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1 Department of Physiology II, To clarify the energy-expenditure mechanism during
Ba2+ contracture of mechanically
unloaded rat left ventricular (LV) slices, we measured myocardial
O2 consumption
(
oxygen consumption; excitation-contraction coupling; sarcoplasmic
reticulum calcium adenosine 5'-triphosphatase; cyclopiazonic
acid; 2,3-butanedione monoxime
WE HAVE INSTITUTED rat left ventricular (LV) slices
(300 µm thick) in a perfusion and electrical stimulation chamber as
an appropriate preparation to analyze myocardial
O2 consumption
( The aim of the present study was, first, to detect an increase in
energy expenditure or There is no direct evidence indicating whether
Ba2+ exclusively activates the
contractile system (4, 13, 17, 22, 23). Because
Ba2+ contracture is induced by
displacing Ca2+ with
Ba2+, an incomplete washout of
Ca2+ during the superfusion with
nominally Ca2+-free Tyrode
solution may be one possibility. Although we have previously reported
that E-C coupling LV Myocardial Slice Preparation
ABSTRACT
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
O2) of quiescent slices in
Ca2+-free Tyrode solution and
O2 during
Ba2+ contracture by substituting
Ca2+ with
Ba2+. We then investigated the
effects of cyclopiazonic acid (CPA) and 2,3-butanedione monoxime (BDM)
on the Ba2+ contracture
O2. The
Ca2+-free
O2 corresponds to that of
basal metabolism (2.32 ± 0.53 ml
O2 · min
1 · 100 g LV1).
Ba2+ increased the
O2 in a
dose-dependent manner (from 0.3 to 3.0 mmol/l) from 110 to 150% of
basal metabolic O2.
Blockade of the sarcoplasmic reticulum (SR)
Ca2+ pump by CPA (10 µmol/l) did
not at all decrease the
Ba2+-activated
O2. BDM (5 mmol/l),
which specifically inhibits cross-bridge cycling, reduced the
Ba2+activated
O2 almost to basal
metabolic O2. These
energetic results revealed that the
Ba2+-activated
O2 was used for the
cross-bridge cycling but not for the
Ca2+ handling by the SR
Ca2+ pump.
INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
O2) under mechanically
unloaded conditions (29, 31, 36). We expected that these
myocardial slices would be virtually free from the mechanical
constraints that they had in situ. Using this new preparation,
we have revealed that an increment in
O2 by 1-Hz stimulation
consists of O2 for
Ca2+ handling in the
excitation-contraction (E-C) coupling (E-C coupling O2) but not for residual
cross-bridge cycling. We also have revealed that
O2 without stimulation
represents a constant basal metabolism (basal metabolic
O2) in both normal and
Ca2+-free Tyrode solution (29,
36). However, the energy expenditure due to the increase in
cross-bridge cycling of these myocardial slices remains to be analyzed.
O2 due
to the increase in cross-bridge cycling by
Ba2+ contracture of myocardial
slices from the rat LV. Our second goal was to analyze the mechanism of
the increased O2 by using a
sarcoplasmic reticulum (SR)
Ca2+-pump blocker, cyclopiazonic
acid (CPA) (2, 29), and a cross-bridge cycling inhibitor,
2,3-butanedione monoxime (29, 36).
O2 is not
detected in nominally Ca2+-free
Tyrode solution, even when electrical stimulation is performed (36),
the possibility that Ba2+ releases
residual Ca2+ from the SR via a
mechanism similar to the
Ca2+-induced
Ca2+-release mechanism (5, 6)
should be considered. However, there is little supporting evidence for
this possibility (13, 25). To the contrary, there is evidence that
Ba2+ blocks SR
Ca2+ release in triadic SR
preparations, even though these preparations are isolated from skeletal
muscle (19, 20). Although there is another possibility that
Ba2+ is sequestered and released
(i.e., cycled) by the SR, just as Ca2+ and
Sr2+ are (26), many studies have
suggested that Ba2+ is not cycled
(1, 9, 14, 15, 19, 32). Displacing Ca2+ with
Ba2+ can activate contractile
protein interactions, but it does not support E-C coupling in skeletal
muscle (9). Furthermore, Ba2+ has
no effect on SR Ca2+ ATPase
activity in skeletal muscle SR (14) and myocardial SR (1) and on
Ca2+ uptake by skeletal muscle SR
(15, 19, 32). In agreement with these studies suggesting the lack of
Ba2+ cycling in SR, we obtained
results of the energetics to reveal that the
O2 of
Ba2+-activated myocardial slices
was mainly used for the cross-bridge cycling but not for the residual
Ca2+ handling by the SR
Ca2+ pump.
