Sustained outward current observed after I(to1) inactivation in rabbit atrial myocytes is a novel Cl- current

1992 ◽  
Vol 263 (6) ◽  
pp. H1967-H1971 ◽  
Author(s):  
D. Y. Duan ◽  
B. Fermini ◽  
S. Nattel
Keyword(s):  
Reversal Potential ◽  
Outward Current ◽  
Cell Voltage ◽  
Cell Swelling ◽  
Disulfonic Acid ◽  
K Channel ◽  
Transient Outward Current ◽  
Channel Blockers ◽  
Atrial Myocytes ◽  
K Concentration

In rabbit atrial myocytes, depolarization of the membrane results in a rapidly activating transient outward current (I(to)) that then decays to a sustained level. The sustained current (Isus) remains constant for at least 5 s during continued depolarization. The present study was designed to identify the ionic mechanism underlying Isus with the use of whole cell voltage-clamp techniques. After exposure to 2 mM 4-aminopyridine (4-AP), the 4-AP-sensitive transient outward current (I(to1)) was abolished, but Isus was unaffected. Isus was not blocked by the K+ channel blockers tetraethylammonium chloride and Ba2+, was not changed by increasing superfusate K+ concentration, and was still present when K+ was replaced by Cs+ in both the superfusate and the pipette. Isus was significantly reduced by the Cl- transport blockers 4-acetamido-4'-isothiocyanatostilbene-2.2'-disulfonic acid and 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid. The current-voltage relations of Isus showed outward rectification, and the reversal potential of Isus shifted with changes in the transmembrane Cl- gradient in the fashion expected for a Cl- current. We conclude that Isus in rabbit atrium is due to a noninactivating Cl- current which, unlike previously described cardiac Cl- currents, is manifest in the absence of exogenous stimulators of adenosine 3',5'-cyclic monophosphate formation, cytosolic Ca2+ transients, or cell swelling.

Role of chloride currents in repolarizing rabbit atrial myocytes

1995 ◽  
Vol 268 (5) ◽  
pp. H1992-H2002 ◽  
Author(s):  
Z. Wang ◽  
B. Fermini ◽  
J. Feng ◽  
S. Nattel
Keyword(s):  
Action Potential ◽  
Reversal Potential ◽  
Outward Current ◽  
Cell Voltage ◽  
Disulfonic Acid ◽  
Transient Outward Current ◽  
Atrial Myocytes ◽  
Atrial Cells ◽  
K Current

Rabbit atrial cells manifest a prominent transient outward K+ current (Ito1), but this current recovers slowly from inactivation and is unlikely to be important at physiological rates (3-5 Hz). Depolarization of rabbit atrial cells also elicits a transient Ca(2+)-dependent outward Cl- current (Ito2). To compare the relative magnitude of these transient outward currents at various rates, we applied whole cell voltage-clamp techniques to isolated rabbit atrial myocytes. Whereas peak Ito1 exceeded Ito2 at slow rates (0.1 Hz), Ito1 was strongly reduced as rate was increased (by 97 +/- 2%, mean +/- SE, at 4 Hz), while Ito2 was slightly reduced (by 28 +/- 4%, 4 Hz). The reversal potential of transient outward tail currents at 0.07 Hz was -49 +/- 9 mV, while at 2.5 Hz the reversal potential became -18 +/- 7 mV (calculated Cl- reversal potential -18 mV). The addition of the Cl- transport blocker 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS; 150 microM) or the replacement of external Cl- with methanesulfonate inhibited a large part of the transient outward current elicited by depolarization at 4 Hz. DIDS and Cl- replacement increased action potential duration in both single rabbit atrial cells and multicellular rabbit atrial preparations. We conclude that the Ca(2+)-dependent Cl- current is substantially larger than the transient K+ current at physiological rates in the rabbit and is likely to play a more important role in action potential repolarization than the latter current in this tissue in vivo.

