Clinical Science ( 1990) 78, 159- 163
I59
Response to dynamic exercise in cardiac transplant recipients:
implications for control of the sodium regulatory hormone
atrial natriuretic peptide
D. R. J. SINGER, N. R. BANNER", A. COX", N. PATEL", M. BURDON", M. G. BUCKLEY,
G. A. MAcGREGOR AND M. H. YACOUB"
Blood Pressure Unit, Department of Medicine, St George's Hospital Medical School, London, and *Cardiothoracic Unit, Harefield
Hospital, Harefield, Middlesex, U.K.
(Received 28 March/l August 1989; accepted 13 September 1989)
SUMMARY
1. To study the importance of cardiac innervation in
the regulation of atrial natriuretic peptide, plasma atrial
natriuretic peptide levels were measured during
symptom-limited, graded exercise on a cycle ergometer in
seven male orthotopic cardiac transplant recipients.
2. Resting plasma atrial natriuretic peptide was significantly, higher in the transplant recipients than in two
control groups, one matched to the transplant recipients
(group 1) and the other to the age of the donor heart
(group 11).
3. The response to exercise of the cardiac transplant
recipients was compared with the response of control
group 11. Mean maximal work load achieved with exercise
was around 40% lower in the cardiac transplant
recipients. During exercise, plasma atrial natriuretic
peptide levels increased in both the cardiac transplant
recipients and the control subjects. The increase in
plasma atrial natriuretic peptide with exercise was greater
in absolute, but less in percentage, terms in transplant
recipients than in the control subjects.
4. The increase in mean arterial pressure with exercise
was similar in patients and in control subjects; however,
heart rate increased in the patients by only 33% compared with a rise of 151% in the control group.
5. These results provide insight into the control of the
sodium regulatory hormone atrial natriuretic peptide.
First, factors other than a change in heart rate appear of
importance in the regulation of atrial natriuretic peptide.
Secondly, these findings suggest that cardiac innervation
is not of dominant importance in the modulation of atrial
natriuretic peptide secretion.
Correspondence: Dr D. R. J. Singer, Blood Pressure Unit.
Department of Medicine. St George's Hospital Medical School,
London SW I7 ORE.
Key words: atrial natriuretic peptide, blood pressure,
cardiac transplantation, exercise.
Abbreviation: ANP, atrial natriuretic peptide.
INTRODUCTION
Atrial natriuretic peptide (ANP) is a cardiac hormone of
importance in the regulation of sodium and water balance
[l].The atria have a complex neural system capable of
discriminating between changes in atrial stretch and in
pressure [2,3]. However, the role of cardiac innervation in
modulating ANP secretion in man is not clear. After
orthotopic cardiac transplantation, the heart is denervated and re-innervation has not so far been reported
in cardiac transplant recipients [4, 51. Exercise in man is
associated with an increase in plasma ANP levels in proportion to the severity of the exercise [6, 71. In order to
study the importance of cardiac innervation in the regulation of ANP secretion, we therefore studied the plasma
ANP response to exercise in healthy cardiac transplant
recipients and in normal subjects.
METHODS
Subjects
We studied seven male orthotopic cardiac transplant
recipients who gave their informed consent to the study
which was approved by the local ethical committee. Two
separate control groups were also studied.
Control group I. Resting plasma ANP levels in the
seven transplant recipients (see Table 1) were compared
with levels in a group of seven healthy subjects matched
for age (47.45~4.7years, m e a n k ~ ~sex,
~ ) ethnic
,
group
and sitting blood pressure (138/98 k 5/3 mmHg).
Control group 11. Resting plasma ANP levels and the
response to exercise in the same seven cardiac transplant
160
D. R. J. Singer et al.
recipients were compared with a separate group of seven
healthy male volunteers, who were matched for ethnic
group and whose age was intermediate between the age of
the cardiac transplant recipients and the age of the donor
hearts (see Table 1).
The patients were studied at 11-52 weeks after transplantation. One transplant recipient who was initially
studied 11 weeks after transplantation was restudied after
29 weeks. They were well, with no clinical features of
cardiac failure and had no evidence of rejection on their
most recent cardiac biopsy. Plasma creatinine was < 165
pmol/l in all patients. Immunosuppressive therapy was
with cyclosporin A (seven), azathioprine (seven) and
prednisone (three). Other drugs were anti-platelet (seven),
nifedipine (two), acyclovir (two), diuretic (one) and
allopurinol (one). The patient studied on two occasions
was during the first study on treatment with cyclosporin
A, azathioprine and prednisonc. The steroid was discontinued several weeks before the second study.