METHODS
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INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
Ba2+-Activated
O2 of Myocardial Slices
O2 for these quiescent
slices was measured for 6 min in an oximetric chamber at 30°C. The
subsequent superfusion with Ba2+
Tyrode solution was employed to induce
Ba2+ contracture of the slices
without electrical stimulation. We observed
Ba2+ contracture under an inverted microscope.
O2 of a set of six
myocardial slices on average was measured polarographically with an
O2 electrode (1T-125; Instech
Laboratories, Plymouth Meeting, PA), the head of which faced the
solution in the air-tight (oximetric) chamber. The electrode was
connected to a current amplifier (model 102; Instech Laboratories), and its output was recorded with a flatbed recorder (model FBR-252A; TOA
Electronic, Tokyo, Japan). This oximetric system was calibrated at
three different O2 concentrations
[0 mg/l (5%
Na2SO3
solution), 7.3 mg/l (Tyrode solution saturated with air), and 35.1 mg/l
(Tyrode solution saturated with 100%
O2)] at 30°C in each experiment.
Assessment of O2 of
Myocardial Slices
O2 with no myocardial
slices in the Tyrode-filled chamber at the beginning and end
[0.42 ± 0.15 and 0.39 ± 0.15 ml
O2/min, respectively;
n = 52 sets of 4-6 slices;
P > 0.05] of each measurement
of the same slices at 30°C. Myocardial
O2 was obtained by
subtracting the background O2
(A; in scales/min) from the measured
O2
(B; in scales/min) with myocardial
slices present in the oximetric chamber. Myocardial O2 (in ml
O2 · min1 · 100 g LV1) was calculated as
[(B 
A) × 0.399/slice wet weight
(in grams)] × volume of Tyrode solution in the
chamber (in liters) × [22.4 liters/32 g] × 100 g (1 scale of the reading of the trace = 0.399 mg/l in the present
study). We determined the wet weight of each set of slices [58.7 ± 13.5 mg; n =52 sets of
slices] so that the ratio of B
to A became higher than 2.0. The
maximum sensitivity of the O2
measurement was obtained when
B/A
was 2.0, i.e., 0.92 µl O2/min in
this chamber. We have previously confirmed that the myocardial slice
O2 was reproducible in
three repeated measurements in the same protocol without stimulation at
30°C (29).
Experimental Protocol
Any Tyrode solutions were gassed with 100% O2 and preheated to 30°C in a water bath. Six slices on average were placed in the air-tight chamber filled with the prepared 2.62 ± 0.04 ml (n = 52 sets of slices) Tyrode solution gassed with 100% O2. These slices were preincubated and then incubated in Ca2+-free Tyrode solution without and with Ba2+ throughout the experiment. E-C couplingO2
was not detected under these conditions, even when electrical
stimulation was performed (36). Myocardial
O2 of
Ba2+-activated slices without
electrical stimulation was not different from that with stimulation.
Therefore, we measured the
O2 of all slices without
stimulation. We used the following protocols in four different groups
of slices.
Protocol 1.
There were 31 sets of slices from 20 rats in protocol 1. After 10-min
incubation of a set of slices in
Ca2+-free Tyrode solution, the
first O2 measurement was
performed for 6 min. This value was used as a control value. After the
first washout of the slices with
Ba2+ Tyrode solution for 10 min,
the second O2 measurement
of Ba2+-activated slices in this
solution was performed for 6 min. The concentration of
Ba2+ was increased from 0.3 (n = 6), 0.5 (n = 10), and 1 (n = 6) mmol/l to 3 mmol/l
(n = 9). After the
O2 of
Ba2+-activated slices was
measured, the slices were washed out with Ca2+-free Tyrode solution for 10 min and the O2 of the
noncontracted slices was measured for 6 min. These values were used as
back control values.
