(Investigative Ophthalmology and Visual Science. 2002;43:780-783.)
© 2002
by The Association for Research in Vision and Ophthalmology, Inc.
Effect of Pituitary Adenylate CyclaseActivating Peptide on Isolated Rabbit Iris Sphincter and Dilator Muscles
Takeshi Yoshitomi1,
Kazutsuna Yamaji1,
Hitoshi Ishikawa2 and
Yoshitaka Ohnishi1
1 From the Department of Ophthalmology, Wakayama Medical University, Wakayama, Japan; and the
2 Department of Ophthalmology Kitasato University School of Medicine, Sagamihara, Kanagawa, Japan.
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Abstract
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PURPOSE. Pituitary adenylate cyclaseactivating peptide (PACAP) is a sensory
neuropeptide in the eye that is released by noxious stimuli and
considered to be a mediator of the neurogenic ocular injury response,
including miosis. The purpose of this study was to clarify the
functional role of PACAP in iris sphincter and dilator muscles.
METHODS. Iris sphincter and dilator muscles were isolated from rabbit eyes, and
the effect of PACAP on mechanical responses of these muscles using
isometric tension-recording methods was investigated.
RESULTS. The iris sphincter responded to electric field stimulation with
contractions composed of fast twitch and subsequent slow components.
Both PACAP 27 and PACAP 38 enhanced the twitch response, but neither
had an effect on the slow response. The effect of both PACAPs on the
twitch response was dose dependent. Neither PACAP had an effect on the
amplitude of contraction evoked by exogenously applied Ach. For the
iris dilator muscle, PACAP 27 inhibited the contractions induced by
field stimulation or phenylephrine, whereas PACAP 38 had no effect.
CONCLUSIONS. Both PACAP 27 and PACAP 38 enhance cholinergic transmission in
sphincter muscle. The PACAP 27 induces relaxation of the dilator muscle
by a direct effect on the muscle itself. The PACAP released during an
ocular inflammatory response may induce miosis by the enhancement of
cholinergic stimulation of the iris sphincter and by direct relaxation
of the dilator muscles.
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Introduction
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Pituitary adenylate cyclaseactivating peptide (PACAP) is
a novel neuropeptide with two molecular forms: one with 27 amino acid
residues (PACAP 27) and one with 38 residues (PACAP 38).1
Each has significant structural homology with vasoactive intestinal
peptide (VIP).2
There are two types of receptors for
PACAP. One has a higher affinity for PACAP than for VIP; the other has
a similar affinity for PACAP and VIP. The anterior uvea of the rabbit
has mainly the latter type of PACAP receptors.3
PACAP immunoreactive nerve fibers have been identified in the central
nervous system4
5
as well as peripheral tissues including
lung,6
pancreas,7
and gastrointestinal
tract,8
suggesting that PACAP acts as a neurotransmitter
or neuromodulator. PACAP immunoreactive nerve fibers are also present
in ocular tissues, including iris-ciliary body, choroid, cornea, and
sclera.9
10
Wang et al.10
reported that the
distribution pattern of PACAP-immunoreactive nerve fibers in the eye is
similar to calcitonin gene-related peptide (CGRP) immunoreactivity, a
known component of sensory C-fiber neurons. Tajti et al.11
demonstrated the presence of CGRP, substance P (SP), and PACAP
immunoreactivity in the human trigeminal ganglion. These results
indicate that PACAP is a sensory neuropeptide in the eye.
Trigeminal nerve stimulation induces inflammatory responses in the
rabbit eye.12
These include vasodilation, breakdown of the
bloodaqueous barrier, and miosis. Sensory nerve fibers are likely to
play important roles in these responses, because SP and CGRP are
released from such nerve terminals and evoke these
responses.13
14
PACAP is also a mediator of the neurogenic
ocular injury response. Intravitreal administered PACAP causes
breakdown of the bloodaqueous barrier, conjunctival hyperemia, and
decreased pupil diameter of the rabbit eye.10
Capsaicin,
which causes release of SP from trigeminal sensory nerves in
rabbits,15
also releases CGRP and PACAP in the rabbit uvea
in vitro.16
Thus, it is likely that SP, CGRP, and PACAP
coexist in sensory nerve fibers and are released by noxious stimuli.
