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1 Research Institute of Environmental Medicine, Nagoya University, Nagoya 464-01, Japan; 2 Department of Physiology, University of Leeds, Leeds LS2 9JT, United Kingdom; and 3 Department of Physiology, Iran University of Medical Sciences, Tehran, Iran
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ABSTRACT |
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 Top
 Abstract
 Introduction
Methods
Results
Discussion
References
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Effects of block of the rapid delayed rectifier K+ current (IK,r) by E-4031 on the electrical activity of small ball-like tissue preparations from different regions of the rabbit sinoatrial node were measured. The effects of partial block of IK,r by 0.1 µM E-4031 varied in different regions of the node. In tissue from the center of the node spontaneous activity was generally abolished, whereas in tissue from the periphery spontaneous activity persisted, although the action potential was prolonged, the maximum diastolic potential was decreased, and the spontaneous activity slowed. After partial block of IK,r, the electrical activity of peripheral tissue was more like that of central tissue under normal conditions. One possible explanation of these findings is that the density of IK,r is greater in the periphery of the node; this would explain the greater resistance of peripheral tissue to IK,r block and help explain why, under normal conditions, the maximum diastolic potential is more negative, the action potential is shorter, and pacemaking is faster in the periphery.
heart; cardiac; pacemaking
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INTRODUCTION |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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THE ACTION POTENTIAL is first initiated within a small area of the sinoatrial node. In the rabbit, Bleeker et al. (2) estimated the leading pacemaker cell group to be ~5,000 cells with an area of ~0.1 mm2. Because the total area of the rabbit sinoatrial node is ~12 mm2 (19), this represents only ~1% of the total area. The leading pacemaker cell group is not at a fixed point within the sinoatrial node; mapping of the activation sequence in the rabbit shows pacemaker shift in response to many interventions, including neurotransmitters (19). In normal human subjects, the configuration of the P wave of the electrocardiogram changes routinely (often associated with changes in rate) (7). Changes in the P wave in the human have also been noted in exercise and myocardial infarction (8, 10). The changes in the P wave have been attributed to changes in atrial excitation, perhaps as a result of pacemaker shift (21). Intraoperative mapping in the human confirms that the leading pacemaker site is dynamic (21). The shifting of the leading pacemaker site is the result of marked heterogeneity in the electrophysiology of the sinoatrial node; in different regions of the sinoatrial node, the action potential and pacemaker activity and their response to interventions vary (2, 14). The leading pacemaker site is the site showing the fastest pacemaker activity, and, because of the heterogeneity, this depends on the prevailing conditions.
The ionic mechanisms responsible for this heterogeneity are beginning to be understood; evidence suggests that the densities of the hyperpolarization-activated current (If) and the 4-aminopyridine (4-AP)-sensitive transient and sustained outward current (Ito) are greater in the periphery of the sinoatrial node than in the center (5, 9). The tetrodotoxin (TTX)-sensitive Na+ current (INa), although present in the periphery, is thought to be absent from the center (9). The putative regional differences in the currents correlate with the effects of channel blockers; If blockers (Cs+, UL-FS-49, ZD-7288), 4-AP, and TTX exert greater effects in the periphery than in the center (4, 15-17). These differences in membrane currents help explain the regional differences in pacemaker activity in the sinoatrial node.
In the rabbit sinoatrial node, the rapid delayed rectifier K+ current (IK,r) plays an important role in pacemaking (18, 22). In the present study, we have investigated the effect of a blocker of IK,r, E-4031, on small ball-like tissue preparations from different regions of the sinoatrial node of the rabbit. The results show that sensitivity to E-4031 varies in the different regions of the sinoatrial node.
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METHODS |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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Experiments were carried out on small ball-like preparations of sinoatrial node tissue or on the intact sinoatrial node.
Small ball-like preparations of sinoatrial node tissue.
