Cell-attached patch clamp study of the electropermeabilization of amphibian cardiac cells

Recent advances in microfluidic-based electroporation techniques for cell membranes

Electroporation is a fundamental technique for applications in biotechnology. To date, the ongoing research on cell membrane electroporation has explored its mechanism, principles and potential applications. Therefore, in this review, we first discuss the primary electroporation mechanism to help establish a clear framework. Within the context of its principles, several critical terms are highlighted to present a better understanding of the theory of aqueous pores. Different degrees of electroporation can be used in different applications. Thus, we discuss the electric factors (shock strength, shock duration, and shock frequency) responsible for the degree of electroporation. In addition, finding an effective electroporation detection method is of great significance to optimize electroporation experiments. Accordingly, we summarize several primary electroporation detection methods in the following sections. Finally, given the development of micro- and nano-technology has greatly promoted the innovation of microfluidic-based electroporation devices, we also present the recent advances in microfluidic-based electroporation devices. Also, the challenges and outlook of the electroporation technique for cell membrane electroporation are presented.

The microglia response to electrical overstimulation of the retina imaged under a transparent stimulus electrode

OBJECTIVE

We investigated using the morphological response of retinal microglia as indicators of tissue damage from electrical overstimulation by imaging them through an optically transparent stimulus electrode.

APPROACH

To track the microglia, we used a transgenic mouse where the microglia expressed a water soluble green fluorescent protein (GFP). The clear stimulus electrode was placed epiretinally on the inner limiting membrane and the microglia layers were imaged using time-lapse confocal microscopy. We examined how the microglia responded both temporally and spatially to local overstimulation of the retinal tissue. Using confocal microscope vertical image stacks, the microglia under the electrode were imaged at 2.5min intervals. The retina was overstimulated for a 5 minute period using 1msec 749μC/cm2/ph biphasic current pulses and changes in the microglia morphology were followed for 1 hour post stimulation. After the imaging period, a label for cellular damage was applied to the retina.

MAIN RESULTS

The microglia response to overstimulation depended on their spatial location relative to the electrode lumen and could result in 3 different morphological responses. Some microglia were severely injured and became a series of immotile ball-like fluorescent processes. Other microglia survived, and reacted rapidly to the injury by extending filopodia oriented toward the damage zone. This response was seen in inner retinal microglia outside the stimulus electrode edge. A third effect, seen with the deeper outer microglia under the electrode, was a fading of their fluorescent image which appeared to be due to optical scatter caused by overstimulation-induced retinal edema.

SIGNIFICANCE

The microglial morphological responses to electrical overstimulation injury occur rapidly and can show both direct and indirect effects of the stimulus electrode injury. The microglia injury pattern closely follows models of the electric field distribution under thinly insulated disc electrodes.

A critical evaluation of whole cell patch clamp studies on electroporation using the voltage sensitive dye ANNINE-6

The patch clamp technique in the whole cell configuration is potentially a powerful tool to investigate electroporation (electric-field-induced membrane permeabilization). Membrane polarization beyond certain threshold voltages leads to a steep conductance increase either indicating field-induced pore formation or being due to patch clamp artifacts (seal resistance breakdown). Protoplasts derived from tobacco culture cell lines (Bright Yellow-2, BY-2; Virginia bright Italian-0, VBI-0) were stained with the voltage-sensitive dye ANNINE-6. After establishing the whole cell patch clamp configuration 50-ms command voltage (Ucomm) steps ranging from -500 mV to +500 mV were applied while simultaneously exposing protoplasts to light at 475 nm wavelength. Pulse-induced currents and fluorescence intensity (known to be linearly related to the trans-membrane voltage, Um) were recorded. Plotting fluorescence intensity against Ucomm revealed saturation of the curve at values<-300 mV and >+300 mV and close correlation with theoretical Um values calculated on the basis of membrane pore formation. For BY-2 and VBI-0 protoplasts ANNINE-6 voltage sensitivity was calculated to be -0.0014 mV(-1) and -0.0012 mV(-1), respectively. Voltage ramp experiments revealed cation-selectivity of field-induced pores. Anions are conducted poorly independent of their size. In conclusion, the patch clamp technique is validated as a useful tool in electroporation research.

Hyperpolarization of the membrane potential in cardiomyocyte tissue slices by the synchronization modulation electric field

Our previous studies have shown that a specially designed, so-called synchronization modulation electric field can entrain active transporter Na/K pumps in the cell membrane. This approach was previously developed in a study of single cells using a voltage clamp to monitor the pump currents. We are now expanding our study from isolated single cells to aggregated cells in a 3-dimensional cell matrix, through the use of a tissue slice from the rat heart. The slice is about 150 μm in thickness, meaning the slices contain many cell layers, resulting in a simplified 3-dimensional system. A fluorescent probe was used to identify the membrane potential and the ionic concentration gradients across the cell membrane. In spite of intrinsic cell-to-cell interactions and the difficulty in stimulating cell aggregation in the tissue slice, the oscillating electric field increased the intracellular fluorescent intensity, indicating elevation of the cell ionic concentration and hyperpolarization of the cell membrane. Blockage of these changes by ouabain confirmed that the results are directly related to Na/K pumps. These results along with the backward modulation indicate that the synchronization modulation electric field can influence the Na/K pumps in tissue cells of a 3-dimensional matrix and therefore hyperpolarize the cell membrane.

Ionic mechanism of shock-induced arrhythmias: role of intracellular calcium

BACKGROUND

Strong electrical shocks can cause focal arrhythmias, the mechanism of which is not well known. Strong shocks have been shown to produce diastolic Ca(i)(2+) increase, which may initiate focal arrhythmias via spontaneous Ca(i)(2+) rise (SCR), activation of inward Na(+)/Ca(2+) exchange current (I(NCX)), and rise in membrane potential (V(m)). It can be hypothesized that this mechanism is responsible for generation of shock-induced arrhythmias.

OBJECTIVE

The purpose of this study was to examine the roles of SCRs and I(NCX) in shock-induced arrhythmias.

METHODS

The occurrence of SCRs during shock-induced arrhythmias was assessed in neonatal rat myocyte cultures.

RESULTS

Simultaneous V(m)-Ca(i)(2+) optical mapping at arrhythmia source demonstrated that V(m) upstrokes always preceded Ca(i)(2+) transients, and V(m)-Ca(i)(2+) delays were not different between arrhythmic and paced beats (5.5 ± 0.9 and 5.7 ± 0.4 ms, respectively, P = .5). Shocks caused gradual rise of diastolic Ca(i)(2+) consistent with membrane electroporation but no significant Ca(i)(2+) rises immediately before V(m) upstrokes. Application of the Ca(i)(2+) chelator BAPTA-AM (10 μmol/L) decreased the duration of shock-induced arrhythmias whereas application of the I(NCX) inhibitor KB-R7943 (2 μmol/L) increased it, indicating that, despite the absence of SCRs, changes in Ca(i)(2+) affected arrhythmias. It is hypothesized that this effect is mediated by Ca(i)(2+) inhibition of outward I(K1) current and destabilization of resting V(m). The possible role of I(K1) was supported by application of the I(K1) inhibitor BaCl(2) (0.2 mmol/L), which increased the arrhythmia duration.

