Responsiveness of canine bronchial vasculature to excitatory
stimuli and to cooling
Luke J.
Janssen,
Hwa
Lu-Chao, and
Stuart
Netherton
Asthma Research Group, Firestone Institute for Respiratory Health,
St. Joseph's Hospital and Department of Medicine, McMaster
University, Hamilton, Ontario, Canada L8N 3Z5
 |
ABSTRACT |
Changes in bronchial vascular tone, in part due
to cooling during ventilation, may contribute to altered control of
airflow during airway inflammation, asthma, and exercise-induced
bronchoconstriction. We investigated the responses of canine bronchial
vasculature to excitatory stimuli and cooling. Electrical stimulation
evoked contractions in only some (8 of 88) tissues; these were
phentolamine sensitive and augmented by
N
-nitro-L-arginine. However,
sustained contractions were evoked in all tissues by phenylephrine
[concentration evoking a half-maximal response (EC50)
2
µM] or the thromboxane A2 mimetic U-46619
(EC50
5 nM) and less so by
,
-methylene-ATP or
histamine. Cooling to room temperature markedly suppressed (
75%)
adrenergic responses but had no significant effect against U-46619
responses. Adrenergic responses, but not those to U-46619, were
accompanied by an increase in intracellular Ca2+
concentration. Chelerythrine (protein kinase C antagonist) markedly antagonized adrenergic responses (mean maxima reduced 39% in artery and 86% in vein) but had no significant effect against
U-46619, whereas genistein (a nonspecific tyrosine kinase inhibitor)
essentially abolished responses to both agonists. We conclude that
cooling of the airway wall dramatically interferes with adrenergic
control of bronchial perfusion but has little effect on
thromboxane-mediated vasoconstriction.
contraction; adrenergic response; norepinephrine; thromboxane
A2; intracellular Ca2+; protein kinase C; tyrosine kinase; mitogen-activated protein kinase
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INTRODUCTION |
THE BRONCHIAL
ARTERIES originate from the aorta and/or the intercostal,
internal mammary, or pericardial arteries and wind around the airways
to form an adventitial or peribronchial plexus (10). The
latter, in turn, sends branches down through the airway smooth muscle
layer to form a submucosal plexus beneath the epithelium and then
empties into the pulmonary vein (10). It is as yet unclear
whether these two vascular beds flow in series, in parallel, or in some
combination of the two. Blood flow to the trachea, on the other hand,
is supplied by the tracheal arteries and veins.
The bronchial and tracheal vascular bed serves at least three important
physiological functions: 1) nourishment of the airway wall,
2) conditioning of inspired air (warming and
humidification), and 3) defense and clearance of the airways
(entry point for inflammatory cells; removal of autacoids and
mediators) (27). Some propose that defects in bronchial
vascular function contribute to asthma, particularly exercise- and cold
and/or dry air-induced bronchoconstriction (27). Also,
restoration of bronchial blood flow is now recognized to be vital to
success of lung transplantation (25).
Like most systemic arteries, bronchial arteries receive excitatory
sympathetic and inhibitory parasympathetic innervation (5,
9). Excitatory input is mediated by the action of norepinephrine (NE) on
1-adrenoceptors (2, 20, 24, 30) and
of neuropeptides coreleased with NE, such as neuropeptide Y
(21). Neurotransmitters and inflammatory mediators can
also act through the endothelium, causing it to release excitatory
(endothelin) and inhibitory (nitric oxide) autacoids (6,
30).
