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THE JOURNAL OF PHARMACOLOGY AND EXPERIMENTAL THERAPEUTICS
Copyright © 1999 by The American Society for Pharmacology and Experimental Therapeutics
JPET 291:474 –481, 1999
Vol. 291, No. 2
Printed in U.S.A.
ATP-Sensitive Potassium Channel Blocker HMR 1883 Reduces
Mortality and Ischemia-Associated Electrocardiographic
Changes in Pigs with Coronary Occlusion
KLAUS J. WIRTH, BJÖRN ROSENSTEIN, JÖRG UHDE, HEINRICH C. ENGLERT, ANDREAS E. BUSCH, and
BERNWARD A. SCHÖLKENS
Hoechst Marion Roussel, Frankfurt am Main, Germany
Accepted for publication July 6, 1999
This paper is available online at http://www.jpet.org
During regional ischemia, myocardial ATP-sensitive potassium (KATP) channels open (Noma, 1983) and extracellular
potassium rises, leading to enhanced automaticity and a
shortening of the refractory period. The arrhythmogenic potential is strongly enhanced by the local nature of myocardial
ischemia, which translates into spatial heterogeneities in
excitability, conduction, and refractoriness favoring reentrant arrhythmias. Regional ischemia also leads to typical
electrocardiographic (ECG) changes. The shift of the S-T
segment reflects heterogeneity in the plateau phase of action
potentials as a consequence of the accelerated repolarization
through opening of KATP channels. Intraventricular conduction is disturbed in both space and time (Gettes and Cascio,
1992), leading to an altered spectrum of the QRS complexes
(Hatala et al., 1995) and an increase in Q-J time, which is
Received for publication February 17, 1999.
including Q-T interval, under baseline conditions and no effect
on hemodynamics during occlusion. In control animals, left
anterior descending coronary artery occlusion lead to a prompt
and significant depression of the S-T segment (20.35 mV) and
a prolongation of the Q-J time (146 ms), the former reflecting
heterogeneity in the plateau phase of the action potentials and
the latter reflecting irregular impulse propagation and delayed
ventricular activation. Both ischemic ECG changes were significantly attenuated by HMR 1883 (S-T segment, 20.14 mV; Q-J
time, 115 ms), indicating the importance of KATP channels in
the genesis of these changes. In conclusion, the KATP channel
blocker HMR 1883, which had no effect on hemodynamics and
ECG under baseline conditions, reduced the extent of ischemic
ECG changes and sudden death due to ventricular fibrillation
during coronary occlusion.
mainly due to the slower, or even blocked, impulse conduction in the ischemic region. Several factors may be involved
in the delay in ventricular activation, including an inexcitability of cells in the ischemic area, a decrease in diastolic
membrane potential leading to a decreased availability of
fast sodium channels with a lower conduction velocity, and
an increase in the intercellular electrical resistance by uncoupling (Kleber et al., 1987). The ischemically induced increase in extracellular potassium concentration due to the
opening of KATP channels is discussed as contributing to the
deterioration of each of these changes (Hill and Gettes, 1980;
Bekheit et al., 1990; Gettes and Cascio, 1992). Therefore,
blocking the ischemia-induced opening of KATP channels
seems to be a promising antiarrhythmic approach. In fact,
the KATP channel blocker glibenclamide has been shown to
inhibit ventricular fibrillation (VF) in various models of ischemia (Ballagi-Pordan et al., 1987; Kantor et al., 1987; Hom-
ABBREVIATIONS: KATP, ATP-sensitive potassium; BP, blood pressure; BPs, systolic blood pressure; ECG, electrocardiographic (electrocardiography, electrocardiogram); dp/dt, left ventricular contractility; HR, heart rate; LVEDP, left ventricular end-diastolic pressure; CO, cardiac output;
SV, stroke volume; TPR, total peripheral resistance; LVSW, left ventricular stroke work; LVP, left ventricular systolic pressure; LAD, left anterior
descending coronary artery; VF, ventricular fibrillation.
