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504
Differences in Cardiovascular Responses to
Isoproterenol in Relation to Age and Exercise
Training in Healthy Men
John R. Stratton, MD; Manuel D. Cerqueira, MD; Robert S. Schwartz, MD;
Wayne C. Levy, MD; Richard C. Veith, MD;
Steven E. Kahn, MB, ChB; and Itamar B. Abrass, MD
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Background. Cardiac aging is characterized by a reduced heart rate response to (-agonist stimulation
with isoproterenol, but whether the ejection fraction and other cardiovascular responses are reduced in
humans is largely unknown. In addition, whether reduced (-agonist responses can be improved with
exercise training has not been determined in humans.
Methods and Resuls. Cardiovascular responses to graded isoproterenol infusions (3.5, 7, 14, and 35
ng/kg/min for 14 minutes each) were assessed in 15 older (age, 60-82 years) and 17 young (age, 24-32 years)
rigorously screened healthy men. Thirteen older and 11 young subjects completed 6 months of endurance
training and were retested. At baseline, the older group had reduced responses to isoproterenol for heart rate
(+65% older versus +92% young, p<0.001), systolic blood pressure (+9%Y versus +24%, p<0.001), diastolic
blood pressure (-12% versus -24%, p<0.05), ejection fraction (+12 versus +20 ejection fraction units,
p<O.OOl), and cardiac output (+70%o versus + 100%, p<0.001). The mean plasma isoproterenol concentrations achieved during the infusions were marginally higher (p=0.07) in the older group (128±58, 227±64,
354±114, and 700±125 pg/ml) than in the young (79±20, 178±49, 273±79, and 571±139 pg/ml). Intensive
training increased maximal oxygen consumption by 21% in the older group (28.9±4.6 to 35.1±3.8 ml/kg/min,
p<O.OOl) and by 17% in the young (44.5±5.1 to 52.1±63 ml/kg/min,p<0.001), but training did not augment
any of the cardiovascular responses to isoproterenol in either group. The mean plasma isoproterenol
concentrations at the four infusion doses were unchanged after training in both groups.
Conclusions. We conclude that there is an age-associated decline in heart rate, blood pressure, ejection
fraction, and cardiac output responses to (-adrenergic stimulation with isoproterenol in healthy men.
Altered (3-adrenergic responses probably contribute to the reduced cardiac responses to maximal exercise
that also occur with aging. Furthermore, intensive exercise training does not increase cardiac responses
to (-adrenergic stimulation with isoproterenol in either young or older men. The reduced (3-adrenergic
response appears to be a primary age-associated change that is not caused by disease or inactivity.
(Circulation 1992;86:504-512)
KEY WoRDs * aging * responses, (3-adrenergic * ejection fraction * cardiac output * exercise
T he age-associated decline in cardiovascular performance is more apparent during stress than
at rest. The hallmarks of cardiovascular aging
are a reduced maximal heart rate, ejection fraction, and,
in most studies, cardiac output with exercise stress.1-6
,B-Adrenergic pathways are a primary means of mediating the increased cardiovascular performance in response to stress,7-9 and abnormalities of ,B-adrenergic
modulation of cardiovascular function may partially
explain the cardiovascular changes of aging.10'11
From the Divisions of Cardiology, Gerontology, and Geriatric
Medicine, Departments of Medicine and Psychiatry and Behavioral Sciences, Seattle VA Medical Center and University of
Washington.
Supported by the Medical Research Service of the Department
of Veteran Affairs and by National Institutes of Health grant
AG06581.
Address for correspondence: John R. Stratton, MD, Cardiology
(111-C), Seattle VA Medical Center, 1660 South Columbian Way,
Seattle, WA 98108.
Received January 8, 1992; revision accepted May 6, 1992.
In animal models, catecholamine-stimulated heart
rate, myocardial contractility, and vasodilatation all
decline with age.1"'12 There are fewer data in humans,
and many prior studies have been confounded by the
failure to rigorously exclude underlying occult cardiovascular disease, which might alter 13-adrenergic responses. Available studies in humans have documented
reduced blood pressure or venodilation responses to
epinephrine or isoproterenol13'4 and smaller heart rate
responses to isoproterenol"3 '5-17 in older than in
younger subjects. It is unclear whether other cardiovascular responses, including ejection fraction and cardiac
output, are also reduced with aging in humans. A
reduction in ,B-adrenergic responses with aging may be
an important mechanism for age-associated decreases
in cardiovascular performance during exercise stress.
In normal young or older subjects, dynamic exercise
training maintains or improves exercise cardiac function, yet resting and submaximal levels of catecholamines are unchanged or reduced, whereas vagal tone
may become more dominant.18-2' The maintenance or
Stratton et al Aging, Exercise Training, and B-Adrenergic Responses
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improvement in exercise cardiac function with reduced
catecholamine concentrations suggests that the cardiac
response to 13-adrenergic stimulation may be increased
with training. However, the extent to which the cardiovascular improvements of training may be a result of
improved ,3-adrenergic responses versus other mechanisms, such as training-induced hypertrophy, is unclear.
Sensitivity to /3-adrenergic stimulation is clearly affected
by exercise training in some systems; for example,
training increases ,l-adrenergic-induced lipolysis.22,23
Results in animal models have been conflicting, with
some training studies suggesting increased cardiovascular catecholamine responses24-27 and others showing no
change or a decrease.28-3' There are only limited and
conflicting data regarding training effects on cardiovascular responses to l3-adrenergic stimulation in young
humans32-38 and no information in older humans. Information in older subjects is of particular interest
because of the reduction in ,3-adrenergic cardiovascular
responsiveness that occurs with aging. Several of the
changes noted with aging are related to disuse and
normalize with increased activity.39
This study had two purposes. The first was to determine whether rigorously screened sedentary healthy
older men indeed do have reduced heart rate, blood
pressure, ejection fraction, and cardiac output responses to ff-adrenergic stimulation with isoproterenol
compared with healthy young men. The second purpose
was to determine whether intensive endurance exercise
training increases cardiovascular responses to isoproterenol in either young or older men. To minimize the
possibility of underlying occult cardiac disease, which is
common in elderly men," we used rigorous cardiac
screening techniques. Our results document reduced
cardiovascular responses to isoproterenol in healthy
older men but no improvement in isoproterenol responses after a 6-month intensive endurance exercise
training program in either young or older men.
