Am J Physiol Heart Circ Physiol 298: H594 –H600, 2010.
First published December 4, 2009; doi:10.1152/ajpheart.00908.2009.
Progressive muscle metaboreflex activation gradually decreases spontaneous
heart rate baroreflex sensitivity during dynamic exercise
Javier A. Sala-Mercado,1,3 Masashi Ichinose,1,4,5 Matthew Coutsos,1 Zhenhua Li,1,7 Dominic Fano,1
Tomoko Ichinose,1,6 Elizabeth J. Dawe,2 and Donal S. O’Leary1
Departments of 1Physiology and 2Surgical Research Services and 3Cardiovascular Research Institute, Wayne State University
School of Medicine, Detroit, Michigan; 4Human Integrative Physiology Laboratory, School of Business Administration, Meiji
University, Tokyo; 5Laboratory for Applied Human Physiology, Faculty of Human Development, Kobe University, Kobe;
6
Laboratory for Human Performance Research, Osaka International University, Osaka, Japan; and 7Department of
Cardiology, Qilu Hospital of Shandong University, Shandong, China
Sala-Mercado JA, Ichinose M, Coutsos M, Li Z, Fano D,
Ichinose T, Dawe EJ, O’Leary DS. Progressive muscle metaboreflex
activation gradually decreases spontaneous heart rate baroreflex sensitivity during dynamic exercise. Am J Physiol Heart Circ Physiol
298: H594 –H600, 2010. First published December 4, 2009;
doi:10.1152/ajpheart.00908.2009.—Ischemia of active skeletal muscle elicits a pressor response termed the muscle metaboreflex. We
tested the hypothesis that in normal dogs during dynamic exercise,
graded muscle metaboreflex activation (MMA) would progressively
attenuate spontaneous heart rate baroreflex sensitivity (SBRS). The
animals were chronically instrumented to measure heart rate (HR),
cardiac output (CO), mean and systolic arterial pressure (MAP and
SAP), and left ventricular systolic pressures (LVSP) at rest and during
mild or moderate treadmill exercise before and during progressive
MMA [via graded reductions of hindlimb blood flow (HLBF)]. SBRS
[slopes of the linear relationships (LRs) between HR and LVSP or
SAP during spontaneous sequences of ⱖ3 consecutive beats when HR
changed inversely vs. pressure] decreased during mild exercise from
the resting values (⫺5.56 ⫾ 0.86 vs. ⫺2.67 ⫾ 0.50
beats 䡠 min⫺1 䡠 mmHg⫺1, P ⬍0.05), and in addition, these LRs were
shifted upward. Progressive MMA gradually and linearly increased
MAP, CO, and HR; linearly decreased SBRS; and shifted LRs upward
and rightward to higher HR and pressures denoting baroreflex resetting. Moderate exercise caused a substantial reduction in SBRS
(⫺1.57 ⫾ 0.38 beats 䡠 min⫺1 䡠 mmHg⫺1, P ⬍0.05) and both an upward
and rightward resetting. Gradual MMA at this higher workload also
caused significant progressive increases in MAP, CO, and HR and
progressive decreases in SBRS, and the LRs were shifted to higher
MAP and HR. Our results demonstrate that gradual MMA during mild
and moderate dynamic exercise progressively decreases SBRS. In
addition, baroreflex control of HR is progressively reset to higher
blood pressure and HR in proportion to the extent of MMA.
exercise reflexes; pressor response; arterial baroreflex sensitivity
WHOLE BODY DYNAMIC EXERCISE can elicit profound changes in
autonomic activity. Two negative feedback reflexes implicated
in mediating these responses are the arterial baroreflex and the
muscle metaboreflex, which negate perturbations in arterial
pressure and blood flow to active skeletal muscle, respectively
(34 –36). The arterial baroreflex operates via modulation of
both cardiac and peripheral vascular function, but it does so
with markedly different time courses. The buffering of rapid
(seconds) changes in arterial pressure occurs via rapid, para-
Address for reprint requests and other correspondence: D. S. O’Leary, Dept.
of Physiology, Wayne State Univ. School of Medicine, 540 East Canfield Ave.,
Detroit, MI 48201 (e-mail: doleary@med.wayne.edu).
H594
sympathetically induced changes in heart rate (HR) that usually
elicits proportional changes in cardiac output (CO). Ogoh et al.
(25) showed that virtually all of the initial compensatory
responses to transient activation of the carotid sinus baroreceptors are due to reflex changes in CO. In contrast, as the
baroreflex perturbation is maintained past the initial few seconds, most of the reflex changes in arterial pressure now occur
via changes in the peripheral vasculature (25, 27).
