Vol. 279, Issue 3, G567-G574, September 2000
Role of cGMP as a mediator of nerve-induced motor functions of
the opossum esophagus
W.
Shahin,
J. A.
Murray,
E.
Clark, and
J. L.
Conklin
Department of Internal Medicine, University of Iowa College of
Medicine and Department of Veterans Affairs Medical Center, Iowa
City, Iowa 52242
 |
ABSTRACT |
Stimulation of esophageal nerves produces
biphasic relaxation of the lower esophageal sphincter (LES) and an off
response of circular esophageal muscle. Previously, we proposed that
cGMP mediates nerve-induced hyperpolarization of circular LES muscle but not LES relaxation. These experiments explore whether cGMP mediates
LES relaxation or the off response. Strips of muscle from the opossum
esophagus and LES were connected to force-displacement transducers,
placed in tissue baths containing oxygenated Krebs solution at 37°C,
and stimulated by an electrical field.
1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one (ODQ), a
selective inhibitor of guanylyl cyclase, antagonized the off response,
shortened its latency, and blocked the first phase of LES relaxation.
ODQ also antagonized LES relaxation by exogenous nitric oxide (NO) but
not relaxations by vasoactive intestinal polypeptide (VIP). Part of the
nerve-induced LES relaxation and the off response appear to be mediated
by the second messenger cGMP. These studies indicate that VIP-induced
LES relaxation is not mediated by cGMP and therefore do not support the
hypothesis that VIP produces LES relaxation by causing the generation
of NO.
nitric oxide; gastrointestinal motility; smooth muscle; enteric
nervous system; vasoactive intestinal polypeptide; guanylyl
cyclase
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INTRODUCTION |
SMOOTH MUSCLE FROM THE
BODY of the esophagus and lower esophageal sphincter
(LES) produce distinctive mechanical and electrophysiological responses
to activation of their intrinsic myenteric innervation. Circular muscle
of the LES generates tone at rest and relaxes on intrinsic nerve
stimulation (4, 25). During nerve stimulation, circular
muscle from the body of the esophagus does not generate a mechanical
response (25, 38). Cessation of the stimulus is followed
by a short period of mechanical quiescence before a transient circular
muscle contraction. The time from the end of the stimulus to the
beginning of the contraction is called the latency period, and the
delayed contraction is called the off response. Circular smooth muscle
cells from the esophagus and LES hyperpolarize during intrinsic nerve
stimulation: the period of hyperpolarization correlates with relaxation
of the LES and the latency period in the esophagus (11, 31,
32). Longitudinal smooth muscle from the esophagus contracts
during intrinsic nerve stimulation and is referred to as the on response.
We now know that nitric oxide (NO) is the neurotransmitter that
participates in the nerve-induced LES relaxation, smooth muscle membrane hyperpolarization, and in the timing of the off response (8, 15, 25, 34). In previous studies (9, 26),
we proposed that the activation of guanylyl cyclase by NO is
responsible for nerve-induced hyperpolarization of esophageal muscle
but not LES relaxation. This was somewhat troublesome because nerve
stimulation or exogenously applied NO increases cGMP concentrations in
this muscle, and analogs of cGMP relax LES muscle (1, 33).
In retrospect, these inhibitors of guanylyl cyclase (cystamine and methylene blue) may not have been an adequate solution to the question.
Cystamine appears to inhibit only the particulate guanylate cyclase
(30, 37). Methylene blue, a putative inhibitor of soluble
guanylate cyclase, may not be a potent inhibitor of the cyclase,
and it has several nonspecific effects, including inhibition of NO
synthase (NOS) (3, 20, 23, 24).
In the studies reported here, we used
1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one (ODQ), an
inhibitor of soluble guanylyl cyclase (2, 20, 28), to once
again test the hypothesis that NO from intrinsic esophageal nerves
controls LES relaxation by activating guanylyl cyclase. We also
explored the role of guanylyl cyclase in the control of the esophageal
off response and longitudinal muscle contraction.
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MATERIALS AND METHODS |
Fasted adult opossums of either sex weighing 2-4 kg were
anesthetized with ketamine HCl (30 mg/kg) and acepromazine (0.3 mg/kg) given intramuscularly and pentobarbital sodium (50 mg/kg) given intraperitoneally. The chest and abdomen were opened, and the entire
intrathoracic and intra-abdominal esophagus was marked and measured in
situ. The esophagus was transected at the proximal mark and excised,
along with a cuff of gastric tissue. The entire tissue was opened in
the long axis of the esophagus along the lesser curve of the stomach.
