Effect of reduced Na gradient on electrical activity in isolated bovine and feline ventricular myocytes

1988 ◽  
Vol 66 (2) ◽  
pp. 222-232 ◽  
Author(s):  
Magda Horackova ◽  
Andrzej Beresewicz ◽  
Gerrit Isenberg
Keyword(s):  
Electrical Activity ◽  
Ventricular Myocytes ◽  
Ionic Current ◽  
Outward Current ◽  
Free Solution ◽  
Initial Part ◽  
Inward Current ◽  
Ca Inward Current ◽  
Sudden Decrease ◽  
K Currents

We have studied changes in electrical activity resulting from abrupt alterations of the Na gradient, using ventricular myocytes isolated from feline and bovine hearts. Attempting to investigate the ionic current possibly generated by Na–Ca exchange, we studied the effects of the changes in [Na]o in the presence of 20 mM CsCl to inhibit K currents. To facilitate the effect of Cs, we also used a K-free solution and a patch electrode filled with 150 mM cesium glutamate. The application of 20 mM Nao resulted in hyperpolarization and the action potential duration was reduced. Under voltage clamp, 20 or 45 mM Nao generated an outward current at all membrane potentials investigated. The initial part (100–200 ms) of this current was only partially inhibited by 5 mM NiCl2 which is known to fully block the Ca inward current. However, the outward current generated by the reduced [Na]o was fully inhibited by 20 mM MnCl2 (which presumably inhibits Na–Ca exchange). Our observations extend the work on multicellular cardiac preparations indicating that the outward current elicited by a sudden decrease in Na gradient could be generated by Na–Ca exchange. Although the characteristics of this outward current support certain concepts of the Na–Ca exchange in cardiac muscle, we cannot at present exclude a contribution of other membrane current(s).

Membrane currents recorded from a fragment of rabbit intestinal smooth muscle cell

1986 ◽  
Vol 251 (3) ◽  
pp. C335-C346 ◽  
Author(s):  
Y. Ohya ◽  
K. Terada ◽  
K. Kitamura ◽  
H. Kuriyama
Keyword(s):  
Smooth Muscle ◽  
Longitudinal Muscle ◽  
Outward Current ◽  
Ionic Currents ◽  
Muscle Layer ◽  
Dependent Manner ◽  
Rabbit Ileum ◽  
Longitudinal Muscle Layer ◽  
Inward Current ◽  
K Currents

Properties of ionic currents in smooth muscle membranes of the longitudinal muscle layer of the rabbit ileum were investigated using the single electrode voltage clamp method. In the present experiments, this method was applicable only to the smooth muscle ball (fragment) and not for the dispersed whole cell, because of incompleteness of the voltage clamping. A voltage step elicited a transient inward current followed by an outward current. This outward current was partly inhibited by Mn2+ or nisoldipine or by a reduction in the extracellular [Ca2+] ([Ca2+]o). Tetraethylammonium (TEA) reduced the delayed outward current in a dose-dependent manner, but 50 mM TEA did not produce a complete block of a residual current. When the pipette contained K+-free (Cs+ with TEA+) solution, the residual outward current was abolished. The inward current was elicited at -30 mV (holding potential of -60 mV) and reached the maximal value at +10 mV; the polarity was reversed at +60 mV. This inward current depended on the [Ca2+]o and was blocked by Mn2+ or nisoldipine. Ba2+ also permeated the membrane, and the inward current evoked by Ba2+ was also blocked by Mn2+ or nisoldipine. Reduction of [Na+]o in a solution containing 2.4 mM Ca2+ neither modified the current-voltage relation nor the decay of the inward current, but when [Ca2+]o was reduced to below 1 microM, Na+ permeated the membrane and was blocked by nisoldipine. In conclusion, ionic currents were recordable from the fragmented ball of the longitudinal muscle of rabbit ileum. There were at least two K+ currents as the outward current (Ca2+-dependent K+ and delayed K+ currents) and a Ca2+ current as the inward current. The property of the Ca2+ channel was similar to that observed with other preparations.

