Download Inverse Relationship Between Heart Rate and Blood

Inverse Relationship Between Heart Rate and
Blood Pressure Variabilities in Rats
ALBERTO U. FERRARI, ANNA DAFFONCHIO, FRANCESCO ALBERGATI,
AND GIUSEPPE MANCIA
Downloaded from http://hyper.ahajournals.org/ by guest on April 29, 2017
SUMMARY The interplay of heart rate variability, baroreceptor control of heart rate, and blood
pressure (BP) variability was examined in chronically instrumented, unanesthetized, freely moving
rats in which the efferent neural influences on heart rate were pharmacologically altered. In each rat,
BP was recorded continuously for 90 minutes in the control condition and in one or more of the
following conditions: 1) /3-adrenergic receptor blockade by propranolol, 1 mg/kg; 2) cholinergic
blockade by atropine, 0.75 mg/kg, and 3) combined blockade by propranolol plus atropine. Each BP
recording was analyzed beat-to-beat by a computer that calculated heart rate and BP variabilities,
both expressed as variation coefficients. In addition, under each condition the sensitivity of the arterial
baroreceptor control of heart rate was assessed by measuring the reflex changes in pulse interval in
response to BP changes induced by bolus i.v. injections of phenylephrine and nitroprusside. As
compared with the control condition, 1) propranolol (n = 10) reduced heart rate variability by
23 ± 4% (p<0.01), only slightly impaired baroreceptor reflex sensitivity, and did not significantly
modify BP variability ( +11 ± 7%); 2) atropine (n = 11) reduced heart rate variability by 30 ± 7%
(p<0.01), drastically impaired baroreceptor reflex sensitivity, and increased BP variability
( + 40 ± 8 % , p<0.01); 3) combined blockade (n= 10) caused variability and baroreceptor reflex
changes similar to those induced by atropine alone. Thus, heart rate variability depends on both vagal
and sympathetic influences. However, only the former component affects BP variability, that is, it
plays an antioscillatory role. This role is likely to originate from arterial baroreceptor modulation of
vagal cardiac drive. (Hypertension 10: 533-537, 1987)
KEY WORDS • blood pressure variability • cardiac parasympathetic nervous system
cardiac sympathetic nervous system • arterial baroreceptors • rats
S
Despite these considerations, our understanding of
the mechanisms controlling BP variability is limited.
Neurogenic influences are responsible for a substantial
proportion of this phenomenon,6 but it is unclear
whether its overall magnitude depends more on central
modulation of autonomic cardiovascular nerves than
on the buffering action of arterial baroreceptor reflexes.7 A further open question concerns the relationship between BP variability and spontaneous heart rate
variability, that is, whether heart rate variability plays
no role in, enhances, or buffers the BP variations.
In the present study pharmacological interventions
interfering with the cardiac parasympathetic and sympathetic influences were used to examine the relationship of baroreceptor reflex sensitivity, heart rate variability, and BP variability. The study was conducted in
unanesthetized, freely moving rats subjected to continuous BP and heart rate recordings to adequately
assess the variability phenomena.
TUDIES in several animal species have demonstrated that, in the unanesthetized state, blood
pressure (BP) is characterized by a large, spontaneous variability. 12 This variability also has been
documented in humans and may have clinical relevance. For example, BP variability is responsible for
the poor correlation between cuff BP measurements
and 24-hour or daytime BP values.3-4 Furthermore, at
any given BP mean, a greater 24-hour BP variability is
accompanied by a greater rate and severity of target
organ damage than is a smaller BP variability.5
From the Centro di Fisiologia Clinica e Ipertensione, Ospedale
Maggiore, CNR, Cattedra di Semeiotica Medica, and Istituto di
Clinica Medica Generale e Terapia Medica, Universita di Milano,
Milano, Italy.
Address forreprints:Dr. Alberto Ferrari, Centro Fisiologia Clinica e Ipertensione, Ospedale Policlinico, Via F. Sforza 35, 20122
Milano, Italy.
Received December 15, 1986; accepted June 5, 1987.
533
534
HYPERTENSION
Materials and Methods
The study used 17 male and female Sprague-Dawley
rats (Charles River Italia SpA, Calco, Italy) with a
mean age of 11.9 ±0.6 (SE) weeks. With the rats
under ketamine anesthesia, 80 mg/kg i.p., PE-50 catheters were implanted in the femoral artery and vein,
tunneled subcutaneously, exteriorized at the dorsal
neck region and kept patent by flushing with appropriate heparin solution (0.01% vol/vol). The animals
were allowed 2 days to recover from the operation and
to get acquainted with the experimental environment,
which consisted of wide individual cages in which rats
could walk, explore, and eat and drink ad libitum.