In contrast to most other vascular smooth muscle beds in which
temperature is maintained at 37°C, bronchial vascular temperature is
expected to vary considerably, particularly during exercise. This is
because heat is transferred to the inspirate to warm it to
physiological temperatures; heat is also lost from the airway wall
during the process of humidification of the inspired air. The magnitude
of this heat transfer increases substantially during inspiration of
very cold air and/or during exercise (when ventilation can increase
from 5 to 200 l/min). Some, but not all, of this heat and moisture are
recaptured during expiration; any deficit must ultimately be
compensated for by bronchial perfusion. To our knowledge, there have
not yet been any direct measurements of the temperature of blood coming
out of the bronchial perfusion. However, measurements have been made of
the airstream at various points within the lungs: the average
temperature in the trachea can be
32°C during quiet breathing of
room air and drop to
29°C and
20°C during increased
ventilation with room air or frigid air, respectively
(23); corresponding temperatures in the subsegmental bronchi can be
34°C and less than 30°C, respectively (12,
22, 23). The effect that these temperature changes might have on bronchial vascular muscle function has not yet been investigated in
detail. We hypothesized that the effects could include altered neurogenic control (which has not been investigated in any detail) and
sensitivity to autacoids (e.g., inflammatory mediators). We therefore
sought to investigate the responsiveness of canine bronchial and
tracheal vasculature to various physiologically relevant agonists and
to cooling.
 |
METHODS |
Preparation of tissues.
Whole lobes of lung and tracheas were obtained from dogs that had been
euthanized using pentobarbital sodium (100 mg/kg). After the overlying
loose connective tissue was removed, sections of tracheal vein
(0.5-2 mm OD) were excised and cut into ring segments (4-5 mm
in length); the tracheal artery was usually too small to be used. Lobes
of lung were treated in similar fashion; overlying connective tissue
and some parenchyma were removed, after which sections of the bronchial
vasculature (0.5-2 mm OD) were excised and cut into ring segments
(4-5 mm long). Unless indicated otherwise, the bronchial
vasculature that we excised was presumed to represent bronchial artery
(because bronchial veins are generally sparse, do not travel far down
the bronchial tree, and are easily distinguished when they are present
from the bronchial artery with which they ramify) (27).
Tissues were either used immediately or stored at 4°C for use the
next day; we found no functional differences in tissues that were
studied immediately compared with those used after 24-h refrigeration.
Muscle bath technique.
Ring segments were mounted into 2-ml muscle baths using stainless steel
hooks inserted into the lumen; care was taken not to damage the
endothelium while inserting the hooks. One hook was fastened to a Grass
FT.03 force transducer using silk thread (Ethicon 4-0); the other
was attached to a Plexiglas rod, which served as an anchor. Tissues
were bathed in Krebs-Ringer buffer (see Solutions and
chemicals for composition) containing 10 µM indomethacin,
bubbled with 95% O2-5% CO2, and maintained at
37°C; tissues were passively stretched to impose a preload tension of
0.5 g (determined to allow maximal responses). Isometric changes in
tension were digitized (2 samples/s) and recorded using an on-line
program (DigiMed System Integrator; MicroMed, Louisville, KY). Tissues
were equilibrated for 2 h before the experiments were started,
during which time the tissues were challenged with 60 mM KCl at least
once to assess the functional state of each tissue. Electrical field
stimulation (EFS) was delivered via two platinum rods (4 mm apart) on
either side of the tissue. Electrical pulses (0.5-ms duration,
50-70 V) were delivered in pulse trains with frequencies of
1-40 pulses/s (pps).
Fura 2 fluorometry.
Ring segments (0.5-1.0 g wet wt) of vasculature were minced and
transferred to dissociation buffer (see Solutions and
chemicals for composition) containing collagenase (type IV, 2.7 U/ml), elastase (type IV, 12.5 U/ml), and BSA (1 mg/ml) and then were
either used immediately or stored at 4°C for use the next day. We did
not notice any functional differences in cells that were studied
immediately compared with those used after 24 h of refrigeration.
To liberate single cells, tissues in enzyme-containing solution were
incubated at 37°C for 60-120 min and then gently triturated.
Cells were studied using a filter-based photometer-driven system
(DeltaScan; Photon Technology International, South Brunswick, NJ).
After being settled on a glass coverslip mounted on a Nikon TMD
inverted microscope, cells were loaded with the membrane-permeant form
of fura 2 (fura 2-acetoxymethyl ester, 2 µM for 30 min at 37°C) and
then superfused continuously with Ringer buffer (2-3 ml/min) at
37°C. Cells were illuminated alternately (0.5 Hz) at the excitation
wavelengths, and the emitted fluorescence (measured at 510 nm) induced
by 340-nm excitation (F340) and that induced by 380-nm
excitation (F380) were measured using a photomultiplier
tube assembly (13). Agonists were applied by pressure
ejection from a puffer pipette (Picospritzer II; General Valve,
Fairfield, NJ).
