Vol. 274, Issue 5, H1560-H1568, May 1998
Effect of BAY y 5959 on myocardial function and metabolism in
normal and failing hearts
Koji
Todaka,
Jie
Wang,
Geng-Hua
Yi,
Anguo
Gu,
Shu-Ming
Zhu,
Hui
Zhang, and
Daniel
Burkhoff
Division of Circulatory Physiology, Department of Medicine, College
of Physicians and Surgeons, Columbia University, New York, New York
10032
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ABSTRACT |
BAY y 5959 is a dihydropyridine derivative
with positive inotropic actions mediated by a direct increase in
intracellular calcium. We characterized the direct myocardial actions
of this new agent in hearts isolated from seven normal dogs and from
five dogs with repeated coronary microembolization-induced heart
failure. Inotropic actions of BAY y 5959 were accompanied by little
effect on duration of contraction and by prolongation of the monophasic action potential (MAP); in contrast, isoproterenol decreased
contraction and MAP durations. Whereas inotropic responsiveness to
isoproterenol was blunted in embolized hearts, these actions of BAY y
5959 were relatively preserved in the heart failure state.
Isoproterenol increased heart rate, whereas BAY y 5959 had little
effect. Changes in coronary vascular resistance also decreased
similarly for isoproterenol and BAY y 5959. Finally, for comparable
inotropy, increases in myocardial oxygen consumption were similar for
isoproterenol and for BAY y 5959. In summary, preserved inotropic
responsiveness and lack of positive chronotropic actions are two
clinically favorable features of this type of inotropic agents compared
with a typical 
-adrenergic agonist.
heart failure; inotropic agents; calcium; oxygen consumption; heart
rate; coronary vascular resistance; action potential
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INTRODUCTION |
TRADITIONAL INOTROPIC agents used in the treatment of
congestive heart failure (CHF), such as catecholamines and
phosphodiesterase inhibitors, enhance myocardial contractile force via
adenosine 3',5'-cyclic monophosphate (cAMP)-dependent
pathways, which are downregulated in CHF (2, 7). Accordingly, inotropic
responsiveness is frequently depressed in CHF patients, and tolerance
may develop during longer term infusions. In addition, these agents
increase heart rate and may enhance ventricular ectopy, both of which
are undesirable side effects. The existence of a class of
dihydropyridine compound derivatives that exert positive inotropic
actions by enhancing systolic transsarcolemmal calcium flux through
L-type calcium channels has been recognized for a long time; the most prominent of these is BAY K 8644 (21). However, lack of myocardial specificity of that compound greatly limited its potential clinical utility, mainly because of its vasoconstricting actions (10, 12).
More recently, BAY y 5959, a dihydropyridine derivative with
cardioselective calcium-channel agonistic activity, has been discovered
(1, 11). BAY y 5959 binds with high affinity to the cardiac
dihydropyridine receptor component of the calcium channel and prolongs
the single-channel open time (1). Preliminary studies of this compound
in various models have shown its inotropic actions to be preserved in
heart failure (20, 24), that it has bradycardic actions (with
associated action potential prolongation) (6, 17), and that it may
exert a relative oxygen-saving effect compared to inotropism with
-agonists (4, 5, 20). These features have renewed interest in this
class of compounds as a potential new therapy for heart failure (16,
17). However, interpretation of results of studies in intact, awake
dogs are complicated by direct or indirect effects of drug infusion on baroreflexes and potentially on vascular properties (preload and afterload), and the degree to which the identified characteristics reflect direct vs. indirect myocardial effects on the heart, while of
primary importance, is not certain.
In view of the potential clinical application of this class of
inotropic agents for treating heart failure, the purpose of this study
was to test in normal and failing hearts the direct myocardial actions
of BAY y 5959 on ventricular contractile state, coronary vascular
resistance, myocardial oxygen consumption
(M
O2), heart rate, and action
potential duration. To provide a relevant framework of interpreting the
findings, we made comparisons to the effects of isoproterenol.
Potential confounding effects of autonomic reflexes and changes in
afterload and preload were obviated because the physiological
conditions of an intact heart and blood perfusion were retained by
performing these studies in isolated cross-perfused canine hearts.
