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Journal of Exercise Physiologyonline
October 2015
Volume 18 Number 5
Editor-in-Chief
Official Research Journal of
Tommy
the American
Boone, PhD,
Society
MBA
of
Review
Board
Exercise
Physiologists
Todd Astorino, PhD
Julien Baker,
ISSN 1097-9751
PhD
Steve Brock, PhD
Lance Dalleck, PhD
Eric Goulet, PhD
Robert Gotshall, PhD
Alexander Hutchison, PhD
M. Knight-Maloney, PhD
Len Kravitz, PhD
James Laskin, PhD
Yit Aun Lim, PhD
Lonnie Lowery, PhD
Derek Marks, PhD
Cristine Mermier, PhD
Robert Robergs, PhD
Chantal Vella, PhD
Dale Wagner, PhD
Frank Wyatt, PhD
Ben Zhou, PhD
Official Research Journal
of the American Society of
Exercise Physiologists
ISSN 1097-9751
JEPonline
Cardiopulmonary Reflex and Blood Pressure
Response after Swimming and Treadmill Exercise in
Hypertensive Rats
Nádia L. Totou1, Renato W. M. Sá2, Andreia C. Alzamora2, Leonardo
M. Cardoso2, Lenice K. Becker1
1Sports
Center, CEDUFOP, Federal University of Ouro Preto,
Brazil/NUPEB, 2Federal University of Ouro Preto, Department of
Biological Sciences/NUPEB
ABSTRACT
Totou NL, Sá RWM, Alzamora AC, Cardoso LM, Becker LK.
Cardiopulmonary Reflex and Blood Pressure Response after
Swimming and Treadmill Exercise in Hypertensive Rats. JEPonline
2015;18(5):86-95. Cardiopulmonary sensitivity was evaluated after
exercise training through swimming and running in spontaneously
hypertensive rats (SHR) that were divided into three groups: (a) run
exercise; (b) swim exercise; and (c) sedentary. For 8 wks, the run
exercise was performed on a treadmill while the swim exercise was
performed by swimming. Cardiopulmonary reflex was evaluated by
chemical and mechanical pathways through the injections of
phenylbiguanide (PBG) (5.0 mg·kg-1) and volume expansion with
isotonic saline (0.75% of body weight), respectively. Both types of
exercise training decreased systolic blood pressure (SBP) compared
to the sedentary group. The swim trained group reduced SBP faster
than the run trained group. The sensitivity of the chemically activated
endings of the cardiopulmonary reflex was increased in both
exercise-trained groups for hypotensive response. The exercise
training groups had higher levels of urine output after acute volume
expansion. The production of urine showed that swimming and
treadmill training were more efficient than the sedentary group.
These results indicate that: (a) exercise improved cardiopulmonary
reflex sensitivity; and (b) swim training led to a faster SBP reduction
and a more sensitive reflex response to pressure stimuli.
Key Words: Cardiopulmonary reflex, Treadmill exercise, Swimming
exercise, Spontaneously hypertensive rats
87
INTRODUCTION
Exercise training is considered an efficient non-pharmacological treatment for hypertension in
spontaneously hypertensive rats (SHR) (5) and humans (12). In particular, moderate-intensity
exercise has been shown to be efficient in reducing blood pressure (5,19). However, other
studies have proposed that, depending on the environment, water exercise can produce
different acute and chronic hemodynamic changes compared to land exercise (2,6-8,19).
Studies have shown that water immersion causes an immediate translocation of blood from
the limbs and an increase in the intrathoracic blood volume that augments cardiac output by
increasing end-diastolic volume and stroke volume (14). Such an increase in central volume
leads to cardiopulmonary reflex activation that decreases the sympathetic outflow (17), which
prevents big increases in arterial blood pressure.
Thornton and colleagues (20) and Widdop et al. (24) indicated that a decrease in sensitivity
of cardiopulmonary reflexes induced both chemorreflex and volume-sensitive reflexes,
respectively, in SHR. Bertagnolli et al. (3) have shown that treadmill exercise training
increases (or restores) cardiopulmonary reflex responses when assessed by a bolus injection
of serotonin (3).
Endlich et al. (6) tested the hypothesis that different types of exercise training could induce
similar hormonal control of the blood pressure in SHR by evaluating blood atrial natriuretic
peptide (ANP) levels. They found that ANP increased only after swim training, suggesting this
type of exercise in fact induces an increase in central volume that stretches the atrial
chamber and possibly activates the cardiopulmonary receptors (6).
