| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
Departments of Internal Medicine and Pharmacology and Pediatrics, University of California, Davis, California 95616
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
ACKNOWLEDGEMENTS
FOOTNOTES
REFERENCES
ABSTRACT 
Bonham, A. C., K. S. Kott, and J. P. Joad. Sidestream
smoke exposure enhances rapidly adapting receptor responses to substance P in young guinea pigs. J. Appl.
Physiol. 81(4): 1715-1722, 1996.
We determined
the effect of sidestream tobacco smoke (SS) exposure on responses of
lung rapidly adapting receptors (RARs), peak tracheal pressure (Ptr),
and arterial blood pressure (ABP) to substance P in young guinea pigs.
Guinea pigs were exposed to SS or filtered air from
day 8 to days
41-45 of life. They were then anesthetized and
given three doses of intravenous substance P (1.56-4.94 nmol/kg).
SS exposure augmented substance P-evoked increases in RAR activity
(P = 0.029 by analysis of variance) but not substance P-evoked increases in peak Ptr or decreases in ABP.
Neurokinin 1-receptor blockade (CP-96345, 400 nmol/kg) attenuated
substance P-evoked increases in RAR activity
(P = 0.001) and ABP
(P = 0.009) but not in peak Ptr
(P = 0.06). Thus chronic exposure to
SS in young guinea pigs exaggerates RAR responsiveness to substance P. The findings may help explain the increased incidence of airway
hyperresponsiveness and cough in children chronically exposed to
environmental tobacco smoke.
environmental tobacco smoke; airways; vagus; tachykinins
INTRODUCTION CHILDREN LIVING IN HOMES where they are exposed to
environmental tobacco smoke have increased respiratory symptoms
including greater wheeze and sputum production (4), more coughing with colds (4, 6), and increased risk of respiratory illness-related hospitalizations (6). Such children also exhibit airway obstruction (21), an increased airway reactivity, an increased rate of asthma, and
an earlier onset of asthma (16). In children who have asthma, environmental tobacco smoke exposure is associated with an increased severity of the asthma and greater airway hyperreactivity (28). Collectively, the data indicate that children exposed to environmental tobacco smoke have increased respiratory symptoms of cough, mucus secretion, airway obstruction, and airway hyperreactivity.
These respiratory symptoms may result, at least in part, from a chronic
stimulation of rapidly adapting (irritant) receptors (RARs)
and/or C-fiber receptors in the lungs and airways. Stimulation of either the RARs or the C-fiber receptors elicits airway reflex defense mechanisms that include cough, bronchoconstriction, and increased mucus secretion (3). Moreover, electrophysiological recordings of single-fiber afferent activity have shown that acute inhalation of mainstream tobacco smoke is a potent stimulus for both
the RARs and C-fiber receptors (13, 20). Less is known about the effect
of environmental tobacco smoke on the responsiveness of these
receptors. Bonham et al. (1) have previously shown that
exposing young guinea pigs to environmental tobacco smoke over a period
of 5 wk during their development markedly diminished the responsiveness
of the RARs to the acute inhalation of mainstream tobacco smoke. These
data were consistent with the findings by Swanny et al. (24) that
chronic exposure to mainstream tobacco smoke over a similar period of
4-8 wk attenuated the reflex apnea and tachypnea produced by acute
inhalation of tobacco smoke in adult rats, suggesting a decreased
responsiveness of the vagal sensory receptors. Together, the data
suggest that with chronic exposure to either sidestream or mainstream
tobacco smoke, lung sensory receptors become less reactive to an acute
inhalation of mainstream tobacco smoke. In both cases, the diminished
responsiveness may have been specific for the acute stimulus of
mainstream tobacco smoke. In that regard, Karlsson et al. (11) reported
that exposure of adult guinea pigs to mainstream tobacco smoke for 1 h
twice daily for 2 wk actually enhanced coughing induced by nebulized citric acid and capsaicin but not by tobacco smoke. The collective observations of Swanny et al. (24) and Karlsson et al. (11) suggest
that periodic exposure to mainstream tobacco smoke may augment the
responsiveness of lung sensory receptors to noxious stimuli other than
acutely inhaled mainstream smoke while simultaneously either
diminishing or not affecting the responsiveness of the receptors to the
acute inhalation of mainstream tobacco smoke. There are no data on the
effect of chronic exposure to sidestream smoke on the responsiveness of
lung sensory receptors to stimuli other than acute inhalation of
mainstream tobacco smoke. Thus the purpose of the present study was to
determine the effect of chronic exposure to environmental tobacco smoke
in the young guinea pig on the responsiveness of RARs to a stimulus
other than mainstream tobacco smoke. Substance P was selected because
it reproducibly stimulates RARs (2, 17) and causes bronchoconstriction,
neurogenic inflammation, and cough (14, 15, 30), which may be relevant to the increased respiratory problems in children raised with environmental tobacco smoke. Young guinea pigs, age equivalent to
prepubescent human children, were exposed to either sidestream smoke (a
surrogate for environmental tobacco smoke) or to the control condition
of filtered air for ~5 wk. The responses of the RARs, peak tracheal
pressure, and arterial blood pressure to increasing doses of substance
P were then examined.
