Download Muscle metaboreflex increases ventricular performance in

Muscle metaboreflex increases ventricular
performance in conscious dogs
DONAL S. O’LEARY AND ROBERT A. AUGUSTYNIAK
Department of Physiology, Wayne State University School of Medicine, Detroit, Michigan 48201
skeletal muscle ischemia; sympathetic nervous system; muscle
afferents; arterial blood pressure; skeletal muscle blood flow
WHEN OXYGEN delivery to active skeletal muscle is
insufficient for ongoing metabolic demands, metabolites accumulate and stimulate group III and IV afferent neurons within the active muscle, eliciting a reflex
increase in efferent sympathetic nerve activity and
systemic arterial pressure (SAP) termed the muscle
metaboreflex (1, 5, 7–9, 11, 16). Data from Wyss et al.
(18) indicate that when this reflex is activated in
conscious dogs via progressive reductions in hindlimb
perfusion during treadmill exercise, a considerable
fraction of the metaboreflex pressor response is due to
an increase in cardiac output (CO) in addition to
peripheral vasoconstriction. Other studies utilizing
anesthetized animal models have shown that static
muscle contraction elicits increases in the first derivative of left ventricular pressure (LV dP/dt) (14), indicating that reflexes arising from skeletal muscle afferents
can induce significant increases in ventricular contractility, although with static muscle contraction it can be
difficult to differentiate the relative roles of mechanosen-
H220
sitive vs. metabosensitive afferents in mediating these
responses.
Previously, O’Leary (7) observed that in conscious
dogs during dynamic exercise, the majority of the
metaboreflex-induced increase in heart rate (HR) occurs via an increase in sympathetic activity to the
heart. In that study, b-adrenergic blockade markedly
decreased the tachycardic response to hindlimb ischemia. In addition, after b-adrenergic blockade the magnitude of the metaboreflex-induced pressor response
was reduced, leading the author to conclude that the
reflex tachycardia contributes importantly to the increase in arterial blood pressure. However, CO was not
measured in that study. Data from Wyss et al. (18)
indicate that muscle metaboreflex-induced increases in
CO occur via increases in HR with little change in
stroke volume (SV). Inasmuch as White et al. (17)
demonstrated using conscious dogs that changes in HR
over a wide range (from ,100 to 180 beats/min) per se
do not elicit changes in CO due to the inverse relationship between HR and SV, muscle metaboreflex-induced
tachycardia by itself likely would not elicit substantial
increases in CO. Thus the data from Wyss et al. (18)
indirectly allude to a reflex increase in ventricular
performance; that is, muscle metaboreflex-induced increases in ventricular contractility could offset the
reduction in ventricular filling time (concomitant with
the increases in HR), maintaining SV essentially constant despite the tachycardia and thereby increasing
CO. Alternatively, activation of the muscle metaboreflex could elicit increases in filling pressure that could
maintain SV via a Frank-Starling mechanism. Thus
the present study was designed to determine whether
the muscle metaboreflex elicits substantial increases in
ventricular performance.
METHODS
All experiments were performed using five conscious dogs
of either gender (22–28 kg) trained to run on a motor-driven
treadmill. All procedures were reviewed and approved by the
Institutional Animal Care Committee and conformed to National Institutes of Health guidelines.
Surgical preparation. The animals were prepared in a
series of surgical sessions with at least 1 wk of recovery
between surgeries and between the last surgery and the first
experiment. For all procedures anesthesia was induced with
Pentothal Sodium and maintained with isoflurane. Immediately before and at the completion of each surgery the
animals were treated with cefazolin (500 mg iv) and then with
cephalexin (30 mg/kg by mouth, 2 times/day) for at least 1 wk
postoperatively. Postoperative discomfort was controlled using acepromazine (0.1 mg/kg im) and buprenorphine (0.015
mg/kg iv) whenever deemed necessary.