Normal hearts and hearts rendered myopathic by repeated coronary
microembolization were studied. The constellation of myocardial actions
of BAY y 5959 identified in this study supports the notion that this
class of agents offers certain advantages over traditional inotropic
agents in the treatment of heart failure.
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METHODS |
Isolated heart preparation.
Hearts of seven normal dogs (22.2 ± 2.0 kg) and of five dogs with
repeated coronary microembolization heart failure (25.3 ± 4.1 kg,
methods described below) were studied using a standard isolated heart
preparation. Details of this preparation have been provided previously
(3). Briefly, the heart from the dog of interest was excised and
metabolically supported by blood provided from a second support dog.
The femoral arteries and veins of the support dog were cannulated and
connected to a perfusion circuit consisting of a peristaltic pump, a
heater, a blood filter, and an air trap. The pressure in the aortic
root of the isolated heart, which is the perfusion pressure for
coronary flow, was measured. Coronary blood flow was adjusted at the
beginning of the protocol to provide a perfusion pressure of ~100
mmHg and was kept constant throughout the experiment. Blood traveled
through the coronary vasculature of the isolated heart and returned to
the support dog by gravity. Coronary flow was collected through a
wide-bore cannula placed in the right atrium and right ventricle and
was measured by an in-line ultrasonic flowmeter (Transonic Systems model T108, Ithaca, NY). The difference between arterial and venous oxygen content (a-vO2) was
measured on-line by a commercially available spectrophotometer (AVOX
Systems, San Antonio, TX). Oxygen consumption of the whole heart
(MO2) was determined by
multiplying coronary flow by
a-vO2.
A water-filled balloon was placed within the left ventricle (LV) via
the mitral valve. The volume of the balloon, and therefore of the LV,
was controlled by a piston pump servosystem. A micromanometer (model
SPC-360, Millar Instruments, Houston, TX) placed within the balloon was
used to measure LV pressure. The heart was paced from the LV apex at
constant rate (154 ± 11 beats/min) and was constrained to contract
isovolumically. Blood temperature was kept at ~37°C by a heat
exchanger.
Monophasic action potentials (MAPs) were recorded from the anterior or
lateral surface of the LV by a commercially available MAP electrode
(8). MAP amplitudes are expressed in relative terms.
Repeated coronary microembolization heart failure model.
The details of the procedures used to induce heart failure using
repeated coronary microembolization have been provided elsewhere (13,
14, 18). Five mongrel dogs were anesthetized (1-2% inhaled
isoflurane) and underwent sterile surgery for chronic instrumentation
via a left thoracotomy. A solid-state pressure transducer (Konigsberg
model P6.5, Konigsberg Instrument, Pasadena, CA) was inserted into the
LV through the apex. Fluid-filled catheters were inserted into the left
atrium and the aorta. Another thin [Tygon catheter, diameter
1.8-2.3 (ID 1.0-1.3) mm, Cardiovascular Instruments, Boston,
MA], fluid-filled catheter was introduced into the
proximal portion of the left circumflex coronary artery for injections
of microspheres over the ensuing weeks. After at least 10 days
recovery, baseline hemodynamics were recorded while the dogs were awake
and resting comfortably on a laboratory table. Approximately
25,000-50,000 glass microspheres (diameter ~90 µm) suspended
in saline were injected daily for ~30 days (average total no. of
microspheres 1,050,000 ± 209,000) until measurements of
peak rate of rise of LV pressure
(dP/dtmax), LV
end-diastolic pressure (LVEDP), and resting heart rate were consistent
with a state of heart failure (detailed below). The animals were then observed for between 7 and 10 days; at the end of this observation period, hemodynamic measurements were repeated with the dogs lying quietly on their side to ensure persistence of the heart failure state.
The results, summarized in Table 1, show
that there was a significant decrease in
dP/dtmax and a
significant elevation in LVEDP (P < 0.05, Wilcoxon signed-rank sum test); the changes in these
parameters suggest that the animals were in a state of moderate heart
failure. The hearts of these animals were studied in isolation as
described above and comprised the CHF group.
Isolated heart protocols and data analysis.
