Department of Physiology and Cell Biology, University of Nevada, Reno, Nevada 89557
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ABSTRACT |
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The present study investigated the spatial
organization of electrical activity in the canine rectoanal region and
its relationship to motility patterns. Contraction and resting membrane
potential (Em) were measured from strips of
circular muscle isolated 0.5-8 cm from the anal verge. Rapid
frequency [25 cycles/min (cpm)] Em
oscillations (MPOs, 12 mV amplitude) were present across the thickness
of the internal anal sphincter (IAS; 0.5 cm) and
Em was constant (52 mV). Between the IAS and
the proximal rectum an 18 mV gradient in Em
developed across the muscle thickness with the submucosal edge at
70
mV and MPOs were replaced with slow waves (20 mV amplitude, 6 cpm).
Slow waves were of greatest amplitude at the submucosal edge.
Nifedipine (1 µM) abolished MPOs but not slow waves. Contractile
frequency changes were commensurate with the changes in pacemaker
frequency. Our results suggest that changing motility patterns in the
rectoanal region are associated with differences in the characteristics
of pacemaker potentials as well as differences in the sites from which
these potentials emanate.
interstitial cells of Cajal; smooth muscle; membrane potential; internal anal sphincter; rectum
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INTRODUCTION |
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THE RECTOANAL REGION REPRESENTS the final site for controlling the storage, transport, and evacuation of gastrointestinal contents. The internal anal sphincter (IAS) is a thickening of the circular muscle layer at the distal end of the rectum. Under most circumstances, the IAS is closed and contributes to maintaining continence of fecal matter, liquids, and gases, whereas during the defecation reflex, it briefly relaxes to allow the passage of fecal matter. In contrast, although the rectum is generally empty, it can serve as a final site of storage before defecation as well as participating in the defecation reflex. The IAS and rectum are, therefore, anatomically linked but subserve different physiological roles (7, 13, 16, 18). These functional differences are likely to be associated with significant differences in the motility patterns in the two regions as well as the mechanisms controlling this activity.
Spontaneous phasic contractile activity in gastrointestinal smooth muscle is generally associated with pacemaker potentials of varying time course and amplitude. In recent years, there has been mounting evidence that this activity is generated by specialized cells referred to as interstitial cells of Cajal (ICC) (14). In the human and canine colonic circular muscle layer, for example, slow waves arise from a specific population of ICC located at the submucosal edge of the muscle layer, and these give rise to a characteristic 1-cpm (4) or 6-cpm contractile rhythm (17), respectively. In contrast, although the IAS exhibits spontaneous contractile activity (8), much less is known about the mechanisms underlying this activity or the role that ICC play in this process.
The goal of the present study was to characterize pacemaker potentials of the canine rectoanal region over an 8-cm distance from distal IAS to proximal rectum and to correlate this with the spontaneous contractile activity occurring in this region. Special attention was given to determining the spatial characteristics of electrical activity in an effort to predict the sites from which pacemaker potentials may arise. Our results suggest that the characteristics of pacemaker potentials and the site from which they emanate changes from IAS to rectum. In Ref. 2 the anatomic distribution of putative pacemaker cells is examined.
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MATERIALS AND METHODS |
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Mongrel dogs of either sex (0.5-2 yr of age) were killed
with an overdose of pentobarbital sodium (100 mg/kg) administered into
the femoral vein. The pelvic rectoanal region was exposed by sawing
through the midline of the pelvic bone. Incisions were made on either
side of the rectum and through the skin adjacent to the anus to allow
removal of the last 10- to 12-cm portion of the gastrointestinal tract.
The dissected segment was cut open from anus to proximal rectum and
fecal material was removed. All adhering skeletal muscle and glands
were then removed after pinning the segment in a dissecting dish. Krebs
bicarbonate solution of the following composition was used (in mM)
118.5 NaCl, 4.7 KCl, 2.5 CaCl2, 1.2 MgCl2, 23.8 NaHCO3, 1.2 KH2PO4, and 11.0 dextrose. This solution had a pH of 7.4 at 37°C when bubbled to
equilibrium with 95% O2-5% CO2. All
experiments, unless otherwise stated, were performed in the presence of
1 µM atropine, 1 µM phentolamine, and 100 µM
N-nitro-L-arginine
(L-NNA) to remove the potential contributions of
cholinergic, adrenergic, and nitrergic nerves.