Methods
Subjects
Two age groups (18-32 years and 60-85 years old) of
healthy men were studied. Subjects were excluded if
they had any history of angina, myocardial infarction,
stroke, hypertension, chronic pulmonary disease, diabetes, current medication use (prescription or over the
counter), current smoking, exercise-limiting orthopedic
impairment, or participation in a regular exercise program in the previous year. Entry laboratory requirements included a normal hematocrit, fasting blood
glucose, total cholesterol, creatinine, resting ECG,
M-mode and two-dimensional echocardiograms, and a
Bruce protocol maximal exercise test, which included
immediate postexercise and redistribution tomographic
thallium imaging in all older subjects. No young subjects
were excluded on the basis of exercise testing, but 22 of
37 potential older subjects were excluded (four with
fixed thallium defects, seven with reversible thallium
defects, five with ST segment depression with normal
thallium, five with frequent premature beats, and one
with claudication).
Seventeen young (age, 24-32 years) and 15 older men
(age, 60-82 years) entered the study and were evaluated before exercise training. Eleven of the young and
505
13 of the older subjects were also evaluated after
exercise training. Six of the young subjects were not
restudied for various reasons (withdrawal from training
in four, refusal in one, technical problems in one). One
of the older subjects underwent baseline testing but did
not enter the training program, and one older subject's
postexercise training data were not included because of
the development of a significant intercurrent illness.
The posttraining results, however, are not significantly
different if this subject is included. All studies were
conducted at least 36 hours after the last episode of
exercise training to avoid the acute effects of exercise.
This study was approved by the Human Subjects Committee of the University of Washington, and all subjects
gave informed consent.
Training Program and Maximal Oxygen Consumption
The 6-month training program was supervised and
monitored and consisted of walking, jogging, and bicycling
for 45 minutes per session four or five times per week in a
supervised setting. Training began at 50-60% of heart
rate reserve, increased to 80-85% by the third or fourth
month, and continued at that level for the remaining time.
Maximal oxygen consumption was measured with a maximal Bruce treadmill protocol exercise test. The mean
expiratory respiratory exchange ratio was 1.23±0.09 on
the tests before training and 1.24±0.05 (p=NS) on the
tests after training, indicating good effort.
Study Protocol
Intravenous catheters were inserted into a right hand
vein and a right antecubital vein of each subject, after
which they rested supine for 30 minutes before collection of baseline data. All studies were performed with
the subject supine, and pretraining and posttraining
studies were performed at the same time of day (10 AM
to 12 noon). After the collection of baseline data, serial
infusions of isoproterenol hydrochloride at 3.5, 7, 14,
and 35 ng/kg/min were given for 14 minutes, each with
a Harvard infusion pump (Harvard Apparatus, South
Natick, Mass.). The infusion solution was prepared by
diluting a sufficient amount of isoproterenol in 0.5N
saline to achieve a total injectate volume of 20 ml at
each infusion level. Goldstein et a140 have documented
steady-state levels of isoproterenol at this duration of
infusion. No complications occurred, and all subjects
received all four doses on all studies.
Data Collection and Processing
At rest and during the final 2 minutes of each infusion
dose, cardiac blood pool images, heart rate, and left arm
automated sphygmomanometer blood pressure
(Paramed Model 9350, Palo Alto, Calif.) were recorded.
For radionuclide angiography, blood was obtained at
the time of intravenous catheter placement and labeled
with 20-30 mCi of 99"Tc as previously described.4'
Images were acquired in the left anterior oblique projection, which offered the best septal definition, with a
high-sensitivity parallel hole collimator and a General
Electric 300 small-field-of-view camera interfaced to a
Microdelta imaging terminal. Radionuclide images
were acquired in 20-msec frames by forward and backward reconstruction with +20% arrhythmia rejection; a
single beat was dropped after each rejected beat.42
Ejection fraction, end-diastolic volume, and end-systolic
Circulation Vol 86, No 2 August 1992
506
volume were calculated by previously described methods.43 Cardiac output was obtained by multiplying the
stroke volume times the mean heart rate during the
acquisition. The correlation between left ventricular
volume measurements by our radionuclide angiographic
method of left ventricular volume determination and
invasive contrast angiography in 19 subjects was r=0.90
with a mean difference of 1.6 ml and an SEM of 31.4
ml.43 Moreover, the interobserver reproducibility of this
method for left ventricular volume determination is
excellent (r=0.99; mean difference, 3.8%).
Plasma isoproterenol concentrations were measured
by high-performance liquid chromatography with electrochemical detection after alumina extraction.44 The
sensitivity of the assay for isoproterenol is 10 pg/ml with
a coefficient of variation of 3.4%. Resting plasma norepinephrine and epinephrine concentrations before and
after training were measured with a single-isotope radioenzymatic assay.45,46
10050-
0*
-,S n
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vO
HR
SBP
EF
DBP
CO
FIGURE 1. Bar graph showing mean percentage change for
several of the measurements in the young and older groups
induced by the 35-nglkglmin isoproterenol dose before training. The response to isoproterenol was reduced for all variables in the older group (all p<O.05). HR heart rate; SBP,
systolic blood pressure; DBP, diastolic blood pressure; EF,
ejection fraction; CO, cardiac output.
isoproterenol infusion rate, the older group had a
smaller increase in heart rate than the young (+65%
older versus +92% young, p<0.001) (Figures 1 and 2,
Table 1). At resting baseline, systolic and diastolic blood
pressures were somewhat higher in the older group.
During isoproterenol infusions, however, systolic blood
pressure increased less in the older group (+9% versus
+24% at the highest dose, p<0.001); systolic blood
pressure at the highest dose was 151 mm Hg in the older
group and 159 mm Hg in the young. In addition, diastolic
blood pressure decreased significantly less during the
infusions in the older group (-12% versus -24%,
Results
Isoproterenol Responses at Baseline Before Training
At rest during the baseline condition, the older and
young groups had similar heart rates. During the highest
-X Y Pre -X Y Post 1 O Pre
o OLD
1O
Statistical Analysis
Results are expressed as the mean±SD. The results in
all young and older subjects before training were compared by ANOVA for repeated measures. The effects of
training in each of the groups were also assessed by
ANOVA for repeated measures. The reported probability
values are those for the interaction terms (old versus
young times dose or pretraining versus posttraining times
dose). A value of p<0.05 was considered significant.