Protocol 2.
Eight sets of slices from eight rats were used in
protocol 2. To confirm the stability
of O2 of
Ba2+-activated slices, the
O2 was repeatedly measured
in the same set of slices after the measurement of
O2 of the slices in
Ca2+-free Tyrode solution. After
10-min incubation of a set of slices in
Ca2+-free Tyrode solution, the
first O2 measurement was
performed for 6 min. This value was used as a control value. After the
washout of the slices with Ba2+
Tyrode solution for 10 min, the second
O2 measurement of
Ba2+-activated slices in this
solution was performed for 6 min. The concentration of
Ba2+ was 1 mmol/l. After the
O2 of
Ba2+-activated slices was
measured, the activated slices were again superfused with
Ba2+ Tyrode solution for 5 or 10 min and the third O2
measurement was performed for 6 min. Total time of treatment with
Ba2+ was at least 27-32 min.
Protocol 3.
Seven sets of slices from seven rats were used in
protocol 3, and CPA (10 µmol/l) in
1.3% dimethyl sulfoxide was used as a vehicle. Control values were
obtained as in protocol 1. After washout of the slices with Ba2+ (1 mmol/l) Tyrode solution for 10 min, the second
O2 measurement of
Ba2+-activated slices was
performed for 6 min. This value was used as the
Ba2+ (1 mmol/l)
O2 value. Pretreatment of
CPA (10 µmol/l) in Ca2+-free
Tyrode solution for 5 min completely blocked
Ba2+ contracture by
Ba2+ (1 mmol/l) Tyrode solution in
the presence of CPA (10 µmol/l) for 10 min (unpublished observation).
Therefore, we added CPA (10 µmol/l) after the second
O2 measurement of
Ba2+-activated slices as follows:
after washout of the slices with Ba2+ (1 mmol/l) Tyrode solution
containing CPA (10 µmol/l) for 5 min, the third
O2 measurement in this
solution was performed for 6 min. This value was used as the CPA
O2 value. To confirm the recovery from activation with Ba2+
(1 mmol/l) Tyrode solution containing CPA (10 µmol/l) after the third
measurement, four of the seven sets of slices were washed out with
Ca2+-free Tyrode solution for 10 min and the O2 of the same
slices was measured for 6 min. These values were used as back control values.
Protocol 4.
Six sets of slices from six rats were in
protocol 4 and treated with
2,3-butanedione monoxime (BDM; 5 mmol/l). The
control value was obtained as in protocol
1. After washout of the slices with
Ba2+ (1 mmol/l) Tyrode solution
for 10 min, the second O2
measurement of Ba2+-activated
slices was performed for 6 min. These values were used as
Ba2+ (1 mmol/l)
O2 values. After washout of
the slices with Ba2+ (1 mmol/l)
Tyrode solution containing BDM (5 mmol/l) for 10 min, the third
O2 measurement in this
solution was performed for 6 min. This
O2 value was used as the
BDM O2 value.
Statistics
All data are presented as means ± SD. Data were analyzed by one-way and repeated-measures ANOVA and followed, as necessary, by multiple comparisons using Bonferroni t-tests. In all statistical tests, P values <0.05 were considered statistically significant.| |
RESULTS |
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DISCUSSION
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O2 for
Ca2+-Free
Quiescent Slices and
Ba2+-Activated
Slices
O2 in
Ca2+-free Tyrode solution was 2.32 ± 0.53 ml
O2 · min1 · 100 g LV1 (basal metabolism)
and was expressed as 100%. The
O2 during Ba2+ (1 mmol/l;
n = 6) contracture significantly
increased to 140% of basal metabolism
(P < 0.05). The
O2 of
Ba2+-activated slices minus the
basal metabolic O2
(
O2) was 42.5 ± 12.7%
of basal metabolism. After washout of
Ba2+,
O2 of the same slices in
Ca2+-free Tyrode solution returned
to the basal metabolic O2.