Protocol
Downloaded from http://hyper.ahajournals.org/ by guest on April 29, 2017
All recordings were performed during the daytime.
The arterial catheter was connected to a Statham
P23Dc pressure transducer (Oxnard, CA, USA) for
arterial pressure recording; the venous catheter was
connected to a long extension so that drug administration could be performed out of the animal's sight. Care
was taken to keep the room noiseless and to maintain a
low degree of illumination.
For the assessment of BP and heart rate variability,
the pressor trace was chart-displayed on a Grass polygraph (Quincy, MA, USA) and recorded on a Racal
Store 4 tape recorder (Southampton, Hampshire, England) for subsequent analysis. A continuous recording
lasting at least 90 minutes was obtained under each
experimental condition.
After the continuous BP recording was completed,
the baroreceptor control of heart rate was examined.
To this aim, the pressor trace was ink-written at a paper
speed of 2.5 mm/sec and simultaneously derived to
trigger a tachograph for beat-to-beat heart rate display.
Bolus i.v. injections of phenylephrine and nitroprusside (1—4 /ig/kg for both drugs) were administered
respectively to stimulate and to deactivate arterial baroreceptors, thus eliciting reflex heart rate changes.
Two to three boluses of each drug were given in a
random order.
Both the study of BP and heart rate variability and
the baroreceptor reflex study were performed under
control conditions and during 1) cholinergic blockade
by atropine sulfate, 0.75 mg/kg i.v. (n = 11); 2) £adrenergic blockade by propranolol, 1 mg/kg i.v.
(n = 10); and 3) combined blockade by propranolol
plus atropine (n = 10). The effectiveness of autonomic
blockades was checked immediately after the atropine
or propranolol injection and every 30 minutes thereafter. /3-Adrenergic blockade was considered effective
if an i.v. injection of isoproterenol, 0.2 Atg/kg, caused
no tachycardic response. Cholinergic blockade was
considered effective if the small, residual bradycardic
response to phenylephrine injection that was observed
immediately after atropine injection showed no recovery. Supplemental doses of either blocking agent were
given as needed.
The experimental session under control conditions
was the initial one in each rat. The session employing
either atropine or propranolol was then performed and
VOL 10, No 5, NOVEMBER
1987
was followed by the session using combined blockade.
Finally, after a 24-hour drug washout interval, the
session in which a single drug was omitted (atropine or
propranolol) was performed. Only four rats went
through all four experimental sessions, however. Of
the remaining rats, seven underwent the study with a
single drug and six were additionally given combined
blockade.
Data Analysis
To measure BP and heart rate variability, each 90minute BP recording was analyzed beat-to-beat by a
PDP11 computer (Digital Equipment, Maynard, MA,
USA) that scanned the trace every 60 msec and calculated 1) the mean values of heart rate and systolic,
diastolic, and mean arterial pressures for each 10-minute period during the 90-minute cycle, 2) the corresponding variation coefficients (i.e., the standard deviation divided by the mean multiplied by 100), and 3)
the average of the nine 10-minute values to obtain a
single measure for each variable examined.
Baroreceptor control of heart rate was analyzed by
calculating the ratio of the peak change in pulse interval to the peak change in mean arterial pressure induced by the vasoactive drug. In each rat, the ratios
obtained by the various injections of phenylephrine
were averaged. The same procedure was adopted for
the nitroprusside injections.
The statistical comparison of control and drug treatment conditions was performed by the nonparametric
Wilcoxon rank sum test (variability data) and the
paired t test (all other data). Because more than one
comparison was made for the same group of animals
(control vs 3 types of autonomic blockade), the/j level
of statistical significance was set below 0.01.
Results
As shown in Table 1, under control conditions mean
arterial pressure and heart rate were similar in the three
groups. Autonomic interventions did not significantly
alter BP, whereas they induced the expected heart rate
changes; namely, a slight bradycardia and a more pronounced tachycardia accompanying, respectively,
propranolol and atropine administration. During combined blockade a modest but significant tachycardia
was observed. After administration of all autonomic
blocking drugs, the behavior of the animal (physical
activity, grooming, eating, drinking, exploring patterns) showed no apparent variations from that observed during control conditions.
