Vol. 84, Issue 4, 1198-1207, April 1998
Differential respiratory muscle recruitment induced by
clonidine in awake goats
Michael S.
Hedrick1,
Melinda R.
Dwinell2,
Patrick L.
Janssen2,
Josue
Pizarro2, and
Gerald E.
Bisgard2
1 Department of Biological
Sciences, California State University, Hayward, California 94542;
and 2 Department of Comparative
Biosciences, University of Wisconsin, Madison, Wisconsin
53706
 |
ABSTRACT |
The purpose of this study was to test the
hypothesis that dysrhythmic breathing induced by the
2-agonist clonidine is
accompanied by differential recruitment of respiratory muscles. In
adult goats (n = 14) electromyographic
(EMG) measurements were made from inspiratory muscles (diaphragm and
parasternal intercostal) and expiratory muscles [triangularis
sterni (TS) and transversus abdominis (Abd)]. EMG of the
thyroarytenoid (TA) muscle was used as an index of upper airway
(glottal) patency. Peak EMG activities of all spinal inspiratory and
expiratory muscles were augmented by central and peripheral
chemoreceptor stimuli. Phasic TA was apparent in the postinspiratory
phase of the breathing cycle under normoxic conditions. During
dysrhythmic breathing episodes induced by clonidine, TS and Abd
activities were attenuated or abolished, whereas diaphragm and
parasternal intercostal activities were unchanged. There was no tonic
activation of TS or Abd EMG during apneas; however, TA activity became
tonic throughout the apnea. We conclude that
1) 2-adrenoceptor stimulation
results in differential recruitment of respiratory muscles during
respiratory dysrhythmias and 2) apneas are accompanied by active glottic closure in the awake goat.
control of breathing; electromyograms; apnea; thyroarytenoid
muscle
| |
INTRODUCTION |
MOTOR OUTPUTS to specific muscles responsible for
generating airflow and regulation of upper airway resistance are the
effectors of the central rhythm-generating network. The various muscle
groups that generate or regulate airflow are subject to control by a variety of factors, including central and peripheral chemoreceptor stimulation (26, 35-37), vagal feedback (1), anesthesia (12), and
posture (9). However, there is little information on specific neurochemical regulation of respiratory muscle activity in awake animals.
Previous studies from our laboratory have shown that systemic
administration of 2-agonists,
such as clonidine and guanabenz, causes profound breathing
instabilities in awake and anesthetized goats (14-16). In awake,
standing goats, 2-agonists
induce dysrhythmic ventilatory patterns characterized by alternating
episodes of tachypnea and apneas [increased expiratory time
(TE)] of variable lengths. Phrenic activity measured in vagotomized, chemodenervated goats anesthetized with chloralose is highly irregular, with apneas of
variable lengths, but tachypneic breathing is abolished.
We have also shown that efferent activity of the recurrent laryngeal nerve (RLN) becomes tonic throughout the apneic period in anesthetized goats (15). These findings are consistent with several other studies
that show that induction of apneas by a variety of maneuvers, principally by mechanically induced hypocapnia, often results in
glottic closure in sleeping humans (24) and in awake, nonsedated animals (22) or results in increased RLN activity in anesthetized animals (8). However, a recent study in lambs (31) showed that barbiturate-induced apneas also resulted in tonic thyroarytenoid (TA) electromyogram (EMG) activity, suggesting that a general depression of central respiratory drive causes upper airway (glottic) closure. Because 2-agonists are
routinely used as sedatives (32), it is possible that
2-agonist-induced apneas in
awake goats (16) are produced by a similar mechanism. At present,
although it is clear that low dosages of
2-adrenoceptor agonists induce
dysrhythmic breathing, tonic RLN activity, and possibly upper airway
obstructions in goats, it is not clear to what extent these agents
affect the central motor output to respiratory muscles.
This study examined the effects of
2-adrenoceptor stimulation on
EMG activity of selected inspiratory and expiratory muscles in the
awake, standing goat. In this study, clonidine preferentially attenuated EMG activity of spinal (thoracic and abdominal) expiratory muscles; in addition, apneas were always associated with activation of
the TA muscle throughout the length of the apnea, thus suggesting active glottic closure.
| |
METHODS |
Animal preparation.
Studies were conducted on 14 adult goats of mixed breed (mean wt 52.8 kg). All goats were prepared under general anesthesia (halothane,
nitrous oxide, and O2) with
bilateral common carotid artery translocations to a subcutaneous
position to facilitate insertion of arterial catheters for experimental
procedures. Some goats with carotid artery translocations from a
previous study were prepared during a second surgery with EMG wires
inserted into specific muscles for this experiment. The remaining goats were implanted with EMG wires and received carotid artery
translocations in the same surgical procedure. In general, two sets of
bipolar, Teflon-coated, stainless steel EMG wires were implanted per
muscle, except in the case of the TA muscle, in which only one set of wires was implanted. The technique for implantation of EMG wires in
goats has been described previously in detail by Smith et al. (37). For
the TA muscle, a small C-shaped incision was made with a scalpel in the
thyroid cartilage to expose the muscle, and the EMG wire was sewn in
place with the use of direct visualization (22). The wire was led out
of the thyroid cartilage, and the cartilage was closed with a single
suture. All EMG wires were sewn into a subcutaneous position to
facilitate access for recording on the day of the experiment. EMG
measurements were taken from five different muscles in the 14 animals;
at most, EMG measurements were taken from four muscles in any one
animal. Successful EMG recordings were obtained from two inspiratory
pump muscles [the costal diaphragm (Dia;
n = 14) and the parasternal
intercostal (PS; n = 4)] and
from two expiratory pump muscles [the triangularis sterni (TS;
n = 7) and the transversus abdominis
(Abd; n = 12)]. These muscles
were chosen on the basis of previous studies in goats (10, 37). The EMG
was recorded from one upper airway (glottal adductor) muscle, the TA
(n = 7), on the basis of its function
during apneas in humans and animals (19, 22, 24) and on the basis of
our previous studies on the effects of
2-agonists on RLN activity in
goats (14).