CONCLUSION

Shock-induced arrhythmias in neonatal rat myocyte monolayers are not caused by SCRs and inward I(NCX). However, these arrhythmias depend on Ca(i)(2+) changes, possibly via Ca(i)(2+)-dependent modulation of outward I(K1) current.

A patch clamp study on the electro-permeabilization of higher plant cells: Supra-physiological voltages induce a high-conductance, K+ selective state of the plasma membrane

Permeabilization of biological membranes by pulsed electric fields ("electroporation") is frequently used as a tool in biotechnology. However, the electrical properties of cellular membranes at supra-physiological voltages are still a topic of intensive research efforts. Here, the patch clamp technique in the whole cell and the outside out configuration was employed to monitor current-voltage relations of protoplasts derived from the tobacco culture cell line "Bright yellow-2". Cells were exposed to a sequence of voltage pulses including supra-physiological voltages. A transition from a low-conductance (~0.1 nS/pF) to a high-conductance state (~5 nS/pF) was observed when the membrane was either hyperpolarized or depolarized beyond threshold values of around -250 to -300 mV and +200 to +250 mV, respectively. Current-voltage curves obtained with ramp protocols revealed that the electro-permeabilized membrane was 5-10 times more permeable to K+ than to gluconate. The K+ channel blocker tetraethylammonium (25 mM) did not affect currents elicited by 10 ms-pulses, suggesting that the electro-permeabilization was not caused by a non-physiological activation of K+ channels. Supra-physiological voltage pulses even reduced "regular" K+ channel activity, probably due to an increase of cytosolic Ca2+ that is known to inhibit outward-rectifying K+ channels in Bright yellow-2 cells. Our data are consistent with a reversible formation of aqueous membrane pores at supra-physiological voltages.

Electroporation-induced inward current in voltage-clamped guinea pig ventricular myocytes

Electroporation induced by high-strength electrical fields has long been used to investigate membrane properties and facilitate transmembrane delivery of molecules and genes for research and clinical purposes. In the heart, electric field-induced passage of ions through electropores is a factor in defibrillation and postshock dysfunction. Voltage-clamp pulses can also induce electroporation, as exemplified by findings in earlier studies on rabbit ventricular myocytes: Long hyperpolarizations to ≤-110 mV induced influx of marker ethidium and irregular inward currents that were as large with external NMDG(+) as Na(+). In the present study, guinea pig ventricular myocytes were bathed with NMDG(+), Na(+) or NMDG(+) + La(3+) solution (36°C) and treated with five channel blockers. Hyperpolarization of myocytes in NMDG(+) solution elicited an irregular inward current (I (ep)) that reversed at -21.5 ± 1.5 mV. In myocytes hyperpolarized with 200-ms steps every 30 s, I (ep) occurred in "episodes" that lasted for one to four steps. Boltzmann fits to data on the incidence of I (ep) per experiment indicate 50% incidence at -129.7 ± 1.4 mV (Na(+)) and -146.3 ± 1.6 mV (NMDG(+)) (slopes ≈-7.5 mV). I (ep) amplitude increased with negative voltage and was larger with Na(+) than NMDG(+) (e.g., -2.83 ± 0.34 vs. -1.40 ± 0.22 nA at -190 mV). La(3+) (0.2 mM) shortened episodes, shifted 50% incidence by -35 mV and decreased amplitude, suggesting that it inhibits opening/promotes closing of electropores. We compare our findings with earlier ones, especially in regard to electropore selectivity. In the Appendix, relative permeabilities and modified excluded-area theory are used to derive estimates of electropore diameters consistent with reversal potential -21.5 mV.

Semi-loose seal Neurobiotin electroporation for combined structural and functional analysis of neurons

Intracellular sharp-electrode, whole-cell patch clamp and juxtacellular labeling methods have previously been developed for combined analysis of neuronal structure and function. We describe a novel electroporation technique for labeling neurons with Neurobiotin, using patch electrodes in a semi-loose seal configuration (R = 100-300 MOmega) with very small amplitude pulses (50 mV). The addition of 2% Neurobiotin to the intracellular solution in the patch electrode reduces the dielectric membrane breakdown voltage threshold by about threefold. The resulting pore formation allows for (1) the stable recording of spontaneous and light-evoked postsynaptic potentials without significant cytoplasmic washout and (2) the passage of dye without spillover. The efficiency and reliability of the method makes it particularly suitable for the serial recording and labeling of multiple neurons in a small area of tissue.

Cellular recovery from electroporation using synchronisation modulation as a rescue model for electrically injured cells

Electroporation of the plasma membrane resulting in a decrement in transmembrane potential is offered as a model in the study of the rescuing effects of the synchronisation modulation technique by electrically activating sodium potassium adenosine triphosphatase. Living cells were first electrically damaged by a pulsed intensive electric field, resulting in cell membrane electroporation, ion leakages and membrane potential depolarisation. Their recovery rate in natural conditions was compared with that of cells in a synchronisation modulation electric field. Fluorescence readings were taken using confocal microscopy and a potentiometric dye. Significantly more rapid recovery was observed after synchronisation modulation, with cell membranes actually polarised to levels higher than the original resting potential, a feature never seen in naturally recovering cells.

Atria are more susceptible to electroporation than ventricles: implications for atrial stunning, shock-induced arrhythmia and defibrillation failure

BACKGROUND

Defibrillation shock is known to induce atrial stunning, which is electrical and mechanical dysfunction.

OBJECTIVE

We hypothesized that atrial stunning is caused by higher atrial susceptibility to electroporation vs ventricles. We also hypothesize that electroporation may be responsible for early recurrence of atrial fibrillation.

METHODS

We investigated electroporation induced by 10-ms epicardial high-intensity shocks applied locally in atria and ventricles of Langendorff-perfused rabbit hearts (n = 12) using optical mapping.

RESULTS

Electroporation was centered at the electrode and was evident from transient diastolic depolarization and reduction of action potential amplitude and maximum upstroke derivative. Electroporation was voltage-dependent and polarity-dependent and was significantly more pronounced in the atria vs ventricles (P <.01), with a summary 50% of Effective Dose (ED50) for main measured parameters of 9.2 +/- 3.6 V/cm and 13.6 +/- 3.2 V/cm in the atria vs 37.4 +/- 1.5 V/cm and 48.4 +/- 2.8 V/cm in the ventricles, for anodal and cathodal stimuli, respectively. In atria (n = 5), shocks of both polarities (27.2 +/- 1.1 V/cm) transiently induced conduction block and reentry around the inexcitable area. Electroporation-induced ectopic activity was a possible trigger for reentry. However, in the thicker ventricles, electroporation and resulting conduction slowing and block were restricted to the surface only, preventing complete block and arrhythmia. The upstroke morphology revealed that the wave front dived below the electroporated region and resurfaced into unaffected epicardial tissue.

