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Meeting Abstracts  |   March 1995
Effects of Hypothermia, Potassium, and Verapamil on the Action Potential Characteristics of Canine Cardiac Purkinje Fibers 
Author Notes
  • (Sprung) Staff Anesthesiologist, Department of Anesthesiology, Cleveland Clinic Foundation.
  • (Laszlo) Postdoctoral Research Fellow, Department of Anesthesiology, Medical College of Wisconsin.
  • (Turner) Associate Professor, Department of Anesthesiology, Medical College of Wisconsin.
  • (Kampine) Professor, Departments of Anesthesiology and Physiology: Chair, Department of Anesthesiology, Medical College of Wisconsin.
  • (Bosnjak) Professor, Departments of Anesthesiology and Physiology; Director, Laboratory of Cellular Biology, Medical College of Wisconsin.
  • Received from the Departments of Anesthesiology and Physiology, Medical College of Wisconsin, Milwaukee, Wisconsin, and the Department of Anesthesiology, Cleveland Clinic Foundation, Cleveland, Ohio. Accepted for publication November 3, 1994. Supported by the Society of Cardiovascular Anesthesiologists Starter Grant (to JS). National Institutes of Health grant HI.-39776 (to JB) VA Medical Research Funds, and Anesthesiology Research Training grant GM08377.
  • Address reprint requests to Dr. Bosnjak: Department of Anesthesiology, Medical College of Wisconsin, 8701 West Washington Plank Road, Milwaukee, Wisconsin 53226.
Article Information
Meeting Abstracts   |   March 1995
Effects of Hypothermia, Potassium, and Verapamil on the Action Potential Characteristics of Canine Cardiac Purkinje Fibers 
Anesthesiology 3 1995, Vol.82, 713-722. doi:
Anesthesiology 3 1995, Vol.82, 713-722. doi:
Key words: Calcium channel, antagonist; verapamil. Heart, electrophysiology: action potential duration; maximum diastolic potential; maximum rate of phase 0 depolarization. Ions, potassium: extracellular. Temperature: hypothermia.
HYPOTHERMIA can occur accidentally after exposure to a cold environment or operating room or may be intentionally induced to protect tissues against hypoxia during surgery. [1 ] Various cardiac arrhythmias, including ventricular fibrillation and asystole, occur at temperatures below about 30 degrees Celsius. [2 ] An anesthesiologist may be faced with this problem in the critical care setting while treating the patient with accidental hypothermia, intraoperatively during recovery from low temperature cardioplegia, or even during irrigation of the thoracic cavity with cold solution. [4 ] Major electrocardiographic manifestations of hypothermia include decreased myocardial conduction velocity with increases in PR and QT intervals and QRS complex duration. [2 ] At least in part, these changes can be attributed to alterations in the electrophysiologic properties of the cardiac Purkinje fibers [4 ] or serum electrolyte changes [5 ] induced by the hypothermia. [6,7 ] In vivo, nonhomogeneous tissue cooling slows action potential propagation and can induce differences in repolarization at various sites in the ventricular conducting system. [8 ] This repolarization heterogeneity facilitates nonuniform distribution of impulse conduction throughout the myocardium and may result in reentrant arrhythmias. [8–11 ] In addition, low temperature modifies cation conductance across cell membranes [12 ] and may result in clinically significant hypokalemia, which can be attributed to a temperature-mediated redistribution of Potassium sup + among body compartments. [6,7 ] The resting membrane potential of cardiac Purkinje fibers may depolarize in deep hypothermia, [13 ] and an increase in the action potential duration (APD) is seen even during mild hypothermia and hypokalemia. [14 ] These electrophysiologic changes can depress impulse conduction and may induce reentry arrhythmias, abnormal forms of automaticity, and heart block. [5 ].
The exact mechanism by which hypothermia increases cardiac irritability is unknown, but the hypothermia-induced electrophysiologic changes in Purkinje fiber action potentials may be involved. In addition to affecting Potassium sup + homeostasis. [6,7 ] hypothermia also increases intracellular myocardial Calcium2+ concentration ([Calcium2+]i). [15 ] Inhibition of the Sodium sup +-Potassium sup + pump activity by low temperature leads to an increase in intracellular Sodium sup + concentration ([Sodium sup +]i), which may promote Calcium2+ influx during the plateau of the action potential or oppose a Calcium2+ efflux during diastole by the Sodium sup +-Calcium2+ exchange mechanism. [16 ] The excessive accumulation of intracellular Calcium2+, termed “Calcium2+ overload,”[17 ] may result in delayed afterdepolarizations, causing severe cardiac arrhythmias. [5,17–19 ] Furthermore, it has been suggested that hypothermia, by delaying inactivation of the inward Calcium2+ current (ICa) and maintaining the ICafor a longer time, contributes to action potential lengthening and arrhythmias based on abnormal impulse propagation. [20 ].
We hypothesized that if increased [Calcium2+]iunderlay important changes in electrophysiologic characteristics of Purkinje fibers in hypothermia, especially prolongation of the APD, we should be able to alter their course with a Calcium2+-channel blocking agent. Because hypothermia alters Potassium sup + homeostasis, [6,7 ] we also examined the effects of hypothermia on action potential characteristics over the wide range of external [Potassium sup +]([Potassium sup +]o) in the absence and presence of verapamil. The reduction of APD with verapamil, if present, might be expected to reduce the incidence of hypothermia-induced arrhythmias, specifically dysrhythmias based on regional differences in myocardial repolarization.
Materials and Methods
This study was approved by the Medical College of Wisconsin Animal Care Committee and conformed with standards set forth in the Guide for Care and Use of Laboratory Animals.*
Adult mongrel dogs (10–22 kg, of either sex)(n = 45) were anesthetized with 30 mg/kg pentobarbital sodium. The heart was quickly excised, and the anterior false tendon with attached papillary muscle from the left ventricle was removed and immersed in modified Krebs' solution (22 degrees Celsius) equilibrated with 97% Oxygen2and 3% CO2. Small (< 1-cm2) preparations with free-running strands of Purkinje fibers were dissected from this tissue and pinned to the silicone elastomere floor of a 2-ml chamber and superfused at a rate of 4 ml/min with modified Krebs' solution (37 degrees Celsius) containing 2.3, 3.9, or 6.8 mM KCl with or without 1 micro Meter verapamil and equilibrated with a 97% Oxygen2-3% CO2gas mixture. The millimolar composition of Krebs' solution was 137 NaCl, 12 NaHCO3, 1.8 NaH2PO4, 1.8 CaCl2, 0.5 MgCl2, 5.5 glucose, and 0.05 EDTA, with a pH of 7.4.
