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1 Department of Medicine, Allegheny General Hospital, MCP-Hahnemann University School of Medicine, Pittsburgh 15212; and 2 Merck Research Laboratories, West Point, Pennsylvania 19486
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ABSTRACT |
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 TOP
 ABSTRACT
 INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Incessant tachycardia
induces dilated cardiomyopathy in humans and experimental models;
mechanisms are incompletely understood. We hypothesized that excessive
chronotropic demands require compensatory contractility reductions to
balance metabolic requirements. We studied 24 conscious dogs during
rapid right ventricular (RV) pacing over 4 wk. We measured hemodynamic,
coronary blood flow (CBF), myocardial O2 consumption
(M
O2) responses, myocardial nitric oxide
(NO) production, and substrate utilization. Early pacing (6 h) resulted
in decreased heart rate (HR)-adjusted coronary blood flow (CBF),
MO2 (CBF/beat: 0.33 ± 0.02 to
0.19 ± 0.01 ml, P < 0.001, MO2/beat: 0.031 ± 0.002 to
0.016 ± 0.001 ml O2, P < 0.001), and
contractility [left ventricular (LV) first derivative pressure
(dP/dt)/LV end-diastolic diameter (EDD): 65 ± 4 to
44 ± 3 mmHg · s
1 · mm1,
P < 0.01], consistent with flow-metabolism-function
coupling, which persisted over the first 72 h of pacing (CBF/beat:
0.15 ± 0.01 ml, MO2/beat:
0.013 ± 0.001 ml O2, P < 0.001).
Thereafter, CBF per beat and MO2 per
beat increased (CBF/beat: 0.25 ± 0.01 ml,
MO2/beat: 0.021 ± 0.001 ml
O2 at 28 days, P < 0.01 vs. 72 h).
Contractility declined [(LV dP/dt)/LVEDD: 19 ± 2 mmHg · s1 · mm1,
P < 0.0001], signifying flow-function mismatch.
Cardiac NO production, endothelial NO synthase expression, and fatty
acid utilization decreased in late phase, whereas glycogen content and
lactate uptake increased. Incessant tachycardia induces contractile,
metabolic, and flow abnormalities reflecting flow-function matching
early, but progresses to LV dysfunction late, despite restoration of flow and metabolism. The shift to flow-function mismatch is associated with impaired myocardial NO production.
nitric oxide; cardiomyopathy; stunning; hibernation; myocardial metabolism
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INTRODUCTION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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BOTH INCESSANT
TACHYCARDIA in humans (16, 17, 38, 46, 59) and rapid
pacing in experimental animal models (6, 55, 62, 69)
produce contractile dysfunction and dilated cardiomyopathy (DCM).
Although structural and functional myocardial abnormalities such as
distortion of contractile elements (34), impaired regional
coronary flow reserve (54), abnormal
sarcoplasmic calcium handling (47), or enhanced
proapoptotic signaling (24) have been described in
such experimental models, the precise mechanisms for the contractile
dysfunction and these structural changes remain incompletely
understood. Despite severe contractile dysfunction after weeks of rapid
pacing, functional recovery is observed after cessation of the
tachycardia. This suggests that the contractile dysfunction reflects a
compensation designed to limit irreversible myocardial injury.
Recently, investigation has focused to the role those myocardial
energetic imbalances and shifts in metabolic substrate utilization may
play in the pathogenesis of human (23, 30, 64) or
experimental heart failure (28, 70). Nitric oxide (NO) has been implicated to play a critical role in modulating myocardial contractility (10), as well as myocardial
O2 consumption (MO2) and
myocardial preferences for metabolic substrates in experimental models
of heart failure (49, 50).
We hypothesized that excessive chronotropic demands imposed by fixed rapid right ventricular (RV) pacing require compensatory reductions in myocardial contractility to balance the myocardial metabolic requirements over time. Therefore, the goal of our study was to characterize the left ventricular (LV), systemic hemodynamic, coronary blood flow (CBF), and metabolic alterations associated with continuous rapid RV pacing that results in DCM in conscious dogs. A second goal was to investigate the role of myocardial NO metabolite production in mediating these dynamics over time.