CONCLUSION

We showed that the atria are more vulnerable to electroporation and resulting block and arrhythmia than the ventricles.

Effect of electroporation on cardiac electrophysiology

Defibrillation shocks are commonly used to terminate life-threatening arrhythmias. According to the excitation theory of defibrillation, such shocks are aimed at depolarizing the membranes of most cardiac cells, resulting in resynchronization of electrical activity in the heart. If shock-induced transmembrane potentials are large enough, they can cause transient tissue damage due to electroporation. In this review, evidence is presented that electroporation of the heart tissue can occur during clinically relevant intensities of the external electrical field and that electroporation can affect the outcome of defibrillation therapy, being both pro- and antiarrhythmic.Here, we present experimental evidence for electroporation in cardiac tissue, which occurs above a threshold of 25 V/cm as evident from propidium iodide uptake, transient diastolic depolarization, and reductions of action potential amplitude and its derivative. These electrophysiological changes can induce tachyarrhythmia, due to conduction block and possibly triggered activity; however, our findings provide the foundation for future design of effective methods to deliver genes and drugs to cardiac tissues, while avoiding possible side effects such as arrhythmia and mechanical stunning.

Electric field-induced changes in membrane proteins charge movement currents

Our previous study showed that thermal effects induced by Joule heating did not play the pivotal role in damage of membrane proteins when cell membranes were shocked by a pulsed membrane potential up to 500 mV. Our analytical study of ion channel currents further indicated that a brief electric shock may cause protein conformational damage in the channel gating system, resulting in a reduction in the number of limiting gating charge particles. In this paper, we present the results of our study into electric shock-induced changes in the intramembrane charge movement currents. We found that a brief electric shock may significantly alter the characteristics of the charge movement currents of the membrane proteins, including reducing the magnitudes of two components Q(beta) and Q(gamma), broadening the hump shape of Q(gamma), and increasing its time delay. This study suggests that a brief intensive electric shock may cause proteins to structurally alter, reducing the amount of movable charge particles and therefore decreasing the protein functions. These results indicate that electro-coupled structural damage in membrane proteins is an important mechanism involved in electrical injury, especially in a field range not sufficient to cause thermal damage.

On the effect of prestin on the electrical breakdown of cell membranes

The voltage-dependent activity of prestin, the outer hair cell (OHC) motor protein essential for its electromotility, enhances the mammalian inner ear's auditory sensitivity. We investigated the effect of prestin's activity on the plasma membrane's (PM) susceptibility to electroporation (EP) via cell-attached patch-clamping. Guinea pig OHCs, TSA201 cells, and prestin-transfected TSA cells were subjected to incremental 50 mus and/or 50 ms voltage pulse trains, or ramps, at rates from 10 V/s to 1 kV/s, to a maximum transmembrane potential of +/-1000 mV. EP was determined by an increase in capacitance to whole-cell levels. OHCs were probed at the prestin-rich lateral PM or prestin-devoid basal portion; TSA cells were patched at random points. OHCs were consistently electroporated with 50 ms pulses, with significant resistance to depolarizing pulses. Although EP rarely occurred with 50 mus pulses, prior stimulation with this protocol had a significant effect on the sensitivity to EP with 50 ms pulses, regardless of polarity or PM domain. Consistent with these results, resistance to EP with depolarizing 10-V/s ramps was also found. Our findings with TSA cells were comparable, showing resistance to EP with both depolarizing 50-ms pulses and 10 V/s ramps. We conclude prestin significantly affects susceptibility to EP, possibly via known biophysical influences on specific membrane capacitance and/or membrane stiffness.

Supra-physiological membrane potential induced conformational changes in K+ channel conducting system of skeletal muscle fibers

The effects of a supra-physiological membrane potential shock on the conducting system of the delayed rectifier K(+) channels in the skeletal muscle fibers of frogs were studied. An improved double Vaseline gap voltage clamp technique was used to deliver stimulation pulses and to measure changes in the channel currents. Our results showed that a single 4 ms, -400 mV pulsed shock can cause a reduction in the K(+) channel conductance and a negative-shift of the channel open-threshold. Following the Boltzmann theory of channel voltage-dependence, we analyzed the shock-induced changes in the channel open-probability by employing both two-state and multi-state models. The results indicate a reduction in the number of channel gating particles after the electric shock, which imply possible conformational changes at domains that gate the channels proteins. This study provides further evidence supporting our hypothesis that high intensity electric fields can cause conformational changes in membrane proteins, most likely in the channel gating system. These structural changes in membrane proteins, and therefore their dysfunctions, may be involved in the mechanisms underlying electrical injury.

Hyperpolarization and lysophosphatidylcholine induce inward currents and ethidium fluorescence in rabbit ventricular myocytes

Strong electric pulses produce reversible or irreversible membrane breakdown (electroporation). We analysed the permeation properties of minute pores caused by hyperpolarization or lysophosphatidylcholine (LPC) by comparing the amount of charge carried by irregular inward currents (I(hi)) with changes in ethidium bromide (EB) fluorescence in isolated rabbit ventricular myocytes. Forty-second negative pulses from a holding potential of -20 mV induced I(hi) whose conductance increased with hyperpolarization; the mean conductance (G(hi)) was 63.6 +/- 9.9 pS pF(-1) (mean +/- S.E.M., n = 9) at -160 mV. EB fluorescence increased during voltage pulses in parallel with the time integral of I(hi) (Q(hi)), with the magnitude of the increases in nuclear EB fluorescence being 5.3 times greater than in the cytoplasm at -160 mV. Similar hyperpolarization-induced parallel increases in I(hi) and EB fluorescence were also obtained in Na(+)-free, N-methyl-D-glucamine (NMDG) solution. LPC (10 microM) induced large (101.2 +/- 21.2 pS pF(-1), n = 16), rapid (rise times, 1-10 ms) I(hi) with slow relaxation rates at -80 mV that reflected increases in G(hi) to 94.3 +/- 24.8 pS pF(-1) (n = 8) at 6 min. Plots of EB fluorescence vs. Q(hi) were well fitted by a common Hill's equation with a Hill coefficient of 0.97. Taken together, our findings indicate that hyperpolarization and LPC produced pores having the same filter properties for the permeation of small ions, including ethidium(+), and that I(hi) (carried in part by Ca(2+)) generated by membrane breakdown are capable of supplying sufficient ions to evoke abnormal excitation and contraction in cardiac myocytes.

Artifacts, assumptions, and ambiguity: Pitfalls in comparing experimental results to numerical simulations when studying electrical stimulation of the heart

Insidious experimental artifacts and invalid theoretical assumptions complicate the comparison of numerical predictions and observed data. Such difficulties are particularly troublesome when studying electrical stimulation of the heart. During unipolar stimulation of cardiac tissue, the artifacts include nonlinearity of membrane dyes, optical signals blocked by the stimulating electrode, averaging of optical signals with depth, lateral averaging of optical signals, limitations of the current source, and the use of excitation-contraction uncouplers. The assumptions involve electroporation, membrane models, electrode size, the perfusing bath, incorrect model parameters, the applicability of a continuum model, and tissue damage. Comparisons of theory and experiment during far-field stimulation are limited by many of these same factors, plus artifacts from plunge and epicardial recording electrodes and assumptions about the fiber angle at an insulating boundary. These pitfalls must be overcome in order to understand quantitatively how the heart responds to an electrical stimulus. (c) 2002 American Institute of Physics.

