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(Circulation. 1998;98:2919-2929.)
© 1998 American Heart Association, Inc.
Basic Science Reports |
Mechanoenergetic Alterations During the Transition From Cardiac Hypertrophy to Failure in Dahl Salt-Sensitive Rats
From the Cardiology Unit, Department of Medicine (T.K., Z.C., S.P.B., P.v.B., M.M.L.), and the Department of Molecular Physiology and Biophysics (D.M.), The University of Vermont College of Medicine, Burlington, Vt.
Correspondence to Martin M. LeWinter, MD, Cardiology Unit, Fletcher Allen Health Care/MCHV Campus, 111 Colchester Ave, Burlington, VT 05401. E-mail martin.lewinter{at}vtmednet.org
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Abstract |
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 Top Abstract IntroductionMethodsResultsDiscussionReferences |
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Background—The time course and mechanisms of altered mechanoenergetics and depressed cross-bridge cycling in hypertrophied and failing myocardium are uncertain.
Methods and Results—We studied mechanoenergetics in Dahl
salt-sensitive (DS) rats fed high-salt diet (HS) for 6 (HS-6) and 12
(HS-12) weeks to produce compensated hypertrophy and
failure. The slope of the end-systolic pressure-volume relation
(E'max) was similar in HS-6 and low-salt controls (LS-6),
but reduced in HS-12 compared with controls (LS-12). Efficiency
[1/slope of oxygen consumption
(
O2)–pressure-volume area
(PVA) relation] was similar in HS-6 and LS-6 but higher in HS-12
versus LS-12 (59±16% versus 44±7%,
P<0.05). Economy [1/slope of the force-time
integral (FTI)–O2 relation] was
similar in HS-6 and LS-6 but higher in HS-12 versus LS-12 (218±123
versus 74±39x103 g · s · mL
O2-1 · g; P<0.05).
Compared with controls, myofibrillar ATPase activity was reduced by
24% in HS-6 and 44% in HS-12. V3 Isomyosin was increased in HS-6
(40±12% versus 9±8%; P<0.05) and further increased
in HS-12 (76±10% versus 22±18%; P<0.05).
Hypothyroid LS-12 rats had 100% V3 isomyosin, yet efficiency, economy,
and ATPase values were intermediate between LS-12 and HS-12. HS-12 rats
demonstrated increased troponin T3 isoform (17±2 versus
23±2%, P<0.05). There were no changes in troponin I
or tropomyosin isoforms. However, the proportion of
phosphorylated troponin T was reduced in HS-12 versus
LS-12 hearts (P<.001).
Conclusions—In DS rats, the transition to failure is associated with depressed E'max and increased efficiency and economy. These findings are linked to myofibrillar ATPase activity and suggest that mechanisms other than isomyosin switching are important determinants of ventricular energetics. A troponin T isoform switch is one potential mechanism.
Key Words: myocardium • hypertrophy • myofibrillar ATPase activity • myosin • troponin T
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Introduction |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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Depressed cross-bridge cycling is a hallmark of overloaded myocardium.1 2 3 4 The time course of this derangement, its relation to the transition to failure, manifestations in the ventricle and mechanism are poorly understood. In small mammals, myosin isoform switching from V1 to the slower V3 form accounts only in part for depressed cross-bridge cycling.2 4 Myosin isoform switching during overload has not been thought to occur in large mammals, including humans,5 who have been considered to have exclusively V3 myosin. Thus, the mechanism of depressed cross-bridge cycling in these species is especially poorly understood.2 6 7 Recently, mRNA for V1 myosin was detected in nonfailing human myocardium, with a decrease in failure.8 The latter report pointed out that technical difficulties make it difficult to separate human V1 and V3 isoforms. Thus, myosin isoform switching has reemerged as a possible mechanism of depressed cross-bridge cycling in human myocardium. A proposed mechanism of non–isomyosin mediated depressed cross-bridge cycling in failing myocardium is an alteration in thin filament regulatory proteins,9 but supporting evidence is scant.
The goals of this study were (1) to delineate the time course of depressed cross-bridge cycling in a small-animal model of cardiac hypertrophy and the transition to failure, (2) to test the hypothesis that, in the ventricle, depressed cross-bridge cycling is manifest as alterations in parameters proposed to reflect the mechanoenergetic behavior of the contractile machinery, and (3) to test preliminarily the hypothesis that thin-filament isoform shifts or altered phosphorylation state contributes to depressed cross-bridge cycling in overloaded myocardium.
We used Dahl salt-sensitive (DS) rats,10 which
develop combined pressure and volume overload when fed a high-salt diet
(HS). A renin gene polymorphism11 and a
mutation of the 
1
Na+-K+ ATPase
gene12 have been identified in this strain. With
HS, DS rats develop compensated hypertrophy and progress to
a dilated, hypokinetic left ventricle (LV) without significant
myocardial cell loss and/or extensive fibrosis.13
With failure, there is increased mortality, pulmonary
congestion, increased plasma ANP and norepinephrine
concentration, and decreased ß-adrenergic
responsiveness.14
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Methods |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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Animal Model
Procedures were approved by the Institutional Animal Care and Use Committee of the University of Vermont. We used 148 male DS rats (Brookhaven National Laboratory, Upton, NY). At 6 weeks of age, rats were divided into populations receiving either HS (8% NaCl) or a low-salt diet (LS, 0.3% NaCl) (Harlem Teklad). Each group was divided into subgroups that underwent isolated heart studies at either 6 or 12 weeks after starting diet. Thus, the rats were divided into 4 groups by diet and duration (in weeks): those given HS for 6 or 12 weeks (HS-6 [n=11] or HS-12 [n=16] or LS for 6 or 12 weeks (LS-6 [n=15] or LS-12 [n=15]). Because of high mortality, 109 rats were initially randomized to the HS-12 group. Blood pressure was measured (tail cuff) every 2 weeks after starting diet. In an additional group, 9 weeks after beginning LS, hypothyroidism was induced by adding 0.8 mg/mL propylothiouracil (PTU) to drinking water for 3 weeks (n=9),15 with isolated heart studies at 12 weeks. PTU treatment was included to convert from predominantly V1 to V3 myosin, allowing comparisons to HS groups with similar shifts.
Isolated Heart Preparation
Rats were anesthetized (sodium pentobarbital, 90 mg/kg
IP), a tracheotomy was performed, and ventilation was initiated with a
Harvard respirator. The chest was opened at the midline,
pulmonary hila and superior and inferior venae
cavae were ligated, and a perfusion tube was inserted into the
ascending aorta. Coronary arteries were perfused via the aortic
root with warmed perfusate (composition below), and the heart
was removed from the chest. Interruption of coronary perfusion
was <20 s. Blood was obtained for BUN and creatinine
measurement.
After the heart was hung from the perfusion tube, the LV was vented to
drain thebesian effluent. A flexible tube was inserted from the right
atrium (RA) into the right ventricle (RV), and the RA and
pulmonary artery were ligated. The left atrium was
opened and a collapsed, thin, high-density polyethylene (HDPE) balloon
(see below) mounted on a Y connector was placed in the LV through the
mitral orifice. A 2.5F micromanometer
(Millar Instruments) was inserted at the center of the balloon via a
side port. A 500-µL graduated syringe (Hamilton) was connected to
another port. Pacing electrodes connected to a stimulator (Grass
Instruments Co) were attached to the LV surface. The RV was kept
collapsed by hydrostatic drainage. The heart was placed in a heated
chamber and maintained at 35°C to 37°C. Coronary flow was
measured by collection of venous drainage from the RV. Coronary
arteriovenous O2 content difference
(AVO2
) was measured with a
platinum O2 electrode system (Instech). Sodium
dithionate, which extracts O2 from solution, was
used to zero the electrode at the start of each experiment. The gain
was calibrated using perfusate saturated with 100%
O2. The oxygen solubility in this solution was
used to calculate O2
content.16
Perfusate
Perfusate composition (mmol/L) was 108.0 NaCl, 4.0 KCl,
1.4 KH2PO4, 25.0
NaHCO3, 11.0 dextrose, 10.0 Na-pyruvate, and 2.5
CaCl2 (Sigma Chemical Co). Perfusate was
filtered, equilibrated with 95% O2+5%
CO2, warmed to 37°C, and adjusted to pH 7.4 by
changing percent CO2. Perfusion pressure was
controlled by a pressurized reservoir connected to a pressure regulator
and compressed air. Perfusate temperature was maintained at
35°C to 37°C with water jackets around the reservoir.
