Departments of Large Animal Clinical Sciences and Physiology, Michigan State University, East Lansing, Michigan 48824-1314
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ABSTRACT |
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To investigate the
effects of changes in intracellular cAMP on
2-adrenoceptor (AR)-induced
inhibition of airway acetylcholine (ACh) release, we examined the
effects of the
2-AR agonist
clonidine on electrical field stimulation-evoked ACh release from
equine tracheal parasympathetic nerves before and after treatment with 8-bromo-cAMP or forskolin. We also tested whether charybdotoxin (ChTX)-
or iberiotoxin (IBTX)-sensitive
Ca2+-activated
K+ channels mediate
2-AR-induced inhibition by
examining the effect of clonidine in the absence and presence of ChTX
or IBTX on ACh release. The amount of released ACh was measured by HPLC
coupled with electrochemical detection. Clonidine
(10
7 to
10
5 M) dose dependently
inhibited ACh release before and after treatment with 8-bromo-cAMP
(10
3 M) or forskolin
(3 × 10
5
M). ChTX and IBTX, both at the concentration of 5 × 10
7 M, significantly
increased ACh release; however, they did not alter the magnitude of
clonidine-induced inhibition. These results indicated that in equine
tracheal parasympathetic nerves,
2-AR-induced inhibition of ACh
release is via an intracellular cAMP-independent pathway. Activation of
both ChTX- and IBTX-sensitive
Ca2+-activated
K+ channels inhibits the
electrical field stimulation-evoked ACh release, but these channels are
not involved in the
2-AR-induced inhibition of ACh
release.
adenosine 3',5'-cyclic monophosphate; potassium ion; 2-adrenoceptor; acetylcholine; clonidine; charybdotoxin; iberiotoxin; 8-bromoadenosine
3',5'-cyclic monophosphate; forskolin; cholinergic
neurotransmission
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INTRODUCTION |
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ACETYLCHOLINE (ACh) release from airway parasympathetic
nerves is modulated prejunctionally by autoreceptors and a variety of
heteroreceptors. By direct measurement of ACh release, our laboratory
and other investigators have confirmed that muscarinic autoreceptors,
the M2 subtype in most species,
provide negative feedback that normally inhibits ACh release (6, 18)
and that inhibitory
2-adrenoceptors (ARs) (3, 22,
24) inhibit ACh release more potently in the trachea than in the
smaller bronchi (22). Zhang and colleagues (23, 24, 26)
have also discovered an excitatory
2-AR, activation of which
increases ACh release.
In comparison with information available from other parts of the
nervous system, the intracellular transduction pathways coupling these
prejunctional receptors to ACh release are largely unexplored in the
airway. Baker et al. (2) have shown that ACh release is dependent on
N-type, -conotoxin-sensitive
Ca2+ channels. We (25) have
recently demonstrated that increasing the intracellular concentration
of cAMP by means of the adenylyl cyclase (AC) activator forskolin, the
lipid-soluble cAMP analog 8-bromo-cAMP, or the phosphodiesterase
inhibitor 3-isobutyl-1-methylxanthine (IBMX) facilitates ACh release.
Furthermore, by examining the additive effects of 8-bromo-cAMP or
forskolin with the
-AR agonist isoproterenol, we (25) confirmed that
2-AR-induced augmentation of
ACh release is via a cAMP-dependent pathway.
In many tissues, 2-receptors
are coupled to AC via an inhibitory G protein so that their activation
can decrease intracellular levels of cAMP (7, 9, 16). It is therefore
possible that
2-receptors
inhibit ACh release via a decrease in cAMP. However, recent studies
(11, 15) demonstrated that activation of
2-receptors inhibits
norepinephrine (NE) release from the sympathetic nervous system via a
cAMP-independent mechanism. For example, activation of the
2-AR inhibits NE release from
cultured rat ganglion cells even when intracellular levels of cAMP are
fixed by use of analogs of cAMP (15). In addition, it has been clearly
demonstrated by patch-clamping techniques that the inhibition of the
N-type Ca2+ channels and of NE
release that follow activation of the
2-AR in sympathetic neurons is
independent of a soluble, diffusible intracellular messenger such as
cAMP (11). Therefore, one of the purposes of our present study was to
investigate the role of intracellular cAMP on
2-AR-induced inhibition of ACh
release from airway parasympathetic nerves. We wanted to determine
whether the
2-AR-mediated
inhibition is via intracellular cAMP-dependent pathways by examining
the effect of the
2-AR agonist
clonidine on ACh release before and after the intracellular
concentration of cAMP was fixed by 8-bromo-cAMP
(10
3 M) or increased by
forskolin (3 × 10
5
M). If inhibition of ACh release by
2-AR agonists is primarily via
cAMP-independent pathways, clonidine should inhibit ACh release before
and after the intracellular concentration of cAMP is elevated. If a
significant component of
2-AR-mediated inhibition is via cAMP-dependent pathways, inhibition should be significantly reduced by
forskolin or abolished after the intracellular concentration of cAMP is
clamped by 8-bromo-cAMP.
