Download The Responses of Cardiac Sympathetic Nerve Activity to Changes in

Page
1 of 30in PresS. Am J Physiol Regul Integr Comp Physiol (July 16, 2008). doi:10.1152/ajpregu.00824.2007
Articles
The Responses of Cardiac Sympathetic Nerve Activity to Changes in
Circulating Volume Differ in Normal and Heart Failure Sheep
R. Ramchandra, S.G. Hood, A.M.D. Watson, and C.N. May
Howard Florey Institute, University of Melbourne, Parkville, Australia 3010
Short title: CSNA responses to changes in circulating volume
Address for corresponding author:
Dr C. N. May
Howard Florey Institute,
University of Melbourne,
Parkville,
Victoria, 3010
AUSTRALIA
Phone: +61 3 8344 7302
Fax:
+61 3 9348 1707
E-mail: [email protected]
Total word count: 6074 words
Copyright © 2008 by the American Physiological Society.
Page 2 of 30
Abstract
The factors controlling cardiac sympathetic nerve activity (CSNA) in the normal state and
those causing the large increase in activity in heart failure (HF) remain unclear. We
hypothesized, from previous clinical findings, that activation of cardiac mechanoreceptors by
the increased blood volume in HF may stimulate SNA, particularly to the heart via cardiocardiac reflexes. To investigate the effect of volume expansion and depletion on CSNA we
have made multiunit recordings of CSNA in conscious normal sheep and sheep paced into
HF. In HF sheep (n=9), compared to normal sheep (n=9), resting levels of CSNA were
significantly higher (21±1 vs. 11±2 spikes/s; P<0.05), mean arterial pressure was lower (76±3
vs. 87±2 mmHg; P<0.05) and central venous pressure (CVP) was greater (3.0±1.0 vs. 0.0±1.0
mmHg; P<0.05). In normal sheep (n=6), hemorrhage (400 mL over 30 min) was associated
with a significant increase in CSNA (179±16%) with a decrease in CVP (2.7±0.7 mmHg).
Volume expansion (400 mL Gelofusine over 30 min) significantly decreased CSNA
(35±12%) and increased CVP (4.7±1.0 mmHg). In HF sheep (n=6), the responses of CSNA to
both volume expansion and hemorrhage were severely blunted, with no significant changes in
CSNA or heart rate with either stimulus. In summary, these studies in a large conscious
mammal demonstrate that in the normal state directly recorded CSNA increased with volume
depletion and decreased with volume loading. In contrast, both these responses were severely
blunted in HF, with no significant changes in CSNA during either hemorrhage or volume
expansion.
Keywords: heart failure, sympathetic nervous system, hemorrhage, cardiac volume
2
Page 3 of 30
Introduction
Cardiac sympathetic nerve activity (CSNA) plays an important role in the control of normal
cardiac function and also in cardiovascular pathology, where for example increased levels in
heart failure (HF) can promote the progression of the disease and lead to development of
arrhythmias and sudden death (23). Despite these critical roles, the factors controlling CSNA
in the normal state and those leading to the increased CSNA in HF remain relatively poorly
understood. In recent studies, in which we directly recorded CSNA, we found that the arterial
baroreflex control of CSNA was not desensitised in a pacing model of HF in conscious sheep
(36). This raised the possibility that cardiac neural reflexes, specifically the cardiac sympathoexcitatory mechano-reflex driven by the fluid retention and increased cardiac filling pressures,
contribute to the increased CSNA in HF.
Previous studies have demonstrated that cardiac reflexes responding to changes in cardiac
filling pressures can selectively alter CSNA. In anaesthetized normal dogs left atrial stretch
increased CSNA, but decreased renal SNA and had no effect on other sympathetic outflows
(19). These data suggested that an increase in cardiac filling pressures may selectively
stimulate CSNA via the Bainbridge reflex, which is mediated by left and right atrial stretch
receptors (4). In healthy subjects changes in cardiac filling pressures resulted in reciprocal
changes in muscle SNA (7, 33), but the effect on CSNA is unclear. Studies in normal subjects
indicate that reductions in cardiac filling pressures resulted in either no change (3) or an
increase in cardiac norepinephrine spillover (29).
In HF patients, there are a number of lines of evidence indicating that the cardiovascular and
SNA responses to activation of cardiopulmonary mechanoreceptors are abnormal. In HF,
upright tilt (20) and lower body negative pressure (13), which lower cardiac filling pressures,
are associated with forearm vasodilatation or attenuated vasoconstriction compared with the
vasoconstrictor response in normal subjects (1). Reductions in cardiac filling pressures,
3
Page 4 of 30
induced by sodium nitroprusside or lower body negative pressure, increased total
norepinephrine spillover, but caused a paradoxical decrease or no change in cardiac
norepinephrine spillover in patients with HF (3, 21, 29, 31).
These findings raise the possibility that the CSNA responses to changes in cardiac filling
pressures are impaired in HF compared to the healthy state, but CSNA has never been directly
recorded in response to blood volume changes. We have, therefore, examined the effect of
changes in blood volume on directly recorded CSNA in conscious normal sheep and in sheep
with pacing induced HF. Studies were performed in conscious animals to avoid the depressant
effect of anaesthesia, which is more potent on CSNA than on other sympathetic outflows (26).