In 4 HS-12 and LS-12 hearts, bovine red blood cells (RBCs) suspended in
perfusate (hematocrit 20%) were used to increase
O2-carrying capacity. In these hearts,
coronary AVO2 was
measured using the AVOX system, calibrated against a Lex
O2 Con hemoximeter. Results are reported
separately and in combination with those from non–RBC containing
perfusate.
LV Balloon
Balloons were constructed by stretching a small piece of HDPE
with a round-tip device, which was then mounted on a 15-gauge Luer
adapter with a thin latex seal. At LV volume 0.03 to 0.05 mL, peak
isovolumic pressure was always 
0. Before each experiment, the
pressure-volume (PV) relation of the balloon was measured to ensure
that pressure was 0 at volume >0.2 mL. To test the accuracy of balloon
volume, we measured residual volume between the balloon and the inner
LV surface in 5 hearts fixed in 10% formalin using methods similar to
those in canine17 and
rabbit18 hearts. Residual volume was 7.1±1.7
µL (±SD) (8.6±1.5% of total LV volume, determined as the volume of
water within the balloon plus volume of the balloon walls and connector
within the LV plus residual volume) at an
intraventricular pressure of 120 mm Hg,
9.2±1.9 µL (11.7±3.8%) at 40 mm Hg, 16.9±6.9 µL
(19.8±4.0%) at 10 mm Hg, and 23.3±7.3 µL (27.9±5.8%) at
5 mm Hg. The effects of these errors on mechanoenergetics are
discussed subsequently. This system showed nearly optimal damping
(coefficient, 0.541) with a high natural frequency (undamped frequency,
222 Hz).19
Experimental Protocol
Hearts were stimulated at 240 bpm (LV pacing). Three PTU hearts
were paced at 180 bpm due to pulsus alternans at 240 bpm. Measurements
of LV and perfusion pressure, coronary flow, and
AVO2 were made at various balloon
volumes during steady-state isovolumic contractions. LV volume was
varied between that at which peak pressure was 0 and maximal volume
(0.15 mL or diastolic pressure >25 mm Hg). Perfusion
pressure was maintained constant and as close as possible to 90
mm Hg for LS and PTU hearts and 120 mm Hg for HS hearts,
simulating in vivo diastolic values. Oxygen consumption
(O2) for basal
metabolism was measured after KCl
arrest.20 Then, LV, RV, and organs were weighed
as rapidly as possible, and LVs were quick-frozen and stored at
-70°C until ATPase, protein, or phosphorylation
analyses was performed.
Mechanoenergetic Parameters
LV and coronary perfusion pressure and
AVO2 were stored on a hard disk
at 2-ms sampling intervals for off-line analyses.
O2 per minute was calculated as
coronary flow (mL/min) ·
AVO2 (vol%) divided by heart rate and
normalized per gram of LV to yield total
O2 per beat per gram (in mL). LV
volume was determined as volume of water within the balloon plus volume
of balloon walls and connector within the LV. LV developed pressure was
the difference between peak and minimum pressure.
End-diastolic pressure (EDP) was taken when LV+dP/dt
reached 10% of maximum.
LV Systolic and Diastolic Function
Analyses at Matched Stress
We compared systolic and diastolic function
at matched stress. Fiber stress (
) was calculated at end-systole
(ES) and end-diastole (ED) assuming a thick-walled
spherical
chamber.21 (g/cm2)=1.36
LVP
ri2/(ro2-ri2)
where ri and
ro are internal and external radii,
respectively. Radii were determined from the equation for the volume of
the respective sphere of interest:
r=(3V/4).1 3 V is LV (or
wall) volume, and LVP is left ventricular pressure. LV wall
volume was set equal to LV mass divided by specific gravity (1.04
mL/g).
Systolic function was quantified as LV developed pressure and maximal dP/dt at matched ED stress of 10 g/cm2. Diastolic function was quantified by EDP and minimal dP/dt at matched ES stress of 100 g/cm2.
End-Systolic PV Relation, O2
PV Area Relation, and Efficiency
LV systolic function was also quantified as slope of the
nonlinear ES PV relation (ESPVR), E'max (in
mm Hg · mL-1 ·
g).22 P=E'max
(V-V0)+
(V-V0),2
where P and V are LV pressure and volume,
V0 is the volume axis intercept, and is
a constant.
Total energy output was quantified as PV area
(PVA),23 the area circumscribed by ESPVR, EDPVR,
and the systolic PV trajectory, normalized per gram LV (in
mm Hg · mL · beat-1 ·
g-1).
O2 was plotted as a function
of PVA as LV volume was varied and a linear regression analysis
(O2=aPVA+b)
performed. Slope a is O2 cost of PVA,
and intercept b is
O2 at 0 PVA (unloaded
O2). Slope-1
is the efficiency of conversion of
O2 to PVA (contractile efficiency)
after conversion of PVA and O2 to
joules.22
The effects of the error in volume measurement on ESPVR (8.3% to
10.0% overestimation of E'max and 9.1% to
14.0% underestimation of V0, based on
average ESPVR of each group) and
O2PVA (10.7% to 12.4%
overestimate of slope and estimation error of -4.0% to 0.2% of
O2 intercept, based on average
O2-PVA relation of each group) were
small and similar among the groups.
Force-Time Integral and Economy
Force-time integral (FTI) is the time integral of developed
force through 1 cardiac cycle. Based on the force equilibrium equation
for a sphere, total developed force was calculated as
follows: F=1.36xPxA=1.64xPxV2/3
where P is LV developed pressure,
A is lumen cross-sectional area, and V is LV
volume. F was integrated throughout 1 cardiac cycle to
obtain FTI.24 Economy was determined as
slope-1 of the linear
O2-FTI relation (g ·
s-1 · mL
O2-1 · g) and is
analogous to thermomechanical economy in myothermal
studies.15 25
Myofibrillar ATPase Activity
Maximal myofibrillar ATPase activity (pCa5) was determined by
the method of White26 in 6 hearts of each group.
Protein concentration was determined by the Lowery
method.27
Myosin Isoforms
Myosin isoforms were determined with pyrophosphate gel
electrophoresis15 using frozen tissue. Fifteen gels were
stained with Coomassie Blue R-250 and destained in acetic acid.
Destained gels were scanned with a PDI laser densitometer. Areas of
each protein peak were determined by analysis of total peak
area using Peakfit curve-fitting software (Jandel Scientific, Inc) and
fitting with a gaussian shape.
