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Circulation Research
JUNE
VOL. XXVIII
1971
NO. 6
An Official Journal of the American Heart Association
Pressor Reflexes Produced by Stimulation of
Afferent Fibers in the Cardiac Sympathetic
Nerves of the Cat
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By D. Fred Peterson and Arthur M . Brown
ABSTRACT
Afferent fibers in cardiac sympathetic nerves were stimulated electrically in
an attempt to evoke circulatory reflexes. A pressor response was always elicited
during stimulation of the central end of the cut left inferior cardiac or
pericoronary nerve in vagotomized intact-brain or spinal cats. The maximum
blood pressure rise was 21.5 mm Hg during inferior cardiac nerve stimulation
and 14.1 mm Hg during pericoronary nerve stimulation. Heart rate and
respiration were unaffected by stimulation. The alpha-receptor-blocking agent
phenoxybenzamine hydrochloride abolished the pressor resp3nse. Sequential
sectioning of cardiac nerves arising from either the stellate ganglion or the
thoracic sympathetic trunk, or of white rami 1 through 4, indicated that each
branch carried afferents contributing to the pressor response. Evoked potentials
revealed that excitation of ASfibersin the inferior cardiac nerve elicited a weak
pressor response, and excitation of Cfibersprovoked a much stronger response.
C fiber continuity between the pericoronary nerve and the inferior cardiac
nerve was demonstrated.
KEY WORDS
evoked potential
pericoronary nerve
sympathetic reflex
vagotomy
phenoxybenzamine hydrochloride
inferior cardiac nerve
• Spinal sympathetic reflexes initiated by
physiological stimulation of cardiovascular
receptors have been described recently (1-3).
They include an excitatory spinal sympathetic
reflex elicited by increases in coronary pressure and a similar reflex provoked by myocardial ischemia (1, 2). Some receptors lie in or
near the coronary arteries (4). The nerve
fibers to which they are connected are in the
From the Departments of Physiology and Medicine,
University of Utah Medical Center, Salt Lake City,
Utah 84112.
This study was supported by U. S. Public Health
Service Grant HE 05875-01, IROI HE 10977-01, and
IROI NS 09545-01. Dr. Brown is an Established
Investigator of the American Heart Association.
Received January 14, 1971. Accepted for publication March 4, 1971.
Reiewcb, Vol. XXV1I1, Jane 1971
cardiac sympathetic nerves and appear to
enter the spinal cord through the upper five
dorsal roots (5). Efferent sympathetics from
which these reflexes have been recorded in
spinal animals are in the third thoracic ramus
communicans and the inferior cardiac nerve.
The evidence for such excitatory spinal
sympathetic reflexes originating from cardiac
receptors has thus far been wholly electroneurographic. Reflex effects on blood pressure
and heart rate due to changes in coronary
pressure have not been established in previous
experiments on spinal animals. There are
several reasons for this. (A) Accumulation of
vasoactive substances or vasodilator metabolites secondary to extensive surgery (6). (B)
Change in cardiac performance when coro605
606
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nary flow is altered, particularly when flow is
reduced. (C) Depression of nervous and
circulatory systems following acute spinal
transection.
We have circumvented the first two of these
problems by electrically stimulating the central end of the cut pericoronary nerve in
vagotomized, intact-brain or spinal cats. However, in addition to stimulating afferent fibers
from receptors in or near the coronary
arteries, fibers from receptors elsewhere in the
heart projecting to the spinal cord through the
cardiac sympathetic nerves (7) were also
stimulated. The results show that stimulation
of these afferent fibers produces a rise in blood
pressure due to sympathetic vasoconstriction.
They confirm and extend the finding that
cardiac spinal sympathetic reflexes are primarily excitatory. This must be contrasted with
cardiac and cardiovascular reflexes mediated
through the vagi, carotid sinus nerves, and
medulla that mainly inhibit sympathetic activity.
Methods
Experiments were performed on 23 cats
weighing 2.5 to 5.0 kg anesthetized with sodium
pentobarbital (0.5 ml/kg) injected intraperitoneally. The femoral vein was cannulated for
subsequent administration of anesthetic or drugs.
The trachea was cannulated and in all except five
cats (see below), intermittent positive pressure
respiration was begun. The stroke of the pump
was adjusted to suppress spontaneous respiration
by the animal. Loose ligatures were placed
around the cervical vagi to facilitate sectioning
later.
Arterial blood pressure was recorded by a
pressure transducer (Statham, Model P23dB)
connected to a polyethylene catheter that had
been passed through a femoral artery to the aorta.
Body temperature was maintained with an
electric heating pad placed under the animal.