474
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ABSTRACT
ATP-sensitive potassium (KATP) channels are activated during
myocardial ischemia. The ensuing potassium efflux leads to a
shortening of the action potential duration and depolarization of
the membrane by accumulation of extracellular potassium favoring the development of reentrant arrhythmias, including ventricular fibrillation. The sulfonylthiourea HMR 1883 was designed as a cardioselective blocker of myocardial KATP
channels for the prevention of arrhythmic sudden death in
patients with ischemic heart disease. We investigated the effect
of HMR 1883 on sudden cardiac arrhythmic death and electrocardiography (ECG) changes induced by 20 min of left anterior
descending coronary artery occlusion in pentobarbital-anesthetized pigs. HMR 1883 (3 mg/kg i.v.) protected pigs from
arrhythmic death (91% survival rate versus 33% in control
animals; n 5 12; p , .05). Ischemic areas were of a similar size.
The compound had no effect on hemodynamics and ECG,
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KATP Channel Blocker HMR 1883 Reduces Arrhythmias in Pigs
Materials and Methods
Two separate studies were performed in anesthetized pigs of the
German Landrace. In the first study, the effects of HMR 1883 on
hemodynamics and ECG were investigated under baseline conditions. In the second study, we attempted to determine the effect of
the KATP channel blocker on survival and ECG changes, S-T-segment
deviation, and Q-J time. Detailed protocols for the two studies,
including anesthesia, are given below.
The pigs were ventilated with room air and oxygen by a Bird
Mark-7 respirator. Blood gas analysis (pO2 and pCO2) was performed at regular time intervals to control oxygen supply via the
respirator to maintain pO2 at .100 mm Hg and pCO2 at ,35 mm
Hg.
Observations and Measurements
To measure hemodynamic parameters, tip catheters (Millar PC
350) were inserted into the left femoral artery ]systolic blood pressure (BPs) and diastolic BP] and into the left ventricle via the right
carotid artery [left ventricular pressure (LVP), left ventricular enddiastolic pressure (LVEDP), and heart rate (HR)]. The maximal rate
of LVP increase (dp/dt) was derived by an analog differentiator. The
LVP signal also triggered a cardiotachometer (HR).
Cardiac output (CO) was measured continuously by an ultrasound
flow probe placed around the pulmonary artery and connected to an
active redirection transit time flowmeter (model 206; ART2, Triton
Technology Inc., San Diego, CA). Bipolar body surface ECGs were
recorded using subcutaneous needle electrodes in the classic lead III
arrangement. Hemodynamic parameters together with the ECG
were recorded on an 8-channel polygraph (model TA 4000; Gould,
Cleveland, OH) at a paper speed of either 5 mm/min (continuously)
or 50 mm/s (intermittent for ECG evaluation). In some experiments,
the ECG was also digitized at a sample rate of 1 kHz and periodically
stored on a computer hard disk for later evaluation (Data Acquisition
System MP 100WSW; AcqKnowledge Software, Harry Fein, World
Precision Instruments, Berlin, Germany).
Effect of HMR 1883 on Hemodynamic and ECG under Baseline Conditions. Pigs of either sex (20 –35 kg) were premedicated
with 1.3 ml of a mixture of Tilest 500 and Rompun i.m. (75.6 mg of
tiletamin HCl, 73.28 mg of zolacepam HCl, and 30.32 mg of xylacin
HCl) and anesthetized with an intravenous bolus of 20 to 30 mg/kg
pentobarbital sodium followed by a continuous infusion of 12 to 17
mg of pentobarbital/kg/h i.v. to maintain anesthesia. When stabile
hemodynamic conditions and blood gas values were achieved for at
least 20 min, the control values for the hemodynamic and ECG
parameters were taken.
HMR 1883 was administered cumulatively to seven pigs at 1 and
3 mg/kg i.v. separated by an interval of 30 min. A dose of 10 mg/kg
was administered to a separate group.
Calculated Parameters
Stroke volume (SV; ml/beat) is defined as SV 5 CO/HR. Total
peripheral resistance (TPR; dyne p s p cm25) is defined as TPR 5
(BPmean/CO) p 79.9. Left ventricular stroke work (LVSW; J/beat) is
defined as LVSW 5 (BPmean 2 LVEDP) p SV p 0.133 p 1023. Left
ventricular minute work (J/min) is defined as LVSW p HR. Myocardial oxygen consumption (MV̇O2; ml O2/min/100 g) is defined as
MV̇O2 5 K1 p BPs p HR 1 K2[(0.8 BPs 1 0.2 diastolic BP) p HR p SV)/
BW] 1 1.43, where K1 5 4.08 p 1024, K2 5 3.25 p 1024, and BW 5
body weight (kg; Rooke and Feigl, 1982). Corrected Q-T (Q-Tc; ms) is
defined as Q-Tc 5 Q-T/(R 2 R)1/2 (Bazett, 1920).