The present basal metabolic
O2 corresponded to our
previously measured constant basal metabolism in
Ca2+-free or normal Tyrode
solution without stimulation (29, 36). The observed full recovery from
Ba2+-induced increase in
O2 of myocardial slices was
in accordance with the results obtained in the superfused papillary
muscle and Langendorff-perfused heart; the
Ba2+ contracture was attenuated as
the Ba2+ was washed out of the
cardiac cells when Ba2+ Tyrode
solution was displaced by normal Tyrode solution (17, 23).
Concentration Dependence of
Ba2+
Activation of Slice O2
O2 values
of quiescent slices and back control of the same slices in
Ca2+-free Tyrode solution were
almost constant at 2.25 ± 0.54 ml
O2 · min1 · 100 g LV1 in all series of
protocol 1. As shown in Fig.
1, the
O2 values of the slices
activated by Ba2+ increased in a
dose-dependent manner from 110% of control at 0.3 mmol/l
Ba2+
(n = 6) to 150% of control at 3 mmol/l Ba2+
(n = 9). The
O2 significantly increased
from that at 0.5 mmol/l Ba2+
(P < 0.05) and gradually increased
up to that at 3 mmol/l Ba2+.
However, the O2 at 3 mmol/l
Ba2+ did not significantly
increase from that at 1 mmol/l
Ba2+
(P > 0.05). Therefore, we used 1 mmol/l Ba2+ in
protocols 2-4.
|
Constancy of Repeated Measurement of
O2 Value in
Ba2+-Activated
Slices
O2 of the quiescent slices
was measured in Ca2+-free Tyrode
solution (2.55 ± 0.55 ml
O2 · min1 · 100 g LV1), we repeatedly
measured the O2 of the same
Ba2+-activated slices. The
respective second and third
O2 values in 1 mmol/l
Ba2+ were almost constant (3.34 ± 0.57 vs. 3.41 ± 0.47 ml
O2 · min1 · 100 g LV1). The constancy of
repeatedly measured O2 during
Ba2+ contracture enabled us to
compare the second O2 with
the third O2 following
treatment with CPA (protocol 3) or
BDM (protocol 4).
Effects of CPA on O2 of
Ba2+-Activated
Slices
O2 in
Ca2+-free Tyrode solution was
2.30 ± 0.61 ml
O2 · min1 · 100 g LV1. The
O2 of
Ba2+-activated slices
significantly increased to 141.5 ± 12.1% of the control
O2
(P < 0.05) (Fig.
2). The
O2 of
Ba2+-activated slices treated with
CPA was 142.1 ± 14.1% of the control O2 (Fig. 2). Thus CPA did not
decrease the O2 of
Ba2+-activated slices
(P > 0.05). To confirm the recovery
of Ba2+-activated slices treated
with CPA, the slices were washed out with
Ca2+-free Tyrode solution and the
fourth O2 of the same slices
in the absence of extracellular
Ba2+ was measured
(n = 4). The fourth
O2 of the slices was 2.13 ± 0.60 ml
O2 · min1 · 100 g LV1
(n = 4). The first
O2 of basal metabolism was
1.95 ± 0.59 ml O2 · min1 · 100 g LV1. These values were
not significantly different from each other (P > 0.05).
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Effects of BDM on O2 of
Ba2+-Activated
Slices
O2 of
Ba2+-activated slices was
significantly (P < 0.05)
reduced from 141.8 ± 21.3 to 110.1 ± 6.7% of the control
O2 value (2.23 ± 0.56 ml
O2 · min1 · 100 g LV1) by treatment with
BDM (Fig. 3).
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DISCUSSION |
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
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The most important results are 1)
our method for measurement of myocardial
O2 using mechanically
unloaded myocardial slices from rat LV (29, 36) showed a stable and
reversible increase in O2 due
to Ba2+ contracture similar to
that in Langendorff-perfused hearts and superfused papillary muscle
(17, 23, 24); 2) the increase in
O2 by
Ba2+ was dose dependent between
0.3 and 3 mmol/l; 3) the increased O2 was not reduced by the SR
Ca2+ pump inhibitor CPA (10 µmol/l); and 4) the increased
O2 was, however, reduced by
the cross-bridge cycling inhibitor BDM (5 mmol/l). Therefore, an
increase in O2 by
Ba2+ contracture was due to
increased cross-bridge cycling but not due to increased
Ca2+ handling in E-C coupling
mainly used for the SR Ca2+ pump
(29).
