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Vol. 294, Issue 3, 1009-1016, September 2000
Department of Medicine, Division of Gastroenterology and Hepatology, Jefferson Medical College, Thomas Jefferson University, Philadelphia, Pennsylvania
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Abstract |
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 Top
 Abstract
 Introduction
Materials and Methods
Results
Discussion
References
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We examined the actions of different heme oxygenase (HO) inhibitors on the basal HO activity in the opossum internal anal sphincter (IAS), rectum, and liver tissues and the IAS smooth muscle relaxation in response to nonadrenergic noncholinergic (NANC) nerve stimulation and different agonists. All the tissues examined were found to have significant levels of basal HO activity. Among different HO inhibitors, tin protoporphyrin IX (SnPP IX) was found to be most selective in inhibiting the HO activity in the IAS smooth muscle. Conversely, in the liver, all the HO inhibitors except SnPP IX caused significant inhibition of HO activity. Consistent with HO activity inhibition, the IAS smooth muscle relaxations caused by NANC nerve stimulation and vasoactive intestinal polypeptide also were inhibited by zinc protoporphyrin IX and SnPP IX. Zinc protoporphyrin IX also caused a significant attenuation of the IAS smooth muscle relaxation caused by isoproterenol. The IAS smooth muscle relaxation caused by nitric oxide was not significantly modified by any of the HO inhibitors. The data show the presence of HO activity in the IAS and other gastrointestinal tissues. The differential attenuation of HO activity by different HO inhibitors in the IAS smooth muscle and liver confirms the presence of different isozymes of HO in different tissues. Suppression of basal HO activity and the IAS smooth muscle relaxation induced by NANC nerve stimulation or VIP but not NO suggest that some of the stimuli that cause IAS smooth muscle relaxation may involve HO activity.
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Introduction |
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Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
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It
is now well known that carbon monoxide (CO) causes smooth muscle
relaxation by a direct action in a number of preparations (Furchgott
and Jothianandan, 1991
; Rattan and Chakder, 1993; Zygmunt et al., 1994;
Ny et al., 1995b), including the lower esophageal sphincter (LES) (Ny
et al., 1996) and internal anal sphincter (IAS) (Rattan and Chakder,
1993). CO is produced endogenously by the interaction between heme and
heme oxygenase (HO) (Maines, 1988, 1997). Different isoforms of HO have
been identified (Maines, 1999). Among these, HO-2 is localized
primarily in the neural and HO-1 in the non-neural tissues (Zakhary et
al., 1997; Appleton et al., 1999; Maines, 1999). Metabolism of heme by
HO leads to the simultaneous production of biliverdin, CO, and free
iron (Fe2+). Zinc protoporphyrin IX (ZnPP IX) has
been suggested to be a selective inhibitor of HO in a number of systems
(Maines, 1981; Luo and Vincent, 1994). Other reports (Luo and Vincent,
1994; Undem et al., 1996; Grundemar and Ny, 1997; Maines, 1997; Fan et
al., 1998a), however, suggest that HO inhibitors such as ZnPP IX may
have effects unrelated to HO inhibition. Future studies with HO-2 and
HO-1 antisense may provide specific information on the role of HO in
the gastrointestinal smooth muscle (Wagener et al., 1999).
The exact physiological role of CO and HO pathway in gastrointestinal
function is not presently known. One of the major impediments in
determining the role of HO pathway in gastrointestinal inhibitory neurotransmission is the lack of a selective HO inhibitor. For example,
in the LES (Ny et al., 1996) and the IAS (Rattan and Chakder, 1993;
Tottrup et al., 1995), ZnPP IX caused an attenuation of the smooth
muscle relaxation induced by nonadrenergic noncholinergic (NANC) nerve
stimulation and vasoactive intestinal polypeptide (VIP). The
observations raised two possibilities. First, that HO is important in
the NANC nerve-mediated smooth muscle relaxation and there is an
interaction between VIP and HO pathway. Second, the actions of the HO
inhibitor on the smooth muscle relaxation are completely unrelated to
HO inhibition and are due to their nonselective action by blocking the
smooth muscle relaxation by NANC nerve stimulation and VIP.