Protocol
To obtain peripheral venous blood samples during
exercise, a cannula was inserted into a forearm vein and
kept patent by intermittent flushing with heparinized
saline ( 154 mmol/l NaCI). Exercise was performed on an
electronically braked cycle ergometer (Corival; Lode,
Groningen, The Netherlands). Subjects rested for 5 min,
seated on the exercise cycle, before the first blood sample
was obtained. The exercise protocol consisted of an initial
warm-up period of cycling with a nominal work load of
10 W. The work load was then increased in 10 W steps at
1 min intervals. No adverse events occurred during
exercise and all subjects exercised to a symptom-limited
maximum level. The heart rate and electrocardiogram
were recorded at 1 min intervals using a microcomputerassisted cardiograph (Marquette MAC 11. Marquette,
Manchester, U.K.). Blood pressure was recorded using a
mercury column sphygmomanometer at rest and every 3
min during exercise (30 W, 60 W, etc.). Blood samples
were obtained from the indwelling venous cannula at 2
min intervals during exercise. Samples were taken into
ethylenediaminetetra-acetate (potassium salt)/Trasylol,
separated immediately and the plasma stored at - 20°C
until radioimmunoassay for ANP [8].
for both parametric and non-parametric tests. Significant
levels given in the text are the lower levels obtained in the
above tests.
RESULTS
Basal blood pressure, heart rate and plasma ANP levels
Group I. In the seven cardiac transplant recipients,
resting plasma ANP levels (Table 1) were higher than in
the group of seven subjects matched for age, sex, ethnic
group and blood pressure (plasma ANP 9.5 k 1.6 pg/ml,
I’< 0.0 I ).
Group 11. In the separate group of seven control
subjects who were also studied during exercise. the
average age was intermediate between the age of the
patients and of their donor hearts (Table I).
Resting sitting
mean arterial blood pressure (109 f 2 mmHg, 1’<0.05)
and heart rate ( 10 1 k 5 beats/min, I>< 0.0 1) were significantly elevated in the patients compared with the control
subjects (95f4 mmHg, 74 f 5 beats/min). Resting
plasma ANP levels in the patients were significantly
clevated at over threefold higher than in control group 11
before exercise ( I > < 0.0 1, Table I ).
Work output, blood pressure and heart rate during
exercise
The maximal work output achieved was lower in rhe
cardiac transplant recipients, at around 60% that
achieved by the control subjects ( I ) < 0.0 1).
Blood pressure increased similarly during exercise in
each group (Fig. 1 ) and there was no significant difference
in the maximal increase in mean arterial pressure with
exercise in the two groups (Table 1). Although basal heart
rate was higher in the cardiac transplant recipients. heart
rate increased only 33 f9% during exercise in the transplant recipients, compared with a 15 I f 18% increase in
the control subjects (1’<0.01, Fig. I).
Table 1. Measurements before exercise in control
subjects (group 11) and cardiac transplant recipients
Results are means fSEM. Statistical significance: “ I ) < 0.01
compared with control subjects.
Statistical analysis
Results are reported as mean values fSEM for the seven
studies in control subjects and seven of the eight studies in
cardiac transplant recipients. For the transplant recipient
studied twice, the data for the two exercise studies are
shown in the Figures; in the pooled analysis, only data
from the second exercise study off corticosteroid treatment was included. Statistical analysis was performed
using Student‘s [-test for paired and unpaired data, and
non-parametric testing with the Wilcoxon signed-rank
test for matched pairs and the Mann-Whitney U-test for
unpaired data. Results are reported as significant when
the level of significance was /’<0.05 in a two-tailed test
Subject age (years)
Donor age (years)
Weight (kg)
Resting blood pressure
(mmHg)
Maximum work output
(W)
Resting plasma ANP
(pg/ml)
Maximal increase in
plasma ANP (W,)
Maximal increase in mean
arterial pressure (Yo)
Control
subjects ( 1 1 = 7)
Cardiac transplant
32.6 f 3.2
72.1 f 4.7
I22/82 k 215
45.9 f 3.4
25.3 f 2.8
68.8 f 3.2
I41*/93 f 313
176f14
103f 12*
8.0 f I .6
3 I .3 k 3.0’
204 f 34
151 f 3 5
29.6 f 4.8
22.6 f 5.3
recipients ( 1 1 = 7)
Cardiac innervation and atrial natriuretic peptide
160
3
E
-f
1
Rest
Max.