The animals were allowed at least 1 wk recovery from surgery before any
experiments began. During this 1-wk period, we trained the animals to
stand quietly in a stanchion while wearing a muzzle mask. One day
before an experiment, arterial catheters were inserted into each
carotid artery for anaerobic blood sampling for blood-gas measurements
and measurement of blood pressure. A catheter (PE 90) was also placed
into one jugular vein for drug injections. All catheters were flushed
with heparinized saline and closed until the day of the experiment. On
the day of the experiment, the incision overlying the EMG wires was
infiltrated liberally with local anesthetic (2% lidocaine), and the
wound was reopened to expose the wires.
Measurements.
Small alligator clips were used to connect the EMG wires to a
differential alternating current-coupled amplifier (CWE or A-M Systems). The EMG signal was amplified ×10,000,
filtered (band pass 10-5 kHz) and recorded on tape (Vetter or
Hewlett-Packard) for off-line analysis. Ventilatory data were recorded
with goats wearing a tightly fitting muzzle mask attached to a
low-resistance one-way breathing valve (Hans Rudolph). Inspired gases
were delivered to the animal via 3-cm ID tubing. Expired gases were
collected in a spirometer (120 liters) for measurements of minute
ventilation (
E). Inspired ventilatory
flow was measured with a pneumotachograph (Fleisch) and was
electronically integrated to obtain tidal volume (VT). An
O2 analyzer (Applied
Electrochemistry) was used to monitor inspired
O2 fraction
(FIO2), and
expired CO2 was monitored with an
infrared CO2 analyzer (Anarad).
A six-channel polygraph (Gilson) was used to record end-tidal
CO2,
VT,
E, and systemic arterial blood
pressure. Arterial blood samples were analyzed for pH,
PCO2, and
PO2 with a blood-gas analyzer (model
ABL3M, Radiometer). A rectal thermistor probe was used to monitor body
temperature throughout the experiment for blood-gas temperature
correction.
Protocol.
After the face mask was placed on the animal, the goat breathed room
air for 20-30 min to allow measurement of baseline control values. Once a stable ventilatory baseline was
established, the animal was subjected to a hypercapnic stimulus with
inspired CO2 fraction
(FICO2) raised to 0.03 for
10 min and then to 0.065 for 10 min before returning the animal to
control (room air) conditions. The 6.5%
CO2 stimulus was used as a
reference stimulus to which all EMG signals for that animal were
compared (37). All EMG values were therefore expressed as a percentage of reference in subsequent analyses.
After baseline E was reestablished, the
animal was subjected to isocapnic hypoxia [arterial
PO2
(PaO2) ~37 Torr] for 10-20
min. Arterial blood samples were collected frequently during the
hypoxic stimulus to verify isocapnic hypoxic conditions. After the
hypoxic stimulus was completed, the animal was once again returned to
room air until baseline conditions were reestablished. Peripheral
chemoreceptor stimulation was also accomplished with an intra-arterial
bolus injection of NaCN (1,000 µg in 0.2 ml saline).
E and EMG values
were measured during the 5- to 20-s interval after NaCN injection, the
time when these values reach peak levels. After this, the ventilatory
responses to three doses of dopamine (DA; 0.5, 1.0, and 5.0 µg · kg
1 · min1)
were measured. The order of the infusions was randomized, with sufficient time between infusions to return to baseline
E. Drug infusions were carried
out at a rate of 5 ml/min for 1 min into a carotid artery. During the
infusion, E and EMG activity were measured during the 15- to 45-s interval, the period of maximal E inhibition by DA (33). An arterial
blood sample was taken immediately before the infusion started and just
before the end of the infusion.
Drugs.
Clonidine (Sigma Chemical) was dissolved in saline to obtain a stock
solution (1 mg/ml) that was diluted 1:10 in saline for intravenous (iv)
administration. Clonidine was given by bolus injection via the jugular
catheter at concentrations ranging from 0.5 to 10 µg/kg (cumulative
final dose) to achieve maximal ventilatory effects without eliciting
the excessive sedation that can occur with these drugs (16).
Data analysis and statistics.
