Departments of Large Animal Clinical Sciences and Physiology, Michigan State University, East Lansing, Michigan 48824-1314
![]() |
ABSTRACT |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
The
2-agonists currently used as
bronchodilators are racemic mixtures of R- and S-enantiomers. In the
present study, we examined the effects of enantiomers of the
2-agonists albuterol and
formoterol on acetylcholine (ACh) release from equine trachealis
parasympathetic nerves. ACh release was evoked by electrical
field stimulation (20 V, 0.5 ms, 0.5 Hz) and measured by
high-performance liquid chromatography coupled with electrochemical
detection. We also tested the effects of enantiomers of albuterol and
formoterol on equine tracheal smooth muscle (TSM) contraction in
response to exogenous ACh. R- and RS-albuterol
(10
8 to
10
5 M) and RR- and
RR/SS-formoterol (10
8 to
10
5 M) augmented ACh
release in a concentration-dependent manner. Beginning at
10
6 M, SS-formoterol
significantly increased ACh release, and at 10
5 M, release increased by
71.9 ± 8.7% over baseline. This effect was only observed, however,
when the prejunctional muscarinic autoinhibitory effect of ACh was
prevented with atropine. Both the RR- and SS-formoterol-induced
increases in ACh release were abolished by the
2-antagonist ICI-118551 (3 × 10
7 M). The effect
of S-albuterol on ACh release was variable, and the mean
increase induced by 10
5 M
was 30.8 ± 16.1% in the presence of atropine. In the muscle tension study, R- and RS-albuterol and RR- and RR/SS-formoterol (10
8 to
10
5 M) but not the
S-enantiomers inhibited TSM contraction. Even though
R-enantiomers augment ACh release, they potently inhibit TSM
contraction. Because racemic
2-agonists are bronchodilators on acute administration, the postjunctional spasmolytic effects of
R-enantiomers predominate over the spasmogenic effect evoked via increased ACh release. The S-enantiomers, in contrast, do not
inhibit TSM contraction and therefore would not contribute to the
observed bronchodilation of the racemate. The S-enantiomers do
prejunctionally facilitate ACh release when prejunctional muscarinic autoreceptors are dysfunctional, suggesting a potentially deleterious effect.
acetylcholine; albuterol; formoterol; cholinergic neurotransmission; airway hyperresponsiveness; muscarinic receptor
![]() |
INTRODUCTION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
EPIDEMIOLOGIC STUDIES suggest a possible correlation
between increased asthma morbidity and mortality and the use of
2-agonists in continuous or
excessive amounts (18). Clinically, a loss of protective efficacy with
a subsequent deterioration of asthma control is observed after regular
use of
2-agonists (16, 19). Sears et al. (16) and Taylor et al. (19) reported that chronic use of
inhaled
2-agonists resulted in
more exacerbations, a significant decline in baseline lung function,
and an increase in airway responsiveness to methacholine in asthmatic
subjects but did not alter bronchodilator responsiveness. These results
indicated that regular treatment with inhaled
2-agonists may be deleterious
in the long-term control of asthma but that dysfunction of the receptor
on airway smooth muscle was unlikely to be involved in the
2-agonist-induced
hyperresponsiveness of the asthmatic airway.
2-Agonists currently used in
the clinic are racemic mixtures of R- and S-enantiomers. Experimental
studies in the guinea pig have demonstrated that prolonged use of a
racemic mixture (7, 10, 12, 14, 22) or acute administration of the
S-enantiomer of
2-agonists (12,
14) can induce hyperresponsiveness to inhaled spasmogens or antigens in
ovalbumin-sensitized airways. Bilateral vagotomy before application of
2-agonists prevents development
of airway hyperresponsiveness; however, the mechanism of
hyperresponsiveness remains unknown (12, 14).