Intracellular calcium activates a chloride current in canine ventricular myocytes

1994 ◽  
Vol 267 (5) ◽  
pp. H1984-H1995 ◽  
Author(s):  
A. C. Zygmunt
Keyword(s):  
Potassium Current ◽  
Ventricular Myocytes ◽  
Single Cells ◽  
Reversal Potential ◽  
Outward Current ◽  
Chloride Current ◽  
Disulfonic Acid ◽  
Transient Outward Current ◽  
Triggered Arrhythmias ◽  
Inward Currents

The contribution of chloride and potassium to the 4-aminopyridine (4-AP)-resistant transient outward current was investigated in dog cardiac myocytes. Whole cell currents were recorded at 37 degrees C in single cells dissociated from epicardial and midmyocardial regions of the canine ventricle. Sodium-calcium exchange current and voltage-dependent transient outward potassium current (IA) were blocked in sodium-free solutions containing 2 mM 4-AP; sodium channels were inactivated by the -50-mV holding potential. When patch pipettes contained 0.4–0.8 mM ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid, voltage-clamp steps over the range -20 to +50 mV activated an inward calcium current (ICa) and a Ca(2+)-activated chloride current [ICl(Ca)]. ICl(Ca) was blocked by 200 microM 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid, 1 mM 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (SITS), or reduction of external chloride. Independent of the presence of potassium, the reversal potential of the SITS-sensitive current varied with extracellular chloride, as predicted for a chloride-selective conductance. The bell-shaped current-voltage relation of ICl(Ca) has a threshold of -20 mV and a peak at +40 mV. No evidence could be found for a Ca(2+)-activated potassium current or a Ca(2+)-activated nonspecific cation current under these conditions. ICl(Ca) contributed to oscillatory inward currents at diastolic potentials in cells superfused by isoproterenol and high Ca2+, suggesting a role for this current in triggered arrhythmias associated with delayed afterdepolarizations. In the normal heart, ICl(Ca) is likely to contribute to rate- and rhythm-dependent repolarization of the cardiac action potential.

Acetylcholine potentiates acetylcholine-induced increases in K+ current in cat atrial myocytes

1995 ◽  
Vol 268 (3) ◽  
pp. H1313-H1321 ◽  
Author(s):  
Y. G. Wang ◽  
S. L. Lipsius
Keyword(s):  
Reversal Potential ◽  
Membrane Conductance ◽  
K Channel ◽  
Channel Blockers ◽  
Atrial Myocytes ◽  
Initial Exposure ◽  
Recovery Interval ◽  
Cell Recording ◽  
K Current ◽  
K Currents

A nystatin-perforated patch whole cell recording method was used to study the effects of acetylcholine (ACh) on ACh-induced K+ currents in atrial myocytes isolated from cat hearts. The general protocol involved an initial 4-min exposure to ACh (ACh1), followed by a 4-min washout in ACh-free Tyrode solution and then a second 4-min ACh exposure (ACh2). Voltage ramps (40 mV/s) between -130 and +30 mV were used to assess changes in total membrane conductance. ACh2 (10 microM) induced an increase in K+ conductance that was significantly larger than that induced by ACh1 (10 microM) at voltages both negative and positive to the reversal potential. The potentiated current induced by ACh2 reversed at about -80 mV and inwardly rectified at voltages positive to the reversal potential. External Ba2+ (5 mM) or tetraethylammonium (10 mM) abolished all ACh2-induced increases in membrane conductance. The sensitivity to K+ channel blockers, reversal potential, and the rectifying properties indicate that the current potentiated by ACh2 is a K+ current. Atropine (1 microM) blocked all effects of ACh on K+ currents. Potentiation of K+ current by ACh2 required 1) ACh1 concentrations > or = 1 microM, 2) ACh1 duration > or = 2 min, and 3) recovery interval > or = 2 min. We conclude that an initial exposure to ACh potentiates subsequent ACh-induced increases in K+ current. ACh-induced potentiation depends on the concentration and duration of the initial ACh exposure and the recovery interval between consecutive ACh exposures.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrogenic Hyperpolarization-Elicited Chloride Transporter Current in Blue Cones of Zebrafish Retinal Slices

1997 ◽  
Vol 77 (3) ◽  
pp. 1447-1459 ◽  
Author(s):  
Shih-Fang Fan ◽  
Stephen Yazulla
Keyword(s):  
Time Course ◽  
Reversal Potential ◽  
Outward Current ◽  
Inhibitory Effect ◽  
Disulfonic Acid ◽  
Channel Blockers ◽  
Voltage Range ◽  
Bath Application ◽  
Cell Recording ◽  
Retinal Slices

