Download Respiratory and circulatory effects of parietal pleural afferent

J Appl Physiol 100: 1539 –1546, 2006;
doi:10.1152/japplphysiol.01422.2005.
Respiratory and circulatory effects of parietal pleural afferent stimulation
in rabbits
Yves Jammes and Stéphane Delpierre
Laboratoire de Physiopathologie Respiratoire (Unité Propre de Recherche de l’Enseignement Supérieur, Equipe
d’Accueil 2201), Institut Jean Roche, Faculté de Médecine, Université de la Méditerranée, Marseille, France
Submitted 9 November 2005; accepted in final form 29 December 2005
pleura; sensory innervation; phrenic activity; circulatory control
IN A PREVIOUS STUDY IN RABBITS,
we identified afferents in
internal intercostal nerves that were activated by mechanical
and/or chemical stimulation of the thoracic pleura (19). The
nerve responses were attributed to the selective activation of
pleural afferents because they existed after complete removal
of all muscles covering the studied intercostal spaces. Because
the conduction velocity of these afferents ranged between 0.5
and 14 m/s, we deduced that they belonged to the group III
(thin myelinated) and IV (unmyelinated) fibers. Most units
(97%) were activated by mechanical stimulation of the pleura
(local positive pressure range ⫽ 4.5 to 8.5 cmH2O), and we
found a linear relationship between the discharge rate of
afferents and the force applied to the thoracic wall. However,
only 29% of this nerve population was purely mechanosensitive. The other units also responded to lactic acid and/or
inflammatory mediators and were considered multimodal receptors. The response to lactic acid was roughly proportional to
its concentration.
Address for reprint requests and other correspondence: Yves Jammes,
Laboratoire de Physiopathologie Respiratoire (UPRES EA 2201), Faculté de
Médecine, Boulevard Pierre Dramard, 13916 Cedex 20 Marseille, France
(e-mail: [email protected]).
http://www. jap.org
Pleural diseases cause pain, dyspnea, tachypnea, and rapid
shallow breathing, and sometimes hypotension, which only
occurs under specific circumstances, e.g., tension pneumothorax, overwhelming infection of the pleural space causing sepsis, and associated severe cardiac dysfunction (26, 38). The
human study by Capps (6) showed that mechanical stimuli
applied to the peripheral margin of the diaphragm of awake
subjects elicited referred pain to the thorax, and the author
speculated about the role of intercostals nerves in producing
the sensations experienced by these patients. Besides, in the
human as well as animal literature, we found no experimental
data investigating cardiorespiratory reflex effects that may be
attributable to activation of pleural afferents.
On the basis of our previous data of the behavior of pleural
afferents in internal intercostal nerves (19), we conducted an
experimental study in the same species to explore the consequences of mechanical or chemical stimuli applied to the
parietal pleura on the breathing pattern (phrenic discharge) and
the circulatory function (blood pressure, heart rate). Some data
exist in the literature on the respiratory effects of internal
intercostal nerve stimulation, but they are only based on
electrical stimulation of the whole nerve trunk or mechanical
activation of nerve endings, but not on their chemical activation.
MATERIALS AND METHODS
Animal Care and General Preparation
The animal experiments were performed in accordance with the
requirements of the ethic committee of the Jean Roche Institute
(School of Medicine, Marseille, France). Our laboratory has been
granted a license from the French government to conduct animal
research.
Seven adult rabbits (body weight ⫽ 2.5 to 3.0 kg) were used. They
were anesthetized by an injection of ethyl carbamate, 1 g/kg (urethane) in the marginal ear vein. The external jugular vein was
cannulated to continue anesthesia by subsequent injections and to
perfuse the animals with saline containing adrenalin (0.3
␮g 䡠 kg⫺1 䡠 h⫺1) to maintain the systolic blood pressure in the range
100 –130 mmHg after thoracotomy. A heating pad maintained rectal
temperature in the range 37–38°C. The animals were paralyzed at
hourly intervals by intravenous injections of pancuronium bromide
(Pavulon, Organon Technika, France, 0.4 mg/kg).