Solutions and chemicals.
Dissociation buffer contained (in mM) 125 NaCl, 5 KCl, 1 CaCl2, 1 MgCl2, 10 HEPES, 0.25 EDTA, 10
D-glucose, and 10 L-taurine, pH 7.0. Single cells
were studied in Ringer buffer containing (in mM) 130 NaCl, 5 KCl, 1 MgCl2, 1 CaCl2, 20 HEPES, and 10
D-glucose, pH 7.4. Intact tissues were studied using Krebs-Ringer
buffer containing (in mM) 116 NaCl, 4.2 KCl, 2.5 CaCl2, 1.6 NaH2PO4, 1.2 MgSO4, 22 NaHCO3, and 11 D-glucose, bubbled to maintain
pH at 7.4. Indomethacin (10 µM) was also added to the latter to
prevent generation of cyclooxygenase metabolites of arachidonic acid.
Chemicals were obtained from Sigma Chemical. The 10 mM stock solutions
were prepared in aqueous medium [phenylephrine (PE), N
-nitro-L-arginine
(L-NNA), and phentolamine], DMSO (chelerythrine and
genistein), or 95% EtOH (U-46619); the final bath concentration of
DMSO and EtOH did not exceed 0.1%, which we have found elsewhere to
have little or no effect on mechanical activity.
Data analysis.
Responses are reported as means ± SE; n refers to the
number of animals. Statistical comparisons were made using Student's t-test; P < 0.05 was considered
statistically significant.
 |
RESULTS |
Excitatory regulation.
We first sought to establish those autacoids that mediate excitation of
these tissues. In general, vascular tissues receive excitatory neural
input from the sympathetic innervation, which releases the
neurotransmitters NE and/or ATP (2, 20, 24); NE mediates
its excitatory effects via
-adrenoceptors (2, 20, 24,
30). We therefore examined the mechanical responses to
electrical stimulation in our excised tissues.
Over the course of the 2-h equilibration period, "resting" tone
increased spontaneously from the initial preload tension of 0.5 g
to a mean value of 1.1 ± 0.1 g; development of this tone was
dramatically accelerated on warming the baths to 37°C (e.g., see Fig.
1). Other than this spontaneous tonic
activity, the tissues were quiescent, showing no spontaneous phasic
activity. Only a minority of tissues contracted when stimulated
electrically (5 of 36 bronchial arteries but none of 47 tracheal veins;
see Fig. 1). These contractions reached a peak within 10 s and
resolved to baseline within 1 min after the onset of electrical
stimulation. The magnitudes of these responses ranged considerably
(from 0% of the response to 60 mM KCl in most tissues up to nearly
100% of KCl in a few tissues). Thus we did not calculate the mean
magnitudes of these responses. In a group of tissues from two animals,
we were able to examine the frequency-response relationship of these contractions, finding an optimal frequency of 2-5 pps (Fig.
2). L-NNA (10
4
M), an inhibitor of endogenous nitric oxide production, enhanced the
magnitude of responses to electrical stimulation without altering their
time course (note that all tissues had already been exposed to
10
5 M indomethacin for several hours); in the bronchial
artery, L-NNA augmented EFS contractions in three of the
five "responders" and unmasked contractions in two of the 36 "nonresponders" (Fig. 3), whereas in
the tracheal vein it unmasked small EFS contractions in one of the 47 tracheal vein segments. These EFS-evoked responses were highly
sensitive to the
-adrenoceptor antagonist phentolamine (10
6 M, Fig. 3, n = 4).

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Fig. 1.
Actual tension recordings from a bronchial artery ( 200 µm
diameter) and the bronchial vein ( 250 µm diameter) that
accompanied it. Tissues were mounted in the muscle baths at room
temperature, and preload tension was set to 0.5 g; spontaneous tone
developed in the artery and to a much lesser extent in the vein.
