Isoproterenol, DBcAMP, and forskolin inhibit cardiac sodium current

1989 ◽  
Vol 256 (6) ◽  
pp. C1131-C1137 ◽  
Author(s):  
K. Ono ◽  
T. Kiyosue ◽  
M. Arita
Keyword(s):  
Ventricular Myocytes ◽  
Sodium Current ◽  
Cell Voltage ◽  
Ionic Currents ◽  
Na Channel ◽  
Intracellular Camp ◽  
Channel Blockers ◽  
H Infinity ◽  
Inactivation Curve ◽  
Gating Mechanism

We studied the effects of isoproterenol (ISP), dibutyryl adenosine 3',5'-cyclic monophosphate (DBcAMP), and forskolin on the sodium current (INa) of guinea pig ventricular myocytes using the tight-seal, whole cell voltage-clamp method. The extracellular [Na+] [( Na+]o) was decreased to 60 mM by replacing NaCl with sucrose (temperature, 32-33 degrees C). Ionic currents other than Na+ were suppressed using appropriate channel blockers. Depolarizing clamp pulse (duration, 30 ms) was applied at a rate of 0.2 Hz from a holding potential of -80 mV. ISP (1 microM) decreased the peak INa by 34% from 6.1 +/- 1.9 (SD) nA (control) to 4.0 +/- 1.5 nA (n = 7). The inhibition was more prominent at less negative potentials and disappeared in the presence of a beta-blocker (10 microM atenolol). The effects of DBcAMP (1-5 mM) and forskolin (3 microM) mimicked those of ISP and depressed the peak INa reversibly. DBcAMP (5 mM) shifted the inactivation curve of INa [h infinity-membrane potential (Em) relationship] to a hyperpolarizing direction, by 3.4 +/- 0.8 mV (n = 5). These findings suggest that ISP inhibits the cardiac INa+, probably by altering the gating mechanism of the Na+ channel, and that the effect is secondary to the increased levels of intracellular cAMP, with possible acceleration of cAMP-dependent phosphorylation of the channel.

Sodium current in isolated human ventricular myocytes

1993 ◽  
Vol 265 (4) ◽  
pp. H1301-H1309 ◽  
Author(s):  
Y. Sakakibara ◽  
T. Furukawa ◽  
D. H. Singer ◽  
H. Jia ◽  
C. L. Backer ◽  
...  
Keyword(s):  
Time Course ◽  
Heart Surgery ◽  
Voltage Dependence ◽  
Ventricular Myocytes ◽  
Sodium Current ◽  
Open Heart Surgery ◽  
External Solution ◽  
Cell Voltage ◽  
H Infinity ◽  
Voltage Dependent

Although fast sodium current (INa) plays a major role in the generation and conduction of the cardiac impulse, the electrophysiological characteristics of INa in isolated human ventricular myocytes have not yet been fully described. We characterized the human ventricular INa of enzymatically isolated myocytes using whole cell voltage-clamp techniques. Sixty myocytes were isolated from ventricular specimens obtained from 22 patients undergoing open-heart surgery. A low temperature (17 degrees C) and Na+ concentration in the external solution (5 or 10 mM) allowed good voltage control and facilitated the measurement of INa. Cs+ was substituted for K+ in both internal and external solutions to block K+ currents, and F- was added to the internal solution to block Ca2+ current. INa was activated at a voltage threshold of approximately -70 mV, and maximal inward current was obtained at approximately -30 mV (holding potential = -140 mV). The voltage dependence of steady-state INa availability (h infinity) was sigmoidal with half inactivation occurring at -97.3 +/- 1.1 mV and a slope factor of 5.77 +/- 0.10 mV (n = 60). We did not detect any significant differences in these parameters in cells from patients with a variety of disease states, with or without congestive heart failure. The overlap in voltage dependence of h infinity and Na+ conductance suggested the presence of a Na+ "window" current. An inactivation time course was voltage dependent and was fitted best by the sum of two exponentials. The rate of recovery from inactivation also was voltage dependent and fitted by the sum of two exponentials.(ABSTRACT TRUNCATED AT 250 WORDS)