Throughout and after the operative procedure, the adequacy of the
level of anesthesia was judged from the changes in blood pressure, heart
rate, and pupil size evoked by noxious stimuli, the changes in these
variables governing the injection of hourly doses of ethyl carbamate. At
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1539
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Jammes, Yves, and Stéphane Delpierre. Respiratory and circulatory effects of parietal pleural afferent stimulation in rabbits. J Appl
Physiol 100: 1539 –1546, 2006; doi:10.1152/japplphysiol.01422.2005.
—Respiratory symptoms accompanying pleural diseases combine
dyspnea, tachypnea, rapid shallow breathing, and sometimes hypotension. There are no experimental data on the changes in respiratory
and circulatory functions elicited by the activation of pleural afferents.
After removal of all muscles covering the 5th to 10th intercostal
spaces, we investigated in paralyzed, vagotomized rabbits the changes
in phrenic discharge, transpulmonary pressure, and systemic arterial
pressure in response to an outwardly directed force exerted on the
parietal pleura or the local application of solutions containing lactic
acid or inflammatory mediators. Mechanical stimulation of the pleura
induced an immediate decrease in both integrated phrenic discharge
and arterial blood pressure, the responses being positively correlated
with the magnitude of force applied on the pleura. No accompanying
changes in ventilatory timing, transpulmonary pressure, or heart rate
were measured. Lactic acid solution also elicited an inhibition of
phrenic activity and a fall in blood pressure. Section of the internal
intercostal nerves supplying the stimulated intercostal spaces totally
abolished the responses to mechanical stimulation or lactic acid. An
inflammatory mixture elicited only modest respiratory and circulatory
effects. We concluded that an acute mechanical distension of the
parietal pleura as well as its chemical stimulation by lactic acid elicit
a marked inhibition of phrenic motoneurons combined to a reduction
of the sympathetic outflow to the circulatory system.
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FUNCTIONAL ROLES OF PLEURAL AFFERENT
the end of the experiments, the rabbits were killed by an intravenous
injection of a hyperosmolar potassium chloride solution.
Surgical Procedure
Because mechanical stimulation of the visceral pleura and/or the
heart could sometimes occur when a piece of gauze was applied
against the thoracic wall, both vagus nerves were cut at the cervical
level to eliminate the major cardiopulmonary innervation.
A large median sternotomy was performed from the xyphoid to the
manubrium sterni to bilaterally expose the entire thoracic pleural area.
On both sides, the thoracic muscles covering the 5th to 10th intercostal spaces, such as the pectoralis major and minor, serratus anterior,
triangularis sterni, and abdominal muscles (transversus and rectus
abdominis and internal oblique) were dissected and removed. In these
interspaces, we used an operating microscope (⫻40, OPM 11 Zeiss)
to carefully remove the external and internal intercostal muscles and
to leave the parietal pleura intact. The more proximal muscles attaching to the ribs (dorsal parts of the intercostals, iliocostalis, or levator
costae) were not removed, but all the muscles distal to the points of
nerve sections performed at the end of the protocol were removed.
A tracheotomy was performed, and the animals were ventilated at
constant volume (10 ml/kg) and frequency (24 –25 min⫺1) with a
Harvard volumetric pump. O2 and CO2 fractions were respectively
measured with rapid pyrolytic and infrared gas analyzers. End-tidal
CO2 fraction was maintained between 0.03 and 0.04, and inspired O2
concentration was fixed at 0.30.
The tracheal pressure was measured with an electromanometer
(Statham PM5) connected to a side arm of the tracheal cannula.
Because the chest was largely opened and the animals were mechanically ventilated, tracheal pressure measurements reflect the variations
of lung resistance. Lung compliance was maintained stable by performing frequent pulmonary inflations (3 ⫻ stroke volume of the
pump) (25).