Measurement of smooth muscle tension in guinea pig airway has suggested
that blockade of Ca2+-activated
K+
(KCa) channels with
charybdotoxin (ChTX) prevents prejunctional inhibition of cholinergic
neurotransmission by the 2-AR
agonist clonidine (13). However, by direct measurement of ACh release, Baker et al. (3) demonstrated that
2-AR-induced inhibition of ACh
release from guinea pig airway parasympathetic nerves is not mediated
via iberiotoxin (IBTX)-sensitive
K+ channels. One possible
explanation for the above conflicting results may be the different
pharmacological characteristics of ChTX and IBTX. Therefore, in the
second protocol, we investigated whether the
2-AR-mediated inhibition is via
KCa channels by examining the
effect of clonidine before and after the blockade of
KCa channels with ChTX or IBTX. If
the
2-AR-mediated inhibition is
via these K+ channels, ChTX
and/or IBTX should attenuate the inhibition.
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MATERIALS AND METHODS |
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Tissue. Tissue was collected from 15 horses (aged 7.9 ± 1.5 yr, body wt 415.1 ± 15.5 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 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. Tracheal smooth muscle strips with epithelium intact were cut with a template along the fiber direction before being 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 Instruments, 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 electrical field stimulation-induced
ACh release. Four tissue strips (each measuring 2 × 15 mm) were cut and tied together at both of their ends with
3-0 surgical silk thread. Each tracheal-strip bundle (wet weight 198.0 ± 3.0 mg; n = 156 strips) was
suspended in a 2-ml tissue bath with 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 (17). After an ~120-min
equilibration period, the tissues were incubated for 60 min with the
cholinesterase inhibitor neostigmine
(106 M), the sympathetic
nerve blocker guanethidine
(10
5 M), and the
muscarinic-autoreceptor antagonist atropine
(10
7 M). These agents were
present during the remainder of the experiment. Endogenous prostanoids
and nitric oxide from an inhibitory nonadrenergic noncholinergic system
do not affect electrical field stimulation (EFS)-induced ACh release
from our tissue preparations (23, 26).
The ACh concentration in the tissue bath liquid was measured by HPLC 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 HPLC 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 Refs. 17, 24).
Study design. EFS (0.5 Hz, 0.5 ms, 20 V) was applied to all the tissues for four 15-min periods, with a 30-min resting interval between consecutive stimuli. During the first EFS, we determined the baseline release of ACh. Then the tested drugs were added to the baths 10-30 min (see protocols 1 and 2 for details) before subsequent EFS. To eliminate any ACh that may have been released during the incubation period (17), the tissue baths were drained and refilled with fresh KH solution containing the tested drugs immediately before the EFS was begun. The tissue bath solution was collected on the completion of each EFS for the measurement of ACh. The tissues were rinsed four times with the KH solution immediately after collection of the samples. At the end of the experiment, the tissues were blotted dry and weighed.
Protocol 1: effect of
2-AR-agonist clonidine on
EFS-induced ACh release before and after increasing the intracellular
concentration of cAMP with 8-bromo-cAMP or forskolin.
Data analysis in this study involved calculation and comparison of the
percent inhibition of ACh release by clonidine in the absence and
presence of 8-bromo-cAMP or forskolin. Both EFS frequency and baseline
ACh release may affect the magnitude of
2-AR-agonist-induced inhibition
of ACh release (11, 22). In our preliminary experiments, we
demonstrated that clonidine-induced inhibition of ACh release did not
significantly differ at 0.5- and 1-Hz EFS frequencies. Both
8-bromo-cAMP (10
3 M) and
forskolin (5 × 10
5 M)
alone doubled the EFS-evoked ACh release (25). To keep the baseline ACh
release the same before the
2-AR-agonist was added, we
stimulated the control tissues (in the absence of cAMP analogs) at 1 Hz
and the 8-bromo-cAMP- and forskolin-treated tissues at 0.5 Hz.