Thin-Filament Regulatory Proteins
Troponin T (TnT) isoforms were determined in LV
myocardium from 6 hearts of each group and 1 heart from a
neonatal DS rat, as described previously.28
One-dimensional SDS-PAGE (30% acrylamide, 1.1%
bis-acrylamide) was carried out according to
Laemmli29 using an 8% separating and a 3.3%
stacking gel. During electrophoresis, we initially used constant
current at 9 mA per gel. After the dye front had run through the
stacking gel, current was increased to 15 mA until the dye front had
run to the bottom. Gels were then silverstained according to Giulian et
al.30
Immunoblotting was carried out according to Twobin et al.31 After electrophoresis was performed, gels were equilibrated in transfer buffer (25 mmol/L Tris, 192 mmol/L glycine, 20% vol/vol MeOH, pH 8.3) for 15 minutes, sandwiched with nitrocellulose, and transferred electrophoretically at constant current of 0.8 mA/cm2 for 1 hour. After rinsing with distilled water, nitrocellulose was incubated in blocking solution (PBS containing 137 mmol/L NaCl, 2.7 mmol/L KCl, 4.3 mmol/L Na2HPO4, and 1.4 mmol/L KH2PO4) with 3% BSA for 1.5 hours. Primary antibody (monoclonal TnT antibody 13-11, provided by Dr Page Anderson, Duke University, Durham, NC) at a predetermined dilution was added to 1% BSA in PBS. After 1.5 hours of incubation, nitrocellulose was washed in PBS and incubated in secondary antibody (alkaline phosphate–conjugated rabbit anti-mouse, Jackson ImmunoResearch) for 1 hour. Nitrocellulose was washed in PBS and TPBS (PBS with 0.05% Tween 20). Color development was performed with the Promega ProtoBlot System. Protein bands in the nitrocellulose were scanned with a PDI laser densitometer and relative amounts of TnT isoforms determined by integrating the areas under the curve.
Troponin I (TnI) isoforms were determined in 6 LS-12 and 6 HS-12 hearts using methods similar to those for TnT, except 1-dimensional SDS-PAGE was performed with 0.8% rather than 1.1% bis-acrylamide, and gels were stained with 0.025% Coomassie Blue. For immunoblotting, primary antibody was a C5 monoclone specific for both cardiac and skeletal TnI (Advanced ImmunoChemical Inc) and secondary was goat anti-mouse (Promega). Tropomyosin (Tm) isoforms were assessed by scanning Coomassie stained myofibrillar gels for proteins of MW range 33 to 34 kDa. Tm appears as a discrete band(s) at this MW.32
TnT Phosphorylation
We modified the methods of Anderson et
al33 to assess TnT
phosphorylation. Paired samples of myofibrils prepared
as for ATPase assays from each of 6 LS-12 and 6 HS-12 hearts were
dephosphorylated with alkaline phosphatase
(AP)34 or subjected to the same protocol without
AP. Two-dimensional electrophoresis of myofibrillar gels was performed,
followed by Coomassie Blue staining. Anderson et
al33 demonstrated that TnT appears as 2 spots on
2-dimensional electrophoresis. After AP, only the more basic spot
remains. We reproduced this in each specimen. Accordingly, we assumed
the more basic spot was nonphosphorylated and performed
laser densitometry of the 2 spots in gels not subjected to AP to
quantify phosphorylated versus
nonphosphorylated TnT.
Statistics
Data are reported as mean±SD. The least squares method was used
to estimate ESPVR and O2-PVA
and O2-FTI relations. One-way
ANOVA was used to detect differences in BP, body weight, organ weight,
renal function, mechanoenergetics, myofibrillar ATPase, and contractile
proteins. A Student-Newman-Keuls test was used for multiple
comparisons. Relations between contractile efficiency or economy and
ATPase were tested by linear regression analysis. Survival
curves were compared by the generalized Wilcoxon test. A value
of P<0.05 was taken to indicate
significance.
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Results |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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Blood Pressure, Mortality, Organ Weight, and Renal Function
The Table
shows
effects of dietary salt on systolic BP, body weight, organ
weight, renal function, and mortality. Hypertension developed with HS.
At 6 weeks, there was a marked increase in LV/body weight ratio, with
only a modest additional increase at 12 weeks. Between 6 and 12 weeks,
HS rats showed increased mortality, lung weight, blood urea nitrogen,
and creatinine, consistent with a transition to
failure. Diastolic BP averaged 83±9 mm Hg in LS and
122±31 mm Hg in HS rats, with no differences at 6 versus 12
weeks.
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LV Mechanics
Figure 1 shows
representative PV,
O2-PVA, and
O2-FTI relations from one LS-12 and
one HS-12 rat. As expected, the ESPVR was curvilinear.
O2-PVA and
O2-FTI relations were highly linear
(for HS-6, r=0.92±0.08 and r=0.94±0.07;
LS-6, r=0.91±0.06 and r=0.93±0.06; HS-12,
r=0.81±0.19 and r=0.89±0.09; LS-12,
r=0.88±0.11 and r=0.91±0.08, and PTU,
r=0.90±0.08 and r=0.89±0.07, respectively
[mean±SD]).
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) and 1 Dahl rat fed HS for 12 weeks (HS-12, 
).
Middle, representative
At perfusion pressure averaging 91±5 mm Hg for LS and PTU and
119±12 mm Hg for HS, coronary flow (mL ·
min-1 · g-1) was
12.9±2.5 in LS-6, 13.2±1.7 in LS-12, and 12.8±2.7 in PTU rats. Flow
was modestly decreased in HS-6 (11.5±3.1) and HS-12 (10.0±2.3) rats.
The decrease in HS-12 was significant versus LS-12 or PTU
(P<0.05), but the decrease for HS-6 was not compared with
LS-6. Compared with LS controls, developed pressure, maximum dP/dt (at
matched diastolic stress), and E'max
were significantly decreased in the HS-12 group (Figure 2). These parameters were
also reduced in the PTU group versus age-matched LS-12 controls.
Maximum dP/dt was decreased in HS-6 versus LS-6 rats, but developed
pressure and E'max were not significantly
different. E'max was decreased in HS-12 versus
HS-6 rats (P<0.05).
|
Compared with age-matched LS controls, EDP at matched ES stress was increased in HS rats at 6 (11.0±4.7 versus 5.2±3.5 mm Hg, P<0.05) and 12 (12.3±4.7 versus 5.0±5.5 mm Hg, P<0.05) weeks, respectively. Minimum dP/dt at matched ES stress was decreased in HS-6 (909±295 versus 1327±598 mm Hg/s, P<0.05) and HS-12 (660±201 versus 959±399 mm Hg/s, P<0.05) rats. Minimum dP/dt was also decreased in PTU (344±74 mm Hg/s) rats versus controls (P<0.05) and HS-12 (P<0.05). There was a trend toward higher EDP in PTU rats (9.5±7.6 mm Hg), but this was not statistically significant.
LV Energetics
Figure 3 shows values for
O2 intercept of the
O2-PVA relation, efficiency, and
economy. The intercept was decreased in PTU versus age-matched LS-12
controls. There was a trend toward a decrease in HS-12 versus LS-12
(P=0.11). Efficiency was significantly
increased in HS-12 versus LS-12. There were no differences between LS-6
and HS-6. Efficiency was not significantly different in PTU compared
with other groups, although there was a trend toward a significantly
lower value versus HS-12 (P=0.08). Economy was strikingly
increased in HS-12 versus age-matched controls and the HS-6 group
(P<0.01). In the PTU group, economy was significantly
increased compared with LS-12. Economy in PTU was lower than in HS-12
but this difference was not statistically significant
(P=0.21). Basal metabolism during KCl arrest was
similar among the groups (HS-6 3.07±0.84, LS-6 3.10±1.15, HS-12
2.59±0.34, LS-12 3.20±1.15, PTU
2.35±0.86x10-3mL O2
· min-1 · g-1).
In hearts perfused with RBCs, HS-12 hearts also revealed increased
efficiency (0.62±0.07) and economy (132±15x103
g · s · mL
O2-1 · g) versus LS-12
(0.44±0.07 and 69±28x103 g · s ·
mL O2-1 · g,
respectively).
|
ATPase Activity and Myosin Isozymes
Values for myofibrillar ATPase activity and isomyosins are shown
in Figure 4. ATPase activity was
decreased in HS-6, HS-12, and PTU groups versus age-matched controls.