The left carotid artery was catheterized for measurements of blood
pressure and heart rate, with an electromanometer (Statham P23 Db),
and also for blood gas analyses (Radiometer ABL 330, Copenhagen,
Denmark).
A phrenic root was dissected free in the neck on each side. The
dissected roots were left intact and placed sequentially on a monopolar tungsten electrode, but in two rabbits both phrenic nerves were
simultaneously recorded during application of test agents on a hemithorax. The nerve activity was referred to a nearby ground electrode,
amplified (50 –100,000), and filtered (30 Hz to 10 kHz) by a differential amplifier. The phrenic neurogram was integrated with a voltagefrequency analog converter (13). We measured the peak amplitude
and duration of integrated phrenic activity and the total breath duration (ventilatory period) from chart recordings of the phrenic discharge.
A strain gauge (Narco-Bio Systems, Detroit, MI), linear from 0 to
50 g, was used to measure the force produced during the gauze
application against the thoracic pleura. A steel hook was placed in a
costal cartilage of one of the ribs (6th or 7th) bounding the space
stimulated. Application of force to the pleura was performed dorsally
to the hook with a gauze covering the 6th to 8th interspaces. The hook
was connected to the strain gauge with a nylon cable. Thus force was
measured perpendicularly to the thoracic wall.
Protocol
The following sessions were performed for each thoracic side
during which transpulmonary and arterial blood pressures and the
ipsilateral phrenic activity were continuously recorded.
Mechanical stimulation (touch stimulus) of the thoracic parietal
pleura (7 rabbits; n ⫽ 29). A small piece of gauze (10 ⫻ 20 mm)
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Physiological Measurements
soaked in buffered isotonic saline (NaCl 0.9%; pH ⫽ 7.0) was placed
for a 10-s period close to the internal thoracic wall at the level of the
studied intercostal spaces. The gauze was applied against the thoracic
wall by use of a glass rod for 15–35 s (mean duration: 28 ⫾ 4 s). This
procedure allowed variation of the applied force as measured by the
strain gauge. The values of force elicited by touching ranged from 2
to 45 g, corresponding to a pressure of 1 to 22.5 g/cm2 (area of the
gauze ⫽ 2 cm2), i.e., 1 to 22.5 cmH2O. Because of the relatively large
value of the gauze area (2 cm2), the mechanical stimulation concerned
at least two spaces. For the maximal mechanical stimulation (50 g),
we measured a 1-cm outward motion of the ribs. Because force was
not measured at the pleura but by displacement of the ribs, one may
suppose that the true value of force applied on the pleura could be
more. However, underestimation of the pleural mechanical stimulation should be minimized by the position of the hook used for force
measurement, which was placed adjacent to the pleura being
stretched.
Response to lactic acid (7 rabbits). A gauze piece (10 ⫻ 20 mm)
soaked in lactic acid solution (40 mM) was positioned for a 10-s
period at the same thoracic level. No pressure was continuously
exerted on the gauze piece. In five animals, we also compared the
response to 20 mM (n ⫽ 7; pH ⫽ 5.10) and 40 mM (n ⫽ 11; pH ⫽
4.7) lactic acid solutions. These lactic acid concentrations are often
measured in pleurisies (5).
Response to the application of a gauze piece soaked in an inflammatory mixture composed of bradykinin-5-HT-histamine-PGE2 at
10⫺5 M concentration (pH ⫽ 6.7) (6 rabbits, n ⫽ 9). The composition
of this mixture was the same as in our previous study (19) and also
that by Wedekind (36). The contact of the pleura with the inflammatory mixture solution was maintained for a 30-s period.
Lactic acid and inflammatory mixture tests were randomized
among animals.