Fan, Shih-Fang and Stephen Yazulla. Electrogenic hyperpolarization-elicited chloride transporter current in blue cones of zebrafish retinal slices. J. Neurophysiol. 77: 1447–1459, 1997. Voltage-activated currents in blue cones of the retinal slice of zebrafish were characterized using whole cell recording techniques. Depolarizing-elicited currents were recorded: an outward tetraethylammonium (TEA)-sensitive K+ current ( I Kx), an outward Ca2+-activated Cl− current ( I Cl(Ca)), from which we inferred an inward Ca2+ current ( I Ca) as well as a hyperpolarizing-elicited nonselective inward cation current ( I h). In addition, hyperpolarizing steps elicited an outward current ( I out-h) in about one-third of the blue cones. I out-h seems to be carried by inward transported Cl− because it was abolished by equimolar substitution of bath Cl− with acetate; equimolar substitution of Na+ with choline or TEA had no effect; it was not affected by Cl− channel blockers, anthracene-9-carboxylic acid, 4,4′-diisothiocyanostilbene-2,2′-disulfonic acid, N-phenylanthranilic acid (DPC), niflumic acid, and 4-acetamido-4′-isothiocyanostilbene-2,2′-disulfonic acid but was suppressed by Cl− transporter blockers acetalzolamide, bumetanide, N-ethylmaleimide, furosemide, and vanadate, and no reversal potential was found. In addition, this current was suppressed by ouabains but unrelated to their Na+-K+-ATPase inhibitory effect, was not suppressed by Co2+ or nifedipine, was not affected by the gap junction decoupler, 2-octanol, was increased by bath application of Cs+, presumably due to suppression of I h, which was masked by I out-h, and was suppressed by intense light. Similar current also was found in the short cones and double cones. As I out-h operates over the same voltage range, and with similar magnitude and time course as I h, we suggest that I out-h contributes to the modulation of the photoresponse of cones.

Hodgkin-Huxley and partially coupled inactivation models yield different voltage dependence of block

1997 ◽  
Vol 272 (4) ◽  
pp. H2013-H2022 ◽  
Author(s):  
S. Liu ◽  
R. L. Rasmusson
Keyword(s):  
Voltage Dependence ◽  
Channel Model ◽  
Outward Current ◽  
Blocking Agent ◽  
K Channel ◽  
Transient Outward Current ◽  
Channel Blockers ◽  
Channel Block ◽  
Current Channel ◽  
Voltage Dependent

K+ channel blockers have been shown to exhibit complex time- and voltage-dependent effects on cardiac K+ currents. Whereas much attention has been focused on the state dependence of K+ channel block, how a particular channel model can alter the predicted time and voltage dependence of channel block remains unexplored. In this study, using two different model formalisms for the same cardiac transient outward current channel, we compare the effects of a theoretical open-state specific channel blocker on macroscopic currents. Model 1 is a Hodgkin-Huxley-like model, in which inactivation is an intrinsically voltage-dependent process and occurs independently of activation. Model 2 is a "partially coupled" model, in which inactivation is intrinsically voltage insensitive but requires channel activation before it can proceed. In the absence of drug (blocking agent), the two models reproduce the macroscopic current data. In the presence of blocking agent, the two models can differ substantially, with model 1 displaying much less block than model 2. We also examine simple mathematically convenient modifications to the Hodgkin-Huxley formalism, which reproduce some, but not all, of the use-dependent properties of block. Thus model formalism is important for analysis and simulation of state-specific drug-channel interactions.