Basis for tetrodotoxin and lidocaine effects on action potentials in dog ventricular myocytes

1988 ◽  
Vol 254 (6) ◽  
pp. H1157-H1166 ◽  
Author(s):  
J. A. Wasserstrom ◽  
J. J. Salata
Keyword(s):  
Action Potential ◽  
Action Potentials ◽  
Ventricular Myocytes ◽  
Ionic Currents ◽  
Detectable Effect ◽  
Na Channel ◽  
Na Current ◽  
Voltage Range ◽  
Purkinje Fibers ◽  
Ventricular Cells

We studied the effects of tetrodotoxin (TTX) and lidocaine on transmembrane action potentials and ionic currents in dog isolated ventricular myocytes. TTX (0.1-1 x 10(-5) M) and lidocaine (0.5-2 x 10(-5) M) decreased action potential duration, but only TTX decreased the maximum rate of depolarization (Vmax). Both TTX (1-2 x 10(-5) M) and lidocaine (2-5 x 10(-5) M) blocked a slowly inactivating toward current in the plateau voltage range. The voltage- and time-dependent characteristics of this current are virtually identical to those described in Purkinje fibers for the slowly inactivating inward Na+ current. In addition, TTX abolished the outward shift in net current at plateau potentials caused by lidocaine alone. Lidocaine had no detectable effect on the slow inward Ca2+ current and the inward K+ current rectifier, Ia. Our results indicate that 1) there is a slowly inactivating inward Na+ current in ventricular cells similar in time, voltage, and TTX sensitivity to that described in Purkinje fibers; 2) both TTX and lidocaine shorten ventricular action potentials by reducing this slowly inactivating Na+ current; 3) lidocaine has no additional actions on other ionic currents that contribute to its ability to abbreviate ventricular action potentials; and 4) although both agents shorten the action potential by the same mechanism, only TTX reduces Vmax. This last point suggests that TTX produces tonic block of Na+ current, whereas lidocaine may produce state-dependent Na+ channel block, namely, blockade of Na+ current only after Na+ channels have already been opened (inactivated-state block).

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.

Ca2+ mobilization by extracellular ATP in rat cardiac myocytes: regulation by protein kinase C and A

1992 ◽  
Vol 263 (5) ◽  
pp. C933-C940 ◽  
Author(s):  
J. S. Zheng ◽  
A. Christie ◽  
M. N. Levy ◽  
A. Scarpa
Keyword(s):  
Protein Kinase C ◽  
Protein Kinase ◽  
Ventricular Myocytes ◽  
Cell Voltage ◽  
Extracellular Atp ◽  
Camp Level ◽  
Intracellular Camp ◽  
Beta Adrenergic ◽  
Kinase C ◽  
Integrated Response

Activation of protein kinase C (PKC) modulates the mobilization of intracellular Ca2+ induced by extracellular ATP in rat ventricular myocytes. Pretreatment of myocytes with PKC activators attenuated both the ATP-induced Ca2+ transient and the noradrenergic potentiation of the Ca2+ response. Various PKC activators decreased both the basal cAMP level and the cAMP levels that had been elevated by norepinephrine, forskolin, or 3-isobutyl-1-methylxanthine. The inhibitory effects of PKC activators were reversed by the PKC inhibitor staurosporine. The ATP-induced Ca2+ response is an integrated response resulting from ATP eliciting an inward cation current (IATP), cellular depolarization, Ca2+ influx through Ca2+ channels, and Ca2+ release from the sarcoplasmic reticulum. We used the whole cell voltage-clamp technique to investigate which steps of this integrated response are affected by PKC. PKC activators did not significantly affect the IATP. In contrast, PKC activators decreased the basal Ca2+ current (ICa) or Ba2+ current and the beta-adrenergic-stimulated ICa. These results suggest that PKC-induced suppression of the ATP-induced Ca2+ response and the beta-adrenergic-potentiated Ca2+ response is achieved at least partially by decreasing the intracellular cAMP level and ICa.

scholarly journals Kinetic and steady-state properties of Na+ channel and Ca2+ channel charge movements in ventricular myocytes of embryonic chick heart.