We hypothesized increasing cardiac filling pressures in normal animals would increase CSNA
and this response would be attenuated in the HF group. We also expected reducing filling
pressures would decrease CSNA in HF animals to a greater extent than normal animals.
4
Page 5 of 30
Methods
Adult merino ewes (35–49 kg body wt) were housed in individual metabolism cages in
association with other sheep. Experiments began when sheep were accustomed to laboratory
conditions and human contact. Sheep were fed a diet of oaten chaff (800 g/day), and water
was offered ad libitum. All experiments were approved by the Animal Experimentation Ethics
Committee of the Howard Florey Institute.
Surgical Procedures
Prior to the studies, sheep underwent 2 or 3 aseptic surgical procedures, each separated by 2
weeks recovery. Anaesthesia was induced with intravenous sodium thiopental (15 mg/kg) and
following intubation was maintained with 1.5-2.0% isoflurane/O2. In the first stage sheep
were prepared with a carotid arterial loop. In the sheep to be induced into HF, a pacemaker
lead (Medtronic Inc., Minneapolis, MN, USA) was inserted under fluoroscopic guidance via
the right jugular vein into the right ventricle. The lead was exteriorized on the neck and
connected to an external pacemaker. In a separate further operation, intrafascicular electrodes
were implanted in the left cardiothoracic nerves as previously described (37). The electrodes
were implanted in 9 normal animals and 9 HF animals. Experiments were conducted on
standing, conscious sheep, and to minimise any effect of surgical stress experiments were not
started until the fourth day after implantation of the electrodes.
In all operations, animals were treated with antibiotics (Ilium Propen; procaine penicillin,
Troy Laboratories Ptd Ltd, Smithfield, NSW, Australia or Mavlab, Qld, Australia) at the start
of surgery and then for 2 days postoperatively. Post-surgical analgesia was maintained with
intramuscular injection of flunixin meglumine (1 mg/kg) (Troy Laboratories or Mavlab, Qld,
Australia) at the start of surgery, then 4 and 16 hours post-surgery.
5
Page 6 of 30
On the day before implantation of recording electrodes, arterial and venous cannulae were
inserted into the carotid artery and jugular vein as described previously (36, 37). The cannulae
for measurement of arterial pressure (AP) and central venous pressure (CVP) were connected
to separate pressure transducers (TDXIII, Cobe) tied to the wool on the sheep’s back. The
pressures were corrected to compensate for the height of the transducers above the level of the
heart.
Experimental protocols
The development of HF was assessed by measurement of ejection fraction and fractional
shortening using short axis M-wave echocardiography on conscious sheep lying on their right
side. Following placement of ventricular pacing leads, a basal measurement was made before
the start of ventricular pacing, at 200-220 beats/min. Echocardiography was then performed
weekly with the pacing switched off. Sheep were considered to be in HF when ejection
fraction had fallen to <40%. All experiments were conducted with the pacing switched off.
CSNA was recorded differentially between the pair of electrodes with the best signal-to-noise
ratio. The signal was amplified (x 100,000) and filtered (bandpass 300–1,000 Hz), displayed
on an oscilloscope, and passed through an audio amplifier and loud speaker. Sympathetic
nerve activity (5000 Hz), AP (100 Hz) and CVP (100 Hz) were recorded on computer using a
CED micro 1401 interface and Spike 2 software (Cambridge Electronic Design, UK).
Four days after surgical implantation of cardiac sympathetic nerve electrodes, a 5 minute
recording of resting CSNA, AP and CVP was made in conscious sheep in the normal state
and in HF. The animals were then subjected to either hemorrhage (n=6 in each group) or
volume expansion (n=6 in each group) on a randomly allocated basis. Following a recovery
period of at least 24 hours the second protocol was performed. Both protocols could not be
performed on all sheep due to deterioration of the CSNA in some animals. For the volume
6
Page 7 of 30
expansion series, gelofusine (Braun Australia Pty Ltd, New South Wales, Australia) was
infused until the total volume infused reached 400 ml. The rate was titrated to between 350500 ml/30 min to try and keep arterial pressure constant, although this was not always
possible towards the end of the volume expansion. For the hemorrhage protocol, the venous
cannula was connected to a blood infusion bag and blood was removed under gravity at a rate
of 20 ml/min, until 450 ml of blood was removed. Following the end of the hemorrhage, the
blood was reinfused back into the animal.
Data Analysis
Data were analysed on a beat-to-beat basis using custom written routines in the Spike 2
program. For each heartbeat the program determined diastolic, systolic and mean arterial
blood pressures and the number of discriminated spikes above threshold between the
following diastolic pressures. The threshold was set at a level so that spikes from small bursts
were counted. The threshold for each animal was checked over the 60 sec control recording to
ensure that each burst crossed the threshold and that spikes from the smallest bursts were
counted. These data were used to calculate CSNA both as total nerve activity (spikes/sec), and
as burst incidence (bursts/100 heart beats). The control level of CSNA, determined as
spikes/sec during 60 sec of control, was taken as 100% and changes in CSNA are reported as
percentage change from control. Cardiovascular variables were calculated over 60 sec periods
at the end of the 5-min control and when 100, 200, 300 and 400 ml of gelofusine had been
infused or blood removed.