Effect of HMR 1883 on Survival and ECG during Coronary
Occlusion
Twenty-four pigs of the German Landrace (castrated males,
28 – 40 kg) were anesthetized with 10 ml of Ketavet (1 g of ketamine
base) i.m. and 30 to 40 mg/kg pentobarbital sodium as i.v. bolus plus
a continuous infusion of 12 to 17 mg pentobarbital/kg/h i.v. to maintain anesthesia.
A left thoracotomy was performed at the fifth intercostal space,
the lung was retracted, the pericardium was incised, and the heart
was suspended in a pericardial cradle. To induce ischemia, a small
segment of the LAD, approximately 1 cm distal from its origin, was
dissected from surrounding tissue and encircled loosely with a snare
occluder. Occlusion was maintained for 20 min, and then the occluder was loosened to allow for reperfusion (20 min).
Experimental Protocol. When stabile hemodynamic conditions
and blood gas values were achieved for at least 20 min, the control
values were taken for the hemodynamic parameters and for the
ECG. The animals were randomly assigned to one of two groups. In
group 1, the animals received an i.v. bolus of physiological saline,
whereas in group 2, HMR 1883 was administered at the dose 3.0
mg/kg i.v. Five minutes later, the LAD was occluded for 20 min,
followed by a 20-min reperfusion period.
Evaluation of Area at Risk. A dye exclusion method (Evans
blue) was applied to delineate the ischemic tissue from the nonischemic one. Briefly, at the end of the experiment, the LAD was occluded, the heart was quickly removed, and the aorta was cannu-
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burg et al., 1991; Gwilt et al., 1992; Billman et al., 1993;
Wilde et al., 1993; Baczko et al., 1997), and more recently, the
new compound HMR 1883, a cardioselective blocker of myocardial KATP channels (Gögelein et al., 1998), has been reported to inhibit sudden death due to VF in conscious dogs
(Billman et al., 1998) and anesthetized rats (Linz et al.,
1998a). In support of the profibrillatory action of sarcolemmal KATP channel opening as a consequence of myocardial
ischemia, the KATP channel opener pinacidil was found to be
profibrillatory in different animal models of cardiac ischemia
(Chi et al., 1990, 1993; Tosaki et al., 1992).
HMR 1883 differs from the prototype glibenclamide by
several properties, which could make it appropriate for the
prevention of severe ischemic arrhythmias in patients. It is
well tolerated, devoid of an insulin-releasing and vasoconstrictor effect at antifibrillatory doses, and does not inhibit
the mitochondrial KATP channel (T. Sato, N. Sasaki, J. Seharaseyon, B. O’Rourke, E. Marbán, unpublished observations;
Linz et al., 1998b) in contrast to the sarcolemmal KATP channel. The mitochondrial KATP channel is held responsible for
the beneficial effect of ischemic preconditioning. Because
myocardial ischemia is a major cause of sudden death due to
VF (Gillum, 1989; Green, 1990), HMR 1883 has a life-saving
potential against ischemically induced arrhythmias. Because
KATP channels will be closed during normoxia, a KATP channel blocker would be expected to be electrophysiologically
silent, having no effect on the ECG (particularly the Q-T
interval) under baseline conditions. Therefore, it should be
devoid of the typical proarrhythmic effects of class III potassium channel-blocking compounds, the occurrence of early
afterdepolarizations and torsade de pointes, that arise from
prolonged repolarization reflected by a longer Q-T interval in
the ECG. If opening of KATP channels were significantly
involved in shaping the ischemic ECG, KATP channel blockade should attenuate the typical effects of ischemia on the
ECG.
In the current study, we investigated the antiarrhythmic
effect of HMR 1883 in anesthetized pigs subject to left anterior descending coronary artery (LAD) occlusion and compared its ECG and hemodynamic effects during normoxia
and ischemia.