Isoproterenol activates a chloride current, not the transient outward current, in rabbit ventricular myocytes

1989 ◽  
Vol 257 (6) ◽  
pp. C1177-C1181 ◽  
Author(s):  
R. D. Harvey ◽  
J. R. Hume
Keyword(s):  
Ventricular Myocytes ◽  
Reversal Potential ◽  
Outward Current ◽  
Chloride Current ◽  
Equilibrium Potential ◽  
K Channel ◽  
Transient Outward Current ◽  
Induced Current ◽  
Adrenergic Stimulation ◽  
Rabbit Ventricular Myocytes

The effects of beta-adrenergic stimulation on the Ca2(+)-insensitive transient outward current (Ito) in rabbit ventricular myocytes were examined. Exposure to isoproterenol (ISO; 1 microM) activated a time-dependent current at positive membrane potentials. To determine whether this ISO-induced current was associated with Ito, sensitivity to the K+ channel antagonist, 3,4-diaminopyridine (DAP; 200 microM) was compared before and after exposure to ISO. The DAP-sensitive current was not enhanced by ISO, suggesting that the ISO-induced current was not a component of Ito. Ito and the ISO-induced current could also be dissociated by changing the membrane holding potential. Positive holding potentials, which produced significant inactivation of Ito, had little effect on the ISO-induced membrane current. Furthermore, the ISO-induced current could be observed when K+ was replaced by Cs+. The reversal potential of the ISO-induced current agreed with the predicted Cl- equilibrium potential, and exposure to Cl(-)-free extracellular solutions eliminated the response to ISO. Therefore, we conclude that ISO does not directly activate Ito in rabbit ventricular myocytes, but instead, activates a time-independent chloride current (ICl) similar to that recently described in guinea pig ventricular myocytes and shown to be regulated by adenylate cyclase (R. D. Harvey and J. R. Hume. Science Wash. DC 244: 983-985, 1989).

Voltage Clamp Studies on Insect Skeletal Muscle: II. The Outward Currents

1981 ◽  
Vol 92 (1) ◽  
pp. 13-22
Author(s):  
DAISUKE YAMAMOTO ◽  
HIROSHI WASHIO
Keyword(s):  
Voltage Clamp ◽  
Time Course ◽  
Reversal Potential ◽  
Outward Current ◽  
Transient Outward Current ◽  
Muscle Fibres ◽  
Activation Process ◽  
Outward Currents ◽  
K Concentration ◽  
Insect Skeletal Muscle

Two components of outward currents were investigated under voltage clamp conditions in Tenebrio muscle fibres. The instantaneous current-voltage relation for the transient outward current showed outward rectification. The tail currents for the delayed outward currents were made up of either one or two exponential components. The activation process for the delayed current was analysed using positive tails that decayed with a simple exponential time course. The delayed current was half-activated at about + 35 mV. Two rate constants for activation are both monotonic functions of membrane potential. The reversal potential for the delayed current was only partially dependent on the external K-concentration. The role of the two outward currents in the production of the action potential was discussed.

Whole cell current analyses of pancreatic acinar AR42J cells. I. Voltage- and Ca(2+)-activated currents

1991 ◽  
Vol 260 (5) ◽  
pp. C934-C948 ◽  
Author(s):  
K. Kusano ◽  
H. Gainer
Keyword(s):  
Reversal Potential ◽  
Outward Current ◽  
Pancreatic Acinar Cells ◽  
Equilibrium Potential ◽  
Channel Blockers ◽  
Whole Cell ◽  
Inward Current ◽  
Tail Current ◽  
Ar42j Cells ◽  
Conductance Increase

Voltage- and Ca(2+)-activated whole cell currents were studied in AR42J cells, a clonal cell line derived from rat pancreatic acinar cells, using a patch electrode voltage-clamp technique. Four kinds of ionic currents were identified by their ionic dependencies, pharmacological properties, and kinetic parameters: 1) an outward current flow due mainly to a voltage-dependent K(+)-conductance increase, 2) an initial transient inward current due to an Na(+)-conductance increase, 3) transient and long-duration inward current due to a Ca(2+)-conductance increase, and 4) a slowly activating inward current that persists over the duration of the depolarizing pulse and deactivates slowly upon repolarization, producing a slow inward tail current. The slow inward tail current was particularly robust and was interpreted as due to a Ca(2+)-activated Cl(-)-conductance increase, since 1) the generation of this current was blocked by removing the extracellular Ca2+, applying Ca(2+)-channel blockers (Cd2+, nifedipine), or by lowering the intracellular Ca2+ concentration [( Ca2+]i) with EGTA; and 2) the reversal potential (Erev) of the slow inward tail current was close to 0 mV in the control condition (152 mM [Cl-]o/154 mM [Cl-]i), and changes of the [Cl-]o/[Cl )i ratio shifted the Erev toward the predicted Cl- equilibrium potential.