It was washed with warmed, aerated Krebs solution and pinned flat in a
tissue bath at its dimensions in situ with the mucosal surface facing
up. The bath contained oxygenated (95% O2-5%
CO2) Krebs solution maintained at 37°C and pH 7.4. The
mucosa and most of the submucosa were removed, and the LES was
recognized as a thickened band of circular muscle at the
gastroesophageal junction. Transversely oriented strips measuring
2 × 0.2-0.3 cm were prepared from the LES and the body of
the esophagus 1 cm above the LES, so that the long axis of the muscle
strip paralleled the long axis of smooth muscle cells constituting the
circular muscle layer. Muscle strips of like dimensions were prepared
in the long axis of the esophagus so that the long axis of the muscle
strip paralleled the long axis of smooth muscle cells constituting the
longitudinal muscle layer. Muscle strips were attached with silk suture
to force-displacement transducers, positioned between platinum
electrodes placed 4 mm apart, and lowered into 8-ml jacketed tissue
baths filled with Krebs solution maintained at 37°C and bubbled
continuously with 95% O2-5% CO2. Electrical
field stimulation (EFS) was accomplished by connecting the electrodes
to the output of a Grass S11 stimulator that delivered 4-s trains of
1.0 ms, 50 V square-wave pulses at 10-20 Hz. These stimulus
parameters were previously shown to produce activation of intrinsic
esophageal nerves (5, 25, 38). Each force-displacement
transducer was attached to a rack-and-pinion device that allowed
sequential stretching of the muscle strips. The output of the
force-displacement transducers was processed through a Maclab 8 analog-digital converter and recorded on a Macintosh IICi computer.
Each muscle strip was stretched rapidly until 100 mg of force was
generated. This was taken as the initial length. Muscle strips were
then sequentially stretched to 130% of initial length. The strips were
equilibrated for 1 h in the warmed, oxygenated Krebs solution
before experimentation. Only LES strips generated tone at rest and
relaxed on stimulation, and muscle strips from the body of the
esophagus produced an off response. Muscle strips were stimulated at
5-min intervals beginning 15 min before experimentation. Only muscle
strips showing reproducible responses to EFS were used. All drug
concentrations listed are final concentrations in the tissue bath.
The modified Krebs solution used in these experiments contained (in mM)
138.5 Na+, 4.6 K+, 2.5 Ca2+, 1.2 Mg+, 125 Cl
, 21.9 HCO3, 1.2 H2PO4, 1.2 SO4, and
11.5 glucose. It was maintained at 37°C and bubbled continuously with
95% O2-5% CO2 to maintain a pH of 7.4 throughout the experiment.
Saturated solutions of NO were prepared by equilibrating deoxygenated
water and NO in a sealed bottle at a pressure slightly above
atmospheric pressure. The concentration of NO in these solutions, as
measured by using a stream of nitrogen to purge the solution directly
into a chemiluminescence NO analyzer, was 2.4 ± 0.1 mM. Solutions
of NO prepared in this way are minimally contaminated by other
nonvolatile vasoactive nitrogen oxide products (35).
Ketamine was obtained from Aveco (Fort Dodge, IA). Pentobarbital sodium
was obtained from University of Iowa Pharmaceutical Service. The
following agents were purchased from Sigma Chemical (St. Louis, MO):
vasoactive intestinal polypeptide (VIP), atropine sulfate, ODQ, and DMSO.
DMSO was used as vehicle for ODQ, but at the concentrations used in
these studies, it did not alter EFS-induced responses significantly.
All physiological recordings were made and analyzed with MacLab
software. Data are expressed as means ± SE; n
represents the number of animals from which observations were made.
Statistical comparisons were made with the Tukey-Kramer honest
significant difference test or Dunnett's method when appropriate.
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RESULTS |
Effect of ODQ on EFS-induced relaxation of LES muscle.
EFS-induced relaxation of LES is biphasic, with the prominence of each
component of the relaxation depending on the frequency of the stimulus
(Fig. 1) (22, 36). There is
a transient relaxation that occurs during the stimulus and becomes
prominent at lower stimulus frequencies (R1), and there is a
relaxation that lasts well after the end of the stimulus and becomes
more prominent at higher frequencies of stimulation (R2).
Both components of the relaxation are sensitive to TTX, but
R1 is much more sensitive to inhibitors of NOS. We
used ODQ, an inhibitor of soluble guanylyl cyclase, to determine if
either phase of nerve-induced LES relaxation is mediated by cGMP.