VIP and isoproterenol are known to activate adenylate cyclase (AC) via
G-protein-coupling and forskolin directly on the catalytic subunit of
AC (Tang et al., 1992; Burks, 1997; Dessauer et al., 1997; Grundemar
and Ny, 1997). Nitric oxide (NO) is considered to be primarily
responsible for the NANC nerve-mediated relaxation of the IAS (Rattan
and Chakder, 1992; Rattan et al., 1992) via the activation of
intracellular soluble fraction of guanylate cyclase (GC) (Chakder and
Rattan, 1993a,b; Lincoln et al., 1996; McDonald and Murad, 1996; Murad,
1996). The systematic examination of the effects of different agonists
that work at different levels, before and after HO inhibitors, will
shed light on their mechanisms in inhibiting the smooth muscle relaxation.
NO plays a major role in the NANC nerve-mediated relaxation of the IAS
(Burleigh, 1992; Rattan and Chakder, 1992; Rattan et al., 1992; Tottrup
et al., 1995). VIP also may be partly involved in the NANC relaxation
of the IAS (Biancani et al., 1985; Nurko and Rattan, 1988). There is
evidence to suggest the participation of HO pathway in the relaxation
of the IAS and other gastrointestinal smooth muscles (Rattan and
Chakder, 1993; Zakhary et al., 1997; Farrugia et al., 1998; Miller et
al., 1998), although its exact role remains to be identified.
To evaluate the role of HO pathway in gastrointestinal inhibitory neurotransmission and the effectiveness of HO inhibitors, it is important to examine the presence of HO activity in the tissues in the basal state. It is equally important to systematically compare the effects of different HO inhibitors on the gastrointestinal smooth muscle relaxation by the appropriate stimuli.
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Materials and Methods |
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Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
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Preparation of Smooth Muscle Strips.
The smooth muscle
strips from the IAS from opossum were prepared for the recording of
isometric tension as described previously (Moummi and Rattan, 1988;
Rattan and Moummi, 1989). Briefly, the animals were anesthetized by
pentobarbital (40-50 mg/kg i.p.) and the entire anal canal was
isolated and transferred to oxygenated (95% oxygen plus 5% carbon
dioxide) Krebs' physiological solution of the following composition:
118.07 mM NaCl, 4.69 mM KCl, 2.52 mM CaCl2, 1.16 mM MgSO4, 1.01 mM
NaH2PO4, 25 mM
NaHCO3, and 11.10 mM glucose. The LES was
carefully freed of the striated muscle fibers of the external anal
sphincter and other extraneous tissues such as large blood vessels. The
anal canal was then opened and pinned flat with the mucosal side up on
a dissecting tray containing oxygenated Krebs' solution. The mucosal
and submucosal layers were removed by a sharp dissection and IAS
circular smooth muscle strips (~1 × 10 mm) were prepared as
described previously (Moummi and Rattan, 1988; Rattan and Moummi,
1989).
Measurement of Isometric Tension.
The smooth muscle strips
were secured at both ends with silk sutures and transferred to 2-ml
muscle baths containing oxygenated Krebs' solution (37°C). One end
of the muscle strip was anchored at the bottom of the muscle bath and
the other end was attached to a force transducer (model FTO3; Grass
Instruments Co., Quincy, MA) for the measurement of isometric tension
on a Dynograph recorder (model R411; Beckman Instruments, Schiller
Park, IL). The muscle strips were stretched initially at 1 g of
tension and then allowed to equilibrate for at least 1 h with
regular washings at 20-min intervals. Only the strips that developed
spontaneous steady tension and relaxed in response to electrical field
stimulation (EFS) were used. The optimal length and the baseline of the
smooth muscle strips were determined as described previously (Moummi
and Rattan, 1988).