Protocol 2: effect of the
2-AR-agonist clonidine on
EFS-induced ACh release in the absence and presence of the
KCa-channel blockade with ChTX or IBTX.
Three tissue-strip bundles were used; one did not receive any drug
treatment and served as a time control. After the first stimulation (20 V, 0.5 ms, 0.5 Hz), the second tissue-strip bundle received ChTX (5 × 10
7 M) 30 min
before the second EFS, and ChTX remained in the tissue bath during the
subsequent EFS. Clonidine
(10
6 to
10
5 M) was then applied 10 min before the third and fourth EFS. A comparison of ACh release rate
before (first EFS) and after (second EFS) ChTX was made to evaluate the
effect of blockade of KCa channels on basal EFS-evoked ACh release. The ACh release rate during the second
EFS was then used as a baseline to calculate the clonidine-induced percent inhibition of ACh release in the presence of ChTX. A similar protocol was performed in the third set of tissue-strip bundles except
in the absence of ChTX. The percent inhibition of ACh release by
clonidine was compared in the absence and presence of ChTX. This
protocol was repeated with IBTX (5 × 10
7 M) instead of ChTX.
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RESULTS |
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Protocol 1: effect of
2-AR-agonist clonidine on
EFS-induced ACh release before and after increasing the intracellular
concentration of cAMP with 8-bromo-cAMP or forskolin.
Baseline ACh release, i.e., release during first EFS period, averaged
6.53 ± 1.24 and 11.46 ± 1.76 pmol · g
1 · min
1
at 0.5 and 1 Hz, respectively. In 8-bromo-cAMP
(10
3 M)- and forskolin (5 × 10
5 M)-incubated
tissues, EFS (0.5 Hz)-evoked ACh release was 11.59 ± 2.17 and 12.92 ± 2.277 pmol · g
1 · min
1, which is in the
similar release level as that evoked by 1-Hz EFS. In all time-control
tissues, ACh release remained constant throughout the four
stimulations. The
2-AR-agonist
clonidine (10
7 to
10
5 M) inhibited ACh
release in a concentration-dependent manner in both the absence
(control; Fig. 1A) and presence of
8-bromo-cAMP (Fig. 1B) or forskolin
(Fig. 1C). At
10
5 M, clonidine caused
50.1 ± 8.0, 41.2 ± 6.6, and 50.0 ± 7.4% inhibition of ACh
release in control, 8-bromo-cAMP-pretreated, and forskolin-pretreated tissues, respectively (Fig. 2). When the
percent inhibition was compared at the concentrations of clonidine of
10
7,
10
6, and
10
5 M, there were no
significant differences between the control and 8-bromo-cAMP- or
forskolin-pretreated groups (n = 6 animals; Fig. 2).
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Protocol 2: effect of the
2-AR-agonist clonidine on
EFS-induced ACh release in the absence and presence of a
KCa-channel blockade with ChTX or IBTX.
Application of ChTX (5 × 10
7 M) increased
EFS-induced ACh release from 6.40 ± 0.51 to 14.73 ± 2.42 pmol · g
1 · min
1
(n = 6 animals; Fig.
3A).
Iberiotoxin (5 × 10
7
M) also significantly increased EFS-induced ACh release from 5.81 ± 0.78 to 9.35 ± 0.98 pmol · g
1 · min
1
(n = 4 animals; Fig.
4A).
Even though both ChTX and IBTX significantly facilitated ACh release,
they did not alter the magnitude of clonidine-induced inhibition. The
percent inhibition of ACh release by clonidine was identical in the
absence and presence of ChTX (n = 6 animals; Fig. 3B) or IBTX
(n = 4 animals; Fig.
4B), indicating clonidine-induced inhibition of ACh release is independent of both ChTX- and
IBTX-sensitive KCa channels.