Data from our laboratory by Fabri and colleagues (7) show that after volume expansion,
aquatic-trained normotensive rats exhibited higher urine and sodium excretion. This finding
suggests that aquatic exercise was more efficient in adjusting the plasma volume after an
acute expansion (7). This efficiency might be related to a higher sensitivity of reflexes
involved in plasma volume adjustment as the cardiopulmonary reflex. Studies performed in
humans showed that a reduced sensitivity of cardiopulmonary reflex may precede the
development of hypertension (10). Grassi et al. (10) reported depressed responses to
changes in cardiac filling in subjects with borderline hypertension. In addition, Widgren et al.
(25) concluded that young normotensive subjects with a family history of hypertension have
delayed sodium excretion after an acute saline infusion.
These findings suggest a role for cardiopulmonary reflexes in the genesis of hypertension.
Thus, the purpose of this study was to determine the influence of different types of exercise
training (treadmill and swimming) on changes in blood pressure and the cardiopulmonary
reflex sensitivity of SHR. The swim exercise and the run exercise programs were used in
present work to describe the role of each in the efficiency to hypertensive animal models
(1,9). Both exercise programs were performed under aerobic metabolic conditions (9,15).
METHODS
Ethical Approval
The research procedures were carried out in compliance with the guidelines for the ethical
use of animals in scientific research as stated by the Federation of the Brazilian Society of
88
Experimental Biology. The Ethics Committee for Animal Use of the Federal University of Ouro
Preto approved the study (02/2013).
Animals
Male SHR, weighing 350 to 400 g, were provided by the Laboratory of Hypertension, Federal
University of Minas Gerais. The rats were housed in individual cages. Each rat received
regular chow and water ad libitum, and was maintained at constant temperature and a 12-hr
light/dark cycle. The rats were divided into three groups: (a) run exercise (n = 6); (b) swim
exercise (n = 8); and (c) sedentary (n = 9).
Procedures
Run Exercise Group
In the adaptation week, the run exercise group started the 1st d running 10 min at 18 m·min-1.
The running time was progressively increased by 10 min each day up to 60 min on the 6th d.
During this period, running speed was kept at 18 m·min-1. From the 1st wk to the 3rd wk,
running time was kept at 60 min·d-1, 5 d·wk-1 with the speed at 18 m·min-1. After that, the
speed was progressively increased to 20 m·min-1 (4th wk), 22 m·min-1 (5th and 6th wks) and
24 m·min-1 (7th and 8th wks) (1,15). Running time was kept at 60 min during this period.
Swim Exercise Group
Swimming training was performed in an apparatus adapted for rats that contained
warm water (30 to 32° C). The swim exercise group was also allowed to adapt for 1 wk.
They started the 1st d swimming 10 min without any tail load. Swimming time was
progressively increased by 10 min each day up to 60 min on the 6th d. After the adaptation
week, the rats were allowed to swim without any tail load for 60 min each day, 5 d·wk-1 during
the 1st, 2nd, and 3rd wks. In the 4th wk, a 2% body mass load was added to the rat’s tail
before exercise. In the 5th and 6th wks, the load was increased to 4% of body mass and, in
the 7th and 8th wks, the load was increased to 6% of body mass (9). The rats were then
allowed to swim with the loads for the entire 60 min of exercise each day.
Sedentary Group
The sedentary group did not participate in either the run exercise or the swim exercise during
the training period.
Cardiovascular Measurements
The time course of heart rate (HR) and blood pressure (BP) changes induced by exercise
training was followed by an indirect tail-cuff via a plethysmography sensor attached to a
computerized data acquisition system. Data were acquired on the 15th, 30th, and 45th d of
training and stored for further analysis.
After 8 wks of exercise training, the animals were anesthetized with a mixture of ketamine (80
mg·kg-1) and xylazine (10 mg·kg-1). Polyethylene cannulas filled with a solution of heparin in
isotonic saline were implanted in the femoral artery and vein to record BP and HR (artery),
and the drug infusion (vein). Each animal recovered from the anesthesia for 24 hrs in
individual cages with access to water and food. Before starting the recordings, the animals
remained in the recording room for at least 1 hr to adapt to the environmental conditions of
the laboratory. Immediately before registration began, a solution of heparin in isotonic saline
was injected into the femoral artery to prevent the formation of clots. The arterial cannula was
89
connected to a digital acquisition system, PowerLab biological signals 400 (AD Instruments,
Sydney, Australia). Pulsatile arterial blood pressure (PAP) was acquired by Chart 4.0 for
Windows software. Mean arterial blood pressure (MAP) and HR were calculated with the
same software. Baseline levels of SBP and HR were evaluated for a period of 40 min in the
three groups.
Cardiopulmonary Reflex Sensitivity
Chemosensitive
The responses to the stimulation of chemosensitive cardiopulmonary receptors (the BezoldJarisch reflex) were determined by a bolus injection of Phenylbiguanide (PBG) at a dose of
5.0 mg·kg-1. Maximal changes in BP and HR were evaluated.