This time-dependent difference in baroreflex mechanisms
extends from rest through heavy whole body dynamic exercise
as the baroreflex is progressively reset to a higher pressure as
workload rises (5, 6, 20, 32). Activation of skeletal muscle
afferents may be a key factor involved in this resetting (8, 11,
13, 17, 28, 30, 33). Recently, several groups have investigated
baroreflex control of HR using the spontaneous baroreflex
technique (3, 4, 15, 26, 37, 48). This technique takes advantage
of spontaneously occurring fluctuations in arterial pressure and
the resultant baroreflex-mediated changes in HR. These rapid
baroreflex responses occur solely via changes in parasympathetic activity (10, 12). Employing this spontaneous HR
baroreflex sensitivity (HR-SBRS) technique, our group has
recently observed that during dynamic exercise, muscle
metaboreflex activation (MMA) causes not only resetting of
the arterial baroreflex but also a decrease in HR-SBRS (40). In
our study we activated the muscle metaboreflex by imposing a
set decrease in skeletal muscle blood flow to the hindlimbs in
running dogs (⬃50% during mild exercise and ⬃30% during
moderate exercise). Although these imposed decreases in skeletal muscle blood flow were clearly sufficient to activate the
muscle metaboreflex, previous studies using this or similar
preparations have shown that the magnitude of the metaboreflex pressor response is proportional to the extent of decrease
in skeletal muscle blood flow (14, 43, 49). When the metaboreflex is activated, quite linear increases in arterial pressure, HR,
and CO occur with progressive further decreases in skeletal
muscle blood flow. Whether resetting of the baroreflex and
decreases in HR-SBRS occur proportionately with progressive
MMA is unknown. Also unknown is whether there is a threshold level of MMA for effects on the baroreflex, whether the
effects on the baroreflex saturate at a certain level of MMA,
and whether there are differential effects of MMA on baroreflex resetting vs. the effects on reduced HR-SBRS. The purpose of the present study was to directly address these questions in normal dogs during mild and moderate dynamic
exercise. We hypothesized that graded MMA would progres-
0363-6135/10 $8.00 Copyright © 2010 the American Physiological Society
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Submitted 24 September 2009; accepted in final form 25 November 2009
MUSCLE METABOREFLEX MODULATION OF BAROREFLEX SENSITIVITY
sively reset the arterial baroreflex and attenuate HR-SBRS in
direct proportion to the extent of MMA.
MATERIALS AND METHODS
AJP-Heart Circ Physiol • VOL
ment for data collection was performed, every animal was refamiliarized (⬃5 times) to run on the motor-driven treadmill. Before each
experiment, one animal was brought to the laboratory and allowed to
roam freely for ⬃20 min. The animal was then directed to the
treadmill. The CO and HLBF probes were connected to a flow meter
(Transonic Systems). The arterial catheter was connected to a pressure
transducer (Transpac IV; Abbott Laboratories), the LVP telemetered
signal was calibrated, and HR was computed by a cardiotachometer
triggered by the CO signal. All data were recorded on analog-todigital recording systems for subsequent off-line analyses. For a given
experimental session, data were collected at rest and then at a
randomly selected workload [mild exercise: 3.2 km/h, 0% grade
elevation; moderate exercise: 6.4 km/h, 10% grade elevation, which
causes CO to increase to ⬃40 and 70% of maximal levels (1)]. Every
animal successfully performed both experimental protocols (mild
exercise and moderate exercise) on different days, since only one
workload was performed per experimental day. All animals ran freely
with only positive verbal encouragement. Steady-state data were
recorded at rest while the animal was standing on the treadmill, during
exercise (at either mild or moderate workload) with unrestricted blood
flow to the hindlimbs, and after graded reductions of HLBF (via
partial inflations of the terminal aortic occluders) to elicit gradual
metaboreflex activation. Each level of reduction in hindlimb perfusion
was maintained until all parameters reached steady state (3–5 min).
Data analysis. Beat-to-beat CO, HLBF, HR, mean arterial pressure
(MAP), and LVP were continuously collected during each experiment. Stroke volume (SV) was calculated as CO/HR. As previously
stated, data were recorded for 3–5 min of steady state at standing rest,
free-flow exercise (mild or moderate), and each level of HLBF (each
reduction) so that each period spanned multiple respiratory cycles.
Since left ventricular systolic pressure (LVSP) is virtually identical to
systolic pressure in the aortic arch, we used LVSP as the input to the
arterial baroreflex. Spontaneous baroreflex control of HR was dynamically assessed by analyzing the beat-to-beat relationship between HR
and LVSP as previously described (40). Briefly, the beat-to-beat time
series of LVSP and HR were searched for three or more consecutive
beats in which the LVSP and HR of the following beat changed in
opposite direction (i.e., ⫺HR/⫹LVSP and ⫹HR/⫺LVSP). These
sequences were identified as baroreflex sequences. A linear regression
was applied to each individual sequence, and only those sequences in
which r2 was ⬎0.85 were accepted, and subsequently, a slope was
calculated. The mean slope of the LVSP-HR relationship, obtained by
averaging all slopes computed within a given test period, was calculated and taken as a measure of spontaneous baroreflex sensitivity for
that period. Sinoaortic baroreflex denervation virtually abolishes
baroreflex sensitivity assessed via this method, indicating that these
spontaneous reciprocal HR changes that occur as a result of changes
in arterial pressure are mediated by the baroreflex (3, 16).
The nonlinear patterns of the hemodynamic and HR-SBRS responses to graded reductions in HLBF were analyzed by plotting the
variable (e.g., MAP) vs. HLBF during free-flow exercise and at each
level of partial vascular occlusion as shown in Fig. 1. As described in
detail previously (44, 45, 49), during mild exercise, initial reductions
in hindlimb perfusion do not elicit metaboreflex responses; however,
once hindlimb perfusion is reduced below a threshold level, a pressor
response occurs. The threshold was approximated as the intersection
between two regression lines, the initial response line in which no
reflex responses occurred during the initial reductions in hindlimb
perfusion and the pressor response line in which further reductions in
hindlimb perfusion elicited a reflex pressor response. During moderate
exercise, often no apparent threshold exists and the initial reduction in
hindlimb perfusion elicits reflex responses. If no threshold was apparent, then the threshold was ascribed as the free-flow value of
hindlimb perfusion.