Ventilatory measurements of E,
frequency (f), and VT as
well as peak EMG values for each muscle were analyzed with a
repeated-measures ANOVA. Where significant differences were found, data
were further analyzed post hoc with the Student-Newman-Keuls multiple
range test. EMGs were quantified on a breath-by-breath basis
(10-20 breaths per condition per muscle) and were normalized to
the response achieved in each goat while it was breathing an
FICO2 of 0.065 (percentage
of reference). All EMG data expressed as percentages were converted to
their arc sine values before statistical comparisons were made. Paired
t-tests were used to compare
preinfusion control EMG values with EMG values obtained during the 15- to 45-s time interval during intra-arterial DA infusion. Statistical significance was accepted at P < 0.05.
| |
RESULTS |
Preclonidine ventilatory patterns and EMG measurements.
Hypercapnia [mean arterial PCO2
(PaCO2) 51.2 Torr, mean
PaO2 >110 Torr], isocapnic
hypoxia (mean PaO2 37 Torr), and intracarotid NaCN injection resulted in an overall augmentation of peak
EMG activities of inspiratory (Dia) and expiratory (Abd and TS) muscles
compared with control (room air) responses (Fig. 1; Table 1).
The effects of these stimuli were quite uniform among both inspiratory
and expiratory muscles (Table 1). Phasic activity was usually present
in the TA muscle during eupnea; the onset of TA activity occurred in
the early expiratory (i.e., postinspiratory) phase of the respiratory
cycle. In contrast to other respiratory muscle EMG activity, TA
activity was suppressed by hypercapnia; that is, there was an overall
reduction in phasic activity that was normally present in eupneic
conditions.
View larger version (36K):
[in this window]
[in a new window]
|
Fig. 1.
Integrated electromyographic (EMG) activity from diaphragm (Dia),
triangularis sterni (TS), and transversus abdominis (Abd) with NaCN
injection (CN; arrow), isocapnic hypoxia (hypoxia) [arterial PO2
(PaO2) ~40 Torr], and
hypercapnia [inspired O2
fraction (FICO2) = 0.03 and 0.065] (3.0 and 6.5%
CO2, respectively).
|
|
View this table:
[in this window]
[in a new window]
|
Table 1.
Mean values of peak EMG activity for inspiratory (Dia and PS) and
expiratory (TS and Abd) muscles for goats breathing different gas
mixtures, after NaCN injection, and after clonidine
|
|
We occasionally observed augmented breaths that were characterized by a
large expiratory effort followed by an apnea that was three to four
times longer than a normal TE
(Fig. 2). These augmented breaths were
usually accompanied by an increase in TA activity that persisted
throughout the length of the apnea. We did not, however, note any tonic
activation of other expiratory muscles during augmented breaths.
View larger version (28K):
[in this window]
[in a new window]
|
Fig. 2.
Example of tonic thyroarytenoid (TA) EMG activity occurring during a
brief apnea after an augmented breath in isocapnic hypoxia. Note large
expiratory breath in Abd and shortened inspiratory breath in Dia
followed by apnea.
VT, tidal
volume.
|
|
Effects of DA infusion.
Intra-arterial infusion of DA (0.5-5.0
µg · kg1 · min1),
which causes an overall reduction in E by
carotid body (CB) feedback inhibition (5, 33), reduced
E and
VT in this study and, in
addition, caused a significant attenuation of peak inspiratory (Dia and
PS) and expiratory (TS and Abd) EMG activity. Figure 3 shows the mean difference between peak
EMG during control and the mean value during the DA infusion for each
dose. There were significant reductions in peak EMG activity in
response to DA infusion for inspiratory and expiratory muscles. TA EMG
activity generally increased during DA infusion when ventilation was
reduced.
View larger version (24K):
[in this window]
[in a new window]
|
Fig. 3.
Inhibitory effects of intra-arterial infusion of dopamine (DA; 0.5, 1.0, 5.0 µg · kg1 · min1) on peak EMG
activity of inspiratory [Dia and parasternal intercostal (PS)] and expiratory (TS and Abd) muscles in awake goats. Values are mean change ( %reference) in peak EMG activity between control (preinfusion) and DA infusion (see
METHODS). Negative values indicate reductions in peak EMG activity during infusion. Nos. in parentheses, no. of goats for which EMG activity of that muscle was recorded. Significantly different from control values (paired t-test).
* P < 0.05;
** P < 0.01;
*** P < 0.001.
|
|
Effects of iv clonidine.
In response to iv infusion of clonidine (1.0-10.0 µg/kg),
ventilatory patterns in goats became very dysrhythmic with prolonged and variable TE.
PaO2 decreased from a mean control value
of 96 ± 2 (SE) to 81 ± 3 Torr, and mean
PaCO2 increased from a control value of
37 ± 1 to 42 ± 1 Torr. During tachypnea alternating with apneas, the mean PaO2 and
PaCO2 values were 85 ± 3 and 40 ± 1 Torr, respectively. We have previously documented the
ventilatory and cardiovascular responses to clonidine in the awake,
standing goat model (16), and the results here are qualitatively and quantitatively similar. In this study, peak EMG measurements during clonidine-induced dysrhythmic breathing episodes revealed that there is
a significant attenuation of expiratory (TS and Abd) muscle activity
compared with control conditions (Fig. 4).
The attenuation and/or abolition of TS and Abd EMG activities was pronounced despite large increases in
PaCO2 that occurred in some animals as
the result of hypoventilation after clonidine administration (Fig.