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METHODS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Surgical procedure and instrumentation. Twenty-four mongrel dogs of either sex weighing between 16 and 22 kg were sedated with xylazine (10 mg/kg) and anesthetized with halothane (1-1.5 vol%). Through an incision in the fifth intercostal space, Tygon catheters were placed in the descending thoracic aorta and left atrium (LA), and a Silastic catheter was placed in the coronary sinus. A solid-state pressure transducer (Konigsberg Instruments) was implanted in the left ventricle through an apical approach that facilitated high-fidelity recordings of LV pressure (LVP). A Transonics flow probe was placed on the proximal portion of the left circumflex coronary artery for continuous measurement of CBF. A similar Transonics flow probe was placed on the ascending aorta to measure aortic blood flow. Piezoelectric ultrasonic dimension crystals were implanted on the anterior and posterior endocardial surfaces of the left ventricle to measure the internal short-axis diameter in end diastole (LVEDD) and end systole (LVESD). Piezoelectric crystals were implanted on the endocardial and epicardial surfaces of the posterior wall to measure regional wall thickening (WTh). A sutureless pacing lead was implanted on the epicardial surface of the right ventricle.
All catheters were tunneled subcutaneously and externalized infrascapularly, after which the thoracotomy was closed in layers, and the thoracic cavity was evacuated of air. All animals received analgesics as needed for the first 72 h following surgery, and cephalexin (1 g iv) was administered daily for 7 days. The dogs were allowed to recover from the surgical procedure for 2 wk, during which time they were trained to lie quietly on the experimental table in a conscious, unrestrained state. All catheters were flushed daily and filled with a 50% heparin solution to maintain patency. Animals used in this study were maintained in accordance with the guidelines of the Committee of Animals of Allegheny General Hospital and the National Institutes of Health Guidelines for the Care and Use of Laboratory Animals (DHHS Publications No. NIH 85-23, Revised 1985).Heart failure induction protocol: hemodynamic measurements.
Hemodynamic measurements were obtained at the fully conscious state, at
baseline (before initiation of pacing), and during rapid RV pacing (240 beats/min), serially at 10, 30, 60, 120, 180, and 360 min to
characterize the acute hemodynamic responses. The dogs were returned to
their kennels and continued to be paced at the same fixed rate. To
investigate the chronic phase of rapid RV pacing, followup hemodynamic
measurements were obtained at 1, 3, 7, 14, 21, and 28 days of
continuous pacing. All measurements were conducted in the conscious and
fasting state and during rapid RV pacing. Measurements consisted of
LVP, LV first derivative pressure (dP/dt), aortic systolic,
diastolic, and mean pressure, LA and right atrium pressure, cardiac
output, CBF, regional myocardial WTh, LVEDD, and LVESD. Mean CBF was
measured on the circumflex coronary artery.
MO2 was calculated as the product of the
left circumflex CBF and the myocardial arteriovenous difference of O2 content, assessed by an automatic blood gas analyzer
(Radiometer). As such, the calculated
MO2 was an estimate of total
MO2.
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Measurement of cardiac NO metabolites.
Plasma levels of NO metabolites nitrate (NO
Competitive pharmacological inhibition of NO
synthase.
To determine the role of NO in mediating the acute response to rapid
pacing, we studied the LV, CBF, and MO2
responses to acute pacing in the presence and absence of NO synthase
(NOS) inhibition in six additional dogs, instrumented similarly. The responses were recorded in the same animals during control saline injection and after
N
-nitro-L-arginine
(L-NNA: 30 mg/kg iv). The dose of L-NNA was determined by the demonstration of 50% attenuation in the epicardial CBF response to acetylcholine (0.1 µg/kg iv), which was assessed before and after the administration of L-NNA. Each dog was
allowed to recover for at least 7 days between each arm of the
protocol. The L-NNA protocol was conducted last because of
the long-lasting activity of L-NNA. In three dogs, the
pacing protocol was repeated during the infusion of phenylephrine
(2-5
µg · kg1 · min1) over
6 h designed to match the increase in mean arterial pressure seen
with L-NNA.
Myocardial respiratory quotient and metabolic substrate
utilization.