Ischemia But Not Anoxia Evokes Vesicular and Ca2+-Independent Glutamate Release In the Dorsal Vagal Complex In Vitro

2000 ◽  
Vol 83 (5) ◽  
pp. 2905-2915 ◽  
Author(s):  
Anna Kulik ◽  
Stefan Trapp ◽  
Klaus Ballanyi
Keyword(s):  
Glutamate Uptake ◽  
Glutamate Release ◽  
Outward Current ◽  
Cyclopiazonic Acid ◽  
Synaptic Activity ◽  
Dorsal Vagal Complex ◽  
Free Solution ◽  
Inward Current ◽  
Metabolic Arrest

Whole cell recordings of fura-2 dialyzed vagal neurons of brain stem slices were used to monitor interstitial glutamate accumulation within the dorsal vagal complex. Anoxia produced a sustained outward current (60 pA) and a moderate [Ca2+]i rise (40 nM). These responses were neither mimicked by [1S,3R]-1-aminocyclo-pentane-1,3-dicarboxylic acid nor affected by Ca2+-free solution, 6-cyano-7-nitroquino-xaline-2,3-dione (CNQX), 2-amino-5-phosphonovalerate (APV), or tetrodotoxin. Anoxia or cyanide in glucose-free saline (in vitro ischemia) as well as ouabain or iodoacetate elicited an initial anoxia-like [Ca2+]i increase that turned after several minutes into a prominent Ca2+ transient (0.9 μM) and inward current (−1.8 nA). APV plus CNQX (plus methoxyverapamil) inhibited this inward current as well as accompanying spontaneous synaptic activity, and reduced the secondary [Ca2+]i rise to values similar to those during anoxia. Each of the latter drugs delayed onset of both ischemic current and prominent [Ca2+]i rise by several minutes and attenuated their magnitudes by up to 40%. Ca2+-free solution induced a twofold delay of the ischemic inward current and suppressed the prominent Ca2+ increase but not the initial moderate [Ca2+]i rise. Cyclopiazonic acid or arachidonic acid in Ca2+-free saline delayed further the ischemic current, whereas neither inhibitors of glutamate uptake (dihydrokainate,d,l-threo-β-hydroxyaspartate,l-transpyrrolidone-2,4-dicarboxylate) nor the Cl− channel blocker 5-nitro-2-(3-phenylpropyl-amino) benzoic acid had any effect. In summary, the response to metabolic arrest is due to activation of ionotropic glutamate receptors causing Ca2+ entry via N-methyl-d-aspartate receptors and voltage-activated Ca2+ channels. An early Ca2+-dependent exocytotic phase of ischemic glutamate release is followed by nonvesicular release, not mediated by reversed glutamate uptake or Cl− channels. The results also show that glycolysis prevents glutamate release during anoxia.

Ischemia alters the electrical activity of pacemaker cells isolated from the rabbit sinoatrial node

2002 ◽  
Vol 282 (6) ◽  
pp. H2284-H2295 ◽  
Author(s):  
O. Gryshchenko ◽  
J. Qu ◽  
R. D. Nathan
Keyword(s):  
Electrical Activity ◽  
Tyrode Solution ◽  
Solution Ph ◽  
Inward Current ◽  
Inward Currents ◽  
K Current ◽  
Induced Changes ◽  
Rabbit Sinoatrial Node ◽  
Selective Blocker ◽  
K Currents