Each preparation was stimulated at a constant rate (1 Hz) with the use of bipolar-silver wire endocardial surface electrodes. The stimuli were square-wave pulses lasting 2 ms at 1.5 times threshold. Transmembrane action potentials were recorded with conventional microelectrode techniques. Action potential changes stabilized within 5–8 min. Glass microelectrodes (15–30-M Omega resistance) were coupled by Silver-AgCl wire to a preamplifier (World Precision Instruments, New Haven, CT). Action potential signals were recorded on frequency-modulated tape (AR Vetter, Rebersburg, PA) for later analysis of maximum diastolic potential (MDP), maximum rate of phase 0 depolarization (Vmax), and APD at 50%(APD50) and at 95%(APD sub 95) repolarization. These values were displayed and measured electronically directly off the digital oscilloscope (Nicolet 310). The Vmaxwas determined with a differentiator exhihiting a linear response from 100–1,000 V/s. The “zero” potential was obtained at the beginning and at the end of the experiments by withdrawing the microelectrode from the inside of the fiber. Because the ground connection between the bath and the circuit was made through a direct Silver-AgCl connection, the change in temperature may slightly influence the half cell potential in the bath, and this change will sum with the real MDP changes. To verify the significance of this phenomenon we performed additional experiments (n = 8) in which the microelectrode was placed into the bath, the temperature was changed from 37 degrees Celsius to 28 degrees Celsius, and the Silver-AgCl bath junction potential change was measured. The half cell potential caused a hyperpolarization to be measured by the microelectrode over this temperature range by -2.1 plus/minus 0.02 mV and -4.2 plus/minus 0.05 mV at 32 degrees Celsius and 28 degrees Celsius respectively. All measured MDP values at 32 degrees Celsius and 28 degrees Celsius were corrected for the above differences, by subtracting the temperature-induced bath potential from the actual measured potential, and all the statistical analyses were performed with these corrected values.
The tissue bath was surrounded by a thermostatically controlled water bath, maintained at a constant temperature of 37 plus/minus 0.02 degrees Celsius. The low bath superfusate temperatures (32 and 28 plus/minus 0.5 degrees Celsius) were attained by readjusting the setting of the thermostat. The temperature in the tissue bath was gradually decreased from 37 degrees Celsius to 25 degrees Celsius over a 20–30-min period. The temperature of the solution was measured by a small, rapidly responding, custom made thermistor probe placed less than 2 mm from the preparation. The fluid level in the chamber was kept at a constant height (4 mm) by continuous suction.
The preparations were allowed to equilibrate for about 1 h. To determine the effects of hypothermia on action potential characteristics, action potentials were recorded at 37 degrees Celsius, 32 degrees Celsius, and 28 degrees Celsius (plus/minus 0.5 degrees Celsius). To determine the effects of various [Potassium sup +]oon action potentials, experiments were performed with 2.3, 3.9, and 6.8 mM [Potassium sup +] in the superfusate. Verapamil (Sigma, St. Louis, MO) prepared as stock solution (100 micro Meter) was added to measured volumes to achieve the desired 1 micro Meter concentration in the superfusate. The same set of action potential measurements were performed with 1 micro Meter verapamil in the superfusate. Tissues were exposed to each [Potassium sup +]oand to verapamil (1 micro Meter) for 20 min before measurement of action potential characteristics.
Data are expressed as means plus/minus standard error of the mean. Statistical analysis was performed by paired and unpaired t tests and with one-way analysis of variance (analysis of variance repeated measures and factorial analysis), as appropriate, with P < 0.05 considered statistically significant.
Results
Typical effects of hypothermia on the action potential in canine Purkinje fibers are illustrated in the upper panel of Figure 1. Each preparation was stimulated at a constant rate of 1 Hz. Recordings were taken from the same cell at various temperatures at a [Potassium sup +]oof 3.9 mM. Hypothermia significantly increased APD. The bottom panel of Figure 1shows the changes of action potential in hypothermia in the presence of 1 micro Meter verapamil in the superfusate. In the presence of verapamil the Purkinje fiber's action potential is shorter than at the same temperature without the drug (dashed action potential was recorded at 37 degrees Celsius in the absence of verapamil). The slope of phase 2 repolarization is increased by verapamil, a finding that is consistent with the blockade of a slow inward current such as that carried by a Calcium2+ ion. Hypothermia, on the other hand, appears to decrease the slope of phase 2, suggesting that low temperature affects the Calcium2+-dependent ionic mechanisms in opposite direction.
Figure 1. Effect of hypothermia on action potential of canine cardiac Purkinje fiber superfused with normal Krebs' solution (top) and in the presence of verapamil (bottom). See text for details.
Figure 1. Effect of hypothermia on action potential of canine cardiac Purkinje fiber superfused with normal Krebs' solution (top) and in the presence of verapamil (bottom). See text for details.
Figure 1. Effect of hypothermia on action potential of canine cardiac Purkinje fiber superfused with normal Krebs' solution (top) and in the presence of verapamil (bottom). See text for details.
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Maximum Diastolic Potential
The effects of two stages of hypothermia (32 degrees Celsius and 28 degrees Celsius) on the MDPs of Purkinje fibers were recorded during the exposure to low (2.3 mM), normal (3.9 mM), and high (6.8 mM)[Potassium sup +]oin the superfusate (Figure 2). MDPs were higher (hyperpolarized) at low, and lower (depolarized) at high [Potassium sup +]othan at normal [Potassium sup +]o(P < 0.05), regardless of the temperature. During the 20–30 min of gradual cooling from 37 degrees Celsius to 28 degrees Celsius, MDP decreased at normal and high [Potassium sup +]o(P < 0.05), and did not change at low [K sup +]o.
At 37 degrees Celsius and in the presence of verapamil MDP was lower (difference not significant, P > 0.05 vs. control;Figure 2). In hypothermia and at [K sup +]ogreater or equal to 3.9 mM loss of membrane potential was parallel to this of control Purkinje fibers, and reached significant depolarization at 28 degrees Celsius (P < 0.05). At [K sup +]o2.3 mM there was no effect of temperature on MDP in verapamil-superfused Purkinje fibers.
Figure 2. Effect of hypothermia and Potassium sup + variations on maximum diastolic potential (MDP) of Purkinje fiber action potentials with and without verapamil in the superfusate. *P < 0.05 versus 2.3 and 6.8 mM external [K sup +]([K sup +]o) at same temperature. (dagger)P < 0.05 versus 37 degrees Celsius at same [K sup +]o.
Figure 2. Effect of hypothermia and Potassium sup + variations on maximum diastolic potential (MDP) of Purkinje fiber action potentials with and without verapamil in the superfusate. *P < 0.05 versus 2.3 and 6.8 mM external [K sup +]([K sup +]o) at same temperature. (dagger)P < 0.05 versus 37 degrees Celsius at same [K sup +]o.
Figure 2. Effect of hypothermia and Potassium sup + variations on maximum diastolic potential (MDP) of Purkinje fiber action potentials with and without verapamil in the superfusate. *P < 0.05 versus 2.3 and 6.8 mM external [K sup +]([K sup +]o) at same temperature. (dagger)P < 0.05 versus 37 degrees Celsius at same [K sup +]o.