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Fig. 1.
Effect of
1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one (ODQ) on
electrical field stimulation (EFS)-induced lower esophageal sphincter
(LES) relaxation. LES tension is on the y-axis, and time is
on the x-axis. The stimulus artifact from EFS (4-s train of
1-ms square-wave pulses at 20 Hz) is shown at bottom. The
responses after the tissue was exposed to 0.1 (B), 1 (C), and 10 (D) µM ODQ are shown. A:
control response to EFS. The control relaxation appears biphasic, and
ODQ seems to preferentially antagonize the first, more rapid phase of
the relaxation.
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The basal resting tone of muscle strips from the LES was not
significantly altered by ODQ (Fig.
2A). The rapid component of the EFS-induced relaxation (R1) was inhibited in a
concentration-dependent manner by ODQ (Figs. 1 and 2B). At
ODQ concentrations of 10 and 100 µM, R1 was essentially
abolished. The slower component of the relaxation (R2) was
decreased less by ODQ, but the decrease was not statistically
significant (Figs. 1 and 2B).
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Fig. 2.
The effects of ODQ on LES muscle. A: the
concentration-related effect of ODQ on LES tone. Although ODQ appears
to increase LES tone slightly, there is no significant difference from
control. P > 0.05; n = 5. B: the concentration-related effect of ODQ on LES relaxation
caused by nonadrenergic noncholinergic nerve stimulation. ODQ
attenuates the rapid phase of LES relaxation (R1) in a
concentration-dependent manner. * P < 0.05 compared
with control; n = 5. Although the slower phase of
relaxation (R2) was attenuated, at no concentration did the
response differ statistically from control. P > 0.05;
n = 5.
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Effect of ODQ on NO and VIP-induced relaxation of LES muscle.
NO relaxes smooth muscle by activating guanylyl cyclase to increase
cellular concentrations of cGMP. If ODQ antagonizes nerve-mediated relaxation of the LES by inhibiting the activity of guanylyl cyclase, then it should also antagonize LES relaxation by exogenous NO. Exogenous authentic NO produced a 55.8% ± 13.0% relaxation of LES
muscle strips. Treating the tissue with 10 µM ODQ diminished the NO-induced relaxation to 9.8 ± 3.3% (P < 0.05, n = 4) (Fig. 3).
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Fig. 3.
Effect of ODQ on nitric oxide (NO)-induced and vasoactive
intestinal polypeptide (VIP)-induced LES relaxation. A: LES
relaxation produced by 100 µl authentic NO alone and after
pretreatment with 10 µM ODQ. ODQ significantly decreased the
NO-induced LES relaxation. * P < 0.05;
n = 4. B: LES relaxation produced by 60 nM
VIP alone and after pretreatment with 10 µM ODQ. ODQ did not alter
the VIP-induced LES relaxation. P > 0.05;
n = 4.
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VIP relaxes circular muscle strips from the LES with an
EC50 in the range of 50 to 100 nM (33, 36). It
does so by activating adenylyl cyclase to increase intracellular
concentrations of cAMP. To be sure that ODQ does not inhibit LES muscle
relaxation by a nonspecific mechanism, we observed the effect of ODQ on
LES relaxation produced by VIP. VIP at 60 nM (a concentration near the
median effective dose for VIP-induced LES relaxation) produced a 54.3 ± 7.5% decrease in LES tone. Treating the tissue with 10 µM ODQ (a
concentration that diminished NO-induced LES relaxation by ~80%) had
no effect on VIP-induced LES relaxation (54.7 ± 10.0%, P > 0.05, n = 4) (Fig. 3).
Effect of ODQ on EFS-induced contraction of circular esophageal
muscle.
Activation of intrinsic esophageal nerves produced an off response in
circular muscle strips taken from the body of the esophagus (Fig.
4). The amplitude of the off response was
diminished in a concentration-dependent manner by ODQ (Figs. 4 and
5A). The latency of the off
response, the time from the end of the stimulus to the start of the
contraction, was also shortened by ODQ (Figs. 4 and 5B).
Exposing the tissue to 1 µM atropine sulfate ameliorated the effect
of ODQ on both the amplitude and timing of the off response (Fig.
6).
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Fig. 4.
Effect of ODQ on EFS-induced contraction of circular
esophageal smooth muscle. Tension is on the y-axis, and time
is on the x-axis. Stimulus artifacts from EFS (4-s train of
1-ms square-wave pulses at 10 Hz) are shown at bottom.