In the first procedure, through a right thoracotomy at the
fourth intercostal space, a blood flow transducer (Transonic
0363-6135/98 $5.00 Copyright r 1998 the American Physiological Society
Downloaded from http://ajpheart.physiology.org/ by 10.220.32.247 on May 12, 2017
O’Leary, Donal S., and Robert A. Augustyniak. Muscle
metaboreflex increases ventricular performance in conscious
dogs. Am. J. Physiol. 275 (Heart Circ. Physiol. 44): H220–
H224, 1998.—Ischemia of active skeletal muscle stimulates
neuronal afferents within the muscle, which elicits a reflex
increase in systemic arterial pressure (SAP), heart rate (HR),
and cardiac output (CO) termed the muscle metaboreflex. We
investigated whether activation of the muscle metaboreflex
elicits increases in ventricular performance using conscious,
chronically instrumented dogs trained to run on a treadmill
(3.2 km/h, 0% grade). The muscle metaboreflex was activated
via progressive partial vascular occlusion of the terminal
aorta during control experiments and with HR maintained
constant via a pacemaker connected to ventricular electrodes
(225 beats/min). In control experiments, hindlimb ischemia
elicited substantial increases in SAP, HR, and CO (153.9 6
4.3 mmHg, 132.4 6 4.5 beats/min, and 11.57 6 0.22 l/min,
respectively; all changes P , 0.05), whereas stroke volume
(SV) remained unchanged with reflex activation (control
45.9 6 2.3 vs. 46.1 6 2.4 ml, P . 0.05). During metaboreflex
activation at constant HR, SV significantly increased such
that the increases in CO and SAP were not significantly
different from control experiments (11.77 6 0.56 l/min and
157.4 6 3.8 mmHg, P . 0.05 vs. control experiments). No
significant change in central venous pressure occurred in
either experiment, indicating no Frank-Starling effect on SV.
We conclude that muscle metaboreflex-induced increases in
ventricular contractility act to sustain SV despite decreases
in ventricular filling time due to the tachycardia such that the
sustained SV coupled with the tachycardia elicits substantial
increases in CO that contribute importantly to the reflex
increase in SAP.
MUSCLE METABOREFLEX INCREASES VENTRICULAR CONTRACTILITY
drugs for studies unrelated to the present investigation,
respectively. An additional catheter was inserted into the
right jugular vein and advanced to the atrial-caval junction to
monitor central venous pressure (CVP) as an index of ventricular filling pressure in the steady state (4, 10, 15).
Experimental procedures. All experiments were performed
after the animals had fully recovered from surgery and were
active, afebrile, and of good appetite. The animal was brought
to the laboratory and allowed to roam freely for 15–30 min.
The animal was then directed to the treadmill, and the blood
flow transducers were connected to a flowmeter (Transonic
Instruments). The FAP, SAP, and CVP catheters were connected to pressure transducers (Spectromed P-10 EZ). HR
was monitored via a cardiotachometer triggered by the CO
signal. SV was calculated as CO/HR. All data were sampled
by a laboratory computer at 1,000 Hz, and mean values for
each cardiac cycle were saved on hard disk for subsequent
analysis.
Fig. 1. Responses in systemic arterial pressure (SAP), heart rate (HR), cardiac output
(CO) and stroke volume (SV) to progressive
reductions in femoral arterial pressure (FAP)
and terminal aortic blood flow (TAQ) via
graded, partial vascular occlusion of the terminal aorta from 1 animal during a control
experiment. Note that with metaboreflex activation marked increases in SAP, HR, and CO
occurred with little change in SV. KPH, km/h;
bpm, beats/min.
Downloaded from http://ajpheart.physiology.org/ by 10.220.32.247 on May 12, 2017
Instruments) was placed on the ascending aorta to monitor
CO and SV. Stainless steel electrodes were sutured to the
apex of the left ventricle for subsequent ventricular pacing.
The pericardium was reapproximated and the chest closed in
layers. The wires were tunneled subcutaneously and exteriorized between the scapulae.
In the second procedure, through either a midventral
abdominal or retroperitoneal approach, a blood flow transducer (Transonic Instruments) and vascular occluder were
placed on the terminal aorta. All side branches between the
iliac arteries and the flow probe were ligated and severed. A
catheter was placed in a side branch of the aorta above the
flow probe and occluder to monitor SAP. The catheter, flow
probe wires, and occluder tubing were tunneled subcutaneously and exteriorized between the scapulae.
In the final procedure, arterial and venous catheters were
implanted into small side branches of the femoral artery to
monitor femoral arterial pressure (FAP) and for infusion of
H221
H222
MUSCLE METABOREFLEX INCREASES VENTRICULAR CONTRACTILITY
RESULTS
Figure 1 shows a typical control experiment. As
described previously (7–9, 11, 18), initial reductions in
hindlimb perfusion during mild exercise do not elicit
metaboreflex responses. However, once hindlimb perfusion is reduced below a threshold level, substantial
increases in SAP, HR, and CO occur. Note that little
change in SV occurred.