In an attempt to clarify the functional role of PACAP on the iris
smooth muscles, we isolated iris sphincter and dilator muscles from
rabbit eyes and investigated the mechanical properties of these
muscles, using isometric tension recording methods.
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Methods
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General
All animals were treated in accordance with the ARVO Statement
for the Use of Animals in Ophthalmic and Vision Research. Male albino
rabbits weighing 2 to 3 kg were killed with an overdose of intravenous
pentobarbital sodium (Abbott Laboratories, North Chicago, IL). The eyes
were immediately enucleated and placed in Krebs solution composed of
(in millimolar): NaCl, 94.8; KCl, 4.7;
MgSO4, 1.2; CaCl2,
2.5; KH2PO4, 1.2;
NaHCO3, 25.0; and glucose, 11.7 and gassed with
95% O2 and 5% CO2. Under
microscopic observation, ring-shaped iris sphincter muscle specimens (1
mm wide) and dilator muscle specimens (1 mm wide, 34 mm long) were
prepared according to a method previously reported.17
18
Isometric Tension Recording Experiments
The ends of each specimen were tied with silk thread and mounted
vertically in a 1.5-mL organ bath. One end was connected to an
isometric tension transducer (EF-601G; Nihon Koden Ltd., Tokyo, Japan),
and the other end was secured to a hook at the bottom of the organ
bath. The initial loads were 100 and 50 mg for sphincter and dilator,
respectively. The organ bath was perfused continuously (0.17 mL/sec)
with oxygenated Krebs solution warmed to 37°C, as we described
previously.19
Experiments were started after a 60-minute
equilibration period. The responsiveness of each preparation was tested
initially by application of 1 mM Ach for sphincter or 10 µM
phenylephrine for dilator at least three times, to confirm that the
same amplitude of contraction was observed each time. Specimens that
did not show the same amplitude of contractions in response to agonists
were excluded from the study. After 1 to 2 hours of washout period,
test drugs were added to the perfusing solution. Transmural electrical
field stimulations were applied through a pair of platinum electrodes
separated by 11 mm and placed in the organ bath, so that the current
pulse would pass transversely across the tissue. Pulse stimuli of 10 V
at 20 Hz were applied for 2.0 ms.
Data Analysis
All data are expressed as the mean ± SD. Students
t-test was used for statistical evaluation of the
differences between means. P < 0.05 was considered
significant.
Drugs and Chemicals
The following drugs and chemicals were used in this study: PACAP
27 and PACAP 38 (Peptide Institute, Inc., Osaka, Japan) and Ach,
tetrodotoxin (TTX), and phenylephrine (all from Wako Chemical Inc.,
Osaka, Japan). Peptides were prepared in aliquots and stored at
-30°C.
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Results
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The iris sphincter and dilator muscles, mounted in an organ bath,
gradually relaxed to a steady tension during 60 minutes of
equilibration. Muscle tone subsequently remained constant for several
hours. The electrical field stimulus (70 pulses) evoked a biphasic
contraction of the iris sphincter muscle characterized by an initial
twitch followed by long-lasting slow contraction. For the dilator
muscle, the same electrical field stimuli evoked only twitch
contractions. These responses were abolished by pretreatment with TTX
(0.1 µM), suggesting that the responses were neurogenic in origin
(data not shown).
Effect of PACAPs on the Mechanical Properties of Iris Sphincter
Muscle
PACAP 27 and PACAP 38 (10 nM) enhanced the amplitude of twitch
component of the sphincter contraction evoked by field stimulation but
had no effect on the slow component (Fig. 1)
. This effect of PACAP 27 and PACAP 38 on the fast component of
contraction evoked by field stimulation was investigated further. To
inhibit the prolonged, slow component of contraction, field stimulation
composed of 5 pulses was applied every 1.5 minutes. Both PACAP 27 and
PACAP 38 enhanced in a dose-dependent manner the amplitude of the
twitch contraction evoked by field stimulation (Figs. 2
3)
. At 10 nM for each peptide, there was a transient inhibition of the
contraction immediately after application (Fig. 2)
. The maximum
enhancement of contraction evoked by field stimulation for 10 nM PACAP
27 and PACAP 38 was 41.1% ± 17.1% and 25.8% ± 11.9%
(n = 6), respectively.

