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THE JOURNAL OF PHARMACOLOGY AND EXPERIMENTAL THERAPEUTICS
Copyright © 1999 by The American Society for Pharmacology and Experimental Therapeutics
JPET 290:146 –152, 1999
Vol. 290, No. 1
Printed in U.S.A.
Electrophysiological Effects of LU111995 on Canine Hearts:
In Vivo and In Vitro Studies1
EUGENE A. SOSUNOV, RAVIL Z. GAINULLIN, PETER DANILO, JR., EVGENY P. ANYUKHOVSKY,
MICHAEL KIRCHENGAST,2 and MICHAEL R. ROSEN
Departments of Pharmacology (E.A.S., R.Z.G., P.D., E.P.A., M.R.R.) and Pediatrics (M.R.R.), College of Physicians and Surgeons of Columbia
University, New York, New York
Accepted for publication March 8, 1999
This paper is available online at http://www.jpet.org
In some instances, drug-induced long Q–T intervals are
associated with torsade de pointes and sudden death (Tan et
al., 1995). The mechanism generally assumed to be operative
is reduction in the repolarizing K1 currents, IKr and/or IKs;
prolongation of the cardiac action potential duration (APD),
and initiation of early afterdepolarizations (EADs) that lead
to the classic torsade de pointes arrhythmia (Roden and
Hoffman, 1985; Hondeghem and Snyders, 1990; Funck-Brentano, 1993). These untoward events have led regulatory
agencies to require the use of established intact animal and
isolated tissue models (or the development of new models) to
determine the likelihood of proarrhythmia occurring on administration of new drugs that prolong repolarization.
LU111995 [(1)-(1S,5R,6S)-exo-3-[2-[6-(4-fluorophenyl)-3aza-bicyclo[3.2.0]heptan-3-yl]ethyl]-1H,3H-quinazoline-2,4dione fumarate; Fig. 1) is a recently identified antipsychotic
agent with high 5-hydroxytryptamine2 and dopamine D4 receptor affinities as well as D4 versus D2 receptor selectivity
(Steiner et al., 1998). Because of concern over the effects of
some antipsychotic agents to excessively prolong repolarizaReceived for publication January 7, 1999.
1
This study was supported in part by U.S. Public Health Service National
Heart, Lung, and Blood Institute Grant HL53956 and by Knoll AG.
2
Present address: Knoll AG, Ludwigshaten, Germany.
independent decrease of V̇max in M cells. In PFs and M cells, it
produced reverse use-dependent lengthening of action potential duration (APD). In PFs at long CL, the drug exhibited a
biphasic concentration-dependent effect on APD: maximum
prolongation (by 26% at a CL of 2000 ms) was attained at 1 3
1026 M, and a decrease of APD occurred at higher concentrations. In M cells, the maximum effect on APD occurred at 3 3
1026 M. Early afterdepolarizations were seen in 50% of M cell
preparations but only at CL of 2000 ms. Triggered activity did
not occur. In summary, LU111995 prolongs the Q–T interval to
a limited degree and is not arrhythmogenic over the physiological range of CLs.
tion and induce proarrhythmia, the purpose of the present
study was to determine the effects of LU111995 on ventricular repolarization in vivo and in vitro. Therefore, we investigated the actions of LU111995 on the ECG and the ventricular effective refractory period (ERP) in conscious dogs in
which chronic atrioventricular block had been induced to
permit measurements over a range of heart rates that encompass the physiological range. In vitro, the electrophysiological properties of LU111995 were investigated in Purkinje
fibers and mid myocardial cells (M cells) (Sicouri and Antzelevitch, 1991). M cells were studied because they constitute
a significant fraction of ventricular myocardium (Antzelevitch et al., 1994; Sicouri and Antzelevitch, 1995; Anyukhovsky et al., 1996), they contribute importantly to T-wave
configuration and the Q–T interval, and, due to their exceptional sensitivity to APD-prolonging agents, they develop
EADs more readily than epicardial or endocardial cells (Antzelevitch and Sicouri, 1994; Antzelevitch et al., 1996;
Shimizu and Antzelevitch, 1997).