Chemical stimulation obliged us to transiently apply the gauze
soaked in solutions against the thoracic wall and thus to elicit an initial
mechanical stimulation. We preferred to use this mean and not to rinse
the parietal pleura with chemical solutions to avoid any spreading of
chemicals over the thorax. We also avoided spreading of chemicals
over the internal thoracic wall by limiting the soaking of gauze in each
solution. Between two successive applications of chemicals, the
thoracic side was rinsed with a large amount of warmed saline. Fifteen
minutes lapsed between two successive test agents. The chemical
solutions in which the application gauze was soaked were maintained
at 37°C in a thermostatic water bath.
Blank tests for chemical stimulation consisted of application of
gauze soaked in warmed saline serum for the same duration. They
were reproduced by touch stimuli eliciting the same mechanical
stimulation.
A whole trial in one pleural side consisted of four to seven
mechanical stimulations, which were followed by one blank chemical
test and then by one or two applications of lactic acid solution (20 mM
then 40 mM) and one application of the inflammatory mixture.
In two animals, mechanical stimulation was applied on the thoracic
pleura when recording both the ipsi- and contralateral phrenic nerves.
At the end of each experiment, the internal intercostal nerves in 5th
to 10th spaces were dissected and freed from surrounding tissues as
described by De Troyer and Legrand (11). The final section of internal
intercostal nerves was distal to the branch point for the lateral nerve
branch innervating external abdominal oblique muscle. Before section
of intercostal nerves, one mechanical stimulation was repeated to
assess the persistency of control phrenic and circulatory responses.
After nerve sections, we repeated mechanical stimulations and application of the 40 mM lactic acid solution on the thoracic pleura and
measured the response of the ipsilateral phrenic nerve and the changes
in blood pressure and heart rate.
FUNCTIONAL ROLES OF PLEURAL AFFERENT
1541
Fig. 1. In the same animal, 2 examples of the
changes in raw and integrated phrenic activity
(E Phr) and arterial blood pressure in response
to a positive force applied on the parietal pleura
(same side for pleural stimulation and phrenic
nerve recording). In right tracings, force was
out of range.
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Statistical Analysis
The baseline values of transpulmonary pressure, systolic and diastolic blood pressure, heart rate, peak integrated phrenic discharge,
and total breath duration were averaged for a 30-s period before
application of each test agent and were expressed as means ⫾ SE.
Then, the significance of maximal changes in each variable induced
by each test agent was determined with respect to the corresponding
averaged baseline value. For phrenic nerve recording, the changes in
amplitude and duration of the phrenic discharge were assessed once
per cycle to determine their maximal variations. For the majority of
mechanical and chemical tests, the depression of phrenic activity
remained stable for a few breaths (5–12). Thus reported data corresponded to the averaged changes in amplitude and duration of phrenic
discharge. We used an analysis of variance for repeated measures
followed by Student-Newman-Keuls post hoc test to indicate the
direction and magnitude of the variations between the different conditions. Data processing was conducted on absolute values by using a
software program (SigmaStat, Jandel, Chicago, IL). Because there
were no significant differences between the baseline values of physiological variables measured in each animal throughout a whole
experiment, we expressed the changes in each variable elicited by
pleural stimulation as a percentage of the corresponding prestimulation level. Regression analyses between force applied on the thoracic
pleura and the changes in phrenic discharge and blood pressure were
also performed.
RESULTS
Responses to Mechanical Stimulation of the Thoracic Pleura
As illustrated in Fig. 1, the response to mechanical stimulation resulted in an immediate decrease (1.2 ⫾ 0.3 s) in peak
amplitude of integrated phrenic discharge and a slightly longer
latency (2.4 ⫾ 0.5 s) for the first drop in arterial pressure, the
circulatory response often adapting throughout the period of
mechanical stimulation. The decrease in amplitude of phrenic
discharge and also systolic and diastolic blood pressures was
positively correlated with the magnitude of force (Fig. 2). We
found no correlation between the stimulus duration and the
magnitude of either phrenic or blood pressure response. Both
baseline spontaneous respiratory frequency (36 ⫾ 8 min⫺1),
counted from integrated phrenic nerve recording, and phrenic
J Appl Physiol • VOL
Fig. 2. Correlations between the decrease in peak integrated phrenic discharge
(Phr integrated) or the systolic (Pa sys.) and diastolic (Pa dias.) arterial
pressures and the force applied to the parietal pleura (data obtained in 7
rabbits). Regression lines with 95% confidence intervals are only shown for the
phrenic response because the relationships between systolic or diastolic blood
pressure vs. force overlapped.