Myocardial respiratory quotient (RQ) was measured by the method
previously described by Recchia et al. (49, 50).
Measurements were made at baseline and during acute (10-360 min)
and chronic (1-28 days) rapid RV pacing. Total myocardial
CO2 production was calculated as the sum of measured plasma
HCO
ao) total CO2/(ao cs)
O2 content, where cs is coronary sinus and ao is aorta.
Myocardial tissue analysis for endothelial NOS activity. LV samples were obtained at the time of euthanasia in three control dogs and in four dogs with advanced DCM, manifest after 27 ± 4 days of RV pacing. Briefly, LV tissue was rapidly removed and frozen in liquid nitrogen. Cryostat sections (6 µm thick) were immunostained with monoclonal anti-canine endothelial NOS (eNOS) antibody. Additional frozen LV tissue samples were homogenized for eNOS protein detection by Western blotting (140 kDa). The methods for eNOS myocardial immunohistochemical (IHC) staining and protein analysis by Western blotting have been previously described elsewhere in detail (11, 15, 53).
Myocardial glycogen content. Glycogen content was determined using 4% buffered formalin-fixed tissues, dehydrated in ethanol, embedded in paraffin, and cut at 3-µm sections. We stained the sections with periodic acid Schiff (PAS) stain to detect glycogen and also performed digestion with amylase followed by PAS staining verifying the PAS-positive material being glycogen (68). These histological sections of LV myocardium were evaluated quantitatively for glycogen content using morphological analysis with a Nikon microscope connected to a computer with Metamorph imaging software. Sections stained with PAS were examined by using the green image from a color RGB Spot camera where 10-100 fields of LV (from subendomyocardial to subepimyocardial) were examined by using the ×10 objective of the microscope. Glycogen volume percent per area for each animal was expressed as the average of all fields examined.
Statistical analysis.
Hemodynamic and metabolic (MO2 and RQ)
parameters were expressed as means ± SE and compared by using
repeated-measures ANOVA. Myocardial NO production was expressed as
coronary sinus-aortic concentration difference (in µM), as well as
temporally related indexes reflecting myocardial NO production "per
minute" (in nmol/min) or "per heartbeat" (in nmol/beat). These
indexes were derived by multiplying coronary sinus minus aorta
concentration times CBF (ml/min) and dividing this product by the heart
rate (baseline native sinus rate or 240 beats/min), respectively. All
indexes have been previously utilized to investigate myocardial NO
production (49) and were also compared over time using
repeated measures ANOVA. Myocardial eNOS expression from paced animals
was compared with controls from our laboratory, using qualitative
analysis of IHC and semiquantitative analysis of optical densitometry
units (ODU) in Western blotting protein gel electrophoresis. Myocardial glycogen was expressed as a volume percentage per area of myocardium stained positive as described above. A level of P value
<0.05 was accepted to indicate statistical significance.
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RESULTS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Induction of DCM-systemic hemodynamics.
The temporal changes in systemic hemodynamic parameters during rapid RV
pacing are depicted in Table 1.
Initiation of rapid RV pacing resulted in immediate (10 min) increases
in LV end-diastolic pressure and decreases in LV dP/dt and
stroke volume. These changes persisted through the acute phase of RV
pacing and intensified during the chronic phase, leading to
progressively worsening heart failure. LVEDD demonstrated a trend to
decrease during the acute phase of rapid pacing, which was followed by
LV dilatation in the chronic phase after 7 days of pacing. To account
for the preload dependence of the index of isovolumic contraction, LV
dP/dt was normalized by LVEDD for serial comparisons. There
was a modest trend to an increase in WTh on initiation of pacing (10 min). Overall, WTh did not change during the acute phase, but there was
a significant decrease in WTh by 7 days that persisted throughout the
chronic phase.
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Myocardial flow-function relationship.
Upon initiation of rapid RV pacing, there was a significant decline in
LV contractile function [(LV dP/dt)/LVEDD index] from 65 ± 4 to 44 ± 3 mmHg · s1 · mm1 after
6 h of pacing (P < 0.01). This was accompanied by
a parallel, significant decline in both CBF and
MO2, normalized for the change in heart
rate (CBF/beat: from 0.33 ± 0.02 ml to 0.19 ± 0.01 ml,
MO2/beat: from 0.031 ± 0.002 to
0.016 ± 0.001 ml O2 at 6 h, respectively;
P < 0.001, Table 2).