A: control response to EFS. B: response after the
tissue was exposed to 1 µM ODQ. C: response after 1 µM
atropine was added to the bath. The control contraction is an off
response. ODQ antagonizes the off response and uncovers an on response.
Atropine abolishes the on response and allows the off response to
increase in magnitude.
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Fig. 5.
Effect of ODQ on EFS-induced off response of circular
esophageal smooth muscle. A: the concentration-related
effect of ODQ on the off response amplitude (expressed as %maximum
contraction). ODQ attenuates the amplitude of the off response in a
concentration-dependent manner. * P < 0.05 compared
with control; n = 5. B:
concentration-related effect of ODQ on the latency of the off response.
ODQ shortens the off response latency (expressed as %control) in a
concentration-dependent manner. * P < 0.05 compared
with control; n = 5.
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Fig. 6.
Effect of cholinergic blockade on ODQ-induced changes in
the off response. A: the effects of ODQ and atropine on the
amplitude of the off response. ODQ (1 µM) decreased the amplitude of
the off response, and 1 µM atropine attenuated this effect.
* P < 0.05 compared with control; n = 4. B: the effect of ODQ on the timing of the off response.
ODQ (1 µM ) decreased the latency of the off response, and 1 µM
atropine attenuated this effect. * P < 0.05 compared with control; n = 4.
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Exposing the circular esophageal smooth muscle to ODQ also uncovered an
on response: a contraction that was distinct from the off response and
that occurred during the stimulus (Fig. 4). This on response was
abolished by 1 µM atropine sulfate (n = 4) (Fig. 4).
The amplitude of the on response increased as a function of ODQ
concentration up to a concentration of 10 µM (Fig.
7A), and the timing of the on
response was also altered by ODQ. The time from the beginning of the
stimulus to the initiation of the contraction decreased as a function
of the ODQ concentration (Fig. 7B).
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Fig. 7.
Effect of ODQ on EFS-induced on response of circular
esophageal smooth muscle. A: concentration-related effect of
ODQ on the on response amplitude (expressed as %maximum contraction).
ODQ increased the amplitude of the on response in a
concentration-dependent manner. * P < 0.05 compared
with control; n = 5. B:
concentration-related effect of ODQ on the on response timing (the
delay between the beginning of the stimulus and the onset of the on
response, expressed as %control). ODQ shortens the time from the
beginning of the stimulus to the onset of the on response in a
concentration-dependent manner. * P < 0.05 compared
with control; n = 5.
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Effect of ODQ on EFS-induced contraction of longitudinal esophageal
muscle.
Nerve-induced contraction of longitudinal esophageal smooth muscle is
largely cholinergic (4). The amplitude of EFS-induced contraction of longitudinal esophageal muscle was significantly increased only at an ODQ concentration of 100 µM (Fig.
8A). The timing of the
longitudinal muscle contraction was also altered by ODQ; the time from
the beginning of the stimulus to the initiation of the contraction
decreased as a function of the ODQ concentration (Fig. 8B).
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Fig. 8.
Effect of ODQ on EFS-induced contraction of longitudinal
esophageal smooth muscle. A: concentration-related effect of
ODQ on the amplitude of longitudinal muscle contraction (expressed as
%maximum contraction). ODQ significantly increased the amplitude of
contraction only at an ODQ concentration of 100 µM.
* P < 0.05 compared with control; n = 5. B: concentration-related effect of ODQ on the
longitudinal muscle contraction timing (the time from the beginning of
the stimulus to the onset of the contraction, expressed as %control).
ODQ shortens the time from the beginning of the stimulus to the onset
of longitudinal muscle contraction in a concentration-dependent manner.
* P < 0.05 compared with control; n = 5.
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DISCUSSION |
Transverse muscle strips taken from the LES and smooth muscle
esophagus respond to activation of their intrinsic myenteric innervation with stereotyped responses (1, 4, 5, 7, 25,
38). The LES, which is tonically contracted at rest, relaxes during nerve stimulation. Depending on the stimulus parameters, all or
a major portion of the relaxation is due to the generation of NO by
myenteric neurons (36). Esophageal muscle does not respond
during the stimulus period but contracts shortly after the end of the
stimulus. This contraction is termed the off response, and the time
between the end of the stimulus and the initiation of the contraction
is called the latency. NO released from myenteric neurons controls both
the amplitude and the timing of the off response in the distal smooth
muscle esophagus (25). It also mediates nerve-induced
hyperpolarization of circular esophageal and LES smooth muscle
(8, 15).