Activation of 2-adrenoceptors
(
2-ARs) by isoproterenol (Iso)
augments acetylcholine (ACh) release from guinea pig (3) and equine
(23, 24, 26) airway parasympathetic nerves. It is therefore plausible
to hypothesize that the S-enantiomer of
2-agonists may possess the
ability to increase ACh release from airway parasympathetic nerves and
that this augmentation of ACh release may contribute to airway
hyperresponsiveness induced by
2-agonists. In the present
study, we tested the effects of enantiomers of the
2-agonists albuterol and
formoterol on ACh release evoked by electric field stimulation (EFS)
from equine trachealis parasympathetic nerves in both the absence and
presence of the muscarinic-autoreceptor blocker atropine. To make a
simultaneous comparison of their pre- and postjunctional effects, we
also examined the postjunctional effects of enantiomers of albuterol
and formoterol on equine tracheal smooth muscle (TSM) contraction
induced by exogenous ACh.
![]() |
MATERIALS AND METHODS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Tissue
Tissue was collected from 16 horses (body weight 412.0 ± 11.6 kg) for this study, which was approved by the All-University Committee on Animal Use and Care of Michigan State University (East Lansing). Other investigators also used tissues from the same animals for a variety of studies. The horses had no clinical signs of respiratory disease for several weeks before they were killed by injection of an overdose of pentobarbital sodium through the jugular vein. Postmortem examination revealed that the lungs and airways were normal in gross appearance. A segment of the trachea between the 6th and 30th cartilaginous rings above the carina was quickly collected, immersed in Krebs-Henseleit (KH) solution (composition in mM: 118.4 NaCl, 25.0 NaHCO3, 11.7 dextrose, 4.7 KCl, 2.6 CaCl2 · 2H2O, 1.19 MgSO4 · 7H2O, and 1.16 KH2PO4), and gassed with 95% O2-5% CO2 during the whole experiment.The trachea was opened longitudinally by dissection of the cartilages in its anterior aspect and was pegged flat on a paraffin block submerged in KH solution. TSM strips with epithelium intact were cut with a template along the fiber direction before they were suspended in tissue baths. The temperature within the baths was maintained at 37°C, and the KH solution was changed every 15 min. Square-wave electrical impulses were produced by a stimulator (S88, Grass Instrument, Quincy, MA) and passed through a stimulus power booster (Stimu-Splitter II, Med-Lab Instruments, Loveland, CO) to electrodes in the tissue baths.
Measurement of EFS-Induced ACh Release
Four tissue strips (each measuring 2 × 15 mm) were cut and tied together at both ends with 3-0 surgical silk thread. Each trachealis strip bundle (wet weight 196.8 ± 4.0 mg; n = 72 strips) was suspended in a 2-ml tissue bath by a pair of parallel platinum wire electrodes built against the wall of the bath in the vertical direction (Radnoti Glass Technology, Monrovia, CA). One end of the tissue strip was secured to the bottom of the bath with a glass tissue holder; the other end was attached to an 8-g weight via a surgical thread that passed over a steel bar above the tissue bath (21). After an ~120-min equilibration period, the tissues were incubated for 60 min with the cholinesterase inhibitor neostigmine (10The ACh concentration in the tissue bath liquid was measured by high-performance liquid chromatography coupled with electrochemical detection. The mobile phase contained 100 mM Na2HPO4 (pH = 8.0), and the flow rate was 0.35 ml/min. The samples were filtered through 0.2-µm nylon membrane filters (Acrodiscs 13, Gelman Sciences, Ann Arbor, MI) and injected into the high-performance liquid chromatography column at a volume of 25 µl/injection. An external ACh standard (2.5 pmol in 25 µl) was injected every six samples, and the concentration of ACh in the samples was calculated based on the bracketed calibration. For details of this technique, see Wang et al. (21).
