AJP - Heart AJP: Gastrointestinal and Liver Physiology
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

Am J Physiol Heart Circ Physiol 283: H412-H422, 2002. First published March 28, 2002; doi:10.1152/ajpheart.01042.2001
0363-6135/02 $5.00
This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow All Versions of this Article:
283/1/H412    most recent
01042.2001v1
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Right arrow Citation Map
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via ISI Web of Science (18)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Komukai, K.
Right arrow Articles by Orchard, C. H.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Komukai, K.
Right arrow Articles by Orchard, C. H.
Vol. 283, Issue 1, H412-H422, July 2002

Electrophysiological response of rat ventricular myocytes to acidosis

Kimiaki Komukai, Fabien Brette, Caroline Pascarel, and Clive H. Orchard

School of Biomedical Sciences, University of Leeds, Leeds LS2 9NL, United Kingdom


    ABSTRACT
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

The effects of acidosis on the action potential, resting potential, L-type Ca2+ (ICa), inward rectifier potassium (IK1), delayed rectifier potassium (IK), steady-state (ISS), and inwardly rectifying chloride (ICl,ir) currents of rat subepicardial (Epi) and subendocardial (Endo) ventricular myocytes were investigated using the patch-clamp technique. Action potential duration was shorter in Epi than in Endo cells. Acidosis (extracellular pH decreased from 7.4 to 6.5) depolarized the resting membrane potential and prolonged the time for 50% repolarization of the action potential in Epi and Endo cells, although the prolongation was larger in Endo cells. At control pH, ICa, IK1, and ISS were not significantly different in Epi and Endo cells, but IK was larger in Epi cells. Acidosis did not alter ICa, IK1, or IK but decreased ISS; this decrease was larger in Endo cells. It is suggested that the acidosis-induced decrease in ISS underlies the prolongation of the action potential. ICl,ir at control pH was Cd2+ sensitive but 4,4'-disothiocyanato-stilbene-2,2'-disulfonic acid resistant. Acidosis increased ICl,ir; it is suggested that the acidosis-induced increase in ICl,ir underlies the depolarization of the resting membrane potential.

action potential; chloride current; potassium current; perforated patch


    INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

CARDIAC MUSCLE can become acidic in pathological conditions such as coronary artery disease and some systemic disorders. However, these conditions differ because in coronary artery disease (e.g., myocardial infarction) acidosis only occurs in the ischemic region, whereas during systemic disorders (e.g., diabetes and respiratory depression) acidosis occurs homogeneously.

Acidosis alters action potential configuration (3, 27, 43) so that localized acidosis will alter action potential dispersion, which could be proarrhythmic. Homogeneous acidosis may also change action potential dispersion because the shape of the cardiac action potential is different in different regions of the heart (1, 41) as a consequence of regional differences in channel expression. Thus acidosis-induced changes in the currents carried by these channels may lead to heterogeneous changes in action potential configuration and hence dispersion.

Regional variation in the response of action potential configuration to acidosis might also explain the different acidosis-induced changes in action potential configuration reported in previous studies (3, 27, 43), which did not discriminate the region from which the cells were obtained. There are, however, other possible explanations for these differences. These include the severity and type of acidosis, and the recording conditions used [for example, L-type Ca2+ current (ICa) is decreased by acidosis when intracellular Ca2+ is buffered, as during whole cell recording, but not during perforated patch recording; 8, 15, 18, 26, 27].

The present study was designed, therefore, to investigate the electrophysiological response to acidosis of cells from different regions of the heart recorded under standard conditions. Preliminary results have been presented to the Physiological Society (25).


    METHODS
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Cell isolation. Male Wistar rats (~220-250 g) were stunned and then killed by cervical dislocation. The heart was rapidly removed, and cells were isolated by collagenase digestion of the Langendorff-perfused heart as described previously (26), with the following variation: after 7-min perfusion of the heart with collagenase-containing solution, small pieces of subepicardial (Epi) and subendocardial (Endo) muscle (<1 mm and <2 mm thick, respectively) were dissected from the left ventricular free wall. These were incubated separately in collagenase-containing isolation solution plus 1% bovine serum albumin for 5 min. The tissue was then filtered and the filtrate centrifuged; the supernatant was removed, and the cells were resuspended in isolation solution containing 0.75 mM Ca2+ and stored at room temperature. This procedure was repeated four to five times with the remaining tissue.

Experimental set up. An aliquot of cells was transferred to the experimental chamber, which was mounted on the stage of an inverted microscope (Diaphot TMD, Nikon; Tokyo, Japan) and continuously perfused with the experimental solutions (see Solutions and chemicals). The perforated patch-clamp technique was used for most of the experiments in this paper, as described previously (26). Electrodes (2-4 MOmega ) were used; the tip of the electrode was filled with pipette solution (see Solutions and chemicals) and back filled with pipette solution containing 200-400 µg/ml amphotericin B. The pipette solution contained 1 mM CaCl2 to ensure that accidental rupture of the membrane resulted in cell death. After electrical access was obtained and the capacity transients became constant, series resistance was compensated, and measurements were made. Changing between control and acid solutions while the electrode tip was in the bath solution was used to ensure the absence of artifactual changes of potential. Voltage and current signals were filtered at 1 kHz (low pass) and digitized at 2 kHz; all experiments were performed at room temperature.

Measurement of action potentials. Action potentials were measured in current clamp configuration. Current pulses of 4-6 ms, sufficient to trigger an action potential, were injected every 2 s.

Measurement of ICa. Holding potential was set to -40 mV to inactivate Na+ current (INa), and a 200-ms depolarizing pulse to 0 mV was applied every 2 s. Current-voltage relationships were obtained using a series of test pulses between -30 and +50 mV. The amplitude of the current was measured as the difference between peak inward current and current remaining at the end of pulses. KCl in the pipette solution and perfusate was replaced with CsCl to block K+ currents.

Measurement of inward rectifier K+ current. Holding potential was set to -40 mV to inactivate INa. The current-voltage relationship was obtained using a series of 500-ms test pulses from -120 to +20 mV; pulses were applied every 5 s. The current was measured at 500 ms (7). CdCl2 (0.1 mM) was added to the perfusate to block ICa.

