Activation of contraction in cat ventricular myocytes: effects of low Cd2+ concentration and temperature

1999 ◽  
Vol 277 (2) ◽  
pp. H488-H498 ◽  
Author(s):  
J. Andrew Wasserstrom ◽  
Ana-Maria Vites
Keyword(s):  
Action Potential ◽  
Voltage Clamp ◽  
Voltage Dependence ◽  
Ventricular Myocytes ◽  
Control Cell ◽  
Ionic Currents ◽  
Experimental Conditions ◽  
Voltage Range ◽  
Cell Shortening ◽  
Reverse Mode

The effects of Cd2+ (20 μM) and different bath temperatures were used to study the contributions of two separate triggering mechanisms, L-type Ca2+current ( I Ca) and reverse mode Na+/Ca2+exchange, to excitation-contraction (E-C) coupling in cat ventricular myocytes. Ionic currents and cell shortening were studied with patch pipettes filled with K+-containing internal solution and discontinuous (“switch”) voltage clamp. Superfusion with Cd2+ blocked cell shortening that closely mirrored the block of I Ca; the voltage dependence of Cd2+-induced reduction in contraction was bell-shaped, displaying minima at test potentials below −10 mV and above +50 mV and a maximum at about +20 mV. Cd2+-insensitive cell shortening was blocked by ryanodine (10 μM) and Ni2+ (4–5 mM). When an action potential was used as the command waveform for the voltage clamp (action potential clamp), Cd2+reduced contraction to ∼60 ± 7% of control cell shortening ( n = 7). The remaining contraction was blocked by ryanodine and Ni2+. Superfusion with nifedipine (10 μM) caused nearly identical effects to Cd2+. The voltage dependence of contraction was sigmoidal at temperatures above 34°C but bell-shaped below 30°C. When Cd2+ was added to superfusate, contraction was abolished at 25°C (to 6 ± 3% of control) but reduced only modestly at 34°C (to 65 ± 13% of control, test potential +10 mV, n = 4, P < 0.01). These results indicate that 1) there is a component of contraction that is sensitive to I Ca antagonists, and the block is equivalent with either organic or inorganic antagonists; 2) the contribution of Na+/Ca2+exchange to triggering of contraction under our experimental conditions is fairly linear throughout the entire voltage range tested; 3) the contribution of I Ca is superimposed on this background component contributed by the Na+/Ca2+exchanger; and 4) triggering via the exchanger is temperature-dependent, providing a major contribution at physiological temperatures but failing at temperatures below 30°C in a nearly all-or-none fashion.

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).

Effects of estrogen on action potential and membrane currents in guinea pig ventricular myocytes

1999 ◽  
Vol 277 (2) ◽  
pp. H826-H833 ◽  
Author(s):  
Seiko Tanabe ◽  
Toshio Hata ◽  
Masayasu Hiraoka
Keyword(s):  
Guinea Pig ◽  
Action Potential ◽  
Action Potentials ◽  
Voltage Dependence ◽  
Ventricular Myocytes ◽  
Membrane Currents ◽  
Patch Clamp Technique ◽  
17Β Estradiol ◽  
Electrophysiological Effects ◽  
Ionic Basis

To explore a possible ionic basis for the prolonged Q-T interval in women compared with that in men, we investigated the electrophysiological effects of estrogen in isolated guinea pig ventricular myocytes. Action potentials and membrane currents were recorded using the whole cell configuration of the patch-clamp technique. Application of 17β-estradiol (10–30 μM) significantly prolonged the action potential duration (APD) at 20% (APD20) and 90% repolarization (APD90) at stimulation rates of 0.1–2.0 Hz. In the presence of 30 μM 17β-estradiol, APD20 and APD90 at 0.1 Hz were prolonged by 46.2 ± 17.1 and 63.4 ± 11.7% of the control ( n = 5), respectively. In the presence of 30 μM 17β-estradiol the peak inward Ca2+ current ( I CaL) was decreased to 80.1 ± 2.5% of the control ( n = 4) without a shift in its voltage dependence. Application of 30 μM 17β-estradiol decreased the rapidly activating component of the delayed outward K+ current ( I Kr) to 63.4 ± 8% and the slowly activating component ( I Ks) to 65.8 ± 8.7% with respect to the control; the inward rectifier K+ current was barely affected. The results suggest that 17β-estradiol prolonged APD mainly by inhibiting the I Kcomponents I Krand I Ks.