Statistical analysis. By utilizing the averaged responses for each
animal, statistical analyses were performed with Systat software
(Systat 11.0). An ␣ level of P ⬍ 0.05 was set to determine statistical
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Experiments were performed on seven adult, mongrel dogs (weight
⬃20 –25 kg) of either sex (4 males, 3 females). The protocols
employed in the present study were reviewed and approved by the
Wayne State University Animal Investigation Committee and conform with the National Institutes of Health guidelines.
Surgical preparation and procedures. On arrival at the laboratory,
all animals were accustomed to human handling and during ⬃10
sessions were individually trained to comfortably run freely on a
motor-driven treadmill at different speeds. Once the animals were
successfully trained to run on the treadmill, two surgical procedures
were performed on each animal (left thoracotomy and left flank
abdominal surgery separated by at least 10 days).
Before each surgery, for tranquilization, the animals received an
intramuscular injection of acepromazine (0.2 mg/kg). The dogs were
anesthetized with thiopental sodium (25 mg/kg iv). After endotracheal
intubation, anesthesia was maintained with isoflurane gas (1–3%).
Before the surgery, the animals received cefazolin (antibiotic, 500 mg
iv), carprofen (analgesic, 2.0 mg/kg iv), and buprenorphine (analgesic,
0.1 mg/kg im), and a 72-h transdermal fentanyl patch was applied
(analgesic, 125–175 ␮g/h). In addition, before the left thoracotomy,
selective intercostal nerve blocks were performed with bupivacaine
hydrochloride (2.0 mg/kg). After each surgical procedure, the dogs
received a second intravenous dose of cefazolin (500 mg), and
antibiotics were continued for the length of the experimental protocol
at an oral dose of cephalexin (30 mg䡠kg⫺1 䡠12 h⫺1) to prevent infections.
Moreover, after each surgical procedure, for the following 12 h, buprenorphine and acepromazine were administered (0.05 and 0.5 mg/kg iv,
respectively) as needed to control any type of discomfort. Thereafter,
carprofen was administered orally (4 mg䡠kg⫺1 䡠day⫺1) for 10 days.
In the first surgical procedure, under sterile conditions, a left
thoracotomy (4th intercostal space) was performed. A fully implantable telemetered blood pressure transducer (model PAD-70; Data
Sciences International) was placed subcutaneously 10 cm caudal to
the thoracotomy incision. Its catheter was tunneled into the thoracic
cavity through the seventh intercostal space and located inside the left
ventricle for measuring left ventricular pressure (LVP). To measure
CO, a 20-mm blood flow transducer (Transonic Systems) was placed
around the ascending aorta. For studies unrelated to the present
investigation, three stainless steel ventricular pacing electrodes (OFlexon; Ethicon) were sutured to the right ventricular free wall,
vascular occluders were placed on the superior and inferior venous
cava, and two pairs of sonomicrometry crystals were placed on the left
ventricular endocardium. The pericardium was reapproximated
loosely, and the chest was closed in layers. After at least 10 days
(recovery period), a second surgical procedure (left abdominal retroperitoneal surgery) was performed on each dog. A 10-mm blood flow
transducer (Transonic Systems) was placed on the terminal aorta for
measuring hindlimb blood flow (HLBF). All side branches between
the iliac arteries and the flow probe were ligated and severed, and two
10-mm vascular occluders (DocXS Biomedical Products) were placed
on the terminal aorta distal to the flow probe to allow us to reduce flow
to the hindlimbs during the experiments (via partial external inflation)
and elicit the muscle metaboreflex. In addition, a catheter was placed
in a lumbar side branch of the aorta above the flow probe and
occluders to monitor arterial pressure. All flow probe cables, pacing
wires, vascular occluder tubings, and the aortic catheter were tunneled
subcutaneously and exteriorized between the scapulae at the end of its
corresponding surgical procedure.
Experimental procedures. All experiments were performed individually and after the animals had fully recovered from the surgeries
and were alert, active, afebrile, and of good appetite. After the animals
had fully recovered from the instrumentation and before an experi-
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MUSCLE METABOREFLEX MODULATION OF BAROREFLEX SENSITIVITY
significance. One-way analysis of variance for repeated measures was
used for comparing hemodynamic data obtained at rest and during
exercise under free-flow conditions at threshold and at maximal levels
of HLBF reduction during mild and moderate workloads. If a significant interaction term was found, a test for simple effects post hoc
analysis (C-Matrix) was performed to determine significant group
mean differences. We compared the slope of the linear regression line
between HR-SBRS and HLBF after threshold between mild and
moderate exercise using a paired t-test. Data are means ⫾ SE.
RESULTS
Figure 2 shows data from one animal during both protocols
(mild and moderate exercise). From rest to mild exercise, the
relationship between HR and LVSP was shifted upward and
the linear relationship was less steep, which represents a
decrease in SBRS. With the initial partial occlusion of the
terminal aorta and imposed reductions in HLBF, no metaboreflex pressor response was engaged and there was little change
in the HR-LVSP relationship, and thus HR-SBRS remained
essentially constant. However, once HLBF was reduced below
the metaboreflex threshold, HR and blood pressure increased.