In addition to analysing the data using spike counts, the data were analysed using full-wave
rectification and integration of the CSNA signal as described previously (30). The same 60
sec of control in each animal was used to calculate the control levels of CSNA (arbitrary
7
Page 8 of 30
values), which was taken as 100% and changes in CSNA were calculated as percentage
changes from control. There were no differences in the conclusions drawn using either
integration (data not shown) or spike counts.
Data are expressed as means ± SEM and were analysed using two-way repeated measures
ANOVA (SigmaStat, Access Softek, version 2.03). If significant, further appropriate post hoc
tests were carried out and multiple comparisons corrected for using the Bonferroni
adjustment. P<0.05 was considered statistically significant. To examine the effect of CVP
changes on CSNA, linear regression lines showing the relationship between changes in CVP
and CSNA were drawn for individual animals using data at selected time points. The average
of the slopes and intercepts were used to draw an average regression line. The slopes of the
regression lines in the HF and normal groups, during hemorrhage and volume expansion,
were compared using an unpaired t-test.
8
Page 9 of 30
Results
Resting Hemodynamics and CSNA in Normal and Heart Failure Sheep
Left ventricular ejection fraction and fractional shortening, measured in conscious sheep by
echocardiography, gradually decreased over 6-8 weeks of rapid ventricular pacing at 200-220
beats/min. In HF animals, 1-2 days before implantation of cardiac sympathetic recording
electrodes, ejection fraction had decreased from 81 ± 2 % to 37 ± 2 % (P<0.001), and
fractional shortening had decreased from 48 ± 2 % to 17 ± 1 % (P<0.001).
Resting levels of CSNA and arterial pressure were calculated on a beat-to-beat basis from
data collected during the control periods. In heart failure animals (n=9), mean arterial pressure
(MAP) was significantly lower (P<0.05), CVP was significantly higher (P<0.05) and heart
rate (HR) tended to be higher, than in normal animals (n=9) (Table 1). The resting level of
CSNA measured as burst incidence was significantly higher in the HF group compared to the
normal group (P<0.05) (Table 1).
CSNA Responses in Normal and Heart Failure Sheep during Hemorrhage
Hemorrhage was conducted in 6 animals in each group and was associated with a gradual
decrease in CVP and a small fall in MAP, with the magnitude of the changes being similar in
the normal and HF sheep (Figures 1 & 2). There was a tendency for pulse pressure to decrease
over time in the normal group, but this did not reach statistical significance (p=0.12). There
was no change in pulse pressure during hemorrhage in the HF group. During hemorrhage
there were progressive, significant increases in heart rate and CSNA in normal sheep, but no
significant changes in HF sheep. After 400 mL of blood had been withdrawn in the normal
and HF groups there were respective decreases in CVP of 2.7 ± 0.7 and 4.0 ± 1.5 mmHg, and
in MAP of 6 ± 1 and 6 ± 2 mmHg. At this time, in the normal group heart rate had increased
9
Page 10 of 30
by 29 ± 4 bpm and CSNA had increased by 179 ± 16%, whereas there were no significant
changes in heart rate or CSNA in the HF group (Figure 2). The responses of HR and CSNA
over time were significantly different between the two groups (significant interaction between
group and time, P<0.05).
Linear regression lines relating the change in CSNA for a given change in CVP were drawn
for individual sheep in both groups. For a given change in CVP, the change in CSNA was
significantly less in HF animals than in normal animals (Figure 3). This was reflected in the
slopes of the average regression lines which were significantly lower in the HF group (-2.87 ±
2.1; r2 =0.74 ± 0.07) than in the normal group (-18.2 ± 3.6; r2 = 0.91 ± 0.01) (P<0.05).
CSNA Responses in Normal and Heart Failure Sheep during Volume Expansion
Volume expansion with 400 mL gelofusine was associated with an increase in CVP but no
significant change in MAP or heart rate in normal (n=6) or HF (n=6) sheep (Figures 4 & 5).
There was no change in pulse pressure in either of the groups over time. These changes were
associated with a significant decrease in CSNA in the normal animals but no change in CSNA
in the HF group (Figures 4 & 5). After 400 ml of gelofusine had been infused into the normal
and HF groups there were respective increases in CVP of 4.7 ± 1.0 and 5.9 ± 1.0 mmHg. At
this time, in the normal group CSNA had significantly decreased by 35 ± 12%, whereas there
was no significant change in CSNA in the HF group (Figure 5). The changes in CSNA over
time were significantly different between the two groups (P<0.05). The slopes of the average
regression lines relating the change in CSNA for a given change in CVP were lower in the HF
animals than the normal animals, but this did not reach statistical significance (1.4 ± 1.8 for
the HF group vs. -6.6 ± 3.4 for the normal group; P=0.06) ( r2 = 0.74 ± 0.06 for HF animals
and 0.92 ± 0.03 for normal animals (Figure 3).