In our previous studies (9, 26), we proposed that NO
activation of guanylyl cyclase is responsible for nerve-mediated hyperpolarization of esophageal muscle but not for LES relaxation. At
the time, this was problematic because we knew that nerve stimulation produced a rise in intracellular cGMP concentrations that coincided with LES relaxation, the cGMP analog 8-bromo-cGMP relaxed the LES, and
specific cGMP phosphodiesterase inhibitors, which increase intracellular concentrations of cGMP, caused LES relaxation (1, 33). In retrospect, the purported inhibitors of guanylyl cyclase we used (cystamine and methylene blue) may not have been adequate. Cystamine appears to inhibit only the particulate guanylate cyclase (30, 37). Methylene blue, a putative inhibitor of soluble guanylate cyclase, may not be a potent inhibitor of this cyclase, and
it has several nonspecific effects, including inhibition of NOS
(3, 20, 23, 24). In the studies reported here, we used
ODQ, a newer and more reliable inhibitor of soluble guanylyl cyclase
(2, 20, 28), to once again explore the hypothesis that NO
from intrinsic esophageal nerves controls LES relaxation by activating
guanylyl cyclase. We also explored the role of guanylyl cyclase in
control of the esophageal off response and contraction of longitudinal
esophageal smooth muscle.
That portion of the LES relaxation that we (36) and Jury
et al. (22) previously showed to be NO dependent was
inhibited by ODQ in a concentration-dependent fashion; that is, it
mimicked the effect of inhibiting NOS. In addition, ODQ inhibited LES
relaxation caused by exogenous NO but not that caused by VIP. This
observation indicates that the inhibitory effect of ODQ is a specific
effect, because VIP-induced relaxation of this muscle is mediated by
activation of adenylyl cyclase (33). Some studies
(18, 21, 27) suggest that VIP relaxes gastrointestinal
smooth muscle by stimulating the production of NO. This study and other
findings by us (36), Daniel et al. (12), and
Tottrup et al. (34) do not support the hypothesis that VIP
relaxes LES muscle by the production of NO in either the nerve
terminals or the muscle of the LES.
Nerve-induced contraction of circular esophageal muscle was also
altered by ODQ. It attenuated or abolished the off response, shortened
its latency, and uncovered a cholinergic contraction, the on response.
In previous studies (25), inhibiting NOS with NG-nitro-L-arginine
(L-NNA) produced the same result: diminution of the off
response, shortening of its latency, and uncovering of a cholinergic on
response. Together, these observations suggest that cGMP is the second
messenger that mediates NO nerve-induced changes in esophageal motor function.
Shortening of the off response latency by ODQ was ameliorated by
atropine. This observation and the changes in the off response produced
by ODQ or inhibitors of NOS suggest a complex interplay of the
excitatory cholinergic and the inhibitory NO-guanylyl cyclase systems
in the control of esophageal motor function. In fact, there are studies
suggesting the importance of both systems in the control of esophageal
peristalsis. The cholinergic innervation plays a role in the generation
of peristaltic contractions in the opossum smooth muscle esophagus
because atropine and the M2 receptor antagonist
4-diphenylacetoxy-N-(2-chloroethyl)-piperidine hydrochloride
decrease the amplitude of peristaltic pressure waves produced by
swallowing, vagal stimulation, or intrinsic nerve stimulation
(10, 13, 14, 17). They also delay the onset of
swallow-induced peristaltic pressure waves at all levels of the smooth
muscle esophagus, and in high doses they increase their velocity of
propagation. In more recent studies, Yamato et al. (39)
explored the effects of inhibitors of NOS and muscarinic cholinergic
neurotransmission on peristalsis in the opossum esophagus produced by
swallowing or vagal nerve stimulation. Inhibitors of NOS increased the
velocity of peristalsis by preferentially shortening the time between
swallowing or vagal stimulation and the appearance of peristaltic
pressure waves in the distal smooth muscle esophagus. Adding atropine
after the inhibitor of NOS increased the latency period slightly, but
by about the same amount along the entire smooth muscle segment.