With the generation of the pressor response, there was a
Fig. 2. A and C: prevailing heart rate (HR)
and left ventricular systolic pressure (LVSP)
with corresponding mean slopes at rest, during free-flow exercise, and during exercise ⫹
HLBF step reductions in 1 animal during
mild (A) and moderate exercise (C). bpm,
Beats/min. B and D: in the same animal, the
heart rate spontaneous baroreflex sensitivity
level (HR-SBRS) during free-flow exercise
and during exercise ⫹ HLBF step reductions
in mild (B) and moderate (D) exercise.
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Fig. 1. Example of the nonlinear pattern of 1 animal’s hemodynamic (mean
arterial pressure) response to graded reductions in hindlimb blood flow
(HLBF). Free-flow ex, free-flow exercise.
progressive shifting of the HR-LVSP relationship upward and
to the right, and the slope progressively flattened. This resulted
in a quite linear relationship between HR-SBRS and HLBF as
the metaboreflex became progressively more engaged. Similar
responses were observed during moderate exercise with even
more pronounced falls in HR-SBRS to ⬃20% of resting levels
at maximal metaboreflex activation. At both workloads, the
decrease in the strength of HR-SBRS was linearly related to the
reduction in HLBF over the entire range of metaboreflex
activation. As we have previously shown (40), neither exercise
nor metaboreflex activation affected the number of SBRS
occurrences observed per minute (rest, ⫺8.3 ⫾ 1.8; mild
exercise, ⫺8.5 ⫾ 1.3; mild exercise ⫹ MMA, ⫺10.3 ⫾ 1.8;
moderate exercise, ⫺8.3 ⫾ 1.0; moderate exercise ⫹ MMA,
⫺9.5 ⫾ 2.2; ANOVA, P ⬎ 0.05).
Figure 3 shows HR, SV, CO, MAP, LVSP, and HR-SBRS at
rest and during mild and moderate exercise without any imposed reductions in HLBF (free flow) at threshold and at
approximately maximum activation of the muscle metaboreflex. As expected and in agreement with previous studies (41,
42) from rest to mild exercise, although MAP and LVSP did
not change significantly, we observed increases in HR and SV
(as a result in CO) and also a rise in HLBF. Thus the increase
in CO is offset by a rise in total vascular conductance due to the
active vasodilation in the exercising skeletal muscles, resulting
in little change in MAP. In addition, a substantial decrease in
HR-SBRS occurred compared with standing rest. MMA at this
workload generated a progressive rise in MAP and LVSP, a
significant tachycardia, and a small rise in SV, and thus CO
substantially increased. Moreover, MMA during mild exercise
caused considerable changes in HR-SBRS with a pattern very
similar to the changes induced in most other hemodynamic
parameters; that is, a clear threshold existed for the effects on
HR-SBRS, and the changes progressed linearly with progressive reductions in HLBF and did not saturate despite marked
reductions in HLBF that were the maximum we could impose
and obtain steady-state data. Similar results were observed
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MUSCLE METABOREFLEX MODULATION OF BAROREFLEX SENSITIVITY
Table 1. Average slope of the linear regressions lines
between HLBF and HR-SBRS during mild and moderate
exercise
HR-SBRS/HLBF, beats 䡠 min⫺1 䡠 mmHg⫺1/
(l 䡠 min⫺1)
Correlation coefficient
Mild Exercise
Moderate
Exercise
⫺4.15⫾1.09
⫺0.98⫾0.01
⫺1.12⫾0.29*
⫺0.96⫾0.01
Values are means ⫾ SE of average slope of linear regression lines for all
animals between hindlimb blood flow (HLBF) and spontaneous heart rate
baroreflex sensitivity (HR-SBRS) during mild and moderate exercise. The high
correlation coefficient denotes the close linear relationship between the two
variables. *P ⬍ 0.05, mild vs. moderate exercise.
Fig. 3. Average HR, stroke volume (SV), cardiac output (CO), mean arterial
pressure (MAP), LVSP, and HR-SBRS at rest and during mild and moderate
exercise without any imposed reductions in HLBF (free flow) at threshold and
approximately maximum activation of the muscle metaboreflex (maximal
possible HLBF reduction). *P ⬍ 0.05, significant increase from rest to mild or
moderate exercise. †P ⬍ 0.05, significant increase from mild or moderate
exercise threshold to approximately maximum activation of the muscle
metaboreflex.
during moderate exercise with the exception that the prevailing
level of HLBF was much closer to metaboreflex threshold (and
no threshold was observed in some experiments). Again, the
pattern of the changes HR-SBRS with metaboreflex activation
mirrored those that occurred in most hemodynamic parameters
(e.g., HR, CO, MAP, LVSP). Table 1 shows the average slope
(of all animals) of the linear regression line during mild and
moderate exercise between HLBF and HR-SBRS. The average
AJP-Heart Circ Physiol • VOL
Fig. 4. HLBF at metaboreflex threshold for HR-SBRS, HR, CO, LVSP, and
MAP for mild (top) and moderate (bottom) exercise. Note that no statistical
difference was found.