5) (16). The attenuation of TS and Abd EMG
activity occurred consistently and independently of the prevailing
breathing pattern. That is, whether apnea or tachypnea occurred, there
was a significant attenuation of expiratory EMG activity, whereas
inspiratory (Dia and PS) muscle EMG amplitude was variably affected
but, on average, was unchanged relative to control conditions (Fig.
6). As in our previous study (16),
clonidine injection decreased mean arterial blood pressure ~20 Torr
(control 108 ± 4 to 88 ± 5 Torr with clonidine). This decrease
was not likely to influence the ventilatory response to clonidine,
because a decreased baroreceptor input would most likely stimulate,
rather than inhibit, ventilation (6).
View larger version (32K):
[in this window]
[in a new window]
|
Fig. 4.
Effects of iv clonidine on EMG activity in 2 awake goats.
Top: EMG activity in Dia, TS, and Abd
in control condition and after clonidine infusion.
Bottom: EMG activity in Dia, PS, and
Abd in control condition and after clonidine infusion. Note presence of
apneas interspersed with tachypnea after clonidine infusion in both
animals; also note attenuation/abolition of EMG activity preferentially
in expiratory muscles (TS and Abd).
|
|
View larger version (21K):
[in this window]
[in a new window]
|
Fig. 5.
Attenuation of peak EMG activity in expiratory muscles (TS and Abd)
after clonidine administration in awake goat. Two doses of clonidine
(3.0 and 5.0 µg/kg) produced significant increases in arterial
PCO2
(PaCO2) resulting from
hypoventilation. Peak EMG of Dia increased with increased hypercapnia,
whereas there was little change in attenuation of TS and Abd peak EMG activity despite extreme increase of
PaCO2.
|
|
View larger version (28K):
[in this window]
[in a new window]
|
Fig. 6.
Summary of peak EMG activities (%reference) of inspiratory (Dia and
PS) and expiratory (TS and Abd) muscles in individual awake goats. Each
mean control value (preclonidine) is connected to peak mean EMG value
after iv clonidine. Means ± SE for each muscle group before and
after clonidine are also indicated. *Significant reductions in peak EMG
activity were noted in expiratory but not in inspiratory muscles
(ANOVA; see Table 1).
|
|
Effects of clonidine on TA activity.
The normal response to clonidine in awake goats results in periods of
tachypnea interspersed with apneas of variable lengths. In each goat in
which TA EMG activity was measured, during apneic periods there was a
clear increase of tonic TA activity that was maintained throughout the
length of the apnea (Fig. 7). In extreme instances, apneas up to 40 or 50 s occurred and were always accompanied by tonic TA activity that persisted throughout the length of the apnea.
Prolonged apneas were potentiated when peripheral chemoreceptor drive
was attenuated by DA infusion into the carotid artery (Fig. 8) or by 100%
O2 (Dejours test) given for four
to five breaths during DA infusion (data not shown). In some cases,
glottal closure, as indicated by tonic TA activity, appeared to be
powerful enough to prevent airflow despite inspiratory and expiratory
ventilatory efforts (Fig. 9).
View larger version (34K):
[in this window]
[in a new window]
|
Fig. 7.
Raw EMG activity in awake goat after clonidine. Note appearance of
apneas as indicated in integrated Dia and tidal volume (VT). Each apnea is
accompanied by increased continuous TA EMG activity throughout each
apnea.
|
|
View larger version (40K):
[in this window]
[in a new window]
|
Fig. 8.
Synergistic effect of clonidine and DA infusion (arrows).
Top: 1-min infusion of DA (5.0 µg · kg1 · min1)
during control. Note attenuation of Dia activity and increased tonic
and phasic TA activity. Bottom: same
dose of DA after iv clonidine in same animal. Prolonged apnea during
infusion is accompanied by tonic TA activity. Single breath near end of
infusion caused break in TA activity during the breath.
|
|
View larger version (18K):
[in this window]
[in a new window]
|
Fig. 9.
Tonic TA activity during inspiratory and expiratory efforts in awake
goat. Raw TA activity (top) persists
during brief (5.5 s) apnea. Note the 2 inspiratory
efforts indicated in integrated Dia trace and the expiratory effort in
integrated TS and Abd traces without any measurable change in air flow
indicated in VT trace.
|
|
| |
DISCUSSION |
The important findings from this study are as follows.
1) Clonidine preferentially
attenuates or abolishes activity of spinal expiratory muscles (TS and
Abd) during dysrhythmic breathing. Moreover, we find no evidence for
tonic activation of these muscles during prolonged apneas induced by
these drugs. 2) Apneas were accompanied by tonic activation of the TA muscle, a glottal adductor, throughout the length of the apnea. After administration of clonidine, the animals clearly exhibited more prolonged apneas with attenuation of
CB feedback by DA infusion. With increased TA activity, it is likely
glottic closure would also be increased under these conditions.
3) Dysrhythmic breathing resulting
from systemic injection of clonidine is correlated with widespread
differential activation and/or attenuation of respiratory pump
and upper airway musculature.
Activity of spinal inspiratory and expiratory muscles.