10
Page 11 of 30
Discussion
This is the first study to directly record CSNA during volume changes in conscious animals in
the normal healthy state and in HF. The main findings were that reductions in cardiac filling
pressures during hemorrhage were accompanied by increases in CSNA in normal animals,
while volume expansion was associated with a significant decrease in CSNA. In contrast,
both these responses were severely blunted in HF, with no significant changes in CSNA
during either hemorrhage or volume expansion.
Decreases in cardiac filling pressures in normal animals
The present finding that hemorrhage caused robust, parallel increases in CSNA and HR,
associated with a reduction in cardiac filling pressures and a small fall in arterial pressure, is
similar to the increase in cardiac norepinephrine spillover found in normal subjects during
hypotensive lower body negative pressure (LBNP)(3). It is likely that these changes in CSNA
and HR are an integrated response to unloading of arterial and cardiopulmonary
baroreceptors. This is supported by the observation that the tachycardia during hemorrhage is
mediated largely by changes in baroreceptor afferent activity (34, 35), and the finding that the
sympathetic response to LBNP is reduced in cardiac transplant patients with denervated
ventricles, but innervated atria and pulmonary veins, indicating an important role for
ventricular mechanoreceptors (27).
Increases in cardiac filling pressures in normal animals
Studies on the effect of increasing cardiac filling pressures on heart rate date back to
experiments by Bainbridge in 1915 who demonstrated that i.v infusion of saline or blood
increased heart rate (4). In human patients, the Bainbridge reflex, in terms of a heart rate
response, has been observed in some but not all studies (5, 16). It is important to note that the
Bainbridge reflex can be counteracted by changes in baroreceptor afferent activity (5), which
11
Page 12 of 30
may account for the different results observed. A role for CSNA in this reflex is suggested by
the demonstration that distension of the left atrium or the pulmonary veins results in an
increase in heart rate (11, 14) that is sympathetically mediated (18), and that activation of
atrial A-type receptors by volume expansion increases efferent sympathetic nerve activity in
the cat (15). Furthermore, in anesthetized dogs, activation of left atrial receptors by balloon
distension increased CSNA, decreased renal SNA and had no effect on splenic or lumbar
SNA (19), although in another study distension of the left atrium resulted in an initial period
of inhibition followed by an excitation of CSNA (24).
A criticism of the above studies is that due to difficulties with recording CSNA in conscious
animals they were completed in anaesthetized animals, and in most cases using an open-chest
preparation. Considering the potent effect of anaesthesia on cardiovascular reflexes, and on
CSNA in particular (26), it is vital to confirm any findings in conscious, unstressed animals. It
is also important to note that experiments that observed sympathoexcitation studied the
specific responses to atrial stretch induced by balloon inflation. Volume expansion is a more
physiological stimulus that activates the ventricular as well as the atrial receptors, which is
important given the evidence that ventricular mechanoreceptors play a role in the responses to
volume changes (27).
In our study, there was no cardiac sympatho-excitation under conditions of volume expansion
in normal conscious sheep. While there was no change in MAP or pulse pressure, it is
possible that the small non-significant increase in MAP may have offset any sympathoexcitatory response mediated by the cardiopulmonary afferents. This study cannot therefore
exclude the possibility that selective activation of atrial receptors may activate the Bainbridge
reflex and induce cardiac sympatho-excitation. Our finding that volume expansion did not
cause cardiac sympatho-excitation indicates that the Bainbridge reflex is absent or relatively
weak with its effects being overridden by the effect of arterial baroreceptors. The finding that
12
Page 13 of 30
there was no change in HR, although CSNA decreased, suggests a decrease in cardiac vagal
activity in response to the volume loading.
Decreases in cardiac filling pressures in heart failure animals
In patients with HF, there is evidence of a positive correlation between pulmonary artery
pressures and cardiac norepinephrine spillover levels (22). These data support the concept that
in HF a reflex responding to increases in cardiac filling pressures may contribute to the
increased CSNA. This possibility has been explored further in HF patients by decreasing
cardiac filling pressures via upright tilt, LBNP and infusions of vasodilators (20, 21). Sodium
nitroprusside infusion caused no change in cardiac norepinephrine spillover in patients with
moderately severe HF, but an increase in normal subjects (29, 31). Similarly, Kaye et al. (21)
observed a reduction in cardiac norepinephrine spillover with infusion of sodium
nitroprusside in HF patients. These studies support the hypothesis that in HF increased cardiac
filling pressures stimulate CSNA, although the fall in MAP during nitroprusside, which would
have activated arterial baroreceptors, complicates the interpretation of these observations.
Using a LBNP stimulus, which reduced filling pressures with no change in arterial pressure, a
reduction in cardiac norepinephrine spillover was observed in HF patients but not normal
subjects (3). Although this study supported the hypothesis that increased cardiac filling
pressures in HF contribute to the increased CSNA in these patients, LBNP has been shown to
causes changes in firing rates of baroreceptor afferents even in the absence of changes in
MAP (17).
While the results obtained from spillover techniques are important, cardiac norepinephrine
spillover is an indirect steady state method and the inability of this technique to distinguish
between direct changes in SNA and the reduced norepinephrine re-uptake that occurs in HF
(10), indicates that it is important to re-examine these responses using direct recordings of
SNA. In addition, HF patients used in the studies of norepinephrine spillover were on
13
Page 14 of 30
medications that may have altered CSNA by a direct action or by actions secondary to the
hemodynamic improvements that they induce.