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Maximum Rate of Phase 0 Depolarization
The effects of hypothermia, Potassium sup + variation, and verapamil on Vmaxare summarized in Figure 3. Hypothermia decreased Vmaxat each [K sup +]o(P < 0.05). Despite significant differences in MDP at various [K sup +]oat 37 degrees Celsius (Figure 2), the respective Vmaxvalues were not different (Figure 3), although there were trends for Vmaxto be lower at 6.8 and higher at 2.3 mM [K sup +]othan at 3.9 mM [K sup +]o(P > 0.05). At [K sup +]oless or equal to 3.9 mM with verapamil in the superfusate no additional effect, besides that of temperature, was noted on Vmax(Figure 3). At 6.8 mM [K sup +]oin hypothermia, Vmaxwas lower in the presence of verapamil (P < 0.05).
Figure 3. Effect of hypothermia and Potassium sup + variations on the maximum rate of phase 0 depolarization (Vmax) of Purkinje fiber action potentials with and without verapamil in the superfusate. *P < 0.05 versus 37, 32, and 28 degrees Celsius; dagger P < 0.05 versus verapamil; double dagger P < 0.05 versus 2.3 and 3.9 mM external [K sup +]+ verapamil.
Figure 3. Effect of hypothermia and Potassium sup + variations on the maximum rate of phase 0 depolarization (Vmax) of Purkinje fiber action potentials with and without verapamil in the superfusate. *P < 0.05 versus 37, 32, and 28 degrees Celsius; dagger P < 0.05 versus verapamil; double dagger P < 0.05 versus 2.3 and 3.9 mM external [K sup +]+ verapamil.
Figure 3. Effect of hypothermia and Potassium sup + variations on the maximum rate of phase 0 depolarization (Vmax) of Purkinje fiber action potentials with and without verapamil in the superfusate. *P < 0.05 versus 37, 32, and 28 degrees Celsius; dagger P < 0.05 versus verapamil; double dagger P < 0.05 versus 2.3 and 3.9 mM external [K sup +]+ verapamil.
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To quantify the effects of changing MDP on Vmax, we examined eight additional Purkinje fiber preparations by gradually increasing [K sup +]oin the superfusate from 0.8 to 10 mM; MDP and Vmaxvalues were measured at 37 degrees Celsius and at 30 degrees Celsius (Figure 4). At both temperatures MDP decreased (less negative) directly with increasing [K sup +]obetween 2.3 and 10 mM (P < 0.05); below 2.3 mM MDP did not further change (P > 0.05). Only between 3.9 and 8.5 mM [K sup +]oMDP was lower at 30 degrees Celsius (P < 0.05), Vmaxdecreased moderately between 2.3 and 6.8 mM [K sup +] sub o, followed by a rapid decrease when [K sup +]oincreased above 6.8 mM (MDP less or equal to -80 mV)(Figure 4). Compared with 2.3 mM [K sup +]o, at 0.8 mM [K sup +]othere was a significant decrease in Vmaxat 37 degrees Celsius, and no change in Vmaxat 30 degrees Celsius. By varying the [K sup +]o, Vmaxat 37 degrees Celsius and 30 degrees Celsius resulted in similar pattern of changes, yet amplitudes were different.
Figure 4. Relation between maximum diastolic potential (MDP) and maximum rate of phase 0 depolarization (Vmax) of Purkinje fiber action potentials at 37 degrees Celsius and at 30 degrees Celsius during the gradual increase in external [K sup +]([K sup +]o) from 0.8 to 10 mM. *P < 0.05 MDP or Vmaxversus lower [K sup +]o. *P < 0.05 MDP37degrees Celsius versus MDP30degrees Celsius at same [K sup +]o.
Figure 4. Relation between maximum diastolic potential (MDP) and maximum rate of phase 0 depolarization (Vmax) of Purkinje fiber action potentials at 37 degrees Celsius and at 30 degrees Celsius during the gradual increase in external [K sup +]([K sup +]o) from 0.8 to 10 mM. *P < 0.05 MDP or Vmaxversus lower [K sup +]o. *P < 0.05 MDP37degrees Celsius versus MDP30degrees Celsius at same [K sup +]o.
Figure 4. Relation between maximum diastolic potential (MDP) and maximum rate of phase 0 depolarization (Vmax) of Purkinje fiber action potentials at 37 degrees Celsius and at 30 degrees Celsius during the gradual increase in external [K sup +]([K sup +]o) from 0.8 to 10 mM. *P < 0.05 MDP or Vmaxversus lower [K sup +]o. *P < 0.05 MDP37degrees Celsius versus MDP30degrees Celsius at same [K sup +]o.
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Action Potential Duration
Hypothermia increased early (APD50) and late (APD95) stages of repolarization (Table 1and Table 2). Figure 5and Figure 6compare the relative changes in APD50and APD95at various temperatures and [K sup +]owith and without verapamil in the superfusate. In hypothermia the increase in APD50(at 28 degrees Celsius) and APD95(at 32 degrees Celsius and 28 degrees Celsius) was inversely related to [K sup +]o; at 6.8 mM [K sup +] sub o in hypothermia APD50and APD95were always closest to normothermic control. Regardless of temperature, verapamil did not affect APD50or APD95at 2.3 mM [K sup +]o. At 28 degrees Celsius and [K sup +]ogreater or equal 3.9 mM verapamil significantly shortened the APD50(Figure 5) and the APD95(Figure 6). At 28 degrees Celsius and 6.8 mM [K sup +]owith verapamil, the APD50was 21% shorter whereas the APD95was 12% longer than normothermic control. Without the verapamil at 28 degrees Celsius, there were no longer relative differences between the increases in APD50and APD95, regardless of the [K sup +]o, indicating that hypothermia prolonged early and late repolarization similarly (Figure 7). After verapamil was introduced to the superfusate, the relative changes in APD became dependent on the actual [K sup +]o; at 2.3 mM [K sup +] APD50and APD95were unchanged (Figure 7(A)); at 3.9 mM [K sup +]othe primary effect was on APD50(Figure 7(B)); at 6.8 mM APD50and APD95decreased, with APD50> APD95(P < 0.05)(Figure 7(C)).
Table 1. Effect of Hypothermia, Potassium sup + Variations, and Verapamil on Action Potential Duration at 50% of Repolarization (APD50, ms)
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Table 1. Effect of Hypothermia, Potassium sup + Variations, and Verapamil on Action Potential Duration at 50% of Repolarization (APD50, ms)
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Table 2. Effect of Hypothermia, Potassium sup + Variations, and Verapamil on Action Potential Duration at 95% of Repolarization (APD95, ms)
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Table 2. Effect of Hypothermia, Potassium sup + Variations, and Verapamil on Action Potential Duration at 95% of Repolarization (APD95, ms)
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Figure 5. Relative changes in action potential duration at 50% repolarization (APD50) by verapamil at various external [K sup +]([K sup +]o) during cooling (control [C] values are normalized to the values obtained at 37 degrees Celsius and at 3.9 mM [K sup +]o). *P < 0.05 V (verapamil) versus C (control) at respective temperature.