Electrical field stimulation [EFS, 20 pulses at 20 pulses/s (pps),
] had no effect in either tissue. When the bath
temperature was increased from 23 to 37°C, both tissues exhibited a
marked increase in tone. The arterial preparation also regained
sensitivity to EFS.
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Fig. 2.
Second half of experiment represented in Fig. 1. Contractions could
be triggered by EFS ( ; 20 pulses at frequencies
indicated) in the bronchial artery but not in the bronchial vein.
However, both exhibited contractile responses to the
1-agonist phenylephrine (PE, 10 5 M), to
temperature (see Fig. 1), and to , -methylene-ATP (data not
shown). *Experimental artifact.
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Fig. 3.
Actual recording from bronchial artery at 37°C showing
mechanical responses to electrical stimulation ( , 10 pulses at 5 pps).
N -nitro-L-arginine
(L-NNA, 10 4 M) increased basal tone and
unmasked contractions that were abolished by phentolamine
(10 6 M).
|
|
Although only a minority of tissues exhibited an excitatory response to
electrical stimulation even in the presence of both indomethacin and
L-NNA, almost all tissues exhibited substantial contractile
responses on application of the
-adrenoceptor agonist PE (Fig.
2). These adrenergic responses were sustained and dose dependent (Fig. 4), with a concentration
evoking a half-maximal response (EC50) value of 2.0 ± 0.5 and 3.0 ± 0.5 µM in the tracheal vein and bronchial artery,
respectively (n = 7).

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Fig. 4.
A: actual recording from a bronchial arterial
segment showing mechanical response to KCl (60 mM), PE
(10 7 to 10 4 M) and to U-46619
(10 9 to 10 6 M), as indicated; W, wash. Mean
concentration-response relationships for PE, U-46619, histamine, and
, -methylene-ATP in bronchial artery and tracheal vein are given
in B and C, respectively (n = 5-8).
|
|
,
-Methylene-ATP, a stable analog of ATP, generally had no effect
on tissues at concentrations below 10
6 M but evoked
transient contractions (lasting <5 min) at higher concentrations (Fig.
4, B and C). Because we were not able to observe
a sustained "plateau" response to
,
-methylene-ATP, we could
not calculate an EC50 value for this agonist.
Vascular tissues are also regulated by a variety of inflammatory
mediators, including thromboxane (Tx)-A2 and histamine. We found that the TxA2 mimetic U-46619 evoked sustained and
dose-dependent contractions in both the tracheal vein and bronchial
artery (Fig. 4), with an EC50 of 6.0 ± 0.5 and
3.0 ± 0.5 nM in the tracheal vein and bronchial artery,
respectively (n = 7). Histamine, on the other hand,
elicited mixed responses: 7 of 8 tracheal vein segments
exhibited contractions (Fig. 4), with an EC50 of
20 ± 1 µM, whereas the remaining tissue relaxed in response to histamine.
Effect of cooling on physiological responses.
In vivo, most vascular beds experience little or no change in
temperature; in contrast, the temperature in the tracheal and bronchial
vasculature can regularly fall substantially below the normal
body temperature of
37°C (12, 22, 23). We
therefore sought to examine the effects of cooling on the physiological responses previously described.
After the water pump of our muscle bath apparatus was turned off, the
bath temperature fell to ambient room temperature (22-25°C) within 5 min, during which time we noticed a drop in basal tension of
the tissues from 1.1 ± 0.1 to 0.8 ± 0.1 g (Fig.
5A); the responsiveness of the
tissues was examined 20-30 min after cessation of warming (Fig.
5). At room temperature, the responses to PE were substantially and
significantly reduced compared with those observed at 37°C (Fig. 5);
this was not due to desensitization of the adrenergic receptors because
full responsiveness was restored when the tissues were rewarmed (Fig.
5, *).

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Fig. 5.