Our study was conducted in conscious animals with stable pacing-induced HF, without the
complication of medication. This animal model of HF has been used extensively and shows
the symptoms of HF seen clinically, including decreased ventricular function and intense
neurohumoral activation (8) (32) (28). Importantly, the level of neurohormonal activation in
pacing models of HF closely parallels that in human HF patients (38).
We used hemorrhage as a stimulus to decrease cardiac filling pressures and observed no
significant change in directly recorded CSNA in HF animals, in contrast to the large increase
in CSNA for a similar reduction in CVP in normal animals. In HF animals, at the end of
hemorrhage CVP had decreased to a level similar to that in normal sheep, but at this point
there was no change in CSNA. As in the clinical studies, the accompanying decrease in MAP
and unloading of arterial baroreceptors is a confounding factor that would stimulate CSNA.
Thus any reduction in the action of the Bainbridge reflex, due to the decreased filling
pressures, may have been offset by an action of the arterial baroreceptors. However,
supporting the notion that cardiac afferent reflexes are not a dominant factor causing
sympatho-excitation in HF is the finding in dogs with pacing-induced HF that cardiac
denervation did not inhibit the increase in plasma norepinephrine (25). Since the degree of
sympathetic activation in HF shows marked regional heterogeneity, it was important to
establish whether decreasing cardiac filling pressures had a selective effect to decrease the
cardiac sympathoexcitation. Our findings, together with those of the cardiac denervation
study (25), suggest that increased cardiac filling pressures are not an important factor driving
the increased CSNA in HF, and if the Bainbridge reflex is evoked in HF its effects are offset
by those of the arterial baroreceptors.
14
Page 15 of 30
The effects of volume depletion on CSNA that we found are in general agreement with those
from clinical studies of cardiac norepinephrine spillover (29, 31). In both cases the degree of
cardiac sympatho-excitation in response to volume depletion in HF is strongly attenuated
compared with the response in the healthy state. The impaired response of CSNA in HF was
not because it was near maximal, since CSNA can be increased a further 40% when arterial
pressure is decreased with sodium nitroprusside (36). Furthermore, the reduced CSNA
response is unlikely to result from a smaller percentage of total blood volume being removed
in the HF group, because the fall in CVP in the HF group tended to be greater than in normal
group and the falls in MAP were similar. It would be expected that the fall in MAP during
hemorrhage would stimulate CSNA via the arterial baroreflex, since in HF the CSNA arterial
baroreflex sensitivity is normal (36). However, the resting level of CSNA is strikingly
elevated and sits on a less steep part of the baroreflex curve so that a fall in MAP induces a
smaller increase in CSNA in HF than in normal animals. An additional factor leading to the
impaired response may be the lack of an increase in heart rate in the HF group, so that for a
given firing frequency activity will increase less than in the normal group in which there was
a large increase in heart rate.
Increases in cardiac filling pressures in heart failure animals
Analogous to the desensitisation of the CSNA response to hemorrhage in HF, the inhibition of
CSNA during volume loading seen in normal animals was completely absent in HF animals.
The similar desensitisation of the reflex inhibition of RSNA by cardiopulmonary reflexes (9,
41), suggests that this is a widespread effect on the sympathetic response to volume
expansion. An explanation for a generalised desensitisation is that it is due mainly to the
reduced sensitivity of atrial vagal afferents, which were shown to become less sensitive in
dogs with chronic high-output HF (40). Further evidence that the afferent pathway responding
15
Page 16 of 30
to volume is abnormal in HF was the lack of activation of neural pathways in the brain that
were normally activated by volume expansion (2).
In conclusion, in normal sheep volume depletion caused parallel increases in CSNA and HR,
and volume loading decreased CSNA but had no effect on HR. In contrast, in a pacing model
of HF in sheep these reflex responses were almost absent. The attenuated response of CSNA
during changes in cardiac filling pressures in the HF animals suggests a striking impairment
of cardiopulmonary mechano-receptors.
Perspectives and Significance
Previous studies by other groups have shown that in HF either sino-aortic or cardiac
denervation did not prevent the increase in plasma norepinephrine (6, 25), suggesting that any
defects in these inhibitory reflexes are not a major cause of the sympathoexcitation in HF. The
findings of our present and previous study (36) have now demonstrated, specifically with
regard to CSNA, that neither the arterial nor the cardiopulmonary baroreflexes are a major
factor determining the cardiac sympatho-excitation in HF. The reduced afferent inhibitory
input from cardiopulmonary receptors that we observed in response to volume expansion in
HF may permit the increase in CSNA in spite of the increased blood volume. Further studies
are required to determine whether other afferent neural reflexes, from the kidneys or skeletal
muscle, play a role. Alternatively the afferent signal may be humoral, possibly by actions of
circulating peptide hormones including angiotensin and endothelin acting on
circumventricular organs (42), blood borne cytokines activating receptors on vascular cells of
the blood brain barrier (12) or a direct central action of aldosterone, which can freely cross the
blood brain barrier.(39).