NANC Nerve Stimulation with EFS. EFS was delivered from a Grass stimulator (model S88; Grass Instruments Co.) connected in series to a Med-Lab Stimu-Splitter II (Med-Lab Instruments, Loveland, CO). The stimusplitter was used to amplify and measure the stimulus intensity with the optimal stimulus parameters for the neural stimulation (12 V, 0.5-ms pulse duration, 200-400 mA, 4-s train) at varying frequencies of 0.5 to 20 Hz. The electrodes used for the EFS consisted of a pair of platinum wires fixed at both sides of the smooth muscle strip. Neurally mediated relaxation of the IAS smooth muscle strips was quantified in response to different frequencies. The above-mentioned parameters of EFS are known to selectively cause relaxation of the IAS smooth muscle via the activation of NANC myenteric neurons.
HO Activity Assay.
HO activity of the extracts of the
opossum IAS, rectum, and liver was determined by their ability to
release 55Fe2+ from
[55Fe2+]hemin.
[55Fe2+]hemin was a
generous gift from Rodney Seaforth and David Ahearn of NEN Life
Sciences Products, Boston, MA. The protocol was modified from the
method described previously (Sierra and Nutter, 1992; Zakhary et al.,
1997).
.
The final volume of the reaction mixture was 20 µl and contained 25 to 50 µg of protein, 2 mM NADPH, and 15 µM
[55Fe2+]hemin in
phosphate buffer. The reaction was started with the addition of
radiolabeled hemin and the incubation continued at 37°C for 30 min.
The incubation was terminated by adding 1.0 ml of 20 mM ice-cold Tris
buffer (pH 7.4). The boiled tissue extracts were used to obtain the
background radioactivity and the values were subtracted from the
radioactivity of the tissue samples collected under control conditions
versus after different stimuli or agonists, before and after the HO
inhibitors. All the experiments were conducted in the dark.
[55Fe2+]hemin was
separated from the reaction mixture by anion exchange chromatography.
The reaction mixture was added to Dowex AG IX-8 resin columns (Bio-Rad,
Hercules, CA) equilibrated with 20 mM Tris buffer, pH 7.4. Biliverdin
and 55Fe2+ were retained in
the column from which free
55Fe2+ was eluted with 20 mM Tris buffer (pH 7.4) containing 1 M sodium chloride. The
radioactivity of the eluent was counted in a scintillation counter
(Beckman Instruments, Fullerton, CA) after adding scintillation fluids
(Scintilsafe; Fisher Scientific, Pittsburgh, PA). The recovery of
55Fe2+ from the columns was
estimated by adding
55Fe2+-ferric chloride (NEN
Life Sciences Products) to the extracts and passing through the
columns. The recovery of
55Fe2+ in the elutes was
30.0 ± 2.0%. All the values for HO activity were corrected for
the recovery and background as explained above and the data are
expressed on a percentile basis.
Drugs and Chemicals. The following chemicals were used in this study: ZnPP IX and isoproterenol hydrochloride (Aldrich Chemical Co., Milwaukee, WI); zinc deuteroporphyrin IX 2,4-bis ethylene glycol (ZnDP IX), tin protoporphyrin dichloride IX (SnPP IX), and coproporphyrin III dihydrochloride (CuPP III) (Porphyrin Products, Inc., Logan, UT); NO (Matheson Gas, Bridgeport, NJ); isoproterenol hydrochloride (Sigma Chemical Co., St. Louis, MO); VIP (Bachem Bioscience Inc., Torrance, CA); and EDTA tetrasodium (Fisher Scientific).