A dynamic action potential model analysis of shock-induced aftereffects in ventricular muscle by reversible breakdown of cell membrane

To elucidate the subcellular mechanism underlying the aftereffects of high-intensity dc shocks, a small pore, which mimics reversible breakdown of the cell membrane (electroporation), was incorporated into the phase-2 Luo-Rudy (L-R) model of ventricular action potentials. The pore size was set to occupy 0.15%-4.25% of the total cell membrane during the 10-ms shock. The pore was assumed to decrease after the shock exponentially with a time constant of 100-1,400 ms to simulate resealing process. In normal myocytes, the pore formation results in a delay of repolarization of the shocked action potential, which is followed by prolonged depolarization and oscillation of membrane potential like early afterdepolarization (EAD). Time- and voltage-dependent changes in the delayed rectifier K+ currents (IKr, IKs) in combination with those of L-type Ca2+ current (ICa,(L)) and ion flux through the pore (I(pore)) are responsible for the potential changes. Spontaneous excitation from the oscillation depends on activation of ICa,(L). In myocytes overloaded with Na+ and Ca2+ secondary to 90% inhibition of Na+-K+ pump, the pore formation results in a delay of repolarization of the shocked action potential, which is followed by slower cyclic depolarization in response to spontaneous release of Ca2+ from the sarcoplasmic reticulum (SR). This delayed afterdepolarization-type oscillation is abolished by complete block of Ca2+ release from the SR. These findings suggest that high-intensity electric field application will cause arrhythmogenic responses through a transient rupture of sarcolemma with different subcellular events in ventricular cells under normal and pathological conditions.

Characterization of single-cell electroporation by using patch-clamp and fluorescence microscopy

Electroporation of single NG108-15 cells with carbon-fiber microelectrodes was characterized by patch-clamp recordings and fluorescence microscopy. To minimize adverse capacitive charging effects, the patch-clamp pipette was sealed on the cell at a 90(o) angle with respect to the microelectrodes where the applied potential reaches a minimum. From transmembrane current responses, we determined the electric field strengths necessary for ion-permeable pore formation and investigated the kinetics of pore opening and closing as well as pore open times. From both patch-clamp and fluorescence microscopy experiments, the threshold transmembrane potentials for dielectric breakdown of NG108-15 cells, using 1-ms rectangular waveform pulses, was approximately 250 mV. The electroporation pulse preceded pore formation, and analyte entry into the cells was dictated by concentration, and membrane resting potential driving forces. By stepwise moving a cell out of the focused field while measuring the transmembrane current response during a supramaximal pulse, we show that cells at a distance of approximately 30 microm from the focused field were not permeabilized.

Nonlinear cell response to strong electric fields

The response of living cells to externally applied electric fields is of widespread interest. In particular, the intensification of electric fields across cell membranes is believed to be responsible, through membrane rupture and reversible membrane breakdown processes, for certain types of tissue damage in electrical trauma cases which cannot be attributed to Joule heating. Large elongated cells such as skeletal muscle fibres are particularly vulnerable to such damage. Previous theoretical studies of field intensification across cell membranes in such cells have assumed the membrane current to be linear in the applied field (Ohmic membrane conductivity) and were limited to sinusoidal applied fields. In this paper, we investigate a simple model of a long cylindrical cell, corresponding to nerve or skeletal muscle cells. Employing the electroquasistatic approximation, a system of coupled first-order differential equations for the membrane electric field is derived which incorporates arbitrary time dependence in the external field and nonlinear membrane response (non-Ohmic conductivity). The behaviour of this model is investigated for a variety of applied fields in both the linear and highly nonlinear regimes. We find that peak membrane fields predicted by the nonlinear model are approximately twice as intense, for low-frequency electrical trauma conditions, as those of the linear theory.

Theoretical modeling of the effects of shock duration, frequency, and strength on the degree of electroporation

Electroporation is becoming an increasingly important tool for introducing biologically active compounds into living cells, yet the effectiveness of this technique can be low, particularly in vivo. One way to improve the success rate is to optimize the shock protocols, but experimental studies are costly, time consuming, and yield only an indirect measurement of pore creation. Alternatively, this study models electroporation in two geometries, a space-clamped membrane and a single cell, and investigates the effects of pulse duration, frequency, shape, and strength. The creation of pores is described by a first order differential equation derived from the Smoluchowski equation. Both the membrane and the cell are exposed to monophasic and biphasic shocks of varying duration (membrane, 10 micros-100 s; cell, 0.1 micros-200 ms) and to trains of monophasic and biphasic pulses of varying frequency (membrane, 50 Hz-4 kHz; cell, 200 kHz-6 MHz). The effectiveness of each shock is measured by the fractional pore area (FPA). The results indicate that FPA is sensitive to shock duration only in a very narrow range (membrane, approximately 1 ms; cell, approximately 0.25 micros). In contrast, FPA is sensitive to shock strength and frequency of the pulse train, increasing linearly with shock strength and decreasing slowly with frequency. In all cases, monophasic shocks were at least as effective as biphasic shocks, implying that varying the strength and frequency of a monophasic pulse train is the most effective way to control the creation of pores.

Regional differences in arrhythmogenic aftereffects of high intensity DC stimulation in the ventricles

Regional differences of the aftereffects of high intensity DC stimulation were investigated in isolated rabbit hearts stained with a voltage-sensitive dye (di-4-ANEPPS). Optical action potential signals were recorded from the epicardial surface of the right and left ventricular free wall (RVep, LVep) and from the right endocardial surface of the interventricular septum (IVS). Ten-millisecond monophasic DC stimulation (S2, 20-120 V) was applied to the signal recording spots during the early plateau phase of the action potential induced by basic stimuli (S1, 2.5 Hz). There was a linear relationship between S2 voltage and the S2 field intensity (FI). S2 caused postshock additional depolarization, giving rise to a prolongation of the shocked action potential. With S2 > or = 40 V (FI > or = approximately 20 V/cm), terminal repolarization of action potential was inhibited, and subsequent postshock S1 action potentials for 1-5 minutes were characterized by a decrease in the maximum diastolic potential and a decrease in the amplitude and a slowing of their upstroke phase. The higher the S2 voltage, the larger the aftereffects. The changes in postshock action potential configuration in RVep were significantly greater than those observed in LVep and IVS when compared at the same levels of S2 intensity. In RVep, 12 of 20 shocks of 120 V resulted in a prolonged refractoriness to S1 (> 1 s), and the arrest was often followed by oscillation of membrane potential. Ventricular tachycardia or fibrillation ensued from the oscillation in five cases. No such long arrest or serious arrhythmias were elicited in LVep and IVS. These results suggest that RVep is more susceptible than LVep and IVS for arrhythmogenic aftereffects of high intensity DC stimulation.