Characterization and functional consequences of delayed rectifier current transient in ventricular repolarization

2000 ◽  
Vol 278 (3) ◽  
pp. H806-H817 ◽  
Author(s):  
Gary A. Gintant
Keyword(s):  
Action Potential ◽  
Ventricular Myocytes ◽  
Outward Current ◽  
Inward Rectifier ◽  
Ventricular Repolarization ◽  
Delayed Rectifier ◽  
Voltage Range ◽  
Delayed Rectifier Current ◽  
Recovery From Inactivation ◽  
Voltage Dependent

Although inactivation of the rapidly activating delayed rectifier current ( I Kr) limits outward current on depolarization, the role of I Kr (and recovery from inactivation) during repolarization is uncertain. To characterize I Krduring ventricular repolarization (and compare with the inward rectifier current, I K1), voltage-clamp waveforms simulating the action potential were applied to canine ventricular, atrial, and Purkinje myocytes. In ventricular myocytes, I Kr was minimal at plateau potentials but transiently increased during repolarizing ramps. The I Kr transient was unaffected by repolarization rate and maximal after 150-ms depolarizations (+25 mV). Action potential clamps revealed the I Kr transient terminating the plateau. Although peak I Kr transient density was relatively uniform among myocytes, potentials characterizing the peak transients were widely dispersed. In contrast, peak inward rectifier current ( I K1) density during repolarization was dispersed, whereas potentials characterizing I K1 defined a narrower (more negative) voltage range. In summary, rapidly activating I Kr provides a delayed voltage-dependent (and functionally time-independent) outward transient during ventricular repolarization, consistent with rapid recovery from inactivation. The heterogeneous voltage dependence of I Kr provides a novel means for modulating the contribution of this current during repolarization.

Modulation of the delayed rectifier, IK, by cadmium in cat ventricular myocytes

1992 ◽  
Vol 262 (1) ◽  
pp. C75-C83 ◽  
Author(s):  
C. H. Follmer ◽  
N. J. Lodge ◽  
C. A. Cullinan ◽  
T. J. Colatsky
Keyword(s):  
Voltage Clamp ◽  
Ventricular Myocytes ◽  
Outward Current ◽  
Membrane Potentials ◽  
Delayed Rectifier ◽  
Voltage Range ◽  
Control Value ◽  
Current Voltage ◽  
Voltage Dependent ◽  
Slope Factor

The effects of cadmium on the delayed outward potassium current (IK) were investigated in isolated cat ventricular myocytes using the single suction pipette voltage-clamp technique. IK activation was examined using peak tail currents elicited after 750-ms voltage-clamp steps to selected membrane potentials from a holding potential of -40 mV. In the presence of Cd2+ (0.2 mM), peak tail currents increased from a control value of 85 +/- 12 to 125 +/- 18 pA (n = 4). Activation curves constructed from the average peak tail-current measurements in all experiments showed that Cd2+ shifted the voltage dependence of activation to more positive potentials by 16.4 +/- 2.0 mV and increased the slope factor of the activation curve from 6.1 +/- 0.2 to 6.9 +/- 0.2 mV. In the absence of Cd2+, increases in holding potential from -30 to -70 mV had no effect on the magnitude of the peak tail currents, suggesting that the Cd(2+)-induced increase was not the result of a voltage-dependent increase in the number of available K+ channels at the holding potential. Slow voltage ramps from -70 to +70 mV revealed that Cd2+ increased the outward current at membrane potentials positive to +20 mV and shifted the voltage range in which IK inwardly rectified to more positive potentials. The fully activated current-voltage relationship was also shifted to more positive potentials by Cd2+. Cd2+ did not alter channel selectivity for K+.(ABSTRACT TRUNCATED AT 250 WORDS)