Inotropic, lusitropic, metabolic, and electrophysiological
effects of BAY y 5959 were assessed in normal and failing hearts. After
the surgical preparation was completed, hearts were allowed to
stabilize for ~30 min before the protocol was started. Ventricular volume was adjusted to provide an end-diastolic pressure (EDP) of ~5
mmHg. Hemodynamic recordings (LV pressure, coronary blood flow,
a-vO2) and MAP recordings were
made at baseline during stepwise increases in BAY y 5959 infusion
(titrated to increase contractile state by ~75%). BAY y
5959-liposome was prepared by diluting a standardized preparation of 25 mg BAY y 5959, sucrose, egg lecithin, and ascorbic acid with a
standardized reconstitution medium containing glycerol and sodium
caprylate in water; this preparation was then diluted in saline to
final concentrations ranging between 10 and 40 µg/ml (23.6 and 94.2 µmol/l), depending on heart sensitivity to BAY y 5959, and was
infused directly into the arterial perfusion line ~1.5 m from the
heart, which allowed ample time for uniform mixing in blood before
reaching the heart. At the highest infusion rate studied, hemodynamic
measurements were made at three different volumes (spanning EDPs
between 0 and 15 mmHg) to construct end-systolic pressure-volume
relationships (ESPVR) and to assess the relationship between workload
and MO2. The results
obtained with BAY y 5959 were compared to those obtained with
isoproterenol titrated to create similar degrees of inotropism. Because
of the relatively long half-life of BAY y 5959, the order of drug
infusion could not be randomized. Therefore, some hearts received only
BAY y 5959, and some hearts received isoproterenol followed by BAY y 5959. Selected results from the two groups were analyzed separately to
test whether isoproterenol pretreatment modified the effects of BAY y
5959. In all, the BAY y 5959 group consisted of seven normal hearts (5 of which were pretreated with isoproterenol) and five CHF hearts (all
of which were pretreated with isoproterenol), and the isoproterenol
group consisted of five normal and five CHF dogs.
Ventricular contractile state was assessed by isovolumic peak developed
pressure (peak minus minimum isovolumic LV pressure) at a fixed volume.
Left ventricular relaxation was assessed by pressure half-time
(t1/2), which
was defined as the time for LV pressure to fall to 50% of its value at
the point of peak rate of decline in LV pressure
(
dP/dtmax).
The duration of contraction was defined as the width of the isovolumic
pressure curve at a pressure level equal to 10% of the peak developed
pressure; this parameter was called
D10.
To assess the metabolic cost of inotropism, we determined the
relationship between MO2
and total mechanical work, indexed by the pressure-volume area (PVA):
MO2 = A × PVA + B. PVA was defined in the usual manner
as the area on the pressure-volume diagram contained within the
triangular region bounded by the linear ESPVR, the end-diastolic
pressure-volume relationship, and the vertical line corresponding to
the volume at which the isovolumic contraction occurred (23). The
MO2 intercept of this
relation (i.e., B; the unloaded
MO2) has been shown to vary
directly with contractile state, whereas the slope,
A, is relatively independent of
contractile state (23). Because changes in contractile state are
generally brought about by changes in intracellular calcium, changes in
B have been hypothesized to reflect
altered energy demands for calcium cycling (23).
Inotropic, lusitropic, and metabolic effects were related to plasma
concentrations of BAY y 5959, which were estimated from the rate of
drug infusion, the measured coronary blood flow (CBF), and a
calibration curve. The calibration curve was derived from 22 freshly
frozen plasma samples spanning a wide range of concentrations (0-800 nmol/l), which were analyzed by high-performance liquid chromatography. Samples were transported overnight on dry ice from our
laboratory to Bayer (West Haven, CT), where the assay was performed.
The formula determined from this procedure was as follows: [BAY y
5959]plasma (in nmol/l) = 0.97[infusion rate (in nmol/min)/CBF (in l/min)], where
0.97 is the empirically determined scaling factor.
Average dose-response curves were computed for each group of hearts by
averaging drug concentrations at the same level of inotropism after
fitting each response to a sigmoidal curve {Boltzmann equation:
y = (A1 A2)/[1 + e(x 
x0)/d] + A2, where
A1,
A2,
x0, and
d are fit parameters}.
Statistical analysis.