METHODS Chronic exposure to sidestream tobacco
smoke. Male Dunkin-Hartley guinea pigs were randomly
assigned to a group exposed to either sidestream smoke or filtered air
for 6 h/day, 5 days/wk, from age 8 days to age 41-45 days of life.
Sidestream smoke is a surrogate for environmental tobacco smoke,
differing only in that it does not contain expired mainstream tobacco
smoke. The guinea pigs were housed in polycarbonate cages (69 × 69 cm cross-sectional area) with wire lids and autoclaved wood carvings
for bedding. They were fed guinea pig chow and water ad libitum,
including during the exposures. Sidestream smoke was generated by a
modified ADL/II smoke-exposure system (Little, Cambridge, MA) from
conditioned 1R4F cigarettes from the University of Kentucky Tobacco and
Health Research Institute (Lexington). Two cigarettes at a
time were smoked under Federal Trade Commission conditions in a
staggered fashion at a rate of 1 puff/min (35 ml, 2-s duration). The
sidestream smoke was diluted in a 1:10 ratio with filtered air in a
mixing chamber, then passed into the stainless steel and glass
Hinners-type exposure chamber 0.44 m3 in size. The exposure chamber
was characterized by the following parameters expressed as means ± SD: relative humidity of 42 ± 9%, temperature of
23 ± 1°C, total suspended particulate concentration of 1.03 ± 0.17 mg/m3, carbon
monoxide concentration of 6.4 ± 0.9 parts/million, and nicotine
concentration of 169 ± 89 µg/m3. Relative humidity and
temperature were sampled continuously. Nicotine was sampled for 15 min
twice during each 6-h exposure period. The total suspended particulate
concentration was sampled with the piezobalance technique for 30 min
out of every hour.
Experimental procedures. During the
data-acquisition period, the investigators were blinded as to whether
the guinea pigs had been exposed to filtered air or sidestream smoke.
Each guinea pig was anesthetized with an intraperitoneal injection of
1.7 g/kg of urethan and then given supplemental doses of pentobarbital sodium (4 mg/kg iv) about every hour as needed. Catheters were introduced into the jugular vein for administering fluids and drugs and
into the carotid artery for monitoring arterial blood pressure and
withdrawing samples for arterial blood gases. The trachea was
cannulated below the larynx, and a catheter was connected to a side
port of the endotracheal tube to monitor intratracheal pressure. The
guinea pigs were prepared with bilateral pneumothoraces by incisions
made in the chest wall and mechanically ventilated with oxygen-enriched
humidified air with a tidal volume of 8 ml/kg at a rate of 30-33
breaths/min. The expiratory line of the ventilator was placed under 2 cmH2O. Arterial blood gases and pH
were maintained within normal limits by adjusting the ventilator rate
and by infusing sodium bicarbonate. The animals were paralyzed with
gallamine (3 mg/kg) every hour as needed. During neuromuscular
blockade, the adequacy of anesthesia was continuously assessed by
monitoring the animal for spontaneous fluctuations in arterial blood
pressure. About every hour, the animals were allowed to recover from
the gallamine, at which time we assessed the adequacy of anesthesia by
testing for the absence of an increase in systemic arterial pressure
and heart rate that occurred in response to a paw pinch. Body
temperature was monitored with a thermistor and maintained with a
servo-controlled water blanket.
For recording RAR activity, the left cervical vagus nerve was
transected below the nodose ganglion and the distal end was placed on a
dissecting platform in a pool of mineral oil. Afferent nerve activity
was recorded from nerve bundles dissected away from the transected
vagus nerve. The contralateral vagus nerve was left intact. A nerve
bundle containing a RAR fiber was split so that the fiber was the only
active fiber discernible or had a signal-to-noise ratio that
was sufficient to differentiate its activity from the noise by use of a
window discriminator. The RARs were identified by their rapid
adaptation to a fast-rising, then maintained, hyperinflation
(~2-3 tidal volumes). The receptors were localized at the end of
each experiment by careful probing of the lungs and airways with a
small glass rod.
In pilot studies, RAR responses to substance P were compared when the
peptide was administered intravenously in one-half, one-third, and
one-fourth log increments. Substance P-evoked increases in RARs
exhibited a steep dose-response curve, so the smallest increment of
one-fourth log was chosen. The lowest dose of 1.56 nmol/kg was selected
because it was the highest dose that produced a <2 impulse/bin
increase in RAR activity in control animals in pilot studies. This dose
was considered the near-threshold dose. Sequential one-fourth log doses
of 2.78 and 4.94 nmol/kg were administered into the jugular vein every
5 min. Doses > 4.94 nmol/kg were not given because of profound
effects on arterial blood pressure and peak tracheal pressure. The 3 doses of substance P were administered to 17 filtered air-exposed
guinea pigs and 21 sidestream smoke-exposed guinea pigs.