Measurement of delayed rectifier K+ and steady-state currents. Depolarization induces two components of outward current in rat ventricular myocytes (2): transient outward current (ITO) and sustained current, which in turn consists of two components (35): one of which, the delayed rectifier K+ current (IK), shows steady-state inactivation, whereas the other, steady-state current (ISS), does not (7, 19). To evoke IK and ISS, holding potential was set to -80 mV and a series of 500-ms test pulses to voltages between -40 and +40 mV was applied; each test pulse was preceded by a 40-ms prepulse to -40 mV to inactivate INa, and pulses were applied every 2 s. IK was calculated by subtracting ISS from the current at the end of the pulse (7): ISS alone was evoked using same the protocol except that the holding potential was -20 mV and a prepulse was not used (at this holding potential IK and INa are inactivated; Refs. 7 and 35). 4-Aminopyridine (4-AP) was not used because of its inhibitory effects on inward rectifier K+ current (IK1) (9), IK (35), and ISS (35). However, even in the absence of 4-AP, ITO is inactivated within the pulse duration so that the current at 500 ms will be unaffected (4). CdCl2 (0.5 mM) was included in the perfusate to block ICa.

Measurement of inwardly rectifying Cl- current. Conventional whole cell (ruptured) patch clamp was used to measure Cl- currents (ICl). A 3 M KCl-agar bridge-AgCl pellet was used as the bath electrode. Holding potential was set to -40 mV. Two-second test pulses to voltages between -120 and +40 mV (in 20-mV increments) followed by 400-ms pulses to +40 mV were applied; pulses were applied every 10 s (10). Current was measured at 2 s.

Solutions and chemicals. The solutions used for cell isolation were as described previously (26). The solution used during the experiments contained (in mM) 108 NaCl, 5 KCl, 1 Na2HPO4 · 12H2O, 1 MgSO4 · 7H2O, 20 sodium acetate, 10 glucose, 1 CaCl2, and 10 HEPES, with 5 U/l insulin; pH adjusted to 7.40 or 6.50 using NaOH. The pipette solution contained (in mM) 130 KCl, 10 NaCl, 1.4 MgCl2 · 6H2O, 1 CaCl2, and 5 HEPES; pH adjusted to 7.10 using KOH. A 50 mg/ml stock solution of amphotericin-B was made in dimethylsulfoxide immediately before the experiments; this was diluted into the pipette solution before use.

The perfusate during ICl measurement contained (in mM) 120 TEACl, 10 CsCl, 1 MgCl2 · 6H2O, 10 HEPES, 2 BaCl2, 10 glucose, 1 CaCl2, 2 4-AP, and 0.01 nifedipine; pH adjusted to 7.4 or 6.5 using TEAOH. The pipette solution contained (in mM) 110 CsCl, 20 TEACl, 5 MgATP, 5 EGTA, and 5 HEPES; pH adjusted to 7.1 using CsOH. The osmolality of the perfusate and the pipette solution for ICl measurement was 270 ± 5 mosmol/kgH2O. A 10 mM stock solution of nifedipine and a 100 mM stock solution of 4,4'-disothiocyanato-stilbene-2,2'-disulfonic acid (DIDS) were made with DMSO and kept in light-resistant containers. A 10 mM stock solution of strophanthidin was made in ethanol. All stock solutions were diluted into the perfusate immediately before use. Addition of DIDS (final concentration, 0.1 mM) did not alter the pH of the solution.

Statistical analysis. Data are expressed as means ± SE for n cells. Paired or unpaired t-tests (two-tailed) were performed as appropriate. When an unpaired t-test was used, the variances were tested by the F-test. Statistical significance was taken as P < 0.05.


    RESULTS
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Effect of acidosis on action potentials in Epi and Endo cells. Figure 1 shows action potentials recorded from representative Epi (A) and Endo (B) cells at control pH and during acidosis. At control pH, resting membrane potential and the amplitude of the action potential were not significantly different in Epi and Endo cells. However, the times for 25, 50, and 90% repolarization of the action potential (APD25, APD50, and APD90, respectively) were significantly longer in Endo cells than in Epi cells (Fig. 1 and Table 1).


View larger version (9K):
[in this window]
[in a new window]
  		Fig. 1.   Action potential of representative subepicardial (Epi, A) and subendocardial (Endo, B) cells at control pH and during acidosis.


                              
View this table:
[in this window]
[in a new window]
  		Table 1.   Effect of acidosis on action potential parameters in subepicardial and subendocardial ventricular myocytes

Acidosis caused a small (~3 mV), but significant, depolarization of the resting membrane potential (Table 1) and decreased the amplitude of the action potential in Epi and Endo cells (Table 1); these changes were not significantly different between Epi and Endo cells. Acidosis also significantly prolonged APD50 in Epi and Endo cells; this prolongation was significantly greater in Endo cells than in Epi cells (P < 0.05), although acidosis did not significantly alter APD25 and APD90 in either Epi or Endo cells (Table 1). Although the effect of acidosis on APD50, particularly in the Epi cells, was small, the measurements at control pH and during acidosis were made on the same cells (i.e., paired data) making artifactual differences unlikely.

Subsequent experiments were designed to investigate the current(s) that contribute to the acidosis-induced change in the action potential configuration and resting membrane potential. Currents are presented as current density; cell capacitance was calculated from capacitance transients during 10-ms, 10-mV hyperpolarizing pulses from -80 mV. Cell capacitance was 136 ± 4 pF in Epi (n = 23) and 145 ± 7 in Endo cells (n = 23); these values are not statistically different.

Effect of acidosis on ICa in Epi and Endo cells. Figure 2A shows original records of ICa elicited in representative Epi (left) and Endo (right) cells at control pH and during acidosis by depolarizing pulses from -40 to 0 mV. Figure 2B shows that the mean ICa density-voltage relationships were not significantly different in Epi and Endo cells: maximum current density was 3.1 ± 0.3 pA/pF (n = 7) in Epi and 2.8 ± 0.5 pA/pF (n = 7) in Endo cells (not significant, NS).


View larger version (22K):
[in this window]
[in a new window]
  		Fig. 2.   A: L-type Ca2+ current (ICa) recorded in representative Epi (left) and Endo (right) cells during a depolarizing pulse from -40 mV to 0 mV at control pH and during acidosis. Arrows indicate zero current level. Cell capacitance was 123 pF (left) and 155 pF (right). B: mean (±SE) current density-voltage relationship of ICa in Epi and Endo cells. C and D: mean (±SE) current-voltage relationship of ICa in Epi (C) and Endo (D) cells at control pH and during acidosis.