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slope after muscle metaboreflex threshold during mild exercise
was about ⫺4.15, meaning that for every liter per minute
decrease in HLBF beyond metaboreflex threshold, HR-SBRS
decreased 4.15 beats 䡠min⫺1 䡠mmHg⫺1. The high value of the
correlation coefficient indicates a very close linear relationship
between these two variables. Moreover, although the slope was
significantly reduced during moderate exercise, there was still
a very close relationship between HLBF and HR-SBRS as
shown by a high correlation coefficient value in this condition.
To compare whether the effects of MMA on spontaneous
baroreflex control of HR occurred concurrently with the effects
of the muscle metaboreflex on the other hemodynamic parameters, we compared the HLBF at metaboreflex threshold, which
is separately calculated for each of the variables. Figure 4
shows that at either workload, there was no significant difference in HLBF at the calculated MMA threshold for HR-SBRS,
HR, CO, LVSP, and MAP, indicating that the muscle
metaboreflex modulates the heart, blood pressure, and the
baroreflex in concert. In addition, for each experiment, the
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MUSCLE METABOREFLEX MODULATION OF BAROREFLEX SENSITIVITY
relationship between HR-SBRS and LVSP beyond metaboreflex threshold was also quite linear at both workloads (Fig. 5).
On average, HR-SBRS decreased ⬃10% for every 10 mmHg
increase in LVSP during steady-state metaboreflex activation
at both workloads.
DISCUSSION
Fig. 5. Relationship between HR-SBRS and LVSP beyond metaboreflex
threshold for each experiment at mild (top) and moderate (bottom) workloads.
The thick solid line represents the average of the slopes and intercepts of the
individual lines.
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To our knowledge, this is the first study to show that 1) graded
MMA during dynamic exercise not only progressively resets the
arterial baroreflex operating point but also gradually decreases
HR-SBRS in direct proportion to the extent of muscle metaboreflex activation; 2) the metaboreflex threshold levels of HLBF for
effects on HR-SBRS were not different between the hemodynamic parameters and the arterial baroreflex, and the effects of the
muscle metaboreflex on HR-SBRS occurred proportionately with
the reflex effects on hemodynamic parameters such as HR, CO,
and MAP; 3) the effects of MMA on HR-SBRS occur progressively with graded metaboreflex activation and do not saturate
over a wide range of MMA; and 4) the effects of MMA on
baroreflex resetting were coincident with the effects on reduced
HR-SBRS.
Feedback reflexes responsible for autonomic modulation
during whole body dynamic exercise. Overall, in addition to the
feedforward role of central command, two negative feedback
reflexes likely responsible for the autonomic modulation during dynamic exercise are the arterial baroreflex and reflexes
arising from activation of skeletal muscle afferents (mechanosensitive and metabosensitive). A fall in skeletal muscle oxygen delivery and flow leads to accumulation of metabolic
by-products within the active muscle that stimulate group III
and IV afferent neurons, which evokes reflex changes in
autonomic nerve activity and release of vasoactive hormones,
termed the muscle metaboreflex. This reflex can trigger a
significant increase in CO and/or also cause peripheral vasoconstriction, which in turn raises MAP. The arterial baroreflex
is the primary short-term regulator of arterial blood pressure by
altering peripheral vasoconstriction and cardiac output, via
adjustments of sympathetic and parasympathetic nerve activity
(38). In addition, it is well known that the baroreflex control of
HR and blood pressure is reset during exercise. One known
mechanism responsible for this baroreflex resetting during
exercise is the feedback from skeletal muscle afferents. In
animal and human studies, it has been shown that neural
feedback from the skeletal muscles modulates the arterial
baroreflex function (19, 28, 46). Employing the HR-SBRS
technique to study the interaction between these two reflexes,
our group recently observed that during mild and moderate
dynamic exercise, MMA causes not only resetting of the
arterial baroreflex but also a decrease in HR-SBRS (40). In that
study, MMA was obtained via a one-step reduction in HLBF
(reduced to ⬃50% and ⬃70% of exercising blood flow level
for mild and moderate workload, respectively). However, it
remained unknown whether the muscle metaboreflex affects
the arterial baroreflex in fashion similar to the cardiovascular
effects of the metaboreflex on hemodynamic parameters. Previous studies from our and other laboratories have shown that
the stimulus-response relationship between O2 delivery or
blood flow to active skeletal muscle is quite linear once beyond
threshold and that no saturation occurs at submaximal workloads over a wide range of metaboreflex activation (14, 43, 49).
The question remained whether the effects of MMA on HRSBRS would show a similar trend, and we found that this was
the case. Progressive MMA caused progressive increases in
HR and MAP and progressively reset the HR-LVSP relationship upward and to the right with a decrease in baroreflex HR
sensitivity as indexed by the spontaneous method. The effects
of MMA on HR-SBRS were quite linear when analyzed as
both the relationship between the imposed reductions in HLBF
and HR-SBRS and the metaboreflex-induced increases in
LVSP vs. HR-SBRS. The effects of the muscle metaboreflex
became apparent at the same threshold level of HLBF as all
other hemodynamic parameters, and no saturation of the effects of MMA on HR-SBRS was evident over the range of
MMA employed (which reflected the largest activation of the
muscle metaboreflex we could obtain in which steady state
could be achieved with the animal freely exercising on the
treadmill). Thus the present study shows that the muscle
metaboreflex control of cardiac function at either mild or
moderate exercise is induced and progresses linearly in concert
with baroreflex modulation, since HLBF values at the threshold for HR-SRBRS, CO, MAP, LVSP, and HR were not
different and all responded linearly with further reductions in
HLBF.