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Figure 1. Iris sphincter muscle recordings of contractile responses evoked by 70
pulses of electrical field stimuli. Twitch and slow responses
(A) before and (B) after 10 nM PACAP 27. Twitch
and slow responses (C) before and (D) after 10 nM
PACAP 38.
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Figure 2. Iris sphincter muscle recordings of contractile responses evoked by
five pulses of electrical field stimuli applied every 1.5 minutes.
Effect of various concentrations of (A) PACAP 27 and
(B) PACAP 38 on the mechanical responses during field
stimulation. All four traces were obtained from the same specimens
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Figure 3. Doseresponse relationship between PACAP concentration and the
amplitude of contractions evoked in sphincter muscle by field
stimulation (five pulses). The mean of the last five contractions
before application of PACAPs was defined as the relative amplitude of
100%. The amplitude of contraction after the application of PACAPs was
defined as mean of the five contractions, beginning 15 minutes after
application. *P < 0.05 and
P < 0.01
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The effect of PACAP 27 and PACAP 38 on the amplitude of sphincter
contraction evoked by field stimulation and 0.1 mM Ach were compared.
Because the median effective concentration (EC50)
for contractile response to Ach in this tissue is approximately 0.3
mM,20
a concentration of 0.1 mM Ach was chosen for this
experiment. Both PACAPs enhanced the twitch contraction evoked
by field stimulation but had no effect on the Ach-induced contraction
(Fig. 4
, Table 1
). These results indicate that the PACAP-dependent increase of the
cholinergic responses were due to enhancement of prejunctional
cholinergic transmission, not enhancement of postjunctional Ach
sensitivity.

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Figure 4. The effect of (A) PACAP 27 and (B) PACAP 38 on
the contraction evoked by 0.1 mM Ach and field stimulation
(arrow, 70 pulses) in the sphincter muscle.
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Effect of PACAPs on the Mechanical Properties of Iris Dilator
Muscle
The effects of PACAP on the contraction evoked by field
stimulation of dilator muscle were also determined. Neither PACAP had
an effect on the basal tone of the dilator muscle. PACAP 27 reduced in
a dose-dependent manner the amplitude of twitch contraction evoked by
field stimulation (Figs. 5A
5C
). The maximum reduction in amplitude, achieved with 10 nM PACAP
27, was 42.0% ± 9.7%. PACAP 38 did not significantly change the
amplitude of contraction at any concentration tested (Figs. 5B
5C)
.

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Figure 5. Iris dilator muscle recordings of contractile responses evoked by field
stimulation with 50 pulses every 1.5 minutes Arrows:
application of 10 nM (A) PACAP 27 and (B) PACAP
38. (C) Doseresponse relationships between PACAP
concentration and the amplitude of contractions evoked in dilator
muscle by field stimulation (50 pulses). The mean of the last five
contractions before application of PACAPs was defined as relative
amplitude of 100%. The amplitude of contraction after the application
of PACAPs was defined as the mean of the five contractions beginning 5
minutes after application. *P < 0.01.
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PACAP 27 also induced a dose-dependent relaxation of the dilator muscle
that was precontracted by 10 µM phenylephrine (Fig. 6A , Table 2
). However, PACAP 38 did not have such an effect (Fig. 6B
, Table 2
). The
results indicate that PACAP 27 but not PACAP 38 directly induced
relaxation of the dilator muscle.

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Figure 6. Concentration-dependent effect of (A) PACAP 27 and
(B) PACAP 38 on the mechanical properties of iris dilator
muscle precontracted with 10 µM phenylephrine.