Figure 2, C and D, shows mean ICa current density-voltage relationships in Epi (Fig. 2C) and Endo (Fig. 2D) cells at control pH, and during acidosis, showing that acidosis did not alter the voltage dependence or current density of ICa in either Epi and Endo cells: maximum ICa density in Epi cells was 3.12 ± 0.29 pA/pF in control and 3.57 ± 0.24 pA/pF in acidosis (NS, n = 5, paired t-test). The corresponding values in Endo cells were 2.70 ± 0.53 pA/pF in control and 2.84 ± 0.48 pA/pF in acidosis (NS, n = 6, paired t-test).

Thus it appears unlikely that ICa underlies the difference in action potential duration in Epi and Endo cells or the acidosis-induced prolongation of the action potential.

Effect of acidosis on IK1 in Epi and Endo cells. Figure 3A shows original traces of the current elicited by 500-ms test pulses from a holding potential of -40 mV in representative Epi (left) and Endo (right) cells; only currents elicited by test pulses between -120 and -40 mV are shown for clarity. Figure 3B shows that the current density-voltage relationship of the sustained current (i.e., current at the end of the pulse) was not significantly different in Epi and Endo cells. The current elicited by test pulses negative to -40 mV is dominated by IK1 (22, 37, 44) and will be presented here. The current elicited by test potentials positive to -40 mV also contains ISS, which may explain the changes in the current observed at more depolarized potentials (see below). Current density at -120 mV was -3.83 ± 0.48 pA/pF (n = 9) in Epi cells and -3.53 ± 0.29 pA/pF (n = 6) in Endo cells (NS).


View larger version (20K):
[in this window]
[in a new window]
  		Fig. 3.   A: original records of current elicited by hyperpolarizing voltage-clamp pulses in representative Epi (left) and Endo (right) cells. Arrows indicate zero current level. Cell capacitance was 120 pF (left) and 139 pF (right). B: mean (±SE) current density-voltage relationship of sustained current in Epi and Endo cells. C and D: mean (±SE) current-voltage relationship of sustained current in Epi (C) and Endo (D) cells at control pH and during acidosis. IK1, inward rectifier K+ current.

Figure 3, C and D, shows current density-voltage relationships of the sustained current at control pH and during acidosis in Epi (Fig. 3C) and Endo (Fig. 3D) cells. Acidosis did not significantly alter the sustained current elicited by test pulses to more negative potentials in either Epi or Endo cells: current density at -120 mV in Epi cells was -3.99 ± 0.51 pA/pF at control pH and -3.70 ± 0.34 pA/pF during acidosis (NS, n = 8). The corresponding values in Endo cells were -3.53 ± 0.29 pA/pF in control and -3.75 ± 0.61 pA/pF in acidosis (NS, n = 6).

Thus it appears unlikely that IK1 underlies the difference in action potential duration in Epi and Endo cells, or the acidosis-induced change in the action potential or resting potential.

Effect of acidosis on IK and ISS in Epi and Endo cells. Figure 4 shows original traces of the total outward current (left), ISS (middle), and IK (right) in representative Epi (top) and Endo (bottom) cells. Test pulses of 500 ms to potentials between -40 and +40 mV were first applied from a holding potential of -80 mV to evoke total outward current (left). Pulses to the same test potentials were then applied from a holding potential of -20 mV to inactivate IK and ITO (44) and evoke ISS alone (7; Fig. 4, middle). ISS was then subtracted from the total outward current to obtain ITO plus IK (right). However, ITO inactivates within the pulse duration (4) so that the current at the end of the pulse represents IK. Note, however, that ITO was present in Epi cells but not in Endo cells. Figures 5A and 6A show mean current density-voltage relationships for IK and ISS, respectively, of Epi and Endo cells. IK was significantly larger in Epi cells than in Endo cells at most potentials positive to -10 mV (Fig. 5A): IK current density at +40 mV was 2.18 ± 0.20 pA/pF (n = 7) in Epi cells and 1.68 ± 0.12 pA/pF in Endo cells (n = 7; P < 0.05). In contrast, ISS was not significantly different in Epi and Endo cells (Fig. 6A): ISS current density at -20 mV was 0.79 ± 0.06 pA/pF (n = 7) in Epi cells and 0.83 ± 0.05 pA/pF (n = 10) in Endo cells (NS).


View larger version (15K):
[in this window]
[in a new window]
  		Fig. 4.   Original records of IK and steady-state current (ISS) in representative Epi (top) and Endo (bottom) cells. Left: cells were depolarized from a holding potential of -80 mV to evoke total outward current. Middle: cells were depolarized from a holding potential of -20 mV to evoke ISS. Right: IK was calculated by subtracting ISS from total outward current. Arrows indicate zero current level. Insets show voltage protocols. Cell capacitance was 146 pF (top) and 189 pF (bottom).



View larger version (16K):
[in this window]
[in a new window]
  		Fig. 5.   A: mean (±SE) current density-voltage relationship of IK in Epi and Endo cells. * P < 0.05 vs. Epi cells. B and C: mean (±SE) IK current-voltage relationship in Epi (B) and Endo (C) cells at control pH and during acidosis. IK, delayed rectifier K+ current.



View larger version (22K):
[in this window]
[in a new window]
  		Fig. 6.   A: mean (±SE) current density-voltage relationship of ISS in Epi and Endo cells. B: mean current density relationship elicited by the same protocol used in A, but K+ replaced by Cs+, at control pH and during acidosis. C and D: mean ISS current-voltage relationship in Epi (C) and Endo (D) cells at control pH and during acidosis. * P < 0.05 vs. control. ** P < 0.01 vs. control.

Figure 5 also shows mean IK density-voltage relationships at control pH and during acidosis in Epi (Fig. 5B) and Endo (Fig. 5C) cells. Acidosis had no significant effect on IK in either Epi and Endo cells at any voltage: IK current density at +40 mV in Epi cells was 2.30 ± 0.19 pA/pF at control pH and 2.34 ± 0.20 pA/pF in acidosis (NS, n = 6). The corresponding values in Endo cells were 1.77 ± 0.19 pA/pF at control pH and 1.78 ± 0.19 pA/pF during acidosis (NS, n = 6).

Figure 6 shows the corresponding relationships for ISS at control pH and during acidosis in Epi (Fig. 6C) and Endo (Fig. 6D) cells. Acidosis decreased ISS at test potentials negative to 0 mV in Epi cells and at all potentials tested (from -40 to +40 mV) in Endo cells: acidosis decreased ISS density at -20 mV from 0.82 ± 0.07 to 0.66 ± 0.04 pA/pF (P < 0.05, n = 6) in Epi cells and from 0.83 ± 0.04 to 0.61 ± 0.04 pA/pF (P < 0.01, n = 6) in Endo cells.