New Zealand White rabbits weighing 1.5-2 kg were anesthetized with
intravenous pentobarbital sodium (30-40 mg/kg). The chest was
opened, and the heart was rapidly excised into Tyrode solution at
~32°C. The right atrium was separated from the rest of the heart
and opened by a longitudinal incision in the free wall to expose the
endocardial surface. The right atrium was trimmed to leave a
preparation ~15 × 15 mm square, which included the whole sinoatrial node and some of the surrounding atrial muscle. The sinoatrial node is located in the intercaval region between the superior and inferior venae cavae (Fig.
1A).
Laterally, it is bounded by the atrial septum on one side and the
crista terminalis and atrial appendage on the other (Fig.
1A). Conduction from the leading
pacemaker site within the sinoatrial node occurs in an oblique cranial
direction to the atrial muscle of the crista terminalis (conduction
toward the atrial septum is blocked). Sinoatrial node tissue and atrial
muscle meet on the surface of the crista terminalis, and this boundary
is conveniently marked by the right branch of the sinoatrial ring
bundle.
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. Membrane
potential was corrected for the microelectrode tip potential, which was
usually only a few millivolts. The action potential duration at
30 mV and the spontaneous cycle length (time interval between
successive spontaneous action potentials) were measured electronically
(12). All signals were recorded using a chart recorder (Gould 2600S), a
personal computer (via a CED 1401, Cambridge Electronic Design,
Cambridge, UK) running Signal Averager software (Cambridge Electronic
Design), and a Racal Store 7DS tape recorder (Racal Recorders, Hythe,
UK) for later analysis.
Intact sinoatrial node. Some experiments were carried out on the intact sinoatrial node using an ~15 × 15-mm preparation, which included the whole sinoatrial node and some of the surrounding atrial muscle (see Small ball-like preparations of sinoatrial node tissue for details of dissection, etc.). A typical preparation is illustrated in Fig. 9. The preparation (endocardial surface up) was fixed in a tissue bath superfused with modified Krebs-Ringer solution at 32°C. Solution flowed under the action of gravity at a rate of 20-25 ml/min through a heat exchanger into the chamber. The bath temperature was monitored using a miniature thermistor to ensure that the temperature remained at 32°C. Activation maps of the intact sinoatrial node were made by recording extracellular potentials from 90 to 100 sites with a pair of modified bipolar electrodes (see Fig. 9). The electrodes were positioned using a calibrated XYZ-micromanipulator with 0.1-mm precision. Another pair of modified bipolar electrodes was used to record the extracellular potential from the atrial muscle as a reference signal. Each pair of modified bipolar electrodes consisted of two 100-µm stainless steel wires (one wire 1 mm shorter than the other) insulated to the tip and taped together. High gain (50-88 dB) amplification and filtering (0.5- to 30-Hz bandpass filter used) of the signals from the modified bipolar electrodes resulted in a sharp negative deflection at the instant of activation of the recording site (confirmed by action potential recording by conventional glass microelectrodes). The time interval between the time of activation at the recording site and the time of activation at the reference site on the atrial muscle was measured (average time interval over 10 beats measured). The site showing the earliest activation (at which this interval was longest) was taken to be the leading pacemaker site. The time of activation of other sites with respect to the time of initiation of the action potential at the leading pacemaker site was shown as a series of isochrones at 5- to 10-ms intervals. The activation pattern was stable in all experiments reported. During the mapping procedure, cycle length was measured 10 times at 5-min intervals (from the beginning to the end of the mapping procedure), and the mean ± SE of cycle length was then calculated from the 10 values. Data were recorded using Axotape software for later analysis. Modified Krebs-Ringer solution contained (in mM) 120 NaCl, 4 KCl, 1.3 MgSO4, 1.2 NaH2PO4, 1.2 CaCl2, 25.2 NaHCO3, and 4 glucose. The solution was equilibrated with 95% O2-5% CO2 to give a pH of 7.4. E-4031 was added to the solution when required.
Data are presented as means ± SE for n preparations. SigmaStat (Jandel Scientific Software) was used for statistical analysis. Student's t-test or the Mann-Whitney rank-sum test was used to test differences. A difference was considered significant if P < 0.05.| |
RESULTS |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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Effect of E-4031 on electrical activity in peripheral and central
balls of sinoatrial node tissue.