Materials and Methods
All experimental procedures conformed to the “Guiding Principles
of the Care and Use of Animals” of the American Physiologic Society
ABBREVIATIONS: MDP, maximum diastolic potential; APD, action potential duration; EAD, early afterdepolarization; Vmax, maximum rate of rise
of phase 0; ERP, effective refractory period; PF, Purkinje fiber; CL, cycle length.
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ABSTRACT
We studied the electrophysiological effects of LU111995 (1–15
mg/kg p.o.) in conscious dogs with chronic atrioventricular
block and ventricular pacing at 50 to 130 beats/min. LU111995
had no effects on idioventricular rhythm, QRS duration, and
ventricular conduction time. It significantly prolonged Q–T interval (by 5– 8%) and effective refractory period (ERP) (by
5–12%) with the maximal effect at 4 h after a 10 mg/kg dose. At
10 and 15 mg/kg, it increased the ERP/Q–T ratio. In vitro, the
effects of LU111995 (1 3 1027 to 1 3 1025 M) on action
potentials of Purkinje fibers (PFs) and M cells were studied at
cycle lengths (CL) of 300 to 2000 ms. It had no effects on
maximum diastolic potential and action potential amplitude in
either tissue. High concentrations induced a moderate, rate-
1999
Electrophysiology of LU111995
147
Fig. 1. Chemical structure of LU111995.
and were in accordance with the “Guide for the Care and Use of
Laboratory Animals” [DHEW (DHHS) publication (NIH) 85-23, revised 1985].
In Vivo Experiments. Conditioned mongrel dogs of either sex
weighing 20 to 25 kg were fasted overnight and anesthetized with
propafol (6 mg/kg i.v.). After tracheal intubation, anesthesia was
maintained with isoflurane (2%) inhalation. Under sterile conditions, a thoracotomy was performed in the right forth intercostal
space, and the heart was positioned in a pericardial cradle. To
achieve a low heart rate, complete heart block was produced by the
injection of 0.1 to 0.3 ml of 40% formalin into the region of the AV
node (Scherlag et al., 1967; Anyukhovsky et al., 1996). An epicardial
screw-type electrode (model 6917; Medtronic) was placed in the left
ventricle near the apex. The electrode lead was connected to a programmable Medtronic pacemaker (MINIX 8340) that was placed in a
s.c. pouch. For studying ERP and for monitoring ventricular conduction time, three platinum bipolar electrodes were sewn to the epicardium of the right ventricle (electrodes were placed along a baseapex axis approximately 1–2 cm apart). The wires from the
electrodes were secured to the ribs, tunneled s.c., and exteriorized
through a small incision between the scapulae. The pericardium was
left open, the chest wall was closed in layers, and the pneumothorax
was evacuated by a chest tube. Each dog was fitted with a jacket to
prevent damage to the pacemaker and wires. Animals were allowed
to recover for 2 weeks. After recovery from surgery and during 1
week before the drug protocols were begun, each dog was trained to
stand quietly in an animal sling during ECG monitoring. At this
time, the ECG had completely stabilized. During the periods of
recovery and training and between drug administration protocols,
ventricular pacing was maintained at 60 beats/min.
ECG parameters were measured at heart rates of 50 (when idioventricular rhythm permitted), 60, 70, 80, 110, and 130 beats/min.
The low heart rates were considered of particular importance because drugs that are reverse use dependent and induce torsade de
pointes have a propensity to do so at long cycle lengths (CLs) (e.g.,
Funck-Brentano, 1993). Variability of the Q–T interval over the
period of time of the study was minimal (i.e., at CL 5 1000 ms, ,5
ms; Fig. 2). To ensure a steady state, each rate was maintained for 3
min. To ensure a steady state, each rate was maintained for 3 min
before data were collected (Anyukhovsky et al., 1996). ERP was
determined at heart rates of 60, 80, and 130 beats/min. In these
experiments, the heart was stimulated with 1-ms rectangular pulses
of amplitude 2 3 threshold (Pulsemaster A300, with isolation unit
A360; WPI, Sarasota, FL) through the right ventricular epicardial
electrode that was closest to the apex. After every 10 beats, an
extrastimulus was delivered, the delay for which was gradually
decreased by 10- and then 1-ms decrements. The earliest premature
response that propagated to the most distant electrogram site was
assumed to represent the end of the ERP. Conduction time was
measured as the interval between the two bipolar right ventricular
electrodes recorded when that located closest to the apex was stimulated. ECG and local electrogram signals were digitized with an
analog-to-digital converter (D-210; DATAQ Instruments Inc., Akron,
OH) and stored in a personal computer for further analysis.