The purpose of this study was to investigate the mechanisms responsible for ischemia-induced changes in spontaneous electrical activity. An ischemic-like Tyrode solution (pH 6.6) reversibly depolarized the maximum diastolic potential (MDP) and reduced the action potential (AP) overshoot (OS). We used SNARF-1, which is an indicator of intracellular pH (pHi), and perforated-patch techniques to test the hypothesis that acidosis caused these effects. Acidic but otherwise normal Tyrode solution (pH 6.8) produced similar effects. Basic Tyrode solution (pH 8.5) hyperpolarized the MDP, shortened the AP, and slowed the firing rate. In the presence of “ischemic” Tyrode solution, hyperpolarizing current restored the MDP and OS to control values. HOE-642, an inhibitor of Na/H exchange, did not alter pHi or electrical activity and did not prevent the effects of ischemic Tyrode solution or recovery after washout. Time-independent net inward current but not hyperpolarization-activated inward current was enhanced by ischemic Tyrode solution or by 30 μM BaCl2, a selective blocker of inward-rectifying K currents at this concentration. The results suggest that 1) acidosis was responsible for the ischemia-induced effects but Na/H exchange was not involved, 2) the OS was reduced because of depolarization-induced inactivation of inward currents that generate the AP upstroke, and 3) reduction of an inward-rectifying outward K current contributed to the depolarization.

Role of Na-Ca exchange in the action potential changes caused by drive in cardiac myocytes exposed to different Ca2+ loads

1999 ◽  
Vol 77 (6) ◽  
pp. 383-397
Author(s):  
Qi-Ying Liu ◽  
Mario Vassalle
Keyword(s):  
Action Potentials ◽  
Ventricular Myocytes ◽  
Outward Current ◽  
Time Dependent ◽  
Inward Current ◽  
Tail Current ◽  
Outward Currents ◽  
Inward Currents ◽  
Single Ventricular Myocytes

The role of Na-Ca exchange in the membrane potential changes caused by repetitive activity ("drive") was studied in guinea pig single ventricular myocytes exposed to different [Ca2+]o. The following results were obtained. (i) In 5.4 mM [Ca2+]o, the action potentials (APs) gradually shortened during drive, and the outward current during a train of depolarizing voltage clamp steps gradually increased. (ii) The APs shortened more and were followed by a decaying voltage tail during drive in the presence of 5 mM caffeine; the outward current became larger and there was an inward tail current on repolarization during a train of depolarizing steps. (iii) These effects outlasted drive so that immediately after a train of APs, currents were already bigger and, after a train of steps, APs were already shorter. (iv) In 0.54 mM [Ca2+]o, the above effects were much smaller. (v) In high [Ca2+]o APs were shorter and outward currents larger than in low [Ca2+]o. (vi) In 10.8 mM [Ca2+]o, both outward and inward currents during long steps were exaggerated by prior drive, even with steps (+80 and +120 mV) at which there was no apparent inward current identifiable as ICa. (vii) In 0.54 mM [Ca2+]o, the time-dependent outward current was small and prior drive slightly increased it. (viii) During long steps, caffeine markedly increased outward and inward tail currents, and these effects were greatly decreased by low [Ca2+]o. (ix) After drive in the presence of caffeine, Ni2+ decreased the outward and inward tail currents. It is concluded that in the presence of high [Ca2+]o drive activates outward and inward Na-Ca exchange currents. During drive, the outward current participates in the plateau shortening and the inward tail current in the voltage tail after the action potential.Key words: ventricular myocytes, repetitive activity, outward and inward Na-Ca exchange currents, caffeine, nickel.

scholarly journals Voltage-dependent ionic currents in taste receptor cells of the larval tiger salamander.

1990 ◽  
Vol 96 (4) ◽  
pp. 809-834 ◽  
Author(s):  
K Sugimoto ◽  
J H Teeter
Keyword(s):  
Single Channel ◽  
Outward Current ◽  
Tiger Salamander ◽  
Inward Current ◽  
Receptor Cells ◽  
Outward Currents ◽  
Taste Cells ◽  
Voltage Dependent ◽  
Single Channel Currents ◽  
K Currents