Figure 2 shows action potentials on fast
and slow time bases recorded from small ball-like preparations of
peripheral (ball A) and central
(balls E and
F) sinoatrial node tissue. Under
control conditions (start of traces) the action potential upstroke was faster, the action potential peak was more positive, the action potential amplitude was greater, the action potential duration was
less, the maximum diastolic potential was more negative, and the
spontaneous activity was faster in the peripheral tissue. All of these
differences between peripheral and central tissue are typical (15).
Figure 2 shows the effect of 0.1 and 1 µM E-4031 on the peripheral
and central balls. E-4031 at a concentration of 0.1 µM is sufficient
to cause partial block of
IK,r, whereas 1 µM is sufficient to cause near-complete block (11, 22) (see DISCUSSION). Near-complete block of
IK,r by 1 µM
E-4031 caused prolongation of the action potential followed by the
cessation of spontaneous activity in all balls studied, regardless of
whether they were from the periphery or center of the sinoatrial node (n = 16 preparations) (Fig. 2,
bottom traces). After block of activity, the membrane potential settled at approximately -34 mV in all
balls; this point is considered in more detail later in
RESULTS. On washoff of E-4031, the
effects of E-4031 were reversible, although over a long time scale.
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77 mV in ball B from the
periphery and 63 mV in ball F
from the center. Electrical activity after the application of 0.1 µM
E-4031 is also shown in Fig. 3. In the central tissue, 0.1 µM E-4031
again abolished the action potential. In the peripheral tissue, 0.1 µM E-4031 did not abolish spontaneous activity, although it did
depolarize the membrane (maximum diastolic potential reduced), slow the
action potential upstroke, prolong the action potential, and slow
spontaneous activity (Fig. 3A). In
the presence of 0.1 µM E-4031, the action potential in the peripheral
tissue in some respects became more like that in the central tissue
under control conditions.
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Effect of E-4031 on resting potential.
Figure 6A
(left bars) shows the mean value of the maximum
diastolic potential in balls A and
B from the periphery and
balls D,
E, and
F from the center under control
conditions. The significant difference
(P < 0.001) between the two is 10.9 mV and is shown in Fig. 6B. Figure
6A (right bars)
also shows the membrane potential after near-complete block of
IK,r by 1 µM
E-4031. The membrane potential is the resting potential, because 1 µM
E-4031 abolished activity in tissue from all regions of the sinoatrial
node (Fig. 2). There is no significant difference
(P = 0.32) between the resting
potentials in the presence of 1 µM E-4031 in the peripheral and
central balls (Fig. 6). Figure 6A
(middle bars) also shows the resting potential of
the peripheral and central balls when the action potential and
pacemaker activity were terminated by the application of 6 µM
nifedipine and 3 µM TTX; block of both the L-type
Ca2+ current
(ICa) and
INa is required
to stop spontaneous activity in all balls (15). The resting potentials
were significantly greater (P 
0.003) when spontaneous activity was terminated by the use of TTX and
nifedipine rather than 1 µM E-4031 (Fig.
6A). The resting potential of the
peripheral balls when spontaneous activity was terminated by nifedipine
and TTX was significantly greater than that of the central balls (Fig.
6A). Figure
6B shows the significant differences
(P 0.009) between the peripheral and central balls in maximum diastolic potential (10.9 mV or 15% of
the value in peripheral balls) and resting potential when activity was
terminated by TTX and nifedipine (6.4 mV or 13%), as well as the small
difference (3 mV or 8%; not significant) between the peripheral and
central balls in the resting potential when activity was terminated by
near-complete block of
IK,r by 1 µM E-4031.
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Differences in response to E-4031 in superior-inferior direction.