When the protocol used to monitor ISS was repeated with K+ replaced by Cs+, the current was inhibited and acidosis had no significant effect (Fig. 6B). Thus it appears that ISS is carried predominantly by K+, and the acidosis-sensitive component is carried completely by K+. Because acidosis had no effect on the Cs+-insensitive current, it has not been subtracted from the other currents shown in Fig. 6.

It appears likely, therefore, that the lower density of IK in Endo cells contributes to the longer action potential observed in these cells, whereas the more pronounced inhibition of ISS by acidosis in Endo cells will contribute to the more marked prolongation of the action potential produced by acidosis in these cells.

Effect of acidosis on ICl in ventricular cells. A novel inwardly rectifying Cl- current (ICl,ir) has recently been described in cardiac myocytes (10). This current is sensitive to Cd2+ and resistant to DIDS, which, with its inward rectification, distinguishes it from Cl- currents previously described in the heart. ICl,ir is thought to be carried by ClC-2 chloride channels, which, when expressed in Xenopus oocytes, are sensitive to pH (21). To investigate whether this current might contribute to the response to acidosis, particularly the depolarization of the resting membrane potential observed in Epi and Endo cells (Table 1), we have investigated whether acidosis alters ICl,ir in ventricular myocytes. Because the effect of acidosis on resting membrane potential is not significantly different in Epi and Endo cells (see Table 1), these experiments were performed on myocytes isolated from the whole ventricle, using K+- and Na+-free, nifedipine- and Cs+-containing solutions (see METHODS) to avoid contamination by currents carried by K+, Na+, and Ca2+, and possible complications due to effects of Cd2+ and DIDS on currents carried by these ions (e.g., 40).

Figure 7A shows representative current traces at control pH, during acidosis, and after recovery at control pH. At control pH, the current showed slow activation, which could be fitted using two exponentials (10) with time constants of 162 ± 29 ms and 1,719 ± 563 ms (n = 7), and showed strong inward rectification (Fig. 7B). Acidosis increased this current (Fig. 7, A and B): the current at -120 mV increased from -4.7 ± 1.0 pA/pF at control pH to -7.5 ± 1.5 pA/pF during acidosis (n = 8, P < 0.01) but did not significantly alter the time constants of activation. The effects of acidosis were completely reversible (Fig. 7A).


View larger version (26K):
[in this window]
[in a new window]
  		Fig. 7.   Effect of acidosis on inwardly rectifying Cl- current (ICl,ir). A: original traces of ICl,ir from a representative cell (capacitance 153 pF) at control pH (left), during acidosis (middle), and after recovery (right) with 130 mM pipette [Cl-]. Protocol is shown schematically in inset (see text for detail). Arrow indicates zero current level. B: mean (±SE) current density-voltage relationships of ICl,ir at control pH and during acidosis when pipette [Cl-] was 130 mM or 20 mM. * P < 0.05 vs. control pH, ** P < 0.01 vs. control. C: mean (±SE) acidosis-induced current of ICl,ir with 130 mM pipette [Cl-] and 20 mM [Cl-]. * P < 0.05 vs. 130 mM pipette [Cl-], ** P < 0.01 vs. 130 mM pipette [Cl-].

To confirm that the current being measured was carried by Cl, the Cl concentration in the pipette solution ([Cl-]pip) was decreased from 130 to 20 mM by replacing CsCl with equimolar Cs-aspartate. This decreased current and caused a negative shift in the reversal potential (Fig. 7B): the current at -120 mV decreased from -4.3 ± 0.7 pA/pF (n = 17) at 130 mM [Cl-]pip to -0.4 ± 0.1 pA/pF (n = 5, P < 0.01) at 20 mM [Cl-]pip. Reversal potential, after correction for liquid junction potential, increased from -6 mV (predicted Cl- equilibrium potential, -1 mV) at 130 mM [Cl-]pip to -36 mV (-49 mV) at 20 mM [Cl-]pip (Fig. 7B). Reducing [Cl-]pip to 20 mM also reduced the acidosis-induced increase in membrane current: at -120 mV, the acidosis-induced change was -2.8 ± 0.6 pA/pF (n = 8) at 130 mM [Cl-]pip and -0.2 ± 0.1 pA/pF (n = 5) at 20 mM [Cl-]pip (P < 0.01, Fig. 7C).

Effect of CdCl2 and DIDS on ICl,ir in response to acidosis. The original traces in Fig. 8, A and B, show the effect of 0.1 mM CdCl2 and 0.1 mM DIDS, respectively, on ICl,ir, the response to acidosis in the presence of these agents, and the subsequent recovery from acidosis in their continuing presence.


View larger version (30K):
[in this window]
[in a new window]
  		Fig. 8.   Effect CdCl2 and DIDS on response of ICl,ir to acidosis. A and B: original traces of ICl,ir in (from left) control, the presence of CdCl2 (A; cell capacitance 123 pF) or DIDS (B; cell capacitance 143 pF), during acidosis in the presence of the drug, and after recovery in the presence of the drug. Arrow indicates zero current level. Protocol was same as in Fig. 7. C: mean (±SE) current density-voltage relationship of ICl,ir in control, in the presence of 0.1 mM CdCl2, and during acidosis in the presence of CdCl2. +P < 0.05 vs. control. ++P < 0.01 vs. control. * P < 0.05 vs. CdCl2. ** P < 0.01 vs. CdCl2. D: mean (±SE) current density-voltage relationship of ICl,ir in control, in presence of 1 mM DIDS, and during acidosis in the presence of DIDS. * P < 0.05 vs. DIDS. ** P < 0.01 vs. DIDS. E: mean (±SE) acidosis-induced current without drug, in the presence of CdCl2, and in the presence of DIDS. * P < 0.05 vs. control.

CdCl2 almost completely inhibited the current at control pH (Fig. 8, A and C); however, in the presence of CdCl2, acidosis still increased the current to a level that was not significantly different from control (Fig. 8C): the current at -120 mV was -4.3 ± 1.0 pA/pF in control, -1.0 ± 0.2 pA/pF in the presence of CdCl2 (P < 0.01 vs. control), and -5.3 ± 0.8 pA/pF during acidosis in the presence of CdCl2 (P < 0.001 vs. in the presence of CdCl2, NS vs. control, n = 8).