Data are presented as means ± SD. Wilcoxon's signed-rank sum test
was used for determination of differences in in vivo hemodynamic parameters between baseline and CHF states for animals that underwent repeated coronary embolizations. Multiple linear regression was used to
test for statistical significance of differences in trends such as
effects of drug concentration on hemodynamic parameters. P < 0.05 was regarded significant.
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RESULTS |
Effects of BAY y 5959 on LV pressure and MAPs.
Representative MAP and isovolumic LV pressure recordings from a heart
isolated from an animal that underwent repeated coronary embolizations
in response to increasing doses of isoproterenol are shown in Fig.
1. As is well known, isoproterenol
shortened the MAP duration (Fig.
1A) while enhancing contractile
force dose dependently (Fig. 1B). In
addition, a decrease in the total duration of contraction is also
evident. At the highest dose of isoproterenol studied, peak developed
pressure increased by 65 ± 19% in normal hearts and 52 ± 9%
in failing hearts, and this was associated with an average 9.3 ± 2.5 and 10.1 ± 2.6% decrease in
D10
(index of overall duration of contraction), respectively. BAY y 5959 also created a significant inotropic response but, in contrast to
isoproterenol, lengthened the MAP (Fig. 2,
A and
B; data from same heart as presented
in Fig. 1). In contrast to isoproterenol, contraction duration was
little affected by BAY y 5959. With a comparable average of 61 ± 17 and 57 ± 10% increase in developed pressure in normal and failing
hearts,
D10
was decreased by only 7.0 ± 2.5 and 6.6 ± 1.7%, respectively
(P < 0.01 vs. normal group by
multiple linear regression ).
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Fig. 1.
Effects of isoproterenol on monophasic action potential (MAP,
A) and on isovolumic left
ventricular (LV) pressure waves (B)
in a representative heart isolated from a dog with moderate heart
failure due to repeated coronary microembolization. As is well known,
inotropism with this -agonist is associated with decreases in action
potential duration and decreases in duration of contraction. Estimated
blood concentrations of isoproterenol
([isoproterenol]est)
were 0, 1.87, 3.32, and 6.7 nM.
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Fig. 2.
Effects of BAY y 5959 on MAP (A) and
on isovolumic LV pressure waves (B)
in representative heart isolated from a dog with moderate heart failure
due to repeated coronary microembolization (same heart as in Fig. 1).
Inotropism with this myocardial-specific calcium agonist was associated
with increased action potential duration and no significant change in
overall duration of contraction. Estimated blood concentrations of BAY
y 5959 ([BAY y
5959]est) were 0, 50, 120, 230, and 460 nM.
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Average contractile responses to both agents in normal and CHF groups
are summarized in Fig. 3. As reported and
well recognized previously, the response of failing hearts to
isoproterenol is depressed markedly as evidenced by the statistically
significant rightward shift of the dose-response curve of the CHF group
(Fig. 3A,
* P = 0.007, normal vs. CHF). In
contrast, the dose-response curve of the CHF group to BAY y 5959 was
shifted by a smaller, statistically nonsignificant
(P = 0.413) degree relative
to that of the normal group (Fig.
3B). Figure
3B,
inset, shows that isoproterenol pretreatment did not affect the effectiveness of BAY y 5959 in any
discernable way.
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Fig. 3.
Average (±SD) dose-response curves of normal and congestive heart
failure (CHF) animals to intracoronary infusions of isoproterenol
(A) and BAY y 5959 (B). Whereas isoproterenol
responsiveness was markedly blunted (normal vs. CHF,
* P = 0.007 by
multiple-regression analysis), responsiveness to BAY y 5959 was not
significantly different between the 2 groups (normal vs. CHF,
P = 0.413 by multiple-regression
analysis). Inset: pretreatment of
normal hearts with isoproterenol (IP) did not influence subsequent
dose-response curve to BAY y 5959.