As well as differences in the response to 0.1 µM E-4031 in the
transverse direction (i.e., from periphery to center), differences in
the longitudinal direction (i.e., from the superior part of the
sinoatrial node to the inferior part) were observed. The difference was
observed in balls B,
C, and
D (i.e., all balls apart from the most
peripheral and most central). For example, in the case of
strand
1 from the more superior part of the
sinoatrial node, 2 of 10 balls (20%) were stopped by 0.1 µM E-4031;
in the case of strand
2, 3 of 11 balls (27%) were stopped;
in the case of strand
3, 3 of 9 balls (33%) were stopped;
and in the case of strand
4 from the more inferior part of the
sinoatrial node, 6 of 8 balls (75%) were stopped. Figure
7A shows
the effect of 0.1 µM E-4031 on ball
1D (i.e., ball D in
strand
1) from the more superior part of
the sinoatrial node. Although Figs. 4 and 5 show that
ball
D is sensitive to 0.1 µM E-4031, in
this example it was resistant (Fig.
7A). Figure
7B shows the effect of partial block
of IK,r by 0.1 µM E-4031 on ball
4B (i.e.,
ball
B in
strand 4) from the more inferior part of
the sinoatrial node. Although Figs. 4 and 5 show that
ball
B is resistant to 0.1 µM E-4031, in
this example it was sensitive (Fig.
7B). These results suggest that the
inferior part of the sinoatrial node is more sensitive to E-4031 than
the superior part. This is confirmed by Fig.
8, which shows mean action potential
amplitude and rate of spontaneous activity in the presence of 0.1 µM
E-4031 (as a percentage of control) for strands
1-4 (data have been plotted against the distance of the strand from strand 1). Data
for balls B,
C, and
D have been combined. There is a
significant linear correlation (P < 0.05) between action potential amplitude in the presence of E-4031 (as
a percentage of control) and distance (Fig.
8A), as well as the rate of
spontaneous activity in the presence of 0.1 µM E-4031 (as a
percentage of control) and distance (Fig.
8B). This confirms that tissue from
the inferior part of the sinoatrial node is more sensitive to 0.1 µM
E-4031 than tissue from the superior part.
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27%/mm
(P < 0.01) in the inferior direction
and by 63%/mm (P < 0.005) in the central direction. With a multiple linear regression, the
rate of spontaneous activity in the presence of 0.1 µM E-4031 changes
by 29%/mm (P < 0.05) in the inferior direction and by 47%/mm
(P < 0.05) in the central direction.
Effect of E-4031 on intact sinoatrial node. In the intact sinoatrial node, the changes in the intrinsic pacemaker activity of different regions caused by E-4031 could result in pacemaker shift. The leading pacemaker site is the site showing the fastest pacemaker activity. Although inspection of Fig. 5A suggests that the periphery will be the leading pacemaker under control conditions, this is not the case, because in the intact sinoatrial node the periphery is suppressed by the atrial muscle and the leading pacemaker site is in the central region (13). In the presence of E-4031, the intrinsic pacemaker activity of the central region is suppressed to a greater extent than that of the periphery (Fig. 5), and, therefore, the leading pacemaker site is expected to shift away from the center (i.e., toward the atrial muscle of the crista terminalis). Furthermore, because E-4031 suppresses the intrinsic pacemaker activity of the more inferior region of the sinoatrial node to a greater extent than that of the more superior region (Fig. 8), E-4031 is expected to shift the leading pacemaker site toward the superior region of the sinoatrial node. These predictions were tested in a series of experiments on the intact sinoatrial node.
Activation maps were constructed as described in METHODS under control conditions and after the application of 1 µM E-4031. A result is shown in Fig. 9. The isochrones show the extent of propagation of the action potential in a given time (in ms) after the action potential was first initiated at the leading pacemaker site (0-ms isochrone); the set of isochrones shows the sequence of activation. Under control conditions, spontaneous excitation first occurred at a site 0.9 mm from the right branch of the sinoatrial ring bundle (Fig. 9A); this is typical (2). After the application of E-4031, the mean cycle length was changed from 580 ± 4 to 759 ± 5 ms, and the leading pacemaker site shifted (the original leading pacemaker site is shown by a filled circle and highlighted by an arrow in the righthand diagram) in the superior direction and toward the crista terminalis (Fig. 9B). This is consistent with our predictions. In four preparations, the leading pacemaker site shifted by 1.6 ± 0.5 mm. The shift was composed of a 1.4 ± 0.4-mm movement along the crista terminalis in the superior direction and a 0.5 ± 0.1-mm movement toward the crista terminalis.