Two surgical approaches were used.
1. In each of 18 experiments, the animal was
placed on its back and the thoracic cavity
exposed via a bilateral incision between the fourth
and fifth ribs. When necessary, ribs on the left
side were removed to fully expose the region
between the left stellate ganglion and the middle
of the heart. This made possible access to the
inferior cardiac nerve as well as to the pericoronary nerve. Anatomical arrangement of the
innervation of this region is shown in Figure 1.
PETERSON, BROWN
Vl\|
Schematic drawing of the left thoracic sympathetic
trunk, stellate ganglion and their neural connections
to the heart. A = aorta; CA = coronary artery; CAS =
caudal limb of the ansa subclavia; CN = cardiac
nerve; CSN = cervical sympathetic nerve; ICN = inferior cardiac nerve; PA = pulmonary artery; PCN =
pericoronary nerve; Rj-R4 = white and gray rami;
RAS = rostral limb of the ansa subclavia; SG = stellate ganglion; TSN = thoracic sympathetic nerve
trunk; VN = vertebral nerve; X = vagus nerve. The
usual arrangement of the stimulating (S) and recording
(R) electrodes are shown.
The inferior cardiac nerve was prepared by
separating a length of 5 to 10 mm from the
surrounding connective tissue.
In 11 experiments, the left auricular flap was
reflected to expose the left atrioventricular groove,
and the left pericoronary nerve was separated
from the surrounding connective tissue. Because
of difficulties in making permanent mineral oil
pools, the exposed nerves were covered with
warm, saline-soaked gauze. During periods of
Circulation Research, Vol. XXVIII, June 1971
CARDIAC SYMPATHETIC PRESSOR REFLEX
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stimulation, nerves were dried and suspended in
air.
2. In five experiments, the left stellate ganglion
and its major branches were exposed without
entering the thoracic cavity, as described by
Brown (5). This permitted the animal to breathe
spontaneously during stimulation of the inferior
cardiac nerve. Respiration was recorded with a
concentric, bipolar electrode inserted into an
external intercostal muscle.
Bipolar hook electrodes (platinum-iridium)
were used for both stimulating and recording.
Square pulses were generated from a Grass SD9
stimulator. Intensity, frequency, and duration of
the stimulus were varied in order to determine the
optimum response for each animal. Except when
otherwise noted, responses were elicited by
rectangular pulses of 2.5 or 5.0 volts at a
frequency of 15/sec and a duration of 5 msec.
The period of stimulation was about 30 seconds.
Blood pressure was monitored continuously on a
Honeywell Visicorder (Model 1508), and in some
experiments was stored on tape using an Ampex
Tape Recorder (Model 1300).
Evoked potentials were recorded from the
inferior cardiac nerve after suitable amplification
using a Grass DP9 a-c preamplifier (10-3KC). In
two cats, the potentials were evoked by stimulating the pericoronary nerve, and in two other cats,
the distal end of the inferior cardiac nerve was
stimulated. These potentials were recorded on a
storage oscilloscope record camera (Fairchild
Type 450 A).
In three experiments, phenoxybenzamine hydrochloride (3 to 5 mg/kg Dibenzyline, Smith,
Kline and French) was administered intravenously to block alpha-receptor sites. The effectiveness
of blockade was demonstrated by injection of
norepinephrine (5/xg/kg DL-arterenol HC1, Sigma).
In two experiments, the spinal cord was
sectioned at the Cj level. Nerve stimulation was
begun about two hours after spinal section.
Periodically, corneal reflexes were tested and, if
necessary, supplemental doses of pentobarbital
(1/6 to 1/8 the initial dose) were given.
Results
It has been shown that changes in coronary
arterial pressure excite afferent sympathetic
fibers in the inferior cardiac nerve (1, 5).
Therefore, the circulatory effects of stimulating these fibers were studied initially. However,
this nerve contains afferent fibers from thoracic viscera other than the heart. For this
reason, the effects of stimulating cardiac
afferent fibers in the pericoronary nerve were
Circulation Research, Vol. XXVIII, Jane
1971
607
Blood pressure responses to stimulation of the central
end of the cut inferior cardiac nerve. A: Response
with all other nerves intact; 2.5 v, IS/sec, 5 msec
duration. B: Response after bilateral cervical vagotomy; 2.5 v, 15/sec, 5 msec duration. C: Response
after section of all central sympathetic connections
from the stellate ganglion; 10 v, 15/sec, 5 msec duration. D: Response after bilateral cervical vagotomy in
a spinal animal; 5 v, 15/sec, 5 msec duration.
examined. This nerve contains afferent fibers
which come solely from cardiac receptors
(8).