Dogs were randomly assigned to receive 1, 3, 10, and 15 mg/kg
LU111995 p.o. The highest dose used in early phase 2 studies was
200 mg/patient/day (Knoll, 1998), corresponding roughly to the 3
mg/kg dose in dogs. Cardiovascular assessments, which required
about 20 min in toto, were performed at control (before drug administration) and 1, 2, 4, and 8 h after drug. Blood samples were drawn
at appropriate intervals for measurements of serum levels of
LU111995 and its major metabolites, LU195730 and LU273973,
which are generated by glucuronydation (Steiner et al., 1998).
Serum drug concentrations were measured as follows: an HPLC
method was used for the simultaneous determination of LU111995,
LU195730, and LU273973 in dog plasma. LU81028 was the internal
standard. After a liquid-liquid plasma extraction procedure, chromatographic separation was performed on a 4-mm Superspher 60,
RPselect B analytical column (250 3 4.0 mm) using a mixture of 0.05
M KH2PO4 (pH 5)/acetonitrile (770:183, g/g) as the mobile phase
with a flow rate of 0.9 ml/min at 35°C. The chromatographic peaks
were measured by UV detection at 240 nm. The calibration curves
were linear with correlation coefficients of 0.999 or better from 5
ng/0.5 ml to 80 ng/0.5 ml for all three compounds in plasma. Other
endogenous components did not interfere with this method. Testing
of the method with 0.5 ml of plasma at a working range of 5 to 80
ng/ml yielded coefficients of variation of 3.8% to 17.8% for LU111995,
7.0% to 11.0% for LU195730, and 6.1% to 13.3% for LU273973.
Throughout the working range, the method yielded accurate values
from 93.4% to 107.0% for LU111995, 92.7% to 107.9% for LU195730,
and 95.4% to 107.5% for LU273973.
Dogs were restudied at weekly intervals, permitting a 7-day washout period before the next dose was administered. Ten successful
experiments were performed.
In Vitro Experiments. Mongrel dogs of either sex weighing 20 to
22 kg were anesthetized with sodium pentobarbital (30 mg/kg i.v.).
Their hearts were removed through a left lateral thoracotomy and
immersed in cold Tyrode’s solution equilibrated with 95% O2/5% CO2
and containing 131 mM NaCl, 18 mM NaHCO3, 4 mM KCl, 2.7 mM
CaCl2, 0.5 mM MgCl2, 1.8 mM NaH2PO4, and 5.5 mM dextrose. Two
types of preparations were used: free-running Purkinje fibers (PFs)
obtained from either ventricle and myocardial slices (;10 3 5 3 1
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Fig. 2. Q–T intervals measured over a time period equal to that of the
total duration of the study. No significant changes occurred in Q–T.
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Vol. 290
Results
In Vivo Study. The idioventricular rhythm, QRS duration, conduction time, and Q–T duration all were stable and
unchanging throughout the duration of the 3-week study
period (Table 1). LU111995 had a consistent effect on the
Q–T interval, as shown in Fig. 3. Q–T prolongation ranged
from 5% to 8% with the maximum at 4 h after the 10 mg/kg
dose (Fig. 3A). The magnitude of prolongation diminished at
TABLE 1
Effects of LU111995 on idioventricular rhythm, QRS duration, and
ventricular CT
Control values were measured in 1-week intervals immediately before LU111995
administration. LU111995 values are at 2 h postdrug administration. QRS duration
in control and after drug administration, conduction time in control, and its changes
after drug administration DCT at heart pacing rate of 80 beats/min is presented.