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 Calcium handling in zebrafish ventricular myocytes

2011 ◽  
Vol 300 (1) ◽  
pp. R56-R66 ◽  
Author(s):  
Ping-Cheng Zhang ◽  
Anna Llach ◽  
Xiao Ye Sheng ◽  
Leif Hove-Madsen ◽  
Glen F. Tibbits
Keyword(s):  
Ventricular Myocytes ◽  
Cardiac Development ◽  
Calcium Transient ◽  
Membrane Potentials ◽  
Calcium Handling ◽  
Patch Clamp Technique ◽  
Cell Shortening ◽  
Calcium Dependent ◽  
Reverse Mode ◽  
Zebrafish Heart

The zebrafish is an important model for the study of vertebrate cardiac development with a rich array of genetic mutations and biological reagents for functional interrogation. The similarity of the zebrafish ( Danio rerio) cardiac action potential with that of humans further enhances the relevance of this model. In spite of this, little is known about excitation-contraction coupling in the zebrafish heart. To address this issue, adult zebrafish cardiomyocytes were isolated by enzymatic perfusion of the cannulated ventricle and were subjected to amphotericin-perforated patch-clamp technique, confocal calcium imaging, and/or measurements of cell shortening. Simultaneous recordings of the voltage dependence of the L-type calcium current ( ICa,L) amplitude and cell shortening showed a typical bell-shaped current-voltage ( I-V) relationship for ICa,L with a maximum at +10 mV, whereas calcium transients and cell shortening showed a monophasic increase with membrane depolarization that reached a plateau at membrane potentials above +20 mV. Values of ICa,L were 53, 100, and 17% of maximum at −20, +10, and +40 mV, while the corresponding calcium transient amplitudes were 64, 92, and 98% and cell shortening values were 62, 95, and 96% of maximum, respectively, suggesting that ICa,L is the major contributor to the activation of contraction at voltages below +10 mV, whereas the contribution of reverse-mode Na/Ca exchange becomes increasingly more important at membrane potentials above +10 mV. Comparison of the recovery of ICa,L from acute and steady-state inactivation showed that reduction of ICa,L upon elevation of the stimulation frequency is primarily due to calcium-dependent ICa,L inactivation. In conclusion, we demonstrate that a large yield of healthy atrial and ventricular myocytes can be obtained by enzymatic perfusion of the cannulated zebrafish heart. Moreover, zebrafish ventricular myocytes differed from that of large mammals by having larger ICa,L density and a monophasically increasing contraction-voltage relationship, suggesting that caution should be taken upon extrapolation of the functional impact of mutations on calcium handling and contraction in zebrafish cardiomyocytes.

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.

Excitable properties and voltage-sensitive ion conductances of horizontal cells isolated from catfish (Ictalurus punctatus) retina

1986 ◽  
Vol 56 (1) ◽  
pp. 32-49 ◽  
Author(s):  
R. Shingai ◽  
B. N. Christensen
Keyword(s):  
Membrane Potential ◽  
Cell Membrane ◽  
Action Potential ◽  
Voltage Clamp ◽  
Ictalurus Punctatus ◽  
Horizontal Cells ◽  
Resting Potential ◽  
Patch Type ◽  
Voltage Range ◽  
Plateau Potential