A: actual recording from bronchial artery showing
mechanical response to PE at 37°C. When the bath was cooled to
25°C, basal tone decreased and subsequent responses to PE were
markedly reduced; the latter returned to control levels when the bath
was rewarmed to 37°C (*). Mean concentration-response relationships
for PE (left) and U-46619 (right) in bronchial
artery (B) and tracheal vein (C) recorded at
37°C ( ) and at 25°C ( ) are shown;
n = 5 for all. The mean increase in tone observed when
bath was rewarmed to 37°C is indicated by * and dotted lines.
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Given that U-46619 responses were largely irreversible on washout of
the agonist, we could not use this same protocol to compare U-46619
sensitivity both before and after cooling. However, we did find that
U-46619 could evoke substantial contractions at room temperature in
tissues that had not been stimulated previously; these responses were
not significantly different from those obtained in other tissues
studied at 37°C (Fig. 5), and these did not demonstrate as dramatic
an increase in magnitude on rewarming as did the PE responses (Fig. 5).
Mechanisms underlying excitatory responses.
In an attempt to understand the differential sensitivity of PE and
U-46619 contractions to cooling, we next examined the
intracellular signaling mechanisms underlying these
responses. Generally, contraction in smooth muscle is a
Ca2+-dependent event; we therefore examined the effect of
these agonists on intracellular Ca2+ concentration
([Ca2+]i) in freshly dissociated single cells.
In freshly dissociated bronchial arterial smooth muscle cells
loaded with the Ca2+ indicator dye fura 2, caffeine was
generally able to evoke a large transient elevation of
[Ca2+]i (Fig.
6). These responses were similar in
amplitude and time course to those that have been examined in many
other vascular and nonvascular preparations, and we did not examine
them in any more detail here. Fluorometric responses could also be
elicited by PE (10
5 M, n = 9, Fig.
6B) but not by U-46619 (10
7 to
10
5 M, n = 12, Fig. 6A).
,
-Methylene-ATP (10
4 M) had no effect on 13 of 16 cells tested (n = 4); 3 of the remaining cells showed a
small fluorometric response.

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Fig. 6.
Fluorometric recordings obtained in single freshly
dissociated bronchial arterial smooth muscle cells loaded with fura 2. A: in one cell, while caffeine (10 mM) evoked large
transient elevations of intracellular Ca2+ concentration
([Ca2+]i), U-46619 (10 6 M)
evoked a much smaller and more slowly developing response.
B: in another cell, PE (10 5 M) evoked a series
of [Ca2+]i oscillations.
F340/F380, ratio of 340-nm to
380-nm fluorescence.
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These observations indicate that U-46619, though a far more
potent and efficacious spasmogen than PE, does not act by mobilizing Ca2+. Instead, it may increase the Ca2+
sensitivity of the contractile apparatus (26); in other
preparations, such an event is mediated by one or more protein
kinases (26). We therefore examined the effects
of inhibitors of protein kinase C (chelerythrine, 10
6 M)
and of tyrosine kinases (genistein, 10
4 M) on responses
to U-46619 and to PE. Chelerythrine suppressed basal tone (to 0.7 ± 0.1 and 0.6 ± 0.1 g in bronchial artery and tracheal
vein, respectively) and reduced PE contractions substantially in the
case of the bronchial artery and completely in the case of tracheal
vein. In contrast, U-46619 contractions in the vein were only slightly
(and not significantly) reduced, whereas those in the artery were
unaffected. Interestingly, the degree of inhibition produced by
chelerythrine in these four test groups approximated that caused by
cooling. Genistein, on the other hand, exerted substantial suppression
of basal tone (to 0.6 ± 0.1 g in both bronchial artery and
tracheal vein) as well as of the contractile responses to U-46619 and
PE (Fig. 7).

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Fig. 7.
Mean concentration-response relationships for PE and
U-46619 (left and right, respectively) in
bronchial artery (A) and tracheal vein (B) in the
presence or absence of chelerythrine (10 6 M) or genistein
(10 4 M), as indicated; n = 4-7.
Temperature in all cases was 37°C.