To determine whether the substance P-evoked increases in RAR activity
were mediated by neurokinin 1 (NK1) receptors, the RAR responses to substance P were compared before and after intravenous administration of the nonpeptide high-affinity
NK1-receptor antagonist CP-96345
(Pfizer). Pilot studies were performed to determine the optimal time
for repeating the substance P dose-response curves. The first substance
P dose-response curve was generated by administering the three doses of
substance P in 5-min intervals. At 10-, 20-, and 30-min intervals after
the final dose of substance P, normal saline was injected
intravenously, and 5 min later, the substance P dose-response curve was
repeated. The optimal time interval was 30 min; thus all protocols
using the antagonist or the inactive enantiomer were performed with
this time interval. From pilot experiments, we determined that 400 nmol/kg of CP-96345 were necessary to significantly attenuate the
substance P-induced increases in RAR activity. These doses were not
exceeded because higher doses (~10-fold higher) have been associated
with effects unrelated to antagonism of
NK1 receptors on L-type calcium
channels (19). In addition, the responses of three RARs to substance P
were examined before and after injection of 400 nmol/kg of the inactive
(2R,3R) enantiomer of CP-96345, referred to as CP-96344 (Pfizer).
Because investigators were blinded as to exposure treatment during the
experiments, protocols in which either CP-96345, the inactive
enantiomer CP-96344, or normal saline were used were performed in both
filtered air and sidestream smoke-exposed guinea pigs. The results were
not different between the two groups, so the data were combined.
Data analysis. RAR activity was
expressed as impulses per 3-s bin to capture moment-to-moment changes
in the receptor activity. To determine the sustained responses of the
RARs and peak tracheal pressure to increasing steps of one-fourth log
doses of substance P, both RAR activity and peak tracheal pressure were
averaged over an initial control period (60 s) and after each dose of
substance P (30 s). Peak tracheal pressure was also averaged for 90 s
after each dose of substance P to capture more sustained increases. For
mean arterial blood pressure, the pressure was determined in the
control period (60 s) and at the point of maximal decrease after each
dose of substance P. For the dose-response curves, RAR activity was log
transformed to stabilize variances because the variances at the highest
dose of substance P were greater than threefold the variances in the
control period. The baseline RAR activity, peak tracheal pressure, and
arterial blood pressure were compared for filtered air- vs. sidestream
smoke-exposed guinea pigs by using an unpaired
t-test. The substance P dose-response curves in filtered air- vs. sidestream smoke-exposed animals were compared with a repeated-measures analysis of variance with treatment group (sidestream smoke or filtered air) as a between-subject effect
and dose level as a within-subject effect. The substance P
dose-response curves before and after
NK1-receptor blockade with
CP-96345, the inactive enantiomer CP-96344, and normal saline were
compared with a repeated-measures analysis of variance with treatment
(CP-96345, CP-96344, or normal saline) as one within-subject effect and
dose level as another within-subject effect (SAS, SAS Institute). The
effects of CP-96345 on baseline RAR activity and peak tracheal pressure
were compared with a paired t-test.
To describe the temporal relationship of the responses of the RARs and
tracheal pressure, we evaluated the onset latency, the latency to the
peak response, and the duration of the response to the highest dose of
substance P. We chose the highest dose because of variable responses at
the lower doses. The onsets for the increases in RAR activity and peak
tracheal pressure were defined as the first observable increases after
the commencement of the injection of substance P. The peak responses
were defined as the largest increase after commencement of the
substance P injection. The peak latencies were defined as the time to
the peak response from the commencement of the injection. The duration was defined as the time from the onset of the responses to the time
when the responses recovered to ~10% of baseline values.
The data are reported as means ± SE unless otherwise indicated.
Significance is claimed when P < 0.05.
RESULTS RARs were recorded in 21 guinea pigs exposed to sidestream smoke and 17 guinea pigs exposed to filtered air. One RAR was recorded per animal,
and all were localized to the upper or middle lobe near the hilum. The
weights and arterial blood gases were not different between the two
groups (Table 1).
Table 1.
Weight and blood gases for animals in filtererd air- and sidestream
smoke-exposed groups
Filtered Air
Sidestream Smoke
Weight, g
467 ± 13
482 ± 13
PO2, Torr
340 ± 28
390 ± 26
PCO2, Torr
43 ± 1.1
40 ± 1.0
pH
7.38 ± 0.01
7.39 ± 0.01
Values are means ± SE. All P > 0.05.
Responses of RARs to substance P in the filtered air-
and sidestream smoke-exposed guinea pigs. Substance P
produced dose-dependent increases in RAR activity; however, the
increases were significantly greater in guinea pigs chronically exposed
to sidestream smoke. An example of the effect of substance P on RAR
activity in a guinea pig from the filtered air control group is shown
in Fig. 1. The lowest dose of substance P
(1.56 nmol/kg) had very little effect on either RAR activity or peak
tracheal pressure but lowered arterial blood pressure (Fig.
1A); the second dose (2.78 nmol/kg) increased RAR activity, slightly increased peak tracheal
pressure, and decreased arterial blood pressure (Fig.
1B); and the highest dose (4.94 nmol/kg) produced a larger excitatory effect on RAR activity and peak
tracheal pressure and decreased arterial blood pressure (Fig. 1C). Figure
2 shows an example of the effect of
substance P on RAR activity in a sidestream smoke-exposed guinea pig.
The lowest dose of substance P slightly increased RAR activity,
produced a delayed increase in peak tracheal pressure, and decreased
arterial blood pressure (Fig. 2A);
the second dose produced a substantially larger increase in RAR
activity, a delayed increase in peak tracheal pressure, and a decrease
in arterial blood pressure (Fig.