Mechanosensitive
The mechanical pathway was evaluated by acute venous expansion by saline 0.9%. Each 3
min 0.75% of body weight was infused in vein, and each 6 min the urine production was
measure through cannula implanted inside the bladder. The urine volume was collected
during the 6th, 12th, 18th, 24th, and 30th-min during acute expansion (26).
Statistical Analyses
The experimental data were expressed as the mean ± standard error of the mean (SEM) and
the statistical analysis was performed using GraphPad Prism 6. The normal distribution was
evaluated by the Shapiro-Wilk test. A one-way analysis of variance (ANOVA) was used to
analyzed the data, which was then followed by the Newman-Keuls multiple comparison post
hoc tests. Any changes in the inter-group and intra-group were compared by a two-way
ANOVA followed by the Newman-Keuls multiple comparisons post hoc tests. The variables
without a normal distribution were analyzed by Kruskal-Wallis tests followed by Dunn's
multiple comparison tests. Pairs of means were considered different when the probability of a
Type I error was less than 5% (P<0.05).
RESULTS
The body weight of trained SHR was smaller in both the swim (321 ± 8.5 g) and the run (315
± 10 g) protocols in comparison with the sedentary rats (381 ± 5 g).
Blood Pressure and Heart Rate Levels
Figure 1 shows systolic (A) and diastolic (B) blood pressure levels measured by tail cuff
plethysmography of the SHR at the 15th, 30th, and 45th-d after the beginning of swim and
run exercises as well as the sedentary groups. Both types of exercise training (186 ± 5
mmHg for the run exercise group P<0.05 and 170 ± 3.5 mmHg for the swim exercise group
P<0.01) resulted in a decrease in SBP when compared to the sedentary group (211 ± 8
mmHg). The swim exercise was faster than the run exercise in decreasing SBP, which
produced significant reductions in SBP only at the 45th-d (as shown in Figure 1A). No
significant changes in DAP were observed among groups.
Data from the direct measurement of arterial blood pressure revealed that both types of
exercise (swim and run) reduced SAP (145 ± 3 mmHg for the run group P<0.001) and (145 ±
2 mmHg for the swim group P<0.001) in comparison with the sedentary group (183 ± 4
mmHg) (Figure 2A). Mean arterial pressure (MAP) (147 ± 2 mmHg for the sedentary group
90
versus 134 ± 8 mmHg for the run exercise group and 128 ± 3 mmHg for the swim exercise
group P<0.01) was significantly reduced only in the swim exercise group, without differences
between swim and run groups as shown in Figure 2B. Heart rate showed the same behavior,
at 397 ± 14 beats·min-1 for the sedentary group versus 310 ± 9 beats·min-1 for the run
(P<0.001) exercise group and 300 ± 4 beats·min-1 for the swim group P<0.001 to about the
same levels as shown in Figure 2C.
A
S e d e n ta r y
S w im
R un
S A P (m m H g )
240
220
200
180
*
#
160
*
15
30
45
D ays
B
D A P (m m H g )
200
180
160
140
120
15
30
45
D ays
*
150
*
100
50
0
200
500
150
*
100
50
0
S e d e n ta ry
R un
S w im
H R (b p m )
S A P (m m H g )
200
M A P (m m H g )
Figure 1. Time Course of Systolic (A) and Diastolic (B) Arterial Pressure in Sedentary Group
vs. the Run and Swim Groups (was followed by an indirect tail-cuff via a plethysmography sensor
on the 15th-d, 30th-d, and 45th-d of training). *P<0.01 in comparison with the sedentary group,
#P<0.05 in comparison with the run group and the sedentary group.
400
300
*
*
R un
S w im
200
100
0
S e d e n ta ry
R un
S w im
S e d e n ta ry
Figure 2. Heart Rate, Systolic and Mean Arterial Pressure in Sedentary Group vs. the Run and
Swim Groups. *P<0.0001 in comparison with the Sedentary Group.
91
Cardiopulmonary Reflex Sensitivity
Cardiopulmonary reflex sensitivity was evaluated by acute volume expansion and PBG
intravenous injection. The acute expansion effects were evaluated by the urine volume in μL
divided by body weight of each animal. The acute volume expansion show that the swim
group (437 ± 80 μL·mL-1, P<0.001) and the run group (278 ± 65 μL·mL-1, P<0.001) were more
efficient than the sedentary group (61 ± 2 μL·mL-1) to correct the volume expansion. These
data suggest that the exercise training groups had a more efficient way to keep volume under
control compared to the sedentary group. In addition, after 30 min of the acute expansion, the
swim training group demonstrates a higher urine production (Figure 3).
S e d e n ta r y
#
R un
8000
6000
(  L /g k W .)