Muscle metaboreflex and baroreflex interaction. It is highly
likely that muscle metaboreflex and baroreflex interaction occurs centrally. Previous studies have shown that central modulation of the baroreflex circuitry may be responsible for
mediating resetting of the baroreflex during exercise (7, 8, 11,
17, 19, 30). Among the different possible central sites that are
part of the central baroreflex arc, the nucleus tractus solitarii
(NTS) is a region where an interaction between these two
reflexes can take place, for this nucleus, apart for being known
MUSCLE METABOREFLEX MODULATION OF BAROREFLEX SENSITIVITY
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ACKNOWLEDGMENTS
We thank Jody Helme-Day and Erin Krengel for expert technical assistance
and care of the animals.
GRANTS
This research was supported by National Heart, Lung, and Blood Institute
Grant HL-55473.
REFERENCES
1. Augustyniak RA, Collins HL, Ansorge EJ, Rossi NF, O’Leary DS.
Severe exercise alters the strength and mechanisms of the muscle
metaboreflex. Am J Physiol Heart Circ Physiol 280: H1645–H1652, 2001.
2. Barraco RA. Nucleus of the Solitary Tract. Boca Raton, FL: CRC, 1993.
3. Bertinieri G, Di Rienzo M, Cavallazzi A, Ferrari AU, Pedotti A,
Mancia G. Evaluation of baroreceptor reflex by blood pressure monitoring in unanesthetized cats. Am J Physiol Heart Circ Physiol 254: H377–
H383, 1988.
4. Burger HR, Chandler MP, Rodenbaugh DW, Dicarlo SE. Dynamic
exercise shifts the operating point and reduces the gain of the arterial
baroreflex in rats. Am J Physiol Regul Integr Comp Physiol 275: R2043–
R2048, 1998.
5. Coote JH, Dodds WN. The baroreceptor reflex and the cardiovascular
changes associated with sustained muscular contraction in the cat. Pflügers
Arch 363: 167–173, 1976.
6. Dicarlo SE, Bishop VS. Onset of exercise shifts operating point of arterial
baroreflex to higher pressures. Am J Physiol Heart Circ Physiol 262:
H303–H307, 1992.
7. Fadel PJ, Ogoh S, Watenpaugh DE, Wasmund W, Olivencia-Yurvati
A, Smith ML, Raven PB. Carotid baroreflex regulation of sympathetic
nerve activity during dynamic exercise in humans. Am J Physiol Heart
Circ Physiol 280: H1383–H1390, 2001.
8. Gallagher KM, Fadel PJ, Smith SA, Norton KH, Querry RG, Olivencia-Yurvati A, Raven PB. Increases in intramuscular pressure raise
arterial blood pressure during dynamic exercise. J Appl Physiol 91:
2351–2358, 2001.
9. Hammond RL, Augustyniak RA, Rossi NF, Lapanowski K, Dunbar
JC, O’Leary DS. Alteration of humoral and peripheral vascular responses
during graded exercise in heart failure. J Appl Physiol 90: 55– 61, 2001.
10. Iellamo F. Neural mechanisms of cardiovascular regulation during exercise. Auton Neurosci 90: 66 –75, 2001.
11. Iellamo F, Legramante JM, Raimondi G, Peruzzi G. Baroreflex control
of sinus node during dynamic exercise in humans: effects of central
command and muscle reflexes. Am J Physiol Heart Circ Physiol 272:
H1157–H1164, 1997.
12. Iellamo F, Sala-Mercado JA, Ichinose M, Hammond RL, Pallante M,
Ichinose TK, Stephenson LW, O’Leary DS. Spontaneous baroreflex
control of heart rate during exercise and muscle metaboreflex activation in
heart failure. Am J Physiol Heart Circ Physiol 293: H1929 –H1936, 2007.
13. Kaufman MP, Rybicki KJ, Waldrop TG, Mitchell JH. Effect on
arterial pressure of rhythmically contracting the hindlimb muscles of cats.
J Appl Physiol 56: 1265–1271, 1984.
14. Kim JK, Sala-Mercado JA, Rodriguez J, Scislo TJ, O’Leary DS.
Arterial baroreflex alters strength and mechanisms of muscle metaboreflex
during dynamic exercise. Am J Physiol Heart Circ Physiol 288: H1374 –
H1380, 2005.
15. Kuo TBJ, Yang CCH. Sleep-related changes in cardiovascular neural
regulation in spontaneously hypertensive rats. Circulation 112: 849 – 854,
2005.
16. Legramante JM, Raimondi G, Massaro M, Cassarino S, Peruzzi G,
Iellamo F. Investigating feed-forward neural regulation of circulation
from analysis of spontaneous arterial pressure and heart rate fluctuations.
Circulation 99: 1760 –1766, 1999.
17. McIlveen SA, Hayes SG, Kaufman MP. Both central command and
exercise pressor reflex reset carotid sinus baroreflex. Am J Physiol Heart
Circ Physiol 280: H1454 –H1463, 2001.
18. McWilliam PN, Shepheard SL. A GABA-mediated inhibition of neurones in the nucleus tractus solitarius of the cat that respond to electrical
stimulation of the carotid sinus nerve. Neurosci Lett 94: 321–326, 1988.
19. McWilliam PN, Yang T, Chen LX. Changes in the baroreceptor reflex at
the start of muscle contraction in the decerebrate cat. J Physiol 436:
549 –558, 1991.