Inhibition of Na,K-ATPase by external electrical cardioversion in a sheep model of atrial fibrillation

INTRODUCTION

Electrical external cardioversion commonly used to treat atrial fibrillation (AF) is associated with myocardial membrane damage and disturbances in ionic homeostasis (hemodynamically unstable). The present study was designed to investigate whether alterations in ionic homeostasis observed were due in part to changes in the myocardial activity of Na,K-ATPase.

METHODS AND RESULTS

AF was induced by pacing in ten anesthetized sheep divided into two groups. Group I (n = 4) received a single external countershock of 360 J after three episodes of AF lasting 10 minutes. Group II (n = 6) served as controls. Activity, responsiveness to ouabain, and membrane expression of catalytic alpha and beta subunits of Na,K-ATPase in sarcolemmal myocardial membrane fractions were investigated. Membrane fluidity and fatty acid composition, and plasma levels of atrial natriuretic factor (ANF) also were measured. One shock after episodes of AF significantly decreased ventricular Na,K-ATPase activity up to 50% (P < 0.001) without modification of atrial activity at the membrane level. Sites with low affinity to ouabain showed a fivefold lower affinity for ouabain in the cardioversion group than in the control group (IC50 = 7.9 micromol/L vs 40 micromol/L ouabain, P < 0.05). Plasma levels of ANF were significantly increased in the cardioversion group compared with the control group. These changes were independent of membrane modulation in terms of expression of Na,K-ATPase, membrane fluidity, and fatty acid composition.

CONCLUSION

This study suggests that left ventricular perturbation of ionic homeostasis subsequent to transthoracic cardioversion could result from inactivation of Na,K-ATPase activity.

Arrhythmogenic changes in action potential configuration in the ventricle induced by DC shocks

Failure of defibrillation by direct current (DC) shocks is the result in part of new ventricular tachyarrhythmias induced by the shocks. We investigated the arrhythmogenic substrate produced by the shocks. Fluorescent action potential (AP) signals were recorded from rabbit hearts perfused in vitro with the use of our original optical recording system. Localized application of 10-ms shocks (S2) during the plateau phase of APs by basic stimuli (S1) caused field intensity (FI)-dependent changes in APs: (a) S2 > 7 V/cm caused additional depolarization, giving rise to a prolongation of AP duration (APD); (b) With S2 > 20 V/cm, terminal repolarization was inhibited, and subsequent postshock S1 APs for 1 to 5 min were characterized by decreases in the maximum diastolic potential and amplitude of APs; and (c) S2 > 30 V/cm often resulted in a prolonged refractoriness, oscillation of membrane potential leading to ventricular tachycardia or fibrillation (VT/VF). The right ventricle was more susceptible than other regions for the aftereffects of high-intensity shocks. Using an 8-channel recording system, we compared the effect of 10-ms monophasic (M) and 5/5-ms biphasic (B) shocks applied to the whole ventricles with FI of 1 to 20 V/cm at the signal recording sites. B shocks were less potent than M shocks in the FI-dependent action potential duration (APD) prolongation, and in the shock-induced enhancement of APD dispersion. Incidence and duration of VT/VF induced by M shocks were significantly greater than those by B shocks. These findings suggest that DC shocks will cause two types of arrhythmogenic substrate: one induced at sites of high FI, and the other at sites with moderate FI. The former would produce local block or focal repetitive excitation due to prolonged depolarization and oscillation of membrane potential, and the latter circuitous movement of wavefronts through an enhancement of spatial inhomogeneity of repolarization.

Biological effects of electric shock and heat denaturation and oxidation of molecules, membranes, and cellular functions

Direct exposure of cells in suspension to intense electric pulses is known to produce damages to cell membranes and supramolecular organizations of cells, and denaturation of macromolecules, much like injuries and tears seen in electric trauma patients. Thus, the system has been used as a laboratory model for investigating the biochemical basis of electric injury. An intense electric pulse can produce two major effects on cells--one caused by the field, or the electric potential, and the other by current, or the electric energy. The field-induced transmembrane potential can produce electro-conformational changes of ion channels and ion pumps and, when the potential exceeds the dielectric strength of the cell membrane (approximately 500 mV for a pulse width of a few ms), electro-conformational damages and electroporations of membrane proteins and lipid bilayers. These events lead to passage of electric current through the membrane-porated cells and to heating of cell membranes and cytoplasmic contents. The subsequent denaturation of cell membranes and cytoplasmic macromolecules brings about many complex biochemical reactions, including oxidation of proteins and lipids. The combined effects may cripple the cells beyond repair. This communication will focus on the thermal effects of electric shock. After a brief review of the current state of knowledge on thermal denaturation of soluble enzymes and muscle proteins, this paper will describe experiments on the thermal denaturation of cellular components and functions, such as nucleosomes, and the electron transport chain and ATP synthetic enzymes of the mitochondrial inner membranes. Data will show that lipid peroxidation and the subsequent loss of the energy-transducing ability of the cells may occur even at moderate temperatures between 40 degrees C and 45 degrees C. However, lipid peroxidation may be prevented with reducing reagents such as mercaptoethanol, dithiothreitol, and ascorbic acid. Reactivation of denatured cellular proteins and functions may also be possible and a strategy for doing so is discussed.

Dynamics of membrane sealing in transient electropermeabilization of skeletal muscle membranes

Large supraphysiologic transmembrane electrical potentials are known to alter the molecular organization of the bilayer lipid component of cell membranes, leading to ionic permeabilization or "electroporation". Typically, membrane electroporation is followed by several orders of magnitude increases in electrical conductance and diffusive permeability to low-molecular-weight solutes. Electroporation may be transient or stable depending on whether the membrane eventually seals or remains permeabilized. Factors that control sealing have not been well characterized. This paper describes the kinetics of membrane sealing following electroporation by pulses over a range of supraphysiologic potentials. The increase in membrane conductance is highly nonlinear during a -440-mV, 4-ms pulse and reaches two orders of magnitude greater than baseline. Electroporation and relaxation sealing kinetics are quite different, reflecting a significant hysteresis effect. Thus, it appears that the magnitude and duration of the field pulse are important factors in sealing.

Changes in skin structure and electrical properties following high voltage exposure

Human skin experiences extensive alterations when exposed to a strong electric field. In high voltage electric shocks, tissues in the current path undergo large-scale changes; the stratum corneum (SC), the outermost layer of the skin, loses its barrier function against ionic transport. The primary mechanism of electric field interaction with the skin is the creation of aqueous pathways, which increase the permeability of the skin to ions and macromolecules. Changes in skin structure and electrical properties were characterized by measuring the transport of fluorescent molecules in an in vitro preparation of human cadaver skin in a permeation chamber. Application of high voltage pulses (Uskia > 150 V, tau pulse = 1 ms) decreased the skin resistance per unit area by three orders of magnitude from its initial value of 100 k omega-cm-2. The resistance reached a quasi-steady state after about 20 pulses. After the cessation of pulsing, Rskin recovered immediately to about 4 k omega-cm-2 and remained low for 2 h thereafter. Transport of fluorescent molecules by high voltage pulses occurs in highly localized transport regions (LTRs), with diameters not exceeding 100 microns for short pulses (tau pulse < 5 ms).