A
L*
150-
Post
-x Y Pre
B
X
Y Post G 0 Pre vOOPost
120
p=0.0003 YoungiOld*Dose
110
Systolic pressure
100
90
80
70
60:
au
Rest
C
3.5
x.ypre
A rwc
-
14
7
PostYPvrpre
X.
X ruKras
-
-
35
-U"
p=O.0001 Young/Old*D~
85-
Rest
Ejection fraction
x
10000
-
9000
-
3.5
7
YPP-x YPr
t
10000
80007000 -=
6000 ;
50OOO 1-
75.
-=.---------
-
65
-
14
35
Pre -O OPost
Young/OldiDose
,p=0.0001
Cardiac output
-------
4000
3000 1,I,
J;
Rest
3.5
7
14
35
ReSt
3.5
7
14
35
FIGURE 2. Graphs showing mean pretraining (pre) and posttraining (post) heart rate (beats per minute panel A), systolic blood
pressure (mm Hg, panel B), ejection fraction (%o, panel C), and cardiac output (ml/min, panel D) responses to the serial
isoproterenol doses (Rest, 3.5, 7, 11, and 35 nglkg/min) for the 11 young and 13 older subjects who underwent exercise training.
The probability (p) values are for the young/old *dose term obtained from ANOVA for repeated measures. The older group had
significantly reduced responses to isoproterenol on all of these measures.
Stratton et al Aging, Exercise Training, and 13-Adrenergic Responses
507
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TABLE 1. Mean Pretraining Responses to Isoproterenol in Young (n= 17) and Older (n=15) Men
Rest
3.5 ng/kg/min
7 ng/kg/min
14 ng/kg/min
35 ng/kg/min
Heart rate* (bpm)
Young
61±7
69±11
74±11
86±14
117±15
Older
65±10
72±10
76± 11
89±12
107±12
Systolic BP* (mm Hg)
Young
127±9
131±11
141±13
149±15
158±21
Older
138±11
144±20
146±17
152±18
151±18
Diastolic BP* (mm Hg)
Young
76+5
70±5
65+6
65±9
58±10
Older
85±9
86±9
79±7
77±7
75±9
Mean BP (mm Hg)
Young
93±4
90±5
91±6
93±7
92±9
Older
103±9
106±11
102±10
102±10
100±11
Ejection fraction* (%)
Young
61±7
71±5
73±6
79±7
81±8
Older
59±5
65+7
66±5
69±5
71±6
Peak ejection rate* (EDV/sec)
Young
-3.3±0.5
-4.1±0.5
-4.8±0.7
-5.6±0.8
-7.5±+1.3
Older
-3.0±0.6
-3.7±0.5
-4.3±0.4
-4.7±0.5
-5.8±1.1
EDV (ml)
Young
132±39
129±45
126±42
117±39
105±30
Older
98±23
96±19
95±21
91±18
84±26
ESV (ml)*
Young
52±21
39±18
35±16
26±13
22±12
Older
41±13
34±11
32+10
29±8
25±11
Stroke volume (ml)
Young
80±21
90±28
91±28
91±30
84±20
Older
57±13
62±12
63±14
62±11
59±17
Cardiac output (l/min)*
4.9±+1.3
Young
6.2±1.9
6.7±1.9
7.7±2.5
9.8+2.5
Older
3.7±0.8
4.5±1.1
4.7±1.1
5.5±1.2
6.3±1.7
bpm, Beats per minute; BP, blood pressure; EDV, end-diastolic volume; ESV, end-systolic volume. Values are
mean±SD.
*p<0.05 by ANOVA for repeated measures (old vs. young x isoproterenol dose).
p<0.05). The mean arterial blood pressure response was
not significantly different between the two groups (-3%
versus -1% at the highest dose).
The mean resting ejection fraction was similar in the
two groups. However, the ejection fraction response to
isoproterenol was significantly smaller in the older
group than the young (+ 12 ejection fraction units older
versus +20 ejection fraction units young at the highest
dose, p<0.001). The peak ejection rate response was
also reduced in the older group (+93% older versus
+127% young, p<0.001). In addition, the older men
had a smaller increase from rest to the highest dose in
the ratio of systolic blood pressure to end systolic
TABLE 2. Mean Pretraining Isoproterenol Levels in Young
(n=17) and Older (n=15) Men
14
7
3.5
35
ng/kg/min ng/kg/min ng/kg/min ng/kg/min
Isoproterenol
79±20
Young
Older
128±58
Values are mean±SD.
178±49
227±64
273±79
354±114
571±139
700±125
volume (+88% older versus +280% young, p<0.05).
The end-diastolic volume response to isoproterenol was
not significantly different between the two groups
(- 14% older versus -20% young, p=NS), whereas the
end-systolic volume response was significantly smaller in
the older group (-39% older versus -58% young,
p<O.OOl). The stroke volume response to isoproterenol
was not significantly different between groups, but the
cardiac output response was reduced in the older men
(+ 70% older versus + 100% young, p< 0.001).
The mean plasma isoproterenol concentrations
achieved during the infusions were marginally higher in
the older group (p=0.07) (Table 2). The older group
had higher mean resting concentrations of norepinephrine (343+124 versus 244+78 pg/ml, p<0.05) and epinephrine (134 + 27 versus 95 + 57 pg/ml, p < 0.05).
Endurance Training Results
Endurance training increased maximal oxygen consumption by 21% in the older men (28.9+±4.6 versus
35.1+3.8 ml/kg/min,p<0.001) and by 17% in the young
men (44.5+5.1 versus 52.1+6.3 ml/kg/min, p<0.001).
