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Departments of 1 General and Digestive Surgery, Centre for Digestive Sciences2 Medical Biochemistry3 Human Physiology, Flinders University, Flinders Medical Centre, Bedford Park, SA 5042, Australia
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(Received 17 January 2004;
accepted after revision 21 May 2004;
first published online 28 May 2004)
Corresponding author G. T. P. Saccone: Department of General and Digestive Surgery, Centre for Digestive Sciences, Flinders Medical Centre, Flinders Drive, Bedford Park, South Australia 5042, Australia. Email: gino.saccone{at}flinders.edu.au
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Introduction |
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Clinically, high common bile duct (CBD)–SO basal pressure is associated with pain, chronic and acute pancreatitis and biliary stones. A significant proportion of patients (
5%) suffer recurrent pain following cholecystectomy, and others experience recurrent idiopathic pancreatitis. Some of these patients are diagnosed with ‘SO dysfunction’ characterized by a high SO basal pressure or dyskinesia (Toouli & Craig, 2000; Corazziari, 2003).
The biliary-SO in the Australian possum is approximately 13 mm long and connects the non-muscular CBD to the duodenum. The majority of the SO (10 mm) is external to the duodenum (extraduodenal); this anatomical feature permits accurate measurements of pressures within the SO without interference from duodenal contractions. The remaining papilla-SO region narrows, passes obliquely through the wall of the duodenum (intraduodenal) and protrudes into the lumen (Padbury et al. 1993a). At the proximal (juxta-CBD) region of the SO (proximal-SO) is a narrow band of thicker circular muscle that corresponds to the superior choledochal sphincter (Boyden, 1957). The extraduodenal-SO has alternating periods of quiescence and spontaneous phasic contraction. These contractions are observed commencing at the proximal-SO and propagating through the body-SO (middle) in a peristaltic-like manner toward the papilla-SO. Similar motility is observed in humans and other animals (Toouli et al. 1982; Toouli et al. 1983).
Traditionally the SO is thought to control bile flow by behaving primarily as a pump or as a resistor depending on the species (Calabuig et al. 1990; Liu et al. 1992; Sand et al. 1997; Toouli & Craig, 2000). Species have been classified as having a resistor-type SO if trans-sphincteric flow (TSF) is slowed during the SO contraction period. In the Australian possum our previous study determined that the SO acts as a resistor at a CBD pressure of 7 mmHg (Liu et al. 1992). However, it has been suggested that the SO may act as either a pump or a resistor depending on the imposed CBD pressure (Toouli et al. 1983; Torsoli, 1988).
The complex motility observed in the SO of both humans (Toouli et al. 1982) and animals (Toouli et al. 1983) may be important in understanding abnormal motility patterns which are associated with disease. However, total flow constraints and the small diameter of the SO, particularly the papilla-SO in animals, has restricted manometric recording to one or two sites, usually in the proximal-SO and body-SO regions, and hence limited our understanding. We recently developed a perfused four-lumen pico-manometry catheter (Craig et al. 2000) that permits simultaneous pressure recordings at four sites within the possum SO–CBD. Temporal correlation of these multiple pressure recordings with concurrent TSF allows assessment of how SO regions affect TSF at various CBD and duodenal pressures. In vitro SO preparations were used to obviate the complexity of extrinsic neural and hormonal signals.
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Methods |
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Animals used in these experiments were treated in accordance with the animal ethics requirements of Flinders University. Fasted Australian brush-tailed possums (Trichosurus vulpecula) of either sex, weighing 1.2–2.1 kg (n= 18) were anaesthetized with intramuscular ketamine (20 mg kg–1; Parke-Davis Pty, Ltd, Caringbah, NSW, Australia) and xylazine (5 mg kg–1; Bayer Australia Ltd, Botany, NSW, Australia). After removal of tissues the possums were killed with a bolus intravenous dose of Lethabarb® (Virbac Pty, Ltd, Baronia, Vic, Australia).