Figure 5. Relative changes in action potential duration at 50% repolarization (APD50) by verapamil at various external [K sup +]([K sup +]o) during cooling (control [C] values are normalized to the values obtained at 37 degrees Celsius and at 3.9 mM [K sup +]o). *P < 0.05 V (verapamil) versus C (control) at respective temperature.
Figure 5. Relative changes in action potential duration at 50% repolarization (APD50) by verapamil at various external [K sup +]([K sup +]o) during cooling (control [C] values are normalized to the values obtained at 37 degrees Celsius and at 3.9 mM [K sup +]o). *P < 0.05 V (verapamil) versus C (control) at respective temperature.
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Figure 6. Relative changes in action potential duration at 95% repolarization (APD95) by verapamil at various external [K sup +]([K sup +]o) during cooling (control [C] values are normalized to the values obtained at 37 degrees Celsius and at 3.9 mM [K sup +]o). *P < 0.05 V (verapamil) versus C (control) at respective temperature.
Figure 6. Relative changes in action potential duration at 95% repolarization (APD95) by verapamil at various external [K sup +]([K sup +]o) during cooling (control [C] values are normalized to the values obtained at 37 degrees Celsius and at 3.9 mM [K sup +]o). *P < 0.05 V (verapamil) versus C (control) at respective temperature.
Figure 6. Relative changes in action potential duration at 95% repolarization (APD95) by verapamil at various external [K sup +]([K sup +]o) during cooling (control [C] values are normalized to the values obtained at 37 degrees Celsius and at 3.9 mM [K sup +]o). *P < 0.05 V (verapamil) versus C (control) at respective temperature.
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Figure 7. Effect of verapamil (closed symbols) on the relative relation between action potential duration at 50%(APD50) and 95%(APD95) repolarization at 28 degrees Celsius at various external [K sup +]([K sup +]o). Values are normalized to their respective controls at 37 degrees Celsius (open symbols). *P < 0.05 describes the significance of relative changes between action potential duration at 50%(APD50) versus 95%(APD95) repolarization after verapamil.
Figure 7. Effect of verapamil (closed symbols) on the relative relation between action potential duration at 50%(APD50) and 95%(APD95) repolarization at 28 degrees Celsius at various external [K sup +]([K sup +]o). Values are normalized to their respective controls at 37 degrees Celsius (open symbols). *P < 0.05 describes the significance of relative changes between action potential duration at 50%(APD50) versus 95%(APD95) repolarization after verapamil.
Figure 7. Effect of verapamil (closed symbols) on the relative relation between action potential duration at 50%(APD50) and 95%(APD95) repolarization at 28 degrees Celsius at various external [K sup +]([K sup +]o). Values are normalized to their respective controls at 37 degrees Celsius (open symbols). *P < 0.05 describes the significance of relative changes between action potential duration at 50%(APD50) versus 95%(APD95) repolarization after verapamil.
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Discussion
Induced hypothermia is commonly used to protect myocardium and other tissues against ischemia during cardiopulmonary bypass. [1 ] At the same time hypothermia is known for its arrhythmogenic potential, including ventricular fibrillation. [1–3 ] The common mechanisms underlying cardiac arrhythmias are the loss of the MDP in Purkinje fibers, [5 ] diastolic Calcium2+ overload, [16 ] and altered dispersion of repolarization, [11 ] all being reported in hypothermia. [4,13,14,20,21 ] Hypokalemia has been encountered during hypothermia, [6,7 ] and rebound hyperkalemia has occurred during rewarming. [7 ] This study examined electrophysiologic alterations of MDP, Vmax, and APD during hypothermia and hypothermic hypokalemia and their reversibility after verapamil treatment and Potassium sup + supplementation.
Maximum Diastolic Potential
In the current study, the shift in MDP caused by variation of [K sup +]obetween 2.3 and 10 mM was consistent with values predicted by the Nernst equation [14 ]: depolarization was present at high, and hyperpolarization at low [K sup +]o(Figure 2and Figure 4). Below [K2+]o2.3 mM, specifically at 0.8 mM, MDP did not further hyperpolarize, but also did not depolarize as could be expected. [14 ] We do not have a plausible explanation for the disparity between ours and previously reported findings. [14 ].
We found moderate loss of membrane potential (depolarization) with reduction of temperature to 28 degrees Celsius but only at [K sup +]ogreater or equal to 3.9 mM. A very small decrease in the resting membrane potential of canine Purkinje fibers was described at 25 degrees Celsius, [13 ] but this decrease was considerably larger below 20 degrees Celsius. [4 ] The decreases in MDP in hypothermia have been attributed to inhibition of adenosine triphosphate-dependent Sodium sup +-Potassium sup + pump. [22–24 ] In our study the Calcium24-channel blockade with 1 micro Meter verapamil had no significant additional effect on MDP regardless of the temperature, and when [K sup +]owas greater or equal to 3.9 mM moderate depolarization at 28 degrees Celsius was parallel to that of control Purkinje fibers. This tendency toward depolarization when [Potassium sup +]owas greater or equal to 3.9 mM may not be incidental, because verapamil is known to decrease MDP at higher concentrations. [25,26 ] Dersham and Han described a nonsignificant decrease in MDP after 1 micro Meter verapamil, and a statistically significant decrease (from 86 to 80 mM) after 2 micro Meter. [9 ] Similar results after 2 micro Meter of verapamil were reported by Amerini et al. [19 ] The mechanism responsible for MDP decrease after verapamil was attributed to the reduced Potassium sup + conductance, [19,27 ] but we cannot confirm this mechanism because the increase in K sup + conductance by increasing [Potassium sup +][8 ] in our study did not reduce the amount of depolarization induced by verapamil. It is unlikely that small changes in MDP, present at normal serum [Potassium sup +]([nearly equal] 3.9 mM), have any clinical significance. Our findings suggest that hypothermic cardiac arrhythmias, which occur around 28 degrees Celsius, cannot be attributed to hypothermia-induced membrane depolarization, providing serum Potassium sup + is in normal range.
Maximum Rate of Phase O Depolarization
The magnitude of Vmaxis determined by the fast inward Sodium sup + current (INa), and has traditionally been used as an index of Sodium sup + channel availability. [28,29 ] In our experiments, hypothermia decreased Vmax(Figure 3), which may be explained by the temperature-mediated decrease in INa. [21,30 ] Hypothermia decreases Sodium sup +-Potassium sup + adenosine triphosphatase activity, which then results in increased [Sodium sup +] sub j and decrease in driving force for INa. [31–33 ] At [Potassium sup +]oless or equal to 3.9 mM in the normothermic and hypothermic states, Calcium2+-channel blockade with 1 micro Meter verapamil did not alter Vmax. Although verapamil in lower concentrations has no significant effect on Vmax, [9,25,34,35 ] the Calcium2+-channel blockade may decrease Vmaxwhen given in higher concentrations. [30,37 ] However, at 6.8 mM [Potassium sup +]oin hypothermia, we found that verapamil decreased Vmax(P < 0.05)(Figure 3). It is possible that the cumulative effect of hypothermic INablockade, low MDP caused by hyperkalemia, and verapamil-induced inhibition of INa, [36,37 ] possibly potentiated by low temperature, resulted in a reduction of Vmax.