U rin e v o lu m e
S w im
*
4000
*
2000
*
0
6
12
18
24
30
T im e ( m i n u t e s )
Figure 3. Urine Volume (µL/gkw) in Response to Acute Expansion with Infusion Intravenous of
Isotonic Saline in Sedentary Group, Run Group, and Swim Group. *P<0.001 in comparison with
Sedentary Group #P<0.05 in comparison with Run Group.
The sensitivity of the chemically activated endings of the cardiopulmonary reflex was
increased in both exercise-trained groups for a hypotensive response (-79 ± 3 mmHg for
swim and -52 ± 4 mmHg for run) compared to the sedentary group (-38 ± 3 mmHg) P<0.001
by the end of the 8 wks (Figure 4A). In particular, swimming induced higher hypotensive
responses compared with the run group, suggesting that swimming produces higher
sensitization of the chemically activated endings of the cardiopulmonary reflex. In addition,
both exercise training protocols (152 ± 14 beats·min-1 for swim, 123 ± 13 beats·min-1 for run)
induced a higher bradycardic response in comparison with the sedentary group (-82 ± 2
beats·min-1) (Figure 4B).
R un
S w im
S e d e n ta r y
0
-2 0
-4 0
-6 0
*
-8 0
S w im
-5 0
-1 0 0
-1 5 0
#
-1 0 0
R un
0
 H R (b p m )
 A P (m m H g )
S e d e n ta r y
-2 0 0
*
*
Figure 4. Decrease in AP (A) and HR (B) after PBG Injection 5.0 (µg·Kg-1) in the Sedentary, Run,
and Swim Groups. *P<0.0001 in comparison with Sedentary Group; #in comparison with the Run
Group.
92
DISCUSSION
This study corroborates the idea that run exercise and swim exercise training decreases the
blood pressure levels of SHR. Resting bradycardia is considered a consistent marker for
exercise training adaptation in both humans and SHR (4,13). Thus, the bradycardia found in
the exercise-trained rats clearly demonstrates the effectiveness of the exercise protocol used
in the present study. These protocols were based on methods that have been previously
described in the literature that ameliorated the blood pressure response in the SHR model
(3,6). Although we did not document that the exercise intensities were same in the swim
exercise group and the run exercise group, the bradycardic response induced by both
exercise programs was equivalent, which suggests that the cardiovascular adaptation were
similar between the two exercise groups.
The results obtained from the direct BP recordings showed that both types of training
produced bradycardia and a reduction in SBP. Also, the data indicates that the swim exercise
program produced a decrease in SAP earlier than did the run exercise program and that it
further sensitized chemically activated endings of the cardiopulmonary reflex. These findings
are in accordance with previous work that show decreases in HR after 8 wks of a running
program and a swimming program on SHR (6) and normotensive rats (18). The efficiency of
swim training to reduce BP faster than the treadmill training may relate to the sensitivity of
reflexes controlling BP. In this study, we observed a higher hypotensive response in the swim
exercise group after the PBG injection, which indicates that chemically activated endings of
the cardiopulmonary reflex are further sensitized in this group.
According to Verberne et al. (22), cardiopulmonary reflex activation results in cardiovagal
activation and sympathetic vasomotor inhibition. Together, these mechanisms produced large
decreases in ABP. This efficiency in the cardiopulmonary reflex can be attributed to a better
autonomic control. The volume expansion method showed an improvement in eliminating
volume by urine formation in both training types. However, the swim exercise group showed
a higher urine production during the 30th-min after the acute volume expansion. This
indicates a more efficient elimination of volume in this group.
In addition, the chemical evaluation shows that the swim exercise group demonstrated a
higher hypotensive response in comparison to the run exercise group. The mechanical and
chemical cardiopulmonary activation use different pathways, as supported by the work of
Weelken et al. (21) who showed that chemical cardiopulmonary blockade by 5-HT3 receptors
did not influence responses to cardiopulmonary mechanoreceptor stimulation. The data
indicate that the cardiopulmonary activation blunted barorreflex responses while the central
volume expansion induced a new barorreflex operating point (11,23).
CONCLUSIONS
The findings in this study support the notion that swim exercise may be better than running
exercise for lowering blood pressure in hypertension. Swimming improved cardiopulmonary
reflex sensitivity, thus it should be considered as a viable non-pharmacological alternative to
the standard treatment for hypertension.
93
ACKNOWLEDGMENTS
Research supported by CNPq, INCT-Nanobiofar and Pronex CNPQ/FAPEMIG CCBB-APQ04758-10 grants. We also thank Maria José Campagnole-Santos and Robson A. S. Santos
for SHR.
Address for correspondence: Lenice Kappes Becker, PhD, Centro Desportivo da UFOP
Morro do Cruzeiro, 35400000 – CEDUFOP – UFOP, Ouro Preto, MG, Brasil, Email:
lenice@cedufop.ufop.br, phone: 55 31 88976327
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