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The MAP duration (MAPD) was indexed by
MAPD90, the duration at 90%
repolarization. Average results for both isoproterenol and BAY y 5959 are summarized in Fig. 4; to make the
comparison easier, we normalized
MAPD90 to its baseline value and
plotted it against normalized developed pressure, which indexed the
degree of inotropism. The baseline values are as follows:
MAPD90 was 200 ± 25 ms in
normal hearts and 211 ± 8 ms in CHF hearts
(P = NS by unpaired
t-test); developed pressure was 68 ± 15 mmHg in normal hearts and 40 ± 14 mmHg in CHF hearts
(P < 0.01 by unpaired t-test). As shown in the
representative case, isoproterenol decreased MAPD90, whereas it was prolonged
by BAY y 5959 in both normal and CHF hearts for comparable degrees of
inotropism. Multiple-regression analysis showed that the differences in
these relations between isoproterenol and BAY y 5959 were statistically
significant in both normal and CHF hearts
(P < 0.001).
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Fig. 4.
Average changes in MAP duration (MAPD) [indexed by duration at
90% repolarization
(MAPD90)] plotted as a
function of contractile state for BAY y 5959 and for isoproterenol.
Contractile state was indexed by developed pressure normalized to its
baseline value before drug infusion. In both normal and CHF hearts,
MAPD90 decreased in response to
isoproterenol and increased in response to BAY y 5959 (* P < 0.01 by multiple linear
regression for both groups of hearts).
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Effects on relaxation.
Although BAY y 5959 enhances peak intracellular calcium, we found that
the rate of relaxation was increased by this agent. For comparable
degrees of inotropy,
t1/2 was
decreased by both agents to roughly the same degree
(P = 0.83 by multiple linear regression analysis, Fig. 5). Baseline
values of t1/2
were 31.3 ± 5.5 ms for the normal group and 29.8 ± 4.6 ms for
the CHF group (P = 0.57 by unpaired
t-test).
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Fig. 5.
Average changes in rate of relaxation [indexed by half-time of
pressure fall
(t1/2)]
plotted as a function of contractile state for BAY y 5959 and for
isoproterenol. Contractile state was indexed by developed pressure
normalized to its baseline value before drug infusion. In both normal
and CHF hearts,
t1/2 decreased as
developed pressure increased by either drug
(P < 0.01 by multiple linear
regression), but there were no differences in the slopes of any of
these relations, indicating a comparable effect of these drugs on
dynamic aspects of relaxation.
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Effects on heart rate.
To test and compare the chronotropic actions of BAY y 5959 and
isoproterenol, we interrupted the pacing for several minutes to examine
the native heart rate at each drug infusion rate. Results, summarized
in Fig. 6, show that positive chronotropic
actions of isoproterenol were identified, with heart rate increasing in relation to the degree of inotropism achieved. However, heart rate did
not increase with BAY y 5959 infusion
(P = NS by multiple linear regression)
despite the same degree of inotropism as achieved with isoproterenol.
Statistical analysis showed that this difference in heart rate
responses was statistically significant in both normal and CHF hearts
(P < 0.001). Baseline values of
heart rate were 108 ± 21 beats/min in normal hearts and 92 ± 17 beats/min for CHF hearts (P = 0.07 by
unpaired t-test).
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Fig. 6.
Effect of BAY y 5959 and isoproterenol on native heart rate (HR) in
both normal and CHF hearts as a function of contractile state. Both HR
and developed pressure (the index of inotropic response) are normalized
to their respective baseline values before drug infusion. Isoproterenol
increased HR in both normal and CHF hearts
(* P < 0.01 by multiple linear
regression analysis for both groups). There was no statistically
significant effect of BAY y 5959 on HR in either normal or CHF hearts,
although a trend for an HR reduction in CHF hearts is
noted.
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Effects on coronary resistance.
To test whether BAY y 5959 affects smooth muscle tone in the heart, we
examined its effect on coronary vascular resistance. As summarized in
Fig. 7, coronary resistance decreased with
increasing degrees of inotropism caused by BAY y 5959. Furthermore, the
changes in resistance in both normal and CHF hearts were similar to
those observed after isoproterenol infusion for comparable degrees of inotropism. Multiple linear regression revealed no difference in these
effects between isoproterenol and BAY y 5959. Baseline values
of coronary resistance were 1.12 ± 0.39 mmHg · min · ml1
for normal hearts and 1.35 ± 0.50 mmHg · min · ml1
for CHF hearts (P = 0.25 by unpaired
t-test). Although metabolic coronary
autoregulation-mediated dilation in response to enhanced contractility
may have masked possible vasoactive properties of BAY y 5959, any such
effect would be unlikely in view of the fact that our result showed no
difference in the relationship between resistance and inotropic state
between the two agents in either normal or heart failure states.