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for participating in many viscerosensory systems, receives
inputs from both the baroreceptor afferents and spinal somatosensory input and contains a complex network of excitatory
and inhibitory interneurons (2). Indeed, neural feedback from
skeletal muscle afferents have been shown to activate a
GABAergic mechanism within the NTS that reduces the rapid
bradycardic responses to transient excitability of baroreceptor
activation (18, 31). A different or additional possible central
site for this reflexes interaction is the rostral ventrolateral
medulla, for it has been shown that skeletal muscle afferents
can cause direct excitation of sympathetic premotor neurons in
this brain stem region (22, 29, 47). Another possibility for the
muscle metaboreflex and baroreflex interaction is that muscle
metaboreflex-induced increase in plasma norepinephrine attenuates baroreflex modulation of HR (21), since it has previously
been shown that MMA elicits a rise in plasma norepinephrine
and that high plasma norepinephrine concentration attenuates
parasympathetic control of HR (9, 23). However, whether
more central regions are involved, as well as the cellular
mechanisms responsible for the interaction, are still uncertain.
Limitations of the study. Our approach to evaluate the
arterial baroreflex control of HR based on spontaneous fluctuations in blood pressure and HR has advantages and disadvantages, which have previously been described in detail (39, 40).
Briefly, the spontaneous baroreflex technique enables a qualitative and quantitative estimate of the baroreceptor-cardiac
response relationships during spontaneous blood pressure fluctuations without the necessity of any mechanical or pharmacological intervention. Sympathostimulatory reflexes by
stretch of cardiac chambers after phenylephrine-induced increase of afterload or a direct ␤-adrenergic stimulation at the
sinus node level by high doses of the drug may affect baroreflex sensitivity determination. Alternatively, the autonomic
mechanisms mediating these rapid baroreflex-induced changes
in HR are likely only parasympathetic in nature (24, 26), and
in addition, this approach only examines the baroreflex sensitivity over a relatively modest range of pressure, which therefore does not allow the calculation of the entire sigmoidal
baroreflex stimulus-response relationship. It is possible, if not
likely, that at least a portion of the reduction in HR-SBRS with
exercise and metaboreflex activation stemmed from a shift in
the operating point of the baroreflex from the high slope
section near the middle of the stimulus-response relationship
toward a flatter part of the sigmoid curve (33). Previous studies
have shown that HR-SBRS is virtually abolished after arterial
baroreceptor denervation, which shows the reflex nature of the
HR responses (3, 16).
Conclusions. In conclusion, MMA during mild and moderate dynamic exercise progressively resets the arterial baroreflex
to higher blood pressure and HR in direct proportion to the
extent of MMA. As the muscle metaboreflex is engaged, it
simultaneously resets and progressively depresses HR-SBRS
in concert with the increases in HR, CO, and arterial blood
pressure. Thus the central interactions occur concurrently with
the efferent responses. The fall in SBRS with metaboreflex
activation indicates that the ability of the baroreflex to rapidly
respond to perturbations in arterial pressure via changes in HR
progressively decreases as muscle ischemia ensues. This may
be particularly important during exercise in patients with claudication and in other patients with high sympathetic activity
such as those with heart failure and hypertension.
H599
H600
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AJP-Heart Circ Physiol • VOL
36. Rowell LB, O’Leary DS. Reflex control of the circulation during exercise: chemoreflexes and mechanoreflexes. J Appl Physiol 69: 407– 418,
1990.
37. Ryan S, Ward S, Heneghan C, McNicholas WT. Predictors of decreased
spontaneous baroreflex sensitivity in obstructive sleep apnea syndrome.
Chest 131: 1100 –1107, 2007.
38. Sagawa K. Baroreflex control of systemic arterial pressure and vascular
bed. In: Handbook of Physiology. The Cardiovascular System. Peripheral
Circulation. Bethesda, MD: Am. Physiol. Soc., 1983, sect. 2, vol. III, pt.
2, p. 453– 496.
39. Sala-Mercado JA, Ichinose M, Hammond RL, Coutsos M, Ichinose T,
Pallante M, Iellamo F, O’Leary DS. Spontaneous baroreflex control of
heart rate versus cardiac output: altered coupling in heart failure. Am J
Physiol Heart Circ Physiol 294: H1304 –H1309, 2008.
40. Sala-Mercado JA, Ichinose M, Hammond RL, Ichinose TK, Pallante
M, Stephenson LW, O’Leary DS, Iellamo F. Muscle metaboreflex
attenuates spontaneous heart rate baroreflex sensitivity during dynamic
exercise. Am J Physiol Heart Circ Physiol 292: H2867–H2873, 2007.
41. Sala-Mercado JA, Hammond RL, Kim JK, McDonald PJ, Stephenson
LW, O’Leary DS. Heart failure attenuates muscle metaboreflex control of
ventricular contractility during dynamic exercise. Am J Physiol Heart Circ
Physiol 292: H2159 –H2166, 2007.
42. Sala-Mercado JA, Hammond RL, Kim JK, Rossi NF, Stephenson LW,
O’Leary DS. Muscle metaboreflex control of ventricular contractility
during dynamic exercise. Am J Physiol Heart Circ Physiol 290: H751–
H757, 2006.