Voltage-dependent membrane currents of cells dissociated from tongues of larval tiger salamanders (Ambystoma tigrinum) were studied using whole-cell and single-channel patch-clamp techniques. Nongustatory epithelial cells displayed only passive membrane properties. Cells dissociated from taste buds, presumed to be gustatory receptor cells, generated both inward and outward currents in response to depolarizing voltage steps from a holding potential of -60 or -80 mV. Almost all taste cells displayed a transient inward current that activated at -30 mV, reached a peak between 0 and +10 mV and rapidly inactivated. This inward current was blocked by tetrodotoxin (TTX) or by substitution of choline for Na+ in the bath solution, indicating that it was a Na+ current. Approximately 60% of the taste cells also displayed a sustained inward current which activated slowly at about -30 mV and reached a peak at 0 to +10 mV. The amplitude of the slow inward current was larger when Ca2+ was replaced by Ba2+ and it was blocked by bath applied CO2+, indicating it was a Ca2+ current. Delayed outward K+ currents were observed in all taste cells although in about 10% of the cells, they were small and activated only at voltages more depolarized than +10 mV. Normally, K+ currents activated at -40 mV and usually showed some inactivation during a 25-ms voltage step. The inactivating component of outward current was not observed at holding potentials more depolarized -40 mV. The outward currents were blocked by tetraethylammonium chloride (TEA) and BaCl2 in the bath or by substitution of Cs+ for K+ in the pipette solution. Both transient and noninactivating components of outward current were partially suppressed by CO2+, suggesting the presence of a Ca2(+)-activated K+ current component. Single-channel currents were recorded in cell-attached and outside-out patches of taste cell membranes. Two types of K+ channels were partially characterized, one having a mean unitary conductance of 21 pS, and the other, a conductance of 148 pS. These experiments demonstrate that tiger salamander taste cells have a variety of voltage- and ion-dependent currents including Na+ currents, Ca2+ currents and three types of K+ currents. One or more of these conductances may be modulated either directly by taste stimuli or indirectly by stimulus-regulated second messenger systems to give rise to stimulus-activated receptor potentials. Others may play a role in modulation of neurotransmitter release at synapses with taste nerve fibers.(ABSTRACT TRUNCATED AT 400 WORDS)

Involvement of anion channel(s) in the modulation of the transient outward K+ channel in rat ventricular myocytes

2004 ◽  
Vol 287 (1) ◽  
pp. C163-C170 ◽  
Author(s):  
Xiao-Gang Lai ◽  
Jun Yang ◽  
Shi-Sheng Zhou ◽  
Jun Zhu ◽  
Gui-Rong Li ◽  
...  
Keyword(s):  
Steady State ◽  
Ventricular Myocytes ◽  
Ionic Current ◽  
Outward Current ◽  
Anion Channel ◽  
K Channel ◽  
Transient Outward Current ◽  
Inactivation Curve ◽  
Phalloidin Staining ◽  
Steady State Inactivation

The cardiac Ca2+-independent transient outward K+ current ( Ito), a major repolarizing ionic current, is markedly affected by Cl− substitution and anion channel blockers. We reexplored the mechanism of the action of anions on Ito by using whole cell patch-clamp in single isolated rat cardiac ventricular myocytes. The transient outward current was sensitive to blockade by 4-aminopyridine (4-AP) and was abolished by Cs+ substitution for intracellular K+. Replacement of most of the extracellular Cl− with less permeant anions, aspartate (Asp−) and glutamate (Glu−), markedly suppressed the current. Removal of external Na+ or stabilization of F-actin with phalloidin did not significantly affect the inhibitory action of less permeant anions on Ito. In contrast, the permeant Cl− substitute Br− did not markedly affect the current, whereas F− substitution for Cl− induced a slight inhibition. The Ito elicited during Br− substitution for Cl− was also sensitive to blockade by 4-AP. The ability of Cl− substitutes to induce rightward shifts of the steady-state inactivation curve of Ito was in the following sequence: NO3− > Cl− ≈ Br− > gluconate− > Glu− > Asp−. Depolymerization of actin filaments with cytochalasin D (CytD) induced an effect on the steady-state inactivation of Ito similar to that of less permeant anions. Fluorescent phalloidin staining experiments revealed that CytD-pretreatment significantly decreased the intensity of FITC-phalloidin staining of F-actin, whereas Asp− substitution for Cl− was without significant effect on the intensity. These results suggest that the Ito channel is modulated by anion channel(s), in which the actin cytoskeleton may be implicated.

scholarly journals Characterization of two distinct depolarization-activated K+ currents in isolated adult rat ventricular myocytes.