Figure 2 shows average values of SAP, HR, CO, SV,
and CVP as a function of TAQ during free-flow exercise,
at metaboreflex threshold, and at the maximal level
attained with the largest reductions in hindlimb perfusion during control experiments. Metaboreflex activation caused substantial and highly significant (P #
0.002) increases in SAP, HR, and CO. No significant
change in SV or CVP occurred with metaboreflex
activation (P 5 0.74 and 0.67, respectively).
Figure 3 shows the average values of SAP, HR, CO,
SV, and CVP as a function of TAQ during free-flow
exercise, at metaboreflex threshold, and at the maximal level attained with the largest reductions in hindlimb perfusion when HR was maintained constant at
225 beats/min. In the absence of any change in HR,
substantial increases in SAP and CO still occurred with
metaboreflex activation because of the large, significant increases in SV.
Fig. 2. Average levels of SAP, HR, CO, SV, and central venous
pressure (CVP) during control experiments plotted as a function of
TAQ during free-flow exercise (circles at right), at metaboreflex
threshold (circles at middle), and at lowest imposed level of TAQ
(circles at left). Once TAQ was reduced below threshold, substantial
increases in SAP, HR, and CO occurred. No significant changes in
either SV or CVP occurred. * P , 0.05 vs. free-flow exercise; NS, no
significant change from free-flow exercise.
Figure 4 shows the maximal changes in SAP, CO, SV,
HR, and CVP during control experiments and during
constant HR. Ventricular pacing did not significantly
affect the maximal levels of SAP or CO. During control
experiments, SV remained essentially constant with
metaboreflex activation, whereas, at constant HR, SV
increased markedly during hindlimb ischemia such
that the changes in CO and SAP were not different
between the control experiments vs. at constant HR. No
significant change in CVP occurred in either experiment.
DISCUSSION
The major new finding in this study is that activation
of the muscle metaboreflex in conscious dogs during
dynamic exercise elicits marked increases in ventricular performance. Normally, with metaboreflex activation CO increases substantially via an increase in HR
with constant SV. However, we observed that when HR
was maintained constant, SV increased significantly
Downloaded from http://ajpheart.physiology.org/ by 10.220.32.247 on May 12, 2017
The muscle metaboreflex was activated during mild treadmill exercise (3.2 km/h, 0% grade) as described previously
(7–9, 11, 18). Briefly, the treadmill was started and after 3–5
min steady-state levels of SAP, CO, HR, SV, terminal aortic
blood flow (TAQ), and FAP were achieved. Thereafter, hindlimb perfusion was progressively decreased by partially inflating the vascular occluder implanted on the abdominal aorta.
Each level of reduction in hindlimb perfusion was maintained
until all parameters reached steady state (3–5 min).
The experiment was then repeated on a separate day, and
HR was maintained constant at 225 beats/min by connecting
the ventricular electrodes to a pacemaker. This level of HR
was above that induced by metaboreflex activation in all dogs
during control experiments.
Statistical analysis. Each animal served as its own control.
One-minute averages of all variables were taken during
steady-state exercise and at each level of partial vascular
occlusion. The data were analyzed as described by Wyss et al.
(18). At this workload, initial reductions in hindlimb perfusion do not elicit metaboreflex responses. Once TAQ is
reduced below a threshold level, further reductions in hindlimb perfusion elicit substantial increases in SAP, CO, and
HR. Thus the relationship between hindlimb perfusion and
the efferent responses (e.g., SAP, CO, HR) is ‘‘dogleg’’ in shape
at this workload. Therefore, the data were approximated to
two linear regressions, an initial response line wherein no
substantial change in SAP, CO, and HR occurred with the
initial reductions in TAQ and a pressor response line when
further reductions in TAQ elicited substantial increases in
the efferent responses. The intersection of these two lines is
taken as the threshold for the reflex. In control experiments
no change in SV occurred, and therefore these data could not
be fitted by two regression lines, and the values during
free-flow exercise were compared with those at the lowest
level of hindlimb perfusion. The data were analyzed using
repeated-measures ANOVA, and individual means were compared using modified Bonferroni tests.