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Wirth et al.
Vol. 291
Results
Effect of HMR 1883 on Hemodynamics and ECG under
Baseline Conditions (First Study)
HMR 1883, given cumulatively at 1 and 3 mg/kg i.v. separated by an interval of 30 min, had no effect on hemodynamics and ECG (data not shown). A dose of 10 mg/kg was
administered to a separate group (Table 1). Even at 10 mg/
kg, HMR 1883 had no significant effects on hemodynamics,
including BP, LVP, left ventricular contractility, CO, TPR,
LVSW, and left ventricular oxygen consumption and ECG
parameters, including R-R, P-Q (P-R), Q-T, Q-Tc interval
(Table 1), S-T segment, and Q-J time. In addition, the shape
of the ECG waves did not change significantly (Fig. 1).
Effect of HMR 1883 on Survival and ECG during Coronary
Occlusion (Second Study)
Location and Size of Ischemic Area after Occlusion
of LAD. In this separate study, occlusion of the LAD resulted
in severe ventrolateral ischemia that affected significant
parts of the left ventricle and the septum and, to a lesser
extent, the right ventricle. Ischemia was transmural. A welldefined borderline was present between the ischemic zone
and the adjacent nonischemic cardiac tissue.
There was no significant difference in the size of the ischemic area between the control group and the group treated
with HMR 1883. In the control group, ischemic area covered
on the average 31.9 6 1.5% (range, 27– 44%) of the left
ventricular tissue (free wall plus septum) and 13.3 6 1.7%
(range, 4 –22%) of the right ventricular free wall. Corresponding values in the HMR 1883 group were 35.7 6 1.0 and
16.6 6 1.3%.
There was no significant correlation between the time of
survival and the size of the ischemic area in the control
group. In the HMR 1883 group, the protective effect did not
depend on the size of the ischemic area. The compound increased the time of survival (see below) both in animals with
a large ischemic area and in those with a smaller one (data
not shown).
Hemodynamic Effects of LAD Occlusion. In control
pigs within 1 min, occlusion of the LAD resulted in a significant decrease in BP (BPs, 218 mm Hg; diastolic BP, 213
mm Hg), LVP (217 mm Hg), and dp/dt (2559 mm Hg/s; Table
2). HR showed a slight tendency to increase, whereas the
LVEDP markedly increased. These effects persisted, with
minor changes, throughout the entire occlusion period. The
administration of 3 mg/kg HMR 1883 had no significant
effect on the hemodynamic parameters before the coronary
artery occlusion, as already shown in the previous experiment with 10 mg/kg.
Occlusion of the coronary artery in the HMR 1883 group
resulted in hemodynamic changes similar to those observed
in the control group, at least during the first 10 min of
occlusion when a sufficient number of pigs survived in the
TABLE 1
Effects of HMR 1883 (10 mg/kg i.v.) on hemodynamic and ECG parameters in anesthetized pigs during normoxia
Values are mean 6 S.E. of baseline values (0 min) and changes from baseline after the administration of HMR 1883 (n 5 7).
BPs
BPd
LVEDP
LVP
1 dp/dt
2 dp/dt
CO
SV
0
1
5
10
20
30
126 6 7
061
061
062
163
21 6 4
80 6 6
21 6 1
161
161
162
21 6 4
15.5 6 0.8
0.3 6 0.3
20.6 6 0.3
20.9 6 0.3
21.0 6 0.3
20.6 6 0.5
94 6 5
061
061
061
061
22 6 2
1142 6 105
9 6 25
210 6 13
219 6 11
232 6 18
249 6 35
1358 6 82
2179 6 68
260 6 33
236 6 25
214 6 32
2103 6 64
2789 6 391
0 6 62
2113 6 78
231 6 99
286 6 92
25 6 101
30 6 3
061
21 6 1
061
21 6 1
061
TPR
LVMW
MVO2
HR
R-R
P-Q
Q-T
Q-Tc
0
1
5
10
20
30
3004 6 470
260 6 84
103 6 87
23 6 101
95 6 141
241 6 161
29.5 6 4.9
20.5 6 0.9
20.7 6 0.9
0.4 6 1.2
20.2 6 1.2
20.4 6 1.8
11.1 6 1.3
0.0 6 0.2
20.2 6 0.2
20.1 6 0.3
20.2 6 0.3
20.1 6 0.4
94 6 8
21 6 1
21 6 1
22 6 1
22 6 1
21 6 1
663 6 55
367
10 6 5
15 6 6
17 6 11
9 6 12
79 6 7
063
363
263
263
163
346 6 10
25 6 5
25 6 6
22 6 6
25 6 4
27 6 6
428 6 11
27 6 6
28 6 7
25 6 7
210 6 7
210 6 10
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lated. Via this cannula, 400 ml of a Evans blue solution (0.5% in
physiological saline) were infused into the coronary vascular bed at
a pressure of 80 mm Hg, which resulted in a heavy, dark-blue
staining of the nonischemic area. After removal of the atria, the
heart was cut into 8 to 10 sections at right angles to the long axis of
the left ventricle. The nonstained ischemic areas and the bluestained normal areas of the right ventricular free wall and of the left
ventricle plus septum were dissected and weighed separately. The
size of the ischemic zones were expressed as the percentage of the
total left or right ventricular tissue.