2B); and the highest dose produced a
larger excitatory effect on RAR activity, markedly increased peak
tracheal pressure, and lowered arterial blood pressure (Fig.
2C).
The grouped data for the RAR responses in the sidestream smoke-exposed
and filtered air-exposed guinea pigs are shown in Fig. 3. The baseline RAR activity was not
significantly different between the two groups (0.5 ± 0.12 impulses/bin in the filtered air-exposed group and 1.8 ± 0.8 impulses/bin in the sidestream smoke-exposed group;
P = 0.11). Substance P produced
dose-dependent increases in RAR activity
(P = 0.0001, dose). However, the RARs
from the sidestream smoke-exposed animals were significantly more
responsive to substance P than were those exposed to filtered air
(P = 0.029, treatment), showing a
leftward shift in the dose-response curve.
Neither the onset latency, latency to the peak response, nor duration of the RAR responses to the highest dose of substance P varied with the exposure treatment (P > 0.05, unpaired t-test); thus the data were pooled. Substance P-evoked increases in RAR activity exhibited an onset latency of 6 ± 5 (SD) s, a peak latency of 14 ± 6 s, and a duration of 39 ± 20 s.
Effect of NK1-receptor antagonism on RAR responses to substance P. The dose-response curves for the substance P-evoked increases in RAR activity were highly reproducible at 30-min intervals [Fig. 4A; P = 0.58, dose by treatment; n = 12 (6 RARs from the filtered air-exposed group and 6 from the sidestream smoke-exposed group)]. Protocols using the NK1-receptor antagonist and the inactive enantiomer were performed with the same time intervals. NK1-receptor blockade with 400 nmol/kg of CP-96345 had no effect on baseline RAR activity (1.5 ± 0.42 impulses/bin before vs. 1.5 ± 0.42 impulses/bin after antagonist; P = 0.928), but it significantly attenuated the substance P-evoked increases in RAR activity [Fig. 4B; P = 0.0001, dose by treatment; n = 8 (2 RARs from the filtered air-exposed group and 6 from the sidestream smoke-exposed group)]. By contrast, 400 nmol/kg of CP-96344, the inactive enantiomer of the NK1 antagonist, had no effect on the substance P-evoked increases in RAR activity [Fig. 4C; P = 0.70, dose by treatment; n = 3 (2 RARs from the filtered air-exposed group and 1 from the sidestream smoke-exposed group)].
Responses of peak tracheal pressure and arterial blood pressure to substance P in the filtered air- and sidestream smoke-exposed guinea pigs. The baseline peak tracheal pressures were not significantly different in the two groups: 8.28 ± 0.44 cmH2O in the sidestream smoke-exposed guinea pigs (n = 20) compared with 9.12 ± 0.50 cmH2O in the filtered air-exposed group (n = 17; P = 0.21). Substance P produced dose-dependent increases in peak tracheal pressure in both the filtered air- and sidestream smoke-exposed groups (Fig. 5A; P = 0.0001, dose). In contrast to the effects on the RARs, the increases in peak tracheal pressure were not different between the filtered air- and sidestream smoke-exposed groups (Fig. 5A; P = 0.88, treatment).
Neither the onset latency, latency to the peak response, nor the duration of the response of tracheal pressure to the highest dose of substance P varied with the exposure treatment (P > 0.05, unpaired t-test); thus the data were pooled. Substance P-evoked increases in tracheal pressure exhibited an onset latency of 10 ± 5 (SD) s, a peak latency of 22 ± 5 s, and a duration of 146 ± 86 s. The dose-response curves for the substance P-evoked increases in peak tracheal pressure were highly reproducible within 30-min time intervals [Fig. 5B; pre-normal saline vs. post-normal saline; P = 0.49, dose by treatment; n = 11 (6 filtered air-exposed animals and 5 sidestream smoke-exposed animals)]. NK1-receptor blockade did not significantly change resting peak tracheal pressure (8.82 ± 1.2 cmH2O before and 8.68 ± 1.2 cmH2O after NK1-receptor blockade; P = 0.37) nor did it significantly attenuate the substance P-induced increases in peak tracheal pressure [Fig. 5C; dose by treatment; P = 0.064; n = 7 (2 filtered air-exposed animals and 5 sidestream smoke-exposed animals)]. The inactive enantiomer also had no effect [Fig. 5D; P = 0.32, dose by treatment; n = 3 (2 filtered air-exposed animals and 1 sidestream smoke-exposed animal)]. The above analyses were based on averaging peak tracheal pressures over 90 s after each dose of substance P. The analyses of peak tracheal pressure over 30 s did not differ from the 90-s analyses. The baseline arterial blood pressure was not different in the two groups (49 ± 2 mmHg in the sidestream smoke-exposed guinea pigs compared with 50 ± 2 mmHg in the filtered air-exposed group; P = 0.58). Substance P produced dose-dependent decreases in arterial blood pressure that were statistically significant within each group but not different between the two groups (P = 0.0001, dose; P = 0.24, treatment). In the filtered air-exposed animals, the three doses of substance P decreased arterial blood pressure by 11 ± 1, 13 ± 2, and 14 ± 1 mmHg, respectively. In the sidestream smoke-exposed animals, the three doses of substance P decreased arterial blood pressure by 13 ± 2, 15 ± 2, and 17 ± 2 mmHg, respectively. The latency for the peak fall in blood pressure averaged 6 ± 5 s and ranged from 2-22 s after the injections. The substance P-evoked decreases in arterial blood pressure were reproducible over the 30-min time interval between the dose-response curves (P = 0.76, dose by treatment; n = 12) and were significantly attenuated by NK1-receptor blockade (P = 0.009, dose by treatment; n = 6). Before NK1-receptor blockade, the three doses of substance P decreased arterial blood pressure by 10 ± 3, 14 ± 3, and 17 ± 3 mmHg, respectively. After NK1-receptor blockade, the substance P-evoked decreases were 3 ± 2, 4 ± 2, and 3 ± 2 mmHg, respectively. The inactive enantiomer had no effect on the substance P-evoked decreases in blood pressure (P = 0.34, dose by treatment; n = 3).