Stimulation of the central end of the cut
inferior cardiac nerve when all other nerves
were intact produced a pressor response (Fig.
2A). Similar stimulation after unilateral or
bilateral cervical vagotomy also produced an
increase in blood pressure usually of greater
magnitude (Fig. 2B, Table 1). In no case did
stimulation significantly change heart rate. In
the five experiments in which the pleura
remained intact and respiration was spontaneous, there was no significant change in
608
PETERSON, BROWN
Latency and Peak Pressor Response to Nerve Stimulation
in Vagotomized Cats
Latency (sec)
Onset
Peak
Peak response
(mm Hg)
Inferior cardiac
nerve (10)
Pericoronary
nerve (11)
5.4 =±= 0.75
20.4 =*= 2.44
4.1 ± 0.43
14.9 ± 1.46
21.5 =±= 2.39
14.1 ± 1.44
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Values are mean ± SE. Number in parentheses is
number of animals.
respiration rate. The blood pressure and heart
rate responses were similar in these experiments.
Sequential sections of the vertebral nerve
and the first three thoracic white rami reduced
the response but did not eliminate it entirely.
Subsequent section of the thoracic trunk
below the third white ramus always eliminated the response (Fig. 2C). In two cats
with spinal cords previously severed at the Ci
level, the pressor response persisted (Fig.
2D), although it was smaller than in intact
animals. The response was present both
before and after bilateral cervical vagotomy.
In one of these cats, cutting all central
connections of the left stellate ganglion
abolished the pressor response.
Injection of phenoxybenzamine hydrochloride in two cats eliminated the response to
stimulation of the inferior cardiac nerve.
The blocking effect of this dose was verified
by abolition of the pressor response following injection of norepinephrine. Prior to phenoxybenzamine hydrochloride, norepinephrine
produced a rise in blood pressure of 60 to 90
mmHg.
Stimulation of the central end of the cut
pericoronary nerve with all other nerves intact
produced a drop in blood pressure and a
slowing of the heart rate in eight of eleven
experiments (Fig. 3A). In the three remaining
experiments, stimulation produced a pressor
effect with no change in heart rate. We
suspected that the bradycardia and hypotension were due to stimulation of cardiac vagal
afferents in the pericoronary nerve. It was
anticipated that bilateral cervical vagotomy
would convert the depressor response to a
pressor response, since the pericoronary nerve
should also contain cardiac afferent fibers
which run centrally in the cardiac sympathetic
nerves (1, 2). In those eight experiments in
which pericoronary stimulation produced a
depressor effect, left cervical vagotomy caused
no change, but right cervical vagotomy, either
before or after left vagotomy, was followed by
the predicted pressor response (Fig. 3B, Table
1). The pressor response was not accompanied by changes in heart rate.
Complete left stellectomy and section of the
third and fourth white rami eliminated the
pressor response to left pericoronary nerve
stimulation in two of three cases. In the third
case, complete elimination of the response did
not occur until the right stellate ganglion was
excised as well. This is consistent with the
finding that branches emanating from the
right stellate ganglion join the left pericoronary nerve (8).
FIGURE 3
Blood pressure response to stimulation of the central
end of the cut pericoronary nerve. A: Response with
all other nerves intact; 2.5 v, 15/sec, 5 msec duration.
B: Response after bilateral cervical vagotomy; 2.5 v,
15/sec, 5 msec duration. C: Response after receptor
block with phenoxybenzamine hydrochloride, 5
mg/kg.
Circulation Research, Vol. XXV11I, June 1971
CARDIAC SYMPATHETIC PRESSOR REFLEX
609
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FIGURE 4
Correlation between evoked potentials in the inferior cardiac nerve and blood pressure responses.
A: Evoked potential of AS velocity with accompanying slight blood pressure rise (5 mm Hg);
10 v, 15/sec, 0.05 msec duration. B: Evoked potentials of As and C fiber velocities with accompanying large blood pressure change; 10 v, 15/sec, 0.1 msec duration. The larger AS wave
in this record is due to a change in recording conditions. C: Evoked potential in a slip Of fibers
from the inferior cardiac nerve during stimulation of the pericoronary nerve; 5 v, 0.5 msec
duration.
In three experiments, afferent pathways
between the pericoronary nerve and stellate
ganglion or sympathetic trunk were investigated. Pressor response to pericoronary nerve
stimulation was demonstrated after bilateral
cervical vagotomy and right stellectomy.