Anode/cathode make and break phenomena in a model of defibrillation

The goal of this simulation study is to examine, in a sheet of myocardium, the contribution of anode and cathode break phenomena in terminating a spiral wave reentry by the defibrillation shock. The tissue is represented as a homogeneous bidomain with unequal anisotropy ratios. Two case studies are presented in this article: tissue that can electroporate at high levels of transmembrane potential, and model tissue that does not support electroporation. In both cases, the spiral wave is initiated via cross-field stimulation of the bidomain sheet. The extracellular defibrillation shock is delivered via two small electrodes located at opposite tissue boundaries. Modifications in the active membrane kinetics enable the delivery of high-strength defibrillation shocks. Numerical solutions are obtained using an efficient semi-implicit predictor-corrector scheme that allows one to execute the simulations within reasonable time. The simulation results demonstrate that anode and/or cathode break excitations contribute significantly to the activity during and after the shock. For a successful defibrillation shock, the virtual electrodes and the break excitations restrict the spiral wave and render the tissue refractory so it cannot further maintain the reentry. The results also indicate that electroporation alters the anode/cathode break phenomena, the major impact being on the timing of the cathode-break excitations. Thus, electroporation results in different patterns of transmembrane potential distribution after the shock. This difference in patterns may or may not result in change of the outcome of the shock.

Electropermeabilization of cell membranes

A position dependent modulation of the membrane potential difference is induced when an electric field is applied to a cell. When cells are submitted to short lived electric field pulses with an overcritical intensity, a local membrane alteration is induced, which may reseal. Its molecular definition remains unknown. A free exchange of hydrophilic molecules takes place across the membrane. A leakage of cytosolic metabolites is present. However, a loading of polar drugs into the cytoplasm is obtained. A short description of the processes affecting the cell membrane organization is given. Lipids appear as the primary target of the field effect as in the case of liposomes. Nevertheless membrane proteins appear to be affected by a direct or by a back effect. The permeabilized state is long lived. The cell metabolism plays indeed a critical role in the recovery. The cell viability can be nevertheless preserved.

Low K+-induced hyperpolarizations trigger transient depolarizations and action potentials in rabbit ventricular myocytes

1. The effects of large reductions of [K+]o on membrane potential were studied in isolated rabbit ventricular myocytes using the whole-cell patch clamp technique. 2. Decreasing [K+]o from the normal level of 5.4 mM to 0.1 mM increased resting membrane potential (Vrest) from -75.6 +/- 0.3 to -140.3 +/- 1.9 mV (means +/- s.e.m; n = 127), induced irregular, transient depolarizations with mean maximal amplitudes of 19.5 +/- 1.5 mV and elicited action potentials in 56.7 % of trials. The action potentials exhibited overshoots of 37.9 +/- 1.5 mV (n = 72) and sustained plateaux. 3. Addition of 0.1 mM La3+ in the presence of 0.1 mM [K+]o significantly increased Vrest but decreased the amplitude of transient depolarizations and suppressed the firing of action potentials. 4. Replacement of external Na+ or Cl- with N-methyl-D-glucamine or aspartate, respectively, or internal dialysis with 10 mM EGTA or BAPTA had little effect on low [K+]o-induced membrane potential changes. 5. Hyperpolarizing voltage clamp pulses to potentials between -110 and -200 mV activated irregular inward currents that increased in amplitude and frequency with increasing hyperpolarization and were depressed by 0.1 mM La3+. 6. The generation of transient depolarizations by low [K+]o can be explained as being a consequence of decreasing the inward rectifier K+ current (IK1) and the appearance of inward currents reflecting electroporation resulting from strong electric fields across the membrane.

Non-linear microscale alterations in membrane transport by electropermeabilization

The purpose of this study was to quantify the changes in cell membrane conductance in response to electropermeabilization, which may elucidate the mechanisms of tissue injury resulting from high-voltage electrical shock. A high-speed, space-clamp and voltage-clamp experimental configuration was used. The pulse parameters of an imposed transmembrane potential that are instrumental in membrane properties alteration were precisely controlled. The dynamics of the non-linear electroporation response was characterized.

Electric field-induced functional reductions in the K+ channels mainly resulted from supramembrane potential-mediated electroconformational changes

The goal of this study is to distinguish the supramembrane potential difference-induced electroconformational changes from the huge transmembrane current-induced thermal damages in the delayed rectifier K+ channels. A double Vaseline-gap voltage clamp was used to deliver shock pulses and to monitor the channel currents. Three pairs of 4-ms shock pulses were used to mimic the electric shock by a power-line frequency electric field. Each pair consists of two pulses with the same magnitude, starting from 350 to 500 mV, but with opposite polarities. The shock pulse-generated transmembrane ion flux and the responding electric energy, the Joule heating, consumed in the cell membrane, as well as the effects on the K+ channel currents, were obtained. Results showed that huge transmembrane currents are not necessary to cause damages in the K+ channel proteins. In contrast, reductions in the K+ channel currents are directly related to the field-induced supramembrane potential differences. By a comparison with the shock field-induced Joule heating effects on cell membranes, the field-induced supramembrane potential difference plays a dominant role in damaging the K+ channels, resulting in electroconformational changes in the membrane proteins. In contrast, the shock field-induced huge transmembrane currents, therefore the thermal effects, play a secondary, trivial role.

The breakdown of cell membranes by electrical and mechanical stress

We attempted to determine whether mechanical tension and electrical stress couple to cause membrane breakdown in cells. Using cell-attached patches from HEK293 cells, we estimated the mechanically produced tension from the applied pressure and geometry of the patch. Voltage pulses of increasing amplitude were applied until we observed a sudden increase in conductance and capacitance. For pulses of 50 micros duration, breakdown required >0.5 V and was dependent on the tension. For pulses of 50-100 ms duration, breakdown required 0.2-0.4 V and was independent of tension. Apparently two physically different processes can lead to membrane breakdown. We could explain the response to the short, high-voltage pulses if breakdown occurred in the lipid bilayer. The critical electromechanical energy per unit area for breakdown by short pulses was approximately 4 dyne/cm, in agreement with earlier results on bilayers. Our data suggest that, at least in a patch, the bilayer may hold a significant fraction (approximately 40%) of the mean tension. To be compatible with the large, nonlytic area changes of patches, the bilayer appears to be pulled toward the pipette tip, perhaps by hydrophobic forces wetting membrane proteins bound to the glass. Although breakdown voltages for long pulses were in agreement with earlier work on algae, the mechanism(s) for this breakdown remain unclear.