DISCUSSION
This study demonstrated that chronic exposure to sidestream smoke (the surrogate for environmental tobacco smoke) in young guinea pigs augmented the responsiveness of RARs to substance P. The substance P-evoked increases in RAR activity, which occurred in both the sidestream smoke-exposed and filtered air control groups, were mediated by NK1 receptors.
This is the first electrophysiological study to demonstrate that substance P stimulates the RARs in guinea pigs, as shown previously in other species (2, 17), and to implicate NK1 receptors by the use of a high-affinity selective nonpeptide NK1-receptor antagonist. Substance P, contained in afferent C-fiber endings within the airway epithelium and smooth muscle layer, is released by activation of the afferent C fibers. The stimulation of the RARs by substance P represents a potential link between the two airway defense systems, both of which elicit bronchoconstriction, mucus secretion, and cough. Such a link, whereby C-fiber-receptor stimulation with the consequential release of substance P and subsequent stimulation of the RARs, has been proposed previously to explain the overlap of stimuli and reflex effects of both afferent systems (2, 30).
The fundamental mechanism by which substance P stimulates RARs is not completely understood, but prominent substance P-induced effects such as microvascular leak (which alone may increase RAR activity or, if sufficient, may contribute to decreased lung compliance that also stimulates the RARs) and increased airway tone are likely candidates because all can stimulate the RARs (2, 18, 29). The present data appear most consistent with a salient role for microvascular leak. First, the doses of substance P used in this study were similar to those which have been shown to increase airway microvascular leak in the guinea pig (25). Second, CP-96345, in concentrations similar to those that blocked substance P-evoked increases in RAR activity in the present study, has also been shown in the guinea pig to block microvascular leak in the trachea and bronchi produced by either vagus nerve stimulation, intravenous capsaicin, or substance P and to spare the bronchoconstriction produced by vagus stimulation or capsaicin (14). Finally, Bonham et al. (2) have previously provided evidence in the rabbit that substance P increases RAR activity by increasing microvascular permeability. The cellular mechanism(s) by which microvascular leak stimulates the RARs is unknown, but it is tempting to speculate that mechanical distortion of the receptors caused by the fluid flux opens stretch-activated ion channels on the sensory nerve endings, as has been suggested for the carotid baroreceptors (8).
It seems less likely that substance P-evoked increases in airway tone contributed importantly to the increases in RAR activity. First, the NK1-receptor antagonist CP-96345 nearly abolished the RAR responses to substance P despite only tending to reduce the tracheal pressure responses. Second, the temporal nature of the responses of the RARs and tracheal pressure differed: substance P-induced increases in RAR activity tended to commence, peak, and recover earlier than the substance P-induced increases in peak tracheal pressure. Third, chronic sidestream smoke exposure augmented substance P-induced increases in RAR activity while having no such effect on the substance P-evoked increases in tracheal responses. Although microvascular leak is the most likely candidate for mediating the stimulation of RARs by substance P, it is important to acknowledge that other substance P-induced effects, i.e., mucus secretion or activation of inflammatory cells, could also have contributed to the increases in RAR activity. It should also be noted that, because the vagus nerve contralateral to the recording site was intact, substance P-evoked changes in RAR activity and tracheal pressure may have been slightly larger due to centrally mediated reflex effects than if the animals were bilaterally vagotomized.
The substance P-induced increases in RAR activity and decreases in arterial blood pressure appeared to be largely due to activation of NK1 receptors, whereas the increases in peak tracheal pressure may have involved both NK1 and NK2 receptors (7) because NK1 antagonism was only partially effective. A recent report (19) has suggested that CP-96345 interacts with L-type calcium channels by mechanisms unrelated to tachykinin antagonism. However, because CP-96344, the inactive enantiomer nearly void of NK1-receptor antagonist properties, had no effect on substance P-evoked responses, it seems likely that the CP-96345 effects were due to NK1-receptor antagonism.