All chemicals except different porphyrins were dissolved and diluted in Krebs' solution and prepared fresh on the day of the experiment. Stock solutions of the porphyrins were prepared and kept in the dark. The porphyrins were dissolved in 0.2 N sodium hydroxide and diluted with Krebs' solution and their pH was adjusted to 7.4 with 0.2 N HCl. The final dilutions of sodium hydroxide used for porphyrins' in the muscle baths produced no significant effect on the basal IAS smooth muscle tone and relaxation in response to NANC nerve stimulation and different agonists. Saturated solution of NO was prepared at room temperature by injecting the gas into deoxygenated Krebs' physiological solution in a sealed vial at a pressure slightly higher than the atmospheric pressure and mixing well. The saturated solution of NO was considered to be ~3 mM (Shikano et al., 1987), and 1 to 100 µl of NO solution was added to
the muscle bath to achieve the desired concentrations. The
corresponding volumes of deoxygenated Krebs' physiological solution
produced no significant effect on the resting tone of the IAS. The
stock solutions of NO similarly prepared have been used previously in
our laboratory (Rattan and Chakder, 1992, 1993). These solutions were
used within 1 h of preparation. The vials and pipette tips were
siliconized and the muscle baths were treated with 2.5% BSA to reduce
binding of the peptides to the surface of the glass or plastic.
Drug Responses.
Pretreatment with different concentrations
of the porphyrins (1 × 10
6 to 3 × 104 M) was used to examine their effects on the
basal HO activity in the IAS, rectum, and liver, and on the basal IAS
tone and relaxation in response to different agonists.
6 M) and atropine
(1 × 106 M). All the agonists except NO
were given in a cumulative manner. The transient nature of the LES
relaxation in response to NO made it difficult to determine its effects
in a cumulative manner. Once the concentration-response curve for an
agent was determined, the smooth muscle strips were washed at least six
times, and the resting tension was allowed to recover to the
preinjection level.
Data Analysis. The data are expressed as means and S.E. of different experiments. The fall of the resting IAS tension is expressed as the percentage of Emax (100%) in response to supramaximal concentration (5 mM) of EDTA. Statistical significance between different groups was determined by using paired or unpaired t tests where applicable and ANOVA was performed to compare the entire frequency-response or dose-response curves before and after the HO inhibitors. A P value smaller than .05 was considered statistically significant.
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Results |
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Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
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Basal HO Activity. Basal HO activity was determined in the IAS, rectum, and liver homogenates with 25 and 50 µg of the tissue proteins. The basal or control HO activity was expressed as above the background levels measured after boiling the tissue homogenates for 15 min. The HO activity in different tissues increased with the protein concentration in the samples (P < .05; n = 4). The HO activity of 50 µg of protein from the IAS, rectum, and liver in this series of experiments was 329 ± 32, 386 ± 34, and 273 ± 45%, respectively, above the background levels (n = 4) in the basal state. In these series of experiments, the basal HO activity in the IAS, rectum, and liver was 7.25 ± 0.76, 7.87 ± 1.21, and 4.91 ± 0.34 nmol/mg of protein/h, respectively.
Influence of Different HO Inhibitors on HO Activity of IAS and
Liver.
Inhibition of HO activity by different HO inhibitors was
found to be tissue-specific. In the IAS, among the different HO
inhibitors examined, ZnPP IX and SnPP IX caused the maximal and
concentration-dependent inhibition of HO (Fig.
1). The inhibitory effect of ZnDP IX and CuPP III, however, although significant, was limited and not
concentration-dependent (Fig. 1). In these experiments, the basal HO
activity in the IAS was 278 ± 21% above the background levels as
explained under Materials and Methods. In the presence of
ZnPP and SnPP IX (1 × 104 M), HO activity
was significantly decreased from these basal levels to 148 ± 20 and 164 ± 34%, respectively (*P < .05;
n = 4). In the presence of ZnDP IX and CuPP III, HO
activity was 216 ± 20 and 221 ± 14%, respectively
(P < .05; n = 4). Similar effects of
ZnPP IX and SnPP IX also have been shown in other systems (Mitrione et
al., 1988; Kappas et al., 1995).
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Influence of Different HO Inhibitors on IAS Relaxation by NANC
Stimulation.