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FUNCTIONAL ROLES OF PLEURAL AFFERENT
Fig. 3. Comparison of the effects of a positive force applied on
the left parietal pleura on ipsi- and contralateral phrenic nerve
activities. Despite the fact that in this example the force was
irregularly maintained on the thorax (glass rod used to apply the
force was hand held), the phrenic nerve response in the ipsilateral
side was consistent but totally absent in the contralateral one.
In all animals, section of the internal intercostal nerves
supplying the stimulated intercostal spaces abolished both the
phrenic and blood pressure responses (Fig. 4).
Responses to Chemical Stimulation
Lactic acid. Both the 20 and 40 mM solutions elicited
significant phrenic and blood pressure changes (Fig. 5). In the
seven rabbits, application of gauze soaked in lactic acid early
Fig. 4. Denervation of the intercostal spaces [section
of the 5th to 10th internal intercostal (IC) nerves]
abolishes both the inhibition of ipsilateral phrenic activity and the decrease in arterial blood pressure in
response to a positive force applied on the parietal
pleura.
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discharge duration (0.61 ⫾ 0.08 s) did not significantly vary in
response to mechanical stimulation, even when the stimulus was
maximal. No significant variation of transpulmonary pressure was
measured, and HR (baseline: 260 ⫾ 22 min⫺1) did not vary at all.
The phrenic response seems to be strictly unilateral because
no changes in phrenic discharge were measured when the
contralateral phrenic nerve was recorded during mechanical
stimulation of the thoracic pleura. Figure 3 gives an example of
seven trials conducted in two animals.
FUNCTIONAL ROLES OF PLEURAL AFFERENT
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Fig. 5. Changes in phrenic nerve activity (ipsilateral side) and arterial blood pressure in response to the application of a solution containing 20 or 40 mM of lactic acid on the parietal
pleura.
Table 1. Maximal respiratory and blood pressure changes in
response to local application of 20 mM or 40 mM lactic
acid solution on the thoracic pleura
LA 20 mM (n ⫽ 7)
LA 40 mM (n ⫽ 11)
⌬Ephr, %
⌬Pa systolic, %
⌬Pa diastolic, %
⫺29⫾5*
⫺36⫾6
⫺19⫾3*
⫺25⫾2
⫺23⫾4†*
⫺31⫾3‡
Values are means ⫾ SE, given as percent of baseline. Ephr, phrenic activity;
Pa, blood pressure; LA, lactic acid; ⌬, change. *Significant difference (P ⬍
0.05) between the effects of 20 mM and 40 mM solutions. Significant
differences between systolic and diastolic blood pressure variations: †P ⬍
0.05; ‡P ⬍ 0.01.
J Appl Physiol • VOL
totally abolished both the respiratory and circulatory responses
to lactic acid.
Inflammatory mixture. The inflammatory mixture elicited a
modest but significant decrease in both phrenic discharge
(⫺15 ⫾ 4%; P ⬍ 0.05) and blood pressure (mean decrease in
systolic pressure was ⫺13 ⫾ 2%; and mean decrease in
diastolic pressure equaled ⫺10 ⫾ 2%, P ⬍ 0.05). The phrenic
and circulatory response latencies were 5.3 ⫾ 1.4 and 7.8 ⫾
2.0 s, respectively.