MUSCLE METABOREFLEX INCREASES VENTRICULAR CONTRACTILITY
H223
per se, are likely more important in eliciting substantial increases in CO. With metaboreflex activation in
control experiments, SV remained constant despite the
reduction in ventricular filling time due to the reflex
tachycardia. White et al. (17) demonstrated using
conscious dogs that changes in ventricular rate within
the range observed with metaboreflex activation in this
model do not elicit any significant change in CO due to
the reduction in SV. As HR increases, SV decreases
proportionately, presumably due to the reduction in
ventricular filling time, thereby causing the product of
SV and HR (e.g., CO) to remain unchanged. Thus our
Downloaded from http://ajpheart.physiology.org/ by 10.220.32.247 on May 12, 2017
Fig. 3. Average levels of SAP, HR, CO, SV, and CVP during constant
ventricular pacing plotted as a function of TAQ during free-flow
exercise (circles at right), at metaboreflex threshold (circles in
middle), and at lowest imposed level of TAQ (circles at left). Note that
at constant HR, metaboreflex activation elicited a substantial increase in SV such that the increases in SAP and CO were similar to
control experiments. * P , 0.05 vs. free-flow exercise; NS, no significant change from free flow exercise.
with metaboreflex activation such that the increases in
CO and SAP were indistinguishable from control experiments. Thus, with activation of the muscle metaboreflex in the normal animal, despite decreases in ventricular filling time due to the reflex tachycardia, SV remains
unchanged due to the increase in ventricular performance, thereby allowing CO to increase substantially
and contribute importantly to the pressor response.
Previously, O’Leary (7) observed that after b-adrenergic blockade both the reflex tachycardia and pressor
responses to metaboreflex activation in conscious dogs
were significantly attenuated, leading to the conclusion
that the HR component of the reflex contributed importantly to the pressor response. However, CO was not
measured in that study. Because b-adrenergic blockade
will affect both chronotropic and inotropic function, we
utilized ventricular pacing to investigate whether
metaboreflex activation elicits increases in ventricular
performance. The present investigation indicates that
increases in ventricular contractility, rather than HR
Fig. 4. Average maximal changes in SAP, CO, SV, HR, and CVP in
response to metaboreflex activation during control experiments and
during constant HR. Note that maximal increases in SAP and CO
were not significantly different between control experiments and at
constant HR. * P , 0.05, change significantly different from constant
HR; NS, no significant difference.
H224
MUSCLE METABOREFLEX INCREASES VENTRICULAR CONTRACTILITY
The authors thank Sue Harris and Susanne LaPrad for expert
technical assistance.
This study was supported by National Heart, Lung, and Blood
Institute Grants HL-02844 and HL-55473.
Address for reprint requests: D. S. O’Leary, Dept. of Physiology,
Wayne State Univ. School of Medicine, 540 E. Canfield Ave., Detroit,
MI 48201.
Received 22 December 1997; accepted in final form 23 March 1998.
REFERENCES
1. Collins, H. L., and S. E. DiCarlo. Cardiac afferents attenuate
the muscle metaboreflex in the rat. J. Appl. Physiol. 75: 114–120,
1993.
2. Kaufman, M. P., J. C. Longhurst, K. J. Rybicki, J. H.
Wallach, and J. H. Mitchell. Effects of static muscular contraction on impulse activity of groups III and IV afferents in cats. J.
Appl. Physiol. 55: 105–112, 1983.
3. Kaufman, M. P., and K. J. Rybicki. Discharge properties of
group III and IV muscle afferents: their responses to mechanical
and metabolic stimuli. Circ. Res. 61 Suppl. I: I-60–I-65, 1987.
4. Luchsinger, P. C., H. W. Seipp, Jr., and D. J. Patel. Relationship of pulmonary artery-wedge pressure to left atrial pressure
in man. Circ. Res. 11: 315–318, 1962.
5. Mark, A. L., R. G. Victor, C. Nerhed, and B. G. Wallin.
Microneurographic studies of the mechanisms of sympathetic
nerve responses to static exercise in humans. Circ. Res. 57:
461–469, 1985.
6. Nobrega, A. C. L., J. W. Williamson, J. A. Garcia, and J. H.
Mitchell. Mechanisms for increasing stroke volume during
static exercise with fixed heart rate in humans. J. Appl. Physiol.
83: 712–717, 1997.
7. O’Leary, D. S. Autonomic mechanisms of muscle metaboreflex
control of heart rate. J. Appl. Physiol. 74: 1748–1754, 1993.
8. O’Leary, D. S., N. F. Rossi, and P. C. Churchill. Muscle
metaboreflex control of vasopressin and renin release. Am. J.
Physiol. 264 (Heart Circ. Physiol. 33): H1422–H1427, 1993.
9. O’Leary, D. S., and D. D. Sheriff. Is the muscle metaboreflex
important in control of blood flow to ischemic active skeletal
muscle? Am. J. Physiol. 268 (Heart Circ. Physiol. 37): H980–
H986, 1995.
10. Reeves, J. T., B. M. Groves, A. Cymerman, J. R. Sutton, P. D.
Wagner, D. Turkevich, and C. S. Houston. Operation Everest
II: cardiac filling pressures during cycle exercise at sea level.