Bonham et al. (1) have previously shown that chronic exposure of young guinea pigs to sidestream smoke during their development decreased RAR responsiveness to acutely inhaled mainstream tobacco smoke. The finding could be explained by a general diminished responsiveness of RARs to all stimuli or by a specific diminished responsiveness to a constituent of cigarette smoke such as nicotine. The purpose of this study was to determine whether chronic exposure to sidestream smoke diminished RAR responsiveness to a stimulus other than mainstream tobacco smoke. Unexpectedly, RARs were more rather than less responsive to substance P after chronic exposure to sidestream smoke. There are no other previous studies that have directly examined the effects of chronic exposure to sidestream smoke on RAR responsiveness to substance P. However, two studies that utilized similar exposure periods to mainstream smoke relate to our findings. Swanny et al. (24) demonstrated that adult rats exposed to mainstream tobacco smoke for 4-8 wk exhibited decreased reflex apnea and tachypnea to acute inhalation of mainstream tobacco smoke, most likely due to a decreased responsiveness of C-fiber receptors and RARs. Collectively, these data and our previous data suggest that exposure to either mainstream or sidestream tobacco smoke renders the RARs less sensitive to acute inhalation of mainstream smoke. Karlsson et al. (11) found that adult guinea pigs exposed to mainstream tobacco smoke for 1 h twice daily for 2 wk had an increased cough in response to nebulized citric acid and capsaicin but not to acute inhalation of mainstream tobacco smoke. Because both C-fiber and RAR afferent pathways are involved in producing cough (26, 30), the behavioral findings of Karlsson et al. (11) of augmented coughing in response to endogenously released substance P and the present electrophysiological findings of augmented increases in RAR activity in response to exogenously delivered substance P collectively suggest that chronic exposure to either sidestream or mainstream smoke may result in an increased responsiveness of RARs to substance P.
The question is, What is the mechanism(s) whereby chronic exposure of young guinea pigs to sidestream smoke augments the responsiveness of RARs to substance P? Possibilities include increases in NK1-receptor number and/or affinity; decreases in metabolizing enzymes of substance P, such as angiotensin-converting enzyme (22) or neutral endopeptidase (NEP) (5); changes in lung status that may make the RARs in general more responsive, e.g., enhanced microvascular permeability or decreased lung compliance; increased airway tone; and/or perhaps changes in the sensory nerve endings themselves.
Joad et al. (9) have previously examined NK1-receptor number and affinity in the isolated lungs of guinea pigs that were subjected to the same sidestream smoke exposure protocol as used in the present study. They found no differences in either NK1-receptor number or affinity in the sidestream smoke-exposed group compared with the filtered air control group. However, because they examined whole lung tissue, it is conceivable that small regional changes in the number and/or affinity in a subset of NK1 receptors were not detected. Regarding substance P-metabolizing enzymes, Kuo and Lu (12) found that extended exposure to mainstream smoke (100 puffs/day for 7 days) decreased NEP but did not alter the airway responsiveness to neurokinin A, a tachykinin also metabolized by NEP. Dusser et al. (5) found that a brief exposure to mainstream smoke (smoke from either one or two cigarettes delivered 15 min apart) both inactivated NEP and enhanced airway responsiveness to substance P. However, the effect of chronic exposure to sidestream smoke on NEP activity has not been studied, and there are considerable differences in the relative concentrations of the chemical constituents in mainstream and sidestream tobacco smoke (27). Furthermore, it seems reasonable to expect that if sidestream smoke exposure globally decreased NEP activity in the lung, then all substance P-evoked changes in the lung, including airway tone, would be affected. Such was not the case in either this or a previous study by Joad et al. (10).
With regard to the possibility that sidestream smoke exposure may have decreased lung compliance or increased airway tone, thereby making the RARs more responsive, Joad et al. (10) previously studied the lung mechanics of isolated lungs from young guinea pigs exposed to the same smoke-exposure protocol used here. Sidestream smoke exposure did not change lung resistance and increased (by 17%) rather than decreased lung compliance. Moreover, the lungs from the sidestream smoke-exposed animals exhibited the same decreases in compliance and the same increases in pulmonary resistance in response to pulmonary arterial injections of substance P as did the lungs from the filtered air-exposed animals. Finally, there were no morphological indications that the lungs from the sidestream smoke-exposed animals would be less compliant: the lungs from both groups had the same fixed lung volume, total surface area, specific surface area, and mean linear intercept length, with no obvious differences in the relative amounts or distribution of collagen and elastin. Thus it seems unlikely that general differences in compliance or airway caliber explained the differences in responsiveness of RARs to substance P.
To summarize, the existing data seem to argue against global changes in the lung as important mechanisms in explaining how chronic exposure to sidestream smoke in the young guinea pig augments RAR responsiveness to substance P. Perhaps regional changes in the lung microenvironment are more relevant, e.g., small regional changes in the number, affinity, and/or receptor-effector coupling of NK1 receptors; regional changes in the metabolizing enzymes of substance P; a greater susceptibility of certain microvasculature to substance P-evoked increases in permeability; or perhaps plastic changes in the sensory nerve endings themselves.
Whether similar exposure to sidestream smoke in the adult guinea pig renders the RARs more responsive to substance P is unknown. These studies were performed in young guinea pigs because of societal relevance to the increased prevalence of airway hyperresponsiveness in children chronically exposed to environmental tobacco smoke. Like the human, the guinea pig shows advanced development of lung function and morphology at birth (23). The age of puberty in guinea pigs is 35-70 days, and the maximum life span is ~7.5 yr (31). Starting with the eighth day of life, the guinea pigs were exposed to sidestream tobacco smoke for 33-37 days and were tested when they were 41-45 days of age. Thus these guinea pigs were exposed during the equivalent period of human childhood and were tested during the equivalent period of human adolescence. The findings that chronic exposure to sidestream smoke in young guinea pigs results in exaggerated responsiveness of their RARs to substance P may have implications in identifying the physiological causes of the increased respiratory problems in children raised in the homes of smokers.