43. Sheriff DD, Augustyniak RA, O’Leary DS. Muscle chemoreflex-induced increases in right atrial pressure. Am J Physiol Heart Circ Physiol
275: H767–H775, 1998.
44. Sheriff DD, O’Leary DS, Scher AM, Rowell LB. Baroreflex attenuates
pressor response to graded muscle ischemia in exercising dogs. Am J
Physiol Heart Circ Physiol 258: H305–H310, 1990.
45. Sheriff DD, Wyss CR, Rowell LB, Scher AM. Does inadequate oxygen
delivery trigger pressor response to muscle hypoperfusion during exercise? Am J Physiol Heart Circ Physiol 253: H1199 –H1207, 1987.
46. Smith SA, Querry RG, Fadel PJ, Gallagher KM, Strømstad M, Kojiro
I, Raven PB, Secher NH. Partial blockade of skeletal muscle somatosensory afferents attenuates baroreflex resetting during exercise in humans.
J Physiol 551: 1013–1021, 2003.
47. Stornetta RL, Morrison SF, Ruggiero DA, Reis DJ. Neurons of rostral
ventrolateral medulla mediate somatic pressor reflex. Am J Physiol Regul
Integr Comp Physiol 256: R448 –R462, 1989.
48. Waki H, Kasparov S, Katahira K, Shimizu T, Murphy D, Paton JF.
Dynamic exercise attenuates spontaneous baroreceptor reflex sensitivity in
conscious rats. Exp Physiol 88: 517–526, 2003.
49. Wyss CR, Ardell JL, Scher AM, Rowell LB. Cardiovascular responses
to graded reductions in hindlimb perfusion in exercising dogs. Am J
Physiol Heart Circ Physiol 245: H481–H486, 1983.
298 • FEBRUARY 2010 •
www.ajpheart.org
Downloaded from http://ajpheart.physiology.org/ by 10.220.33.1 on September 30, 2016
20. Melcher A, Donald DE. Maintained ability of carotid baroreflex to
regulate arterial pressure during exercise. Am J Physiol Heart Circ Physiol
241: H838 –H849, 1981.
21. Miyamoto T, Kawada T, Takaki H, Inagaki M, Yanagiya Y, Jin Y,
Sugimachi M, Sunagawa K. High plasma norepinephrine attenuates the
dynamic heart rate response to vagal stimulation. Am J Physiol Heart Circ
Physiol 284: H2412–H2418, 2003.
22. Morrison SF, Reis DJ. Reticulospinal vasomotor neurons in the RVL
mediate the somatosympathetic reflex. Am J Physiol Regul Integr Comp
Physiol 256: R1084 –R1097, 1989.
23. Nishiyasu T, Tan N, Morimoto K, Sone R, Murakami N. Cardiovascular and humoral responses to sustained muscle metaboreflex activation
in humans. J Appl Physiol 84: 116 –122, 1998.
24. O’Leary DS, Seamans DP. Effect of exercise on autonomic mechanisms
of baroreflex control of heart rate. J Appl Physiol 75: 2251–2257, 1993.
25. Ogoh S, Fadel PJ, Nissen P, Jans O, Selmer C, Secher NH, Raven PB.
Baroreflex-mediated changes in cardiac output and vascular conductance
in response to alterations in carotid sinus pressure during exercise in
humans. J Physiol 550: 317–324, 2003.
26. Ogoh S, Fisher JP, Dawson EA, White MJ, Secher NH, Raven PB.
Autonomic nervous system influence on arterial baroreflex control of heart
rate during exercise in humans. J Physiol 566: 599 – 611, 2005.
27. Olivier NB, Stephenson RB. Characterization of baroreflex impairment
in conscious dogs with pacing-induced heart failure. Am J Physiol Regul
Integr Comp Physiol 265: R1132–R1140, 1993.
28. Potts JT, Hand GA, Li JH, Mitchell JH. Central interaction between
carotid baroreceptors and skeletal muscle receptors inhibits sympathoexcitation. J Appl Physiol 84: 1158 –1165, 1998.
29. Potts JT, Lee SM, Anguelov PI. Tracing of projection neurons from the
cervical dorsal horn to the medulla with the anterograde tracer biotinylated
dextran amine. Auton Neurosci 98: 64 – 69, 2002.
30. Potts JT, Mitchell JH. Rapid resetting of carotid baroreceptor reflex by
afferent input from skeletal muscle receptors. Am J Physiol Heart Circ
Physiol 275: H2000 –H2008, 1998.
31. Potts JT, Paton JFR, Mitchell JH, Garry MG, Kline G, Anguelov PT,
Lee SM. Contraction-sensitive skeletal muscle afferents inhibit arterial
baroreceptor signalling in the nucleus of the solitary tract: role of intrinsic
GABA interneurons. Neuroscience 119: 201–214, 2003.
32. Potts JT, Shi XR, Raven PB. Carotid baroreflex responsiveness during
dynamic exercise in humans. Am J Physiol Heart Circ Physiol 265:
H1928 –H1938, 1993.
33. Raven PB, Fadel PJ, Ogoh S. Arterial baroreflex resetting during exercise: a current perspective. Exp Physiol 91: 37– 49, 2006.
34. Rowell LB. Reflex control of the circulation during exercise. Int J Sports
Med 13, Suppl 1: S25–S27, 1992.
35. Rowell LB. Human Circulation Regulation During Physical Stress. New
York: Oxford University Press, 1986.