Respir. Physiol. 80: 147–154, 1990.
11. Sheriff, D. D., D. S. O’Leary, A. M. Scher, and L. B. Rowell.
Baroreflex attenuates pressor response to graded muscle ischemia in exercising dogs. Am. J. Physiol. 258 (Heart Circ. Physiol.
27): H305–H310, 1990.
12. Stebbins, C. L. Reflex cardiovascular response to exercise is
modulated by circulating vasopressin. Am. J. Physiol. 263 (Heart
Circ. Physiol. 32): R1104–R1109, 1992.
13. Stebbins, C. L., B. Brown, D. Levin, and J. C. Longhurst.
Reflex effect of skeletal muscle mechanoreceptor stimulation on
the cardiovascular system. J. Appl. Physiol. 65: 1539–1547,
1988.
14. Stebbins, C. L., A. Ortiz-Acevedo, and J. M. Hill. Spinal
vasopressin modulates the reflex cardiovascular response to
static contraction. J. Appl. Physiol. 72: 731–738, 1992.
15. Verel, D., and N. H. Stentiford. Comparision of left atrial
pressure and wedge pulmonary capillary pressure: pressure
gradients between left atrium and left ventricle. Br. Heart J. 32:
99–102, 1970.
16. Victor, R. G., D. R. Seals, and A. L. Mark. Differential control
of heart rate and sympathetic nerve activity during dynamic
exercise. J. Clin. Invest. 79: 508–516, 1987.
17. White, S., C. B. Higgins, S. F. Vatner, D. Franklin, E.
Braunwald, and T. Patrick. Effects of altering ventricular rate
on blood flow distribution in conscious dogs. Am. J. Physiol. 221 :
1402–1407, 1971.
18. Wyss, C. R., J. L. Ardell, A. M. Scher, and L. B. Rowell.
Cardiovascular responses to graded reductions in hindlimb
perfusion in exercising dogs. Am. J. Physiol 245 (Heart Circ.
Physiol. 14): H481–H486, 1983.
Downloaded from http://ajpheart.physiology.org/ by 10.220.32.247 on May 12, 2017
results from the control experiments indirectly indicate
a significant role for increases in ventricular contractility with metaboreflex activation. In the control experiments SV remained unchanged despite significant increases in HR (and thus reductions in ventricular
filling time), thereby allowing CO to increase markedly.
Furthermore, when HR was maintained constant, SV
increased significantly during metaboreflex activation
such that the increases in CO (and SAP) were virtually
the same as in control experiments (Fig. 4). Because no
significant change in CVP occurred in either control
experiments or during constant HR, the rise in SV
during metaboreflex activation at constant HR is not
likely to be a consequence of the Frank-Starling effect
but likely reflects the effects of increases in sympathetic activity to the ventricular myocardium eliciting
increases in ventricular contractility. Thus, in the
previous study by O’Leary (7) in which b-adrenergic
blockade attenuated both the tachycardia and pressor
responses to muscle metaboreflex activation, it is likely
that the reflex increases in ventricular contractility
were attenuated (or abolished) by the b-adrenergic
blockade, and thus the reduced pressor responses were
a consequence of attenuated increases in CO.
In support of the concept that activation of muscle
afferents can elicit changes in ventricular contractility,
Stebbins et al. (12, 14) demonstrated using anesthetized cats that electrically induced static muscle contraction causes significant increases in LV dP/dt. In addition, recently Nobrega et al. (6) observed in humans
with atrioventricular block that SV increased significantly during static knee extension when HR was
maintained constant at the peak level observed in
control experiments when HR was allowed to increase
(via a dual-chamber sensing and pacing pacemaker). In
that experiment, at constant high HR, the increase in
SV during static contraction was accomplished via a
decrease in end-systolic volume with no change in
end-diastolic volume, indicating that the rise in SV and
CO was mediated via an increase in ventricular contractility. However, because static muscle contraction activates both mechano- and metabosensitive afferents (2,
3) and that activation of mechanosensitive afferents
also elicits increases in ventricular contractility (13), to
what extent the ventricular contractility responses
observed with static muscle contraction are attributable to the muscle metaboreflex vs. mechanoreflex is
unclear.
In summary, activation of the muscle metaboreflex
during dynamic exercise elicits increases in ventricular
performance such that SV remains unchanged despite
large increases in HR. The combination of constant SV
with the reflex tachycardia results in marked increases
in CO that, combined with peripheral vasoconstriction,
cause substantial increases in arterial blood pressure.