Neither the absolute increases in maximal oxygen con-
508
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TABLE 3. Mean Pretraining and Posttraining Responses in the Young (n = 11)
3.5 ng/kg/min
7 ng/kg/min
Rest
Heart rate* (bpm)
74+9
62±7
70±9
Pre
56±4
59+7
66±7
Post
Systolic BP (mm Hg)
131±10
140±9
129±8
Pre
144±12
126±7
135±10
Post
Diastolic BP (mm Hg)
75±5
67±4
65±6
Pre
73±7
65±7
62±9
Post
Mean BP (mm Hg)
89±4
90±4
93±4
Pre
91±6
88±6
89±5
Post
Ejection fraction (%)
71±6
73±6
60±6
Pre
74±5
72±6
63+5
Post
EDV (ml)
132±50
138±53
138±46
Pre
152±49
161±52
166±54
Post
ESV (ml)
39±18
44±20
58±23
Pre
43±18
64±22
50±22
Post
Stroke volume (ml)
81±26
95±33
95±33
Pre
111±32
103±34
113±30
Post
Cardiac output (I/min)
6.9±2.3
4.9±1.6
6.5±2.1
Pre
7.3±2.0
6.7±1.9
5.7±1.8
Post
14 ng/kg/min
35 ng/kg/min
87+11
75+10
118+15
100±11
148±15
146±12
161±21
162±21
63±10
59±12
60±12
55±9
91±7
88±7
94±10
91±8
78±7
76±8
80±9
81±9
122±48
142±49
113+34
126±40
30±13
39±21
27±13
28±18
94±36
105±29
88±23
100±24
10.3±2.6
9.9±2.3
bpm, Beats per minute; BP, blood pressure; EDV, end-diastolic volume; ESV, end-systolic volume. Values are
mean±SD.
*p<0.05 by ANOVA for repeated measures (pretraining vs. posttraining times x isoproterenol dose).
sumption nor the percent increase from baseline was
significantly different between the two groups.
In the young men after training, absolute heart rate
responses to isoproterenol infusions were decreased
(p<0.05) (Table 3). Systolic, diastolic, and mean blood
pressure responses to isoproterenol were not altered by
training in the young men. In addition, the ejection
fraction, peak ejection rate, end-diastolic volume, endsystolic volume, stroke volume, and cardiac output
responses to isoproterenol were not altered by training.
In the older men, absolute heart rate responses to
isoproterenol infusions were also decreased with training (p<0.05) (Table 4). None of the other responses to
isoproterenol were altered by training in the older men
(systolic or diastolic blood pressure, ejection fraction,
peak ejection rate, end-diastolic volume, end-systolic
volume, stroke volume, or cardiac output).
The mean plasma isoproterenol concentrations at the
various infusion doses (Table 5) and the mean resting
norepinephrine and epinephrine concentrations were
unchanged after training in both groups. The older
group had no change in resting plasma norepinephrine
concentration (362±+126 pg/ml before versus 313±96
pg/ml after) or in plasma epinephrine concentration
(135±26 pg/ml before versus 135±39 pg/ml after) as a
result of training. Similarly, the young group had no
significant change in resting plasma norepinephrine
8.0±2.9
7.8±2.0
concentration (261±+79 pg/ml before versus 304±81
pg/ml after) or in plasma epinephrine concentration
(77+±39 pg/ml before versus 88±+23 pg/ml after) as a
result of training.
Discussion
There are two major findings of this study. First,
cardiovascular responses to isoproterenol, including
heart rate, ejection fraction, cardiac output, and systolic
and diastolic pressures were reduced in healthy older
men compared with younger men. Second, exercise
training did not augment isoproterenol responses in
these healthy men.
Aging and /3-Adrenergic Responses
The autonomic nervous system mediates, at least in
part, most cardiovascular responses including preload,
contractility, heart rate, afterload, and cardiac output.7-9'47
With aging, cardiovascular responses to stress are, in
general, reduced in both animals and humans.11'2'47'48
Several factors may contribute to the deficiencies of aging,
including occult disease (particularly coronary artery disease), deconditioning, and intrinsic age-associated structural and functional changes. In addition to these factors,
however, there is evidence in animal models of an ageassociated reduction in 8-adrenergic-mediated changes in
heart rate, vascular tone, and myocardial contractili-
Stratton et al Aging, Exercise Training, and P-Adrenergic Responses
509
TABLE 4. Mean Pretraining and Posttraining Responses in the Older Group (n= 13)
Rest
3.5 ng/kg/min
7 ng/kg/min
14 ng/kg/min
35 ng/kg/min
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Heart rate* (bpm)
107+12
89±13
73±10
77±11
Pre
66+10
88+14
71±11
62±9
65+9
Post
57+8
Systolic BP (mm Hg)
153+18
152+18
Pre
138+11
147±18
147+18
155±21
147±16
150±16
Post
136±9
140±11
Diastolic BP (mm Hg)
75±9
79±7
77±7
Pre
85±9
85±8
65±6
73±9
72±11
Post
84±8
76±8
Mean BP (mm Hg)
101±11
102±10
Pre
103±9
106±11
101±10
95±9
98±89
98±11
Post
101±8
98±7
Ejection fraction (%)
71±6
Pre
66±6
67±5
69±5
59±5
71±7
69±7
70±5
Post
61±6
68±5
EDV (ml)
85±26
Pre
97±24
96±20
94±22
90±18
106±30
Post
126±39
115±25
113±25
113±25
ESV (ml)
33±11
31±9
40±13
28±8
25±11
Pre
51±22
38±13
36±13
34±10
30±11
Post
Stroke volume (ml)
63±12
62±12
57±13
63±15
60±17
Pre
78±16
75±23
Post
75±19
77±16
79±17
Cardiac output (1/min)
3.7±0.8
4.6±1.0
4.8±1.1
5.5±1.3
6.4±1.8
Pre
4.3±1.1
4.8±1.2
5.0±1.0
5.6±1.3
Post
6.4±1.4
bpm, Beats per minute; BP, blood pressure; EDV, end-diastolic volume; ESV, end-systolic volume. Values are mean±SD.
*p<0.05 by ANOVA for repeated measures (pretraining vs. posttraining x isoproterenol dose).
ty.10llA47-53 The reduced contractile and vasodilating responses to ,B-adrenergic stimulation are not a generalized
phenomenon but rather are specific for 13-agonists, as the
responses to other agents (calcium or nitrates) are maintained.52-54 Because isoproterenol is not significantly
taken up by storage sites, the reduced responses cannot be
explained by differential uptake or release of the mediator
from storage sites.