Muscle Tension Study
Strips (2 × 10 mm) tied with 3-0 surgical silk thread on both ends were suspended between platinum ring electrodes in 15-ml tissue baths that contained KH solution. The lower thread was attached to a hook on the electrode unit, whereas the upper thread was connected to a force transducer (Grass model FTO3) that was mounted on a manipulator to adjust tissue length. Tension produced by the tissue was measured by the transducer and recorded on a polygraph (Grass model 7E). EFS (16 Hz, 20 V, 0.5 ms) was applied for 2-3 min at 7- to 10-min intervals to equilibrate the muscle. Passive tension (optimal tissue length) was adjusted for each strip to produce the maximal response to EFS. Equilibration was ended after ~90 min when both baseline tension and the response to EFS were stable. After equilibration, the maximal tension response to a 127 mM KCl-substituted KH solution was recorded in grams, and the tissues were repeatedly washed until the muscle tension returned to baseline.Study Design
Protocol 1: Effect of R-, RS-, and S-albuterol and RR-, RR/SS-, and SS-formoterol on EFS-induced ACh release. Seven tissue-strip bundles from each horse were used in the absence of the muscarinic-receptor blocker atropine. One horse did not receive enantiomers ofTo determine whether enantiomers exerted their effect via
2-ARs, we tested the effect of
the specific
2-AR antagonist
ICI-118551 (4, 5) on RR- or SS-formoterol-induced augmentation of ACh release. Five tissue-strip bundles incubated with atropine
(10
7 M) were used. One
bundle received neither
2-agonist nor antagonist and
served as the control. The remainder received either RR-formoterol (10
10 to
10
8 M) or SS-formoterol
(10
6 to
10
4 M) in the absence or
presence of ICI-118551 (3 × 10
7 M). After the first
EFS, ICI-118551 was added to the baths 30 min before and RR- or
SS-formoterol was added 10 min before subsequent EFS.
Protocol 2: Effect of R-, RS-, and S-albuterol and
RR-, RR/SS-, and SS-formoterol on smooth muscle contraction in response to exogenous ACh. Exogenous ACh
(107 to
10
4 M) was applied to each
of the strips to elicit smooth muscle contraction. The first
concentration-response curve (priming curve) was not used for data
analysis. The subsequent five curves were used to determine the
postjunctional effect of enantiomers of albuterol or formoterol
(10
8 to
10
5 M) on smooth muscle
contraction in response to exogenous ACh. Seven TSM strips were used.
One strip served as a time control; the other six each received four
concentrations of either R-, RS-, or S-albuterol or RR-, RR/SS-, or
SS-formoterol (10
8 to
10
5 M) after the first
exogenous ACh-induced contraction. Enantiomer solutions were added to
the baths 10 min before subsequent applications of exogenous ACh.
Before generation of another concentration-response curve to exogenous
ACh, the tissues were washed for 30 min with at least four changes of
KH solution.
Drugs
Atropine sulfate, ACh chloride, neostigmine methylsulfate, guanethidine monosulfate (Sigma Chemical, St. Louis, MO), ICI-118551 hydrochloride (Research Biochemicals International, Natick, MA), and enantiomers of albuterol and formoterol (Sepracor, Marlborough, MA) were dissolved and diluted in KH solution. All the drugs were prepared on the day of experiment. The drug solution was pipetted into the tissue bath at 1% of the bath volume. The final concentration of the drugs was expressed as their bath molar concentration.Data Analysis
The ACh release rate is expressed both in picomoles per gram per minute and as a percentage of the baseline value (first EFS without drug treatment). The contractile responses of the muscle strips were expressed as percentages of their maximal tension responses to a 127 mM KCl-substituted KH solution. Means ± SE for all parameters were calculated. The data were evaluated by repeated-measures analysis of variance and analysis of variance with contrasts by Statview II (Abacus Concepts, Calabasas, CA) for the Macintosh computer. Means were compared by Fisher's (protected) least significant difference test. Significance was accepted at the 0.05 level of probability. ![]() |
RESULTS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Protocol 1: Effect of R-, RS-, and S-Albuterol and RR-, RR/SS-, and SS-Formoterol on EFS-Induced ACh Release
In the absence of atropine, baseline ACh release, i.e., release during the first EFS period, averaged 1.39 ± 0.08 pmol · g
|
In the presence of blockade of muscarinic receptors with atropine
(107 M), baseline ACh
release was increased to 5.50 ± 0.35 pmol · g
1 · min
1
(n = 49 strips). Under these
conditions, R- and RS-albuterol and RR- and RR/SS-formoterol still
augmented ACh release in a concentration-dependent manner. The
augmentation reached a plateau at the concentrations of
10
6 M for R- and
RS-albuterol and 10
8 M for
RR- and RR/SS-formoterol (Fig. 2). Maximal
augmentation, i.e., an approximate doubling of ACh release, was of
similar magnitude to that induced by activation of
2-ARs by Iso (23, 24, 26). Beginning at 10
6 M,
SS-formoterol significantly increased ACh release, and at 10
5 M, release increased by
71.9 ± 8.7% over baseline. The effect of S-albuterol on ACh
release was variable, and the mean increase induced by
10
5 M was 30.8 ± 16.1%
(Fig. 2).