The rectoanal region was pinned taut (but not overtly stretched) in a dissecting dish. The distal-most extension of the IAS was identified and is referred to in this study as the anal verge. Two-millimeter-wide strips of the tunica muscularis were created at various distances from the anal verge by cutting the tissue parallel to the circular muscle fibers with a knife consisting of a pair of parallel scalpel blades set 2 mm apart. The rectoanal lengths in vivo are likely to be slightly less than those reported here, because the muscle in vivo is untethered. All of the mucosa and most of the underlying submucosa was removed from strips by sharp dissection leaving behind enough connective tissue to pin the preparation in place and ensure the integrity of cells at the submucosal edge of the circular muscle layer. For contractile experiments whole muscle strips isolated at 1, 2, 4, and 8 cm from the anal verge were attached with sutures to a stable mount and to a Gould strain gauge and immersed in tissue baths containing 10 ml Krebs bicarbonate solution oxygenated and maintained at 37°C. A basal tension of 1 g was applied to muscle strips. Over the next 30-60 min, this applied tension declined to ~0.2-0.3 g (as assessed from periods of minimum spontaneous rhythmic contraction). Contractile patterns were evaluated once baseline tension had stabilized.
For intracellular measurements, muscle strips isolated 0.5, 3, 5, and 8 cm from the anal verge were pinned cross-sectional to the floor of an
electrophysiological chamber. In this way, the entire thickness of the
circular muscle layer could be visualized, making it possible to record
from specific positions within the muscle layer. The position of each
impalement within the circular muscle layer has been expressed as the
%distance from the submucosal edge. In some experiments, the circular
muscle of the IAS and proximal rectum were further divided into
sections by cutting through the muscle layer parallel to the circular
muscle fibers by using a microsurgical knife. Muscle cells were impaled
with glass microelectrodes filled with 3 M KCl and having resistances ranging from 40 to 80 M. Impalements were accepted on the basis of
previously discussed criteria (15). Membrane potential
(Em) was measured with a high input impedance
electrometer (model Duo 773; World Precision Instruments), and outputs
were displayed on an oscilloscope (model 3091; Nicolet). Analog
electrical signals were digitized and recorded on video tape (model
875; Vetter). Data were also stored and analyzed by computer by using a
data acquisition program (AcqKnowledge; Biopac Systems).
Analysis of data. Several different parameters of electrical activity were tabulated. Resting Em of cells was determined as the most negative potential attained between membrane potential oscillations (MPOs) in the IAS or slow waves in the rectal region. Slow-wave amplitude was determined as the peak level of depolarization attained during the plateau phase. In cases where both MPOs and slow waves were recorded at the myenteric edge of the rectum, only the slow-wave frequency was used to determine pacemaker potential frequency. In addition, when slow waves were very small in amplitude at this edge, slow-wave parameters were measured from those portions of the recording in which slow waves were most readily distinguishable.
Because the tone of the IAS fluctuated, contractile amplitude was determined by averaging the peak contractions achieved during a 20-min time period (3-6 peaks). Basal tension was taken as the minimum level of tension occurring during this period of time. Contractile amplitude therefore included both the slow fluctuation in tone plus the small rapid superimposed contractions. Spontaneous contractile amplitude in rectal segments was determined as the average of the five largest phasic contractions occurring during a 20-min period of time. It was not possible to elicit a maximum agonist contraction in the same tissue in which spontaneous contractions were measured because these tissues were continually bathed in adrenergic and cholinergic antagonists. Furthermore, the addition of high concentrations of norepinephrine in the absence of blockers produces changes in the spontaneous contractile activity of the IAS that persists for hours. Thus maximum contractile amplitude was determined in nine additional tissues from each region that had not been exposed to blockers. Norepinephrine (100 µM) was used for IAS strips (1 and 2 cm) and 1 mM acetylcholine was used for rectal strips (4 and 8 cm), because these agonists and concentrations produce maximum contractions (21). Spontaneous contractile responses were then normalized to these mean maximum responses. Significant differences between means in the four rectoanal regions was determined by using one-way ANOVA followed by a Tukey-Kramer multiple comparisons test. Means were considered significantly different when P < 0.05. Only one muscle strip per rectoanal region was used from any one animal; thus n values represent both the number of animals and the number of muscle strips used.Drugs. Tetrodotoxin, atropine sulfate, phentolamine, L-NNA, acetylcholine, norepinephrine, and nifedipine were all purchased from Sigma (St. Louis, MO). Nifedipine was dissolved in ethanol. Other drugs were dissolved in distilled water.