Values are mean 6 S.E.M. (n 5 10).
Dose
1 mg/kg
3 mg/kg
Idioventricular rhythm (beats/min)
Control
46 6 4
46 6 2
LU111995
46 6 4
44 6 2
QRS duration (ms)
Control
61 6 4
61 6 3
LU111995
61 6 3
62 6 2
DCT (ms)
Control CT
12.3 6 4.0
16.5 6 3.9
LU111995
21.3 6 0.8
0.6 6 0.4
10 mg/kg
15 mg/kg
45 6 3
48 6 3
46 6 2
44 6 3
63 6 4
63 6 4
61 6 3
62 6 2
15.1 6 2.9
0.1 6 0.5
16.0 6 3.3
0.5 6 0.5
Fig. 3. A, time course of LU111995 effects on the Q–T interval at pacing
rates of 60 and 130 beats/min. All doses of LU111995 are shown. Horizontal axis, time in hours after drug administration (points at 0 h represent the Q–T interval before drug administration). Values are mean 6
S.E.M. (n 5 10). * P , .05 for 10 mg/kg compared with respective control
(0 h). B, effects of LU111995 (10 mg/kg) on Q–T interval at various pacing
rates 4 h after drug administration. Values are mean 6 S.E.M. (n 5 10).
* P , .05 compared with control at the same pacing rate. C, dosedependent effects of LU111995 on Q–T interval at pacing rate of 60
beats/min and 4 h after drug administration. Values are mean 6 S.E.M.
(n 5 10). * P , .05 compared with control (0 mg/kg).
15 mg/kg. The maximum effects of 10 mg/kg at 4 h after drug
administration at all driving rates are presented in Fig. 3B.
Prolongation reached significance at 60, 70, and 80 beats/
min. The magnitude of the Q–T interval prolongation is
explored across all doses in Fig. 1C.
The effects of LU111995 on the ERP at heart rates of 60
and 130 beats/min are shown in Fig. 4A. All doses prolonged
the ERP significantly, with the maximal effect occurring at 2
to 4 h after drug administration. A similar picture was observed at 80 beats/min (data not shown). The maximal percent increases were 5% at 1 mg/kg, 7% at 3 mg/kg, 12% at 10
mg/kg, and 9% at 15 mg/kg. The changes in ERP at various
pacing rates at 10 mg/kg and 4 h after drug administration
are shown in Fig. 4B. Note that significant prolongation of
ERP was seen across all rates tested. The magnitude of ERP
prolongation across all doses at a rate of 60 beats/min is
demonstrated in Fig. 4C. As in the case of the Q–T interval,
a maximal effect was observed at 10 mg/kg, and the effect
decreased at 15 mg/kg.
Figure 5 illustrates the effects of LU111995 on the ERP/
Q–T ratio. ERP/Q–T increased at the 10 and 15 mg/kg doses,
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mm) filleted from the posterobasal left ventricular free wall parallel
to the epicardial surface from the depth of 3 to 5 mm beneath it. The
preparations were placed in a tissue bath and superfused with Tyrode’s solution warmed to 37°C (pH 7.35 6 0.05). Solution was
pumped through the tissue bath at a flow rate of 12 ml/min, changing
chamber content three times per minute. The bath was connected to
ground via a 3 M KCl/Ag/AgCl junction.
All preparations were impaled with 3 M KCl-filled glass capillary
microelectrodes that had tip resistances of 10 to 20 MV. The maximum upstroke velocity of the action potential (V̇max) was obtained
through electronic differentiation with an operational amplifier. The
electrodes were coupled by an Ag/AgCl junction to an amplifier with
high input impedance and input capacity neutralization. Transmembrane action potentials and V̇max were displayed on a digital storage
oscilloscope (model 4074; Gould) and stored in digitized form for
subsequent analysis. For stimulation of preparations, standard techniques were used to deliver square-wave pulses 1.0 ms in duration
and 1.53 threshold through bipolar Teflon-coated silver electrodes.