The abdomen was opened via a midline laparotomy. The CBD (to the hepatic ducts junction), pancreatic tissue, SO and 4 cm of attached duodenum were removed in toto and placed in modified Krebs solution (composition (mM): Na+ 151.0, K+ 4.7, Ca2+ 2.8, Mg2+ 0.6, Cl– 143.7, H2PO4– 1.3, HCO3– 16.3, SO42– 0.6, glucose 7.7, pH 7.4) gassed with 95% O2–5% CO2 at room temperature. Careful dissection removed adherent connective tissue, blood vessels and pancreatic tissue. The pancreatic duct(s) were ligated and the duodenum opened along the anti-mesenteric border. The tissue was transferred to a 150 ml capacity organ bath (Fig. 1A) filled with fresh, continuously gassed modified Krebs solution and pinned to the Sylgard base (Dow Corning Corporation, Midland, MI, USA) without stretching.
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Modified Krebs solution in a conical inflow reservoir (3.4 cm maximum diameter x 25.5 cm height) flowed through a heat exchanger and bubble-trap into the CBD via an inflow catheter (1.05 mm o.d. x 0.35 mm i.d., polyvinyl chloride, Fig. 1A). TSF flowed into a custom-built collection cup (13 mm o.d., 9.5 mm i.d., acrylic) that surrounded the papilla and was attached via a silk tie around the duodenum (Liu et al. 1992). The cup was pinned to the Sylgard base at 60 deg angle to the SO longitudinal axis since the papilla-SO passes through the duodenal wall at a similar angle – this minimized manometric artefacts. Fluid flowed from the cup into an outflow reservoir (37 mm i.d. x 90 mm height; a modified Buretol® Extension Set A2C7572, Baxter Healthcare Corporation, Aibonito, Puerto Rico) that was connected to a force displacement transducer (Model FT03, Grass Instrument Company, Quincy, MA, USA), thus allowing continuous gravimetric measurement of TSF (Liu et al. 1992).
Fluid levels in the in- and outflow reservoirs were zeroed at the level of the SO. The outflow reservoir fluid pressure was maintained at 0 mmHg or at preset pressures (4 or 7 mmHg, ±0.07 mmHg) using an automated pump. Imposed CBD pressure was increased by raising the fluid height in the inflow reservoir, via a pump, to 23 cmH2O (17 mmHg). Pressures applied were within a physiological range. Inflow reservoir weight was continuously recorded and fluid height was noted at regular intervals. The data set of weight versus height measurements was curve-fitted with a cubed root trendline using Microsoft Excel. The resultant curve was used to convert inflow reservoir weight to hydrostatic pressure.
The inflow and manometry catheters were tied into the CBD 20 mm proximal to the SO with size 2.0 silk thread over a soft latex strip – to prevent fluid leakage between the catheters. At the conclusion of experiments, the CBD was transected and the inflow rate in the absence of the SO was recorded. This represented the maximum flow rate.
Manometric recording
SO motility was recorded via cannulation of the CBD with a four-lumen pico-manometry catheter (0.8 mm o.d.) constructed as previously described (Craig et al. 2000). Side-holes were located on the outward-facing surface of each lumen at 2, 6, 10 and 14 mm from the assembly tip (unless otherwise specified). Each lumen was perfused with isotonic saline at 0.02 ml min, optimal for the pico-manometric measurement of SO pressure (Craig et al. 2000). Papilla-SO pressures were recorded after visually locating the first manometer side-hole distal to the SO–duodenum junction (Fig. 1B). The pressure recorded with the manometric catheter placed external to but at the level of the SO was used as zero.
Experimental protocol
The bath medium was refreshed before gradually warming to 35 ± 1°C. The experimental protocol was commenced after tissue equilibration for 30–45 min. All preparations displayed regular spontaneous contractions for the duration of the experiment. Imposed CBD pressure was increased continuously from 0 to 17 mmHg. In separate experiments imposed duodenal pressure was increased as well, step-wise from 0 to 4 or 7 mmHg, by elevating the outflow reservoir 5 or 10 cm, respectively. Manometric pressures and TSF were recorded continuously.