Evaluation of S-T Segment Deviation and Q-J Time. Two
parameters were analyzed from the ECG records: the ventricular
activation time measured as the distance between the beginning of
the Q wave and the J point (beginning of the S-T segment) and the
S-T deviation. The latter was defined as the difference between the
baseline of the ECG (T-Q segment) and the actual voltage 100 ms
after the onset of the Q wave. This calculation procedure for the S-T
segment was selected because the S-T segment was sometimes not
parallel to the baseline but rather was downsloping or slow rising
during ischemia.
Statistical Analysis. All data were expressed as mean 6 S.E..
Differences in the mean values within the groups were analyzed with
use of the paired Student’s t test. To analyze the differences between
the groups, either the unpaired Student’s t test (hemodynamics, S-T
segment deviation, ventricular activation time) or the Fisher’s exact
probability test (survival data) were used. A value of P , .05 was
regarded as significant. In the control group of the LAD study, the
animals that died early during ischemia were not included for the
evaluation of the ECG parameters.
Preparation of Test Compound. HMR 1883 (1-[[5-[2-(5-chloroo-anisamido)ethyl]-2-methoxyphenyl]sulfonyl]-3-methylthiourea)
was synthesized in the Department of Medicinal Chemistry (Hoechst
AG, Frankfurt/Main, Germany). For i.v. injection, HMR 1883 was
dissolved immediately before use. Because of the poor solubility of
HMR 1883, a special dissolution procedure had to be performed; 250
mg of the compound and 65 mg of Na2CO3 were added to 50 ml of
distilled water. The suspension was stirred for approximately 30 min
at 70 – 80°C until dissolution was completed.
1999
477
KATP Channel Blocker HMR 1883 Reduces Arrhythmias in Pigs
control group, and thus, a reasonable comparison was possible (Table 2). The only difference between the groups was a
smaller decrease in left ventricular contractility after HMR
1883 (1058 6 114 versus 950 6 87 in control animals after 10
min). However, this difference did not reach the threshold of
statistical significance.
ECG Effects, Arrhythmias, and Sudden Cardiac
Death. Figure 2 (top) shows a representative example of the
changes in the ECG observed during ischemia in a control
pig. The first sign of ischemia was a depression of the S-T
segment, which was detectable at 30 s of the occlusion. The
S-T segment depression increased with time, was maximal at
5 min, and thereafter, showed a slight tendency to return
toward baseline values (Figs. 2 and 3, top). Approximately 1
min after the beginning of the occlusion, the positive R wave
started to decline. At 2 min, the shape of the QRS complex
has completely changed into a Q-S pattern consisting of a
large negative wave that was preceded by the more-or-lessmarked initial Q wave.