Unlike the durable responses to lactic acid or inflammatory
mixture, application of gauze soaked in warmed physiological
saline serum (blank chemical test) only induced transient
inhibitory effects limited to the duration of mechanical
stimulation.
DISCUSSION
The present study reports inhibitory effects of mechanical
and chemical stimulation of the thoracic parietal pleura on both
the phrenic nerve discharge and systemic blood pressure, with
no associated changes in the ventilatory timing and heart rate.
Both the changes in phrenic discharge and the circulatory
response were proportional to the magnitude of mechanical
stimulation and also depended on the concentration of lactic
acid solution applied against the parietal pleura. Application of
a mixture of inflammatory mediators also elicited an inhibition
of the phrenic nerve discharge and a blood pressure decrease.
The response was reflex because the section of internal intercostal nerves totally suppressed both the respiratory and circulatory effects. We supposed that muscle afferents cannot be
involved in the reflex phrenic and circulatory responses elicited
by pleural stimulation. Indeed, we ensured the destruction of
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elicited phrenic inhibition and blood pressure decrease (2.3 ⫾
1.2 and 3.8 ⫾ 1.0 s, respectively), which may be attributed to
the initial modest mechanical stimulation. Then, opposing the
phrenic response to mechanical stimulation that disappeared
after removal of pressure applied to the chest wall (Figs. 1, 3,
and 4), the respiratory and also the circulatory effects elicited
by lactic acid continued to develop despite the absence of
persistent changes in force applied against the thoracic wall
(Fig. 5). The response was significantly greater for trials
performed with the greater lactic acid concentration (Table 1).
As for mechanical stimulation, we did not measure any significant variation of spontaneous respiratory frequency, transpulmonary pressure, and HR. In all cases, the blood pressure fall
lasted more than the phrenic inhibition and, as shown in Table
1, the fall in diastolic blood pressure was significantly higher
than that of systolic pressure. Final section of the internal
intercostal nerves supplying the stimulated intercostal spaces
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FUNCTIONAL ROLES OF PLEURAL AFFERENT
J Appl Physiol • VOL
We were surprised by the absence of changes in phrenic
timing in response to pleural afferents activation, suggesting
their absence of connections with the respiratory centers.
Previous studies on the effects of the mechanical activation of
intercostal mechanoreceptors and costovertebral joint receptors
reported consistent changes in respiratory frequency resulting
from the supraspinal projections of these chest wall afferents
on brain stem respiratory neurons via the cerebellum (35) and
the Bötzinger complex (33) and also directly to the dorsal and
ventral respiratory groups (33). The existence of direct connections between parietal pleural afferents in internal intercostal nerves and phrenic motoneurons is similar to an inhibitory
visceromotor reflex similar to that reported between the
splanchnic afferents and the diaphragm (1, 2, 12). As for the
pleural-to-phrenic reflex here described, splanchnic-to-phrenic
reflex involves the activation of nonmyelinated afferent fibers
(2) directly connected with ipsilateral phrenic motoneurons
because the reflex persists in animals spinalized between C1
and C2 (1, 12). The splanchnic-to-phrenic reflex may explain
inhibition of diaphragm electromyogram after peritoneal effraction with laparotomy (4) or simple laparoscopy (34). Thus
there are numerous analogies between visceromotor reflexes
elicited by mechanical activation of the pleura or peritoneum.
In our study, the chemical activation of pleural afferents also
exerted both respiratory and circulatory responses that developed after a transient mechanical stimulation associated with
gauze application. The magnitude of respiratory response was
higher with the lactic acid solution than with the inflammatory
mixture. Because the pH of lactic acid solutions was rather
low, our lactic acid tests only mimicked the concentration of
this substance measured in the majority of pleural effusions in
which the lactic acid concentration was in the same range (5).