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DISCUSSION |
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When intracellular cAMP levels are elevated by activation of
stimulatory G proteins with cholera toxin, activation of AC by forskolin, administration of membrane-permeable cAMP analogs, or
inhibition of phosphodiesterase, there is an increased release of ACh
from equine (25) and guinea pig (4) airway parasympathetic and phrenic
nerves (21) as well as from cerebral cholinergic neurons (10), rat
superior cervical ganglion (5), and guinea pig ileum myenteric plexus
(1, 20). It is therefore well established that increasing intracellular
cAMP increases ACh release. In many tissues,
2-ARs are coupled to AC via an
inhibitory G protein so that their activation can decrease
intracellular levels of cAMP (7, 9, 16). It is therefore possible that
the inhibition of ACh release from airway parasympathetic nerves by activation of
2-ARs is mediated
via a decrease in cAMP. However, the results of our first experiments
do not support this hypothesis. 8-Bromo-cAMP
(10
3 M) had no significant
effect on clonidine-induced inhibition of ACh release, and the
magnitude of inhibition was similar in control tissues and in the
presence of forskolin (Fig. 2). These results indicate that the
majority of
2-AR-induced
inhibition of ACh release from airway parasympathetic nerves is via
mechanisms that are independent of intracellular cAMP. This is similar
to the situation in the sympathetic nervous system, in which
2-AR-induced inhibition of NE
release is not affected by cAMP analogs and therefore is independent of
the intracellular cAMP pathway (11, 15). We cannot totally exclude a
minor role for cAMP-dependent pathways, however, because there was a
tendency for 8-bromo-cAMP to attenuate the effect of clonidine at
10
6 M.
Activation of prejunctional K+
channels leads to hyperpolarization of the plasma membrane, reduces
Ca2+ influx through
voltage-activated channels, and decreases neurotransmitter release (12,
14). In guinea pig airway, Miura et al. (13) demonstrated that ChTX
(108 M), a blocker of
KCa and voltage-dependent
K+ channels (8), potentiates
EFS-induced smooth muscle contraction without affecting the response to
exogenously applied ACh. These observations were interpreted to suggest
that activation of ChTX-sensitive K+ channels inhibits cholinergic
neurotransmitter release. Our direct measurement of ACh release
confirms the above conclusion. When K+ channels were blocked with
ChTX, EFS-induced ACh release was potentiated. Thus in airway
parasympathetic nerves as in other parts of the nervous system,
neuronal ChTX-sensitive KCa
channels are inhibitory to ACh release. This finding is also in
agreement with a recent demonstration that NS- 1619, a selective
large-conductance KCa-channel
opener, inhibits EFS-evoked
[3H]ACh release from
guinea pig trachea (14). In contradiction to this conclusion is the
observation by Baker et al. (3) that a more selective blocker of
fast-conductance KCa
[KCa(f)] channel (8),
IBTX, at a concentration
(10
7 M) that increased
tracheal tension, failed to alter EFS-evoked ACh release from
guinea pig trachea. The possible explanations for these conflicting
observations have been carefully discussed by Baker et al. (3). In
addition to the methodological difference, ChTX is pharmacologically
less selective than IBTX for
KCa(f) channels; it also inhibits
several other classes of KCa
channels (8). It is possible that although IBTX-sensitive
KCa(f) channels have been
excluded, other classes of ChTX-sensitive
KCa channels may be involved in
the control of ACh release from guinea pig airway parasympathetic
nerves. However, our results demonstrated that IBTX significantly
increased ACh release, indicating that activation of prejunctional
IBTX-sensitive KCa channels
inhibits ACh release from equine airway parasympathetic nerves. Both
Baker et al. (3) and we used HPLC to directly measure the ACh release. Therefore, one possible explanation for the differences between our
results and those of Baker et al. (3) might be the species difference.
The average increases in ACh released from our tissue preparations by
ChTX and IBTX at the concentration of 5 × 10
7 M were 8.33 and 3.54 pmol · g
1 · min
1,
respectively. In agreement with the literature (8), these results
suggest that ChTX may inhibit several other classes of KCa channels in addition to those
that are IBTX sensitive.
If activation of ChTX- or IBTX-sensitive
KCa channels inhibits ACh release,
it is possible that 2-ARs may
exert their effect on ACh release via activation of such channels.