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DISCUSSION |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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It is already known that block of IK,r abolishes pacemaker activity in the sinoatrial node (18, 22). The present study shows that the sensitivity to E-4031 varies in different regions of the sinoatrial node: the sensitivity of central tissue is greater than that of the peripheral tissue, and the sensitivity of tissue from the more inferior part is greater than that of tissue from the more superior part.
Selectivity of E-4031. Verheijck et al. (22) reported that, in rabbit sinoatrial node cells, 0.1 µM E-4031 caused partial block of IK,r (precise value not reported) and 1 µM E-4031 blocked ~96% of IK,r. In the study of Ito and Ono (11) on rabbit sinoatrial node cells, 0.27 µM E-4031 caused half-maximal inhibition of IK,r. In the study of Verheijck et al. (22) on rabbit sinoatrial node cells, 1 µM E-4031 (highest concentration used in present study) had no effect on ICa and 10 µM E-4031 had no effect on If. In the study of Ito and Ono (11) on rabbit sinoatrial node cells, it was concluded that 3 µM E-4031 (a higher concentration than that used in the present study) only blocked IK,r and had no effect on ICa and If. In ventricular cells, ICa has also been reported to be insensitive to E-4031 (20). In summary, all available evidence suggests that E-4031 at the concentrations used is a selective blocker of IK,r, although it remains a possibility that it is having another unknown action.
Action of E-4031. In the studies of Ono and Ito (18) and Verheijck et al. (22) on single cells from the rabbit sinoatrial node, 0.1 µM E-4031 decreased the maximum diastolic potential and action potential amplitude, increased the action potential duration, and slowed or stopped spontaneous activity; 1 and 10 µM E-4031 always stopped spontaneous activity (18, 22). The effects of 0.1 and 1 µM E-4031 in the present study were the same. The decrease in the maximum diastolic potential and increase of action potential duration are likely to be the direct effects of the block of IK,r. In contrast, the decrease of the action potential upstroke velocity and overshoot are likely to be indirect effects resulting from voltage-dependent inactivation of the inward currents (INa and ICa) responsible for the action potential upstroke. The decrease in spontaneous activity caused by E-4031 is again likely to be an indirect effect perhaps caused by the increase in action potential duration, decrease in the takeoff potential as a result of the inactivation of INa and ICa (Figs. 3 and 7), and reduced activation of If during the pacemaker potential as a result of the depolarization.
Peripheral-central differences in sinoatrial node in sensitivity to E-4031. In the present study, the effects of the almost complete block of IK,r by 1 µM E-4031 were the same throughout the sinoatrial node (Fig. 2); this shows that, in both the periphery and center, IK,r is important for pacemaking (for reasons considered in Action of E-4031). However, the effects of partial block of IK,r by 0.1 µM E-4031 varied and were greater in the center of the sinoatrial node compared with the periphery (Figs. 3-5).