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Fig. 7.
Effects of BAY y 5959 and isoproterenol on coronary vascular resistance
as a function of contractile state. Both coronary resistance and
developed pressure (the index of inotropic response) are normalized to
their respective baseline values before drug infusion. As shown,
coronary vascular resistance varied similarly after both isoproterenol
and BAY y 5959 with a vasodilatory response. Statistical analysis
revealed that these responses were similar for the 2 drugs, suggesting
that these effects are mediated by metabolic autoregulation and that
BAY y 5959 did not interfere with this process.
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Effects on oxygen consumption.
Representative results regarding the ESPVR and the
MO2-PVA relationship from a
failing heart are shown in Fig. 8. BAY y 5959 injection to a blood concentration of ~700 nM increased the slope of the ESPVR with little effect on the volume-axis intercept (Fig. 8A). This increase in
contractility was accompanied by an upward shifting of the
MO2-PVA relationship as seen
in Fig. 8B. These characteristics were
confirmed in other hearts examined as summarized in Fig.
9. Multiple linear regression analysis
indicated that there was no significant influence of inotropism by
either isoproterenol or BAY y 5959 on the
MO2-PVA slope (Fig.
9A) in either normal or CHF hearts.
In contrast, both agents significantly increased the intercept values,
B, in both groups (Fig.
9B); furthermore, the increases were
comparable for isoproterenol and BAY y 5959 for similar degrees of
inotropism. Finally, the CHF group had a lower baseline intercept
compared with normal, and this difference was statistically
significant.
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Fig. 8.
Representative end-systolic pressure-volume relationship (ESPVR,
A) and myocardial oxygen consumption
(MO2)-pressurevolume area
(PVA) relationship (B) under control
conditions and after infusion of BAY y 5959 to achieve blood
concentration of ~700 nM in a CHF heart. Consistent with the positive
inotropic action, the slope of the ESPVR increased with little increase
in volume-axis intercept. Similar to reports with other inotropic
agents, BAY y 5959 increased the
MO2 intercept with little
effect on the slope of the relationship.
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Fig. 9.
Impact of BAY y 5959 and isoproterenol on slope
A
(A) and
MO2 intercept
B
(B) in normal and CHF hearts as a
function of the degree of inotropism. As in previous studies, multiple
linear regression analysis revealed no statistically significant effect
on slope A of the relationship, but
there was a consistent inotropic-related effect on intercept
B of the relationship. Furthermore,
changes in intercept B were similar
for isoproterenol and for BAY y 5959. Finally, there was a
statistically significant difference in baseline values of intercept
B between normal and CHF hearts.
* P < 0.05.
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DISCUSSION |
The results of the present studies have provided new insights into the
effects of BAY y 5959 on properties of the intact heart that have not
been obtained from previous studies of hearts in situ. Most notably, it
has been shown that changes in
MO2 created with this agent
are comparable to that observed when the inotropic state is increased
comparably by a -agonist. Mild bradycardic actions observed in our
ex vivo preparation suggest that at least a component of its
bradycardic actions in situ may be mediated by direct myocardial
effects independently of reflexes. Coronary flow measurements revealed
that metabolic autoregulation of the coronary vascular bed in response
to inotropic-mediated increased work was not affected by this calcium
channel promoter. Finally, whereas action potential duration was
increased, there was a mild shortening of the overall contraction
duration and increase in the rate of relaxation under conditions in
which heart rate is maintained constant.
Results of previous in vitro studies have shown that BAY y 5959 enhances L-type calcium channel conductance during electrical depolarization and prolongs the action potential (1). The inotropic actions of this agent have been attributed to changes in intracellular calcium and are independent of pathways involving cAMP. The results of
the present study provide a detailed characterization of the direct
myocardial physiological actions of this agent in intact blood-perfused
canine hearts. Studies have been performed in both normal hearts and in
hearts rendered myopathic by repeated coronary microembolization (13,
14). Hemodynamic measurements have shown that the degree of heart
failure achieved with this model is more moderate than that achieved
with other experimental models of heart failure (e.g., rapid
pacing-induced heart failure). However, this model is relatively stable
over long periods of time (13), and the method of achieving myocardial
damage simulates the most common etiology of chronic heart failure,
making it a good model for studying effects of cardiotonic agents.