|
In the presence of atropine, both RR-formoterol
(1010 to
10
8 M) and SS-formoterol
(10
6 to
10
4 M) facilitated ACh
release in a concentration-dependent manner. The RR- and
SS-formoterol-induced augmentation of ACh release was abolished by the
2-antagonist ICI-118551 (Fig.
3).
|
Protocol 2: Effect of R-, RS-, and S-Albuterol and RR-, RR/SS-, and SS-Formoterol on Smooth Muscle Contraction in Response to Exogenous ACh
Application of exogenous ACh in the tissue bath concentration dependently contracted the TSM. Even though the contractile response to subsequent ACh in the time-control TSM decreased slightly, the difference was not significant (data not shown). R- and RS-albuterol (Fig. 4, A and B) and RR- and RR/SS-formoterol (Fig. 5, A and B) at concentrations of 10
|
|
![]() |
DISCUSSION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
2-Agonists are widely used as
bronchodilators because of their ability to dilate airways via
2-receptors on smooth muscle cells (6). However, there is increasing concern that regular use of
2-agonists has a deleterious
effect on the control of asthma (1, 16, 18, 19). Although a number of
theories have been advanced to explain such an effect, none is well
established (1, 15). In the airways of most species, the
parasympathetic innervation is the predominant neural pathway (2), and
ACh released from cholinergic nerve terminals provides the dominant constrictor mechanism. Zhang and colleagues (23, 24, 26) previously demonstrated that the racemic
-agonist Iso increases ACh
release from equine airway parasympathetic nerves. The Iso-induced augmentation of ACh release is abolished by the
2-antagonist ICI-118551 but not
by the
1-antagonist CGP-20712A,
indicating that
2-ARs mediate
the facilitation of ACh release (23, 24, 26). Activation of
2-ARs also facilitates
[3H]ACh release from
guinea pig tracheal parasympathetic nerves (3). These excitatory
2-ARs may have important
consequences in the treatment of airway obstructive disease because
2-agonists might paradoxically
potentiate the release of ACh while relaxing smooth muscle.
The 2-agonists used for
treatment of airway obstruction currently are the racemic mixture of R-
and S-enantiomers. The differential effects of enantiomers on
neuromuscular regulation of airway tone have been suspected as one of
the possible mechanisms for the
2-agonist-induced deleterious
effects (15). It has been reported that prolonged exposure to a racemic
mixture (7, 10, 22) or acute administration of S-enantiomers (12, 14)
of
2-agonists can induce
hyperresponsiveness of ovalbumin-sensitized guinea pig airways to
spasmogens. However, the S-enantiomer no longer elicits airway
hyperresponsiveness when given after the vagus nerves are sectioned
(12, 14). These observations indicate that induction of airway
hyperresponsiveness by the S-enantiomer of
2-agonists may involve
neurotransmitters released by the vagus nerves. For this reason, we
hypothesized that the differential actions of enantiomers of
2-agonists prejunctionally on
ACh release from airway parasympathetic nerves and postjunctionally on
airway smooth muscle contraction may contribute to the
2-agonist-induced airway
hyperresponsiveness.