ATPase in HS-12 was also decreased versus the HS-6 group. There was
a trend toward higher activity in PTU versus HS-12, but the difference
was not statistically significant (P=0.10). PTU resulted in
conversion to 100% V3 isomyosin. The percent of V3 isomyosin (%V3)
was increased in HS-6, HS-12, and PTU groups compared with age-matched
controls. %V3 was also significantly increased in HS-12 and PTU versus
HS-6 and in PTU versus HS-12. Across all groups, ATPase activity was
significantly correlated with %V3 (r=-0.75,
P<0.001).
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Thin-Filament Isoforms
Four TnT isoforms, TnT1 through
TnT4, were detected in the neonatal DS rat
(Figure 5, left).
TnT2 and TnT4 are dominant;
TnT1 and TnT3 are evident
as faint bands. In LS and HS, 2 isoforms (TnT3
and TnT4) were detected, and
TnT4 was dominant. Group mean percent
TnT3 is shown in Figure 5, right. TnT isoforms
were virtually identical in LS-6, LS-12, HS-6, and PTU (17% to 18%
TnT3). In HS-12, percent
TnT3 increased to 23%, significantly greater
than in any other group. Across all groups, ATPase activity was
inversely correlated with percent TnT3
(r=-0.483, P=0.003).
|
TnI appeared as a single band corresponding to the cardiac isoform (MW
24 kDa) in each LS-12 and HS-12 heart, with no skeletal TnI
detectable. Tm appeared as a single band in the 33 to 34 kDa range in
each LS-12 and HS-12 heart.
TnT Phosphorylation
Examples of 2D gels are shown in Figure 6. Without AP, TnT appears as 2 spots.
After AP, only the more basic spot remains.33 The
percent phosphorylated TnT was 51±10 in LS-12 and
27± in HS-12 hearts (P<0.001).
|
Contractile Proteins and Energetics
Figure 7 shows the relation between
mean values of percent V3 myosin and ATPase activity and
efficiency (left) and economy (right). In hearts not subjected to
overload, the influence of isomyosin content on these
parameters was relatively modest, and can be appreciated by
comparing the 2 LS and PTU groups. Increases in efficiency and economy
in HS-12 were more striking and appear greater than can be accounted
for by isomyosin content alone, especially considering the fact that
values were lower in the PTU group despite higher %V3.
Correspondingly, ATPase activity was more depressed in HS-12 than would
be expected based on isomyosin content alone (Figure 8). (As noted, there were trends toward
differences in efficiency, economy, and ATPase between HS-12 and PTU
hearts, but none were significant at P<0.05.)
|
, LS-6; 
, HS-6;
, LS-12; 
, HS-12;
|
As shown in Figure 7
, bottom, there was an inverse correlation between
ATPase activity and efficiency (r=0.441, P=0.002)
and economy (r=0.576, P<0.001). In contrast to
relationships between %V3 and efficiency/economy and %V3 and ATPase
activity (Figure 8), when ATPase activity was related to efficiency and
economy, no group appeared anomalous.
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Discussion |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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As expected,13 14 DS rats developed compensated hypertrophy after 6 weeks of HS, and then underwent a transition to failure, with increased mortality and lung/body weight ratio, depressed amount and rate of LV pressure development, and reduced E'max. Concomitant with this transition, efficiency and economy increased. These mechanoenergetic alterations are correlated with myofibrillar ATPase activity and occur in conjunction with alterations in myosin and TnT isoforms and TnT phosphorylation.
Mechanoenergetics During the Transition From Compensation to
Failure
In isolated muscle myothermal experiments, economy (tension-time
integral divided by tension-dependent heat) is increased in failing
human and rabbit myocardium,2 3 4 in
association with increased cross-bridge force time integral. Increases
in economy appear greater than can be accounted for by isomyosin
content.2 The time course of altered cross-bridge
cycling during hypertrophy and the transition to failure
have not been assessed, and there are only a few reports delineating
ventricular mechanoenergetics. Izzi et
al35 reported that compensated
hypertrophy did not affect the
O2-PVA relation in canine
aortic banding. Wolff et al21 reported increased
efficiency in pacing failure, but did not evaluate a compensated phase.
The O2-PVA relation has been
studied in failing human hearts,36 37 38 with
inconsistent results. Because there have been no serial
studies, the time course and nature of ventricular
mechanoenergetic alterations during hypertrophy and failure
are uncertain, as is the pathophysiological
significance of the aforementioned isolated muscle
experiments2 3 4 (performed at low temperatures
and contraction frequencies). Our studies reveal favorable
mechanoenergetic alterations when decompensation occurs and are
consistent with increased economy reported in isolated muscle
studies.
The O2 intercept of the
O2-PVA relation is considered
to represent O2 used for
excitation-contraction coupling (mainly sarcoplasmic reticulum
Ca2+ reuptake) and basal
metabolism.23 We observed a trend
toward a decrease in HS-12 rats. Because basal
O2 was unchanged, this likely
reflects depressed Ca2+ cycling, as observed in
failing Dahl rats.39
Determinants of Efficiency and Economy
Efficiency and economy are markers of altered cross-bridge cycling
because they relate mechanical output of the contractile machinery to
chemical energy input. Other measures of ventricular
performance vary with changes in transsarcolemmal signaling and
excitation-contraction coupling as well as cross-bridge cycling. PVA is
appropriate to estimate thermodynamic efficiency because it
represents total mechanical energy
output.40 FTI does not have units of energy.
However, during isovolumic contractions, most mechanical energy is used
to produce force and both parameters should therefore
change in parallel.41 42 A practical difference
is that efficiency ranges between 0% and 100%, whereas economy has no
upper limit. In calculating efficiency, recovery metabolism
(energy required to regenerate high-energy phosphates) is neglected and
assumed to be equal for all energy consuming processes. This results in
systemic underestimation of the thermodynamic efficiency of the
contractile machinery40 and may make it
relatively difficult to detect increases in efficiency and explain the
larger increases in economy in DS rats.
Efficiency and economy are determined by factors that modify the energy requirements of the cross-bridge cycle or its mechanical output. It is useful to consider these systemically. A priori, it seems unlikely that ATPase energy requirements (1 ATP per cycle) decrease during failure. This would require a fundamental change in the thermodynamics of the enzyme. An increase in force produced by each cross-bridge would require a change in the force-producing mechanism, thought to reside in the S1 subunit of myosin43 (and to which the light chains are attached). In this regard, expression of atrial light chains in the right ventricle is observed in congenital heart disease44 ; Margossian et al45 reported changes in the stoichiometry of regulatory to essential light chains in failing human myocardium in association with depressed ATP splitting. Although the latter result has not been confirmed,46 alterations in light chains offer potential explanations for our results. Arguing against this is the fact that actomyosin ATPase activity is normal in failing human myocardium.7 Another possibility is an increased cross-bridge duty cycle (percent time/cycle force is generated), resulting in more force producers acting in concert and more PVA/FTI per ATP split. Duty cycle is determined by the rate constants of the biochemical reactions underlying the cross-bridge cycle. An increase could occur as a result of altered cross-bridge kinetics in failing myocardium.