Data acquisition and analysis
All SO manometry and in- and outflow data were recorded using a MacLab recording system, with Chart 3.6 software (ADInstruments Pty, Ltd, Castle Hill, NSW, Australia). Data of inflow pressure versus rate of TSF was fitted to a Medium Lowess Curve using GraphPad Prism (GraphPad Software, Inc., San Diego, CA, USA). The maximum flow rate as a function of pressure in the inflow reservoir was fitted with a Microsoft Excel (Microsoft Corporation, Redmond, WA. Australia) line of best fit. Group data are presented as mean ± standard error of the mean (S.E.M.).
Interpretation of manometric recordings
Perfused manometry catheters were constructed with side-holes at defined positions, chosen to correspond to the optimal SO locations (Fig. 1B). However, obtaining recordings from the CBD, proximal-SO, body-SO and papilla-SO in each preparation and during the entire experimental procedure was not always possible due to variation in possum SO length and variable shortening of the SO during longitudinal contraction. Consequently, information acquired from many experiments was combined to fully interpret the SO motility events. Several hydrostatic aspects of manometry need to be considered to interpret manometric recordings. Perfused side-hole manometry measures two types of pressures, fluid pressure and lumen-occluding pressure (Fig. 2A). Fluid pressure is recognized when identical pressure profiles are recorded simultaneously from two or more independent adjacent side-holes, indicating that the side-holes are located within fluid enclosed in a common cavity. Pressure applied at any point to a common cavity results in instantaneous increase in pressure at all recording sites within that common cavity. Lumen-occluding pressure that is localized to an individual site is recorded when contraction or pressure at that site results in local closure of the SO lumen and restriction of fluid flow through the side-hole.
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Figure 2C allows interpretation of additional features in manometric recordings. Peristaltic-like activity of the SO was recognized from the sequential occurrence of lumen-occluding contractions within the body-SO. The diagrams also show how small variations in location of recording site can occur as a consequence of SO movement during longitudinal muscle contraction.
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TSF with the SO attached was always less than the maximum flow rate through the inflow catheter (Fig. 3A). The curve of TSF at imposed CBD pressures of 0–17 mmHg (and 0 mmHg duodenal pressure) was a three-component function of imposed pressure with two linear segments and an inflection. The initial linear segment consisted of slow TSF at imposed pressures < 3.0 mmHg. This was followed by an inflection in TSF rate at pressures of 3–5 mmHg. The second linear segment consisted of rapid TSF and occurred at imposed pressures > 5 mmHg.
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Temporal relationships between SO pressure profiles and TSF at low inflow pressures
Concurrent manometric pressure recordings from the CBD, proximal-SO, body-SO and papilla-SO, and TSF data extracted from an experiment are presented in Fig. 4A. The imposed CBD pressure was 1.9 mmHg (0 mmHg duodenal pressure). In order to recognize the temporal relationships between events along the CBD–SO preparation, corresponding segments of these data were superimposed (Fig. 4B). The following sequential events were revealed.
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This sequence of events describes the pumping of fluid through the SO, and involves a propagating contraction through the body-SO accompanied by the opening of the papilla. Figure 4D diagrammatically summarizes our interpretation of the peristaltic-like pump function of the SO. In these experiments, at 1.5 and 2.5 mmHg of imposed CBD pressure, every peristaltic contraction pumped 17 ± 3 and 27 ± 4 µl of fluid, respectively. At these pressures the average duration of proximal-SO contraction (i.e. a pumping event) was 3.0 ± 0.2 s (range 2–4 s; n= 9).
To obtain more information concerning motor activity of the papilla-SO, a manometry catheter was constructed with side-holes positioned at 2, 4, 6 and 8 mm from the tip (Fig. 4E). Recordings with this catheter demonstrated that the papilla-SO was 4 mm in length and revealed significant differences in the pressure profiles of the papilla-SO within this small region (first three recording sites from the catheter tip). Such diverse complex pressure profiles make quantification of papilla-SO contractions by general manometric indices, such as amplitude and frequency, very difficult.
Temporal relationships between SO pressure profiles and TSF at high inflow pressure
When imposed CBD pressure was greater than 5 mmHg, with no duodenal pressure applied, TSF primarily occurred as passive flow during body-SO quiescence (Fig. 5). The following sequence of events was evident.