Compared with the values obtained at 10 min after initiation of rapid
pacing, there was a progressive decline in both CBF per beat and
MO2 per beat over the first 72 h.
This pattern of "matched" decline in myocardial blood flow and
contractile function characterized the acute phase of rapid RV pacing
in our model (Fig. 1A).
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1 · mm1 at 28 days, P < 0.0001), both CBF per beat and
MO2 per beat reached their nadir values
at 3 days of pacing (CBF/beat: 0.15 ± 0.01 ml and
MO2/beat: 0.013 ± 0.001 ml
O2, respectively, P < 0.001 compared with
controls). Thereafter, they increased significantly (CBF/beat:
0.25 ± 0.01 ml, MO2/beat:
0.021 ± 0.001 ml O2 at 28 days of pacing,
P < 0.01, compared with respective values at 72 h
of pacing), returning to levels comparable to those observed immediately (10 min) after initiation of rapid pacing. Despite the
recovery of CBF, LV contractile function was reduced by 65% (Fig.
1B), suggesting "flow-function mismatch" during the
chronic phase of rapid RV pacing (3-28 days). Similarly, we
observed a pattern of preserved myocardial WTh during the early phase
of pacing (3.1 ± 0.3 to 3.2 ± 0.3 mm at 6 and 24 h of
pacing), followed by progressive decline during the chronic phase (to
2.2 ± 0.2 mm at 28 days of pacing, P < 0.03), at
a time where CBF had already started to increase (Fig.
2), consistent with flow-function
mismatch in the later phase of pacing-induced cardiomyopathy.
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Myocardial metabolic requirements during tachycardia.
During the initial 10 min of rapid RV pacing, there was a significant
rise in MO2 (from 2.3 ± 0.1 to
4.6 ± 0.3 ml O2/min, P < 0.001).
This was related to the excessive chronotropic demand imposed by fixed
pacing at 240 beats/min, as the MO2 per
beat declined from 0.031 ± 0.002 to 0.019 ± 0.001 ml
O2 (P < 0.001). Thereafter, there was a
decline in both MO2 and
MO2 per beat, reaching a nadir at 3 days
of rapid RV pacing (Table 2). During the chronic phase, there was a
significant increase in both MO2 (from
3.3 ± 0.2 ml O2/min at 3 days to 4.7 ± 0.2 ml
O2/min at 28 days of pacing, P < 0.01) and
MO2 per beat (from 0.013 ± 0.001 ml O2 at 3 days to 0.021 ± 0.001 ml O2 at
28 days, P < 0.001), resulting in a return to levels
comparable to those observed immediately after initiation of pacing.
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Myocardial metabolic substrate utilization.
Myocardial RQ increased progressively from a baseline value of
0.72 ± 0.06 to an end-stage value of 0.91 ± 0.07 (P < 0.05), suggesting of a shift in myocardial
metabolic substrate utilization from FFA to glucose. Notably, the RQ
increased substantially by 7 days of rapid RV pacing, associated with
increasing MO2 and decreased myocardial
work efficiency.
0.4 ± 1.9 µmol/min, P < 0.03) was significantly impaired in chronic pacing, whereas the lactate extraction and uptake (control: 1.7 ± 1.9 vs. chronic pacing: 9.8 ± 5.8 µeq/min, P ~ 0.07) were
significantly increased in chronic pacing (Fig.
3).
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Myocardial glycogen expression.
Figure 4 depicts PAS staining for
myocardial glycogen in control (n = 4), early
(n = 3), and late (n = 8) phases of
rapid RV pacing. Early rapid pacing (3 days) was associated with
decreased glycogen content. In contrast, chronic pacing resulted in
increased myocardial glycogen content (control: 0.9 ± 0.2 vol%
vs. chronic pacing: 5.3 ± 1.3 vol%, P < 0.05)
to values greater than those observed in controls.
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Myocardial NO production.