In humans, there are fewer data, but studies have
noted smaller changes in systolic and diastolic pressures
during epinephrine or isoproterenol infusions with aging,13'55 reduced isoproterenol-induced venodilation,14
and a reduced heart response to isoproterenol.15'175657
Whether the contractile response to /3-adrenergic stimTABLE 5. Pretraining and Posttraining Isoproterenol Levels in
the Young (n=11) and Older Group (n= 13)
14
35
7
3.5
ng/kg/min
Young
Pre
71±+-17
71+43
Post
Older
Pre
132+62
Post
130±60
Values are mean±SD.
ng/kg/min
ng/kg/min
ng/kg/min
170±44
136±66
268+91
233+57
522±112
223±68
211±78
354±114
316±63
689±124
673±139
511 ± 132
ulation is reduced with aging in humans is uncertain.
One recent study, in which occult coronary artery
disease was not excluded, found a reduced echocardiographically measured shortening fraction response to
isoproterenol.56 We found a reduced ejection fraction
and peak ejection rate response in older men. The
reduced ejection cannot be explained by a greater
systolic pressure response, because the systolic pressure
actually rose more in the young than the older group; at
the highest infusion dose, when the ejection fraction was
10 units higher in the young than in the older group, the
systolic pressure was similar (158+21 mm Hg young
versus 151+± 18 mm Hg older). In addition, the ratio of
systolic pressure to end-systolic volume, which partially
normalizes for differences in afterload, rose by 280% in
the young compared with 88% in the older group.
Although none of the measures of contractility used in
this study are free of significant limitations, taken
together the results suggest a reduced inotropic response of healthy older hearts to ,B-adrenergic
stimulation.
Our subjects were thoroughly screened to exclude any
overt or occult disease, and a high proportion of older
subjects were eliminated because of abnormal screening
exercise tolerance tests. Therefore, underlying but undiagnosed disease is an unlikely explanation for our
results. The reduced responses of the older group were
510
Circulation Vol 86, No 2 August 1992
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also probably not a result of differences in circulating
isoproterenol concentrations, as marginally higher concentrations were present in the older group, possibly
because of a smaller volume of distribution or alterations in clearance. Thus, the present study might have
slightly underestimated the age-associated decline in
isoproterenol responses.
Another possible explanation for the reduced isoproterenol response at baseline in the older group might be
deconditioning.47 However, deconditioning does not
appear to account for the diminished responses in the
older group, because there was no change with training.
Therefore, the most likely reason for the observed
differences is an age-associated diminution in /3-adrenergic responses similar to that noted in animal models.
The available data suggest that the age-associated
decrease in catecholamine responses may be multifactorial and related to changes in both receptor and
postreceptor function.58,59 With aging there are reductions in p-receptor agonist binding affinity in humans,59-62 cardiac adenylate cyclase activity in animals,
and lymphocyte adenylate cyclase in humans,63-65 but
no change in the numbers or antagonist affinity of
p-receptors in humans.61,6266 The receptor alterations
may account, at least in part, for the decreased p-agonist responses with aging.
The reduced responses to isoproterenol in the older
group parallel the changes seen with exercise stress, in
which heart rate and ejection fraction responses are
decreased with aging.4,10,67 Our findings are consistent
with the interpretation that the reduced responses at
peak exercise are caused, at least in part, by reduced
p8-adrenergic responses. Conway et a168 noted that the
age-related differences in cardiac output during exercise
were lessened during 8-blockade, also suggesting that
the differences seen with age result from a decrease in
,8-adrenergic responses.
Exercise Training and p-Adrenergic Responses
Plasma catecholamine concentrations are reduced at
submaximal work rates after training,18-20 but exercise
cardiac output is maintained or increased,21'69 which
could be explained by an increased sensitivity to adrenergic stimulation, among other mechanisms. Training
does clearly increase p-adrenergic responses to epinephrine-induced lipolysis.23'70 Whether training increases cardiovascular sensitivity to catecholamines,
however, remains unclear. Two animal studies showed
increased contractility of isolated papillary muscles to
isoproterenol,24'25 whereas a third study noted no
change in norepinephrine sensitivity.28 In dogs, training
resulted in greater isoproterenol-induced changes in
stroke volume, stroke work, and maximal velocity of
contraction and systemic vascular resistance but not
ejection fraction.26'27 In rats, one group found decreased
vascular sensitivity to norepinephrine after training,29
whereas a second study found no change in hindlimb
vascular resistance to epinephrine.30 Although maximal
heart rate response to isoproterenol was reduced in
trained rats71 and pigs,31 other studies have found
increased or unchanged adrenergic heart rate
sensitivity.72
Previous data in humans regarding the effects of
exercise training on ,B-adrenergic responses are limited
and conflicting. Heart rate increases with isoproterenol
epinephrine have been similar in trained and untrained young subjects.33-35,37,73 Cross-sectional studies
have reported an increase3274 and a reduced73 blood
pressure response to norepinephrine or epinephrine
and no difference in cardiac output to isoproterenol in
trained compared with untrained subjects.36 In another
cross-sectional study, both the vasodilator and systolic
pressor responses to epinephrine were greater in endurance-trained subjects; in a longitudinal component to
this study, however, endurance training failed to alter
the systolic or diastolic pressure responses to epinephrine.37 In longitudinal training studies in humans, no
apparent change was noted in inotropic or chronotropic
sensitivity to epinephrine.33,37 However, the results of
these two studies are hampered by the small numbers
(n =6 and n = 10) and by the relatively crude contractility
measures by echo and systolic time intervals. A recently
reported longitudinal study in 16 young subjects (mean
age, 27 years) noted an improvement in echo-measured
fractional shortening.38 The reasons for the discrepancy
between findings of the study by Spina and colleagues38
and the findings in our young group are unclear but may
relate to measurement techniques (echo versus nuclear), patient populations (men and women versus all
men), or, less likely, training intensity (+20% versus
+ 17% maximal oxygen consumption).
The current study found no evidence of increased
heart rate, blood pressure, ejection fraction, or cardiac
output responses to isoproterenol. The lower heart rate
at all levels of infusion was probably a result of increased vagal tone. Thus, we conclude that regular
exercise does not increase /8-adrenergic responses in
healthy young or older men.
or
Limitations
The observed hemodynamic changes represent both
the primary effects of the infusion and secondary reflex
responses. The relative contributions cannot be determined. Baroreflex sensitivity is decreased with age,75
but differences in baroreflex sensitivity, if anything,
might tend to minimize the age differences observed.