Progressive depolarization: a unified hypothesis for defibrillation and fibrillation induction by shocks

Experimental studies of defibrillation have burgeoned since the introduction of the upper limit of vulnerability (ULV) hypothesis for defibrillation. Much of this progress is due to the valuable work carried out in pursuit of this hypothesis. The ULV hypothesis presented a unified electrophysiologic scheme for linking the processes of defibrillation and shock-induced fibrillation. In addition to its scientific ramifications, this work also raised the possibility of simpler and safer means for clinical defibrillation threshold testing. Recent results from an optical mapping study of defibrillation suggest, however, that the experimental data supporting the ULV hypothesis could instead be interpreted in a manner consistent with traditional views of defibrillation such as the critical mass hypothesis. This review will describe the evidence calling for such a reinterpretation. In one regard the ULV hypothesis superseded the critical mass hypothesis by linking the defibrillation and shock-induced fibrillation processes. Therefore, this review also will discuss the rationale for developing a new defibrillation hypothesis. This new hypothesis, progressive depolarization, uses traditional defibrillation concepts to cover the same ground as the ULV hypothesis in mechanistically unifying defibrillation and shock-induced fibrillation. It does so in a manner consistent with experimental data supporting the ULV hypothesis but which also takes advantage of what has been learned from optical studies of defibrillation. This review will briefly describe how this new hypothesis relates to other contemporary viewpoints and related experimental results.

The relationship between temperature and calcium in acute cell damage after exposure to radiofrequency or thermal energy in isolated neonatal and adult rabbit cardiac myocytes

Radiofrequency (RF) ablation is a nonsurgical technique using catheter-directed RF energy for treating cardiac arrhythmias in children and adults. Previous reports have suggested that sequestration of calcium (Ca2+) by the sarcoplasmic reticulum may partially protect mature cardiac myocytes from the effects of RF energy. The purposes of this study were to determine whether differences exist between neonatal and adult myocyte responses to RF energy and if myocyte damage is a Ca2+-dependent process. Because immature myocardium is functionally deficient in sarcoplasmic reticulum, we hypothesized that immature myocytes would be more susceptible to damage induced by RF energy. Isolated ventricular myocytes were obtained from neonatal and adult New Zealand White rabbits by enzymatic dissociation, then placed in a perfusion chamber designed to deliver RF energy or a heated perfusate solution. Measurements of bath temperature, cell morphology, and contractile response to electrical stimuli were recorded. RF energy application associated with increased perfusate temperature resulted in cell death, but not when the temperature rise was inhibited. Thus, the acute damage to cells exposed to RF energy appears to be mediated by thermal energy. After exposure to thermal energy, neonatal cells underwent contracture at lower temperatures than did adult cells. Perfusion with solutions containing low Ca2+ concentrations, comparable to intracellular diastolic Ca2+ levels, had a protective effect for both neonatal and adult myocytes. These findings indicate that acute cell damage after exposure to RF energy is mediated by a Ca2+-dependent process. Furthermore, immature myocardium is particularly susceptible to RF-mediated cell damage, possibly secondary to reduced Ca2+ sequestration by the sarcoplasmic reticulum.

A model analysis of aftereffects of high-intensity DC stimulation on action potential of ventricular muscle

The mechanism for aftereffects of high-intensity dc stimulation on ventricular muscle was studied by using Beeler-Reuter's action potential model. A leak conductance (Gpore, maximal value from 40 to 80 microS for 1 cm2 of membrane), which mimics reversible dielectric breakdown of the cell membrane by the shock, was incorporated into the model. To simulate resealing process, Gpore was assumed to decrease after the shock exponentially at a time constant (tau pore) of 5-50 s. The simulation results are qualitatively consistent with our experimental observations in guinea pig papillary muscle [1]; they include prolonged depolarization, diastolic depolarization or oscillation of membrane potential leading to a single or multiple spontaneous excitation. The phase-independence and shock intensity-dependence can also be reproduced. Analysis of current components has revealed that: 1) a large inward leak current (Ileak) is responsible for the prolonged depolarization; 2) time-dependent decay of outward current (IX1) in combination with Ileak and slow inward current (I(s)) results in diastolic depolarization or oscillation of membrane potential; 3) spontaneous excitation depends on an activation of I(s). These findings support our hypothesis that strong shocks (> 15 V/cm) will produce abnormal arrhythmogenic responses in ventricular muscle through a transient rupture of sarcolemmal membrane.

Dose-dependent reduction of cardiac transmembrane potential by high-intensity electrical shocks

Cardiac tissue dysfunction can result from high-intensity electrical shocks and is manifested as changes in transmembrane potential (Vm). Ten-millisecond shock pulses (SPs) of varying intensity and polarity were applied to frog ventricle in diastole, and Vm was quantified directly under the stimulating electrode by an optical method using voltage-sensitive dye. As SP intensities were increased, the shock-induced action potential (AP) plateau and AP amplitude (APAs) decreased sigmoidally toward 75-85% of the control AP amplitude (APAc) and zero, respectively. APAs was shifted toward lower current densities for anodal compared with cathodal SPs (half-maximal values 185 and 238 mA/cm2, respectively; P = 0.02). Recovery of APAs was marginally significant 1 s after SP delivery (P = 0.063). The peak change in Vm during SP (across all intensity levels) was -200% APAc for anodal and +125% APAc for cathodal pulses. In conclusion, we show that SP reduces APA in a sigmoidal fashion at strengths > 10-20 x diastolic threshold and is more deleterious for anodal polarities.

Calcium dynamics in cultured heart cells exposed to defibrillator-type electric shocks

Spatial and temporal changes in intracellular calcium ion concentration and transmembrane voltage were recorded optically from single-isolated cultured chick-embryo heart cells exposed to high-voltage, defibrillator-type shocks. Fluorescence changes were measured during 5 msec electric shocks of field strengths up to 56 volts/cm in single myocytes stained with a Ca(++)-sensitive or voltage-sensitive dye. Shocks caused a reversible period of depolarization, elevated cytosolic Ca++, and refractoriness. Intracellular Ca++ elevation had two temporal phases: first, a Ca++ spike with morphology independent of shock intensity; and second, a prolonged Ca++ elevation with a shock-intensity-dependent magnitude and duration, and with greatest Ca++ elevation at the poles of the cell adjacent to the electrodes. The prolonged elevation (second phase) was initiated earlier at the anode-facing pole of the cell than at the cathode-facing pole. These results suggest that postshock Ca++ entry consists of two parts: early normal entry through excitation channels plus a prolonged elevation which may be related to cellular damage.