An alternative mechanism is myocardial remodeling resulting in a mechanical advantage for myofibrils in the hypertrophied/failing LV. This seems an unlikely explanation for mechanoenergetic alterations that are most marked during failure, when parameters of contractile performance become depressed. Another possibility is an alteration in energy metabolism. Hypertrophied or failing myocardium is deficient in energy reserves and shifts from aerobic to glycolytic metabolism.47 48 A decrease in reserves should not affect efficiency or economy because it should not alter the energy input/output ratio of the contractile machinery. However, a switch toward glycolytic metabolism would increase efficiency and economy because of increased ATP production without additional O2 requirements. While increased glycolysis may have contributed to our results, several considerations argue against this as the predominant factor. Glycolytic ATP production is modest compared with aerobic pathways. In failing myocardium, the ratio of initial heat (excitation-contraction coupling plus cross-bridge cycling energy) to recovery heat is unchanged.3 4 This ratio reflects the efficiency of conversion of O2 to ATP and should be altered if there is a quantitatively important increase in ATP production from glycolysis. Finally, the inverse relationship between ATPase activity and efficiency and economy itself suggests that depressed cross-bridge cycling is the main mechanism of our results.
Modeling studies predict that efficiency should reflect cross-bridge level events.41 49 We reported decreased efficiency in hyperthyroidism,24 but there has been little further testing of this concept. The present studies combined with myothermal studies cited earlier2 3 4 support the idea that alterations in ventricular mechanoenergetic parameters reflect underlying changes in cross-bridge kinetics.
Significance of Changes in Contractile Proteins
With overload, rats switch from V1 to mainly V3 myosin. Parallels
between isomyosin distribution, ATPase activity, and myothermal economy
provide evidence of causal links between structural changes in myosin
and mechanical performance of the
myocardium.15 In vitro motility
studies with varying myosin isoforms confirm these findings at the
molecular level.50 As noted previously, however,
increases in myothermal economy appear out of proportion to isomyosin
content.2 In humans, if V1 myosin is present
in significant amounts,8 this would make the
higher than predicted economy in failure more striking because of the
inverse relationship between economy and V1
content.2 Thus, an unresolved question in heart
failure is why cross-bridge cycling and mechanoenergetics are altered
independent of myosin isoforms.
In this study, %V3 myosin increased progressively with
compensated hypertrophy and failure and was correlated with
ATPase activity. A larger increase in %V3 ascribable to overload
occurred during compensated hypertrophy than failure (a
portion of the latter is related to aging, as indicated by increased V3
in LS-12 versus LS-6). To better understand the influence of isomyosin
content, we studied hypothyroid rats, with 100% V3 myosin. The only
other known effect of thyroid status on contractile proteins is a very
small change in TnI isoforms confined to the
neonate.51 Several aspects of our results suggest
that altered ventricular mechanoenergetics and ATPase
activity in DS rats cannot solely be explained by isomyosin content,
analogous to human heart failure. Efficiency and economy were higher
and ATPase lower in HS-12 hearts than would be expected on the basis of
isomyosin content (Figures 7 and 8). For each parameter,
there was a trend toward a statistically significant difference between
HS-12 and PTU, despite the lower %V3 in HS-12 hearts (average 75%
versus 100%). Thus, 2 mechanoenergetic and 1 biochemical
parameters were internally consistent in this
regard. In addition to the relation between isomyosin content and
mechanoenergetics provided by LS-6, LS-12, and PTU rats, the temporal
sequence of altered mechanoenergetics and isomyosins also argues for a
non–isomyosin mediated effect. Thus, the %V3 in HS-6 averaged 44%,
versus 9% in LS-6 rats, amounting to a 35% increment, whereas the
%V3 in HS-12 rats was 75%, for an additional 30% increment during
failure. Despite these comparable increments in V3 content in
compensated versus decompensated hearts, increases in efficiency and
economy occurred exclusively during decompensation. In contrast, ATPase
activity did decrease significantly during compensated
hypertrophy. This suggests that non–isomyosin mediated
depression in cross-bridge cycling occurring during decompensation is
associated with a more important effect on mechanoenergetics than
depressed ATPase related to isomyosin switching.
To explain altered cross-bridge cycling and mechanoenergetics in failure, changes in thin-filament regulatory proteins3 7 9 have been proposed. As a preliminary investigation, we assessed TnT, TnI, and Tm isoforms and TnT phosphorylation. The fact that neither TnI nor Tm manifested isoform shifts during overload is consistent with previous reports.52 53 In contrast, there are reports of reexpression of a fetal TnT isoform in human failure,33 and we recently demonstrated a modest TnT isoform switch in hypertrophied rabbit hearts.28
TnT exists as multiple, developmentally regulated isoforms.33 54 55 Isoform variation is correlated with developmental changes in myofibrillar Ca2+ sensitivity.32 We observed an increase in TnT3 in HS rats that, in contrast to isomyosin switching, was temporally correlated with both the transition to failure and concomitant increases in efficiency and economy. The isoform pattern ( percent TnT4 greater than percent TnT3) is similar to that reported by Reiser et al56 in adult Wistar rats. Results in the neonatal DS rat indicate that TnT3 is detectable at this stage. Thus, a TnT isoform switch may explain the component of altered mechanoenergetics and ATPase activity not accounted for by myosin content. Against this is its small magnitude. However, TnT isoform variation occurs in its highly charged, amino-terminal portion,57 and TnT has been considered to have a role in cooperativity.32 55 56 Thus, a relatively small isoform shift could possibly have a large functional effect.
Changes in phosphorylation may also contribute to depressed cross-bridge cycling in heart failure.46 58 With respect to thin-filament proteins, protein kinase A (PKA) phosphorylation of TnI is reduced in failing myocardium.46 59 However, PKA phosphorylation of TnI influences TnC-calcium affinity without changing maximal force or ATPase activity34 60 61 62 63 64 and hence is not a good explanation for our results. Protein kinase C (PKC) phosphorylates TnI and TnT,64 65 66 with multiple, isoform-specific phosphorylation sites on both.64 Phosphorylation of TnI is dominated quantitatively by PKA, but TnT is phosphorylated exclusively by PKC.64 65 66 67 Thus, TnT phosphorylation should reflect PKC activity. Depending on sites and isoform specificity, increased PKC phosphorylation of TnT can either decrease both maximal ATPase activity and its Ca2+ sensitivity or slightly increase Ca2+ sensitivity without altering maximal activity.64 We found decreased percent phosphorylated TnT in failing hearts. If anything, this would be expected to increase maximal ATPase activity64 65 66 67 and thus does not provide an explanation for depressed cross-bridge cycling. Although we did not assess TnI, decreased PKC phosphorylation would also be expected to increase ATPase activity.64 65 66 67 Assuming that decreased TnT phosphorylation reflects PKC, this result was surprising in view of reports of increased PKC activity in heart failure.68 69 Once again, however, effects on contractile protein phosphorylation are highly isoform-specific,64 and some studies have shown differing results.70
Altered thin-filament regulatory proteins are not the only potential mechanisms for non–isomyosin mediated depressed cross-bridge cycling; we have already mentioned light chain modifications.45 Protein C can be phosphorylated but is thought to have relatively minor effects on ATPase activity.62 71 ,72 Thus, our results support but do not prove a role for TnT isoform shifts and suggest that thin-filament regulatory protein phosphorylation does not cause depressed cross-bridge cycling.
Limitations
We used crystalloid perfusate in most experiments, which
is known to decrease O2-carrying capacity and
could cause ischemia. Decreased coronary perfusion
pressure has been reported to decrease the slope of the
O2-PVA relation in the normal
canine heart,74 a finding attributed to
progressively decreasing nonmechanical
O2 caused by decreased
contractility related to cardiac ischemia
during increased load. To minimize ischemia, we used an RBC
suspension to increase O2-carrying capacity.
Increased efficiency and economy in failing hearts were also evident in
this subset of experiments.
We used a constant perfusion pressure of 90 mm Hg in LS and
PTU hearts and 120 mm Hg in HS hearts to approximate in vivo
diastolic blood pressure. However, decreased
coronary flow was observed in HS rats. This may be explained by
endothelial dysfunction.74
Because rat hearts have a detectable Gregg
effect,75 the lower coronary flow may
have contributed to the mechanoenergetic alterations. However, the
Gregg effect influences the
O2 intercept but not the
slope of the O2-PVA
relation.76 Thus, it is unlikely that perfusion
pressure per se influenced the latter results.