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Discussion
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The twitch and slow contractions evoked by field stimulation in
rabbit iris sphincter muscles result from the activation of cholinergic
and SP-ergic nerve fibers, respectively.21
The present
investigation showed that both PACAP 27 and PACAP 38 enhanced twitch
contraction, but had no effect on slow contraction evoked by field
stimulation. Because PACAP had no effect on the contraction evoked by
exogenously applied Ach in this tissue, we conclude that PACAPs
enhances cholinergic transmission in sphincter muscle. Because SP-ergic
slow contraction was not affected by PACAP (up to the concentration of
10 nM), the prejunctional action of PACAP seems to affect only
cholinergic nerves, not SP-ergic nerves. Prejunctional action of PACAP
is also reported in the guinea pig tenia coli.22
However,
our data are not consistent with previous data showing that 1 µM
PACAP 27 and PACAP 38 had no effect on electrically evoked contractions
in rabbit iris sphincter.10
Although the cause of this
discrepancy is not clear, a likely possibility is that the electrical
stimulation settings may affect the results. Preliminary experiments
showed that the PACAP-dependent enhancement of twitch contraction
evoked by field stimulation was more apparent with the 5-pulse stimulus
than with the 70-pulse. Because the amplitude of contraction by the
70-pulse stimulus was nearly maximum, there may be little room for
greater contractions. We have shown the enhancement of the contraction
evoked by field stimulation from concentrations as low as 1 nM PACAP
27, which is in the range of physiological relevance. However, the
physiological relevance of transient inhibition of contraction
immediately after application of both peptides in the concentration of
10 nM is not clear.
To our knowledge, this is the first report of the effect of PACAPs on
iris dilator muscle. In contrast to the effect on iris sphincter, PACAP
27 inhibited the dilator muscle contractions evoked by both field
stimulation and exogenously applied phenylephrine. The indicates that
PACAP had mainly a direct relaxant effect on the dilator muscle and had
little effect on adrenergic transmission. PACAP causes smooth muscle
relaxation through the nitric oxide pathway in opossum internal anal
sphincter23
24
or by increasing the cAMP level in tenia of
the guinea pig cecum.25
VIP, which has significant
structural homology with PACAP, also relaxes dilator
muscle.26
Thus, PACAPs could induce miosis through two different mechanisms. One
is the enhancement of cholinergic transmission in sphincter muscle, and
the other is the direct relaxant effect on the dilator muscle.
Intravitreal injection of PACAP induces miosis in rabbit, which
supports our findings.10
Trigeminal nerve stimulation induces inflammatory responses including
miosis in the rabbit eye.12
SP, CGRP, and PACAP all exist
in trigeminal nerve fibers and are involved in these
responses.13
14
15
16
SP induces miosis through direct
contractile effect on the sphincter muscle without effect on the
dilator.19
In addition, we have reported that CGRP also
induces miosis through direct relaxation in the dilator muscle without
effect on the sphincter.18
We conclude in the present
study that PACAPs induce miosis through enhancement of cholinergic
transmission in sphincter muscle and by relaxation of dilator muscle.
Therefore, the miosis that occurs during an ocular inflammatory
response may result, at least in part, from neuropeptides released from
the trigeminal nerve. These neuropeptides, SP, CGRP, and PACAP, all
induce miosis at different sites of action.
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Footnotes
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Supported by Japan Society for the promotion of Science Grants-in-Aid
for Scientific Research 12671718 and 12671719.
Submitted for publication August 6, 2001; revised November 2, 2001;
accepted November 13, 2001.
Commercial relationships policy: N.
The publication costs of this article were defrayed in part by page
charge payment. This article must therefore be marked
"advertisement" in accordance with 18 U.S.C.
1734
solely to indicate this fact.
Corresponding author: Takeshi Yoshitomi, Department of
Ophthalmology, Wakayama Medical University, 811-1 Kimiidera,
Wakayama, Wakayama, 641-0012, Japan;
yoshitom{at}wakayama-med.ac.jp
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