The fall in IAS tension by the EFS was examined
before and after three concentrations (3 × 105, 1 × 104, and
3 × 104 M) of the HO inhibitors. The fall
in IAS tension by different frequencies of EFS was significantly
inhibited by ZnPP IX and SnPP IX (*P < .05;
n = 5) but not by ZnDP IX and CuPP III (Fig. 3). On the contrary, ZnDP IX and CuPP III
caused a significant augmentation of the EFS-induced IAS smooth muscle
relaxation especially caused by the lower frequencies of EFS
(*P < .05; n = 5). Percentage of fall
in the IAS tension with 2 Hz of EFS in control experiments for ZnPP IX,
SnPP IX, ZnDP IX, and CuPP III were 69.1 ± 3.4, 58.2 ± 4.6, 71.5 ± 2.7, and 70.7 ± 6.6%, respectively. After 3 × 104 M of these inhibitors, the values were
56.9 ± 4.2, 43.1 ± 8.4, 82.1 ± 3.6, and 86.2 ± 2.7%, respectively. The effect of ZnPP IX and SnPP IX in inhibiting
the EFS-induced IAS relaxation appears to follow the trend similar to
that on the basal HO activity in the IAS, as shown in Fig. 1.
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4 M) was
21.6 ± 2.0, 21.6 ± 3.9, 22.5 ± 2.9, and 17.6 ± 2.0 mN, respectively. These values in the presence of the HO inhibitors were 20.6 ± 2.0, 23.5 ± 3.9, 29.4 ± 2.9, and
19.6 ± 3.9 mN, respectively (P > .05;
n = 5).
Influence of Different HO Inhibitors on Fall in IAS Tension by NO,
VIP, and Isoproterenol.
In comparison to the NANC nerve
stimulation, the overall concentration-effect curve showing the fall in
the IAS tension by NO was not significantly affected by any of the HO
inhibitors investigated (P > .05; n = 7; Fig. 4). Percentage of fall in the basal IAS tension with NO at the concentration of 1 × 105 M in the control experiments for ZnPP IX,
SnPP IX, ZnDP IX, and CuPP III, was 79 ± 3, 73 ± 7, 83 ± 3, and 76 ± 10, respectively. In the presence of these HO
inhibitors, the values for the fall in the basal IAS tone were 58 ± 7, 67 ± 10, 82 ± 2, and 87 ± 2%, respectively.
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4 M),
however, caused a moderate but significant augmentation of the
VIP-induced fall in the basal tension of the IAS smooth muscle (*P < .05; n = 5; Fig. 5D). Percentage
of fall in the basal IAS tension with 1 × 107 and 1 × 106 M
VIP in control versus after CuPP III was 26 ± 4 and 60 ± 10 versus 43 ± 7 and 77 ± 3%, respectively (P < .05; n = 5).
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Discussion |
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Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
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This study suggests the presence of HO activity in different gastrointestinal tissues examined. Furthermore, we compared the effects of different HO inhibitors on the basal HO activity and the IAS smooth muscle relaxation in response to NANC nerve stimulation and different agonists.
In the basal state, the levels of HO activity in the IAS and rectum
were significantly higher than in the liver. This was especially the
case when lower concentrations of tissue proteins were used. Although,
there are no systematic data that compare the levels of HO activity in
the muscularis propria of the gastrointestinal tract versus other
tissues, in general liver has been known to contain lower levels of HO
activity than other tissues such as spleen, brain, and intestine
(Vreman et al., 1991; Boni et al., 1993; Vallier et al., 1993). The
brain characteristically possesses higher levels of HO, particularly
HO-2. It is noteworthy that the HO assay determines the total HO
activity and does not discriminate different HO isozymes. Whether the
higher levels of HO activity in the IAS reflect the combination of both
HO-1 and HO-2 or only the one type, remains to be determined. The
constitutive HO-2 has been suggested to be present in higher levels in
neuronal tissues compared with HO-1 that may be present predominantly
in non-neuronal tissues (Maines, 1997). Whether the higher levels of HO
activity in the anorectal region compared with liver may be
attributable to the presence of the myenteric neurons in this region
remains to be determined. However, our recent studies have actually
shown the presence of HO-2 and HO-1 immunoreactive neurons in the IAS
(Battish et al., 2000).