Clinical Implications
Since the failing heart has limited energy reserves, increased
efficiency and economy should be beneficial adaptations. Thus, in
failing myocardium, depressed cross-bridge cycling may be
viewed as having dual effects. It contributes to abnormal contractile
performance but results in reduced energy expenditure out of
proportion to the decrease in performance.
|
Acknowledgments |
|---|
This work was supported by NIH grant R01 HL52087.
Received April 29, 1998; revision received August 17, 1998; accepted August 20, 1998.
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References |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
|---|
1. Hajjar RJ, Gwathmey JK. Cross-bridge dynamics in human ventricular myocardium: regulation of contractility in the failing heart. Circulation. 1992;86:1819–1826.
2.
Hasenfuss G, Mulieri LA, Blanchard EM, Holubarsch C,
Leavitt BJ, Ittleman F, Alpert NA. Energetics of isometric force
development in control and volume-overload human
myocardium: comparison with animal species. Circ
Res. 1991;68:836–846.
3.
Hasenfuss G, Mulieri LA, Leavitt BJ, Allen PD,
Haeberle JR, Alpert NR. Alterations of contractile function and
excitation-contraction coupling in dilated
cardiomyopathy. Circ Res. 1992;70:1225–1232.
4.
Alpert, NR, Mulieri LA. Increased myothermal economy
of isometric force generation in compensated cardiac
hypertrophy induced by pulmonary artery
constriction in rabbit. Circ Res. 1982;50:491–500.
5.
Mercadier JJ, Bouveret P, Gorza L, Schiaffino S, Clark
WA, Zak R, Swynghedauw B, Schwartz K. Myosin isoenzymes in normal and
hypertrophied human ventricular myocardium.
Circ Res. 1983;53:52–62.
6. Alpert NR, Gordon MS. Myofibrillar adenosine triphosphatase activity in congestive heart failure. Am J Physiol. 1962;202:940–946.
7.
Pagani ED, Alousi AA, Grant AM, Older TM, Dziuban SW
Jr, Allen PD. Changes in myofibrillar content and Mg-ATPase activity in
ventricular tissues from patients with heart failure caused
by coronary artery disease, cardiomyopathy,
or mitral insufficiency. Circ Res. 1988;63:380–385.
8. Nakao K, Minobe W, Roden R, Bristow MR, Leinwand LA. Myosin heavy chain gene expression in human heart failure. J Clin Invest. 1997;100:2362–2370.[Medline] [Order article via Infotrieve]
9. Solaro RJ, Van Eyk J. Altered interactions among thin filament proteins modulate cardiac function. J Mol Cell Cardiol. 1996;28:217–230.[Medline] [Order article via Infotrieve]
10.
Dahl LK, Knudsen KD, Heine MA, Leitl GJ. Effect of
chronic excess salt ingestion. modification of experimental
hypertension in the rat by variation in the diet. Circ Res. 1968;22:11–18.
11.
Rapp JP, Wang S, Dene H. A genetic polymorphism in
the renin gene of Dahl rats cosegregates with blood pressure.
Science. 1989;243:542–544.
12.
Herrera VLM, Ruiz-Opazo N. Alteration of
1Na+,
K+-ATPase86RB+
influx by a single amino acid substitution. Science. 1990;249:1023–1026.
13.
Inoko M, Kihara Y, Morii H, Fujiwara H, Sasayama S.
Transition from compensatory hypertrophy to dilated,
failing left ventricles in Dahl salt-sensitive rats. Am J
Physiol. 1994;267:H2471–H2482.
14. Inoko M, Kihara Y, Sasayama S. Neurohumoral factors during transition from left ventricular hypertrophy to failure in Dahl salt-sensitive rats. Biochem Biophys Res Commun. 1995;206:814–820.[Medline] [Order article via Infotrieve]
15.
Holubarsch C, Goulette RP, Litten RZ, Martin BJ,
Mulieri LA, Alpert NA. The economy of isometric force development,
myosin isoenzyme pattern, and myofibrillar ATPase activity in normal
and hypothyroid rat myocardium. Circ Res. 1985;56:78–86.
16.
Wannenburg T, Schulman SP, Burkhoff D.
End-systolic pressure-volume and
MVO2-pressure-volume area relations of isolated
rat hearts. Am J Physiol. 1992;262:H1287–H1293.
17. Suga H, Sagawa K. Accuracy of ventricular lumen volume measurement by intraventricular balloon method. Am J Physiol. 1979;236:H506–H507.
18.
GotoY, Slinker BK, LeWinter MM. Accuracy of volume
measurement of rabbit left ventricle by balloon method. Am J
Physiol. 1988;255:H394–H396.
19. Grossman, W. Pressure measurement. Cardiac Catheterization and Angiography. 3rd ed. Philadelphia, Pa: Lea & Febiger; 1986:118–134.
20.
Nozawa T, Yasumura Y, Futaki S, Tanaka N, Suga H. No
significant increase in O2 consumption of
KCl-arrested dog heart with filling and dobutamine.
Am J Physiol. 1988;255:H807–H812.
21.
Wolff MR, de Tombe PP, Harasawa Y, Burkhoff D, Bier S,
Hunter WC, Gerstenblith G, Kass DA. Alterations in left
ventricular mechanics, energetics, and contractile reserve
in experimental heart failure. Circ Res. 1992;70:516–529.
22.
Burkhoff D, Sugiura S, Yue DT, Sagawa K.
Contractility-dependent curvilinearity of
end-systolic pressure-volume relations. Am J
Physiol. 1987;252:H1218–H1227.
23.
Suga H. Ventricular energetics.
Physiol Rev. 1990;70:247–277.
24.
Goto Y, Slinker BK, LeWinter MM. Decreased contractile
efficiency and increased nonmechanical energy cost in hyperthyroid
rabbit heart. Relation between O2 consumption and systolic
pressure-volume area or force-time integral. Circ Res. 1990;66:999–1011.
25.
Gibbs CL, Gibson WR. Energy production in
cardiac isotonic contractions. J Gen Physiol. 1970;56:732–750.
26. White HD. Special instrumentation and techniques for kinetic studies of contractile proteins. Methods Enzymol. 1982;85:698–708.
27.
Lowry OH, Rosenbrough NJ, Farr AI, Randall RA. Protein
measurement with folin phenol reagent. J Biol Chem. 1951;193:265–275.
28. Chen Z, Higashiyama A, Yaku H, Bell S, Fabian J, Watkins MW, Maughan DW, LeWinter MM. Altered expression of troponin T isoforms in mild left ventricular hypertrophy in the rabbit. J Mol Cell Cardiol. 1997;29:2345–2354.[Medline] [Order article via Infotrieve]
29. Laemmli UK. Cleavage of structural proteins during the assembly of the head of bacteriophage T4. Nature. 1970;227:680–685.[Medline] [Order article via Infotrieve]
30. Giulian GG, Moss RL, Greaser M. Improved methodology for analysis and quantification of proteins on one-dimensional silver-stained slab gels. Anal Biochem. 1983;129:277–287.[Medline] [Order article via Infotrieve]
31.
Twobin H, Staehelin T, Gorden J. Electrophoretic
transfer of proteins from polyacrylamide gels to nitrocellulose
sheets: procedure and some applications. Proc Natl Acad Sci
U S A. 1979;76:4350–4354.
32.
Nassar R, Malouf NN, Kelly MB, Oakley AE, Anderson PAW.
Force-pCa relation and troponin T isoforms of rabbit
myocardium. Circ Res. 1991;69:1470–1475.
33.