ACKNOWLEDGEMENTS
The authors gratefully acknowledge the excellent technical support provided by Judy Stewart, Stephanie Perino, Sirika Soorma, and Dr. Kent Pinkerton and the clerical support provided by Elizabeth Walker.
FOOTNOTES
Address for reprint requests: A. C. Bonham, Div. of Cardiovascular Medicine, Univ. of California, Davis, TB 172, Bioletti Way, Davis, CA 95616.
Received 29 March 1996; accepted in final form 10 June 1996.
REFERENCES
| 1. | Bonham, A. C., C. T. Kappagoda, K. S. Kott, and J. P. Joad. Exposing young guinea pigs to sidestream tobacco smoke decreases rapidly adapting receptor responsiveness. J. Appl. Physiol. 78: 1412-1420, 1995. |
| 2. | Bonham, A. C., K. S. Kott, K. Ravi, C. T. Kappagoda, and J. P. Joad. Substance P contributes to rapidly adapting receptor responses to pulmonary venous congestion in rabbits. J. Physiol. Lond. 493: 229-238, 1996. |
| 3. | Coleridge, H. M., and J. C. G. Coleridge. Pulmonary reflexes: neural mechanisms of pulmonary defense. Annu. Rev. Physiol. 56: 69-91, 1994. |
| 4. | Dodge, R. The effects of indoor pollution on Arizona children. Arch. Environ. Health 37: 151-155, 1982. |
| 5. | Dusser, D. J., T. D. Djokic, D. B. Borson, and J. A. Nadel. Cigarette smoke induces bronchoconstrictor hyperresponsiveness to substance P and inactivates airway neutral endopeptidase in the guinea pig. J. Clin. Invest. 84: 900-906, 1989. |
| 6. | Ekwo, E. E., M. M. Weinberger, P. A. Lachenbruch, and W. H. Huntley. Relationship of parental smoking and gas cooking to respiratory disease in children. Chest 84: 662-668, 1983. |
| 7. | Foulon, D. M., E. Champion, P. Masson, I. W. Rodger, and T. R. Jones. NK1 and NK2 receptors mediate tachykinin and resiniferatoxin-induced bronchospasm in guinea pigs. Am. Rev. Respir. Dis. 148: 915-921, 1993. |
| 8. | Hajduczok, G., M. W. Chapleau, R. J. Ferlic, H. Z. Mao, and F. M. Abboud. Gadolinium inhibits mechanoelectrical transduction in rabbit carotid baroreceptors. Implication of stretch-activated channels. J. Clin. Invest. 94: 2392-2396, 1994. |
| 9. | Joad, J. P., K. P. Avadhanam, K. C. Watt, K. S. Kott, J. M. Bric, and K. E. Pinkerton. Effects of extended sidestream smoke exposure on components of the C-fiber axon reflex. Toxicology 110: 1-9, 1996. |
| 10. | Joad, J. P., J. M. Bric, and K. E. Pinkerton. Sidestream smoke effects on lung morphology and C-fibers in young guinea pigs. Toxicol. Appl. Pharmacol. 131: 289-296, 1995. |
| 11. | Karlsson, J.-A., C. Zackrisson, and J. M. Lundberg. Hyperresponsiveness to tussive stimuli in cigarette smoked-exposed guinea-pigs: a role for capsaicin-sensitive, calcitonin gene-related peptide-containing nerves. Acta Physiol. Scand. 141: 445-454, 1991. |
| 12. | Kuo, H.-P., and L.-C. Lu. Sensory neuropeptides modulate cigarette smoke-induced decrease in neutral endopeptidase activity in guinea pig airways. Life Sci. 57: 2187-2196, 1995. |
| 13. | Lee, L.-Y., Y. R. Kou, D. T. Frazier, E. R. Beck, T. E. Pisarri, H. M. Coleridge, and J. C. G. Coleridge. Stimulation of vagal pulmonary C-fibers by a single breath of cigarette smoke in dogs. J. Appl. Physiol. 66: 2032-2038, 1989. |
| 14. | Lei, Y., P. J. Barnes, and D. F. Rogers. Inhibition of neurogenic plasma exudation in guinea-pig airways by CP-96,345, a new non-peptide NK1 receptor antagonist. Br. J. Pharmacol. 105: 261-262, 1992. |
| 15. | Maggi, C. A., A. Giachetti, R. D. Dey, and S. I. Said. Neuropeptides as regulators of airway function: vasoactive intestinal peptide and the tachykinins. Physiol. Rev. 76: 277-322, 1995. |
| 16. | Martinez, F. D., M. Cline, and B. Burrows. Increased incidence of asthma in children of smoking mothers. Pediatrics 89: 21-26, 1992. |
| 17. | Prabhakar, N. R., M. Runold, Y. Yamamoto, H. Lagercrantz, N. S. Cherniack, and C. von Euler. Role of the vagal afferents in substance P-induced respiratory responses in anaesthetized rabbits. Acta Physiol. Scand. 131: 63-71, 1987. |
| 18. | Ravi, K., and C. T. Kappagoda. Responses of pulmonary C-fibre and rapidly adapting receptors to pulmonary edema in dogs. Can. J. Physiol. Pharmacol. 70: 68-76, 1992. |