In contrast, DIDS had no significant effect on basal current (Fig. 8, B and D); however, in the presence of DIDS, the effect of acidosis on the current was markedly reduced (Fig. 8, B and D): the current at -120 mV was -4.9 ± 1.3 pA/pF in control, -5.3 ± 1.1 pA/pF in the presence of DIDS (NS vs. control), and -6.4 ± 1.4 pA/pF during acidosis in the presence of DIDS (P < 0.05 vs. in the presence of DIDS, n = 6).

Figure 8E shows the mean current density-voltage relationships for the acidosis-induced current (i.e., the increase in current during acidosis) in the absence of inhibitors and in the presence of CdCl2 or DIDS, showing that the acidosis-sensitive component is resistant to CdCl2 but sensitive to DIDS: acidosis-induced current at -120 mV is -2.8 ± 0.6 pA/pF in control (n = 8), -4.3 ± 1.0 pA/pF in the presence of CdCl2 (n = 8, NS vs. control), and -1.1 ± 0.2 pA/pF in the presence of DIDS (n = 6, P < 0.05 vs. control).


    DISCUSSION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

In the present study, acidosis was produced by decreasing the pH of the perfusate (i.e., extracellular pH) from 7.4 to 6.5. Intracellular pH (pHi) decreases by 0.5 units in response to this maneuver and reaches a new steady state within 3 min (14). The presence of a perforated patch electrode containing 5 mM HEPES does not affect the change of pHi (not shown). Measurements were made after a 5-min exposure to the acid solution (16), when both intracellular and extracellular pH had decreased. This can occur in vivo, for example, during ischemia when both extracellular and pHi decrease as a consequence of metabolite accumulation, and the decrease of pHi in the present study (0.5 pH units) is similar to that reported during global ischemia (0.8 pH units; 12). Acidosis alters the intracellular environment, increasing intracellular Ca2+ concentration ([Ca2+]i) (13), intracellular Na+ concentration ([Na+]i) (13), calcium-calmodulin-dependent kinase (CaMKII) activity via the increase in [Ca2+]i (14), and inhibiting protein phosphatase activity (32). We used the perforated patch-clamp technique to minimize disruption to these normal responses to investigate the physiological response to acidosis.

The methods used to record the currents measured in the present study have been described previously. Pharmacological K+ channel inhibitors were not used because many such inhibitors affect more than one channel (35). However, a depolarized holding potential is known to inhibit ITO and IK (44), allowing measurement of ISS (7; consistent with the noninactivating current shown in Fig. 4). It has also previously been shown that the current at the end of a 200-ms depolarizing pulse is insensitive to 4-AP, and thus that ITO inactivates during the pulse (4) so that the remaining current consists of ISS and IK. The protocol used to monitor IK1 has also been described previously (e.g., 7).

Regional differences in the effect of acidosis on the action potential. Action potential configuration is different in Epi and Endo cells in many species (for review see Ref. 1), and the rat action potential shows similar regional differences to other species, being shorter in Epi cells than in Endo cells (Fig. 1; Refs. 7 and 9). This appears to be due mainly to the higher density of ITO in Epi cells (6, 7, 9, 16, 44), although the present study shows that IK, which was larger in Epi cells, may also contribute to the shorter action potential.

Previous studies of the effect of acidosis on the action potential have given variable results, some showing prolongation and some shortening of the action potential (e.g., 3, 27). In the present study, acidosis prolonged APD50; this change was larger in Endo than in Epi cells. The possible mechanisms are considered below.

Regional differences in the effect of acidosis on ICa. The ICa density-voltage relationship of Epi cells and Endo cells was not significantly different, consistent with previous studies in the rat (6, 9), cat (24), and guinea pig (30).

Acidosis did not alter the current density-voltage relationship of ICa in Epi or Endo cells. This is consistent with previous reports using the perforated patch-recording technique in the rat ventricle (8, 15, 26) and suggests that a change in ICa is unlikely to underlie the change in APD during acidosis.

The possible role of Na2+/Ca2+ exchange in the response to acidosis. Previous work has shown that Na+/Ca2+ exchange current (INa/Ca) affects the slow repolarization phase in the rat ventricle and that inhibition of the exchanger shortens the APD (36). Because Na+/Ca2+ exchange is inhibited during acidosis (15, 26, 39), this would be expected to shorten rather than prolong the action potential. In addition, Na+/Ca2+ exchange expression appears to be the same in Epi and Endo cells (31), making it unlikely that the exchanger can account for the regional differences in the response to acidosis, although it has been reported that exchange current is smaller in Endo cells (34), which, if the exchanger were involved, might be expected to make the response to acidosis less pronounced in these cells.

The possible role of ITO in the response to acidosis. ITO is greater in Epi cells than in Endo cells (Fig. 4; Refs. 6, 7, and 9); this is the major factor underlying the difference in action potential configuration between Epi and Endo cells. The effect of acidosis on ITO has been investigated previously in the same conditions as those used in the present study by Hulme and Orchard (16), who showed that acidosis does not alter ITO when the holding potential is negative to -60 mV, so that changes in ITO would not contribute to the effect of acidosis on the action potential.

Regional differences in the effect of acidosis on IK1. The present study showed no regional differences in IK1. Previous studies have given variable results: some studies have shown no regional differences between Epi and Endo cells from rats (7, 9, 37), guinea pigs (30), and dogs, (29), whereas others have shown that IK1 is greater in Endo cells than in Epi cells (6, 44). IK1 in all of these studies was measured using ruptured patch whole cell recording; because IK1 is sensitive to extracellular Na+ concentration ([Na+]o) and [Ca2+]i, it is possible that the recording conditions may contribute to the observed differences.

Previous studies of the effect of acidosis on IK1 have shown little or no effect during pH changes of the magnitude used in the present study (11, 19, 43). The present data are, therefore, consistent with previous studies and with the idea that Kir2.1/ Kir2.2 underlie IK1 in the rat ventricle because Kir2.1 is insensitive to pH (45). These data also suggest that acidosis-induced changes in IK1 do not play a significant role in the response to acidosis.

Regional differences in the effect of acidosis on IK. Casis et al. (7) reported no regional differences in IK between Epi and Endo cells. However, Yao et al. (44) reported that IK is larger in Epi than in Endo cells, consistent with the present data. It is possible that the use of 4-AP by Casis et al. (7) might have affected IK. The higher density of IK in Epi cells, in conjunction with the higher density of ITO (16), will contribute to the faster repolarization in Epi cells.

However, acidosis had no significant effect on IK either in Epi or Endo cells, consistent with Xu and Rozanski (43), who showed that a 1.2-unit decrease in the pH of the pipette solution in whole cell configuration does not alter IK in rat ventricular cells and making it unlikely that IK contributes to the acidosis-induced changes in the action potential.

