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Response of isolated myocytes to field stimulation
834
Symposium
abstracts
RESPONSES TO A DEFIBRILLATION SHOCK: THE GOOD, THE BAD AND THE UGLY. Douglas L. Jones. Departments of Physiology & Medicine, University of Western Ontario, London, Ontario, Nf3A 5C1, Canada. After a defibrillation shock, fibrillation may continue, sinus rhythm maj be restored, or sinus rhythm may return with compromised hemdynamics. Mechanisms were determined using: standard microelectrode recordings of membrane potentials from papillary muscle and attached Purkinje fibres of dogs, and determination of cardiac calcium Ca) handling by isolated sarcoplasmic reticulum (S k ). Shock voltage-dependent refractoriness was induced in papillary muscle. On the other hand, depolarization and rapid oscillation was induced in Purkinje fibres. Shock voltagedependent decrease in Ca uptake of SR was induced without concomitant alteration in the ATPase activity. Myocyte refractoriness and conduction block is suggested to be responsible for successful defibrillation (the GOOD). Depression of Ca handling of SR is suggested to be responsible for hemodynamic compromise (the BAD). Rapid oscillation of Purkinje fibre is suggested to be a mechanism for reinitiation of rapid fibrillation (the UGLY).
American
September 1992 Heart Journal
RESPONSE OF ISOLATED MYOCYTES TO FIELD STIMULATION. N. Thakor*, M. Fishier*, R. Sweeney**, and P. Reid**, * The Johns Hopkins School of Medicine, 720 Rutland Ave., 701 Traylor Bldg., Baltimore, MD 21205, U.S.A., ** Lilly Research Laboratories, Indianapolis, IN 48285, U.S.A. We studied the effects of externally applied electrical shocks on cardiac myocytes enzymatically isolated from canine right ventricles. Action potentials (AP) were recorded in a current clamp mode using the whole cell patch clamp technique at room APs were first initiated by an Sl temperature. current injection pulse and shock effects were evaluated by delivering an S2 pulse, of 5-20 ms duration and -5 to +5 volts/cm gradient, across Ag-AgCl plates separated by 1 cm. S2 pulses were delivered at 600 ms into the refractory period and AI’ durations at 90% repolarization (AI’D in’ms f s.d.) were compared: Pulse Field
5 ms
10 ms
15 ms
20 ms
Conclusions: 1) Low field gradients (-1 V/cm) simply reschedule the AP, while higher field gradients (-5 V/cm) cause an AI’ prolongation by active reexcitation of the cell membrane. Varying the shock pulse duration does not significantly alter the APD except when the cell is re-excited at higher field strengths. 2) Field polarity has a significant effect on the APD, apparently related to the orientation of the cell with respect to the shock plates.
CURRENT INJECTION EFFECTS ON ISOLATED MYOCYTES STUDIED WITH THE PATCH CLAMP TECHNIQUE. N. Thakor*, M. Fishier*, R. Sweeney**, and P. Reid**, * The Johns Hopkins School of Medicine, 720 Rutland Ave., 701 Traylor Bldg., Baltimore, ** Lilly Research Laboratories, MD 21205, U.S.A. Indianapolis, IN 46285, U.S.A. We present an experimental demonstration of the effects of current injection in isolated cardiac myocytes. Action potentials (AI’) were measured in digested cardiac myocytes by enzymatically employing the whole cell patch clamp technique operating in a current clamp mode. Isolated right ventricular myocytes were maintained at 35-37 C and paced at 500 ms with 20 ms duration Sl pulses. S2 current injection pulses in the 1-9 nA range were delivered at various times during the repolarization period (250, 275,..., ms). AI’ durations measured at 90% repolarization (A.PD90) were compared. Effects of current injection included 1) Rescheduling of AP, resulting from net current injection, occurring at low current strengths during early repolaritation, and 2) AP prolongation, resulting from possible active re-excitation of the cell membrane, occurring at high current strengths during early repolarization and at low current strengths during late repolarization. Our observations of AP prolongation in isolated myocytes by current injection match the results obtained from theoretical and computer models, and also the observations made by others in whole heart during defibrillation and, thus, may be useful in providing a cellular level understanding of defibrillation.
REGIONAL DEPOLARIZATION OF CARDIAC MUSCLE ADJACENT TO AN EPICARDIAL STIMULATING ANODE. Leslie Tung and Michel Neunlist. Biomedical Engineering, Johns Hopkins University, 720 Rutland Ave., Baltimore, MD 21205, U.S.A. During defibrillation or pacing, conventional cable theory predicts the following effects in the near field of the stimulating electrodes (SE): cellular depolarization near the cathode (negative polarity electrode), and hyperpolarization near the anode. To monitor changes in transmembrane potential (Vm) directly under ’ or adjacent to SE, we developed an optical fiber sensor (RoptrodeS) for use with voltage-sensitive fluorescent dyes. Experiments were conducted on a frog ventricular flap stained with di-4-ANEPPS. For 10 msec pulses at low current densities, tissue under a 300 micron diameter, epicardial SE undergoes the expected changes in Vm. However, with anodal stimulation applied at rest at current densities > 5-8X cathodal threshold, an anomalous decay of hyperpolarization occurs during the stimulus pulse, accompanied by direct tissue excitation at the electrode. Similar results were obtained during other phases of the action potential with higher intensity anode1 pulses. An exploring optrode 50 microns from the edge of SE revealed anomalous regions of epicardial depolarization during anodal stimulation. These findings may be explained in part by models which account for regional variation in the anisotropy of the intracellular and extracellular conductances of the tissue.