Anderson PAW, Malouf NN, Oakeley AE, Pagani ED, Allen
PD. Troponin T isoform expression in humans. a comparison among normal
and failing adult heart, fetal heart, and adult and fetal skeletal
muscle. Circ Res. 1991;69:1226–1233.
34. Holroyde MJ, Howe E, Solaro RJ. Modification of calcium requirements for activation of cardiac myofibrillar ATPase by cyclic AMP dependent phosphorylation. Biochim Biophys Acta. 1979;586:63–69.
35.
Izzi G, Zile MR, Gaasch WA. Myocardial oxygen
consumption and the left ventricular pressure-volume area
in normal and hypertrophic canine hearts. Circulation. 1991;84:1384–1392.
36.
Kameyama T, Asanoi H, Ishizaka S, Yamanishi K, Fujita
M, Sasayama S. Energy conversion efficiency in human left ventricle.
Circulation. 1992;85:988–996.
37.
Takaoka H, Takeuchi M, Odake M, Hata K, Hayashi Y, Mori
M, Yokoyama M. Depressed contractile state and increased myocardial
consumption for non-mechanical work in patients with heart failure due
to old myocardial infarction. Cardiovasc Res. 1994;28:1251–1257.
38.
Hayashi Y, Takeuchi M, Takaoka H, Hata K, Mori M,
Yokoyama M. Alteration in energetics in patients with left
ventricular dysfunction after myocardial infarction:
increased oxygen cost of contractility.
Circulation. 1996;93:932–939.
39.
Gomez AM, Valdivia HH, Cheng H, Lederer MR, Santana LF,
Cannell MB, McCune SA, Altschuld RA, Lederer WJ. Defective
excitation-contraction coupling in experimental cardiac
hypertrophy and heart failure. Science. 1997;276:800–806.
40.
Denslow S. Relationship between PVA and myocardial
oxygen consumption can be derived from thermodynamics. Am J
Physiol. 1996;270:H730–H740.
41. Suga H, Goto Y, Hata K, Takasaga T, Saeki A, Taylor TW, Nishioka T. Constant efficiency versus variable economy of cardiac contraction. Jpn Heart J. 1992;33:213–227.[Medline] [Order article via Infotrieve]
42. Suga H, Goto Y, Nozawa T, Yasumura Y, Futaki S, Tanaka N. Force-time integral decreases with ejection despite constant oxygen consumption and pressure-volume area in dog left ventricle. Circ Res. 1987;60:7997–803.
43.
Morano I, Ritter O, Bonz A, Timek T, Vahl CF, Michel G.
Myosin light chain-actin interaction regulates cardiac
contractility. Circ Res. 1995;76:720–725.
44. Morano M., Zacharzowski U, Maier M, Lange PE, Alexi-Meskishvili V, Haase H, Morano I. Regulation of human heart contractility by essential myosin light chain isoforms. J Clin Invest. 1996;98:467–473.[Medline] [Order article via Infotrieve]
45.
Margossian SS, White HD, Caulfield JB, Norton P, Taylor
S, Slayer HS. Light chain 2 profile and activity of human
ventricular myosin during dilated
cardiomyopathy: identification of a causal agent
for impaired myocardial function. Circulation. 1992;85:1720–1733.
46. Wolff MW, Buck SH, Stoker SW, Greaser MW, Mentzer RM. Myofibrillar calcium sensitivity of isometric tension is increased in human dilated cardiomyopathies. J Clin Invest. 1996;98:167–176.[Medline] [Order article via Infotrieve]
47.
Nascimben L, Ingwall JS, Pauletto P, Friedrich J,
Gwathmey JK, Sacks V, Pessina AC, Allen PD. Creatine kinase system in
failing and nonfailing human myocardium.
Circulation. 1996;94:1894–1901.
48. Sylven C, Lin L, Jansson E, Sotonyi P, Fu L, Waagstein F, Hjalmarsson A, Marcus C, Bronnegard M. Ventricular adenosine nucleotide translocator mRNA is upregulated in dilated cardiomyopathy. Cardiovasc Res. 1993;27:1295–1299.[Medline] [Order article via Infotrieve]
49.
Yamaguchi H, Takaki M, Matsubara H, Yasumura S, Suga H.
Constancy and variability of contractile efficiency as a function of
calcium and cross-bridge kinetics: simulation. Am J
Physiol. 1996;270:H1501–H1508.
50. Harris DE, Work SS, Wright RK, Alpert NR, Warshaw DM. Smooth cardiac and skeletal muscle myosin force and motion generation assessed by cross-bridge mechanical interaction in vitro. J Muscle Res Cell Motil. 1994;15:11–19.[Medline] [Order article via Infotrieve]
51.
Dieckman LJ, Solaro RJ. Effect of thyroid status on
thin filament Ca2+ regulation and expression of
troponin I in perinatal and adult rat hearts. Circ Res. 1990;67:344–351.
52.
Toyota N, Shimada Y, Bader D. Molecular cloning and
expression of chicken cardiac troponin C. Circ Res. 1989;65:1241–1246.
53. Murphy AM, Jones L II, Sims HF, Strauss AW. Molecular cloning of rat cardiac troponin I and analysis of troponin I isoform expression in developing rat heart. Biochemistry. 1991;30:707–712.[Medline] [Order article via Infotrieve]
54.
Anderson PAW, Moore GE, Nassar RN. Developmental
changes in the expression of rabbit left ventricular
troponin T. Circ Res. 1988;63:742–747.
55.
Tobacman LS, Lee R. Isolation and functional comparison
of bovine cardiac troponin T isoforms. J Biol Chem. 1987;262:4059–4064.
56. Reiser PJ, Westfall MV, Schiaffino S, Solaro RJ. Tension production and thin-filament protein isoforms in developing rat myocardium. Am J Physiol. 1994;264:H1589–H1596.
57.
Greg A, Hirschberg Y, Anderson PAW, Hainsworth C,
Malouf NN, Oakeley AE, Kay BK. Molecular basis of cardiac troponin T
isoform heterogeneity in rabbit heart. Circ
Res. 1994;74:41–47.
58.
Noland TA Jr, Guo Z, Raynor RL, Jideama NM,
Averyhart-Fuller V, Solaro RJ, Kuo JF. Cardiac troponin I mutants:
phosphorylation by protein kinase C and A and
regulation of Ca2+-stimulated MgATPase of
reconstituted actomyosin S-1. J Biol Chem. 1995;270:25445–25454.
59.
Bodor GS, Oakeley AE, Allen PD, Crimmins DL, Ladenson
JH, Anderson PAW. Troponin I phosphorylation in the
normal and failing adult human heart. Circulation. 1997;96:1495–1500.
60.
Hoffman PA, Lange JH III. Effects of
phosphorylation of troponin I and C protein on
isometric tension and velocity of unloaded shortening in skinned single
cardiac myocytes from rats. Circ Res. 1994;74:718–726.
61. Zhang R, Zhao J, Mandveno A, Potter JD. Cardiac troponin I phosphorylation increases the rate of cardiac muscle relaxation. Circ Res. 1995;1995:76:1028–1035.
62.
Zhang R, Zhao J, Potter JD.
Phosphorylation of both serine residues in cardiac
troponin I is required to decrease the Ca2+
affinity of cardiac troponin C. J Biol Chem. 1995;270:30773–30780.
63. Wattanapermpool J, Guo X, Solaro RJ. The unique amino-terminal peptide of cardiac troponin I regulates myofibrillar activity only when it is phosphorylated. J Mol Cell Cardiol. 1995;27:1383–1391.[Medline] [Order article via Infotrieve]
64.
Jideama NM, Noland TA Jr, Raynor RL, Blobe GC, Fabbro
D, Kazanietz MG, Blumberg PM, Hannun YA, Kuo JF.