| 19. | Schmidt, A. W., S. McLean, and J. Heym. The substance P receptor antagonist CP-96,345 interacts with Ca2+ channels. Eur. J. Pharmacol. 215: 351-352, 1992. |
| 20. | Sellick, H., and J. G. Widdicombe. Stimulation of lung irritant receptors by cigarette smoke, carbon dust, and histamine aerosol. J. Appl. Physiol. 31: 15-19, 1971. |
| 21. | Sherrill, D. L., F. D. Martinez, M. D. Lebowitz, M. D. Holdaway, E. M. Flannery, G. P. Herbison, W. R. Stanton, P. A. Silva, and M. R. Sears. Longitudinal effects of passive smoking on pulmonary function in New Zealand children. Am. Rev. Respir. Dis. 145: 1136-1141, 1992. |
| 22. | Shore, S. A., M. A. Martins, and J. M. Drazen. Effect of the NEP inhibitor SCH32615 on airway responses to intravenous substance P in guinea pigs. J. Appl. Physiol. 73: 1847-1853, 1992. |
| 23. | Sosenko, I. R. S., and L. Frank. Guinea pig lung development: antioxidant enzymes and premature survival in high O2. Am. J. Physiol. 252: R693-R698, 1987. |
| 24. | Swanny, A., R. F. Morton, and L. Lee. Acute effect of cigarette smoke on breathing is attenuated by chronic smoking in rats. J. Appl. Physiol. 74: 333-338, 1993. |
| 25. | Tokuyama, K., T. Yokoyama, A. Morikawa, H. Mochizuki, T. Kuroume, and P. J. Barnes. Attenuation of tachykinin-induced airflow obstruction and microvascular leakage in immature airways. Br. J. Pharmacol. 108: 23-29, 1993. |
| 26. | Ujiie, Y., K. Sekizawa, T. Aikawa, and H. Sasaki. Evidence for substance P as an endogenous substance causing cough in guinea pigs. Am. Rev. Respir. Dis. 148: 1628-1632, 1993. |
| 27. | Weiss, S. T., I. B. Tager, M. Schenker, and F. E. Speizer. The health effects of involuntary smoking. Am. Rev. Respir. Dis. 128: 933-942, 1983. |
| 28. | Weitzman, M., S. Gortmaker, D. K. Walker, and A. Sobol. Maternal smoking and childhood asthma. Pediatrics 85: 505-511, 1990. |
| 29. | Widdicombe, J. G. Regulation of tracheobronchial smooth muscle. Physiol. Rev. 43: 1-37, 1963. |
| 30. | Widdicombe, J. G. Neurophysiology of the cough reflex. Eur. Respir. J. 8: 1193-1202, 1995. |
| 31. | Witschi, E. Development and growth. In: Biology Data Book, edited by P. L. Altman, and D. S. Dittmer. Bethesda, MD: Fed. Am. Soc. Exp. Biol., 1973, p. 173-244. |
This article has been cited by other articles:
|
|
 |
|
  A. C. Bonham, C.-Y. Chen, S.-i. Sekizawa, and J. P. Joad Plasticity in the nucleus tractus solitarius and its influence on lung and airway reflexes J Appl Physiol, July 1, 2006; 101(1): 322 - 327. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 J.-A. Lin, C.-C. Yeh, M.-S. Lee, C.-T. Wu, S.-L. Lin, and C.-S. Wong Prolonged Injection Time and Light Smoking Decrease the Incidence of Fentanyl-Induced Cough Anesth. Analg., September 1, 2005; 101(3): 670 - 674. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 J. P. Joad, P. A. Munch, J. M. Bric, S. J. Evans, K. E. Pinkerton, C.-Y. Chen, and A. C. Bonham Passive Smoke Effects on Cough and Airways in Young Guinea Pigs: Role of Brainstem Substance P Am. J. Respir. Crit. Care Med., February 15, 2004; 169(4): 499 - 504. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
D. R. Bergren Enhanced lung C-fiber responsiveness in sensitized adult guinea pigs exposed to chronic tobacco smoke J Appl Physiol, October 1, 2001; 91(4): 1645 - 1654. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
T. Mutoh, A. C. Bonham, K. S. Kott, and J. P. Joad Chronic exposure to sidestream tobacco smoke augments lung C-fiber responsiveness in young guinea pigs J Appl Physiol, August 1, 1999; 87(2): 757 - 768. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 R. F. Robledo and M. L. Witten NK1-receptor activation prevents hydrocarbon-induced lung injury in mice Am J Physiol Lung Cell Mol Physiol, February 1, 1999; 276(2): L229 - L238. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
J. P. Joad, K. S. Kott, and A. C. Bonham Exposing guinea pigs to ozone for 1 wk enhances responsiveness of rapidly adapting receptors J Appl Physiol, April 1, 1998; 84(4): 1190 - 1197. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| Visit Other APS Journals Online |