Symposium
abstracts
RESPONSES TO A DEFIBRILLATION SHOCK: THE GOOD, THE BAD AND THE UGLY. Douglas L. Jones. Departments of Physiology & Medicine, University of Western Ontario, London, Ontario, Nf3A 5C1, Canada. After a defibrillation shock, fibrillation may continue, sinus rhythm maj be restored, or sinus rhythm may return with compromised hemdynamics. Mechanisms were determined using: standard microelectrode recordings of membrane potentials from papillary muscle and attached Purkinje fibres of dogs, and determination of cardiac calcium Ca) handling by isolated sarcoplasmic reticulum (S k ). Shock voltage-dependent refractoriness was induced in papillary muscle. On the other hand, depolarization and rapid oscillation was induced in Purkinje fibres. Shock voltagedependent decrease in Ca uptake of SR was induced without concomitant alteration in the ATPase activity. Myocyte refractoriness and conduction block is suggested to be responsible for successful defibrillation (the GOOD). Depression of Ca handling of SR is suggested to be responsible for hemodynamic compromise (the BAD). Rapid oscillation of Purkinje fibre is suggested to be a mechanism for reinitiation of rapid fibrillation (the UGLY).
American
September 1992 Heart Journal
RESPONSE OF ISOLATED MYOCYTES TO FIELD STIMULATION. N. Thakor*, M. Fishier*, R. Sweeney**, and P. Reid**, * The Johns Hopkins School of Medicine, 720 Rutland Ave., 701 Traylor Bldg., Baltimore, MD 21205, U.S.A., ** Lilly Research Laboratories, Indianapolis, IN 48285, U.S.A. We studied the effects of externally applied electrical shocks on cardiac myocytes enzymatically isolated from canine right ventricles. Action potentials (AP) were recorded in a current clamp mode using the whole cell patch clamp technique at room APs were first initiated by an Sl temperature. current injection pulse and shock effects were evaluated by delivering an S2 pulse, of 5-20 ms duration and -5 to +5 volts/cm gradient, across Ag-AgCl plates separated by 1 cm. S2 pulses were delivered at 600 ms into the refractory period and AI’ durations at 90% repolarization (AI’D in’ms f s.d.) were compared: Pulse Field
5 ms
10 ms
15 ms
20 ms
Conclusions: 1) Low field gradients (-1 V/cm) simply reschedule the AP, while higher field gradients (-5 V/cm) cause an AI’ prolongation by active reexcitation of the cell membrane. Varying the shock pulse duration does not significantly alter the APD except when the cell is re-excited at higher field strengths. 2) Field polarity has a significant effect on the APD, apparently related to the orientation of the cell with respect to the shock plates.
CURRENT INJECTION EFFECTS ON ISOLATED MYOCYTES STUDIED WITH THE PATCH CLAMP TECHNIQUE. N. Thakor*, M. Fishier*, R. Sweeney**, and P. Reid**, * The Johns Hopkins School of Medicine, 720 Rutland Ave., 701 Traylor Bldg., Baltimore, ** Lilly Research Laboratories, MD 21205, U.S.A. Indianapolis, IN 46285, U.S.A. We present an experimental demonstration of the effects of current injection in isolated cardiac myocytes. Action potentials (AI’) were measured in digested cardiac myocytes by enzymatically employing the whole cell patch clamp technique operating in a current clamp mode. Isolated right ventricular myocytes were maintained at 35-37 C and paced at 500 ms with 20 ms duration Sl pulses. S2 current injection pulses in the 1-9 nA range were delivered at various times during the repolarization period (250, 275,..., ms). AI’ durations measured at 90% repolarization (A.PD90) were compared. Effects of current injection included 1) Rescheduling of AP, resulting from net current injection, occurring at low current strengths during early repolaritation, and 2) AP prolongation, resulting from possible active re-excitation of the cell membrane, occurring at high current strengths during early repolarization and at low current strengths during late repolarization. Our observations of AP prolongation in isolated myocytes by current injection match the results obtained from theoretical and computer models, and also the observations made by others in whole heart during defibrillation and, thus, may be useful in providing a cellular level understanding of defibrillation.
REGIONAL DEPOLARIZATION OF CARDIAC MUSCLE ADJACENT TO AN EPICARDIAL STIMULATING ANODE. Leslie Tung and Michel Neunlist. Biomedical Engineering, Johns Hopkins University, 720 Rutland Ave., Baltimore, MD 21205, U.S.A. During defibrillation or pacing, conventional cable theory predicts the following effects in the near field of the stimulating electrodes (SE): cellular depolarization near the cathode (negative polarity electrode), and hyperpolarization near the anode. To monitor changes in transmembrane potential (Vm) directly under ’ or adjacent to SE, we developed an optical fiber sensor (RoptrodeS) for use with voltage-sensitive fluorescent dyes. Experiments were conducted on a frog ventricular flap stained with di-4-ANEPPS. For 10 msec pulses at low current densities, tissue under a 300 micron diameter, epicardial SE undergoes the expected changes in Vm. However, with anodal stimulation applied at rest at current densities > 5-8X cathodal threshold, an anomalous decay of hyperpolarization occurs during the stimulus pulse, accompanied by direct tissue excitation at the electrode. Similar results were obtained during other phases of the action potential with higher intensity anode1 pulses. An exploring optrode 50 microns from the edge of SE revealed anomalous regions of epicardial depolarization during anodal stimulation. These findings may be explained in part by models which account for regional variation in the anisotropy of the intracellular and extracellular conductances of the tissue.