16
Page 17 of 30
Acknowledgements
The authors are grateful to Tony Dornom, Alan McDonald and Craig Thomson for their
excellent technical assistance and to David Trevaks for Spike 2 programming.
Funding Sources
This work was supported by grants from the National Health and Medical Research Council
of Australia (232313) and the National Institutes of Health, USA (5 R01 HL07 4932).
Disclosures
None
17
Page 18 of 30
References
1.
Abboud FM, Eckberg DL, Johannsen UJ, and Mark AL. Carotid and
cardiopulmonary baroreceptor control of splanchnic and forearm vascular resistance during
venous pooling in man. The Journal of physiology 286: 173-184, 1979.
2.
Akama H, McGrath BP, and Badoer E. Volume expansion fails to normally
activate neural pathways in the brain of conscious rabbits with heart failure. J Auton Nerv Syst
73: 54-62, 1998.
3.
Azevedo ER, Newton GE, Floras JS, and Parker JD. Reducing cardiac filling
pressure lowers norepinephrine spillover in patients with chronic heart failure. Circulation
101: 2053-2059, 2000.
4.
Bainbridge FA. The influence of venous filling upon the rate of the heart. The
Journal of physiology 50: 65-84, 1915.
5.
Barbieri R, Triedman JK, and Saul JP. Heart rate control and mechanical
cardiopulmonary coupling to assess central volume: a systems analysis. Am J Physiol Regul
Integr Comp Physiol 283: R1210-1220, 2002.
6.
Brandle M, Patel KP, Wang W, and Zucker IH. Hemodynamic and norepinephrine
responses to pacing-induced heart failure in conscious sinoaortic-denervated dogs. J Appl
Physiol 81: 1855-1862, 1996.
7.
Charkoudian N, Martin EA, Dinenno FA, Eisenach JH, Dietz NM, and Joyner
MJ. Influence of increased central venous pressure on baroreflex control of sympathetic
activity in humans. Am J Physiol Heart Circ Physiol 287: H1658-1662, 2004.
8.
Dibner-Dunlap ME, and Thames MD. Baroreflex control of renal sympathetic nerve
activity is preserved in heart failure despite reduced arterial baroreceptor sensitivity. Circ Res
65: 1526-1535, 1989.
18
Page 19 of 30
9.
DiBona GF, and Sawin LL. Reflex regulation of renal nerve activity in cardiac
failure. Am J Physiol 266: R27-39, 1994.
10.
Eisenhofer G, Friberg P, Rundqvist B, Quyyumi AA, Lambert G, Kaye DM,
Kopin IJ, Goldstein DS, and Esler MD. Cardiac sympathetic nerve function in congestive
heart failure. Circulation 93: 1667-1676, 1996.
11.
Fater DC, Schultz HD, Sundet WD, Mapes JS, and Goetz KL. Effects of left atrial
stretch in cardiac-denervated and intact conscious dogs. Am J Physiol 242: H1056-1064,
1982.
12.
Felder RB, Francis J, Zhang ZH, Wei SG, Weiss RM, and Johnson AK. Heart
failure and the brain: new perspectives. Am J Physiol Regul Integr Comp Physiol 284: R259276, 2003.
13.
Ferguson DW, Abboud FM, and Mark AL. Selective impairment of baroreflex-
mediated vasoconstrictor responses in patients with ventricular dysfunction. Circulation 69:
451-460, 1984.
14.
Furnival CM, Linden RJ, and Snow HM. Reflex effects on the heart of stimulating
left atrial receptors. The Journal of physiology 218: 447-463, 1971.
15.
Hakumaki MO. Function of the left atrial receptors. Acta physiologica Scandinavica
344: 1-54, 1970.
16.
Hakumaki MO. Seventy years of the Bainbridge reflex. Acta Physiol Scand 130:
177-185, 1987.
17.
Hartikainen J, Ahonen E, Nevalainen T, Sikanen A, and Hakumaki M.
Haemodynamic information encoded in the aortic baroreceptor discharge during
haemorrhage. Acta Physiol Scand 140: 181-189, 1990.
19
Page 20 of 30
18.
Kappagoda CT, Linden RJ, and Snow HM. A reflex increase in heart rate from
distension of the junction between the superior vena cava and the right atrium. The Journal of
physiology 220: 177-197, 1972.
19.
Karim F, Kidd C, Malpus CM, and Penna PE. The effects of stimulation of the left
atrial receptors on sympathetic efferent nerve activity. The Journal of physiology 227: 243260, 1972.
20.
Kassis E. Cardiovascular response to orthostatic tilt in patients with severe congestive
heart failure. Cardiovasc Res 21: 362-368, 1987.
21.
Kaye DM, Jennings GL, Dart AM, and Esler MD. Differential effect of acute
baroreceptor unloading on cardiac and systemic sympathetic tone in congestive heart failure. J
Am Coll Cardiol 31: 583-587, 1998.
22.
Kaye DM, Lambert GW, Lefkovits J, Morris M, Jennings G, and Esler MD.