Because Vmaxis voltage-dependent we examined the relation between MDP and Vmaxby varying the [Potassium sup +]obetween 0.8 and 10 mM. Whereas the increase in [Potassium sup +]obetween 2.3 and 10 mM caused an almost linear decrease in MDP (depolarization), at less than 2.3 mM, MDP ceased to decrease further, consistent with a decrease in resting Potassium sup + conductance at low [Potassium sup +]o. [38 ] The decrease in temperature from 37 degrees Celsius to 30 degrees Celsius induced a leftward shift of the V sub max curve. Although there was a decreasing trend, Vmaxdid not significantly change when [Potassium sup +]owas increased from 2.3 to approximately 6 mM, a finding that is consistent with the results described in Figure 3. Therefore, Vmaxwas not greatly affected when the voltage (MDP) was between -100 and -85 mV (2.3–6 mM [Potassium sup +]o), but was markedly decreased when MDP became less negative than -80 mV. In extreme hypokalemia (0.8 mM) at 37 degrees Celsius Vmax, significantly decreased, at least in part mirroring changes in the respective MDP. Our data suggest that during phase 0 of the stimulated action potential at MDP between -80 and -100 mV, the fraction of available Sodium sup + channels remains relatively constant or only slightly increases as MDP becomes more negative. Thus, when MDP reaches -80 mV, the near-maximum capacity for Sodium sup + influx through the fast Sodium sup + channels may be reached (especially in hypothermia) and cannot be further increased by more negative MDP. The decrease in V sub max when MDP drops below approximately -72 mV at 37 degrees Celsius and -80 mV at 30 degrees Celsius, is very abrupt. In addition, hypothermia significantly decreased Vmaxand less affected MDP. These findings are consistent with the existence of two mechanisms that may effect Sodium sup +-channel kinetics: first, a temperature-dependent mechanism that is responsible for the decrease in Vmaxduring cooling, and second, a voltage-dependent mechanism that is active only at MDP less negative than -80 mV. In deep hypothermia and at [Potassium sup +]o6.8 mM, verapamil further decreased Vmax, most likely by the interference with INakinetics by voltage-dependent mechanism. The clinical significance of the decrease in Vmaxby verapamil and [Potassium sup +]oin hypothermia has to be studied, but typically the drugs that are able to decrease the I sub Na [39 ] induce the following chain of events: decrease in [Sodium sup +]i, increase in Calcium2+ efflux and therefore a decrease in sarcoplasmic reticulum Calcium2+ loading. These events may ultimately result in an antiarrhythmic effect of the drugs by decreasing oscillatory afterpotentials. [40 ].
Action Potential Duration
The cardiac action potential plateau is determined by a delicate balance among several distinct time- and voltage-dependent ionic currents, [8,38,41–43 ] After upstroke of the action potential caused by the influx of Sodium sup +, the INabecomes inactivated. This inactivation does not lead to immediate repolarization, because the initial depolarization reduces the inwardly rectifying Potassium sup + conductance and opens voltage-gated Calcium2+ channels thus permitting the intracellular influx of Calcium2+ by the INa. During the action potential plateau, while membrane conductance for all ions is reduced, several currents help to maintain the transmembrane potential at around 0 mV, including ICa, Sodium sup +“window.” Cl and inward (anomalous) rectifying Potassium sup + currents. In addition, currents produced by the electrogenic Sodium sup +-Potassium sup + pump and Sodium sup +- Calcium2+ exchange mechanism help to maintain the action potential plateau. The outward (delayed) rectifying Potassium sup + current (IK) and the inward rectifying Potassium sup + current are responsible for final rapid repolarization (phase 3). Details of the ionic basis of action potential are reviewed elsewhere. [5,16,23,27,38,41,45 ].
The electrophysiologic mechanisms responsible for the marked prolongation of APD seen in hypothermia are not fully understood, although mechanisms based on temperature dependence of IK[46 ] and ICa[47 ] have been proposed. One important mechanism may be the hypothermia-induced reduction of IK, which is a major contributor to membrane repolarization. [46 ] In addition, lengthened APD in hypothermia may be attributed to delayed inactivation of the I sub Ca sup +[47,48 ] which will maintain the ICafor a longer time. [20 ] Finally, prolonged APD may be also attributed to increase in [Calcium2+]ithrough altered Sodium sup +-Calcium2+ exchange. [33 ] The activity of the Sodium sup +-Potassium sup + pump is likely to be reduced in hypothermia [31,32 ] resulting in a net rise in [Sodium sup +]i. [33 ] The increase in [Sodium sup +]jmay promote Calcium2+ influx during the plateau of the action potential or reduce Calcium2+ efflux during diastole by Sodium sup +-Calcium2+ exchange. [49 ] Although the activity of the Sodium sup +-Calcium2+ exchanger is temperature dependent, its relatively low Q10(the rate of exchange produced by changing the temperature 10 degrees Celsius) means that this mechanism may contribute to raising diastolic free [Calcium2+]j. [44 ].
We have demonstrated that hypothermia equally lengthens the APD50and APD95, indicating that low temperature similarly affects the ionic mechanisms that determine the early and late stages of repolarization (Table 1and Table 2and Figure 7), i.e. earlier discussed IKand ICa. The finding that hypokalemia, similarly to hypothermia, prolongs the APD is not surprising, because low [Potassium sup +]odecreases Potassium sup + conductance. [38 ] APD50and APD95were shorter at 6.8 mM [Potassium sup +]othan at either 3.9 or 2.3 mM [Potassium sup +]o(Figure 5and Figure 6) as a result of improved Potassium sup + conductance. [8,40,45,50 ] Interventions that affect ionic currents during the repolarization phase may selectively alter the shape and duration of the cardiac action potential. Verapamil may affect both the ICaand Potassium sup + conductance. [43,51 ] In our study, 1 micro Meter of verapamil significantly shortened both the APD50and the APD95, but only at 28 degrees Celsius and [Potassium sup +]ogreater or equal to 3.9 mM (Figure 5and Figure 6). At this temperature and with 6.8 mM [Potassium sup +]oand verapamil in the superfusate, the relative shortening of APD50exceeded that of APD95(Figure 7(C)). The observation that earlier phases of repolarization (APD50) appear to be more readily affected by verapamil is consistent with the primary blocking action of this agent on ICa. At the same time less affected shortening of APD95by verapamil, and its significant shortening at higher [Potassium sup +]osuggests that another mechanism is involved in the lengthening of APD at low temperature, presumably hypothermia-induced reduction of IK. [46 ] Also, when [Potassium sup +]owas sufficiently high at 28 degrees Celsius, both APD50and APD95were shortened, indicating not only an important role of [Potassium sup +]oin increasing Potassium sup + conductance but also an interaction between Calcium2+-channel blockade and Potassium sup + conductance. Our study indicates that hypokalemia must be corrected if the APD is to be shortened with verapamil. Not only does correction of hypokalemia affects APD through increases of Potassium sup + conductance, [38 ] but in addition, as Cavalie et al. have shown, ICainactivation (which is delayed by hypothermia) depends on voltage and is greater at a more depolarized potential, [52 ] as in our study at 6.8 mM.