The prejunctional autoinhibitory
M2 receptors are dysfunctional in
ovalbumin-sensitized guinea pig airway parasympathetic nerves after
antigen challenge (9) and in virus-infected guinea pigs (8) and also
may be dysfunctional in some asthmatic patients (13). Therefore, in the
present experiment, we first examined the effects of R-, RS-, and
S-albuterol and RR-, RR/SS-, and SS-formoterol on ACh release from
airway parasympathetic nerves in both the absence and presence of the
muscarinic-autoreceptor blocker atropine (107 M). We demonstrated
that in the absence of muscarinic blockade, only R- and RS-albuterol
and RR- and RR/SS-formoterol, but not S-albuterol and SS-formoterol,
augmented ACh release. However, after the blockade of the muscarinic
autoinhibitory receptor, SS-formoterol significantly increased
EFS-induced ACh release, and a similar, although lesser, trend was
noted with S-albuterol. To explain these observations, we considered
the response of the muscarinic autoreceptor to ACh. When a weak
stimulus such as the S-enantiomer causes only a small increase in ACh
release, this increase is totally masked by the negative feedback
originating from the muscarinic autoreceptor. Only when this feedback
is prevented by atropine is it possible to observe the ACh-augmenting
effect of the S-enantiomer. When there is a greater increase in ACh
release, such as that induced by the R-enantiomer, the muscarinic
autoreceptor cannot totally prevent the increase. Hence the
R-enantiomer effect on ACh release is observed even in the absence of
autoreceptor blockade by atropine.
Both RR- and SS-formoterol-induced facilitations of ACh release were
abolished by the specific
2-antagonist ICI-118551,
suggesting that facilitation of ACh release by both RR- and
SS-enantiomers is via activation of
2-ARs. Zhang et al. (25)
previously demonstrated that activation of
2-ARs facilitates ACh release
via the intracellular adenosine 3',5'-cyclic monophosphate
pathway. Thus we concluded that the augmenting effect of both the R-
and S-enantiomers on ACh release is also mediated via the intracellular
adenosine 3',5'-cyclic monophosphate pathway.
To make a simultaneous comparison of their pre- and postjunctional effects, we also tested the effects of enantiomers of both albuterol and formoterol on TSM contraction in response to exogenous ACh. The results revealed that R- and RS-albuterol and RR- and RR/SS-formoterol potently inhibited exogenous ACh-induced smooth muscle contraction, whereas S-albuterol and SS-formoterol had little effect. Johansson et al. (11) also recently reported that R- and RS-albuterol are far more potent than S-albuterol at inhibiting carbachol-induced TSM contraction in guinea pigs.
Because both R- and S-enantiomers of
2-agonists augment ACh release
after blockade of autoinhibitory muscarinic
M2 receptors, it seems likely that
the deleterious effects may be a property of both enantiomers. However,
to truly evaluate the effects of these enantiomers on airway smooth
muscle function, one must consider the combined pre- and postjunctional
effects. Even though R-enantiomers prejunctionally augment ACh release,
they potently inhibit TSM contraction postjunctionally. Because racemic
2-agonists are demonstrably
bronchodilatory on acute administration (12), the postjunctional
spasmolytic effects of R-enantiomers must predominate over the
prejunctional spasmogenic effect that is evoked via increased ACh
release. However, in the presence of prejunctional
M2-receptor dysfunction,
S-enantiomers might enhance rather than inhibit smooth muscle
contraction because they increase ACh release but have no effect on
smooth muscle.
Metabolism of racemic
2-agonists is stereoselective
in humans (17, 20), with minimal absorption and preferential and rapid
elimination of the R-enantiomer. In contrast, the S-enantiomer of
albuterol exhibits a three- to fourfold greater plasma level and a
significantly longer duration. This stereoselective metabolism and
subsequent disproportionate accumulation of the S-enantiomer of
2-agonists may result in the
progressive loss of the protective effectiveness of the
2-agonist mediated by action of
the R-enantiomer on smooth muscle and may thus be responsible for
airway hyperresponsiveness. In our present study, S-enantiomers of
2-agonists increased release of
ACh, an airway spasmogen, from airway parasympathetic nerves in the
presence but not in the absence of muscarinic autoinhibitory receptor
blockade. These results are consistent with the observation that
racemic or S-enantiomer
2-agonist-evoked airway
hyperresponsiveness can be induced more readily in ovalbumin-sensitized
animals in which M2 receptors are
dysfunctional than in control animals in which autoinhibitory
M2 receptors are functional
(12). In the situation of prejunctional
M2-receptor dysfunction
in airways (8, 9, 13), and combined with the preferential
elimination of R-enantiomers (17, 20), S-enantiomer-mediated
facilitation of ACh release may become evident and may contribute to
the
2-agonist-induced airway
hyperresponsiveness.