I(to) and action potential notch are smaller in left vs. right canine ventricular epicardium

1996 ◽  
Vol 271 (2) ◽  
pp. H548-H561 ◽  
Author(s):  
J. M. Di Diego ◽  
Z. Q. Sun ◽  
C. Antzelevitch
Keyword(s):  
Action Potential ◽  
Phase 1 ◽  
Outward Current ◽  
Disulfonic Acid ◽  
Transient Outward Current ◽  
Pharmacological Agents ◽  
Whole Cell Patch Clamp ◽  
Chloride Channel Blocker ◽  
The Right ◽  
Basic Cycle Length

Transmural heterogeneities of repolarizing currents underlie prominent differences in the electrophysiology and pharmacology of ventricular epicardial, endocardial, and M cells in a number of species. The degree to which heterogeneities exist between the right and left ventricles is not well appreciated. The present study uses standard microelectrode and whole cell patch-clamp techniques to contrast the electrophysiological characteristics and pharmacological responsiveness of tissues and myocytes isolated from right (RVE) and left canine ventricular epicardium (LVE). RVE and LVE studied under nearly identical conditions displayed major differences in the early repolarizing phases of the action potential. The magnitude of phase 1 in RVE was nearly threefold that in LVE: 28.7 +/- 6.2 vs. 10.6 +/- 4.1 mV (basic cycle length = 2,000 ms). Phase 1 in RVE was also more sensitive to alterations of the stimulation rate and to 4-aminopyridine (4-AP), suggesting a much greater contribution of the transient outward current (I(to) 1) in RVE than in LVE. The combination of 4-AP plus ryanodine, low chloride, or 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (chloride channel blocker) completely eliminated the notch and all rate dependence of the early phases of the action potential, making RVE and LVE indistinguishable. At +70 mV, RVE myocytes displayed peak I(to) 1 densities between 28 and 37 pA/pF. LVE myocytes included cells with similar I(to) 1 densities (thought to represent subsurface cells) but also cells with much smaller current levels (thought to represent surface cells). Average peak I(to) 1 density was significantly smaller in LVE than in RVE at voltages more than or equal to +10 mV. Our data point to prominent differences in the magnitude of the I(to) 1-mediated action potential notch in cells at the surface of RVE compared with the LVE and suggest that important distinctions may exist in the response of these two tissues to pharmacological agents and pathophysiological states, as previously demonstrated for epicardium and endocardium. Our findings also suggest that a calcium-activated outward current contributes to the early repolarization phase in RVE and LVE and that the influence of this current, although small, is more important in the left ventricle.

Ionic channels in corneal endothelium

1996 ◽  
Vol 270 (4) ◽  
pp. C975-C989 ◽  
Author(s):  
J. L. Rae ◽  
M. A. Watsky
Keyword(s):  
Patch Clamp ◽  
Corneal Endothelium ◽  
Single Channel ◽  
Cell Voltage ◽  
Anion Channel ◽  
Ionic Channels ◽  
K Channel ◽  
Na Channel ◽  
Channel Blockers ◽  
K Currents

Single-channel patch-clamp techniques as well as standard and perforated-patch whole cell voltage-clamp techniques have been applied to the study of ionic channels in the corneal endothelium of several species. These studies have revealed two major K+ currents. One is due to an anion- and temperature-stimulated channel that is blocked by Cs+ but not by most other K+ channel blockers, and the other is similar to the family of A-currents found in excitable cells. The A-current is transient after a depolarizing voltage step and is blocked by both 4-aminopyridine and quinidine. These two currents are probably responsible for setting the -50 to -60 mV resting voltage reported for these cells. A Ca(2+)-activated ATP-inhibited nonselective cation channel and a tetrodotoxin-blocked Na+ channel are possible Na+ inflow pathways, but, given their gating properties, it is not certain that either channel works under physiological conditions. A large-conductance anion channel has also been identified by single-channel patch-clamp techniques. Single corneal endothelial cells have input resistances of 5-10 G omega and have steady-state K+ currents that are approximately 10 pA at the resting voltage. Pairs or monolayers of cells are electrically coupled and dye coupled through gap junctions.

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