Spinal inspiratory and expiratory muscles are clearly influenced by a
variety of factors that help to regulate and optimize ventilation to
adapt to prevailing demands (2). Recent experiments have revealed a
considerable difference in the responses of respiratory muscles to
various stimuli in animals under anesthesia compared with awake
animals. For example, it is generally accepted that a hypoxic
ventilatory stimulus provides little or no augmentation of expiratory
muscle activity compared with an equivalent hyperpnea elicited by
hypercapnia (11, 12). This effect has been attributed, in part, to a
relatively greater stimulation of inspiratory muscles by CB
chemoreceptors or to the depressant effects of central hypoxia and/or hypocapnia. This differential effect on inspiratory and expiratory motor outflow has been termed "the inspiratory shift" (35). However, recent studies in awake dogs (34) and
awake goats (10, 37) have shown that equivalent hypoxic or hypercapnic stimuli result in an overall augmentation of both inspiratory and
expiratory motor output, with no evidence for an asymmetrical recruitment pattern characteristic of anesthetized animals. Therefore, data collected from awake animals argue against an inspiratory shift
and, instead, suggest that both inspiratory and expiratory muscles are
recruited to similar degrees with increased central or peripheral
chemoreceptor drive.
This study is also consistent with the recent studies of awake dogs
(36) that suggest that inhibition of CB feedback by DA infusion results
in an overall reduction in ventilatory motor output to both inspiratory
and expiratory muscles. It thus appears that, under normal conditions
in awake animals, inspiratory and expiratory muscles are tightly
coupled with respect to the prevailing respiratory drive. That is,
increased central or peripheral chemoreceptor drive causes an overall
augmentation of respiratory muscles, whereas inhibition of respiratory
drive causes an overall attenuation of respiratory muscles. Although
the nature of this coupling is unclear, it is presumably the final
integrative output of the brain stem respiratory controller for
respiration and serves to maintain optimal levels of ventilation for
maintenance of respiratory homeostasis (2).
Systemic infusion of clonidine appears to decouple the relationship
between inspiratory and expiratory muscle activity. More specifically,
there is a preferential attenuation of phasic expiratory (TS and Abd)
muscle activity relative to inspiratory (Dia and PS) muscle activity.
Our results do not allow for a complete mechanistic explanation for the
disruption of central respiratory rhythm and differential outflow to
specific respiratory muscle groups. However, several lines of evidence
argue strongly for the importance of 2-adrenoceptors in regulating
central respiratory output. Anatomical studies have shown a widespread
distribution of
2-adrenoceptors throughout the brain stem in mammals (38). More recently, the 2a-adrenoceptor subtype has
been shown to be predominant in areas associated with central
respiratory rhythmogenesis (3, 13). There is considerable evidence that
2-adrenoceptors regulate sympathetic outflow from the brain stem and, therefore, provide the
hypotensive effects of systemically administered clonidine (28, 32).
The actions of 2-agonists on
membrane function have been attributed to increased
K+ conductance and/or
decreased Ca2+ conductance (27).
Application of clonidine to brain stem respiratory-related neurons
causes hyperpolarization of these neurons (7). Bulbospinal expiratory
neurons to the thoracic spinal cord project mainly from the caudal
ventral respiratory group and, more specifically, from the nucleus
retroambigualis (2, 3). Many of the bulbospinal expiratory neurons
projecting to the spinal cord are located in the Bötzinger
complex (2), and anatomical studies have shown the presence of
2-adrenoceptors in this area of
the medulla (38). However, it has recently been shown that
presympathetic neurons and expiratory neurons of the Bötzinger
complex are anatomically and functionally distinct neuronal populations
(21).
Our study does not allow us to distinguish between many different
possibilities that would explain the attenuation of expiratory motor
output by systemically administered
2-agonists, but a direct hyperpolarizing effect of
2-agonists on brain stem
expiratory-related neurons is consistent with our observations. Thus,
although it is clear that, in awake goats, there is a profound
disruption of normal respiratory rhythmogenesis accompanied by
attenuation of expiratory motor output, the manner in which
2-adrenoceptors decouple
inspiratory and expiratory motor output is unclear.
Our study has also revealed an important function of CB feedback drive
under normal conditions that maintains phasic spinal respiratory muscle
activity. Infusion of DA, which reduces
E by a CB inhibitory mechanism (5,
33), resulted in an overall attenuation of inspiratory and
expiratory muscle groups (Fig. 3). To our knowledge, this is the first
evidence for a role of tonic CB afferent feedback providing a
background facilitory drive to respiratory motor activity in an awake
animal. Our results also indicate that there is an additive effect of
DA inhibitory feedback and
2-agonists: DA infusion caused
a greater degree of attenuation of respiratory muscle activity in the
presence of clonidine than in the control condition (Fig. 8). This
synergistic effect may occur peripherally, centrally, or both. In the
cat, guanabenz has a direct inhibitory effect on carotid sinus nerve discharge (23) which would enhance the inhibitory effect of DA on CB
afferent activity. However, we have also shown that the respiratory
responses to 2-agonists are not
affected by CB denervation in awake goats (16); this suggests that
there may be a central interaction between
2-agonists and reduced CB
afferent feedback by DA infusion.
Previous studies have noted tonic activation of spinal expiratory
muscles during hypocapnia-induced apneas. Most notable is the finding
that TS activity in awake or sleeping dogs becomes tonic with
mechanically induced hypocapnia (17, 18). In this study, we found no
evidence for tonic TS activity during apneas in awake, standing goats.