The only pig of the control group that survived both isch-
TABLE 2
Lack of effect of HMR 1883 on hemodynamics during LAD occlusion and reperfusion
N
BPs
Diastolic BP
LVP
LVEDP
HR
dp/dt
Time
C
H
C
H
C
H
C
H
12
12
111 6 5
112 6 5
77 6 4
78 6 5
92 6 5
88 6 5
C
H
C
H
C
H
99 6 4
92 6 4
1542 6 123
1525 6 127
1100 6 92
983 6 71
958 6 78
950 6 87
867 6 120
1217 6 103
1100 6 124
1067 6 113
1058 6 114
1027 6 114
1300 6 404
1050 6 166
825 6 48
800 6 91
800 6 71
(min)
0
661
761
Occlusion
0.5
1
5
10
20
12
12
12
10
3
12
11
12
12
11
102 6 3
93 6 4
93 6 5
90 6 5
77 6 7
3
1
1
1
4
4
4
4
83 6 4
100 6 5
97 6 5
92 6 5
94 6 5
93 6 6
Occlusion
71 6 3
64 6 4
64 6 5
61 6 4
50
68 6 4
67 6 5
64 6 4
65 6 4
66 6 5
81 6 4
76 6 4
75 6 4
76 6 4
60 6 5
80 6 4
77 6 4
73 6 4
74 6 4
75 6 5
761
861
861
861
11 6 2
861
861
961
961
961
102 6 6
103 6 7
104 6 8
104 6 11
82 6 4
53 6 4
54 6 6
58 6 5
61 6 4
61 6 5
55 6 4
75 6 9
74 6 6
76 6 8
76 6 8
10 6 1
10 6 1
10 6 1
10 6 1
10 6 1
82 6 4
Reperfusion
1
5
10
20
83 6 8
90 6 6
93 6 6
90 6 4
95 6 4
95 6 5
96 6 4
97 6 4
98 6 5
Reperfusion
C, controls; H, HMR 1883; N, number of remaining animals.
95 6 6
94 6 5
93 6 4
91 6 4
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Fig. 1. Absence of significant effects of HMR 1883 (3 mg/kg i.v.) on ECG
in an anesthetized pig before occlusion of the LAD.
emia and reperfusion showed less-marked ECG changes consisting mainly of a transient S-T segment depression at the
beginning of both the occlusion and reperfusion. Besides the
S-T segment depression, LAD occlusion also resulted in a
marked prolongation of the ventricular activation time as
revealed by the significantly increased Q-J time (from 47 to
93 ms; Fig. 3, bottom).
HMR 1883 (3 mg/kg i.v.) administered i.v. before LAD
occlusion had no effect on the shape of the ECG and on the
HR during normoxia. Occlusion of the LAD then resulted in
a significant depression of the S-T segment and a prolonged
Q-J time (Fig. 3). However, compared with control animals,
those changes were significantly less pronounced. Q-J time
increased only by 33% (115 ms) compared with the 98%
increase (146 ms) in the control group. Representative ECGs
(Fig. 2) of a control pig and an HMR 1883-treated pig, the
latter with a very large ischemic area, show the strong effect
of this compound: a surprisingly normally looking ECG (Fig.
2, bottom).
The time course of death, which was due to VF, is shown in
Fig. 4. The incidence of arrhythmias during the first 10 min
(Ia arrhythmias) of the ischemic period was low. In the control group, 2 of 12 animals were affected. One pig died from
VF at 8 min of the LAD occlusion. In this pig, VF was
preceded by a few extrasystoles. In the other pig, ventricular
premature beats occurred more frequently; this animal died
from VF at 18 min.
Six of 12 control pigs developed arrhythmias during the
period of 14 to 20 min (Ib arrhythmias) of coronary artery
ligation. The arrhythmic events consisted of one or a few
ventricular ectopic beats with subsequent VF. A typical example of VF of a control pig is shown in Fig. 5.
In control animals, two of the three survivors of the occlusion period died from VF within the first 2 min of reperfusion.
Similar to the situation during ischemia, VF during reperfusion was preceded by a few ventricular ectopic beats or even
just a single premature beat. Thus, of the 12 animals of the
control group, 11 died from sudden cardiac death: 8 during
the occlusion period and 3 during reperfusion.
In the HMR 1883-treated group, only three pigs had ventricular ectopic beats during the ischemia period, and only
one of these three pigs died from VF at 14 min of the LAD
occlusion. In the second pig, some polymorphic ventricular
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Wirth et al.
Vol. 291
Fig. 4. Effect of HMR 1883 (3 mg/kg i.v.) on survival during occlusion of
the LAD and reperfusion. *p , .05 for the end of the occlusion period. Top
curve, HMR 1883. Bottom curve, control animals.