A New Approach of LV Mechanoenergetics Using Mechanically Unloaded Myocardial Slices
We and others have already reported a linear myocardialO2 systolic pressure-volume
area (a total mechanical energy) from the curved end-systolic
pressure-volume relation in the rat LV in the blood-perfused whole
heart preparation (10, 11, 31, 33), similar to that in canine and
rabbit hearts (8, 18, 27, 28, 30). The
O2 intercept of this relation
(minimally loaded O2, not
mechanically unloaded O2) is
mainly composed of the E-C coupling
O2 and basal metabolic
O2 (10, 11, 31). However, the
O2-intercept may include the
O2 residual cross-bridge
cycling (12, 34). Therefore, we have proposed a new approach for
evaluating the mechanically unloaded
O2 using myocardial slices.
Recently, we have reported that BDM (5 mmol/l) markedly attenuated the
1-Hz twitch-free shortening of mechanically unloaded myocardial slices
by 78% of control but did not decrease E-C coupling O2 (29, 34, 36). This
indicates that the O2 for the residual cross-bridge cycling during mechanically unloaded twitches was
negligibly small (29, 36). However, when cross-bridge cycling is
enhanced, the O2 for its
cycling could be detected. Actually, we obtained the expected results
in the present study; the O2
during Ba2+ contracture of
myocardial slices increased with
Ba2+ concentration in a
dose-dependent manner. This increase is due to the increase in
O2 for cross-bridge cycling,
because E-C coupling O2 is
almost completely abolished in
Ca2+-free Tyrode solution (36).
From these results, we conclude that the energy expenditure of
myocardial slices during 1-Hz twitch-free shortening is composed of the
E-C coupling and basal metabolic O2 but is not composed of the
detectable O2 for the
cross-bridge cycling.
Basal metabolism after KCl arrest is ~20 µl
O2 · min1 · g1
(0.02 ml
O2 · min1 · g1)
in an intact rat heart (11). On the other hand, basal metabolic O2 (in
Ca2+-free medium) of rat
myocardial slices (mechanically unloaded) was ~2.3 ml
O2 · min1 · 100 g1 (0.023 ml
O2 · min1 · g1)
in the present study. Therefore, the basal metabolic
O2 in rat myocardial slices
(29, 36) is nearly equal to that in an intact heart (11). This value is
greater than that in dogs and rabbits (11, 18, 29, 36). We have
previously reported a lack of suppression of basal metabolism by SR
Ca2+-ATPase inhibitors (29). From
these results, we have already suggested energy-consuming (wasting)
processes, such as protein synthesis, other than those at the SR as the
possible underlying mechanism for the greater basal metabolism in the
rat myocardium (29).
O2 of
Ba2+-Activated
Slices
O2 of myocardial slices in a
dose-dependent manner. These findings are reasonable considering the
mechanism of Ba2+ contracture;
Ba2+ interacts with troponin C
binding site like Ca2+, in a
dose-dependent manner, and allows cross-bridge formation (21-24),
i.e., an increase in energy expenditure due to an enhanced number of
cross-bridge cycles of attachment and detachment (35).
In the present experiment, the
O2 began to increase by 10%
(0.2 ml
O2 · min1 · 100 g LV1) of the control
O2 of quiescent slices at 0.3 mmol/l of Ba2+, and it gradually
increased up to that at 3 mmol/l
Ba2+. The
Ba2+ contracture of the slice was
also confirmed by the microscopic observation in the present
experiment. Although the steady isometric tension increased as
Ba2+ increased from ~1 µmol/l
to 0.1 mmol/l in the glycerinated skinned papillary muscle (22), the
effective Ba2+ concentration on LV
pressure in Ba2+-perfused rabbit
hearts (17) is similar to that on
O2 due to
Ba2+ contracture of myocardial
slices. Furthermore, the muscle tension developed by
Ba2+ of rabbit papillary muscle
bathed in Tyrode solution reached the maximum above 4 mmol/l
Ba2+ (24).