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 |
DISCUSSION |
The bronchial circulation is important for the supply of oxygen
and nutrients to the various cell types of the airways (epithelium, smooth muscle, and innervation), and it has a key role in the warming
and humidification of inspired air (10). In dogs, it also
is important in cooling the animal because they do not sweat. The
bronchial circulation can also influence airflow in several ways.
Vasodilation of this vascular bed increases the thickness of the mucosa
and the stiffness of the airway wall (5, 7, 16, 17, 29)
and removes spasmogenic agents (inflammatory mediators, allergens,
neurotransmitters, etc.), thereby accelerating recovery from
bronchoconstriction (27). Finally, it contributes to
allergic asthma (as a source of inflammatory cells) and
exercise-induced asthma [exercise-induced vasodilation
leads to thickening of mucosa, narrowing of lumen, and stiffening of
airway walls (27)]. For these reasons, it is important to
gain a better understanding of the control of bronchial perfusion and
its response to cooling.
Excitatory regulation of bronchial vascular tone.
As is true for many vascular beds, we found that the bronchial
vasculature develops spontaneous tonic activity. We did not characterize in detail the mechanisms underlying this tone, but its
insensitivity to phentolamine and to indomethacin indicates little or
no role for adrenergic mechanisms or cyclooxygenase metabolites,
whereas its sensitivity to both chelerythrine and genistein indicates
important roles for protein kinase C and tyrosine kinases. The
target(s) of these kinases is as yet unclear.
In contrast to many other vascular beds, however, we found the
bronchial vasculature to be relatively poorly regulated by excitatory
neurogenic input; only a minor fraction of the excised tissues
exhibited any excitatory response to electrical stimulation even though almost all of them responded to an exogenously applied adrenergic agonist. Responsiveness to electrical stimulation did not
improve dramatically during simultaneous inhibition of nitric oxide
synthase and cyclooxygenase activities, suggesting that the endothelial
factors nitric oxide and prostacyclin were not inhibiting adrenergic
neurotransmission. Furthermore, because we have previously been able to
demonstrate substantial excitatory neurogenic responses in airway and
pulmonary vascular segments (14, 15) even after 1 or 2 days of storage in a refrigerator, we do not believe that the nerve
endings were inadvertently damaged or destroyed in this study. In fact,
in this study, we often observed electrically evoked inhibitory
responses in these tissues (data not shown). Instead, we interpret
these data to mean that the canine bronchial vasculature is poorly
regulated by excitatory innervation. The nerve endings that do exist in
these tissues appear to be primarily adrenergic because the neurogenic
responses were essentially abolished by phentolamine.
Inflammatory cells in the airway mucosa release a wide variety of
mediators, including histamine (28) and several
prostaglandins (2, 17). Histamine can mediate contractions
via an action on H1 receptors (2, 18, 19, 28);
TxA2 is also a potent spasmogen in many vascular beds. In
the bronchial vasculature, we found the TxA2 mimetic
U-46619 to be the most potent and powerful spasmogen, much more so than
either histamine or PE. The receptors through which U-46619 acts in
this tissue (likely TxA2 receptors, given the very low
EC50 value for U-46619) are apparently not coupled to
phospholipase C or protein kinase C because the responses were not
accompanied by a [Ca2+] transient and were completely
insensitive to chelerythrine. On the other hand, the
sensitivity of these responses to genistein suggests that the receptors
are coupled to tyrosine kinase(s); again, the target(s) of these
kinases is as yet unclear.
The endothelium may also release factors that affect vasomotor tone,
nitric oxide and PGI2 being the most widely recognized mediators of its inhibitory effects and endothelin the most popular excitatory endothelial autacoid (6, 30). We did not study these regulatory pathways in detail in the present study but did obtain
evidence of an ongoing synthesis (and action) of nitric oxide, as
indicated by the change in basal tone on addition of L-NNA.
It is as yet unclear how nitric oxide or other endothelial factors
might have influenced the responses to PE or U-46619, or how cooling
might affect the synthesis and/or release of endothelial signaling
molecules (although the potential contributions of PGI2 were prevented using indomethacin). Endothelial regulation of this
tissue needs to be examined in a subsequent study.