However, the pleural fluid pH was markedly higher in pleurisy,
because of the presence of buffers in the pleural fluids components (7). Our experimental animal study was very different
than clinically relevant intervention, but it must be underlined
that numerous other experimental studies on chemosensitive
muscle afferents, including the diaphragmatic ones, are based
on intra-arterial injections of the same low-pH lactic acid
solutions (9, 17, 21). The buffers in blood and interstitial fluid
must rapidly and markedly increase the pH of injected solution.
We also noted that the phrenic and circulatory responses to
inflammatory mediators was markedly less than those to lactic
acid. This observation is not consistent with our previous
electrophysiological data in the same species (19), where we
measured a comparable activation of the pleural afferents in
response to 40 mM lactic acid (⫹485%) and the same concentrations of bradykinin, 5-HT, histamine, and PGE2 (⫹639%).
However, among the recorded pleural afferents in our previous
study, the proportion of purely chemosensitive units was high
(78%) when we tested the effects of lactic acid, but only 14%
in response to the inflammatory mixture. On the whole, the
chemosensitivity to lactic acid of the parietal pleura seems to
prevail. This is not surprising because the bacterial synthesis of
lactic acid is present in most of pleural effusions (27, 29), and
its synthesis may also originate from the pleural inflammatory
mesothelial cells (16, 23). Thus the present observations of
modest respiratory effects elicited by pleural inflammation
compared with the marked inhibitory action of mechanical
pleural stimulation agree with clinical observations that mostly
associate pleuritic symptoms to mechanical stimulation of the
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all thoracic and abdominal muscles covering the stimulated and
also the adjacent interspaces and also of muscles inserted on
the ribs delimiting the studied pleural area. Moreover, intercostal nerves contain a large proportion of fast conducting
fibers connected with muscle spindles and Golgi tendon organs
(33), and solely slow-conducting fibers innervated the parietal
pleura (19).
In our protocol, the mechanical distension of the thoracic
pleura also distended the thoracic wall, and thus could activate
the costovertebral joints receptors described by Godwin-Austen (14). These slowly adapting receptors are localized in the
capsule of the costo-transverse joint and signal the rib joint
position and the direction and velocity of movement. Recording the electromyographic activity in external intercostal inspiratory muscles, De Troyer (10) reported an increased activity when the normal cranial motion of the ribs was reduced by
external force and a decreased inspiratory muscle activity when
the cranial motion of the ribs was augmented. Shannon (32)
measured a marked inhibition of the phrenic nerve activity
during costovertebral joint movements. In our study, the pressure applied to the internal side of the thoracic wall indifferently induced cranial or distal motion of the ribs and thus could
elicit either inhibitory or facilitating inspiratory reflexes mediated through the activation of costovertebral joint receptors.
However, an inhibition of inspiratory activity was always
noted. Another argument against possible reflex responses
elicited by the activation of costovertebral joint receptors in our
study was that the inhibitory phrenic response was abolished
after section of internal intercostal nerves, distal to the branch
point for their lateral branches and thus to the nerves innervating the costovertebral joint receptors recorded in dorsal-root
filaments after laminectomy by Godwin-Austen (14). Moreover, we already showed that no fast-conducting afferents
supplied the innervation of parietal pleura (19) and joint
receptors are only connected with fast-conducting fibers. It
seems also unlikely that mechanoreceptors in the costovertebral joints may be activated by the application of chemicals
(lactic acid, inflammatory mediators, capsaicin) on the thoracic
pleura.
In the present study, the phrenic inhibitory effects exerted by
mechanical or chemical (lactic acid) activation of pleural
afferents were limited to the side ipsilateral to the thoracic
pleura, suggesting that their supraspinal projections were not
involved. Indeed, first we measured no change in phrenic
timing in response to either the mechanical or chemical stimulation, and second the inhibitory effect on the phrenic motor
discharge strictly remained ipsilateral. We found no data on the
effects of thoracic afferents stimulation on both the ipsi- and
contralateral phrenic motoneurons. The mechanical stimulation
of chest wall muscle seems to exert only excitatory influences
on intercostal and phrenic motoneurons (22, 24, 30, 31, 33),
opposing the inhibitory action exerted by thoracic pleural
afferents on phrenic motoneurons here reported. We found one
study in vagotomized dogs and cats that reported phrenic nerve
inhibition in response to rapid compression of the chest with a
pneumatic cuff (28): a possible mechanical activation of thoracic pleural afferents, not discussed in that study, cannot be
discarded. Also Aminoff and Sears (3) reported an inhibition of
ipsilateral inspiratory motoneurons in response to electrical
stimulation of the central end of internal intercostal nerve that
activated all afferents including those innervating the pleura.