Fig. 3. Effect of HMR 1883 on S-T-segment deviation (top) and Q-J time
(bottom) during LAD occlusion. E, control group. F, HMR 1883. *p , .05.
Values are mean 6 S.E. (n 5 8).
extrasystoles occurred between 19 and 20 min of the occlusion period. This animal died from fibrillation at 30 s of
reperfusion. The third pig had extrasystoles before LAD occlusion; nevertheless, this animal survived the entire experiment. Seven of 12 pigs of the HMR 1883-treated group had
no arrhythmias during ischemia; nevertheless, they died
later from reperfusion arrhythmias.
Of the four pigs that survived both ischemia and reperfusion, three had no arrhythmias at all. Altogether, during
ischemia, pretreatment with HMR 1883 reduced the incidence of all ischemia-induced arrhythmias to 3 of 12 and
protected 11 of 12 pigs from sudden death, whereas only 4 of
12 pigs survived in the control group (Fig. 4). HMR 1883 was
less effective against reperfusion-induced cardiac death; 7 of
the 11 survivors of ischemia died during reperfusion compared with 3 of 4 deaths in control animals. Thus, 4 of 12
animals in the HMR 1883-treated group and only 1 of 12
animals in the control group survived both ischemia and
reperfusion (Fig. 4).
Discussion
HMR 1883 significantly protected anesthetized pigs
against ischemically induced arrhythmias and sudden cardiac death. The second major finding was the prevention of
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Fig. 2. ECG effects of LAD occlusion in a control pig (top) and an HMR 1883-treated pig (bottom) at baseline (0 min) and 5, 10, and 15 min after
occlusion. The untreated pig survived 19 min of occlusion (ischemic area, 35% of left ventricle and 7% of right ventricle). The HMR 1883-treated pig
survived the entire experiment (ischemic area, 42% of left ventricle and 20% of right ventricle).
1999
KATP Channel Blocker HMR 1883 Reduces Arrhythmias in Pigs
electrophysiological changes typically associated with myocardial ischemia. HMR 1883 reduced the ischemic S-T segment shift and the increase in Q-J time, the former reflecting
heterogeneity in the plateau phase of action potentials and
the latter reflecting irregular impulse propagation and delayed ventricular activation. In some animals, the ECG appeared quite normal despite large ischemic areas. During
normoxia, however, HMR 1883 was silent, affecting neither
the ECG nor hemodynamics. HMR 1883 had no effect on the
delayed rectifier at 100 mM and no effect on action potential
duration in vitro (Gögelein et al., 1998). Thus, in contrast to
class III compounds, blockers of the delayed rectifier, the
KATP channel blocker did not affect repolarization during
normoxia reflected by an unchanged Q-T-interval, and hence,
it should be devoid of the typical proarrhythmic effects of
class III antiarrhythmic compounds. The protective effect of
the KATP channel blocker HMR 1883 in our study indicates
that opening of KATP channels during ischemia induces
proarrhythmic mechanisms that are responsible for ischemic
sudden cardiac death and for the typical ECG changes during
ischemia.
Protection against sudden death by HMR 1883 in our study
is consistent with results of Billman et al. (1993, 1998), who
could demonstrate that pretreatment with either glibenclamide or HMR 1883 significantly reduced the incidence of
VF induced by the combination of exercise and acute myocardial ischemia in conscious dogs susceptible to life-threatening arrhythmias. Moreover, HMR 1883 inhibited VF dur-
ing ischemia and reperfusion in anesthetized rats (Linz et al.,
1998a).
Effects of HMR 1883 on S-T Segment. One of the interesting findings of the present study was the fact that HMR
1883 could attenuate the S-T segment shift, although the
LAD was completely occluded. This indicates a reduction in
the heterogeneity of repolarization of the heart. A shift of the
S-T segment is generally regarded as a clear indication for
myocardial ischemia (Scher and Spach, 1979), and even a
linear relationship between coronary flow reduction (stepwise down to 50%) and S-T segment deviation could be demonstrated in the anesthetized pig (Watanabe and Buffington,
1994). It is very likely, however, that this compound directly
interfered with the electrophysiological mechanisms that occur as a consequence of reduced flow, the ischemically induced opening of KATP channels, and the ensuing electrophysiological derangements rather than with coronary flow
itself for the following reasons. The pig, similar to healthy
humans, has very few coronary collaterals (Weaver et al.,
1986; Patterson and Kirk, 1983), and hence, residual flow is
very low (6% of the normal blood flow; Muller et al., 1986),
and the transition zone between normal flow and no-flow
regions is less than 2 mm in width (Hearse et al., 1986).