Neurochemical evidence of cardiac sympathetic activation and increased central nervous
system norepinephrine turnover in severe congestive heart failure. J Am Coll Cardiol 23: 570578, 1994.
23.
Kaye DM, Lefkovits J, Jennings GL, Bergin P, Broughton A, and Esler MD.
Adverse consequences of high sympathetic nervous activity in the failing human heart. J Am
Coll Cardiol 26: 1257-1263, 1995.
24.
Kollai M, Koizumi K, Yamashita H, and Brooks CM. Study of cardiac sympathetic
and vagal efferent activity during reflex responses produced by stretch of the atria. Brain
research 150: 519-532, 1978.
25.
Levett JM, Marinelli CC, Lund DD, Pardini BJ, Nader S, Scott BD, Augelli NV,
Kerber RE, and Schmid PG, Jr. Effects of beta-blockade on neurohumoral responses and
neurochemical markers in pacing-induced heart failure. Am J Physiol 266: H468-475, 1994.
20
Page 21 of 30
26.
Matsukawa K, Ninomiya I, and Nishiura N. Effects of anesthesia on cardiac and
renal sympathetic nerve activities and plasma catecholamines. Am J Physiol 265: R792-797,
1993.
27.
Mohanty PK, Thames MD, Arrowood JA, Sowers JR, McNamara C, and
Szentpetery S. Impairment of cardiopulmonary baroreflex after cardiac transplantation in
humans. Circulation 75: 914-921, 1987.
28.
Murakami H, Liu JL, and Zucker IH. Blockade of AT1 receptors enhances
baroreflex control of heart rate in conscious rabbits with heart failure. Am J Physiol 271:
R303-309, 1996.
29.
Newton GE, and Parker JD. Cardiac sympathetic responses to acute vasodilation.
Normal ventricular function versus congestive heart failure. Circulation 94: 3161-3167, 1996.
30.
Ramchandra R, Barrett CJ, Guild SJ, and Malpas SC. Evidence of differential
control of renal and lumbar sympathetic nerve activity in conscious rabbits. Am J Physiol
Regul Integr Comp Physiol 290: R701-708, 2006.
31.
Ros M, Azevedo ER, Newton GE, and Parker JD. Effects of nitroprusside on
cardiac norepinephrine spillover and isovolumic left ventricular relaxation in the normal and
failing human left ventricle. Can J Cardiol 18: 1211-1216, 2002.
32.
Spinale FG, Mukherjee R, Iannini JP, Whitebread S, Hebbar L, Clair MJ,
Melton DM, Cox MH, Thomas PB, and de Gasparo M. Modulation of the reninangiotensin pathway through enzyme inhibition and specific receptor blockade in pacinginduced heart failure: II. Effects on myocyte contractile processes. Circulation 96: 2397-2406,
1997.
33.
Sundlof G, and Wallin BG. Effect of lower body negative pressure on human muscle
nerve sympathetic activity. The Journal of physiology 278: 525-532, 1978.
21
Page 22 of 30
34.
Thrasher TN, and Keil LC. Arterial baroreceptors control blood pressure and
vasopressin responses to hemorrhage in conscious dogs. Am J Physiol 275: R1843-1857,
1998.
35.
Thrasher TN, and Shifflett C. Effect of carotid or aortic baroreceptor denervation on
arterial pressure during hemorrhage in conscious dogs. Am J Physiol Regul Integr Comp
Physiol 280: R1642-1649, 2001.
36.
Watson AM, Hood SG, Ramchandra R, McAllen RM, and May CN. Increased
cardiac sympathetic nerve activity in heart failure is not due to desensitization of the arterial
baroreflex. Am J Physiol Heart Circ Physiol 293: H798-804, 2007.
37.
Watson AM, Mogulkoc R, McAllen RM, and May CN. Stimulation of cardiac
sympathetic nerve activity by central angiotensinergic mechanisms in conscious sheep. Am J
Physiol Regul Integr Comp Physiol 286: R1051-1056, 2004.
38.
Yarbrough WM, and Spinale FG. Large animal models of congestive heart failure: a
critical step in translating basic observations into clinical applications. J Nucl Cardiol 10: 7786, 2003.
39.
Yu Y, Wei SG, Zhang ZH, Gomez-Sanchez E, Weiss RM, and Felder RB. Does
aldosterone upregulate the brain renin-angiotensin system in rats with heart failure?
Hypertension 51: 727-733, 2008.
40.
Zucker IH, Earle AM, and Gilmore JP. The mechanism of adaptation of left atrial
stretch receptors in dogs with chronic congestive heart failure. The Journal of clinical
investigation 60: 323-331, 1977.
41.
Zucker IH, Gorman AJ, Cornish KG, and Lang M. Imparied atrial receptor
modulation or renal nerve activity in dogs with chronic volume overload. Cardiovasc Res 19:
411-418, 1985.
22
Page 23 of 30
42.
Zucker IH, and Pliquett RU. Novel mechanisms of sympatho-excitation in chronic
heart failure. Heart failure monitor 3: 2-7, 2002.