We showed for the intact heart that BAY y 5959 has positive inotropic
actions that are associated with an increase in the action potential
duration and minor reduction in overall duration of contraction when
heart rate is maintained constant by pacing. These effects contrast
with the shortened action potential duration and larger decrease in
contraction duration seen with -agonists. Even though these effects
of -agonism on the dynamics of contraction are heart rate
independent (and relate to phosphorylation of myofilament proteins and
phospholamban), they are generally considered to be the means by which
adequate diastolic duration can be maintained in the face of increased
heart rate. Because BAY y 5959 does not increase heart rate and does
not impair relaxation (as may have been the case for an agent that
enhances intracellular calcium), the lack of a contraction duration
shortening effect should not result in a detrimental effect on overall
hemodynamics. Indeed, preliminary studies in intact animals (both with
and without heart failure) and patients have failed to reveal any
hemodynamic consequences related to long durations of contraction or
relaxation (4, 11, 17, 19, 24). The reason that relaxation is not
impaired by BAY y 5959 has not been elucidated but is consistent with
results obtained previously in isolated muscle strips for both BAY y
5959 and BAY K 8644 (9). It has been presumed that although peak calcium is enhanced by these agents, the rate of calcium sequestration by the sarcoplasmic reticulum is not influenced (positively or negatively), and thus the rate of relaxation is unaltered.
Whereas inotropic responsiveness to isoproterenol was blunted in our
model of moderate heart failure compared with normal hearts, the
effectiveness of BAY y 5959 was preserved. Recent preliminary
studies in conscious dogs have also shown preserved inotropic
responsiveness in two different models of heart failure (rapid pacing
and repeated coronary microembolization) (20, 24). Although it remains
to be determined whether tolerance develops during long-term infusions
[and results of preliminary studies suggest that it may not
develop (24)], the finding of relatively preserved inotropic
effectiveness in heart failure is a potentially important
characteristic of this class of agents.
In the heart, myocardial specificity of BAY y 5959 was evidenced in the
present study by a lack of vasoconstricting action on the coronary bed.
Rather, coronary vasodilation occurred to a similar degree as was
observed with isoproterenol for comparable degrees of inotropism. This
suggests that the observed vasodilation could be related to metabolic
autoregulation of the vascular bed in the face of increased work- load
rather than a direct vasodilatory effect of the drug on the vasculature
(15). Because we did not examine effects on other vascular beds, we
cannot address whether BAY y 5959 is truly myocardial specific or
whether it may affect functioning of other organ systems.
In studies in which cardiac pacing was interrupted, it was shown that,
whereas heart rate was increased dose dependently by isoproterenol,
there was no significant effect (and in CHF animals a trend to decrease
heart rate) in response to BAY y 5959. Bradycardic actions of BAY y
5959 have been noted after administration to conscious dogs, and it has
been debated whether this reflects a direct myocardial effect or
whether this is mediated by autonomic reflexes (19). Results of
preliminary studies of in vitro cells suggest a direct bradycardic
action (6). The observed tendency for a bradycardic action in our
isolated CHF hearts indicates that direct, reflex-independent effects
may be present (presumably effects on the sinus node) because reflexes
are absent in this preparation and heart rate is controlled, as in
situ, by the sinus node, which is intact with our methods of isolation.
The fact that the bradycardic effects were much stronger in the
conscious animals suggests that other factors may be involved, for
example, as proposed, the baroreflexes. A drug with bradycardic or
neutral effects on heart rate despite marked inotropic actions may have advantages for the treatment of heart failure over traditional agents
with which MO2 increases in
proportion to heart rate. On the other hand, lack of a tachycardiac
action may render changes in cardiac output less pronounced with BAY y
5959 than with a -agonist for a given degree of inotropism; this is
because a part of the increased cardiac output seen with -agonists
is due to the increase in heart rate and not to the inotropic effects.