Whether these differences can be attributed to species differences,
posture (9), or the mechanism to produce the apnea is unclear.
Effects of 2-agonists on
TA activity.
A general observation we have made in these studies is that, when
respiratory pump muscles were stimulated by a variety of means, TA
activity was suppressed. Conversely, when the pump muscles were
depressed by clonidine or DA, TA activity was increased. This study has
shown clearly that apneas associated with systemic administration of
clonidine are accompanied by tonic activation of TA and, although we
realize that EMG activity does not give an accurate estimate of
mechanical activity, it appears likely that glottic closure is also
induced in awake goats (Fig. 7). The data from awake goats directly
corroborate our previous study in anesthetized goats (15) which showed
that prolonged apneas induced by clonidine are associated with tonic
RLN activation. The RLN controls glottal adductor activity, and our
results indicate that apneas induced by
2-agonists activate this motor
efferent, with the result of glottic closure in goats. On occasion,
this mechanism of glottic closure appeared to be powerful enough to prevent airflow in the presence of ventilatory efforts (Fig. 9).
Our results are also consistent with recent studies that show glottic
closure and/or TA EMG activity becomes tonic during sleep
apneas (19) and during apneas induced in awake or sleeping animals or
humans by hypocapnic hyperventilation (22, 24). What is
observed in these studies is that TA activity becomes tonic during the
hypocapnia-induced apneic period. One interpretation of these results
is that glottic closure helps to preserve lung gas exchange by
preventing alveolar gas from flowing from the lung and restoring
normocapnia (22). However, a recent study in lambs (31) has shown that
barbiturate-induced apneas also result in tonic TA activity, indicating
that glottic closure accompanies central respiratory depression and
need not require central hypocapnia. Because clonidine and other
2-agonists are clinically
important for inducing anesthesia, sedation, and analgesia (32), our
results appear to be consistent with the results from lambs and may
suggest that glottic closure is a general response to depression of
central respiratory drive (31). In our previous study with anesthetized goats, which showed tonic RLN activity with systemic clonidine and
guanabenz (15), we found that systemic pentobarbital resulted in apneas
that were not accompanied by tonic RLN activity (unpublished results).
Instead, there was a general depression of both respiratory drive and
RLN activity. Thus our results may suggest a specific effect of
2-agonists on RLN and,
therefore, TA activity. Regardless of the precise mechanism for
activation of the TA, it is clear from studies with awake goats and
lambs that glottic closure can occur in the absence of
hypocapnia-induced apneas.
Our observations of tonic TA activity also have important clinical
implications. Recent studies in humans (20, 29) and horses (25) have
indicated that 2-agonists cause
significant upper airway obstructions and apneas which lead to arterial
O2 desaturation. Although the
nature of the upper airway obstruction was not identified in those
studies, the results of the present study suggest that the larynx is
the principal site for obstructive apneas associated with systemically
administered clonidine.
It is unclear what mechanism(s) results in tonic activation of the TA
during apneas in this study. The RLN motoneurons of the vagus, which
control the activity of the TA, originate in the nucleus ambiguus (NA;
Ref. 4). This area of the ventral lateral medulla is associated with
cardiovascular and respiratory integration and is subject to a wide
variety of inputs from areas within the brain stem. Because a
preponderance of evidence indicates that
2-agonists primarily
hyperpolarize neurons, our results showing tonic activation
(depolarization) of TA suggest that RLN motoneurons are disinhibited in
the presence of 2-agonists. At present, there is little direct information on the role of specific neurotransmitters within the NA that may explain this observation. Recently, an electrophysiological study that used preparations of brain
stem slices from the NA of guinea pigs (30) revealed that application
of norepinephrine to brain stem slices blocked a
Ca2+-activated
K+ current that resulted in
greater depolarization of NA neurons. The effect of
norepinephrine on this current was blocked by the 
-adrenoceptor
antagonist propranolol. Although the authors did not investigate
-adrenergic mechanisms on NA neurons, it is clear that
norepinephrine has a direct effect on NA neurons that is consistent
with the tonic activation of motoneurons in this region of the medulla.
Further studies are needed to determine the precise mechanism(s) by
which 2-agonists influence
central respiratory drive, but it is clear that systemic administration
of these agents causes widespread changes in central rhythmic activity
and efferent motor output to specific respiratory muscles in awake
goats.
| |
ACKNOWLEDGEMENTS |
The authors thank Gordon Johnson for excellent technical assistance
and Dr. Ken O'Halloran for critically reading the manuscript.
| |
FOOTNOTES |
This work was supported by National Heart, Lung, and Blood Institute
Grants HL-53969 and HL-07654.
Address for reprint requests: G. E. Bisgard, Dept. of Comparative
Biosciences, Univ. of Wisconsin, Madison, WI 53705 (E-mail:
bisgardg{at}svm.vetmed.wisc.edu).
Received 17 September 1997; accepted in final form 26 November
1997.
| |
REFERENCES |
1.
Ainsworth, D. M.,
C. A. Smith,
B. D. Johnson,
S. W. Eicker,
K. S. Henderson,
and
J. A. Dempsey.
Vagal contributions to respiratory muscle activity during eupnea in the awake dog.