Myocardial NO metabolite production remained essentially unchanged
during the acute phase of rapid RV pacing (2 ± 17 to 18 ± 19 nmol NO/min at 6 h, not significant). Myocardial NO
metabolite production increased between 1 and 7 days of pacing, but the
difference did not reach statistical significance. During the chronic
phase of rapid RV pacing, NO metabolite production decreased
significantly to 174 ± 39 nmol NO/min at 28 days
(P < 0.05 from baseline). The temporal course of
impaired myocardial NO metabolite production coincided with:
1) evidence of flow-function mismatch, 2) higher levels of MO2 and
MO2 per beat (Fig.
5) and impaired myocardial work
efficiency, 3) RQ values indicative of a shift to glucose metabolism, 4) decreased FFA utilization and increased
lactate uptake, and 5) increased myocardial glycogen
content.
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Myocardial eNOS expression.
The decline in myocardial NO metabolite production in chronic pacing
was accompanied by decreased expression of eNOS, compared with control
animals, as assessed by IHC staining. In addition, myocardial eNOS
protein by Western blotting was substantially reduced in chronic pacing
compared with controls (Fig. 6).
Semiquantitative analysis of the Western blot gels from control and
cardiomyopathic dogs with the use of ODU confirmed a significant
decrease in eNOS expression in the chronic pacing (control: 2,412 ± 361 vs. chronic pacing: 1,129 ± 165 ODU, P < 0.05).
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Competitive NOS inhibition protocol.
To determine the role of NO in flow-function match observed during
acute pacing, we studied six dogs during acute pacing in the presence
and absence of NOS inhibition. The mean arterial pressure, CBF, and
MO2 responses were accentuated during
acute rapid pacing (6 h) in dogs following L-NNA, whereas
the normalized LV contractile response (LV dP/dt)/EDD was
depressed to a greater extent compared with controls (Fig.
7). Importantly, the difference was not
attributable to the increase in afterload imposed by NOS inhibition, because the response to pacing was similar between control and during phenylephrine infusion (2-5
µg · kg1 · min1 over
6 h, n = 3), designed to match the increase in
mean arterial pressure associated with L-NNA
administration.
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DISCUSSION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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In normal dogs, continuous rapid pacing imposes an excessive myocardial metabolic demand that eventuates in LV dysfunction, ventricular dilatation, and heart failure.
In the present study, we observed the hemodynamic, CBF and metabolic
dynamics throughout the 28 days of rapid pacing. We observed that the
acute onset of rapid pacing was associated with a progressive decline
in CBF per beat and MO2 per beat and
parallel declines in LV contractility over the first 24-72 h. This
was associated with preservation of myocardial NO metabolites and
depletion of myocardial glycogen stores. Thereafter, during the chronic
phase of rapid pacing there was progressive recovery of the CBF and MO2 responses, yet the LV contractile
response continued to deteriorate. This pattern was accompanied by a
progressive decrease in myocardial eNOS protein and myocardial NO
metabolites, a shift to glycolysis as the preferred metabolic
substrate, and increases in myocardial glycogen stores. Taken together,
these data suggest that normal myocardium subjected to the continuous
stress of rapid pacing develops a profile of energy sparing as opposed
to contractile efficiency, as a compensatory strategy. The early phase
is transient and accompanied by flow-function matching and ultimately
replaced by a pattern of uncoupling of flow and function.