Vagal inhibition of ventricular function probably occurs
in humans.76 An increase in vagal tone as a result of
training (as suggested by the reduced heart rate) might
mask any training-induced improvement in isoproterenol responses. Although we consider this unlikely, we
cannot exclude this possibility because we did not
pretreat with atropine. In addition, our study cannot
differentiate
and 82-adrenergic receptor responses.
Conclusions regarding the lack of a training effect in
,B-adrenergic responses must be tempered by the relatively small number of subjects. When all subjects were
combined (n = 24), however, there was still no evidence
of an enhanced cardiovascular response to isoproterenol after training for any of the measured variables.
The ejection fraction is a relatively crude measure of
contractility. More precise measures require invasive
methods, which were not deemed justifiable in this
healthy sample. The reduced ejection fraction during
isoproterenol in the older group cannot be explained by
differences in heart rate. Increasing the heart rate alone
causes no significant change in the ejection fraction.77 In
this study, we did not measure maximal ,-adrenergic
/3k-
responses
but only submaximal
safety concerns.
responses because of
Stratton et al Aging, Exercise Training, and j-Adrenergic Responses
In conclusion, the heart rate, blood pressure, ejection
fraction, and cardiac output responses to graded doses
of isoproterenol are reduced in healthy older men.
Endurance exercise training does not enhance f-adrenergic responses in either older or young men. Reduced
,f-adrenergic responses probably contribute to the decreased cardiovascular responses to maximal exercise
that occur with aging.
Acknowledgments
The authors thank Kevin Murphy, Jean Hadlock, John
Martin, Candy Sands, Geri Orta, Dale Matsuoka, and Anna
Coleman for their technical help. We thank Robert A. Bruce,
MD, for his help and encouragement in planning and conducting the study and for reviewing the manuscript. We thank Jim
Kousbaugh for his help in preparation of the manuscript.
References
Downloaded from http://circ.ahajournals.org/ by guest on June 11, 2017
1. Raven PB, Mitchell JH: Effect of aging on the cardiovascular
response to dynamic and static exercise, in Weisfeldt M (ed): The
Aging Heart. New York, Raven Press, 1980, chap 10, pp 269-296
2. Hossack KF, Bruce RA: Maximal cardiac function in sedentary
normal men and women: Comparison of age-related changes.
JAppl Physiol 1982;53:799-804
3. Julius S, Amery A, Whetlock LS, Conway J: Influence of age on
the hemodynamic response to exercise. Circulation 1967;36:
222-229
4. Port S, Cobb FR, Coleman RE, Jones RH: Effect of age on the
response of the left ventricular ejection fraction to exercise. N Engl
J Med 1980;303:1133-1137
5. Rodeheffer RJ, Gerstenblith G, Becker LC, Fleg JL, Weisfeldt
ML, Lakatta EG: Exercise cardiac output is maintained with
advancing age in healthy human subjects: Cardiac dilatation and
increased stroke volume compensate for a diminished heart rate.
Circulation 1984;69:203-213
6. Higginbotham MB, Morris KG, Williams RS, Coleman RE, Cobb
FR: Physiologic basis for the age-related decline in aerobic work
capacity. Am J Cardiol 1986;57:1374-1379
7. Neil E: Catecholamines and the cardiovascular system, in Blaschko
H, Sayers G, Smith AD (eds): Handbook of Physiology, Section 7.
Washington, DC, American Physiology Society, 1975, pp 473-489
8. Levy MN, Martin PJ: Neural control of the heart, in Berne RM
(ed): Handbook of Physiology, Section 2. Washington, DC, American Physiology Society, 1979, pp 581-620
9. Christensen NJ, Galbo H: Sympathetic nervous activity during
exercise. Annu Rev Physiol 1983;45:139-147
10. Gerstenblith G, Renlund DG, Lakatta EG: Cardiovascular
response to exercise in younger and older men. Fed Proc 1987;46:
1834-1839
11. Weisfeldt ML, Lakatta EG, Gerstenblith G: Aging and the heart,
in Braunwald E (ed): Heart Disease: A Textbook of Cardiovascular
Medicine, ed 4. Philadelphia, WB Saunders Co, 1992, chap 52,
pp 1656-1669
12. Lakatta EG: Determinants of cardiovascular performance: Modification due to aging. J Chronic Dis 1983;36:15-30
13. van Brummelen P, Buhler FR, Kiowski W, Amann FW: Agerelated decrease in cardiac and peripheral vascular responsiveness
to isoprenaline: Studies in normal subjects. Clin Sci 1981;60:
571-577
14. Hiremath AN, Pershe RA, Hoffman BB, Blaschke TF: Comparison of age-related changes in prostaglandin El and betaadrenergic responsiveness of vascular smooth muscle in adult
males. J Gerontol 1989;44:M13-M17
15. Fitzgerald D, Doyle V, Kelly JG, O'Malley K: Cardiac sensitivity to
isoprenaline, lymphocyte beta-adrenoceptors and age. Clin Sci
1984;66:697-699
16. Klein C, Hiatt WR, Gerber JG, Nies AS: Age does not alter
human vascular and nonvascular beta 2-adrenergic responses to
isoproterenol. Clin Pharmacol Ther 1988;44:573-578
17. Vestal RE, Wood AJJ, Shand DG: Reduced ,-adrenoreceptor
sensitivity in the elderly. Clin Pharmacol Ther 1979;26:181-186
18. Winder WW, Hickson RC, Hagberg JM, Ehsani AA, McLane JA:
Training-induced changes in hormonal and metabolic responses to
submaximal exercise. JAppl Physiol 1979;46:766-771
511
19. Cousineau D, Ferguson RJ, deChamplain J, Gauthie RP, Cote P,
Bourassa M: Catecholamines in coronary sinus during exercise in
man before and after training. JAppl Physiol 1977;43:801-806
20. Hartley LH, Mason JW, Hogan RP, Jones LG, Kotchen TA,
Mougey EH, Wherrg FE, Pennington LL, Ricketts PT: Multiple
hormonal responses to prolonged exercise in relation to physical
training. J Appl Physiol 1972;33:607-610
21. Ehsani AA, Ogawa T, Miller TR, Spina RJ, Jilka SM: Exercise
training improves left ventricular systolic function in older men.