J. Appl. Physiol.
72:
1355-1361,
1992[Abstract/Free Full Text].
2.
Berger, A. L.,
and
M. C. Bellingham.
Mechanisms of respiratory motor output.
In: Regulation of Breathing (2nd ed.), edited by J. A. Dempsey,
and A. I. Pack. New York: Dekker, 1995, vol. 79, p. 71-149. (Lung Biol. Health Dis. Ser.)
3.
Bianchi, A. L.,
M. Denavit-Saubie,
and
J. Champagnat.
Central control of breathing in mammals: neuronal circuitry, membrane properties, and neurotransmitters.
Physiol. Rev.
75:
1-45,
1995[Free Full Text].
4.
Bieger, D.,
and
D. A. Hopkins.
Viscerotopic representation of the upper alimentary tract in the medulla oblongata in the rat: the nucleus ambiguus.
J. Comp. Neurol.
262:
546-562,
1987[Medline].
5.
Bisgard, G. E.,
H. V. Forster,
J. P. Klein,
M. Manohar,
and
V. A. Bullard.
Depression of ventilation by dopamine in goats-effects of carotid body excision.
Respir. Physiol.
41:
379-392,
1980.
6.
Brunner, M. J.,
M. S. Sussman,
A. C. Greene,
C. H. Kallman,
and
A. A. Shoukas.
Carotid baroreceptor reflex control of respiration.
Circ. Res.
51:
624-636,
1982[Abstract/Free Full Text].
7.
Champagnat, J.,
M. Denavit-Saubie,
J. L. Henry,
and
V. Leviel.
Catecholaminergic depressant effects on bulbar respiratory neurons.
Brain Res.
160:
57-68,
1979[Medline].
8.
Cohen, M. I.
Discharge patterns of brain-stem respiratory neurons in relation to carbon dioxide tension.
J. Neurophysiol.
31:
142-165,
1968[Free Full Text].
9.
De Troyer, A.,
and
V. Ninane.
Effect of posture on expiratory muscle use during breathing in the dog.
Respir. Physiol.
67:
311-322,
1987[Medline].
10.
Engwall, M. J. A.,
C. A. Smith,
J. A. Dempsey,
and
G. E. Bisgard.
Ventilatory afterdischarge and central respiratory drive interactions in the awake goat.
J. Appl. Physiol.
76:
416-423,
1994[Abstract/Free Full Text].
11.
Fregosi, R. F.
Influence of hypoxia and carotid sinus nerve stimulation on abdominal muscle activities in the cat.
J. Appl. Physiol.
76:
602-609,
1994[Abstract/Free Full Text].
12.
Fregosi, R. F.,
S. L. Knuth,
D. K. Ward,
and
D. Bartlett, Jr.
Hypoxia inhibits abdominal expiratory nerve activity.
J. Appl. Physiol.
63:
211-220,
1987[Abstract/Free Full Text].
13.
Guyenet, P. G.,
R. L. Stornetta,
T. Riley,
F. R. Norton,
D. L. Rosin,
and
K. R. Lynch.
Alpha2A-adrenergic receptors are present in lower brainstem catecholaminergic and serotonergic neurons innervating spinal cord.
Brain Res.
638:
285-294,
1994[Medline].
14.
Hedrick, M. S.,
M. L. Ryan,
and
G. E. Bisgard.
Modulation of respiratory rhythm in the goat by 2 adrenoceptors.
In: Ventral Brainstem Mechanisms and Control of Respiration and Blood Pressure, edited by C. O. Trouth,
R. M. Millis,
H. Kiwull-Schone,
and M. E. Schlafke. New York: Dekker, 1995, vol. 82, p. 739-754. (Lung Biol. Health Dis. Ser.)
15.
Hedrick, M. S.,
M. L. Ryan,
and
G. E. Bisgard.
Recurrent laryngeal nerve activation by 2 adrenergic agonists in goats.
Respir. Physiol.
101:
129-137,
1995[Medline].
16.
Hedrick, M. S.,
M. L. Ryan,
J. Pizarro,
and
G. E. Bisgard.
Modulation of respiratory rhythm by 2-adrenoceptors in awake and anesthetized goats.
J. Appl. Physiol.
77:
742-750,
1994[Abstract/Free Full Text].
17.
Horner, R. L.,
L. F. Kozar,
R. J. Kimoff,
and
E. A. Phillipson.
Effects of sleep on the tonic drive to respiratory muscle and the threshold for rhythm generation in the dog.
J. Physiol. (Lond.)
474:
525-537,
1994[Abstract/Free Full Text].
18.
Horner, R. L.,
L. F. Kozar,
and
E. A. Phillipson.
Tonic respiratory drive in the absence of rhythm in the conscious dog.
J. Appl. Physiol.
76:
671-680,
1994[Abstract/Free Full Text].
19.
Insalaco, G.,
S. T. Kuna,
G. Catania,
O. Marrone,
B. M. Costanza,
V. Bellia,
and
G. Bonsignore.
Thyroarytenoid muscle activity in sleep apneas.
J. Appl. Physiol.
74:
704-709,
1993[Abstract/Free Full Text].
20.
Issa, F.
Effect of clonidine in obstructive sleep apnea.
Am. Rev. Respir. Dis.