Phosphorylation specificities of protein kinase C
isozymes for bovine cardiac troponin I and troponin T and sites within
these proteins and regulation of myofilament properties. J
Biol Chem. 1996;271:23277–23283.
65.
Noland TA Jr, Kuo JF. Protein kinase C
phosphorylation of cardiac troponin I or troponin T
inhibits Ca2+-stimulated actomyosin Mg ATPase
activity. J Biol Chem. 1991;266:4974–4978.
66. Noland, TA Jr, Kuo JF. Protein kinase C phosphorylation of cardiac troponin I and troponin T inhibits Ca2+-stimulated Mg ATPase activity in reconstituted actomyosin and isolated myofibrils, and decreases actin-myosin interactions. J Mol Cell Cardiol. 1993;25:53–65.[Medline] [Order article via Infotrieve]
67. Noland TA Jr, Guo X, Raynor RL, Jideama NM, Averyhart-Fullard V, Solaro RJ, Kuo JF. Cardiac troponin I mutants: phosphorylation by protein kinases C and A and regulation of Ca2+-stimulated Mg-ATPase of reconstituted actomyosin S-1. J Biol Chem. 1995;270:25445–25454.
68.
Gu X, Bishop SP. Increased protein kinase C and isozyme
redistribution in pressure-overload cardiac hypertrophy in
the rat. Circ Res. 1994;75:926–931.
69.
Lee HC, Cai JJ, Yu H. Effect of protein kinase C
on cyclic 3',5'-adenosine monophosphate-dependent
phosphodiesterase in hypertrophic cardiomyopathic
hamster hearts. J Pharmacol Exp Ther. 1994;270:1171–1176.
70.
Rouet-Benzineb P, Mohammadi K, Perennec J, Poyard M,
Bouanani NEH, Crozatier B. Protein kinase C isoform expression in
normal and failing rabbit hearts. Circ Res. 1996;79:153–161.
71. Hartzell HC. Effects of phosphorylated and unphosphorylated C-protein on cardiac actomyosin ATPase. J Mol Biol. 1985;1985:186:185–195.
73.
Weisberg A, Winegrad S. Alteration of myosin cross
bridge by phosphorylation of myosin-binding protein C
in cardiac muscle. Proc Natl Acad Sci U S A. 1996;93:8999–9003.
74.
Suga H, Goto Y, Yasumura Y, Nozawa T, Futaki S, Tanaka
N, Uenishi M. O2 consumption of dog heart under
decreased coronary perfusion and propranolol.
Am J Physiol. 1988;254:H292–H303.
75.
Lüscher TF, Reij L, Vanhoutte PM.
Endothelium-dependent vascular responses in
normotensive and hypertensive Dahl rats. Hypertension. 1987;9:157–163.
76. Kameyama T, Chen Z, Bell SP, Fabian J, Watkins MW, LeWinter MM. Modulation of ventricular function by coronary perfusion pressure in a rat model of pressure overload and failure. Circulation. 1996;94:I-361. Abstract.
77.
Goto Y, Slinker BK, LeWinter MM. Effect of
coronary hyperemia on Emax and oxygen consumption in
blood-perfused rabbit heart: energetic consequences of Gregg's
phenomenon. Circ Res. 1991;68:482–492.We studied
changes in mechanoenergetics, myofibrillar ATPase activity, and
contractile protein isoforms and phosphorylation in
Dahl salt-sensitive rats fed high-salt diet for 6 or 12 weeks to
produce compensated hypertrophy followed by failure. With
failure contractile efficiency and economy increased. ATPase activity
decreased progressively. These changes were only partially explained by
myosin isoform switching. A troponin T isoform shift is a potential
mechanism.
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W. J. Paulus and J. G. F. Bronzwaer Nitric oxide's role in the heart: control of beating or breathing? Am J Physiol Heart Circ Physiol, July 1, 2004; 287(1): H8 - H13. [Abstract] [Full Text] [PDF]
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D. Yamashita, H. Kohzuki, Y. Kitagawa, T. Nakashima, A. Kikuta, and M. Takaki O2 consumption of mechanically unloaded contractions of mouse left ventricular myocardial slices Am J Physiol Heart Circ Physiol, July 1, 2004; 287(1): H54 - H62. [Abstract] [Full Text] [PDF]
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T. Noguchi, Z. Chen, S. P. Bell, L. Nyland, and M. M. LeWinter Endothelin receptor blockade has an oxygen-saving effect in Dahl salt-sensitive rats with heart failure Am J Physiol Heart Circ Physiol, October 1, 2003; 285(4): H1428 - H1434. [Abstract] [Full Text] [PDF]
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 R. Persinger, Y. Janssen-Heininger, S. S. Wing, D. E. Matthews, M. M. LeWinter, and M. J. Toth Effect of heart failure on the regulation of skeletal muscle protein synthesis, breakdown, and apoptosis Am J Physiol Endocrinol Metab, May 1, 2003; 284(5): E1001 - E1008. [Abstract] [Full Text] [PDF]
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 T. Noguchi, Y. Kihara, K. J. Begin, J. A. Gorga, K. A. Palmiter, M. M. LeWinter, and P. VanBuren Altered Myocardial Thin-Filament Function in the Failing Dahl Salt-Sensitive Rat Heart: Amelioration by Endothelin Blockade Circulation, February 4, 2003; 107(4): 630 - 635. [Abstract] [Full Text] [PDF]
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T. Noguchi, Z. Chen, S. P. Bell, L. Nyland, and M. M. LeWinter Activation of PKC decreases myocardial O2 consumption and increases contractile efficiency in rats Am J Physiol Heart Circ Physiol, November 1, 2001; 281(5): H2191 - H2197. [Abstract] [Full Text] [PDF]
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D. T. McCurdy, B. M. Palmer, D. W. Maughan, and M. M. LeWinter Myocardial cross-bridge kinetics in transition to failure in Dahl salt-sensitive rats Am J Physiol Heart Circ Physiol, September 1, 2001; 281(3): H1390 - H1396. [Abstract] [Full Text] [PDF]
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Y. Takeuchi, Y. Kihara, K. Inagaki, T. Yoneda, and S. Sasayama Endothelin-1 Has a Unique Oxygen-Saving Effect by Increasing Contractile Efficiency in the Isolated Rat Heart Circulation, March 20, 2001; 103(11): 1557 - 1563. [Abstract] [Full Text] [PDF]
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 N Westerhof Cardiac work and efficiency Cardiovasc Res, October 1, 2000; 48(1): 4 - 7. [Full Text] [PDF]
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S. D. Prabhu and G. L. Freeman Altered LV inotropic reserve and mechanoenergetics early in the development of heart failure Am J Physiol Heart Circ Physiol, March 1, 2000; 278(3): H698 - H705. [Abstract] [Full Text] [PDF]
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 J. H. Marden, G. H. Fitzhugh, M. R. Wolf, K. D. Arnold, and B. Rowan From the Cover: Alternative splicing, muscle calcium sensitivity, and the modulation of dragonfly flight performance PNAS, December 21, 1999; 96(26): 15304 - 15309. [Abstract] [Full Text] [PDF]
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K. L. Butler, A. H. Huang, and J. K. Gwathmey AT1-receptor blockade enhances ischemic preconditioning in hypertrophied rat myocardium Am J Physiol Heart Circ Physiol, December 1, 1999; 277(6): H2482 - H2487. [Abstract] [Full Text] [PDF]
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P. VanBuren, S. L. Alix, J. A. Gorga, K. J. Begin, M. M. LeWinter, and N. R. Alpert Cardiac troponin T isoforms demonstrate similar effects on mechanical performance in a regulated contractile system Am J Physiol Heart Circ Physiol, May 1, 2002; 282(5): H1665 - H1671. [Abstract] [Full Text] [PDF]
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