FUNCTIONAL ROLES OF PLEURAL AFFERENT
J Appl Physiol • VOL
be able to modulate phrenic activity and sympathetic neural
drive to arterial blood vessels.
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pleural cavity. Indeed, the clinical review of pleuritic symptoms by Yernault (38) stated that the “pleuritic pain and
dyspnoea are definitely intensified by deep breathing, coughing, laughing, or sneezing and relieved by anything that assists
in immobilizing the affected side.”
There is no doubt as to the existence of specific inhibitory
effects of pleural afferent activation on circulatory control
in humans. In their review of the clinical literature, Noppen
and Schramel (26) report that in primary spontaneous pneumothorax 30% of patients present with hypotensive faintness. In our animal study, hypotension, with parallel decrease in both systolic and diastolic arterial pressure, accompanied phrenic inhibition in response to mechanical
pleural stimulation. During a sustained mechanical stimulation of the thoracic parietal pleura, it is worth emphasizing
that blood pressure inconstantly varied in parallel with the
development of phrenic inhibition. As illustrated in the
figures, the blood pressure decrease either adapted while the
phrenic inhibition progressed (Fig. 1) or the time courses of
both variables were similar (Figs. 3–5). By contrast, lactic
acid stimulation of pleural afferents elicited a durable and
marked decrease in blood pressure, and in this case the fall
in diastolic blood pressure was significantly higher than that
of systolic pressure. We can totally discard the possibility
that the blood pressure decrease accompanying the phrenic
inhibition during application of gauze on the internal thoracic wall could result from an abrupt fall in venous return
because of cardiac and/or diaphragm compression. Indeed,
first, the chest was largely opened and the rabbits were
paralyzed, and second, the hypotension persisted after removal of the outwardly directed force in the case of lactic
acid stimulation. Because the carotid sinus baroreflex was
left intact, hypotension and the phrenic response may be
also linked via a reflex mechanism. However, previous
studies have clearly demonstrated that hypotension elicited
hyper- and not hypoventilation (15, 18) and thus the phrenic
inhibition here reported cannot result from the blood pressure fall that always followed the respiratory response. The
activation of visceral afferents from the lungs also elicits
hypotension. Thus injecting small amounts of bradykinin
(20) or histamine (37) in the bronchial or pulmonary circulation to activate pulmonary vagal chemoreflexes as well as
pulmonary hyperinflation to stimulate vagal mechanoreceptors in the airways (8) produce marked hypotension. It must
be pointed out that vagotomy may have also suppressed any
possible bronchomotor effects of the pleural afferent activation, but this intervention was necessary to discard any
artifact due to the stimulation of pulmonary and also cardiac
vagal afferents.
The present study clearly shows that mechanical or chemical
stimulation of parietal pleural afferents elicits both respiratory
and circulatory effects. However, we were only able to mimic
positive pressure changes in the pleural cavity, a situation that
occurs in rare circumstances such as cough, forced expiration,
pneumothorax, pleurisy, and surgical pleuroscopy. Thus there
is a major difference between our experimental protocol and
the natural conditions in which pleural nerve endings are
normally exposed to a negative (subatmospheric) pressure. We
cannot conclude, therefore, that pleural afferents are activated
during spontaneous breathing movements and that they would
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FUNCTIONAL ROLES OF PLEURAL AFFERENT
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