145:
435-439,
1992[Medline].
21.
Kanjhan, R.,
J. Lipski,
B. Kruszewska,
and
W. Rong.
A comparative study of pre-sympathetic and Bötzinger neurons in the rostral ventrolateral medulla (RVLM) of the rat.
Brain Res.
699:
19-32,
1995[Medline].
22.
Kianicka, I.,
J.-F. Leroux,
and
J.-P. Praud.
Thyroarytenoid muscle activity during hypocapnic central apneas in awake nonsedated lambs.
J. Appl. Physiol.
76:
1262-1268,
1994[Abstract/Free Full Text].
23.
Kou, Y. R.,
P. Ernsberger,
P. A. Cragg,
N. S. Cherniack,
and
N. R. Prabhakar.
Role of 2-adrenergic receptors in the carotid body response to isocapnic hypoxia.
Respir. Physiol.
83:
353-364,
1991[Medline].
24.
Kuna, S. T.,
M. P. McCarthy,
and
J. S. Smickley.
Laryngeal response to passively induced hypocapnia during NREM sleep in normal adult humans.
J. Appl. Physiol.
75:
1088-1096,
1993[Abstract/Free Full Text].
25.
Lavoie, J. P.,
J. R. Pascoe,
and
C. J. Kurpershoek.
Effects of xylazine on ventilation in horses.
Am. J. Vet. Res.
53:
916-920,
1992[Medline].
26.
Ledlie, J. F.,
A. I. Pack,
and
A. P. Fishman.
Effects of hypercapnia and hypoxia on abdominal expiratory nerve activity.
J. Appl. Physiol.
55:
1614-1622,
1983[Abstract/Free Full Text].
27.
Li, Y.-W.,
D. A. Bayliss,
and
P. G. Guyenet.
C1 neurons of neonatal rats: intrinsic beating properties and 2-adrenergic receptors.
Am. J. Physiol.
269 (Regulatory Integrative Comp. Physiol. 38):
R1356-R1369,
1995[Abstract/Free Full Text].
28.
MacMillan, L. B.,
L. Hein,
M. S. Smith,
M. T. Piascik,
and
L. E. Limbird.
Central hypotensive effects of the 2a-adrenergic receptor subtype.
Science
273:
801-803,
1996[Abstract].
29.
Narchi, P.,
D. Benhamou,
J. Hamza,
and
H. Bouaziz.
Ventilatory effects of epidural clonidine during the first 3 hours after caesarean section.
Acta Anaesthesiol. Scand.
36:
791-795,
1992[Medline].
30.
Nishimura, Y.,
M. Muramatsu,
T. Asahara,
T. Tanaka,
and
T. Yamamoto.
Electrophysiological properties and their modulation by norepinephrine in the ambiguus neurons of the guinea pig.
Brain Res.
702:
213-222,
1995[Medline].
31.
Praud, J.,
I. Kianicka,
V. Diaz,
J. Leroux,
and
D. Dalle.
Prolonged active glottic closure after barbiturate-induced respiratory arrest in lambs.
Respir. Physiol.
104:
221-229,
1996[Medline].
32.
Ruffolo, R. R.,
A. J. Nichols,
J. M. Stadel,
and
J. P. Hieble.
Pharmacologic and therapeutic applications of 2-adrenoceptor subtypes.
Annu. Rev. Pharmacol. Toxicol.
32:
243-279,
1993.
33.
Ryan, M. L.,
M. S. Hedrick,
J. Pizarro,
and
G. E. Bisgard.
Carotid body noradrenergic sensitivity in ventilatory acclimatization to hypoxia.
Respir. Physiol.
92:
77-90,
1993[Medline].
34.
Saupe, K. W.,
C. A. Smith,
K. S. Henderson,
and
J. A. Dempsey.
Respiratory muscle recruitment during selective central and peripheral chemoreceptor stimulation in awake dogs.
J. Physiol. (Lond.)
448:
613-631,
1992[Abstract/Free Full Text].
35.
Sears, T. A.,
A. J. Berger,
and
E. A. Phillipson.
Reciprocal tonic activation of inspiratory and expiratory motoneurones by chemical drives.
Nature
299:
728-730,
1982[Medline].
36.
Smith, C. A.,
D. M. Ainsworth,
K. S. Henderson,
and
J. A. Dempsey.
The influence of carotid body chemoreceptors on expiratory muscle activity.
Respir. Physiol.
82:
123-136,
1990[Medline].
37.
Smith, C. A.,
M. J. A. Engwall,
J. A. Dempsey,
and
G. E. Bisgard.
Effects of specific carotid body and brain hypoxia on respiratory muscle control in the awake goat.
J. Physiol. (Lond.)
460:
623-640,
1993[Abstract/Free Full Text].
38.
Unnerstall, J. R.,
T. A. Kopajitic,
and
M. J. Kuhar.
Distribution of 2 agonist binding sites in the rat and human central nervous system: analysis of some functional, anatomic correlates of the pharmacologic effects of clonidine and related adrenergic agents.
Brain Res. Rev.
7:
69-101,
1984.
J APPL PHYSIOL 84(4):1198-1207
8570-7587/98 $5.00
Copyright © 1998 the American Physiological Society
Top
Abstract
Introduction