There are various possible explanations of this finding. 1) The sensitivity of IK,r to E-4031 may vary in the different regions of the sinoatrial node. Although this is an unlikely explanation, it is a possibility. 2) The sensitivity of the tissue to block of IK,r may vary in the different regions of the sinoatrial node. It is possible that the density of background inward current is greater in the center of the sinoatrial node compared with the periphery; this could explain why the maximum diastolic potential is more positive in the center compared with the periphery (Fig. 6), although it could not explain why the action potential is longer and the pacemaker activity is slower in the center. In this case, block of a smaller fraction of IK,r in central tissue compared with peripheral tissue would be required to stop spontaneous activity. However, a difference in the density of background inward current would be expected to result in a difference in the resting potential as well as the maximum diastolic potential, and such a difference would be expected to occur regardless of whether spontaneous activity was terminated by E-4031 or by nifedipine and TTX; although a significant difference in the resting potential was observed after block of spontaneous activity by TTX and nifedipine, a significant difference was not observed after block by E-4031 (Fig. 6). 3) The density of IK,r may vary in the different regions of the sinoatrial node. It is possible that the density of IK,r is lower in the center of the sinoatrial node than in the periphery; this could help explain why the action potential is longer, the maximum diastolic potential is more positive, and pacemaker activity is slower in the center than in the periphery. A lower density of IK,r in the center of the sinoatrial node could explain the greater sensitivity of central tissue to 0.1 µM E-4031 compared with that of peripheral tissue. It may appear paradoxical that because the density of INa, Ito, and If is lower in the periphery of the sinoatrial node (5, 9), block of the currents produces smaller effects on electrical activity in the center (4, 15, 17) and yet a smaller density of IK,r in the center would mean that block of the current (by E-4031) produces a greater effect. This is a consequence of the different roles of the currents in electrical activity. INa, Ito, and If are not required for spontaneous activity to persist; spontaneous activity can continue after complete block of the currents. Block of the currents results in changes in electrical activity, and the magnitude of the changes is proportional to the normal density of the currents. However, IK,r is required for spontaneous activity to persist (as indicated by the fact that complete block of IK,r abolishes spontaneous activity). It follows from this that a minimal density of IK,r is required to sustain spontaneous activity. If the reserve of IK,r, above this minimum, is less in central tissue (in other words the density of IK,r is lower in central tissue) compared with that in peripheral tissue, a smaller fraction of IK,r will need to be blocked in central tissue to abolish spontaneous activity. If the density of IK,r is higher in the periphery of the sinoatrial node, as well as helping to explain why the maximum diastolic potential is more negative in the periphery, it could also explain why the resting potential is significantly more negative in the periphery when spontaneous activity is stopped by TTX and nifedipine but not by E-4031 (Fig. 6). Some additional observations are in favor of this hypothesis. First, using models of central and peripheral action potentials in the sinoatrial node, we have shown that a greater density of IK,r in the periphery can explain the differing effects of E-4031 in the periphery and center (H. Zhang, M. R. Boyett, A. V. Holden, I. Kodama, and H. Honjo, unpublished observations). Second, using an anti-ERG antibody, Brahmajothi et al. (6) studied the distribution of the ERG protein (the channel protein responsible for IK,r) in the ferret heart. Figure 2, panel I5, of Brahmajothi et al. (6) shows that in the intercaval region distant from the crista terminalis (where the center of the sinoatrial node is found in the rabbit at least) little ERG protein was detected, whereas in the intercaval region next to the crista terminalis (where the periphery of the sinoatrial node is found in the rabbit) the ERG protein was abundant. If the distribution of the ERG protein is the same in the rabbit sinoatrial node, it may explain the results obtained in the present study. Although these observations are in favor of the hypothesis, a proper test of the hypothesis must await the direct measurement of the density of IK,r in voltage-clamp experiments in cells from the different regions of the sinoatrial node. The electrical activity of tissue from the periphery in the presence of 0.1 µM E-4031 was in some respects similar to that of tissue from the center under control conditions (Table 1). Thus, in the peripheral tissue after partial block of IK,r by 0.1 µM E-4031, the action potential was no longer shorter, the maximum diastolic potential was no longer more negative, and the spontaneous activity was no longer faster than that of central tissue. It is possible that after partial block of IK,r in the periphery, the density of IK,r was more similar to that in the center and, therefore, the action potential duration, maximum diastolic potential, and rate of spontaneous activity were more similar to those in the center. However, a difference in the density of IK,r (if one exists) is not the only reason for the differences between the periphery and center; there is evidence