Different arrhythmias may arise if the [Calcium2+]iis increased or if cardiac action potential is lengthened. First, increase in [Calcium2+]icaused by hypothermia results in oscillatory release of [Calcium2+] from sarcoplasmic reticulum; this may generate the transient inward current allowing more Sodium sup + and Calcium2+ into the cell creating delayed afterdepolarization. [5,17,38 ] When delayed after-depolarization reaches certain threshold, arrhythmias may be triggered. These arrhythmias can be seen at low [Potassium sup +]o, and when Sodium sup + extrusion from the cell is reduced, [8 ] because both may be encountered during hypothermia. By diminishing [Calcium2+]iinflux, verapamil reduces Calcium2+ overload, delays afterdepolarization amplitude and reduces triggered automaticity. [53,54 ] Verapamil thus may be useful for treatment of hypothermia-induced arrhythmias based on delayed afterdepolarization. Second, reentry arrhythmias may occur during the propagation of slow action potentials through Purkinje fibers [5,8,21,31,55 ] especially in vivo, when, because of uneven tissue cooling, heterogeneity of repolarizations may ensue. [8 ] A shortening of APD with verapamil in hypothermia is both temperature and [Potassium sup +]odependent, and this may have significant clinical implications, but only at higher [Potassium sup +]o: the lower the temperature, the longer the APD, and the greater the shortening effect of verapamil. Because the APD is very dependent on regional myocardial temperature, verapamil selectively affects APD: a greater APD in a cooler region will be decreased more than a lesser APD in a warmer region. By reducing the regional differences in myocardial repolarization this effect may decrease the propensity for development of arrhythmogenic reentry circuits.
In conclusion, there appears to be an increased relative I sub Ca at low temperature, as evidenced by the effective shortening of APD in deep hypothermia by verapamil. Although this shortening may result from an increase in ICa, a decrease in Potassium sup + conductance at low temperature is more likely, as evidenced by the effective shortening of APD when the Potassium sup + conductance is increased by increasing [Potassium sup +]O. Finally, the effectiveness of verapamil to shorten APD in hypothermia was dependent on [Potassium sup +]Osuggesting that the increase in Potassium sup + conductance is an important factor for achieving this verapamil effect. Verapamil and Potassium sup + supplementation in hypothermia may have an antiarrhythmic effect primarily by reducing the dispersion of prolonged APD. Further studies will be needed to evaluate this antiarrhythmic effect in vivo.
The authors thank Edith Sulzer for her help in the preparation of this manuscript.
* Guide for Care and Use of Laboratory Animals. Publication 85–23. Bethesda, MD, Public Health Services, National Institutes of Health, revised 1985.
REFERENCES
Taylor CA: Surgical hypothermia. Pharmacol Ther 38:169-200, 1988.
Davis RF: Etiology and treatment of perioperative cardiac arrhythmias, Cardiac Anesthesia 3rd edition. Edited by Kaplan JA, Philadelphia, WB Saunders. 1993, pp 170-205.
Doyle DJ, Knapp CD: Asystole from unintended myocardial hypothermia (letter). ANESTHESIOLOGY 80:956, 1994.
Deleze J: Possible reasons for drop of resting potential of mammalian heart preparations during hypothermia. Circ Res 8:553-557, 1960.
Atlee JL, Bosnjak ZJ: Mechanisms for cardiac dysrhythmias during anesthesia. ANESTHESIOLOGY 72:347-374, 1990.
Sprung J, Cheng EY, Gamulin S, Kampine JP, Bosnjak ZJ: Effects of acute hypothermia and beta-adrenergic receptor blockade on serum potassium concentration in rats. Crit Care Med 19:1545-1551, 1991.
Koht A, Cane R, Cerullo LJ: Serum potassium levels during prolonged hypothermia. Intensive Care Med 9:275-277, 1983.
Rosen MR, Legato MJ: Repolarization: Physiological and structural determinants, and pathophysiological changes. Eur Heart J 6(suppl):S3-S14, 1985.
Dersham GH, Han J: Effects of verapamil on action potentials of Purkinje fibers, J Electrocardiol 13:67-72, 1980.
Han J, Moe GK: Nonuniform recovery of excitability in ventricular muscle. Circ Res 14:44-60, 1964.
Kuo CS, Munakata K, Reddy P, Surawicz B: Characteristics and possible mechanisms of ventricular arrhythmias dependent on the dispersion of action potential durations. Circulation 67:1356-1367, 1983.
Klein R, Haddow JE, Kind C, Cockburn F: Effect of cold on muscle potentials and electrolytes, Metabolism 17:1091-1103, 1968.
Coraboeuf E, Weidman S: Temperature effects of the electrical activity of Purkinje fibers. Helvetica Physiologica et Pharmacologica Acta 12:32-41, 1954.
Hoffman BF, Cranefield PF: The Purkinje fibers, Electrophysiology of the Heart, New York, Futura, 1976, pp 175-210.
Langer GA, Brady AJ: The effects of temperature upon contraction and ionic exchange in rabbit ventricular myocardium. Relation to control of active state. J Gen Physiol 52:682-713, 1968.
Vaughan-Jones RD: Excitation and contraction in heart. The role of calcium. Br Med Bull 12:113-120, 1986.
Wit AL, Rosen MR: Afterdepolarization and triggered activity. The Heart and Cardiovascular System. Edited by Fozzard HA, Haber E, Jennings RB, Katz AM, Morgan HE, New York, Raven Press, 1986, pp 1449-1490.
Allen DG, Eisner DA, Pirolo JS, Smith GL: The relationship between intracellular calcium and contraction in calcium overloaded ferret papillary muscles. J Physiol (Lond) 364:169-182, 1985.
Amerini S, Giotti A, Mugelli A: Effect of verapamil and diltiazem on calcium-dependent electrical activity in cardiac Purkinje fibres. Br J Pharmacol 85:89-96, 1985.
Bjornstad H, Lathrop DA, Refsum H: Prevention of some hypothermia induced electromechanical changes by calcium channel blockade. Cardiovasc Res 28:55-60, 1994.
Bjornstad H, Tande PM, Lathrop DA, Refsum H: Effects of temperature on cycle length dependent changes and restitution of action potential duration in guinea pig ventricular muscle. Cardiovasc Res 27:946-950, 1993.
Hiraoka M, Hecht HH: Recovery from hypothermia in cardiac Purkinje fibers: Considerations for an electrogenic mechanism Pflugers Arch 339:25-36, 1973.
Isenberg G, Trautwein W: Temperature sensitivity of outward current in cardiac Purkinje fibers: Evidence for electrogeneicity of active transport. Pflugers Arch 358:225-234, 1975.
Reder RF, Miura DS, Danilo P, Rosen MR: The electrophysiological properties of normal neonatal and adult canine cardiac Purkinje fibers. Circ Res 18:658-668, 1981.