Regional differences in the effect of acidosis on ISS. Several investigators have described a sustained current that is not inactivated even at more positive holding potentials in rat ventricular myocytes (7, 20, 35), and there appears to be no regional difference in the distribution of this current (Ref. 7 and this study). When K+ was replaced with Cs+, ISS was largely suppressed (compare Fig. 6, A and B), and acidosis did not alter the current (Fig. 6B), indicating that the measured current is carried predominantly by K+ and that the acidosis-sensitive increase in current is carried by K+.

Acidosis decreased ISS in Epi and Endo cells, although the inhibition was greater in Endo cells (Fig. 6). This current appears to contribute to the repolarization of rat ventricular myocytes (35) so that the acidosis-induced decrease in ISS might explain the prolongation of the action potential observed during acidosis and the greater prolongation observed in Endo cells. Scamps (35) suggested that ISS is carried by Kv1.2 or Kv2.1; more recently James et al. (20) suggested that ISS is carried by TBAK-1/TASK-1, which is inhibited by acidosis (23), consistent with the present data.

One potential problem with this hypothesis is that acidosis inhibits ISS throughout the voltage range studied, but the effect of acidosis on the action potential is most apparent at APD50. It seems possible, however, that the early and late phases of the action potential are dominated by other larger currents so that the effect of ISS on APD will be less apparent at these times.

Effect of acidosis in ICl,ir. The present study showed a marked increase in ICl,ir during acidosis. This increase in inward current may contribute to the prolongation of the action potential, although the small amplitude of the current at more depolarized potentials when [Cl-]pip was in the physiological range (20 mM) suggests that its contribution would be very small. It could, however, underlie the acidosis-induced depolarization of the resting membrane potential. Such depolarization has been a consistent observation (for review see Ref. 33) and appears to be the same in atrial and ventricular cells and in Epi and Endo cells, although the mechanism has remained obscure: acidosis-induced depolarization occurs following inhibition of Ca2+-sensitive currents (including INa/Ca), ICa, K+ currents, and Na+ pump current. The amplitude of the current at -80 mV is sufficient to account for the depolarization: injection of 40 pA current at -80 mV induced a 4.0 ± 0.8 mV (n = 3) depolarization (not shown), giving a cell input resistance of 100 ± 20 MOmega (n = 3), within the normal range for rat ventricular myocytes (20-200 MOmega , Ref. 42). Thus the small (28 pA) acidosis-induced inward shift of ICl,ir at -80 mV would be expected to cause a 2.8-mV depolarization, consistent with the 3-mV depolarization observed during acidosis.

ICl,ir does not appear to be due to protein kinase A-dependent Cl- current, Ca2+-activated Cl- current, or swelling-induced Cl- current, because these do not show inward rectification (for review see Ref. 17). However, ICl,ir shows marked similarities to the current carried by ClC-2 channels. These channels have been found in atrial and ventricular cells (5, 10); the Cl- current carried by these channels shows inward rectification (10, 21), is of comparable magnitude to that recorded in the present study (10), is sensitive to Cd2+ and resistant to DIDS at control pH, and is increased by acidosis when expressed in Xenopus oocytes (21). Thus it appears that ICl,ir is carried by ClC-2.

An interesting observation is that the acidosis-induced increase of current has a different pharmacology from that of the current observed at control pH, showing Cd2+ insensitivity and DIDS sensitivity. In the case of Cd2+, the acid-induced current in the presence of Cd2+ is not significantly different from that in control (Fig. 8E); if anything, it is slightly larger. If Cd2+ and H+ were competing for a common binding site, as suggested for other channels, it might be expected that this current would be smaller in the presence of Cd2+. However, it appears that the stimulatory effect of acidosis on ICl,ir is independent of the inhibitory effect of Cd2+ and thus that Cd2+ and H+ are acting at different sites or that acidosis is altering the concentration of the active form of Cd2+. The observation that ICl,ir is sensitive to Cd2+ at control pH but that acidosis still increases this current in the presence of Cd2+ is germane to the measurements of K+ currents in the present study, in which Cd2+ was present in the bathing solution (see METHODS). However, ICl,ir is small at physiological levels of intracellular [Cl-] and at the range of potentials used in the measurement of K+ currents (e.g., Fig. 7B), particularly compared with the K+ currents being measured. Thus although activation of ICl,ir may have occurred in these experiments, the effect is likely to be very small; the observation that two of the three K+ currents measured did not change during acidosis supports this idea.

There are a number of possible explanation for why DIDS partially inhibited the acidosis-induced increase of ICl,ir. First, it is possible that ICl,ir is already maximal. This is unlikely because DIDS had no effect on current at control pH (Fig. 8D) and acidosis could increase the current from this level in the absence of DIDS (Fig. 7B). Second, DIDS may bind to a pH-sensitive domain of the channel, although this is unlikely because DIDS only inhibited the acid-induced increase of current, rather than all ICl,ir. Third, protonation of DIDS may alter its effectiveness. Finally, acidosis may increase ICl,ir, not by acting on the channel, but by causing a DIDS-sensitive increase of intracellular [Cl-], although the possible mechanism is unclear: the Cl-/OH- exchange (or H+-Cl- coinflux) mechanism described by Sun et al. (38) is DIDS insensitive, whereas the DIDS-sensitive Na+-HCO<UP><SUB>3</SUB><SUP>−</SUP></UP> cotransporter and Cl/HCO3 exchanger (28) would be inhibited in the HEPES-buffered (HCO<UP><SUB>3</SUB><SUP>−</SUP></UP> free) solutions used in the present study (38).

Whatever the mechanism, it is clear that DIDS inhibits the response of ICl,ir (ClC-2) to H+. Thus, acidosis, as well as increasing ICl,ir, also changes the response to pharmacological inhibitors, which may have important consequences for previous studies using these inhibitors during acidosis, for studies of such channels and for the development of therapeutic agents.

In conclusion, no difference in ICa, IK1, and ISS was observed between Epi and Endo cells at control pH. However, IK was larger in Epi cells and will therefore contribute to the shorter action potential in these cells. ICa, INa/Ca, ITO, IK1, or IK are unlikely to be responsible for the prolongation of action potential during acidosis. However, it appears likely that the acidosis-induced decrease of ISS, which regulates action potential configuration (35), plays a major role in the prolongation of the action potential during acidosis, although it may be modulated by small changes in other currents such as ICl,ir. The acidosis-induced increase in ICl,ir could, however, explain the acidosis-induced depolarization of the resting membrane potential.