23
Page 24 of 30
Figure Legends:
Figure 1. Raw figure from one normal (top panel) and one HF animal (bottom panel) showing
the changes in central venous pressure (CVP), arterial pressure (AP) and cardiac sympathetic
nerve activity (CSNA) during control and when 400 ml of blood was removed. arb=arbitrary
units. freq=frequency.
Figure 2. Change in mean arterial pressure (MAP), central venous pressure (CVP), heart rate
(HR) and CSNA (CSNA) during hemorrhage in HF (dark bars) and control (open bars)
animals (n=6 in each group). *-P<0.05 from respective control; #-P<0.05 between groups
over time.
Figure 3. Average and individual regression lines showing the relation between changes in
CSNA as a function of changes in CVP for normal (upper panels) and HF (lower panels)
groups. The panels indicate the regression lines during volume expansion (left panels) and
hemorrhage (right panels) in normal and HF groups. The dotted lines are individual animals
and the straight line is the mean regression line. Comparisons are between the slopes of the
regression lines in the normal and HF groups. The r2 values represent the goodness of fit for
the individual animals.
Figure 4. Raw figure from one normal (top panel) and one HF animal (bottom panel)
showing the changes in central venous pressure (CVP), arterial pressure (AP) and cardiac
sympathetic nerve activity (CSNA) during control and when 400 ml of gelofusine was
infused. arb=arbitrary units. freq=frequency.
Figure 5. Change in mean arterial pressure (MAP), central venous pressure (CVP), heart rate
(HR) and CSNA (CSNA) during volume expansion in HF (dark bars) and control (open bars)
animals (n=6 in both groups). *-P<0.05 from respective control; #-P<0.05 between groups
over time.
24
Page 25 of 30
Normal Animal
Control
5
Hemorrhage
5
CVP (mmHg) 0
0
-5
150
-5
150
100
100
50
100
50
100
50
50
0
0
0.15
0.15
0.00
0.00
-0.15
-0.15
AP (mmHg)
Freq (Hz)
CSNA (arb)
0
5
0
10
5
10
Time (s)
Time (s)
HF Animal
Control
Hemorrhage
10
10
CVP (mmHg)
0
0
150
150
AP (mmHg)
Freq (Hz)
CSNA (arb)
100
100
50
1000
50
1000
500
500
0
0
0.15
0.15
0.00
0.00
-0.15
-0.15
0
5
Time (s)
10
0
5
10
Time (s)
Figure 1
25
Page 26 of 30
Change in
CVP (mmHg)
0
*
-5
*
Change in
MAP (mmHg)
0
-5
*
-10
* *
-15
40
*
Change in
HR (bpm)
30
*
20
#
10
0
*
Change in
CSNA (%)
100
80
*
60
#
40
20
0
100 ml
200 ml
300 ml
400 ml
Figure 2
26
Page 27 of 30
Volume Expansion - Normals
60
Hemorrhage - Normals
100
2
r = 0.92 ± 0.03
Change in CSNA
from control (%)
40
20
60
0
40
-20
20
-40
0
-60
-80
0
60
Change in CSNA
from control (%)
40
2
r =s 0.91 ± 0.01
80
2
4
6
8
10
-20
-12
-10
-8
-6
-4
-2
Change in CVP (mmHg)
Change in CVP (mmHg)
Volume Expansion - HF
Hemorrhage - HF
r2 = 0.74 ± 0.06
100
0
r2 = 0.74 ± 0.07
80
20
60
0
40
-20
-40
20
-60
0
-80
0
2
4
6
8
Change in CVP (mmHg)
10
-20
-12
-10
-8
-6
-4
-2
0
Change in CVP (mmHg)
Figure 3
27
Page 28 of 30
Normal Animal
Control
Volume Expansion
10
10
CVP (mmHg)
0
0
150
150
AP (mmHg)
100
100
50
200
50
100
150
Freq (Hz)
50
100
50
CSNA (arb)
0
0.15
0
0.15
0.00
0.00
-0.15
0
5
-0.15
0
10
5
10
Time (s)
Time (s)
HF Animal
Control
Hemorrhage
10
10
CVP (mmHg)
0
0
150
150
100
100
50
1000
50
1000
500
500
0
0.15
0
0.15
0.00
0.00
AP (mmHg)
Freq (Hz)
CSNA (arb)
-0.15
0
5
Time (s)
-0.15
0
10
5
10
Time (s)
Figure 4
28
Change in
CVP (mmHg)
Page 29 of 30
*
5
* *
*
*
0
Change in
MAP (mmHg)
15
10
5
0
Change in
HR (bpm)
-5
10
5
0
-5
Change in
CSNA (%)
40
20
0
#
-20
-40
100 ml in
200 ml in
300 ml in
*
400 ml in
Figure 5
29
Page 30 of 30
Table I. Mean resting levels in normal animals (n=9) and animals in heart failure (n=9) from 5
min of control data. *-P<0.05.
Normals
Heart Failure
Central venous pressure (mmHg)
0.0 ± 1.0
3.0 ± 1.0*
Mean arterial pressure (mmHg)
87 ± 2
76 ± 3*
Heart rate (bpm)
73 ± 4
81 ± 3
CSNA (spikes/s)
11 ± 2
21 ± 1*
CSNA burst incidence (%)
34 ± 5
93 ± 2*
30