The basal HO activity was inhibited by HO inhibitors in a
concentration- and tissue-specific manner. The HO inhibitors caused a
significant inhibition of HO activity both in the IAS and liver with
some interesting differences. In the liver, the HO activity was found
to be more sensitive to inhibition by ZnPP IX, ZnDP IX, and CuPP III.
On the contrary, in the IAS, ZnPP IX, and SnPP IX were most potent in
inhibiting the HO activity, whereas in the liver, SnPP IX caused only a
modest inhibition. The reverse was the case with ZnDP IX, which was
more potent in the liver than in the IAS. The data are in general
agreement with the effect of HO inhibitors on HO activity in hepatic
versus nonhepatic tissues. ZnPP IX was found to be most potent against
the hepatic HO activity in contrast to the nonhepatic HO activity that
was most sensitive to SnPP IX (Maines, 1997). Because of the
intracellular location of the HO, the issue of transport of different
HO inhibitors in different types of cells being responsible for the
differences in the potencies of HO inhibitors may not be ruled out.
The differences in HO inhibition pattern by the inhibitors in the IAS
versus the liver suggest the presence of different HO isozymes in
different tissues. In light of the literature on other tissues, the
current data suggest the presence of HO-2 primarily in the IAS and that
of HO-1 in the liver. The data, however, do not preclude the presence
of HO-1 and HO-2 in the IAS and liver, respectively. The presence of
HO-2 primarily in the IAS may be related to its presence in myenteric
neurons (Ny et al., 1996; Farrugia et al., 1998) and other structures
such as interstitial cells of Cajal (ICC) (Miller et al., 1998) in the
gastrointestinal tract. A dramatic reduction in the NANC relaxation by
the specific HO-2 knockout model of mice (Zakhary et al., 1997)
provides further support to the hypothesis.
The data further show that the inhibition of the IAS smooth muscle
relaxation by the HO inhibitors is stimulus-specific. ZnPP IX and SnPP
IX inhibited, whereas ZnDP IX and CuPP III augmented the IAS smooth
muscle relaxation by NANC nerve stimulation caused by the lower
frequencies of EFS. These HO inhibitors followed a similar trend for
attenuating the IAS smooth muscle relaxation by VIP with the exception
of CuPP III, which in higher concentrations (3 × 104 M) caused a modest augmentation of VIP
responses. The HO inhibitors however had no significant effect on the
overall IAS smooth muscle relaxation caused by NO and isoproterenol.
The lack of inhibitory effect of ZnPP IX on CO-induced IAS smooth
muscle relaxation has been shown before (Rattan and Chakder, 1993).
Among different HO inhibitors tested, only ZnPP IX caused an inhibition
of isoproterenol-induced relaxation of the IAS smooth muscle.
Whether the inhibition of the IAS smooth muscle relaxation caused by
NANC nerve stimulation and VIP is due to HO pathway in the inhibitory
neurotransmission and neurotransmitters interaction is not currently
known. The difficulty in arriving at a definitive conclusion about the
role of HO pathway in IAS relaxation by NANC nerve stimulation and VIP
is due to the nonselective inhibition of the smooth muscle relaxation
by a classical HO inhibitor such as ZnPP IX (Rattan and Chakder, 1993;
Ny et al., 1995a; Undem et al., 1996; Fan et al., 1998a). We have shown
before that ZnPP IX caused the blockade of the LES smooth muscle
relaxation caused by the agonists that work via G-protein-coupled
receptor activation linked to AC or GC pathways (Fan et al., 1998a). In
an earlier study, we identified the mechanism of action of the HO
inhibitor to be the inhibition of binding of the agonist to the
receptor (Fan et al., 1998b). Other actions of metalloporphyrins
besides HO inhibition in different systems include GC inhibition
(Ignarro et al., 1984), NO synthase inhibition (Meffert et al., 1994), and NO synthase stimulation (Chakder et al., 1996). Interestingly, the
attenuation of VIP-induced IAS smooth muscle relaxation was observed
with all the HO inhibitors examined except CuPP III.