Max.
Rest
161
-
125
P
140 100 -
120 100 -
I
-g
.
h
75-
M
80 -
a.
Y
5d
60 -r
m
-a$
50
25
0
0
I
40
40
80
120
160
200
240
Work output ( W )
d
I
I
Normal subjects
I
I
Cardiac transplant
recipients
Fig. 2. Plasma ANP response to exercise in normal
subjects (0,n = 7) and cardiac transplant recipients (@,
ii = 7), in one of whom exercise was studied at 11 weeks
( B ) and 29 weeks (0)
after transplant.
Fig. 1. Heart rate and blood pressure response to
exercise in normal subjects (0,n = 7) and cardiac transplant recipients (@, i t = 7), in one of whom exercise was
after transplant.
studied at 11 weeks (B) and 29 weeks (0)
100
1
Plasma ANP levels during exercise
Plasma ANP increased with exercise in all subjects
studied ( P < 0 . 0 5 , Fig. 2) with a greater absolute increase
in the transplant recipients compared with control
subjects ( P < 0 . 0 5 , Fig. 3). However, the maximal percentage increase in plasma ANP levels was not significantly different in transplant patients compared with the
control subjects (Table 1).
Plasma ANP levels began to increase at a lower level of
exercise in the cardiac transplant recipients compared
with the control subjects (Fig. 2). In the transplant
recipients, plasma ANP levels at both 60 W exercise
(44.7 f3.9 pg/ml) and 50% maximal exercise (37.8 k 2.2
pg/ml) were significantly increased compared with baseline ( P < 0.05), but not significantly changed compared
with baseline in the control subjects at these levels of
exercise. In the one transplant recipient studied on two
occasions, the maximum level of exercise achieved was
unchanged and the absolute increase in plasma ANP was
similar; however, during the second study, after prednisone had been discontinued, plasma ANP levels were
much less elevated than in the presence of prednisone and
4n
40
.9
2ol
0
C
I
I
"
-
1
Normal subjects
1
Cardiac transplant
recipients
Fig. 3. Increase in plasma ANP from rest to maximal
exercise in response to exercise in normal subjects (0,
n = 7) and cardiac transplant recipients ( 0 , ii = 7), in one
of whom exercise was studied at 11 weeks ( B ) and 29
weeks (0)after transplant.
the proportional increase in plasma ANP was greater
(154% compared with 35% in the initial study; Fig. 2).
DISCUSSION
This study has confirmed that basal plasma ANP levels in
cardiac transplant recipients are significantly greater than
162
D. R. J. Singer et al.
in control subjects [9, 101. Furthermore, the results of the
present study demonstrate that plasma ANP levels in
cardiac transplant recipients increase with exercise and
therefore indicate that cardiac innervation is not essential
for the plasma ANP response to exercise [6,7].
The mechanisms causing the high basal levels of
plasma ANP, and the pathophysiological significance of
these high levels remain unclear. The classical technique
of human cardiac transplantation involves anastomosis of
the inflow tracts of the systemic and pulmonary veins
together with the posterior wall of the left atrium of the
recipient heart to the entire donor heart. There is therefore around 25% more atrial tissue in the heart after
transplantation is complete. This increased atrial tissue
mass may have contributed to the elevated plasma ANP
levels observed [9]. Essential hypertension, particularly
when blood pressure is moderately to severely elevated, is
associated with elevated plasma ANP levels [ l l ] . However, in the present study blood pressure was only mildly
elevated in the cardiac transplant recipients and plasma
ANP levels were greatly elevated compared with a group
of control subjects matched for blood pressure. Drug
treatment with corticosteroids may account in part for the
elevation of plasma ANP in cardiac transplant recipients,
either directly by stimulation of ANP gene expression by
cardiocytes [12] or indirectly as a result of sodium and
water retention. In support of this was our observation
that in the patient studied both on and off steroid treatment, plasma ANP levels, both before and during exercise, were lower after steroid therapy had been stopped.
Plasma ANP levels d o not appear to change appreciably
with long-term cyclosporin A treatment, at least in bone
marrow transplant recipients [ 131. Renal impairment,
attributed in part to cyclosporin nephrotoxicity, is a
common finding in cardiac transplant recipients, and
moderate to severe renal impairment is associated with
elevated plasma ANP levels [14]. However, none of the
patients in the present study had a significant degree of
renal impairment. A further possible mechanism for
elevated plasma ANP levels after cardiac transplantation
is that normal secretion of ANP is under inhibitory neural
control. If ANP secretion were under dominant neural
inhibition, then the denervation after transplantation [4,5]
would be predicted to result in an increase in basal secretion of ANP. Furthermore, a disproportionately large rise
in ANP secretion would be expected, compared with the
response of normal subjects to physiological stimuli
associated with increases in plasma ANP levels.