1992 ◽  
Vol 100 (2) ◽  
pp. 195-216 ◽  
Author(s):  
I R Josephson ◽  
N Sperelakis
Keyword(s):  
Steady State ◽  
Time Constant ◽  
Decay Time ◽  
Ventricular Myocytes ◽  
Ionic Currents ◽  
Decay Time Constant ◽  
Na Channel ◽  
Charge Movement ◽  
Embryonic Chick ◽  
Voltage Range

Nonlinear or asymmetric charge movement was recorded from single ventricular myocytes cultured from 17-d-old embryonic chick hearts using the whole-cell patch clamp method. The myocytes were exposed to the appropriate intracellular and extracellular solutions designed to block Na+, Ca2+, and K+ ionic currents. The linear components of the capacity and leakage currents during test voltage steps were eliminated by adding summed, hyperpolarizing control step currents. Upon depolarization from negative holding potentials the nonlinear charge movement was composed of two distinct and separable kinetic components. An early rapidly decaying component (decay time constant range: 0.12-0.50 ms) was significant at test potentials positive to -70 mV and displayed saturation above 0 mV (midpoint -35 mV; apparent valence 1.6 e-). The early ON charge was partially immobilized during brief (5 ms) depolarizing test steps and was more completely immobilized by the application of less negative holding potentials. A second slower-decaying component (decay time constant range: 0.88-3.7 ms) was activated at test potentials positive to -60 mV and showed saturation above +20 mV (midpoint -13 mV, apparent valence 1.9 e-). The second component of charge movement was immobilized by long duration (5 s) holding potentials, applied over a more positive voltage range than those that reduced the early component. The voltage dependencies for activation and inactivation of the Na+ and Ca2+ ionic currents were determined for myocytes in which these currents were not blocked. There was a positive correlation between the voltage dependence of activation and inactivation of the Na+ and Ca2+ ionic currents and the activation and immobilization of the fast and slow components of charge movement. These complementary kinetic and steady-state properties lead to the conclusion that the two components of charge movement are associated with the voltage-sensitive conformational changes that precede Na+ and Ca2+ channel openings.

scholarly journals Inactivation of batrachotoxin-modified Na+ channels in GH3 cells. Characterization and pharmacological modification.

1992 ◽  
Vol 99 (1) ◽  
pp. 1-20 ◽  
Author(s):  
G K Wang ◽  
S Y Wang
Keyword(s):  
Time Course ◽  
Cell Voltage ◽  
Gh3 Cells ◽  
Na Channel ◽  
Na Channels ◽  
H Infinity ◽  
Recovery From Inactivation ◽  
Na Currents ◽  
Steady State Inactivation ◽  
Pharmacological Modification

Batrachotoxin (BTX)-modified Na+ currents were characterized in GH3 cells with a reversed Na+ gradient under whole-cell voltage clamp conditions. BTX shifts the threshold of Na+ channel activation by approximately 40 mV in the hyperpolarizing direction and nearly eliminates the declining phase of Na+ currents at all voltages, suggesting that Na+ channel inactivation is removed. Paradoxically, the steady-state inactivation (h infinity) of BTX-modified Na+ channels as determined by a two-pulse protocol shows that inactivation is still present and occurs maximally near -70 mV. About 45% of BTX-modified Na+ channels are inactivated at this voltage. The development of inactivation follows a sum of two exponential functions with tau d(fast) = 10 ms and tau d(slow) = 125 ms at -70 mV. Recovery from inactivation can be achieved after hyperpolarizing the membrane to voltages more negative than -120 mV. The time course of recovery is best described by a sum of two exponentials with tau r(fast) = 6.0 ms and tau r(slow) = 240 ms at -170 mV. After reaching a minimum at -70 mV, the h infinity curve of BTX-modified Na+ channels turns upward to reach a constant plateau value of approximately 0.9 at voltages above 0 mV. Evidently, the inactivated, BTX-modified Na+ channels can be forced open at more positive potentials. The reopening kinetics of the inactivated channels follows a single exponential with a time constant of 160 ms at +50 mV. Both chloramine-T (at 0.5 mM) and alpha-scorpion toxin (at 200 nM) diminish the inactivation of BTX-modified Na+ channels. In contrast, benzocaine at 1 mM drastically enhances the inactivation of BTX-modified Na+ channels. The h infinity curve reaches minimum of less than 0.1 at -70 mV, indicating that benzocaine binds preferentially with inactivated, BTX-modified Na+ channels. Together, these results imply that BTX-modified Na+ channels are governed by an inactivation process.