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As imposed CBD pressure was increased, passive TSF became greater than TSF due to pumping (Fig. 5B; 118.0 ± 41.7 versus 58.5 ± 13.5 µl (n= 9) at 7 mmHg imposed pressure).
Effect of elevated duodenal pressure on TSF
When duodenal pressure was elevated, increasing CBD pressure (0–17 mmHg, Fig. 6A and B) also produced the basic three-component TSF relationship (slow and rapid linear, and inflection) that was observed with 0 mmHg duodenal pressure. However, elevated duodenal pressure increased the threshold pressure at which the SO opened. When duodenal pressures were 0, 4 or 7 mmHg the SO opening pressures were 3.4 ± 0.3 (n= 9), 6.4 ± 0.6 (n= 8) or 9.2 ± 0.3 mmHg (n= 7), respectively.
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The SO has been categorized as predominantly functioning as a pump or as a resistor of TSF, and this distinction has traditionally been regarded as species dependent. The SO of the guinea-pig, American opossum, rabbit and prairie dog have been classified as pumps, whereas those of the pig, cat, human and Australian possum have been classified as resistors (Calabuig et al. 1990; Liu et al. 1992; Sand et al. 1997; Toouli & Craig, 2000). Our findings suggest that TSF via pumping or passive flow may not be entirely species specific and is a function of the pressure difference between the CBD and duodenum. Thus pressures utilized during previous experimental investigations may have allowed only one form of TSF to be evident. In a previous study (Toouli et al. 1983) we showed cineradiographically that the American opossum SO displayed pumping as well as passive TSF when pressure was increased within the CBD–SO.
The transition of TSF from pumped to passive flow may be of physiological significance. During the fasted state approximately 25% of bile produced by the liver passes through the SO (Shaffer et al. 1980). This bile flow, which occurs at low CBD pressure, may be achieved by SO pumping and may act to prevent bile stasis and the build-up of ‘sludge’ that is thought to be responsible for biliary stones. Our results suggest that, during gallbladder contraction following a meal, the increased bile flow becomes mainly passive with the papilla-SO acting as a resistor. This passive flow behaviour maintains a comparatively low pressure within the biliary tract. While our data suggest that usually it is the papilla-SO that resists TSF, the proximal-SO and body-SO also can behave as resistors to flow when their contraction frequency is stimulated, preventing SO filling (Toouli et al. 1983; Venu et al. 1983; Helm et al. 1988; Saccone et al. 1992; Huang et al. 1998; Chen et al. 2000). Generally, however, papilla-SO contraction is responsible for resisting flow, causing the accumulation of fluid in the SO–CBD, and thus generating SO basal pressure.
Our findings demonstrate that the activity of the papilla-SO plays a pivotal role in regulating TSF. There have been a few previous descriptions of complex patterns of papilla-SO manometry (Lueth, 1932; Toouli et al. 1983). Reports of papilla-SO muscle relaxation, however, are rare. Shelhamer (1973) reported papilla-SO ‘relaxation’ in the American opossum using manometry. In contrast, in a later study of the American opossum (Toouli et al. 1983), papilla-SO ‘relaxation’ was not demonstrated by manometry; however, our present data suggest that manometric recording of papilla-SO relaxation can be partially masked by fluid pressure during TSF events (see Fig. 4C). It should be noted that manometry is unable to distinguish between the possibilities that the papilla-SO is tonically contracted but undergoes active relaxation, or is simply displaying phasic contractions. Another function attributed to the SO is the prevention of reflux of duodenal contents into the biliary and pancreatic systems. We observed that peak papilla-SO pressure occurs immediately after emptying of the SO, and this may be an important factor in preventing duodenal reflux into the biliary tract during this period when proximal-SO–body-SO pressures are minimal.