A mathematical model of make and break electrical stimulation of cardiac tissue by a unipolar anode or cathode

Numerical simulations of electrical stimulation of cardiac tissue using a unipolar extracellular electrode were performed. The bidomain model with unequal anisotropy ratios represented the tissue, and the Beeler-Reuter model represented the active membrane properties. Four types of excitation were considered: cathode make (CM), anode make (AM), cathode break (CB), and anode break (AB). The mechanisms of excitation were: for CM, tissue under the cathode was depolarized to threshold; for AM, tissue at a virtual cathode was depolarized to threshold; for CB, a long cathodal pulse produced a steady-state depolarization under the cathode and hyperpolarization at a virtual anode. At the end (break) of the pulse, the depolarization diffused into the hyperpolarized tissue, resulting in excitation. For AB, a long anodal pulse produced a steady-state hyperpolarization under the anode and depolarization at a virtual cathode. At the end (break) of the pulse, the depolarization diffused into the hyperpolarized tissue, resulting in excitation. For AB stimulation, decay of the hyperpolarization faster than that of the depolarization was necessary. The thresholds for rheobase and diastolic CM, AM, CB, and AB stimulation were 0.038, 0.41, 0.49, and 5.3 mA, respectively, for an electrode length of 1 mm and a surface area of 1.5 mm2. Threshold increased as the size of the electrode increased. The strength-duration curves for CM and AM were similar except when the duration was shorter than 0.2 ms, in which case the AM threshold rose more quickly with decreasing duration than did the CM threshold. CM and AM resulted in similar strength-frequency curves. The model agrees qualitatively, but (in some cases) not quantitatively, with experiments.

Changes in the passive electrical properties of human stratum corneum due to electroporation

The stratum corneum (SC) is the main barrier to molecular and ionic transport across mammalian skin and has been extensively studied by others at low voltages (U(skin)(t) < 10 V) in order to partially characterize the skin. Here we use one or more exponential pulses (tau pulse = 1 ms) and a temperature of 25 +/- 2 degrees C and found that the low voltage passive electrical properties (impedance) change rapidly and significantly if these pulse result in U(skin),0 > 40 V. In contrast, the dynamic resistance (describing passive electrical behavior in a nonlinear range) changes dramatically by application of pulses between 40 V and 80 V and then it settles at levels between 50 omega and 100 omega. We also found that recovery of the low voltage electrical parameters after pulsing depends mainly on the voltage, and, for multiple pulse protocols, on the number of pulses. For single pulses of U(skin),0 approximately 90 V or less the electrical recovery was almost complete, returning to within 0.90 of the pre-pulse value. In contrast, larger pulses result progressively in decreased recovery. The recovery for pulses > 90 V revealed several characteristic times, suggesting the involvement of different processes. For multiple pulses with U(skin),0 > 130 V almost no recovery of the transdermal resistance, R(skin), was evident (returning to < 0.10 of pre-pulse values), i.e., essentially permanent changes in the stratum corneum occurred. This is similar to that of single bilayer membrane electroporation, for which a transition from reversible to irreversible behavior occurs as transmembrane voltage is increased. Thus, these results are consistent with the hypothesis that 'high-voltage' pulses cause electroporation within the SC, i.e., that elevated transmembrane voltage result in creation of new aqueous pathways ('pores') across SC lipid regions.

Characterization of shock-induced action potential extension during acute regional ischemia in rabbit hearts

INTRODUCTION

Defibrillation shocks produce extension of the myocardial action potential repolarization time (AP extension) in nonischemic myocardium. AP extension may synchronize repolarization in the heart because the extension increases when shock timing is increased. We tested whether AP extension occurs and whether it increases when shock timing is increased in regionally ischemic isolated perfused rabbit hearts stained with the transmembrane voltage sensitive fluorescent dye, di-4-ANEPPS and given diacetyl monoxime to eliminate motion artifacts.

METHODS AND RESULTS

Before and after left anterior descending (LAD) coronary artery occlusion, APs were recorded on the anterior left ventricular epicardium with an epifluorescence measurement system. Hearts were paced with a train of 10 stimuli (S1) and then during the 10th AP were given a defibrillation shock (S2) from epicardial electrodes on either side of the recording region. Before LAD occlusion, duration of the 9th S1-induced AP measured at full repolarization was 171 +/- 11 msec (mean +/- SD). Within 15 minutes after LAD occlusion, the AP duration became shorter (P < 0.05) and more variable (137 +/- 47 msec), and APs with negligible plateaus were observed. Extension of the 10th AP by S2 was significant both before (mean extension of 59 to 65 msec for three S2 waveforms tested) and after LAD occlusion (mean extension of 35 to 41 msec). Unlike the results before LAD occlusion, AP extension after occlusion was independent of absolute shock timing expressed in msec. When timing was expressed as a fraction of individual AP durations, AP extension after occlusion increased with increases in shock timing.

CONCLUSIONS

Shocks extend APs during ischemia; however, absolute time dependence of AP extension is not constant among cells that have different AP durations during ischemia. This may influence postshock repolarization synchrony when different AP durations exist in different parts of regionally ischemic hearts.

Effects of electroporation on transmembrane potential induced by defibrillation shocks

This study uses a one-dimensional model of cardiac strand to investigate the effects of electroporation on transmembrane potential (Vm) induced by defibrillation shocks. The strand is stimulated at the ends by extracellular electrodes. Its membrane, when exposed to large Vm, increases its conductance in a manner consistent with reversible electrical breakdown. Numerical simulations indicate that Vm increases proportionally to the shock strength only until the ends of the strand electroporate. Beyond this point, further increases in shock strength result in only a minor change in Vm. This arrest in the growth of Vm is caused by pores that develop in the cells immediately adjacent to the electrodes and that shunt part of the stimulating current directly into intracellular space. Consequently, only a fraction of the delivered current, Icr, gives rise to Vm; the current in excess of Icr divides itself proportionally between intra- and extracellular space and does not contribute to macroscopic Vm. Thus, electroporation has a beneficial effect: the formation of pores prevents the development of an excessively high Vm and limits the damage to the tissue. In contrast, electroporation does not affect the "sawtooth" component of Vm that reflects polarization of individual cells by electric field. These results indicate that electroporation does not impair the ability of the shock to reach the distant myocardium and may actually aid defibrillation by reducing nonuniformity of electrical conditions between regions close to the electrodes and in the bulk of tissue.

Millisecond measurement of transport during and after an electroporation pulse

Electroporation involves the application of an electric field pulse that creates transient aqueous pathways in lipid bilayer membranes. Transport through these pathways can occur by different mechanisms during and after a pulse. To determine the time scale of transport and the mechanism(s) by which it occurs, efflux of a fluorescent molecule, calcein, across erythrocyte ghost membranes was measured with a fluorescence microscope photometer with millisecond time resolution during and after electroporation pulses several milliseconds in duration. One of four outcomes was typically observed. Ghosts were: (1) partially emptied of calcein, involving efflux primarily after the pulse; (2) completely emptied of calcein, involving efflux primarily after the pulse; (3) completely emptied of calcein, involving efflux both during and after the pulse; or (4) completely emptied of calcein, involving efflux primarily during the pulse. Partial emptying, involving significant efflux during the pulse, was generally not observed. We conclude that under some conditions transport caused by electroporation occurs predominantly by electrophoresis and/or electroosmosis during a pulse, although under other conditions transport occurs in part or almost completely by diffusion within milliseconds to seconds after a pulse.