Sequential sectioning of the left inferior
cardiac nerve and other cardiac nerves indicated in Figure 1 demonstrated that each
nerve contributed to the pressor response to
stimulation. The response was totally abolished only after the final section. Stimulation
of the central end in each of these cut nerves
elicited a pressor response.
Injection of phenoxybenzamine hydrochloride in one vagotomized cat eliminated the
response to stimulation of the pericoronary
nerve (Fig. 3C).
In two animals, occlusion of the blood
supply to the adrenal glands, using ligatures,
did not alter the pressor response to pericoronary nerve stimulation in the vagotomized
cat. Release of the ligatures in one cat was
followed by an increase of 39 mm Hg in blood
pressure.
In one cat in which the spinal cord had
been previously severed at the d level the
Circulation Reiearch, Vol. XXVlll, June 1971
pressor response to pericoronary nerve stimulation was present both before and after
bilateral cervical vagotomy. This response was
not as great as that in the intact animal,
however.
Stimulation of the central end of the cut
inferior cardiac nerve elicited two distinct
compound potentials in the same nerve. In
one animal, the first appeared at 10 v and 0.05
msec and had a conduction velocity of 10 to
20 m/sec (Fig. 4A), and fell in the AS
category of afferent nerve fibers (9). With
stronger currents (10 v, 0.1 msec), a second,
larger evoked potential with a conduction
velocity of 1 m/sec appeared (Fig. 4B). Such
fibers belong to the C class (9), and many of
them are undoubtedly postganglioriic sympathetic sC fibers. However, some of them are
probably afferent or drC fibers (9), as we
shall demonstrate subsequently.
In two experiments, stimulation of the
central end of the cut inferior cardiac nerve at
voltages which were supramaximal for the AS
fibers but below threshold for C fibers, evoked
a small rise in mean arterial blood pressure (5
PETERSON, BROWN
610
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mm Hg) (Fig. 4A). When the voltages were
made supramaximal for the C fibers, much
larger increases in mean blood pressure
occurred (Fig. 4B).
Stimulation of the central end of the cut
pericoronary nerve in two experiments elicited
a C potential in the inferior cardiac nerve
(Fig. 4C). There was no evidence of an A8
component. The rise in blood pressure evoked
by such stimulation occurred when the C
elevation was evoked in the inferior cardiac
nerve (two experiments).
Discussion
The nature and location of the cardiac
receptors that send their afferent fibers centrally in the pericoronary nerve is only
partially known. Brown and Malliani (unpublished observations) found some afferent
fibers in the third thoracic ramus communicans excited by increases or decreases in
coronary flow which were silenced when the
pericoronary nerve was cut. However, other
cardiac receptors located in the atria and
ventricles may also have fibers in this nerve.
Nevertheless, stimulation of this population of
fibers produced a reflex rise in blood pressure
in vagotomized intact-brain and spinal animals; the hypertension was due to reflex
sympathetic vasoconstriction. This result is in
agreement with the finding of Brown and
Malliani (1) that changes in coronary flow in
spinal, vagotomized cats evoked a reflex
increase in sympathetic efferent discharge.
Thus there is further support for the idea that
excitatory sympathetic reflexes restricted at
least in part to the spinal cord are mediated
by afferent fibers originating from cardiac
receptors that run in the cardiac sympathetic
nerves. Moreover, the present experiments
show that a significant rise in blood pressure
results from such reflexes.
The afferent fibers in the pericoronary nerve
run to the stellate ganglion and sympathetic
trunk in at least five branches and probably a
relatively small percent of the fibers are found
in each branch. This fact, combined with the
technical problem of stimulating a short (2 to
3 mm) length of nerve while the heart is
beating, probably accounts for the difficulty in
evoking potentials in the inferior cardiac
nerves from the pericoronary nerve. It is
therefore not possible at this stage to identify
definitely which fiber groups are involved.
However, stimulation of AS and afferent C
fibers in the inferior cardiac nerve elicited a
pressor reflex similar to that elicited from the
pericoronary nerve with the afferent C fibers
making the larger contribution. Since we were
able to evoke a C potential in the inferior
cardiac nerve from the pericoronary nerve,
these fibers (if they are indeed afferent) mav
have been responsible for the effect of
pericoronary nerve stimulation.
Acknowledgment
We thank Dr. Diana L. Kunze for her helpful
criticism of this work and manuscript.
References
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Spinal
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Circulation Research, Vol. XXVlll, June 1971
Pressor Reflexes Produced by Stimulation of Afferent Fibers in the Cardiac Sympathetic
Nerves of the Cat
D. FRED PETERSON and ARTHUR M. BROWN
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Circ Res. 1971;28:605-610
doi: 10.1161/01.RES.28.6.605
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