Rosen MR, Hvento JP, Gelband H, Merker C: Effects of verapamil on electrophysiologic properties of canine cardiac Purkinje fibers. J Pharmacol Exp Ther 189:414-422, 1971.
Danilo P Jr, Hordof AJ, Reder RF, Rosen MR: Effects of verapamil on electrophysiologic properties of blood superfused cardiac Purkinje fibers. J Pharmacol Exp Ther 213:222-227, 1980.
Kass RS, Tsien RW: Multiple effects of calcium antagonists on plateau currents in cardiac Purkinje fibers. J Gen Physiol 66:169-192, 1975.
Cohen CJ, Bean BP, Tsien RW: Maximal upstroke velocity as an index of available sodium conductance. Circ Res 51:636-651, 1984.
Hondeghem LM, Katzung BG: Time- and voltage-dependent interactions of antiarrhythmic drugs with cardiac sodium channels. Biochim Biophys Acta 172:373-398, 1977.
Gettes LS, Reuter H: Slow recovery from inactivation of inward currents in mammalian myocardial fibers. J Physiol (Lond) 240:703-724, 1974.
Isenberg G, Trautwein W: Temperature sensitivity of outward current in cardiac Purkinje fibers. Evidence for electrogenicity of active transport. Pflugers Arch 358:225-235, 1975.
Eisner DA, Lederer WJ: Characterization of the electrogenic sodium pump in cardiac Purkinje fibers. J Physiol (Lond) 303:441-474, 1980.
Chapman RA: Sodium/calcium exchange and intracellular calcium buffering in ferret myocardium: An ion-sensitive microelectrode study. J Physiol (Lond) 373:163-179, 1986.
Cranefield PF, Aronson RS, Witt AL: Effect of verapamil on the normal action potential and on a calcium-dependent slow response of canine cardiac Purkinje fibers. Circ Res 34:204-213, 1974.
Shigenobu K, Schneider JA, Sperelakis N: Verapamil blockade of slow Sodium sup + and Calcium sup ++ responses in myocardial cells. J Pharmacol Exp Ther 190:280-288, 1974.
Bayer R, Kalusche D, Kaufmann R, Mannhold R: Inotropic and electrophysiological actions of verapamil and D600 in mammalian myocardium. Naunyn Schmiedebergs Arch Pharmacol 290:81-97, 1975.
Galper JB, Catteral WA: Inhibition of sodium channels by D600. Mol Pharmacol 15:174-178, 1979.
Lynch C[I]: Cellular electrophysiology of the heart, Clinical Cardiac Electrophysiology: Perioperative Considerations. A Society of Cardiovascular Anesthesiologists Monograph. Edited by Lynch C III. Philadelphia, JB Lippincott, 1994, pp 1-52.
Eisner DA, Lederer WS: A cellular basis for lidocaine's antiarrhythmic action. J Physiol (Lond) 295(suppl):25P-26P, 1979.
Rosen M, Danilo P: Effects of tetrodotoxin, lidocaine, verapamil and AHR-2666 on ouabain-induced delayed afterdepolarization in canine Purkinje fibers. Circ Res 46:117-124, 1980.
Fozzard HA, Wasserstrom JA: Voltage dependence of intracellular sodium and control of contraction. Cardiac Electrophysiology. Edited by Zipes DP, Jalife J, Orlando, Grune and Stratton. 1985, pp 51-57.
Siegelbaum SA, Tsien RW: Calcium-activated transient outward current in calf cardiac Purkinje fibers. J Physiol (Lond) 299:485-506, 1980.
Bassingthwaighte JB, Fry CH, McGuigan JAS: Relationship between internal calcium and outward current in mammalian ventricular muscle: a mechanism for the control of the action potential duration? J Physiol (Lond) 262:15-37, 1976.
Reuter H, Seitz N: The dependence of calcium efflux from cardiac muscle on temperature and on external ion composition. J Physiol (Lond) 195:451-470, 1968.
Carmeliet E: Potassium sup + channels in cardiac cells: Mechanisms of activation inactivation, rectification and Potassium sup + sub e sensitivity. Pflugers Arch 414(suppl):S88-S92, 1989.
Egan TM, Noble SJ, Powell T, Spindler AJ, Twist VW: Sodium-calcium exchange during the action potential in guinea-pig ventricular cells. J Physiol (Lond) 411:639-661, 1989.
Cavalie A, Mcdonald TF, Trautwein W: Temperature-induced transitory and steady-state changes in the calcium current of guinea pig ventricular myocyte. Pflugers Arch 405:294-296, 1985.
Arreola J, Dirksen RT, Shieh RC, Williford DJ, Sheu SS: Calcium sup +2 current and Calcium sup +2 transients under action potential clamp in guinea pig ventricular myocytes. Am J Physiol 261:393-397, 1991.
Shattock MJ, Bers DM: Inotropic response to hypothermia and the temperature-dependence on ryanodine action in isolated rabbit and rat ventricular muscle: Implications foe extitation-contraction coupling. Circ Res 61:761-771, 1987.
Scamps F, Carmeliet E: Delayed Potassium sup + current and external Potassium sup + in single cardiac Purkinje cells. Am J Physiol 257:C1080-C1092, 1989.
Posner P, Miller L, Lambert CR: The effect of verapamil on potassium fluxes in canine cardiac Purkinje fibers. Eur J Pharmacol 34:369-372, 1974.
Cavalie A, Ochi R, Pelzer D, Trautwein W: Elementary currents through Calcium sup 2+ channels in guinea-pig myocytes. Pflugers Arch 398:284-297, 1983.
Ferrier GR: Digitalis arrhythmias: Role of oscillatory after-potentials. Prog Cardiovasc Dis 19:459-474, 1977.
Gallager JD, Bianchi JJ, Gessman LJ: Halothane antagonizes ouabain toxicity in isolated canine Purkinje fibers. ANESTHESIOLOGY 71:695-703, 1989.
Han J: Mechanisms of ventricular arrhythmias associated with myocardial infarction. Am J Cardiol 24:857-864, 1969.
Figure 1. Effect of hypothermia on action potential of canine cardiac Purkinje fiber superfused with normal Krebs' solution (top) and in the presence of verapamil (bottom). See text for details.
Figure 1. Effect of hypothermia on action potential of canine cardiac Purkinje fiber superfused with normal Krebs' solution (top) and in the presence of verapamil (bottom). See text for details.
Figure 1. Effect of hypothermia on action potential of canine cardiac Purkinje fiber superfused with normal Krebs' solution (top) and in the presence of verapamil (bottom). See text for details.
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Figure 2. Effect of hypothermia and Potassium sup + variations on maximum diastolic potential (MDP) of Purkinje fiber action potentials with and without verapamil in the superfusate. *P < 0.05 versus 2.3 and 6.8 mM external [K sup +]([K sup +]o) at same temperature. (dagger)P < 0.05 versus 37 degrees Celsius at same [K sup +]o.