Evidence is accumulating that the possum biliary-SO may be composed of at least two SO regions, the proximal-SO and distal-SO, which respond differently to chemical and electrical stimuli in electrophysiological and in vitro functional studies (Lonovics et al. 1994; Parkman et al. 1998; Woods et al. 2000). Two studies in guinea pig (Vongalis et al. 1989; Hirose & Ito, 1991) have identified three neurally distinct SO regions that appear to correspond to the proximal-SO, body-SO and papilla-SO regions described in this current study. Generally, proximal-SO and body-SO contractions were closely correlated, with most contractions commencing at the proximal-SO and propagating through the body-SO. The body-SO contractions were weaker than proximal-SO contractions. Since papilla-SO opening was coordinated with body-SO contraction, thus facilitating TSF, body-SO contractions do not need to be powerful. Papilla-SO contraction occurred during periods of proximal-SO–body-SO quiescence.
The functional differences observed in the various regions of the possum SO may be due to local variation in the smooth muscle and/or innervation; however, this remains to be established. The findings from the present study suggest that the possum SO displays several functional similarities with other sphincters. For example, propagating proximal-SO–body-SO contractions that pump fluid toward the papilla-SO may be comparable to the peristaltic contractions of the body of the oesophagus. Furthermore the lower oesophageal sphincter is tonically contracted but actively relaxes to allow passage of luminal contents during a wave of oesophageal body contraction (Diamant, 1989); the papilla-SO may function in an analogous fashion. It is interesting that the origin of the musculature along the SO varies. In the possum, proximal-SO–body-SO musculature is predominantly continuous with the muscularis externa of the duodenum, with only a small contribution from muscle originating in tissue continuous with the duodenal submucosa and muscularis mucosa. However, all outer circular and inner longitudinal muscles surrounding the papilla-SO are contiguous with, respectively, duodenal submucosal and muscularis mucosal tissue (Padbury et al. 1993a). As with the papilla-SO, muscularis mucosa-derived muscle is prominent in the lower oesophagus (Clerc, 1983).
Innervation of the SO is complex, involving intricate intramural ganglionated networks of excitatory and inhibitory nerves (Vongalis et al. 1989; Padbury et al. 1993a; Lonovics et al. 1994; Sand et al. 1997; Talmage et al. 1997; Cox et al. 1998; Vogalis & Smith, 2000; Simula et al. 2001). Inhibitory junction potentials and neurones are more frequently found in the distal-SO (Vongalis et al. 1989; Hirose & Ito, 1991; Lonovics et al. 1994; Vogalis & Smith, 2000; Simula et al. 2001) while excitatory junction potentials and neurones are prominent in the proximal-SO (Vongalis et al. 1989; Hirose & Ito, 1991). Extrinsic neural reflexes and pathways between the gallbladder, CBD, duodenum and SO have also been reported by us and others (Wyatt, 1967; Thune et al. 1986, 1989; Padbury et al. 1993b; Saccone et al. 1994; Shibata et al. 1997; Simula et al. 1997; Kennedy & Mawe, 1998; Shafik, 1998; Mawe & Kennedy, 1999; Deng et al. 2000; Kennedy et al. 2000; Konomi et al. 2002). Thus in vivo preparations are likely to display even more complex patterns of motility under the influence of these additional extramural neural reflexes, gut hormones (e.g. CCK, motilin) and input from sympathetic and parasympathetic nerves. The complexity of the neuroanatomy very likely underlies the numerous functions performed by the SO. Despite the extensive intramural neural connections between the SO and the duodenal myenteric plexuses and the apparent continuity of the duodenal muscularis externa over the proximal-SO–body-SO, the results of the current experiments revealed an absence of synchrony between duodenal contractions and SO contractions, indicating that, in vitro, SO motility is temporally independent of the duodenum. It may be that these neural pathways are not involved in SO–duodenum contraction synchrony.
In conclusion, our findings from this study indicate that the SO regulates TSF by acting as a pump at low CBD pressure and as a resistor at higher CBD pressures. The papilla-SO plays a pivotal role in the expression of these functions. The complexity of the SO structure and function is such that clear definition of the SO segment(s) under study is essential to avoid potential confusion in the interpretation of temporal relationships and responses to pharmacological application. In vivo, SO preparations may demonstrate more complex motility patterns when extrinsic neural and hormonal systems are intact.
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References |
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