External horizontal cells were enzymatically dissociated from intact catfish (Ictalurus punctatus) retina and pipetted onto a small chamber attached to the stage of an inverted phase-contrast microscope. Individual horizontal cells were recognized by their large size and restricted dendritic arborization. Low-resistance (3-12 M omega) patch-type electrodes were used to record intracellular potentials and to pass current across the cell membrane under either current or voltage-clamp conditions. The average resting potential of isolated horizontal cells was -67 V + 6.9 mV (mean +/- SD, n = 40). At the resting potential, the cell membrane appears to be mainly permeable to K. A depolarizing current step evoked an action potential in the cell. The maximum rate of rise of the action potential (dV/dt) in normal physiological solution was 6.5 +/- 1.8 V/s (means +/- SD, n = 24) and was reduced to 1.2 +/- 0.39 V/s (means +/- SD, n = 9) in 1-10 micron tetrodotoxin (TTX) and 3.2 +/- 1.4 V/s (means +/- SD, n = 6) in Ca-free solution. The maximum dV/dt was reduced in 10 mM extracellular K concentration [K]o to about half of that seen in standard saline, and values in 30 or 80 mM [K]o were similar to that measured in TTX. Following an action potential, the membrane potential reached a plateau potential of + 17.4 +/- 8.1 mV (means +/- SD, n = 17) and remained depolarized for variable periods of time lasting from less than a second to a few minutes. When the plateau potential was long lasting, the cell repolarized slowly and upon reaching zero rapidly repolarized to the original resting potential. The duration of the plateau potential decreased or was absent in saline containing one of the following calcium channel antagonists: La, Cd, Co, or Ni. The voltage-clamp technique was used to identify the membrane currents responsible for the membrane potential changes seen under current clamp. Experiments were carried out using either a single or two individual electrodes. Fast and steady-state inward currents were recorded from isolated horizontal cells in the voltage range between -20 and +20 mV. These currents were a result of increased membrane conductance to both Na and Ca ions. The Na channels are inactivated at depolarized potentials and are TTX sensitive. Ca channels are partially inactivated at depolarized potentials. The Ca conductance is decreased by Cd, Co, Ni, and La. Ba can substitute for Ca in the channel.(ABSTRACT TRUNCATED AT 400 WORDS)

Palmitoyl-L-carnitine acts like ouabain on voltage, current, and contraction in guinea pig ventricular cells

1995 ◽  
Vol 268 (3) ◽  
pp. H1027-H1036 ◽  
Author(s):  
J. B. Shen ◽  
A. J. Pappano
Keyword(s):  
Guinea Pig ◽  
Ventricular Myocytes ◽  
Present Report ◽  
Ionic Currents ◽  
Pump Current ◽  
Membrane Potentials ◽  
Cell Shortening ◽  
Ventricular Cells ◽  
Ramp Voltage ◽  
Inward Currents

We previously showed that palmitoyl-L-carnitine (L-PC) inhibits the Na/K pump current (INa/K). In the present report, we test the hypothesis that L-PC, like ouabain, should increase myocyte shortening. Membrane potentials or ionic currents were recorded simultaneously with cell shortening in single guinea pig ventricular myocytes at room temperature (22 degrees C). Like ouabain, L-PC (1 microM) reversibly depolarized the resting membrane, decreased action potential duration, and increased the amplitude of myocyte contractions. Neither L-PC nor ouabain had a significant effect on Ca current (ICa). When L-PC increased cell shortening during ramp voltage clamp, membrane current shifted inward at voltages negative to -20 mV and shifted outward at more positive voltages. Similar to toxic concentrations of ouabain, L-PC induced transient inward currents and aftercontractions. At concentrations that inhibit INa/K, L-PC acted like ouabain to produce characteristic effects on membrane potentials, currents, and cell contractions that were unrelated to significant changes in ICa. L-PC reduces surface negative charge of erythrocytes and myocytes (C. Gruver and A. J. Pappano, J. Mol. Cell. Cardiol. 25: 1275–1284, 1993), and we speculate that L-PC inhibits INa/K by this mechanism.

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