1991 ◽  
Vol 97 (5) ◽  
pp. 973-1011 ◽  
Author(s):  
M Apkon ◽  
J M Nerbonne
Keyword(s):  
Steady State ◽  
Ventricular Myocytes ◽  
Outward Current ◽  
Resting Membrane Potential ◽  
Delayed Rectifier ◽  
Patch Clamp Recording ◽  
Rat Ventricular Myocytes ◽  
Adult Rat ◽  
Steady State Inactivation ◽  
K Currents

Depolarization-activated outward K+ currents in isolated adult rat ventricular myocytes were characterized using the whole-cell variation of the patch-clamp recording technique. During brief depolarizations to potentials positive to -40 mV, Ca(2+)-independent outward K+ currents in these cells rise to a transient peak, followed by a slower decay to an apparent plateau. The analyses completed here reveal that the observed outward current waveforms result from the activation of two kinetically distinct voltage-dependent K+ currents: one that activates and inactivates rapidly, and one that activates and inactivates slowly, on membrane depolarization. These currents are referred to here as Ito (transient outward) and IK (delayed rectifier), respectively, because their properties are similar (although not identical) to these K+ current types in other cells. Although the voltage dependences of Ito and IK activation are similar, Ito activates approximately 10-fold and inactivates approximately 30-fold more rapidly than IK at all test potentials. In the composite current waveforms measured during brief depolarizations, therefore, the peak current predominantly reflects Ito, whereas IK is the primary determinant of the plateau. There are also marked differences in the voltage dependences of steady-state inactivation of these two K+ currents: IK undergoes steady-state inactivation at all potentials positive to -120 mV, and is 50% inactivated at -69 mV; Ito, in contrast, is insensitive to steady-state inactivation at membrane potentials negative to -50 mV. In addition, Ito recovers from steady-state inactivation faster than IK: at -90 mV, for example, approximately 70% recovery from the inactivation produced at -20 mV is observed within 20 ms for Ito; IK recovers approximately 25-fold more slowly. The pharmacological properties of Ito and IK are also distinct: 4-aminopyridine preferentially attenuates Ito, and tetraethylammonium suppresses predominantly IK. The voltage- and time-dependent properties of these currents are interpreted here in terms of a model in which Ito underlies the initial, rapid repolarization phase of the action potential (AP), and IK is responsible for the slower phase of AP repolarization back to the resting membrane potential, in adult rat ventricular myocytes.

Calcium currents in normal and hypertrophied isolated feline ventricular myocytes

1988 ◽  
Vol 255 (6) ◽  
pp. H1434-H1442 ◽  
Author(s):  
R. B. Kleiman ◽  
S. R. Houser
Keyword(s):  
Right Ventricular ◽  
Ventricular Myocytes ◽  
Slow Component ◽  
Outward Current ◽  
Calcium Currents ◽  
Inward Current ◽  
Outward Currents ◽  
Hypertrophied Myocardium ◽  
Prolonged Duration ◽  
Slow Inward Calcium Current

The magnitude and kinetics of the slow inward calcium current (Isi) were compared in single right ventricular myocytes that were isolated from normal cats and cats with right ventricular hypertrophy. Peak inward current density was greater in hypertrophy than normal myocytes (-20.4 +/- 15.3 vs. -10.4 +/- 8.8 microA/cm2, P less than 0.05). When we blocked early outward currents with intracellular CsCl, however, the peak magnitude of Isi was shown to be similar in hypertrophy and normal myocytes (-16.4 +/- 11.2 vs. -12.7 +/- 3.0 microA/cm2, P = NS). The increased net inward current in hypertrophy was thus due to a decrease in Cs-sensitive early outward current rather than an increase in the magnitude of Isi. The fast component of inactivation of Isi was similar in hypertrophy and normal myocytes, but the slow component was delayed in hypertrophy (slow time constant; tau slow = 75.9 +/- 14.7 ms vs. tau slow = 60.6 +/- 4.9 ms, P less than 0.05). These abnormalities of Isi may contribute to the prolonged duration of the action potential and of contraction in hypertrophied myocardium, but a defect in excitation-contraction coupling distal to Isi appears to produce the diminished magnitude of contraction.