The concept of perfusion-contraction matching involves several cardiac survival strategies, including preconditioning, hibernation, and stunning (5, 8). These concepts are usually applied to clinical or experimental circumstances in which CBF is reduced partially or completely by an obstructive lesion (3, 4, 9, 39, 40, 66). Our model was devoid of coronary artery obstruction but was predicated on continuous excessive myocardial metabolic demand. As such, a supply-to-demand imbalance was created and resulted in a pattern of reduced metabolic and contractile function observed in acute myocardial hibernation (9, 39, 40, 66). The depletion of myocardial glycogen content during the early period of pacing stress is consistent with these observations. Depletion in myocardial glycogen stores has been associated with increased myocardial NO production (7) through a cGMP-dependent mechanism (7, 42). NO has also been shown to stimulate glycogen phosphorylase (65), contributing to glycogen breakdown. In contrast, during the more chronic phase of rapid pacing, there is perfusion-contraction uncoupling associated with progressive depletion in eNOS and consequently, myocardial NO metabolites. These changes are associated with a metabolic phenotype of increased myocardial glycogen storage and glycolytic flux. This is consistent with the well-characterized regional postischemic contractile dysfunction in patients with collateral-dependent circulation (40). Under these circumstances, limited coronary flow reserve predisposes to brief periods of myocardial ischemia leading to an uncoupling of perfusion and contraction. In the model of pacing-induced heart failure in conscious dogs, our laboratory (54) has shown previously that advanced heart failure (4 wk) is associated with impaired flow reserve. As such, limited coronary flow reserve despite normal resting CBF per beat might predispose to brief repetitive myocardial dysfunction in the face of persistent tachycardia. Under this scenario, we cannot determine whether the decline in eNOS protein and activity is a cause or a consequence of brief bouts of repetitive demand-related ischemia.
Most evidence suggests that NO maintains myocardial contraction
following recovery of ischemia, i.e, stunning (13, 36, 58), and that inhibition of NOS is associated with prolonged, more intense postischemic contractile dysfunction. (3, 4, 19, 21, 26, 33, 35, 67). In contrast, myocardial hibernation has
been associated with NO upregulation, which is generally regarded an
adaptive mechanism (51), minimizing metabolic requirements (MO2) (26, 57, 71).
The role of NO in models of cardiomyopathy not associated with epicardial coronary ischemia is less well understood (10, 27, 31, 72). Studies have reported either increased (14, 18, 41, 44, 45) or decreased (1, 32, 43, 61) myocardial NO in different clinical or experimental settings of heart failure. The issue is further complicated by different temporal, spatial, and preferential isoform expression of NO synthase (eNOS vs. inducible NOS) during the evolution of LV dysfunction from a compensated to a decompensated state (11, 15, 20, 53, 55).
In a similar model, Recchia et al. (49) reported decreased
myocardial NO production in relatively late (7-21 days of rapid RV
pacing) stages of pacing-induced cardiomyopathy in conscious dogs. They
also correlated NO inhibition with increased resting MO2, implying a role of NO in modulating
metabolic requirements in this model. These observations are similar to
what has been described by Heusch et al. (25, 26) in
myocardial hibernation. Furthermore, Recchia et al. (49)
demonstrated an association between a decline in myocardial NO
production and a shift in metabolic substrate preference from FFA to
carbohydrate utilization.
Our study extends the observation of Recchia et al. (49,
50) in two ways. We investigated myocardial NO production and its associations with LV contractile function, CBF, myocardial metabolic requirements (MO2), and
substrate utilization (RQ) serially, during the entire rapid pacing
protocol. We have not limited our experiments to the late stages of
rapid RV pacing when advanced phenotypic changes of cardiomyopathy have
already ensued. Instead, we observed dynamic modifications in flow,
metabolism, and function from the onset of rapid RV pacing and by
recording events serially and at frequent intervals. We compared these
data with those obtained at a more chronic stage, similar to that
described by previous investigators (49, 50). In addition,
all measurements in our study were conducted while the dogs were
actively paced at the fixed, rapid rate (240 beats/min) and not in
their native sinus rhythm. Our study was designed to investigate the
impact of excessive chronotropic demands on metabolic requirements and myocardial function. To normalize for the inevitable influence of heart
rate differences between baseline and paced status on CBF and
MO2, we also reported rate-adjusted
parameters (CBF per beat and MO2
per beat). Similarly, to normalize for preload changes associated with
progressive ventricular dilatation, we calculated
and compared [(LV dP/dt)/LVEDD] as index of
myocardial contractility. Importantly, we extend the observations
regarding the role of NO in metabolic substrate preference to a role in mediating the dynamics of flow-function coupling. Eventually, during
the stress of the chronic rapid pacing, declines in the myocardial
production of NO were associated with flow-function mismatch. Finally,
we observed that the decline in myocardial NO production was associated
with alterations in myocardial eNOS protein.