Circulation 1991;83:96-103
22. Crampes F, Beauville M, Riviere D, Garrigues M, Lafontan M:
Lack of desensitization of catecholamine-induced lipolysis in fat
cells from trained and sedentary women after physical exercise.
J Clin Endocrinol Metab 1988;67:1011-1017
23. Martin WH, Coyle EF, Joyner M, Santensanio D, Ehsani AA,
Holloszy JO: The effects of stopping exercise training on epinephrine induced lipolysis. JAppl Physiol 1984;56:845-848
24. Mole PA: Increased contractile potential of papillary muscles from
exercise-trained rat hearts. Am J Physiol 1978;234(Heart Circ Physiol):H421-H425
25. Wyatt HL, Chuck L, Rabinowitz B, Tyberg JV, Parmley WW:
Enhanced cardiac response to catecholamines in physically trained
cats. Am J Physiol 1978;234(Heart Circ Physiol):H608-H613
26. Ramo P, Kettunen R, Hirvonen L: Hemodynamic effects of endurance training on canine left ventricle. Am J Physiol 1987;252(Heart
Circ Physiol):H7-H13
27. Ramo P, Kettunen R, Timisjarvi J, Takala T, Hirvonen L: Anabolic steroids alter the haemodynamic effects of endurance training on the canine left ventricle. Pfiugers Arch 1987;410:272-278
28. Williams JF, Potter RD: Effect of exercise conditioning on the
intrinsic contractile state of cat myocardium. Circ Res 1976;39:
425-428
29. Wiegman DL, Harris PD, Joshua IG, Miller FN: Decreased vascular sensitivity to norepinephrine following exercise training.
JAppl Physiol 1981;51:282-287
30. Richter EA, Christensen NJ, Ploug T, Galbo H: Endurance training augments the stimulatory effect of epinephrine on oxygen
consumption in perfused skeletal muscle. Acta Physiol Scand 1984;
120:613-615
31. Hammond HK, White FC, Brunton LL, Longhurst JC: Association
of decreased myocardial 3-receptors and chronotropic response to
isoproterenol and exercise in pigs following chronic dynamic exercise. Circ Res 1987;60:720-726
32. LeBlanc J, Boulay M, Dulac S, Jobin M, Labrie A, RousseauMigneron S: Metabolic and cardiovascular responses to norepinephrine in trained and nontrained human subjects. JAppl Physiol
1977;42:166-173
33. Martin WH, Coyle EF, Ehsani AA: Cardiovascular sensitivity to
epinephrine in the trained and untrained states. Am J Cardiol
1984;54:1326-1330
34. Williams RS, Eden RS, Moll ME, Lester RM, Wallace AG: Autonomic mechanisms of training bradycardia: ,-adrenergic receptors
in humans. JAppl Physiol 1981;51:1232-1237
35. Duncan JJ, Farr JE, Upton SJ, Hagan RD, Oglesby ME, Blair SN:
The effects of aerobic exercise on plasma catecholamines and
blood pressure in patients with mild essential hypertension. JAMA
1985;254:2609-2613
36. Lehmann M, Dickhuth HH, Schmid P, Porzig H, Keul J: Plasma
catecholamines, beta-adrenergic receptors, and isoproterenol sensitivity in endurance trained and non-endurance trained volunteers. Eur JAppl Physiol 1984;52:362-369
37. Svedenhag J, Martinson A, Ekblom B, Hjemdahl P: Altered cardiovascular responsiveness to adrenaline in endurance trained subjects. Acta Physiol Scand 1986;126:539-550
38. Spina RJ, Ogawa T, Coggan AR, Holloszy JO, Ehsani AA: Exercise training improves left ventricular contractile response to beta
adrenergic agonist. J Appl Physiol 1992;72:307-311
39. Bortz WM: Disuse and aging. JAMA 1982;248:1203-1208
40. Goldstein DS, Zimlichman R, Stoll R, Keiser HR: Plasma catecholamine and hemodynamic responses during isoproterenol infusions in humans. Pharmacol Ther 1986;40:233-238
41. Cerqueira MD, Harp GD, Ritchie JL, Stratton JR, Walker RD:
Rarity of preclinical alcoholic cardiomyopathy in chronic alcoholics less than 40 years of age. Am J Cardiol 1991;67:183-187
42. Wallis JW, Wu-Connolly L, Rocchini AP, Juni JE: Dynamic
arrhythmia filtration for gated blood pool imaging: Validation
against list mode technique. J Nucl Med 1986;27:1347-1352
43. Levy WC, Cerqueira MD, Matsuoka DT, Harp GD, Sheehan FH,
Stratton JR: Four radionuclide methods of left ventricular volume
512
44.
45.
46.
47.
48.
49.
50.
Downloaded from http://circ.ahajournals.org/ by guest on June 11, 2017
51.
52.
53.
54.
55.
56.
57.
58.
59.
60.
Circulation Vol 86, No 2 August 1992
determination: Comparison of manual and a new automated technique. J Nucl Med 1992;33:763-770
Goldstein DS, Feuerstein G, Izzo JL, Kopin IJ, Keiser HR: Validity and reliability of liquid chromatography with electrochemical
detection for measuring plasma levels of norepinephrine and epinephrine in man. Life Sci 1981;28:467-475
Evans MI, Halter JB, Porte D: Comparison of double- and singleisotope enzymatic derivative methods for measuring catecholamines in human plasma. Clin Chem 1978;24:567-570
Stratton JR, Halter JB, Hallstrom AP, Caldwell JH, Ritchie JL:
Comparative plasma catecholamine and hemodynamic responses
to handgrip, cold pressor and supine bicycle exercise testing in
normal subjects. JAm Coll Cardiol 1983;2:93-104
Filburn CR, Lakatta EG: Age-related alterations in ,B-adrenergic
modulation of cardiac cell function, in Johnson JJ (ed): Aging and
Cell Function. New York, Plenum Publishing, 1984, chap 4,
pp 211-246
Lakatta EG, Yin FC: Myocardial aging: Functional alterations and
related cellular mechanisms. Am J Physiol 1982;242:H927-H941
Lakatta EG, Gerstenblith G, Angell CS, Shock NW, Weisfeldt
ML: Diminished inotropic response of aged myocardium to catecholamines. Circ Res 1975;36:262-269
Dobson JG Jr, Fenton RA, Romano FD: Increased myocardial
adenosine production and reduction of 8-adrenergic contractile
response in aged hearts. Circ Res 1990;66:1381-1390
Yin FC: Aging and aortic impedance in beagle dogs. Mech Ageing
Dev 1988;42:49-62
Guarnieri T, Filburn CR, Zitnik G, Roth GS, Lakatta EG: Contractile and biochemical correlates of beta adrenergic stimulation
of the aged heart. Am J Physiol 1980;239:H501-H508
Deisher TA, Mankani S, Hoffman BB: Role of cyclic AMPdependent protein kinase in the diminished beta adrenergic
responsiveness of vascular smooth muscle with increasing age.