that the density of Ito is greater in the periphery and, after block of the current by 4-AP, the difference in action potential duration is no longer significant and the gradient in the maximum diastolic potential between the periphery and center is reduced (4). This suggests that the shorter action potential and more negative maximum diastolic potential in the periphery may be the result of greater densities of two K+ currents, Ito and IK,r. IK,r also is not the only reason for the faster intrinsic spontaneous activity of the periphery of the sinoatrial node; evidence suggests that the densities of If and INa are greater in the periphery and that block of either If or INa eliminates or even reverses the difference in the speed of spontaneous activity between the periphery and center (15, 17). Table 1 shows that in the presence of 0.1 µM E-4031 the upstroke velocity of the action potential in the peripheral tissue became like that in the central tissue. This was probably the result of the inactivation of INa normally responsible for the action potential upstroke in the periphery. Once INa was inactivated, ICa would have been responsible for the action potential upstroke, as it is in the center under normal conditions (15). In the center, evidence suggests that INa is absent, rather than being inactivated by the low diastolic potentials (1, 9). Although Table 1 shows that the electrical activity of the peripheral tissue became more like that of the center under control conditions when the peripheral tissue was exposed to 0.1 µM E-4031, the action potential peak did not become more similar and the action potential peak remained more positive than that in the central tissue. This suggests that the smaller overshoot of the action potential in the center of the sinoatrial node is not directly or indirectly related to IK,r. Although the intrinsic pacemaker activity of the periphery is faster than that of the center, in the intact sinoatrial node under normal conditions the leading pacemaker site is located in the center of the sinoatrial node, because the pacemaker activity of the periphery is suppressed electronically by the adjacent, more polarized atrial muscle. In the presence of E-4031, the leading pacemaker site is expected to shift to the site, the spontaneous activity of which is least affected (i.e., the cycle length of which is least increased) by E-4031. On the basis of the data in Figs. 5 and 8, the leading pacemaker site is expected to shift to the periphery of the sinoatrial node next to or on the crista terminalis and to the superior part of the sinoatrial node. Consistent with the prediction above, on application of 1 µM E-4031, the leading pacemaker site shifted 0.5 ± 0.1 mm toward the crista terminalis and 1.4 ± 0.4 mm in the superior direction (Fig. 9). E-4031 (1 µM), when used with small balls of tissue, stopped spontaneous activity even in small balls of tissue from the periphery (Fig. 2). It is likely that in the intact sinoatrial node, a higher concentration of E-4031 was tolerated, because the atrial muscle helped to keep the sinoatrial node polarized after block of IK,r.Superior-inferior differences in the sinoatrial node in sensitivity to E-4031. Despite the importance of superior-inferior differences within the sinoatrial node (pacemaker shift almost invariably involves a shift in the superior or inferior direction; see Ref. 19), less is known of the differences in this direction. On average, action potential duration is longer and spontaneous activity is slower in the inferior part of the sinoatrial node (3). The inferior part of the sinoatrial node showed a greater sensitivity to 0.1 µM E-4031 in the present study; as for the peripheral-central differences in E-4031 sensitivity, there are various possible reasons for this. One of these is a reduction in the density of IK,r in the inferior part; this could help explain the longer action potential and slower pacemaking in the inferior part of the sinoatrial node. The longer action potential in the inferior part of the sinoatrial node is not the result of a lower density of Ito (as it may in part be in the center), because block of Ito by 5 mM 4-AP causes a greater prolongation of the action potential in the inferior part of the sinoatrial node than in the superior part (it is feasible, therefore, that the density of Ito is greater in the inferior part) (4).
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ACKNOWLEDGEMENTS |
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This work was supported by the British Heart Foundation, the British Council, the Japanese Ministry of Education, Science, Sports and Culture, and the Japan Society for the Promotion of Science.
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FOOTNOTES |
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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. §1734 solely to indicate this fact.
Address for reprint requests: M. R. Boyett, Dept. of Physiology, Univ. of Leeds, Leeds LS2 9JT, UK.
Received 11 March 1998; accepted in final form 20 October 1998.
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REFERENCES |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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) in MDP
and in RP between peripheral and central balls (calculated from mean
values in A). *Significantly
different from value from peripheral balls ( P < 0.01).
NS, not significant.
, highlighted by arrow). RA, right atrial
appendage; CT, cut end of crista terminalis; SVC, superior vena cava;
SEP, atrial septum; LSARB, left branch of sinoatrial ring bundle; IVC,
inferior vena cava. 
, Position of reference electrode (see
METHODS).