Figure 2. Effect of hypothermia and Potassium sup + variations on maximum diastolic potential (MDP) of Purkinje fiber action potentials with and without verapamil in the superfusate. *P < 0.05 versus 2.3 and 6.8 mM external [K sup +]([K sup +]o) at same temperature. (dagger)P < 0.05 versus 37 degrees Celsius at same [K sup +]o.
Figure 2. Effect of hypothermia and Potassium sup + variations on maximum diastolic potential (MDP) of Purkinje fiber action potentials with and without verapamil in the superfusate. *P < 0.05 versus 2.3 and 6.8 mM external [K sup +]([K sup +]o) at same temperature. (dagger)P < 0.05 versus 37 degrees Celsius at same [K sup +]o.
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Figure 3. Effect of hypothermia and Potassium sup + variations on the maximum rate of phase 0 depolarization (Vmax) of Purkinje fiber action potentials with and without verapamil in the superfusate. *P < 0.05 versus 37, 32, and 28 degrees Celsius; dagger P < 0.05 versus verapamil; double dagger P < 0.05 versus 2.3 and 3.9 mM external [K sup +]+ verapamil.
Figure 3. Effect of hypothermia and Potassium sup + variations on the maximum rate of phase 0 depolarization (Vmax) of Purkinje fiber action potentials with and without verapamil in the superfusate. *P < 0.05 versus 37, 32, and 28 degrees Celsius; dagger P < 0.05 versus verapamil; double dagger P < 0.05 versus 2.3 and 3.9 mM external [K sup +]+ verapamil.
Figure 3. Effect of hypothermia and Potassium sup + variations on the maximum rate of phase 0 depolarization (Vmax) of Purkinje fiber action potentials with and without verapamil in the superfusate. *P < 0.05 versus 37, 32, and 28 degrees Celsius; dagger P < 0.05 versus verapamil; double dagger P < 0.05 versus 2.3 and 3.9 mM external [K sup +]+ verapamil.
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Figure 4. Relation between maximum diastolic potential (MDP) and maximum rate of phase 0 depolarization (Vmax) of Purkinje fiber action potentials at 37 degrees Celsius and at 30 degrees Celsius during the gradual increase in external [K sup +]([K sup +]o) from 0.8 to 10 mM. *P < 0.05 MDP or Vmaxversus lower [K sup +]o. *P < 0.05 MDP37degrees Celsius versus MDP30degrees Celsius at same [K sup +]o.
Figure 4. Relation between maximum diastolic potential (MDP) and maximum rate of phase 0 depolarization (Vmax) of Purkinje fiber action potentials at 37 degrees Celsius and at 30 degrees Celsius during the gradual increase in external [K sup +]([K sup +]o) from 0.8 to 10 mM. *P < 0.05 MDP or Vmaxversus lower [K sup +]o. *P < 0.05 MDP37degrees Celsius versus MDP30degrees Celsius at same [K sup +]o.
Figure 4. Relation between maximum diastolic potential (MDP) and maximum rate of phase 0 depolarization (Vmax) of Purkinje fiber action potentials at 37 degrees Celsius and at 30 degrees Celsius during the gradual increase in external [K sup +]([K sup +]o) from 0.8 to 10 mM. *P < 0.05 MDP or Vmaxversus lower [K sup +]o. *P < 0.05 MDP37degrees Celsius versus MDP30degrees Celsius at same [K sup +]o.
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Figure 5. Relative changes in action potential duration at 50% repolarization (APD50) by verapamil at various external [K sup +]([K sup +]o) during cooling (control [C] values are normalized to the values obtained at 37 degrees Celsius and at 3.9 mM [K sup +]o). *P < 0.05 V (verapamil) versus C (control) at respective temperature.
Figure 5. Relative changes in action potential duration at 50% repolarization (APD50) by verapamil at various external [K sup +]([K sup +]o) during cooling (control [C] values are normalized to the values obtained at 37 degrees Celsius and at 3.9 mM [K sup +]o). *P < 0.05 V (verapamil) versus C (control) at respective temperature.
Figure 5. Relative changes in action potential duration at 50% repolarization (APD50) by verapamil at various external [K sup +]([K sup +]o) during cooling (control [C] values are normalized to the values obtained at 37 degrees Celsius and at 3.9 mM [K sup +]o). *P < 0.05 V (verapamil) versus C (control) at respective temperature.
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Figure 6. Relative changes in action potential duration at 95% repolarization (APD95) by verapamil at various external [K sup +]([K sup +]o) during cooling (control [C] values are normalized to the values obtained at 37 degrees Celsius and at 3.9 mM [K sup +]o). *P < 0.05 V (verapamil) versus C (control) at respective temperature.
Figure 6. Relative changes in action potential duration at 95% repolarization (APD95) by verapamil at various external [K sup +]([K sup +]o) during cooling (control [C] values are normalized to the values obtained at 37 degrees Celsius and at 3.9 mM [K sup +]o). *P < 0.05 V (verapamil) versus C (control) at respective temperature.
Figure 6. Relative changes in action potential duration at 95% repolarization (APD95) by verapamil at various external [K sup +]([K sup +]o) during cooling (control [C] values are normalized to the values obtained at 37 degrees Celsius and at 3.9 mM [K sup +]o). *P < 0.05 V (verapamil) versus C (control) at respective temperature.
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Figure 7. Effect of verapamil (closed symbols) on the relative relation between action potential duration at 50%(APD50) and 95%(APD95) repolarization at 28 degrees Celsius at various external [K sup +]([K sup +]o). Values are normalized to their respective controls at 37 degrees Celsius (open symbols). *P < 0.05 describes the significance of relative changes between action potential duration at 50%(APD50) versus 95%(APD95) repolarization after verapamil.
Figure 7. Effect of verapamil (closed symbols) on the relative relation between action potential duration at 50%(APD50) and 95%(APD95) repolarization at 28 degrees Celsius at various external [K sup +]([K sup +]o). Values are normalized to their respective controls at 37 degrees Celsius (open symbols). *P < 0.05 describes the significance of relative changes between action potential duration at 50%(APD50) versus 95%(APD95) repolarization after verapamil.
Figure 7. Effect of verapamil (closed symbols) on the relative relation between action potential duration at 50%(APD50) and 95%(APD95) repolarization at 28 degrees Celsius at various external [K sup +]([K sup +]o). Values are normalized to their respective controls at 37 degrees Celsius (open symbols). *P < 0.05 describes the significance of relative changes between action potential duration at 50%(APD50) versus 95%(APD95) repolarization after verapamil.
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Table 1. Effect of Hypothermia, Potassium sup + Variations, and Verapamil on Action Potential Duration at 50% of Repolarization (APD50, ms)
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Table 1. Effect of Hypothermia, Potassium sup + Variations, and Verapamil on Action Potential Duration at 50% of Repolarization (APD50, ms)
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Table 2. Effect of Hypothermia, Potassium sup + Variations, and Verapamil on Action Potential Duration at 95% of Repolarization (APD95, ms)
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Table 2. Effect of Hypothermia, Potassium sup + Variations, and Verapamil on Action Potential Duration at 95% of Repolarization (APD95, ms)
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