The trend with the effects of different HO inhibitors, except ZnDP IX, on the IAS smooth muscle relaxation by NANC nerve stimulation and exogenous VIP was somewhat similar. The mechanism of differences in the action of ZnDP IX on the IAS smooth muscle relaxation in response to VIP (an attenuation) and NANC nerve stimulation (a slight augmentation) is not known. The observed effects of different HO inhibitors on the IAS smooth muscle appear to be relatively specific because the smooth muscle relaxation by NO was not affected.
Based on a number of experimental findings, the involvement of HO
pathway in the gastrointestinal smooth muscle relaxation is evident. It
has been shown before that CO, the product of HO activation, causes
smooth muscle relaxation accompanied with hyperpolarization of the
smooth muscle cells (Farrugia et al., 1998), via the activation of GC
pathway (Utz and Ullrich, 1991; Rattan and Chakder, 1993; Ingi et al.,
1996; Maines, 1997). HO immunoreactive neurons have been shown to be
present in a number of gastrointestinal tissues (Ny et al., 1996;
Chakder et al., 1997; Maines, 1997; Zakhary et al., 1997; Farrugia et
al., 1998). The specific knockout of HO-2 gene has been shown to cause
a dramatic reduction in the NANC relaxation and hyperpolarization of
the gastrointestinal smooth muscle (Zakhary et al., 1997; Xue et al.,
2000). However, the exact location of HO, the site of action of HO
pathway in modifying the relaxation in response to NANC nerve
stimulation and VIP, is not known. Interestingly, a recent study has
proposed the involvement of HO pathway at the level of ICC (Miller et
al., 1998). According to the study, carried out in the murine small intestine, CO may serve as a messenger between the ICC and the smooth
muscle cells for the inhibitory neurotransmission.
In summary, studies for the first time show the comparison of different HO inhibitors on the basal HO activity in the gastrointestinal tissues and the smooth muscle relaxation in response to NANC nerve stimulation and VIP that works via G-protein-coupled receptor activation. The data show that although the HO inhibitors exert a wide range of inhibitory effects on the smooth muscle relaxation in response to NANC nerve stimulation and certain agonists, they also inhibit HO activity in a tissue-specific manner. SnPP IX may be most selective in inhibiting the HO activity in the gastrointestinal smooth muscle because it was relatively devoid of significant effects in the liver HO activity and exerted stimulus-specific effects in the IAS smooth muscle relaxation. The exact role of HO in the IAS in the basal state and in response to NANC relaxation remains to be determined.
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Acknowledgment |
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We thank Dr. Ya-Ping Fan for valuable discussion and suggestions.
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Footnotes |
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Accepted for publication May 16, 2000.
Received for publication January 18, 2000.
1 The study was supported by National Institutes of Diabetes and Digestive and Kidney Diseases Grant DK-35385 and an institutional grant from Thomas Jefferson University.
Send reprint requests to: Dr. Satish Rattan, Professor of Medicine and Physiology, Jefferson Medical College, Thomas Jefferson University, 1025 Walnut St., Room 901 College, Philadelphia, PA 19107. E-mail: Satish.Rattan{at}mail.tju.edu
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Abbreviations |
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CO, carbon monoxide; LES, lower esophageal sphincter; IAS, internal anal sphincter; HO, heme oxygenase; ZnPP IX, zinc protoporphyrin IX; NANC, nonadrenergic noncholinergic; VIP, vasoactive intestinal polypeptide; AC, adenylate cyclase; NO, nitric oxide; GC, guanylate cyclase; EFS, electrical field stimulation; ZnDP IX, zinc deuteroporphyrin IX; SnPP IX, tin protoporphyrin IX; CuPP III, coproporphyrin III; ICC, interstitial cells of Cajal.
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References |
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Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
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and 
gamma subunits.
Cold Spring Harbor Symp Quant Biol
57:
135-144
-nitro-L-arginine.
Acta Physiol Scand
152:
137-143[Medline].
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