![]() |
ACKNOWLEDGEMENTS |
---|
We thank Cathy Berney for technical assistance and Victoria Hoelzer-Maddox and MaryEllen Shea for manuscript preparation.
![]() |
FOOTNOTES |
---|
This work was supported in part by Sepracor Inc. and by an endowment from the Matilda R. Wilson Fund.
Address for reprint requests: Dr. X.-Y. Zhang, Dept. of Large Animal Clinical Sciences, Michigan State Univ., East Lansing, MI 48824-1314.
Received 25 June 1997; accepted in final form 23 September 1997.
![]() |
REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
1.
Bai, T. R.
Beta2 adrenoceptors in asthma: a current perspective.
Lung
170:
125-141,
1992[Medline].
2.
Barnes, P. J.
Cholinergic control of airway smooth muscle.
Am. Rev. Respir. Dis.
136:
S42-S45,
1987[Medline].
3.
Belvisi, M. G.,
H. J. Patel,
T. Takahashi,
P. J. Barnes,
and
M. A. Giembycz.
Paradoxical facilitation of acetylcholine release from parasympathetic nerves innervating guinea-pig trachea by isoprenaline.
Br. J. Pharmacol.
117:
1413-1420,
1996[Abstract].
4.
Booze, R. M.,
E. A. Crisostomo,
and
J. N. Davis.
Species differences in the localization of CNS beta adrenergic receptors: rat versus guinea pig.
J. Pharmacol. Exp. Ther.
249:
911-920,
1989[Abstract].
5.
Bristow, M. R.,
R. E. Hershberger,
J. D. Port,
W. Minobe,
and
R. Rasmussen.
1- and
2-adrenoceptor-mediated adenylate cyclase stimulation in nonfailing and failing human ventricular myocardium.
Mol. Pharmacol.
35:
295-303,
1988[Abstract].
6.
Carstairs, J. R.,
A. J. Nimmo,
and
P. J. Barnes.
Autoradiographic visualization of beta-adrenoceptor subtype in human lung.
Am. Rev. Respir. Dis.
132:
541-547,
1985[Medline].
7.
Conolly, M. E.,
D. S. Davies,
C. T. Dollery,
and
C. F. George.
Resistance to -adrenoceptor stimulants (a possible explanation for the rise in asthma deaths).
Br. J. Pharmacol.
43:
389-402,
1971[Medline].
8.
Fryer, A. D.,
and
D. B. Jacoby.
Parainfluenza virus infection damages inhibitory M2-muscarinic receptors on pulmonary parasympathetic nerves in the guinea-pig.
Br. J. Pharmacol.
102:
267-271,
1991[Abstract].
9.
Fryer, A. D.,
and
M. Wills-Karp.
Dysfunction of M2-muscarinic receptors in pulmonary parasympathetic nerves after antigen challenge.
J. Appl. Physiol.
71:
2255-2261,
1991
10.
Jafarian, A.,
D. A. Handly,
and
D. F. Biggs.
Effects of RS-albuterol on the development of antigen-mediated airway hyperreactivity in guinea pigs.
Clin. Rev. Allergy Immunol.
14:
91-100,
1996[Medline].
11.
Johansson, F.,
I. Rydberg,
G. Aberg,
and
R. G. G. Andersson.
Effects of albuterol enantiomers on in vitro bronchial reactivity.
Clin. Rev. Allergy Immunol.
14:
57-64,
1996[Medline].
12.
Mazzoni, L.,
R. Naef,
I. D. Chapman,
and
J. Morley.
Hyperresponsiveness of the airways following exposure of guinea-pigs to racemic mixtures and distomers of 2-selective sympathomimetics.
Pulm. Pharmacol.