As expected, there was an increase in
MO2 that accompanied BAY y
5959 positive inotropism. Furthermore, we demonstrated that the manner
in which BAY y 5959 influenced the
MO2-PVA relationship was
comparable to that observed with isoproterenol. Both agents increased
the intercept of the relationship (which corresponds to an increase in
unloaded oxygen consumption) in a dose-dependent manner. Results of
recent preliminary studies in vivo and in vitro suggested that when BAY
y 5959 and a -agonist were administered at equipotent inotropic
doses, the increase in oxygen consumption was less with
BAY y 5959 than for a -agonist, even when heart rate was controlled
(4, 5, 20). This finding suggested a favorable direct effect of BAY y
5959 on myocardial energetics. The results of the present study, in
which loading conditions are strictly controlled and in which inotropic
actions can be precisely matched (much harder to achieve in situ), did
not reveal any such effect. Whereas -agonists may increase basal
metabolism by elevating cAMP which, in turn, may influence several
subcellular processes (which would not be the case for a calcium
promoter), the sensitivity of the methods used in the present study
could detect no energetic differences between these two types of
agents. Differing results in these different preparations are not
necessarily contradictory because
MO2 is very sensitively
dependent on heart rate and contractility, as well as preload and
afterload; subtle effects of these drugs on each of these factors could
strongly influence the results. Therefore, although BAY y
5959 does not differ from -agonist in the energy cost of inotropism,
this does not necessarily predict an equally costly effect on
myocardial energy metabolism in situ.
The conditions under which isolated canine heart studies are performed
provide control over several important parameters in a setting that is
free of autonomic reflexes. Consequently, it is relatively
straightforward to determine the direct myocardial actions of a
cardiotonic agent. However, several limitations must be acknowledged.
First, the isolation and cross-perfusion depress resting contractile
state so that the studies are performed with the heart slightly
impaired compared with the in situ state. Second, the altered metabolic
state of the support dog attributable to the added stress of perfusing
the isolated heart may alter blood levels of important cardioactive
hormones compared with normal. Finally, questions related to drug
effects in the intact circulation, obviously, cannot be determined. Two
examples that have already been discussed above relate to the
consequences of the lack of effects of BAY y 5959 on relaxation and to
the effects of BAY y 5959 on cardiac output, which will be solely
dependent on inotropic actions in the absence of increased heart rate.
Naturally, many other factors not investigated in the present study
need to be addressed in assessing the potential utility of this agent
as a therapy for heart failure. For example, the implications of the
action potential prolongation and effects on cardiac arrhythmias need
to be defined. The effects of long-term infusions (with attention to
the potential for developing tolerance) also need to be clarified. The
rate of apoptosis, now considered to be an important factor in the
progression of heart failure, may be influenced by a drug that
increases intracellular calcium, especially in our embolization model,
in which apoptosis has been demonstrated to occur (22). Finally, it
will be important to elucidate the effects of BAY y 5959 on arterial
and venous properties in various vascular beds.
In summary, lack of coronary vascular effects, preserved inotropic
actions in heart failure, and lack of positive chronotropic effects are
three aspects of the myocardial actions of BAY y 5959 that are
potentially advantageous characteristics for heart failure therapy.
Lack of influence on the duration of contraction and rate of relaxation
have not been noted to influence systemic hemodynamics, although the in
vivo consequences of these factors need further clarification,
particularly in the heart failure state. Similarly, the implications of
action potential prolongation (also noted previously in isolated
muscle) also need to be studied (16). Finally, the direct metabolic
cost of inotropism was demonstrated to be the same as for
isoproterenol, a finding which was anticipated on the basis of current
understanding of the determinants of
MO2. These findings suggest,
in view of the unique mechanism of action, constellation of myocardial
effects, and lack of effects on coronary vasculature, that further
studies aimed at defining the potential clinical utility of this novel
inotropic agent are warranted.
| |
ACKNOWLEDGEMENTS |
The authors are grateful to Dr. Dave Wood for useful discussions
and suggestions throughout the course of this research. We are grateful
to Drs. Susan Bjorge and George Krol for performing plasma BAY y 5959 concentration assays at Bayer, West Haven, CT.
| |
FOOTNOTES |
Address for reprint requests: D. Burkhoff, Div. of Circulatory
Physiology, Dept. of Medicine, Columbia Univ., 630 W. 168th St., New
York, NY 10032.
Received 22 August 1997; accepted in final form 16 January 1998.
| |
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