Relevance of the bronchial circulation to asthma.
Exercise decreases airflow in >70% of individuals with symptomatic
asthma (27). Two overall mechanisms have been proposed to
account for these changes based on the findings that they can be
mimicked in humans and animals by inspiration of cold and/or dry air
(3) or exposure to hyperosmolar fluids (11).
On the one hand, the cooling and drying associated with increased
ventilation are believed to stimulate mast cells in the airway wall,
either directly or through increased osmolarity of the mucosal lining, leading to degranulation and release of mediators. On the other hand,
some patients still exhibit exercise-induced bronchospasm (EIB) while
breathing air which has already been fully warmed and/or humidified
(1), suggesting that the trigger for EIB is something
other than cooling, drying, or altered osmolarity of the airway
wall. A competing theory posits that bronchial arterial dilation during exercise leads to thickening of the mucosa via engorgement of the vessels per se and/or increased edema formation; this would lead to encroachment of the mucosa into the airway lumen as
well as to decreased airway compliance, both of which would impair
airflow (29). Bronchial arterial dilatation is accompanied
by a near doubling of tracheal mucosal thickness (7) and
impairment of airflow (8) in sheep; similar changes have been described in dogs (5, 16, 17).
Pertinent to this, we found that cooling of the excised tissues
markedly suppressed spontaneous myogenic tone as well as that tone
triggered by activation of adrenoceptors (by electrical stimulation or
exogenous application of PE). Thus during increased ventilation (e.g.,
exercise) or inhalation of cold air, the basal level of resistance to
blood flow as well as the increased resistance evoked by
sympathetic/adrenergic stimulation would be reduced; the resultant vasodilation would then contribute in part to exercise- and cold air-induced asthma. The vasodilatory response to osmotic changes in the
airway wall (e.g., caused by hyperosmolar fluids or dry air), on the
other hand, likely involves a different mechanism(s) because a previous
study has shown that inhalation of warm dry air in dogs causes an
increase in tracheobronchial blood flow, which is not attenuated by
-adrenergic blockade (4). Remarkably, U-46619-evoked
contractions were much less sensitive to cooling, being only moderately
reduced in the tracheal vein and unaffected in the bronchial artery.
In an attempt to understand the basis for the relative differences in
the sensitivity of adrenergic and U-46619 contractions to cooling, we
examined the signaling mechanisms underlying these two responses and
found two important differences: adrenergic responses were accompanied
by an elevation of [Ca2+]i and were highly
sensitive to an inhibitor of protein kinase C (chelerythrine), while
those to U-46619 were not. It is not clear how and/or whether these
functional differences account for the different sensitivity to
cooling, but we noted empirically that the degree of inhibition exerted
by cooling paralleled closely that exerted by chelerythrine. Both
interventions reduced adrenergic contractions dramatically, reduced
U-46619 contractions in the tracheal vein partially (but not
significantly), and had no effect whatsoever on U-46619 contractions in
the bronchial artery.
We conclude that excitation in the canine bronchial vasculature is
exerted primarily by myogenic mechanisms and thromboxanes, whereas
excitatory neurogenic input is relatively unimportant. Furthermore,
cooling of the vasculature markedly suppresses excitatory myogenic and
adrenergic responses but has little effect on tone exerted by
TxA2. These findings are relevant to exercise- and cold
and/or dry air-induced asthma.
 |
ACKNOWLEDGEMENTS |
These studies were supported by an operating grant and Medical
Research Council (MRC) Scientist Award from the MRC of Canada.
 |
FOOTNOTES |
Address for reprint requests and other correspondence: L. J. Janssen, L-314, St. Joseph's Hospital, 50 Charlton Ave. East, Hamilton, Ontario, Canada L8N 4A6 (E-mail:
janssenl{at}mcmaster.ca).
The costs of publication of this
article were defrayed in part by the
payment of page charges. The article
must therefore be hereby marked
"advertisement"
in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Received 15 September 2000; accepted in final form 27 November 2000.
 |
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