In a study by Wilkins et al. [lo] in cardiac transplant
recipients in whom central blood volume was increased
by lower body positive pressure, the percentage rise in
plasma ANP was similar in cardiac transplant recipients
and control subjects. A potential criticism of the above
study [lo] is that all patients were on immunosuppressive
therapy with steroids, which may have accounted in part
for the raised basal levels of plasma ANP, as well as
possibly modulating the ANP response to an increase in
central blood volume [ 101. However, in the present study,
in five out of seven subjects, the response to exercise was
studied in the absence of corticosteroids; although the
absolute increase in plasma ANP levels from basal to
maximal exercise was greater in the cardiac transplant
recipients than in control subjects, the proportional
increase in ANP in the cardiac transplant recipients was
similar or slightly lower than in the control subjects.
Therefore, after cardiac denervation by transplantation,
there appears to be no exaggerated plasma ANP response
either to exercise or to a passive increase in central blood
volume [lo]. Taken together these findings suggest that
denervation hypersensitivity of ANP secretion is not the
principal explanation for the high plasma ANP levels
observed in cardiac transplant recipients.
The heart rate response to exercise is blunted after
cardiac transplantation [ 151 and in the present study the
maximal heart rate with exercise was only 33% above
basal values, compared with an increase of greater than
151% in the control subjects. This suggests that factors
other than heart rate are of importance in modulating
ANP secretion, at least after cardiac denervation. Atrial
stretch is an important stimulus for ANP secretion [16]
and during exercise central blood volume increases [ 171.
Haass et al. [18], who studied patients undergoing elective
cardiac catheterization, found that plasma ANP levels
correlated better with atrial dimensions and an index of
atrial stretch (atrial pressure multiplied by atrial crosssection) than with atrial pressures alone. However, in that
report, no information was provided on blood pressure or
concomitant drug treatment and all subjects had cardiac
disease; therefore the results must be interpreted with
caution. An increase in plasma ANP in the denervated
patients at a milder level of exercise than in control
subjects may indicate that an increase in atrial pressure
and thus atrial wall tension occurs at a milder level of
exercise when the heart rate response to exercise is
blunted by denervation. An interesting speculation by
Brouwer et al. [ 191is that, as a result of Laplace’s law, at a
given atrial pressure higher atrial wall tension may occur
in cardiac transplant recipients compared with matched
normal subjects, because of the larger atrium present after
cardiac transplantation [9]. It is thus possible that the
pressure-stretch relationship in atrial tissue may be
abnormal after transplantation. However, we cannot
comment on this mechanism in the present study as it was
not considered ethically acceptable to measure atrial
pressures during exercise in the control subjects and
cardiac transplant recipients.
The physiological significance of the exercise-related
increase in plasma ANP in both normal subjects and
cardiac transplant recipients is at present unclear and it
was not possible in present study to determine the relative
contribution of secretion from donor and recipient atrium
to plasma ANP levels. The increase in plasma ANP
observed in the patients in this study is in the same range
as has been reported to cause vasodilatation, when reproduced in studies of ANP infusion [20, 211. Therefore the
elevation in plasma ANP, in particular with more severe
exercise, may influence systemic vascular resistance and
the distribution of cardiac output during exercise and thus
possibly protect the renal and other circulatory beds from
ischaemic injury.
Cardiac innervation and atrial natriuretic peptide
Finally, systemic hypertension occurs commonly 'after
cardiac transplantation [22,23] and has been attributed in
part t o effects of cyclosporin A, which is also associated
with hypertension after renal [24] and bone marrow [25]
transplantation. T h e re is indirect evidence that increased
plasma ANP levels, at least in association with increases
in dietary sodium [26], may act to.protect hypertensive
subjects from an otherwise greater increase in blood
pressure as blood volume increases. The raised plasma
ANP levels in cardiac transplant recipients may therefore
contribute t o a homoeostatic response t o the raised blood
pressure in these patients.
ACKNOWLEDGMENTS
D.R.J.S. is a British Heart Foundation Intermediate
Research Fellow. T h i s work was supported in part by a
grant from the National Kidney Research Fund.
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