Previous investigations have reached conflicting conclusions on this
point. Miura et al. (13) demonstrated reversal of clonidine-induced
inhibition of EFS-induced smooth muscle contraction by ChTX and
concluded that ChTX-sensitive K+
channels are involved in the
2-AR-induced inhibition of ACh release from guinea pig airway parasympathetic nerves. However, in
direct measurements of neurotransmitter release, IBTX
(10
7 M) failed to alter
clonidine-induced inhibition of ACh release from guinea pig trachea
(3). Again, the different results may be due to the different
pharmacological characteristics of ChTX and IBTX. Therefore, in the
present study, we tested the effects of ChTX and IBTX on
clonidine-induced inhibition of EFS-evoked ACh release. Our results
clearly demonstrated that even though both ChTX and IBTX potentiate
EFS-induced ACh release, they do not alter the
2-AR-induced inhibition of ACh
release. The reasons for these conflicting conclusions obtained by
direct measurement of ACh release and from functional evaluations (13)
are currently unclear. Nevertheless, as our laboratory (17, 22-24,
26) and other investigators (3) have previously mentioned, functional studies that compare the effect of an agent on EFS- and ACh-induced contractions rely on several assumptions, and a differential effect on
the two contractions can be explained by several possibilities. For
example, a greater number of muscarinic receptors on the airway smooth
muscle cells may be activated by exogenous ACh than by EFS, which, in
turn, could decrease the response to inhibitory agonists. Furthermore,
EFS activates several types of airway nerves and releases several
neurotransmitters in addition to ACh that can affect the response of
the muscle. Finally, many agents, such as
K+-channel blockers, act both
prejunctionally to modulate ACh release and postjunctionally to alter
the response of smooth muscle to the released ACh. For these reasons,
it is essential to directly measure ACh release to accurately
investigate prejunctional modulation of cholinergic neurotransmitter
release.
On the basis of our above observations, we conclude that in equine
tracheal parasympathetic nerves,
2-AR-induced inhibition of ACh
release is via an intracellular cAMP-independent pathway. Both ChTX-
and IBTX-sensitive KCa channels
inhibit the EFS-evoked ACh release but are not involved in the
2-AR-induced inhibition. Therefore, the intracellular mechanisms of
2-AR-induced inhibition of ACh
release are still unclear. In many neurons, the inhibitory regulation
of neurotransmitter release is mediated via direct coupling of
prejunctional receptors to Ca2+
channels (12). The
2-AR-induced
inhibition of NE release from sympathetic neurons is associated with a
reduction in the activity of N-type
Ca2+ channels (11). N-type,
-conotoxin-sensitive Ca2+
channels regulate ACh release in the rat myenteric plexus (19) and
guinea pig (2) and horse (unpublished data) airway
parasympathetic nerves. Therefore, it is likely that N-type,
-conotoxin-sensitive Ca2+-channels may mediate the
2-AR-induced inhibition of ACh
release from airway parasympathetic nerves. Further investigations with an electrophysiological approach are needed to directly measure Ca2+ influx in airway
parasympathetic ganglion cells.
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ACKNOWLEDGEMENTS |
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We thank Cathy Berney for technical assistance and Victoria Hoelzer-Maddox and MaryEllen Shea for manuscript preparation.
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FOOTNOTES |
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This work was supported in part by an endowment from the Matilda R. Wilson Fund.
X.-Y. Zhang is a fellow of the American Lung Association of Michigan.
Address for reprint requests: X.-Y. Zhang, Dept. of Large Animal Clinical Sciences, Michigan State Univ., East Lansing, MI 48824-1314.
Received 11 August 1997; accepted in final form 11 February 1998.
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REFERENCES |
---|
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---|
1.
Alberts, P.,
V. R. Ögren,
and
Å. I. Sellström.
Role of adenosine 3',5'-cyclic monophosphate in adrenoceptor-mediated control of [3H]-noradrenaline secretion in guinea-pig ileum myenteric nerve terminals.
Naunyn Schmiedebergs Arch. Pharmacol.
330:
114-120,
1988.
2.
Baker, D. G.,
H. F. Don,
and
J. K. Brown.
N-type, -conotoxin-sensitive Ca2+ channels mediate electrically evoked release of ACh in guinea pig trachea.
Am. J. Physiol.
264 (Lung Cell. Mol. Physiol. 8):
L581-L586,
1993
3.
Baker, D. G.,
H. F. Don,
and
J. K. Brown.
-Adrenergic and muscarinic cholinergic inhibition of ACh release in guinea pig trachea: role of neuronal K+ channels.
Am. J. Physiol.
266 (Lung Cell. Mol. Physiol. 10):
L698-L704,
1994
4.
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].
5.
Briggs, C. A.,
D. A. McAfee,
and
R. E. McCaman.
Long-term regulation of synaptic acetylcholine release and nicotine transmission: the role of cyclic AMP.
Br. J. Pharmacol.
93:
399-411,
1988[Abstract].
6.