The O2 of 1 mmol/l
Ba2+-activated slices pooled from
all experiments corresponds to ~140% of the control
O2 in
Ca2+-free Tyrode solution.
However, in the rabbit LV preloaded at its end-diastolic pressure of 10 mmHg, mean LV O2 during the first 30 min of Ba2+ contracture
increased to 3.63 ml
O2 · min1 · 100 g LV1 (~300% of basal
metabolic O2 of rabbit
cardiac muscle) (24). Furthermore, Munch et al. (17) have reported a
linear increase in LV pressure due to the increase in LV volume during
Ba2+ contracture of rabbit hearts.
These results suggest that an increase in
O2 under a preloaded
condition would be higher than that under a mechanically unloaded
condition even in the myocardium during
Ba2+ contracture.
Reversibility and Stability of
O2 of Slice During
Ba2+ Contracture
O2 of myocardial slices under
Ca2+-free conditions corresponded
to that under normal Tyrode conditions without stimulation (7, 29, 36)
and that under KCl arrest in the rat heart (11). The
O2 after washout of 1 mmol/l
Ba2+ with
Ca2+-free Tyrode solution returned
to the pre-Ba2+ level of
O2 obtained by the first
measurement in Ca2+-free Tyrode
solution. Thus the Ba2+ that
enters the cell (16) can be extruded from the myocardium (3,
4).
From the measurements of ATP and phosphocreatine during
Ba2+ contracture, Shibata et al.
(24) supported the notion that
Ba2+ induces a form of contracture
that is quite stable but associated with a minimal energy demand,
perhaps primarily for the cross-bridge cycling. In fact, the
O2 of
Ba2+ contracture of mechanically
unloaded slices continued for at least 32 min in the present
experiment, and the value was constant, corresponding to 130-140%
of basal metabolic O2.
Effects of CPA and BDM
Saeki et al. (22, 23) claimed that there is no direct evidence that Ba2+ exclusively activates the contractile system; in fact, they observed that Ba2+ depolarized the membrane potential independently of activating the contractile system. We have already reported that CPA inhibits theO2 of the myocardial slices
during 1-Hz mechanically unloaded contraction in a dose-dependent
manner (1-10 µmol/l); at 10 µmol/l CPA the E-C coupling
O2 was reduced by 70% (2,
29, 31). The present results showing no decreases in
O2 of
Ba2+-activated slices by 10 µmol/l CPA support the postulate that the
O2 of
Ba2+-activated slices is not
primarily consumed by the SR
Ca2+-ATPase. These results, in
agreement with many studies on skeletal muscle SR (9, 14, 15, 19, 20,
32), suggest that Ba2+ is not
cycled in SR of the rat myocardium and does not induce the release of
residual Ca2+ from SR of the rat myocardium.
In the present study, we used BDM (5 mmol/l) to inhibit cross-bridge
cycling. We have already confirmed that this concentration of BDM did
not affect either the E-C coupling or basal metabolic O2 of the myocardial slices
(36). BDM (5 mmol/l) decreased O2 during
Ba2+ contracture approximately to
the basal metabolic O2 level, suggesting the possibility that the increase in
O2 by
Ba2+ contracture exclusively
derived from the increased cross-bridge cycling. Taking the CPA and BDM
data together, we conclude that the increase in
O2 of
Ba2+-activated slices is not due
to SR Ca2+-ATPase but is primarily
due to cross-bridge cycling.
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ACKNOWLEDGEMENTS |
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We greatly thank Yukishige Akagi, a representative of the Okayama Rail Road Model Shop, for generously custom-making our oximetric chamber system.
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FOOTNOTES |
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This study was partly supported by Grants-in-Aid 09670053, 09307029, 09470009, 10770307, 10558136, and 10877006 for Scientific Research from the Ministry of Education, Science, Sports and Culture and a 1997-1998 Frontier Research Grant for Cardiovascular System Dynamics from the Science and Technology Agency.
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.
Address for reprint requests and other correspondence: H. Kohzuki, Department of Physiology II, Nara Medical University, 840 Shijo-cho, Kashihara, Nara 634-8521, Japan (E-mail: hkohzuki{at}naramed-u.ac.jp).
Received 2 November 1998; accepted in final form 26 February 1999.
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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