Although we cannot fully exclude that HMR 1883 may have
somewhat improved the low residual flow in the ischemic
area via collaterals, this is unlikely because the blockade of
vascular KATP channels is a vasoconstrictor mechanism as
shown with glibenclamide. HMR 1883 is devoid of a significant vasoconstrictor effect, however. In contrast to glibenclamide, it neither reduced coronary blood flow during normoxia (Gögelein et al., 1998) nor inhibited the reactive
hyperemia occurring after the opening of an occluded coronary artery in dogs (Billman et al., 1998).
LAD occlusion in dogs and pharmacological opening of
KATP channels in a nonischemic situation by intracoronary
infusion of the KATP channel opener pinacidil (Nakayama et
al., 1990) elevated the S-T segment recorded with epicardial
mapping electrodes (Kubota et al., 1993), although pinacidil
actually doubled the flow in the LAD. Pinacidil and hypoxia
open KATP channels in the cell membrane of cardiac myocytes (Wilde et al., 1990). On the other hand, glibenclamide,
as a blocker of the KATP channel (Faivre and Findlay, 1989),
attenuated the emergence of the S-T segment elevation regardless of whether induced by acute myocardial ischemia or
pinacidil. The effect of HMR 1883 on the S-T segment confirms the role of KATP channels for the ischemically induced
ECG changes and the role of a regional increase in potassium
conductance in causing regional electrical inhomogeneity.
Acute myocardial ischemia is associated with a high incidence of life-threatening arrhythmias (Janse and Wit, 1989).
The initiation of sustained, lethal arrhythmias requires both
a trigger and an electrophysiological substrate to allow for
reentry (Coronel, 1994). Ischemia meets both requirements
by enhancing automaticity and creating the conditions necessary for reentry. In regional ischemic tissue, large differences exist in the diastolic membrane potential as a consequence of heterogeneities in extracellular potassium that
emerge after the opening of KATP channels (Coronel, 1994).
The difference in the membrane potentials at the border zone
will cause electrotonic currents to flow between depolarized
ischemic and normal myocardial cells. This injury current
may bring the normal tissue to threshold earlier than other-
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Fig. 5. Representative example of sudden death from ventricular fibrillation 17 min after LAD occlusion in a control pig.
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Wirth et al.
polarizations, has been mainly held responsible for the induction of reperfusion arrhythmias (Coronel, 1994). It is
unlikely that the failure of HMR 1883 to protect against
reperfusion arrhythmias in this study is due to insufficient
levels of the compound because in pigs, the plasma concentration 25 min after bolus administration still exceeds 2
mg/ml (K.J.W., B.R., J.U., H.C.E., A.E.B., and B.A.S., unpublished data). This concentration was still effective during
ischemia in the dog model of sudden cardiac death (Billman
et al., 1998). We can exclude neither species differences in the
potency of the compound nor the possibility that higher doses
would be needed to inhibit reperfusion arrhythmias compared with arrhythmias during ischemia, however. In contrast to pigs, HMR 1883 and glibenclamide were effective
against reperfusion arrhythmias and reduced the duration of
VF and death but not the incidence of VF in rats (Linz et al.,
1998a). The discrepancy between the two species could be
explained by the fact that the rat, in contrast to the pig, has
a high xanthine oxidase activity that can generate free oxygen radicals, which open KATP channels (Tokube et al., 1996).
In conclusion, KATP channel blockade with the cardioselective compound HMR 1883 reduced ischemically induced arrhythmias and VF in anesthetized pigs. While being silent on
the ECG and hemodynamics during normoxia, the ECG
changes typically associated with regional myocardial ischemia, a shift in S-T segment and a prolongation of Q-J time,
were strongly attenuated, suggesting an improvement of the
ischemically disturbed impulse propagation and ventricular
activation.
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