Using this experimental design, we were able to define two discrete
temporal stages during the process of development of rapid pacing-induced DCM in conscious dogs. The first stage encompassed the
entire acute pacing phase (0-6 h) and the subsequent first 3-7 days of pacing, where a parallel decline of both contractile function [LV dP/dt, (LV dP/dt)/LVEDD] and
metabolic requirements (MO2 per beat,
CBF per beat) was observed. Myocardial NO production was preserved
during this stage. The protocol in which rapid pacing was conducted in
the presence of NOS inhibition supports the mechanistic role of NO in
mediating perfusion-contraction matching during the early phase of
rapid pacing. Importantly, the difference in response was not
attributable to increased load imposed by rapid pacing, because there
was no difference between responses in control and during phenylephrine
infusion. These findings suggest that excessive chronotropic demand
also leads to a state of myocardial hibernation, where inotropy is
reduced, to balance energy requirements. Myocardial NO during this
initial adaptive phase played a mechanistic role in maintaining this
compensatory strategy.
During the evolution of the late or chronic phase (7-28 days) of
rapid pacing, when DCM developed, a gradual increase in O2 consumption (MO2 per beat) was observed
while contractile function continued to decline. This was accompanied
by a sharp decline in myocardial NO production, which was corroborated
by downregulation of myocardial eNOS (NOS-3) protein expression.
Although myocardial RQ demonstrated a modest increase during the early
phase, the RQ increased significantly in the chronic phase, suggestive
of metabolic substrate shift from FFA toward glucose and lactate utilization. This pattern of increased
MO2 and substrate shift is also in
agreement with prior investigations that have focused on the advanced
stage of heart failure (52, 60), although these studies
were conducted after termination of pacing. The association
between defective myocardial NO production and eNOS expression supports
the hypothesis that once the regulatory function of NO is lost, the
fine balance between myocardial function and metabolic demands is
compromised. This is associated with the development of flow-function
mismatch. This sequence suggests that early hibernation is an adaptive
process (5, 51), modulated at least in part by NO, and
late stunning is a sequel to the loss of the "protective"
NO-mediated downregulation in O2 consumption and energy
requirements. Similar MO2-sparing
effects of NO donors (and, conversely, deleterious effect of NO
inhibitors) have been previously established in other settings in vitro
(37) or in vivo, with regard to attenuating the excessive
metabolic demands associated with exercise in normal conscious dogs
(2), or patients subjected to either atrial pacing
(29) or pharmacological dobutamine stress
(60). The application of the physiology of myocardial stunning to the condition of rapid pacing is further supported by the
reversible nature of the chronic pacing-induced hemodynamic changes.
Our assessment of RQ is a well-validated surrogate method for investigating metabolic substrate utilization (49, 50), and it was corroborated by biochemical data indicative of decreased FFA uptake, although these measurements were limited to the control and advanced heart failure stages (where the RQ differences were found to be the greatest). However, either method can be considered rather semiquantitative compared with more accurate radiolabeled substrate infusion techniques described by other investigators (63), which might have examined myocardial metabolic substrate utilization in more detailed terms.
In conclusion, our study demonstrated that rapid RV pacing induces a
dynamic state of progressive contractile dysfunction, characterized by
an early phase of acute hibernation (0-3 days of pacing) followed
by a later phase of progressive myocardial stunning (7-28 days).
Our study also confirmed that NO plays a pivotal role in mediating
these dynamics, because the switch to myocardial stunning physiology
with a disproportionate increase in MO2
and increased preference for glycolytic substrate utilization were
associated with significant declines in myocardial NO production and
eNOS expression.
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ACKNOWLEDGEMENTS |
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This work was supported in part by National Institutes of Health Grants HL-59070 and DA-10480, and by an American Heart Association (PA-DE Affiliate) Fellowship Grant 0020248U (to L. A. Nikolaidis).
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FOOTNOTES |
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Address for reprint requests and other correspondence: R. P. Shannon, Dept. of Medicine, Allegheny General Hospital, 320 E. North Ave., Pittsburgh, PA (E-mail: rshannon{at}wpahs.org).
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. Section 1734 solely to indicate this fact.
Received 4 June 2001; accepted in final form 23 August 2001.
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REFERENCES |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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