    ACKNOWLEDGEMENTS

This work was supported by the British Heart Foundation and Wellcome Trust. K. Komukai also thanks the Uehara Memorial Foundation.


    FOOTNOTES

Address for reprint requests and other correspondence: C. Orchard, School of Biomedical Sciences, Univ. of Leeds, Leeds LS2 9NL, UK (E-mail: c.h.orchard{at}leeds.ac.uk).

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

First published March 28, 2002;10.1152/ajpheart.01042.2001

Received 29 November 2001; accepted in final form 22 March 2002.


    REFERENCES
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

1.   Antzelevitch, C, Sicouri S, Litovsky SH, Lukas A, Krishman SC, Diego JMD, Gintant GA, and Liu DW. Heterogeneity within the ventricular wall. electrophysiology and pharmacology of epicardial, endocardial, and M cells. Circ Res 69: 1427-1449, 1991[Free Full Text].

2.   Apkon, M, and Nerbonne JM. Characterization of two distinct depolarization-activated K+ currents in isolated adult rat ventricular myocytes. J Gen Physiol 97: 973-1011, 1991[Abstract/Free Full Text].

3.   Bethell, HW, Vandenberg JI, Smith GA, and Grace AA. Changes in ventricular repolarization during acidosis and low-flow ischaemia. Am J Physiol Heart Circ Physiol 275: H551-H561, 1998[Abstract/Free Full Text].

4.   Bogdanov, KY, Spurgeon HA, Vinogradova TM, and Lakatta EG. Modulation of the transient outward current in adult rat ventricular myocytes by polyunsaturated fatty acids. Am J Physiol Heart Circ Physiol 274: H571-H579, 1998[Abstract/Free Full Text].

5.   Britton, FC, Hatton WJ, Rossow CF, Duan D, Hume JR, and Horowitz B. Molecular distribution of volume-regulated chloride channels (CLC-2 and ClC-3) in cardiac tissues. Am J Physiol Heart Circ Physiol 279: H2225-H2233, 2000[Abstract/Free Full Text].

6.   Bryant, S, Shipsey SJ, and Hart G. Normal regional distribution of membrane current density in rat left ventricle is altered in catecholamine-induced hypertrophy. Cardiovasc Res 42: 391-400, 1999[Abstract/Free Full Text].

7.   Casis, O, Iriarte M, Gallego M, and Sanchez-Chapula JA. Differences in regional distribution of K+ current densities in rat ventricle. Life Sci 63: 391-400, 1998[ISI][Medline].

8.   Choi, HS, Trafford AW, Orchard CH, and Eisner DA. The effect of acidosis on systolic Ca and sarcoplasmic reticulum calcium content in isolated rat ventricular myocytes. J Physiol 529: 661-668, 2000[Abstract/Free Full Text].

9.   Clark, RB, Bouchard RA, Salinas-Stefanon E, Sanchez-Chapula J, and Giles WR. Heterogeneity of action potential waveforms and potassium currents in rat ventricle. Cardiovasc Res 27: 1795-1799, 1993[Abstract/Free Full Text].

10.   Duan, D, Ye L, Britton F, Horowitz B, and Hume JR. A novel anionic inward rectifier in native cardiac myocytes. Circ Res 86: e63-e71, 2000[Abstract/Free Full Text].

11.   Findlay, I. Effects of pH upon the inhibition by sulphonylurea drugs of ATP-sensitive K+ channels in cardiac muscle. J Pharmacol Exp Ther 262: 71-79, 1992[Abstract/Free Full Text].

12.   Garlick, PB, Radda GK, and Seeley PJ. Studies of acidosis in the ischaemic heart by phosphorus nuclear magnetic resonance. Biochem J 184: 547-554, 1979[ISI][Medline].

13.   Harrison, SM, Frampton JE, McCall E, Boyett MR, and Orchard CH. Contraction and intracellular Ca2+, Na+, and H+ during acidosis in rat ventricular myocytes. Am J Physiol Cell Physiol 262: C348-C357, 1992[Abstract/Free Full Text].

14.   Hulme, JT, Colyer J, and Orchard CH. Acidosis alters the phosphorylation of Ser16 and Thr17 of phospholamban in rat cardiac muscle. Pflügers Arch 434: 475-483, 1997[ISI][Medline].

15.   Hulme, JT, and Orchard CH. Effect of acidosis on Ca2+ uptake and release by the sarcoplasmic reticulum of intact rat ventricular myocytes. Am J Physiol Heart Circ Physiol 275: H977-H987, 1998[Abstract/Free Full Text].

16.   Hulme, JT, and Orchard CH. Effect of acidosis on transient outward potassium current in isolated rat ventricular myocytes. Am J Physiol Heart Circ Physiol 278: H50-H59, 2000[Abstract/Free Full Text].

17.   Hume, JR, Duan D, Collier ML, Yamazaki J, and Horowitz B. Anion transport in heart. Physiol Rev 80: 30-81, 2000.

18.   Irisawa, H, and Sato R. Intra- and Extracellular actions of proton on the calcium current of isolated guinea pig ventricular cells. Circ Res 59: 348-355, 1986[Abstract/Free Full Text].

19.   Ito, H, Vereecke J, and Carmeliet E. Intracellular protons inhibit inward rectifier K+ channel of guinea-pig ventricular cell membrane. Pflügers Arch 422: 280-286, 1992[ISI][Medline].

20.   James, AF, Ramsey JE, Reynolds AM, Hendry BM, and Shattock MJ. Effects of endothelin-1 on K+ currents from rat ventricular myocytes. Biochem Biophys Res Commun 284: 1048-1055, 2001[ISI][Medline].

21.   Jordt, SE, and Jentsch TJ. Molecular dissection of gating in the CIC-2 chloride channel. EMBO J 16: 1582-1592, 1997[ISI][Medline].

22.   Kilborns, MJ, and Fedida D. A study of the developmental changes in outward currents of rat ventricular myocytes. J Physiol 430: 37-60, 1990[Abstract/Free Full Text].

23.   Kim, Y, Bang H, and Kim D. TBAK-1 and TASK-1, two-pore K+ channel subunits: kinetic properties and expression in rat heart. Am J Physiol Heart Circ Physiol 277: H1669-H1678, 1999[Abstract/Free Full Text].

24.   Kimura, S, Bassett AL, Furukawa T, Furukawa N, and Myerburg J. Differences in the effect of metabolic inhibition on action potentials and calcium currents in endocardial and epicardial cells. Circulation 84: 768-777, 1991[Abstract/Free Full Text].