Effects of Autonomic Blockade on BP and
Heart Rate Variabilities
As shown in Figure 1, following atropine administration heart rate increased markedly and became less
variable, whereas BP oscillations were markedly enhanced. This response was apparent in 10 of the 11
animals studied, and the results were significant for the
group as a whole (Figure 2). The average reduction
in heart rate variability induced by atropine was
30 ± 7%, and the corresponding average increase in
BP variability was 40 ± 8%.
BP VARIABILITY AND BARORECEPTOR REFLEXES IN RATS/Ferrari et al.
535
TABLE 1. Average Values of Mean Arterial Pressure and Heart Rate Under Control Conditions and During Treatment
with Autonomic Blocking Drugs
Atropine +
Propranolol
Propranolol
Control
Control
Variable
(n=10)
(71=10)
(71=10)
(n=10)
MAP (mm Hg)
105 ± 3
101 ±2
100±3
100 + 2
106±5
106 + 3
38O±8
334 + 7
417 + 3*
HR (beats/min)
361 ±6
355 + 2
379 + 5*
Entries are means ± SE of the data obtained in single animals by computer analysis of a 90-minute continuous BP
recording under each experimental condition. MAP = mean arterial pressure; HR = heart rate.
*p<0.01, compared with control values.
Control
Atropine
Downloaded from http://hyper.ahajournals.org/ by guest on April 29, 2017
The results obtained after propranolol administration are shown in Figure 3. Propranolol reduced heart
rate variability in nine of the 10 animals studied, and
the average change (— 23 ± 4%) was significant for
the group as a whole. However, the reduction in heart
rate variability was not accompanied by a significant
change in BP variability.
The results of combined autonomic blockade are
shown in Figure 4. The effects of this intervention
closely mimicked those observed after administration
of atropine alone, that is, a marked reduction in heart
rate variability (— 40 ± 5%) and a marked increase in
BP variability (+ 33 ± 7%) occurred.
Effects of Autooomlc Blockade on Baroreceptor
Control of Heart Rate
As shown in Figure 5 (left panels), following atropine administration thereflexincrease andreductionin
pulse interval induced by phenylephrine and nitroprusside, respectively, were drastically blunted. In contrast, propranolol administration (see Figure 5, middle
panels) did not affect the pulse interval response to
phenylephrine, and it reduced the pulse interval response to nitroprusside to a much lesser extent than
was observed after the administration of atropine. The
pulse interval responses to phenylephrine and nitroprusside were almost completely abolished by combined autonomic blockade (see Figure 5, right panels).
Discussion
In our unrestrained rats, a 30% reduction in heart
rate variability induced by cholinergic blockade was
accompanied by an increase in BP variability. In contrast, a comparable reduction in heart rate variability
induced by J9-adrenergic blockade was not accompanied by any changes in BP variability. Finally, combined cholinergic and /3-adrenergic blockade caused
alterations in heart rate and BP variability that were
superimposable on those induced by cholinergic
blockade alone. These results cannot be explained by
behavioral differences induced by the central effects of
atropine or propranolol because no drug-related behavioral changes were observed. Thus, they indicate that
in the unrestrained condition 1) cardiac neural drive is
responsible for a substantial fraction of spontaneous
heart rate variability, 2) the oscillatory role is played
by both the vagi and the sympathetic cardiac nerves,
and 3) only the vagally mediated heart rate oscillations
subserve a BP-stabilizing effect (i.e., display a trend
toward a reduction in the magnitude of the spontaneous
BP oscillations that characterize the unrestrained
state).
As to the mechanisms underlying these results, a
plausible hypothesis is that the vagally mediated oscillations in heart rate are triggered by the arterial baroreceptors, reflecting the ability of this reflex system to
buffer BP changes through opposite changes in cardiac
I ATROPINE 0.75mg/Kg
200
mm
ABP
Hg
0
500
HR
L
15sec
250
FIGURE 1. Original recording showing the effects of atropine administration on arterial blood pressure (ABP)
and heart rate (HR) in an unanesthetized normotensive rat. Note the increase in ABP variability and the decrease
in HR variability after injection of the drug (arrow).