ODQ allowed the expression of a cholinergic on response and altered
both its timing and amplitude in a concentration-dependent manner. The
time between the onset of nerve stimulation and initiation of the on
response became shorter as the ODQ concentration was increased, and the
amplitude of the on response increased with ODQ concentrations. Using
long-duration vagal stimulation, Dodds et al. (13) and
Yamato et al. (39) were able to generate two distinct
peristaltic sequences in the opossum esophagus. One occurred during
vagal stimulation, was primarily cholinergic, and was called the "A
wave." The other occurred after stimulus, was primarily nitrergic,
and was called the "B wave." Thus the A and B waves seen in vivo
are likely to be analogous to the cholinergic on response and a
nitrergic off response seen in vitro after exposure to either
L-NNA or ODQ. Using vagal stimulation, Yamato et al. (39) found that NOS inhibitors increase the velocity of
the cholinergic A wave by decreasing the time for its arrival in the distal esophagus. This is comparable with our observation that inhibiting guanylyl cyclase with ODQ shortens the time from the beginning of EFS to the initiation of the cholinergic on response.
Together, the data from previous studies and those we presented here
demonstrate the functional presence of excitatory cholinergic and
inhibitory NO-guanylyl cyclase signaling systems in the circular smooth
muscle of the opossum esophagus. Whether the nitrergic and cholinergic
signals are integrated at the level of the smooth muscle cell or the
myenteric neuron is not yet known. There is experimental evidence to
support both possibilities. According to Rae et al. (29),
EFS of neurons intrinsic to the circular muscle of the colon produced a
biphasic electrophysiological response consisting of an excitatory
junction potential (EJP) followed by an inhibitory junction potential
(IJP). The EJP was inhibited by atropine. ODQ, L-NNA, and
the protein kinase G inhibitor
Rp-
-phenyl-1,N2-etheno-8-bromoguanosine-3',5'-cyclic
monophosphorothioate all increased the EJP amplitude and attenuated or
abolished the IJP. Potentiation of the EJP by inhibiting NOS
was reversed by NO donors, and this effect was blocked by ODQ.
[14C]ACh was used to measure the ACh from cholinergic
neurons. ODQ, L-NNA, and NO donors had no effect on
[14C]ACh release. These studies suggested that
cholinergic and NO motor neurons innervate the smooth muscle and that
the inhibitory effect of NO on cholinergic neurotransmission is
postjunctional, at the level of the muscle cell. Hebeiss and Kilbinger
(19) used [3H]ACh to explore the hypothesis
that NO alters ACh release from myenteric neurons of the guinea pig
ileum. In their studies (19), ODQ increased the basal and
nerve-stimulated release of ACh, and it increased the amplitudes of
nerve-mediated cholinergic and tachykininergic muscle contraction.
L-NNA produced similar results. An activator of soluble
guanylyl cyclase inhibited the nerve-induced release of ACh and muscle
contraction. Fox-Threlkeld et al. (16) came to a similar
conclusion on the basis of their study of the canine ileum. These
studies support the hypothesis that NO modulates neuromuscular
transmission by inhibiting the release of excitatory neurotransmitters.
In our studies, ODQ also increased the amplitude of longitudinal muscle
contraction and diminished the time from the beginning of the stimulus
to initiation of the contraction. This observation suggests that
guanylyl cyclase activity and cGMP levels may also play a role in
controlling the cholinergic nerve-induced contraction of the
longitudinal muscle. Cholinergic and NOS-containing neurons are present
in the longitudinal muscle of the opossum esophagus (6,
38); however, the functional role of the latter is not known.
Perhaps the motor function of the longitudinal muscle is also under the
control of both systems. This speculation remains to be explored.
In summary, for the most part, ODQ tends to mimic the effects on
esophageal motor function of NOS inhibition: it attenuates the off
response and shortens its latency, and it diminishes nerve-induced LES
relaxation. It also alters the cholinergic contraction of the
longitudinal muscle. These studies support the hypothesis that the
NO-guanylyl cyclase signaling system plays a role in controlling the
nerve-induced motor functions of the esophagus. In particular, they
provide evidence that activation of guanylyl cyclase mediates NO
nerve-induced relaxation of the LES.
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ACKNOWLEDGEMENTS |
This research was supported by a Veterans Affairs Merit Grant to
J. L. Conklin and a scholarship from the Egyptian government to W. Shahin.
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FOOTNOTES |
Address for reprint requests and other correspondence: J. L. Conklin, Dept. of Internal Medicine, 4549 John Colloton Pavilion, Univ. of Iowa Hospitals and Clinics, Iowa City, Iowa 52242.
The costs of publication of this
article were defrayed in part by the
payment of page charges. The article
must therefore be hereby marked
"advertisement"
in accordance with 18 U.S.C. §1734 solely to indicate this fact.
Received 12 November 1999; accepted in final form 31 March 2000.
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ABSTRACT
INTRODUCTION