Depression of excitability by sphingosine 1-phosphate in rat ventricular myocytes

1998 ◽  
Vol 275 (6) ◽  
pp. H2291-H2299 ◽  
Author(s):  
Karen L. MacDonell ◽  
David L. Severson ◽  
Wayne R. Giles
Keyword(s):  
Action Potential ◽  
Ventricular Myocytes ◽  
Sodium Current ◽  
Potassium Conductance ◽  
Ionic Currents ◽  
Stimulus Current ◽  
Muscle Action Potential ◽  
Action Potential Waveform ◽  
Sphingosine 1 Phosphate ◽  
Electrophysiological Effects

Sphingosine 1-phosphate (S-1- P) is a bioactive sphingolipid that is released from activated platelets. Extracellular S-1- P augments an inwardly rectifying potassium conductance in cultured atrial preparations, but the electrophysiological effects of this compound in the ventricle are unknown. The electrophysiological effects of S-1- P were examined in single myocytes from rat ventricular muscle. Action potential waveforms and underlying ionic currents in the presence and absence of 3 μM S-1- P (1–6 min) were recorded. S-1- P increased the minimum stimulus current needed to elicit an action potential by ∼100 pA. Pertussis toxin or preexposure to S-1- P did not alter this effect. The action potential waveform was unchanged by S-1- P. The inward sodium current ( I Na) was examined in a range of membrane potentials just negative to the potential for firing an action potential. S-1- P reversibly inhibited peak I Na by ∼50 pA, whereas the inward rectifier potassium current was not significantly changed. The results of this study suggest that S-1- P inhibits rat ventricular excitability by reducing I Na.

PROPERTIES OF 185TAGE-ACTIVATED IONIC CURRENTS IN CELLS FROM THE BRAINS OF THE TRICLAD FLATWORM BDELLOURA CANDIDA

1993 ◽  
Vol 185 (1) ◽  
pp. 267-286
Author(s):  
K. L. Blair ◽  
P. A. V. Anderson
Keyword(s):  
Cultured Cells ◽  
Sodium Current ◽  
Cell Voltage ◽  
Ionic Currents ◽  
Sodium Currents ◽  
Channel Evolution ◽  
Major Stage ◽  
Fast Transient ◽  
State Component

Cells were dispersed from the brains of the triclad flatworm Bdelloura candida and maintained in primary culture for up to 2 weeks. Cultured cells assumed a variety of morphologies consistent with those of neurones in vivo. Whole-cell voltage-clamp recordings from cultured cells revealed that these cells possess a variety of ionic currents, including a fast transient sodium current, a calcium current and several potassium currents. The sodium current does not inactivate completely but instead decays to a steady-state component which has the same physiology and pharmacology as the fast transient component, suggesting that the two components are carried by the same population of channels. The physiology and pharmacology of these various currents were not remarkable save for the fact that, contrary to earlier reports, all sodium currents examined were sensitive to tetrodotoxin (TTX). These animals are, therefore, the lowest animals known to possess TTX-sensitive sodium currents and, as such, represent a major stage in sodium channel evolution.