HYPERTENSION
536
HR
VOL 10, No 5, NOVEMBER
MAP
p<0 .01
p c C .01
T
a> 3
o
a
o
o
c
o
J
o
o
Control
Atropin*
Control
Downloaded from http://hyper.ahajournals.org/ by guest on April 29, 2017
output. This hypothesis is supported by the observation made in this and in previous studies that the vagus
is primarily responsible for the bradycardia accompanying baroreceptor stimulation8 and for the tachycardia
accompanying baroreceptor deactivation, with the
sympathetic nervous system playing a minor role. It is
also supported by the results of studies conducted in
animals and humans. In animals, the increased BP
variability that follows denervation of the arterial
baroreceptors has been shown to be accompanied by a
reduced heart rate variability.2910 Likewise, the
sensitivity of the baroreceptor-heart rate control has
been found to correlate positively with 24-hour heart
HR
MAP
6
I
o
Control
Propranolol
Control
Proprsnolol
* p<0.01 vs Control
FIGURE 3. Modifications in the heart rate (HR) and mean
arterial pressure (MAP) variation coefficients caused by propranolol-induced fi-adrenergic blockade in 10 individual rats
(
) and in the group as a whole (o
o).
i
1•
%
HR
I
FIGURE 2. Modifications in the heart rate (HR) and mean
arterial pressure (MAP) variation coefficients caused by atropine-induced cholinergic blockade in 11 individual rats (
)
and in the group as a whole (o
o).
36
'
Atroplne
* p < 0 0 1 vs Control
c
e
73
T
T
r s
1987
MAP
I Cont rol
Y//,:i P r o p r a n o l o l
+
Atropine
(n-10)
FIGURE 4. Effects of combined autonomic blockade by propranolol plus atropine on the variation coefficients of heart rate
(HR) and mean arterial pressure (MAP).
rate variability and negatively with 24-hour BP
variability."'12
Three further comments should be made. The first
comment concerns the report by Buchholz and Nathan13 that administration of methylatropine to conscious rats caused a reduction in heart rate variability
but no change in BP variability.13 However, BP variability was measured by analyzing a single pressure
wave every 15 seconds (i.e., 1-2% of all BP waves),
and intermittent sampling is known to limit precise
estimation of this phenomenon.14 Furthermore, methylatropine is endowed with ganglionic blocking properties" that may blunt sympathetic influences on peripheral circulation. This blunting effect may have
prevented an increased BP variability from being
observed.
The second aspect concerns the role of the sympathetically mediated heart rate variability in overall cardiovascular control. This question arises because the
propranolol-induced reduction in heart rate oscillations
did not cause any change in BP variability. As we
mentioned, failure of BP variability to increase following propranolol-induced reduction in heart rate variability can be easily explained by the fact that baroreceptorreflexinfluences on the sinus node are mediated
largely by the vagus. It is more difficult, however, to
explain why a propranolol-induced reduction in the
variability of heart rate (and probably of cardiac output) was not followed by a reduction in BP variability.
One possibility is that, at variance with the sudden
vagally evoked heart rate alterations, the slower-developing changes in heart rate mediated by the sympathetic system are not easily transformed into BP changes.
An alternative possibility is that when the /3-adrenergic
drive is removed, BP oscillations are maintained by
sympathetic influences on a-adrenergic receptors.
The third aspect relates to thefindingthat combined
BP VARIABILITY AND BARORECEPTOR REFLEXES IN RATS/Ferrari et al.
BARORECEPTOR STIMULATION
1
_ 1,5
FIGURE 5. Effects of cholinergic (left panels), fi-adrenergic (middle panels), and combined (right panels) blockade on baroreceptor
control of heart rate as studied by the vasoactive drug technique. Baroreceptor reflex sensitivity was calculated as the ratio of the peak
change in pulse interval (PI) to the peak
change in mean arterial pressure (MAP)
following baroreceptor stimulation by phenylephrine administration (upperpanels) or baroreceptor deactivation by nitroprusside administration (lower panels).
I
-p<0.01 ,
537
BARORECEPTOR DEACTIVATION
.- p<0.01-<
-p<0.01 -
< 1.0
Downloaded from http://hyper.ahajournals.org/ by guest on April 29, 2017
u u
• •
oa
ZZ2 Control
CD Atropin* (»:11)
ropr.nolol (n = 10>
n* (n-.IO)
cholinergic and /3-adrenergic blockade of the heart,
although more effective than selective blockade of either autonomic division, reduced spontaneous heart
rate variability by only slightly more than 40%. Because the effectiveness of total autonomic cardiac
blockade was well established (see Materials and
Methods), this finding indicates that in the rat more
than 50% of this phenomenon is unrelated to neural
modulation, at least as far as the traditional autonomic
pathways are concerned. The factors involved in this
residual heart rate variability are unknown, although
various humoral and physical mechanisms' 6 can be
regarded as possible candidates. Our propranolol data,
however, allow us to exclude the possibility that,
among the humoral factors, circulating catecholamines
play a role.
4.
5.
6.
7.
8.
9.
10.
