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

Altered ion channel conductance and ionic selectivity induced by large imposed membrane potential pulse

The effects of large magnitude transmembrane potential pulses on voltage-gated Na and K channel behavior in frog skeletal muscle membrane were studied using a modified double vaseline-gap voltage clamp. The effects of electroconformational damage to ionic channels were separated from damage to lipid bilayer (electroporation). A 4 ms transmembrane potential pulse of -600 mV resulted in a reduction of both Na and K channel conductivities. The supraphysiologic pulses also reduced ionic selectivity of the K channels against Na+ ions, resulting in a depolarization of the membrane resting potential. However, TTX and TEA binding effects were unaltered. The kinetics of spontaneous reversal of the electroconformational damage of channel proteins was found to be dependent on the magnitude of imposed membrane potential pulse. These results suggest that muscle and nerve dysfunction after electrical shock may be in part caused by electroconformational damage to voltage-gated ion channels.

Electrical stimulation of cardiac tissue: a bidomain model with active membrane properties

Numerical calculations simulated the response of cardiac muscle to stimulation by electrical current. The bidomain model with unequal anisotropy ratios represented the tissue, and parallel leak and active sodium channels represented the membrane conductance. The speed of the wavefront was faster in the direction parallel to the myocardial fibers than in the direction perpendicular to them. However, for cathodal stimulation well above threshold, the wavefront originated farther from the cathode in the direction perpendicular to the myocardial fibers than in the direction parallel to them, consistent with observations of a dog-bone-shaped virtual cathode made by Wikswo et al., Circ. Res. 68:513-530, 1991. The model showed that the virtual cathode size and shape were dependent upon both membrane and tissue conductivities. Increasing the peak sodium conductance or reducing the transverse intracellular conductivity accentuated the dog-bone shape, while the opposite change caused the virtual cathode to become more elliptical, with the major axis of the ellipse transverse to the fiber direction. A cathodal stimulus created regions of hyperpolarization that slowed conduction of the wavefront propagating parallel to the fibers. An anodal stimulus evoked a wavefront with a complex shape; activation originated from two depolarized regions 1 to 2 mm from the stimulus site along the fiber direction. The threshold current strength (0.5 ms duration pulse) for a cathodal stimulus was 0.048 mA, and for an anodal stimulus was 0.67 mA. When the model was modified to simulate the effect of electropermeabilization, which may be present when the transmembrane potential reaches very large values near the stimulating electrode, our qualitative conclusions remained unchanged.(ABSTRACT TRUNCATED AT 250 WORDS)

An improved double vaseline gap voltage clamp to study electroporated skeletal muscle fibers

An improved voltage clamp with a double vaseline gap chamber was designed to study electroporated skeletal muscle fibers. The new clamp eliminated spike overshock of membrane potential when applying step stimulation occurring in the traditional configuration. It allowed greater consistency in membrane potential distribution. After the intracellular resistances of the fiber segment at the vaseline gap area were compensated, it was possible to change membrane potential more quickly. Using this technique, strong electrical pulses used to mimic the situation of electrical shock can be delivered to the cell membrane by voltage clamp. Transmembrane currents of skeletal muscle cell were simultaneously measured during a high pulsed shock and resolved into different components. Distinct transient changes of the transmembrane current, involving the time courses of the formation of electroporation and their recovery time constants, can be recorded. Because of more even membrane potential distribution and faster response to pulsed membrane potential change, this technique is also suitable for membrane study under physiological conditions.

Patch clamp studies of single intact secretory granules

The membrane of secretory granules is involved in the molecular events that cause exocytotic fusion. Several of the proteins that have been purified from the membrane of secretory granules form ion channels when they are reconstituted in lipid bilayers and, therefore, have been thought to form part of the molecular structure of the exocytotic fusion pore. We have used the patch clamp technique to study ion conductances in single isolated secretory granules from beige mouse mast cells. We found that the membrane of the intact granule had a conductance of < 50 pS. No abrupt changes in current corresponding to the opening and closing of ion channels were observed, even under conditions where exocytotic fusion occurred. However, mechanical tension or a large voltage pulse caused the breakdown of the granule membrane resulting in the abrupt opening of a pore with an ion conductance of about 1 nS that fluctuated rapidly and could expand to an immeasurably large conductance or close completely. Surprisingly, the behavior of these pores resembled the pattern of conductance changes of exocytotic fusion pores observed in degranulating beige mast cells. This similarity supports the view that the earliest fusion pore is formed upon the breakdown of a bilayer such as that formed during hemifusion.