25.   Komukai, K, Pascarel C, and Orchard CH. Effect of acidosis on the action potential, ICa and background steady-state current in isolated sub-endocardial and sub-epicardial rat ventricular myocytes (Abstract). J Physiol 526P: 97P, 2000.

26.   Komukai, K, Pascarel C, and Orchard CH. Compensatory role of CaMKII on ICa and SR function during acidosis in rat ventricular myocytes. Pflügers Arch 442: 353-361, 2001[ISI][Medline].

27.   Kurachi, Y. The effects of intracellular protons on the electrical activity of single ventricular cells. Pflügers Arch 394: 264-270, 1982[ISI][Medline].

28.   Leem, CH, Lagadic-Gossmann D, and Vaughan-Jones RD. Characterization of intracellular pH regulation in the guinea-pig ventricular myocyte. J Physiol 517: 159-180, 1999[Abstract/Free Full Text].

29.   Liu, DW, Gintant GA, and Antzelevitch CC. Ionic bases for electrophysiological distinctions among epicardial, midmyocardial, and endocardial myocytes from the free wall of the canine left ventricle. Circ Res 72: 671-687, 1993[Abstract/Free Full Text].

30.   Main, MC, Bryant SM, and Hart G. Regional differences in action potential characteristics and membrane currents of guinea-pig left ventricular myocytes. Exp Physiol 83: 747-761, 1998[Abstract].

31.   McDonald, RL, Colyer J, and Harrison SM. Expression of Na+-Ca2+ exchanger protein is lower in guinea-pig left ventricular mid-myocardial myocytes compared with surface myocytes (Abstract). J Physiol 521P: 28P, 1999.

32.   Mundina-Weilenmann, C, Vittone L, Cingolani HE, and Orchard CH. Effects of acidosis on phosphorylation of phospholamban and troponin I in rat cardiac muscle. Am J Physiol Cell Physiol 270: C107-C114, 1996[Abstract/Free Full Text].

33.   Orchard, CH, and Cingolani HE. Acidosis and arrhythmias in cardiac muscle. Cardiovasc Res 28: 1312-1319, 1994[Free Full Text].

34.   Quinn, FR, McIntosh MA, Cobbe SM, and Smith GL. Sodium-calcium exchange current density is higher in endocardial than epicardial cells (Abstract). Biophys J 78: 224A, 2001.

35.   Scamps, F. Characterization of a beta-adrenergically inhibited K+ current in rat cardiac ventricular cells. J Physiol 491: 81-97, 1996[Abstract/Free Full Text].

36.   Schouten, VJA, and ter Keurs HEDJ The slow repolarization phase of the action potential in rat heart. J Physiol 360: 13-25, 1985[Abstract/Free Full Text].

37.   Shimoni, Y, Severson D, and Giles W. Thyroid status and diabetes modulate regional differences in potassium currents in rat ventricle. J Physiol 488: 673-688, 1995[Abstract/Free Full Text].

38.   Sun, B, Leem CH, and Vaughan-Jones RD. Novel chloride-dependent acid loader in the guinea-pig ventricular myocyte: part of a dual acid-loading mechanism. J Physiol 495: 65-82, 1996[Abstract/Free Full Text].

39.   Teracciano, CMN, and MacLeod KT. Effects of acidosis on Na+/Ca2+ exchange and consequences for relaxation in guinea pig cardiac myocytes. Am J Physiol Heart Circ Physiol 267: H477-H487, 1994[Abstract/Free Full Text].

40.   Wang, HS, Dixon JE, and McKinnon D. Unexpected and differential effects of Cl- channel blockers on the Kv4.3 and Kv4.2 K+ channels. Circ Res 81: 711-718, 1997[Abstract/Free Full Text].

41.   Watanabe, T, Delbridge LM, Bustamante JO, and McDonald TF. Heterogeneity of the action potential in isolated rat ventricular myocytes and tissue. Circ Res 52: 280-290, 1983[Abstract/Free Full Text].

42.   White, RL, Spray DC, Campos de Carvalho AC, Wittenberg BA, and Bennett MVL Some electrical and pharmacological properties of gap junctions between adult ventricular myocytes. Am J Physiol Cell Physiol 249: C447-C455, 1985[Abstract/Free Full Text].

43.   Xu, Z, and Rozanski G. Proton inhibition of transient potassium outward current in rat ventricular myocytes. J Mol Cell Cardiol 29: 481-490, 1992.

44.   Yao, JA, Jiang-M, Fan JS, Zhou YY, and Tseng GN. Heterogeneous changes in K currents in rat ventricles three days after myocardial infarction. Cardiovasc Res 44: 132-145, 1999[Abstract/Free Full Text].

45.   Zhu, G, Liu C, Qu Z, Chanchevalap S, Xu H, and Jiang C. CO2 inhibits specific inward rectifier K+ channels by decrease in intracellular and extracellular pH. J Cell Physiol 183: 53-64, 2000[ISI][Medline].

Am J Physiol Heart Circ Physiol 283(1):H412-H422
0363-6135/02 $5.00 Copyright © 2002 the American Physiological Society


This article has been cited by other articles:


Home page
 Am. J. Physiol. Heart Circ. Physiol.Home page
J. D. O'Brien, J. H. Ferguson, and S. E. Howlett
Effects of ischemia and reperfusion on isolated ventricular myocytes from young adult and aged Fischer 344 rat hearts
Am J Physiol Heart Circ Physiol, May 1, 2008; 294(5): H2174 - H2183.
[Abstract] [Full Text] [PDF]

Home page
 Biophys. JHome page
S. A. Niederer, P. Swietach, D. A. Wilson, N. P. Smith, and R. D. Vaughan-Jones
Measuring and Modeling Chloride-Hydroxyl Exchange in the Guinea-Pig Ventricular Myocyte
Biophys. J., March 15, 2008; 94(6): 2385 - 2403.
[Abstract] [Full Text] [PDF]

Home page
 Exp PhysiolHome page
S.-S. Zhou, L.-B. Zhang, W.-P. Sun, F.-C. Xiao, Y.-M. Zhou, Y.-J. Li, and D.-L. Li
Heart/Cardiac Muscle: Effects of monocarboxylic acid-derived Cl- channel blockers on depolarization-activated potassium currents in rat ventricular myocytes
Exp Physiol, May 1, 2007; 92(3): 549 - 559.
[Abstract] [Full Text] [PDF]