Acknowledgments
Tbe authors gratefully acknowledge the contributions of Dr. Carla Zanchetti (illustrations), Mr. Riccardi Sarri (photography), and
Mrs. Paola Boccaccini (manuscript typing).
References
1. Anderson DE, Yingling JE, Sagawa K. Minute-to-minute covariations in cardiovascular activity in conscious dogs. Am J
Physiol 1979;236:H434-H439
2. Ramirez J, Bertinieri G, Belli G, et al. Reflex control of blood
pressure and heart rate by arterial baroreceptors and by cardiopulmonary receptors in the unanesthetized cat. J Hypertens
1985^:327-335
3. Sokolow M, Werdegar D, Kain HK, Hineman AT. Relationship between level of blood pressure measured casually and by
11.
12.
13.
14.
15.
16.
portable recorders and severity of complications in essential
hypertension. Circulation 1966^6:279-298
Perloff D, Sokolow M, Cowan R. The prognostic value of
ambulatory blood pressure. JAMA 1983;240:2792-2798
Parati G, Pomidossi G, Albini F, Malaspina D, Mancia G.
Relationship of 24 hour blood pressure mean and variability to
severity of target organ damage in hypertension. J Hypertens
1987;5:93-98
Mancia G, Ferrari A, Gregorini L, et al. Blood pressure and
heart rate variabilities in normotensive and hypertensive human beings. Circ Res 1983;53:96-1O4
Mancia G, Zanchetti A. Blood pressure variability. In: Zanchetti A, Tarazi RC, eds. Pathophysiology of hypertension —
cardiovascular aspects. Amsterdam: Elsevier Science Publishers, 1986:125-152 (Handbook of hypertension; vol 7)
Coleman TG. Arterial baroreceptor control of heart rate in the
conscious rat. Am J Physiol 1980;238:H515-H520
Ito CS, Scher AM. Hypertension following arterial baroreceptor denervation in the unanesthetized dog. Circ Res 1981;
48:576-586
Buchholz RA, Hubbard JW, Nathan MA. Comparison of 1hour and 24-hour blood pressure recordings in central or peripheral baroreceptor-denervated rats. Hypertension 1986;8:
1154-1163
Conway J, Boon N, Davies C, Jerres JV, Sleight P. Neural and
humoral mechanisms involved in blood pressure variability. J
Hypertens 1984;2:203-208
Mancia G, Parati G, Pomidossi G, Casadei R, Di Rienzo M,
Zanchetti A. Arterial baroreflcxes and blood pressure and heart
rate variabilities in humans. Hypertension 1986;8:147-153
Buchholz RA, Nathan MA. Chronic lability of the arterial
blood pressure produced by electrolytic lesion of the nucleus
tractus solitarii in the rat. Circ Res 1984|54:227-238
Di Rienzo M, Grassi G, Pedotti A, Mancia G. Continuous vs
intermittent blood pressure measurements in estimating 24hour average blood pressure. Hypertension 1983^:264-269
Fink LD, Cervoni P. Ganglionic blocking action of atropine
and methylatropine. J Pharmacol Exp Ther 1953; 109:372-376
Lange G, Lee HH, Chang A, McBrooks C. Effect of stretch
on the isolated cat sinoarrial node. Am J Physiol 1966;211:
1192-1196
Inverse relationship between heart rate and blood pressure variabilities in rats.
A U Ferrari, A Daffonchio, F Albergati and G Mancia
Downloaded from http://hyper.ahajournals.org/ by guest on April 29, 2017
Hypertension. 1987;10:533-537
doi: 10.1161/01.HYP.10.5.533
Hypertension is published by the American Heart Association, 7272 Greenville Avenue, Dallas, TX 75231
Copyright © 1987 American Heart Association, Inc. All rights reserved.
Print ISSN: 0194-911X. Online ISSN: 1524-4563
The online version of this article, along with updated information and services, is located on
the World Wide Web at:
http://hyper.ahajournals.org/content/10/5/533
Permissions: Requests for permissions to reproduce figures, tables, or portions of articles originally
published in Hypertension can be obtained via RightsLink, a service of the Copyright Clearance Center,
not the Editorial Office. Once the online version of the published article for which permission is being
requested is located, click Request Permissions in the middle column of the Web page under Services.
Further information about this process is available in the Permissions and Rights Question and Answer
document.
Reprints: Information about reprints can be found online at:
http://www.lww.com/reprints
Subscriptions: Information about subscribing to Hypertension is online at:
http://hyper.ahajournals.org//subscriptions/