To investigate frequency dependence of drug effects, the preparations were driven at CLs of 2000, 1000, 500, and 300 ms in sequence.
Each frequency was maintained for 3 min before the data were
collected.
Experiments were started after preparations had fully recovered
and displayed stable electrophysiological characteristics. This required 1 h for PFs (Wyse et al., 1993) and 3 to 4 h for transmural
preparations (Anyukhovsky et al., 1996; Sosunov et al., 1997). After
control records were obtained, the preparations were superfused
with Tyrode’s solution containing graded concentrations (1 3 1027,
1 3 1026, 3 3 1026, and 1 3 1025 M) of LU111995. Because preliminary experiments had shown that steady-state effects on action
potential parameters were achieved in 25 to 30 min, the preparations
were equilibrated at each drug concentration for 30 min.
Statistical Analysis. Data are expressed as mean 6 S.E.M. The
statistical techniques used were one- or two-way ANOVA for repeated or nonrepeated measures, with Bonferroni’s test when the F
value permitted (Winer et al., 1991). Microelectrode data were analyzed from impalements maintained throughout the course of each
experiment. Significance was determined at P , .05.
1999
2 and 4 h after drug administration (Fig. 5A). The increase
was 7%. With an acceleration of pacing rate, the effect decreased; at 130 beats/min, no significant effect on ERP/Q–T
ratio was seen. The changes in ERP/Q–T at various pacing
rates at 10 mg/kg and 4 h after drug administration are
shown in Fig. 5B. Significant prolongation of ERP/Q–T was
seen only at 60 beats/min. The magnitude of effect across all
doses at a stimulation rate of 60 beats/min is demonstrated
in Fig. 5C. The drug induced a dose-dependent increase of
ERP/Q–T ratio that reached significance at 10 and 15 mg/kg.
Figure 6 demonstrates serum concentrations of LU111995
and its metabolites achieved at each dosage schedule. A
maximal level of parent compound was achieved at 4 h after
administration and then slowly decreased. Maximal concentrations of the metabolites LU195730 and LU273973 were
25- and 70-fold less, respectively, than the parent compound.
These levels were achieved at 2 h after LU111995 administration and then decayed faster than that of the parent
compound.
In Vitro Study. Figure 7 illustrates representative PF
and M cell transmembrane potentials recorded in control and
in the presence of two LU111995 concentrations (1026 and
149
Fig. 5. Time course of LU111995 effects on the ratio of ERP to Q–T
interval at a pacing rate of 60 beats/min. A, effects of all doses. B, effect
of the 10 mg/kg dose at 4 h. C, dose-dependent effects at a pacing rate of
60 beats/min and at 4 h. Values are mean 6 S.E.M. (h 5 10). * P , .05
compared with 0 h in A and C and control curve in B.
1025 M), at the longest and shortest CLs. Control maximum
diastolic potential (MDP) at a CL of 1000 ms was 292 6 1 mV
in PFs and 291 6 1 mV in M cells. There was no significant
effect of LU111995 on MDP at LU111995 concentrations up
to 3 3 1026 M at any cycle length (e.g., at CL 5 1000 ms,
MDP 5 291 6 1 mV in PFs and 289 6 1 mV in M cells at 3 3
1026 M, P . .05 compared with control). Only in PFs at the
fast rate of stimulation (300 ms) and at 1 3 1025 M drug was
a significant decrease in MDP seen (to 289 6 1 mV, P , .05).
Similar results were seen for action potential amplitude
(A.P.D.) and V̇max in both tissues: in PFs, amplitude and
V̇max were reduced significantly only at CL of 300 ms and
1025 M drug from 130 6 1 to 126 6 1 mV amplitude and from
543 6 28 to 455 6 31 V/s V̇max (both P , .05). In M cells,
V̇max, but not amplitude, was reduced significantly at 3 3
1026 M and 1 3 1025 M drug at all cycle lengths. For example,
at CL of 1000 ms, control V̇max was 351 6 42 V/s, and this
was reduced to 290 6 20 V/s at 3 3 1026 M and 293 6 23 V/s
at 1 3 1025 M (both P , .05).
