, we measured the extent of anion selectivity from the extracellular end of the channel to eight of the accessible residues. We show that the major site determining anion vs. cation selectivity is near the cytoplasmic end of the channel; it favors anions by ∼25-fold and may involve the residues Arg347 and Arg352. From the voltage dependence of the reaction rates, we calculated the electrical distance to the accessible residues. For the residues from Leu333 to Ser341 the electrical distance is not significantly different than zero; it is significantly different than zero for the residues Thr351 to Gln353. The maximum electrical distance measured was 0.6 suggesting that the channel extends more cytoplasmically and may include residues flanking the cytoplasmic end of the M6 segment. Furthermore, the electrical distance calculations indicate that R352C is closer to the extracellular end of the channel than either of the adjacent residues. We speculate that the cytoplasmic end of the M6 segment may loop back into the channel narrowing the lumen and thereby forming both the major resistance to current flow and the anion-selectivity filter.

Download Full-text

Effects of strychnine on the potassium conductance of the frog node of Ranvier.

The Journal of General Physiology ◽

10.1085/jgp.69.6.897 ◽

1977 ◽

Vol 69 (6) ◽

pp. 897-914 ◽

Cited By ~ 21

Author(s):

B I Shapiro

Keyword(s):

Relaxation Time ◽

Voltage Dependence ◽

Potassium Conductance ◽

Node Of Ranvier ◽

Axon Membrane ◽

Quaternary Derivative ◽

Steady Level ◽

Voltage Dependent

The nature of the block of potassium conductance by strychnine in frog node of Ranvier was investigated. The block is voltage-dependent and reaches a steady level with a relaxation time of 1 to several ms. Block is increased by depolarization or a reduction in [K+]O as well as by increasing strychnine concentration. A quaternary derivative of strychnine produces a similar block only when applied intracellularly. In general and in detail, strychnine block resembles that produced by intracellular application of the substituted tetraethylammonium compounds extensively studied by C.M. Armstrong (1969. J. Gen Physiol. 54:553-575. 1971. J. Gen. Physiol. 58:413-437). The kinetics, voltage dependence, and dependence on [K+]O of strychnine block are of the same form. It is concluded that tertiary strychnine must cross the axon membrane and block from the axoplasmic side in the same fashion as these quaternary amines.

Download Full-text

Repriming of delayed potassium conductance in frog skeletal muscle

Journal of Experimental Biology ◽

10.1242/jeb.87.1.217 ◽

1980 ◽

Vol 87 (1) ◽

pp. 217-228

Author(s):

E. W. Ballou

Keyword(s):

Skeletal Muscle ◽

Time Course ◽

Rana Pipiens ◽

Voltage Dependence ◽

Potassium Conductance ◽

Frog Skeletal Muscle ◽

Semitendinosus Muscle ◽

Sequential Model ◽

Temperature Dependences ◽

High Concentrations

A dissection of the semitendinosus muscle from Rana pipiens was developed for three-microelectrode voltage-clamp studies of the delayed potassium-selective conductance system. The delayed conductance inactivates in muscles bathed in high concentrations of potassium or rubidium, but can be reprimed by hyperpolarizing voltage pulses to membrane potentials beyond −80 mV. The repriming time-course was studied by measuring the delayed conductance that coulde be activated following hyperpolarizing pulses of varying duration. Responses following 20–100 s pulses to potentials between −90 and −140 mV could not be reconciled with an exponential approach to the conductance present in normally polarized fibres. The sigmoid appearance of the early (< 25 s) time course was exaggerated by cooling from 20 to 10 degrees C. This effect was described by a sequential model invoking two inactivated states with different temperature dependences. An explanation is suggested for differences in the kinetics and voltage dependence of repriming between briefly and chronically depolarized muscle cells.

Download Full-text

Divalent cation selectivity for external block of voltage-dependent Na+ channels prolonged by batrachotoxin. Zn2+ induces discrete substates in cardiac Na+ channels.

The Journal of General Physiology ◽

10.1085/jgp.97.1.89 ◽

1991 ◽

Vol 97 (1) ◽

pp. 89-115 ◽

Cited By ~ 46

Author(s):

A Ravindran ◽

L Schild ◽

E Moczydlowski

Keyword(s):