Tedisamil inactivates transient outward K+ current in rat ventricular myocytes

1989 ◽  
Vol 257 (5) ◽  
pp. H1746-H1749 ◽  
Author(s):  
I. D. Dukes ◽  
M. Morad
Keyword(s):  
Ventricular Myocytes ◽  
Sodium Current ◽  
Outward Current ◽  
Cell Voltage ◽  
Transient Outward Current ◽  
Dependent Manner ◽  
Rat Ventricular Myocytes ◽  
K Current ◽  
New Type ◽  
Inwardly Rectifying

The action of tedisamil, a new bradycardiac agent with antiarrhythmic properties, was investigated in single rat ventricular myocytes using the whole cell voltage-clamp technique. Under current clamp conditions, 1-20 microM tedisamil caused marked prolongations of the action potential. Over the same concentration range, in voltage-clamped myocytes, tedisamil suppressed the transient outward current (ito) and enhanced its inactivation in a dose-dependent manner. The half-maximal dose for the effect of tedisamil on ito was approximately 6 microM. Tedisamil had no significant effects on the inwardly rectifying potassium current and calcium current but did suppress the sodium current at concentrations greater than 20 microM. Our findings suggest that tedisamil represents a new type of antiarrhythmic agent that primarily suppresses the transient outward K+ current.

Monensin-Induced Increase in Intracellular Na+ Induces Changes in Na+ and Ca2+ Currents and Regulates Na+-K+ and Na+-Ca2+ Transport in Cardiomyocytes

Pharmacology ◽  
2020 ◽  
pp. 1-15
Author(s):  
Katsuharu Tsuchida ◽  
Hitomi Hirose ◽  
Sachiyo Ozawa ◽  
Haruka Ishida ◽  
Tomomi Iwatani ◽  
...  
Keyword(s):  
Ventricular Myocytes ◽  
Sodium Current ◽  
Pump Current ◽  
Plateau Phase ◽  
Patch Clamp Technique ◽  
Inactivation Curve ◽  
Reverse Mode ◽  
Whole Cell Patch Clamp ◽  
Intracellular Ca ◽  
K Current

<b><i>Background/Aims:</i></b> Monensin, an Na ionophore, increases intracellular Na ([Na]i). Alteration of [Na]i influences ion transport through the sarcolemmal membrane. So far, the effects of monensin on ventricular myocytes have not been examined in detail. The main objective of this study was to elucidate the mechanism via which monensin-evoked increases in [Na]i affect the membrane potential and currents in ventricular myocytes of guinea pigs. Methods: Membrane potentials and currents were measured using the whole-cell patch-clamp technique in single myocytes. The concentration of intracellular Ca ([Ca]i) was evaluated by measuring fluorescence intensity of Fluo-4. Results: Monensin (10<sup>−5</sup>M) shortened the action potential duration (APD) and reduced the amplitude of the plateau phase. In addition, monensin decreased the sodium current (I<sub>Na</sub>) and shifted the inactivation curve to the hyperpolarized direction. Moreover, it decreased the L-type calcium current (I<sub>Ca</sub>). However, this effect was attenuated by increasing the buffering capacity of [Ca]i. The Na-Ca exchange current (I<sub>Na-Ca</sub>) was activated particularly in the reverse mode. Na-K pump current (I<sub>Na-K</sub>) was also activated. Notably, the inward rectifying K current (I<sub>K1</sub>) was not affected, and the change in the delayed outward K current (I<sub>K</sub>) was not evident. Conclusion: These results suggest that the monensin-induced shortened APD and reduced amplitude of the plateau phase are primarily due to the decrease in the I<sub>Ca</sub>, the activation of the reverse mode of I<sub>Na-Ca</sub>, and the increased I<sub>Na-K</sub>, and second due to the decreased I<sub>Na</sub>. The I<sub>K</sub> and the I<sub>K1</sub> may not be associated with the abovementioned changes induced by monensin. The elevation of [Na]i can exert multiple influences on electrophysiological phenomena in cardiac myocytes.

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