The most prominent effect of LU111995 was a lengthening
of the APD in both types of tissues. In PFs, at low stimulation
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Fig. 4. A, time course of LU111995 effects on the ERP at pacing rates of
60 and 130 beats/min. All doses of LU111995 are shown. Horizontal axis,
time in hours after drug administration (points at 0 h represent the ERP
before drug administration). Values are mean 6 S.E.M. (n 5 10). * P , .05
control (0 h) at the same dose. B, effects of LU111995 (10 mg/kg) on the
ERP at various pacing rates 4 h after drug administration. Values are
mean 6 S.E.M. (n 5 10). * P , .05 compared with control at the same
pacing rate. C, dose-dependent effects of LU111995 on the ERP at pacing
rate of 60 beats/min and 4 h after drug administration. Values are
mean 6 S.E.M. (n 5 10). * P , .05 compared with control (0 mg/kg).
Electrophysiology of LU111995
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Sosunov et al.
Vol. 290
Fig. 6. Time courses of serum levels of LU111995 (A) and its metabolites
LU195730 (B) and LU273973 (C). All doses of LU111995 are shown.
Horizontal axis, time in hours after LU111995 administration. Values
are mean 6 S.E.M. (n 5 7 for 1 mg/kg, n 5 10 for 3, 10, and 15 mg/kg).
* P , .05 compared with 0 h (before LU111995 administration).
rates, the compound exhibited a concentration-dependent biphasic effect on APD to 90% repolarization (APD90): the
maximum prolongation was attained at 1 3 1026 M, and a
decrease in APD90 was then seen at higher concentrations
(Fig. 8A). This decrease was accompanied by significant
shortening of APD to 50% repolarization (APD50) (Fig. 8B).
The APD90 prolongation manifested reverse use dependence:
no prolongation took place at high stimulation rates.
As in PFs, LU111995 induced reverse use-dependent prolongation of APD in M cells (Fig. 9). At low stimulation rates,
about the same lengthening of APD90 and APD50 was observed. The maximum effect was attained at 3 3 1026 M. In
contrast to PFs, no APD shortening was seen at high
LU111995 concentrations. In three of six experiments with M
cells, LU111995 of 3 3 1026 M or more induced EADs that
developed at the end of the plateau (Fig. 7). These depolarizations were observed only at the longest CL (2000 ms) and
had low amplitude. Triggered activity was never seen.
Discussion
LU111995 had no effect on the variables measured in vivo
except for the Q–T interval and the ERP. The comparison of
time courses of Q–T interval changes (Fig. 3) and of
LU111995 and its serum metabolite levels (Fig. 9) suggests
that the parent compound rather than its metabolites was
mainly responsible for Q–T prolongation. The extent of Q–T
increase was small, never reaching 10% even at its greatest
magnitude. Moreover, in clinical studies, at a maximum dosage of 200 mg/patient/day, the peak plasma level of parent
compound attained was approximately 500 ng/ml, with
steady-state levels in the range of 50 to 100 ng/ml (Knoll,
1998). This suggests that the 3 mg/kg dose in dogs is the
likely clinical surrogate. This dose had minimal effects on the
Q–T interval.
The fact that the largest increase in Q–T interval occurred
at the 10 mg/kg dose and that the Q–T interval decreased at
the higher dose suggests that in addition to inducing K1
channel blockade, which would prolong the APD, the drug
may reduce inward plateau currents, thereby limiting the
extent of prolongation of the Q–T interval. In vitro results
with PFs and M cells are consistent with this suggestion. The
drug significantly decreased APD50, a result consistent with
inhibition of Na1 and/or Ca21 plateau currents. Although no
APD shortening was observed in M cells, a decrease of the
magnitude of APD lengthening occurred at the highest drug
concentration. Also consistent with an effect on inward Na1
current was the decrease in V̇max of phase 0.
PFs are vulnerable to the effects of APD-prolonging agents.