J Pharmacol Exp Ther 1989;249:812-819
Fleisch JH, Maling HM, Brodie BB: Beta receptor activity in aorta:
Variations with aging and species. Circ Res 1970;26:151-162
Wilkie FL, Halter JB, Prinz PN, Benedetti C, Eisdorfer C, Atwood
B, Yamasaki D: Age-related changes in venous catecholamines
basally and during epinephrine infusion in man. J Gerontol 1985;
40:133-140
Chou HT, Yokota Y, Fukuzaki H: Left ventricular reserve of the
hypertrophied heart in patients with systemic hypertension and
hypertrophic cardiomyopathy: Relation to age and left ventricular
relative wall thickness. Jpn Circ J 1990;54:373-382
Johansson SR, Hjalmarson A: Age and sex differences in cardiovascular reactivity to adrenergic agonists, mental stress and isometric exercise in normal subjects. Scand J Clin Lab Invest 1988;
48:183-191
Fan TH, Banerjee SP: Age-related reduction of beta-adrenoceptor
sensitivity in rat heart occurs by multiple mechanisms. Gerontology
1985;31:373-380
Scarpace PJ: Decreased receptor activation with age: Can it be
explained by desensitization? JAm Geriatr Soc 1988;36:1067-1071
Montamat SC, Davies AO: Physiological response to isoproterenol
and coupling of beta-adrenergic receptors in young and elderly
human subjects. J Gerontol 1989;44:100-1o5
61. Feldman RD, Limbird LE, Nadeau J, Robertson D, Wood AJ:
Alterations in leukocyte beta-receptor affinity with aging: A potential explanation for altered beta-adrenergic sensitivity in the
elderly. N Engl J Med 1984;310:815-819
62. Winther K, Naesh 0: Aging and platelet beta-adrenoceptor function. Eur J Pharmacol 1987;136:219-223
63. Scarpace PJ, Armbrecht HJ: Adenylate cyclase in senescence:
Catecholamine and parathyroid hormone pathways. Rev Clin Basic
Pharm 1987;6:105-118
64. Abrass IB, Scarpace PJ: Catalytic unit of adenylate cyclase:
Reduced activity in aged human lymphocytes. J Clin Endocrinol
Metab 1982;55:1026-1028
65. Scarpace PJ: Forskolin activation of adenylate cyclase in rat myocardium with age: Effects of guanine nucleotide analogs. Mech
Ageing Dev 1990;52:169-178
66. Abrass I, Scarpace PJ: Human lymphocyte beta-adrenergic receptors are unaltered with age. J Gerontol 1981;36:298-301
67. Levy WC, Cerqueira M, Abrass I, Schwartz R, Stratton JR: Stroke
volume is maintained in advanced age by cardiac dilatation despite
decreased contractility with supine bicycle exercise. (abstract)
J Nucl Med 1990;(suppl 31):838
68. Conway J, Wheeler R, Sannerstedt R: Sympathetic nervous activity during exercise in relation to age. Cardiovasc Res 1971;5:
577-581
69. Jennings G, Nelson L, Nestel P, Esler M, Korner P, Burton D,
Bazelmans J: The effects of changes in physical activity on major
cardiovascular risk factors, hemodynamics, sympathetic function,
and glucose utilization in man: A controlled study of four levels of
activity. Circulation 1986;73:30-40
70. Crampes F, Riviere D, Beauville M, Marceron M, Garrigues M:
Lipolytic response of adipocytes to epinephrine in sedentary and
exercise trained subjects: Sex related differences. EurJApplPhysiol
1989;59:249-255
71. Bolter CP, Atkinson KJ: Maximum heart rate responses to exercise
and isoproterenol in the trained rat. Am J Physiol 1988;254:
R834-R839
72. Schaible TF, Scheuer J: Cardiac adaptations to chronic exercise.
Prog Cardiovasc Dis 1985;27:297-324
73. Pavlik G, Frenkl R: Sensitivity to catecholamines and histamine in
the trained and in the untrained organism and sensitivity changes
during digestion. Eur JAppl Physiol 1975;34:199-204
74. Mann SJ, Krakoff LR, Felton K, Yeager K: Cardiovascular
responses to infused epinephrine: Effect of state of physical conditioning. J Cardiovasc Pharmacol 1984;6:339-343
75. Gribbin B, Pickering TG, Sleight P, Peto R: Effect of age and high
blood pressure on baroreflex sensitivity in man. Circ Res 1971;29:
424-431
76. Stratton JR, Pfeifer MA, Halter JB: The hemodynamic effects of
sympathetic stimulation combined with parasympathetic blockade
in man. Circulation 1987;75:922-929
77. Johannessen KA, Cerqueira M, Veith RC, Stratton JR: Influence
of sympathetic stimulation and parasympathetic withdrawal on
Doppler echocardiographic left ventricular diastolic filling velocities in young normal subjects. Am J Cardiol 1991;67:520-526
Differences in cardiovascular responses to isoproterenol in relation to age and exercise
training in healthy men.
J R Stratton, M D Cerqueira, R S Schwartz, W C Levy, R C Veith, S E Kahn and I B Abrass
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Circulation. 1992;86:504-512
doi: 10.1161/01.CIR.86.2.504
Circulation is published by the American Heart Association, 7272 Greenville Avenue, Dallas, TX 75231
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