7:
367-376,
1994[Medline].
13.
Minette, P.,
W. J. Lammers,
C. M. S. Dixon,
M. T. McCusker,
and
P. J. Barnes.
A muscarinic agonist inhibits reflex bronchoconstriction in normal but not in asthmatic subjects.
J. Appl. Physiol.
67:
2461-2465,
1989
14.
Sanjar, S.,
A. Kristersson,
L. Mazzoni,
J. Morley,
and
E. Schaeublin.
Increased airway reactivity in the guinea-pig follows exposure to intravenous isoprenaline.
J. Physiol. (Lond.)
425:
43-54,
1990[Abstract].
15.
Sears, M. R.,
and
D. R. Taylor.
The 2-agonist controversy: observations, explanations and relationship to asthma epidemiology.
Drug Saf.
11:
259-283,
1994[Medline].
16.
Sears, M. R.,
D. R. Taylor,
C. G. Print,
D. C. Lake,
Q. Q. Li,
E. M. Flannery,
D. M. Yates,
M. K. Lucas,
and
G. P. Herbison.
Regular inhaled -agonist treatment in bronchial asthma.
Lancet
336:
1391-1396,
1990[Medline].
17.
Tan, Y. K.,
and
S. J. Soldin.
Analysis of salbutamol enantiomers in human urine by chiral high-performance liquid chromatography and preliminary studies related to the stereoselective disposition kinetics in man.
J. Chromatogr.
422:
187-195,
1987[Medline].
18.
Taylor, D. R.,
M. R. Sears,
and
D. W. Cockcroft.
The -agonist controversy.
Med. Clin. North Am.
80:
719-748,
1996[Medline].
19.
Taylor, D. R.,
M. R. Sears,
G. P. Herbison,
E. M. Flannery,
C. G. Print,
D. C. Lake,
D. M. Yates,
M. K. Lucas,
and
Q. Li.
Regular inhaled beta agonist in asthma: effects on exacerbations and lung function.
Thorax
48:
134-138,
1993[Abstract].
20.
Walle, U. K.,
G. R. Pesola,
and
T. Walle.
Stereoselective sulphate conjugation of salbutamol in humans: comparison of hepatic, intestinal and platelet activity.
Br. J. Clin. Pharmacol.
35:
413-418,
1993[Medline].
21.
Wang, Z.,
N. E. Robinson,
and
M. Yu.
Acetylcholine release from horse airway cholinergic nerves: effects of stimulation intensity and muscle preload.
Am. J. Physiol.
264 (Lung Cell. Mol. Physiol. 8):
L269-L275,
1993
22.
Wang, Z.-L.,
A. M. Bramley,
A. McNamara,
P. D. Pare,
and
T. R. Bai.
Chronic fenoterol exposure increases in vivo and in vitro airway responses in guinea pigs.
Am. J. Respir. Crit. Care Med.
149:
960-965,
1994[Abstract].
23.
Zhang, X.-Y.,
M. A. Olszewski,
and
N. E. Robinson.
2-Adrenoceptor activation augments acetylcholine release from tracheal parasympathetic nerves.
Am. J. Physiol.
268 (Lung Cell. Mol. Physiol. 12):
L950-L956,
1995
24.
Zhang, X.-Y.,
N. E. Robinson,
Z. W. Wang,
and
M. C. Lu.
Catecholamine affects acetylcholine release in trachea: 2-mediated inhibition and
2-mediated augmentation.
Am. J. Physiol.
268 (Lung Cell. Mol. Physiol. 12):
L368-L373,
1995
25.
Zhang, X.-Y.,
N. E. Robinson,
and
F.-X. Zhu.
Potentiation of acetylcholine release from tracheal parasympathetic nerves by cAMP.
Am. J. Physiol.
270 (Lung Cell. Mol. Physiol. 14):
L541-L546,
1996
26.
Zhang, X.-Y.,
F.-X. Zhu,
and
N. E. Robinson.
Excitatory prejunctional 2-adrenoceptor distribution within equine airway cholinergic nerves.
Respir. Physiol.
106:
81-90,
1996[Medline].