Their long APD and steep APD-rate relationship make them
a likely site for the occurrence of EADs and triggered activity
as seen especially at slow stimulation rates (Brachman et al.,
1983). In light of the above, the action potential prolongation
and reverse use dependence of LU111995 effects seen in PFs
are a potential drawback. However, there was a limit of
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Fig. 7. Examples of the effects of LU111995 on PF and M cell action
potentials at CL of 2000 and 300 ms. Top, transmembrane action potential. Bottom, V̇max. Vertical calibration is for the action potential and
V̇max; horizontal calibration is for the action potential. Numbers represent molar concentrations of the compound. C, control.
1999
Electrophysiology of LU111995
151
Fig. 9. Concentration-dependent effects of LU111995 on M cell APD at
90% (A) and 50% (B) of full repolarization at all CLs. C, control. * P , .05
compared with respective control (n 5 6).
LU111995-induced APD prolongation in PFs: maximal prolongation was attained at 1 3 1026 M, and a shortening of
APD occurred at higher concentrations. In this setting of
high LU111995 concentrations, EADs did not occur in PFs.
In contrast, EADs were seen in three of six M cell preparations but only at the highest drug concentrations and at a
cycle length (2000 ms) outside the usual physiological range,
although within the range of long sinus pauses.
In vivo, the effect of LU111995 to prolong ERP was greater
than its effect on Q–T. The results of the experiments in vitro
help explain this action. In M cells, the drug induced a
moderate, concentration-dependent decrease of V̇max that
was independent of simulation rate. Although not reaching a
statistically significant level, a qualitatively similar effect
was observed in PFs. This type of local anesthetic action may
prolong refractoriness and increase the ERP/APD ratio (Bigger and Mandel, 1970; Gintant, 1990). Thus, even if there
were some arrhythmogenic potential of the Q–T prolongation, this would tend to be counteracted by the greater prolongation of the ERP. The inhibition of V̇max implies a decrease of conduction velocity that, however, was not seen in
vivo. This can be explained as follows: The LU111995-induced decrease of V̇max did not exceed 20%. The speed of
impulse propagation is directly related to the square root of
V̇max (Buchman et al., 1985); hence the V̇max inhibition in our
study implies a less than 10% slowing of conduction velocity.
This is why no significant changes in CT and QRS duration
was seen in the experiments in vivo.
The results of the present study relate to an important
additional question: What type of isolated ventricular tissue
adequately reflects the effects of APD-prolonging agents on
ventricular repolarization in vivo? Free running PFs are
widely used in vitro as a multicellular preparation. Because
PFs have long APD and are sensitive to action potential
prolonging effects of antiarrhythmic compounds (Varro et al.,
1986; Anyukhovsky et al., 1997), they are suitable for the
estimation of proarrhythmic propensity. However, to estimate in vitro the effects of drugs on ventricular repolarization in vivo (on Q–T interval), ventricular myocardial tissue
is a more appropriate model. The use of M cells here is reasonable because they constitute a significant fraction of ventricular
myocardium (Antzelevitch et al., 1994; Sicouri and Antzelevitch, 1995; Anyukhovsky et al., 1996) and can contribute importantly to T-wave configuration and the Q–T interval. In addition, APD-prolonging agents produce a marked APD
lengthening, EADs, and triggered activity in M more than in
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Fig. 8. Concentration-dependent effects of LU111995 on PF APD at 90%
(A) and 50% (B) of full repolarization at all CLs. C, control. * P , .05
compared with respective control (n 5 10).
152
Sosunov et al.
endocardial or epicardial cells (Antzelevitch and Sicouri, 1994;
Antzelevitch et al., 1996; Shimizu and Antzelevitch, 1997).
In summary, the results of the present study showed that
in a model in which heart rate was carefully controlled,
LU111995 prolonged the Q–T interval to an extent that is not
considered of clinical importance. Hence, in the absence of
any contradictory information, there should be minimal concern regarding the proarrhythmic potential of this Q–T-prolonging drug when administered to an individual with a
healthy heart.
Acknowledgments
We express our gratitude to Dietmar Schoebel for performing the
HPLC determinations, to Dr. Natalia Egorova for assistance with
certain of the experiments, and to Ms. Eileen Franey for her careful
attention to preparation of the manuscript.
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