D'Agostino, G.,
M. C. Chiari,
E. Grana,
A. Subissi,
and
H. Kilbinger.
Muscarinic inhibition of acetylcholine release from a novel in vitro preparation of the guinea-pig trachea.
Naunyn Schmiedebergs Arch. Pharmacol.
342:
141-145,
1990[Medline].
7.
Exton, J. H.
Molecular mechanisms involved in -adrenergic response.
Trends Pharmacol. Sci.
3:
111-115,
1982.
8.
Garcia, M. L.,
A. Galvez,
M. Carcia-Calvo,
V. F. King,
J. Vazquez,
and
G. J. Kaczorowski.
Use of toxin to study potassium channels.
J. Bioenerg. Biomembr.
23:
615-646,
1991[Medline].
9.
Jumblatt, J. E.
Prejunctional 2-adrenoceptor and adenylyl cyclase regulation in the rabbit iris-ciliary body.
J. Ocul. Pharmacol.
10:
617-621,
1994[Medline].
10.
Katsura, M.,
T. Hashimoto,
and
K. Kuriyama.
Effect of 1,3-di-n-butyl-7-(2-oxopropyl)-xanthine (denbufylline) on metabolism and function of cerebral cholinergic neurons.
Jpn. J. Pharmacol.
55:
233-240,
1991[Medline].
11.
Lipscombe, D.,
S. Kongsamut,
and
R. W. Tsien.
-Adrenergic inhibition of sympathetic neurotransmitter release mediated by modulation of N-type calcium-channel gating.
Nature
340:
639-642,
1989[Medline].
12.
Miller, R. J.
Receptor-mediated regulation of calcium channels and neuro-transmitter release.
FASEB J.
4:
3291-3299,
1990
13.
Miura, M.,
M. G. Belvisi,
C. D. Stretton,
M. H. Yacoub,
and
P. J. Barnes.
Role of K+ channels in the modulation of cholinergic neural responses in guinea-pig and human airways.
J. Physiol. (Lond.)
455:
1-15,
1992[Abstract].
14.
Patel, H. J.,
M. A. Giembycz,
P. J. Barnes,
and
M. G. Belvisi.
Prejunctional modulation of acetylcholine release from guinea pig trachea by NS1619, an activator of large-conductance calcium activated potassium channels (Abstract).
Am. J. Respir. Crit. Care Med.
153:
A847,
1996.
15.
Schwartz, D. D.,
and
K. U. Malik.
Cyclic AMP modulates but does not mediate the inhibition of [3H]norepinephrine release by activation of 2-adrenergic receptors in cultured rat ganglion cells.
Neuroscience
52:
107-113,
1993[Medline].
16.
Umemura, S., N. Hirawa, Y. Toya, K. Minamizawa, G. Yasuda, Y. Ishikawa, S. Hayashi, and M. Ishii.
2-Adrenoceptor stimulation
inhibits cellular cyclic AMP production in microdissected human
glomeruli. Clin. Exp. Hypertens. 11, Suppl. 1: 275-280, 1989.
17.
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
18.
Wang, Z.-W.,
M.-F. Yu,
and
N. E. Robinson.
Prejunctional muscarinic autoreceptors on horse airway cholinergic nerves.
Life Sci.
56:
2255-2262,
1995[Medline].
19.
Wessler, I.,
D. J. Dooley,
J. Werhand,
and
F. Schlemmer.
Differential effects of calcium channel antagonists (-conotoxin GVIA, nifidipine, verapamil) on the electrically-evoked release of [3H]acetylcholine from the myenteric plexus, phrenic nerve and neocortex of rats.
Naunyn Schmiedebergs Arch. Pharmacol.
341:
288-294,
1990[Medline].
20.
Wiley, J.,
and
C. Owyang.
Somatostatin inhibits cAMP-mediated cholinergic transmission in the myenteric plexus.
Am. J. Physiol.
253 (Gastrointest. Liver Physiol. 16):
G607-G612,
1987
21.
Wilson, D. F.
The effects of dibutyryl cyclic adenosine 3',5'-monophosphate, theophylline and aminophylline on neuromuscular transmission in the rat.
J. Pharmacol. Exp. Ther.
188:
447-452,
1974[Medline].
22.
Yu, M.,
Z. Wang,
and
N. E. Robinson.
Prejunctional 2-adrenoceptors inhibit acetylcholine release from cholinergic nerves in equine airways.
Am. J. Physiol.
265 (Lung Cell. Mol. Physiol. 9):
L565-L570,
1993
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].