Surface Charge ◽

Voltage Dependence ◽

Divalent Cations ◽

Na Channel ◽

Canine Heart ◽

Na Channels ◽

Cation Selectivity ◽

Voltage Dependent ◽

Negative Surface ◽

Negative Surface Charge

The mechanism of block of voltage-dependent Na+ channels by extracellular divalent cations was investigated in a quantitative comparison of two distinct Na+ channel subtypes incorporated into planar bilayers in the presence of batrachotoxin. External Ca2+ and other divalent cations induced a fast voltage-dependent block observed as a reduction in unitary current for tetrodotoxin-sensitive Na+ channels of rat skeletal muscle and tetrodotoxin-insensitive Na+ channels of canine heart ventricular muscle. Using a simple model of voltage-dependent binding to a single site, these two distinct Na+ channel subtypes exhibited virtually the same affinity and voltage dependence for fast block by Ca2+ and a number of other divalent cations. This group of divalent cations exhibited an affinity sequence of Co congruent to Ni greater than Mn greater than Ca greater than Mg greater than Sr greater than Ba, following an inverse correlation between binding affinity and ionic radius. The voltage dependence of fast Ca2+ block was essentially independent of CaCl2 concentration; however, at constant voltage the Ca2+ concentration dependence of fast block deviated from a Langmuir isotherm in the manner expected for an effect of negative surface charge. Titration curves for fast Ca2+ block were fit to a simplified model based on a single Ca2+ binding site and the Gouy-Chapman theory of surface charge. This model gave similar estimates of negative surface charge density in the vicinity of the Ca2+ blocking site for muscle and heart Na+ channels. In contrast to other divalent cations listed above, Cd2+ and Zn2+ are more potent blockers of heart Na+ channels than muscle Na+ channels. Cd2+ induced a fast, voltage-dependent block in both Na+ channel subtypes with a 46-fold higher affinity at 0 mV for heart (KB = 0.37 mM) vs. muscle (KB = 17 mM). Zn2+ induced a fast, voltage-dependent block of muscle Na+ channels with low affinity (KB = 7.5 mM at 0 mV). In contrast, micromolar Zn2+ induced brief closures of heart Na+ channels that were resolved as discrete substate events at the single-channel level with an apparent blocking affinity of KB = 0.067 mM at 0 mV, or 110-fold higher affinity for Zn2+ compared with the muscle channel. High-affinity block of the heart channel by Cd2+ and Zn2+ exhibited approximately the same voltage dependence (e-fold per 60 mV) as low affinity block of the muscle subtype (e-fold per 54 mV), suggesting that the block occurs at structurally analogous sites in the two Na+ channels.(ABSTRACT TRUNCATED AT 400 WORDS)

Download Full-text

Cation Activation of the Basolateral Sodium-Potassium Pump in Turtle Colon

The Journal of General Physiology ◽

10.1085/jgp.82.3.315 ◽

1983 ◽

Vol 82 (3) ◽

pp. 315-329 ◽

Cited By ~ 26

Author(s):

D R Halm ◽

D C Dawson

Keyword(s):

Basolateral Membrane ◽

Potassium Conductance ◽

Pump Current ◽

Sodium Potassium ◽

Cation Selectivity ◽

Cation Activation ◽

Sodium Activation ◽

Kinetics Of ◽

Potassium Exchange ◽

Cytoplasmic Side

The current generated by electrogenic sodium-potassium exchange at the basolateral membrane of the turtle colon can be measured directly in tissues that have been treated with serosal barium (to block the basolateral potassium conductance) and mucosal amphotericin B (to reduce the cation selectivity of the apical membrane). We studied the activation of this pump current by mucosal sodium and serosal potassium, rubidium, cesium, and ammonium. The kinetics of sodium activation were consistent with binding to three independent sites on the cytoplasmic side of the pump. The pump was not activated by cellular lithium ions. The kinetics of serosal cation activation were consistent with binding to two independent sites with the selectivity Rb > K > Cs > NH4. The properties and kinetics of the basolateral Na/K pump in the turtle colon are at least qualitatively similar to those ofthe well-characterized Na/K-ATPase of the human red blood cell .

Download Full-text

Analysis of the Bias Dependent Spectral Response of a-SiC:H p-i-n Photodiode

MRS Proceedings ◽

10.1557/proc-715-a7.3 ◽

2002 ◽

Vol 715 ◽

Author(s):

P. Louro ◽

A. Fantoni ◽

Yu. Vygranenko ◽

M. Fernandes ◽

M. Vieira

Keyword(s):

Bias Voltage ◽

Voltage Dependence ◽

Spectral Response ◽

Surface Recombination ◽

Electrical Field ◽

Field Reversal ◽

Current Voltage ◽

Voltage Dependent ◽

And Inversion ◽

Device Operation

AbstractThe bias voltage dependent spectral response (with and without steady state bias light) and the current voltage dependence has been simulated and compared to experimentally obtained values. Results show that in the heterostructures the bias voltage influences differently the field and the diffusion part of the photocurrent. The interchange between primary and secondary photocurrent (i. e. between generator and load device operation) is explained by the interaction of the field and the diffusion components of the photocurrent. A field reversal that depends on the light bias conditions (wavelength and intensity) explains the photocurrent reversal. The field reversal leads to the collapse of the diode regime (primary photocurrent) launches surface recombination at the p-i and i-n interfaces which is responsible for a double-injection regime (secondary photocurrent). Considerations about conduction band offsets, electrical field profiles and inversion layers will be taken into account to explain the optical and voltage bias dependence of the spectral response.

Download Full-text