External anions and volume-sensitive anion current in guinea-pig ventricular myocytes

2000 ◽  
Vol 78 (8) ◽  
pp. 662-668 ◽  
Author(s):  
Lesya M Shuba ◽  
Terence F McDonald
Keyword(s):  
Guinea Pig ◽  
Ventricular Myocytes ◽  
Reversal Potential ◽  
Outward Current ◽  
Cell Voltage ◽  
Dependent Manner ◽  
Free Solution ◽  
Relative Permeabilities ◽  
Voltage Dependent ◽  
Relative Slope

The objective of this study was to determine the effects of anion replacement on volume-sensitive anion current in guinea-pig ventricular myocytes. Myocytes in the conventional whole-cell voltage-clamp configuration were superfused and dialysed with Na+-, K+-, and Ca2+-free solution, and exposed to external 75 mM Cl- solution of one-half normal osmolality. Prolonged exposures to hyposmotic solution promoted the development of outwardly-rectifying currents that were inactivated at high positive potentials and reversed in a Cl--dependent manner (50 mV per decade pipette Cl- concentration). Replacement of external Cl- by iodide and aspartate affected the reversal potential (Erev) and slope conductance of the volume-sensitive current. Relative permeabilities calculated from changes in Erev were 1.49 ± 0.09, 1.00, and 0.29 ± 0.04 for iodide, Cl-, and aspartate, respectively; relative slope conductances between Erev and Erev + 40 mV were 1.21 ± 0.09, 1.00, and 0.43 ± 0.07, respectively. Replacement of Cl- also affected the time dependence of the volume-sensitive current; replacement by iodide reversibly enhanced the decay of outward current at positive potentials, whereas replacement by aspartate reduced it. These results are compared with earlier findings on non-cardiac time- and voltage-dependent anion current activated by hyposmotic solution.Key words: hyposmotic solution, Cl- current, iodide, aspartate, permeability, conductance.

A novel type of depolarization-activated K+ current in isolated adult rat atrial myocytes

1991 ◽  
Vol 260 (4) ◽  
pp. H1236-H1247 ◽  
Author(s):  
W. A. Boyle ◽  
J. M. Nerbonne
Keyword(s):  
Ventricular Myocytes ◽  
Outward Current ◽  
Cdna Libraries ◽  
K Channel ◽  
Delayed Rectifier ◽  
Atrial Myocytes ◽  
Patch Clamp Recording ◽  
K Channels ◽  
Adult Rat ◽  
K Currents

To determine the types of voltage-gated K+ channels controlling action potential repolarization in atrial cells, we have characterized the properties of depolarization-activated K+ channels in isolated adult rat atrial myocytes using the whole cell patch-clamp recording technique. On membrane depolarization, Ca2(+)-independent outward K+ currents in these cells begin to activate at approximately -40mV. At all test potentials, the currents activate rapidly after a delay, and there is little or no decay of the peak outward current amplitude during brief (100 ms) depolarizations. In addition, the currents show little steady-state inactivation at membrane potentials negative to -60 mV. The currents are blocked effectively by 1-5 mM 4-aminopyridine but are relatively insensitive to extracellular tetraethylammonium at concentrations up to 50 mM. Based on the measured time- and voltage-dependent properties and the pharmacological sensitivity of the currents, we suggest that the depolarization-activated K+ channels underlying the macroscopic currents in adult rat atrial myocytes are distinct from those described previously in other myocardial preparations, including adult rat ventricular myocytes. Interestingly, the outward K+ currents characterized here in isolated adult rat atrial myocytes are remarkably similar to those of several recently described "delayed rectifier" K+ channel genes isolated from rat brain cDNA libraries and expressed in Xenopus oocytes, suggesting that similar K+ currents are likely present in cells of the mammalian central nervous system.

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