Cell electropermeabilization: a new tool for biochemical and pharmacological studies

Cell electropermeabilization is the transient permeabilization of the plasma membrane by means of short and intense electric pulses. Under optimized conditions, electropermeabilization is compatible with cell survival. It provides a direct access into the cytosol to ions, small molecules, exogenous drugs and macromolecules. As cells remain functional, a large variety of cell biology questions can be addressed. Such 'in situ biochemistry' opens new possibilities beside the more classical studies dealing with unpermeabilized cells or subcellular extracts. Electropermeabilization also allows pharmacological studies with cells, cultured monolayers and in vivo tissues as well as the design of drug controlled-release systems.

Monitoring electropermeabilization in the plasma membrane of adherent mammalian cells

When an electrical potential of order one volt is induced across a cell membrane for a fraction of a second, temporary breakdown of ordinary membrane functions may occur. One result of such a breakdown is that molecules normally excluded by the membrane can now enter the cells. This phenomenon, generally referred to as electropermeabilization, is known as electroporation when actual pores form in the membrane. This paper presents a unique approach to the measurement of pore formation and closure in anchored mammalian cells. The cells are cultured on small gold electrodes, and by constantly monitoring the impedance of the electrode with a low-amplitude AC signal, small changes in cell morphology, cell motion, and membrane resistance can be detected. Because the active electrode is small, the application of a few volts across the cell-covered electrode causes pore formation in the cell membrane. In addition, the heat transfer is very efficient, and the cells can be porated in their regular growth medium. By this method, the formation and resealing of pores due to applied electric fields can be followed in real time for anchorage-dependent cells.

Influence of electrical axis of stimulation on excitation of cardiac muscle cells

Orthogonal sequential shock can defibrillate the heart with greater efficacy compared with single shock defibrillation. In this study we tested the hypothesis that cardiac cells have a preferred orientation in their response to excitatory extracellular electric fields, so that orthogonal shocks may stimulate distinct populations of cells. A micropaddle electrode system was used to deliver rectangular pulses for extracellular field stimulation of individual heart cells. We found that single frog and guinea pig ventricular myocytes are excitable with rectangular pulse field stimulation over a wide range of pulse durations, ranging from 10 msec to as little as 20 microseconds. The excitation field strength varies inversely with pulse duration as described by the Weiss-Lapicque equation, although the frog myocytes show a significant "notch" at pulse durations of approximately 1-2 msec, and the guinea pig myocytes are more excitable than predicted for pulse durations of less than 0.2 msec. Every myocyte tested was more excitable when the long axis of the cell was oriented parallel to the stimulating field than when perpendicular to the field. For 2-msec pulses, the difference in field strength was a factor of 5.8 +/- 2.0 (n = 30) for frog and 2.6 +/- 0.5 (n = 23) for guinea pig myocytes. Complete excitation strength-duration curves were obtained in seven frog and 14 guinea pig cells for both parallel and perpendicular cell orientations.(ABSTRACT TRUNCATED AT 250 WORDS)

Electroporation of cell membranes

Electric pulses of intensity in kilovolts per centimeter and of duration in microseconds to milliseconds cause a temporary loss of the semipermeability of cell membranes, thus leading to ion leakage, escape of metabolites, and increased uptake by cells of drugs, molecular probes, and DNA. A generally accepted term describing this phenomenon is "electroporation." Other effects of a high-intensity electric field on cell membranes include membrane fusions, bleb formation, cell lysis... etc. Electroporation and its related phenomena reflect the basic bioelectrochemistry of cell membranes and are thus important for the study of membrane structure and function. These phenomena also occur in such events as electric injury, electrocution, and cardiac procedures involving electric shocks. Electroporation has found applications in: (a) introduction of plasmids or foreign DNA into living cells for gene transfections, (b) fusion of cells to prepare heterokaryons, hybridoma, hybrid embryos... etc., (c) insertion of proteins into cell membranes, (d) improving drug delivery and hence effectiveness in chemotherapy of cancerous cells, (e) constructing animal model by fusing human cells with animal tissues, (f) activation of membrane transporters and enzymes, and (g) alteration of genetic expression in living cells. A brief review of mechanistic studies of electroporation is given.