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RESULTS |
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Anatomic characterization of the rectoanal region.
The gross morphology of the rectoanal region was examined at the light
level by cutting a thin strip of muscle perpendicular to the circular
muscle fibers from the distal IAS to the proximal rectum. Strips were
then pinned cross-sectional to reveal the circular and longitudinal
muscle layers throughout this region. In the rectum, a thin, densely
packed, circular muscle layer was apparent. The longitudinal muscle
layer was of approximately the same thickness as the circular muscle
layer and was separated from it by a thin connective tissue space (Fig.
1). In the distal direction, there was a
progressive widening of the separation between longitudinal and
circular muscle layers. At the level of the IAS the longitudinal muscle
layer dissipated, no longer forming a discrete structure. In contrast,
the circular muscle layer becomes thicker in the distal direction,
particularly within the last 2-cm section. There was also a clear
change in the density of muscle fibers from rectum to IAS with the
rectum being compact and dense, whereas the IAS was diffuse and
separated into bundles (Fig. 1).
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Electrical activity in the IAS and rectum.
Em was recorded from strips of muscle isolated
from various sites 0.5-8 cm from the anal verge. Muscle strips
were oriented so that the entire muscle thickness was visible and
recordings were made at 5, 50, and 95% distance from the submucosal
edge. In the IAS (0.5 cm) there was no significant difference in the level of resting Em across the thickness of the
muscle layer (Fig. 3A). In
contrast, an 18-mV gradient in Em was observed
from the submucosal edge (70.2 ± 3 mV) to the myenteric edge
(
51.8 ± 3.2 mV) of the proximal rectum (8 cm). Between these
two extremes, there was a gradual development of more negative
potentials at the submucosal edge, whereas the myenteric edge remained
unchanged (n = 4-6; Fig. 3A).
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Effect of nifedipine on electrical activity.
Nifedipine is often used as a diagnostic tool to evaluate whether an
electrical oscillation is generated by ICC, because in most cases,
pacemaker potentials are resistant to dihydropyridines (24). The ability of nifedipine to inhibit pacemaker
potentials in strips of IAS (1 cm) and rectum (8 cm) was therefore
tested. Nifedipine (1 µM) entirely abolished MPOs recorded near
either the myenteric (95%) or submucosal edge (5%) of the IAS as seen in Fig. 7A (n = 4). There was no significant change in the value of mean resting
Em (51 ± 2 vs.
50 ± 4 mV,
n = 4). In contrast to the IAS, the area of
depolarization occurring during the rectal slow wave (recorded 5% from
the submucosal edge) was only reduced by 51.5 ± 13% in six of
eight tissues, whereas in two tissues, slow waves were abolished. The
reduction in area was predominantly due to a reduction in the plateau
phase of the slow wave (Fig. 7B). Nifedipine did not
significantly change the rectal resting Em
(
71 ± 2 vs.
69 ± 3 mV, n = 8).
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Contractile activity in the rectoanal region.
All muscle strips (1-8 cm; n = 40) exhibited
spontaneous contractile activity. Examples of this activity are shown
in Fig. 8A. The contractile
pattern in the IAS was complex and consisted of a slow fluctuating
level of tone with superimposed high frequency contractions that
averaged 19 ± 1.2 cpm at 1 cm (Fig. 8B). In contrast,
baseline tension between contractions in the rectum remained
constant. Contractile frequency declined in the proximal direction and
averaged 5.7 ± 0.7 cpm in the 8 cm muscle strips (Fig.
8B). There was no significant difference in the amplitude of
spontaneous contractions between muscle regions when raw contractile amplitudes were tabulated (Fig. 8C). However, maximum
contractile amplitude significantly increased in the proximal
direction although each strip was cut to the same width (Fig.
8D). This difference likely reflects the greater
quantity of connective tissue present distally (2). When
spontaneous contractions were normalized to the maximum contraction,
there was a significant decline in the proximal direction (Fig.
8E).
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DISCUSSION |
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The rectum and IAS are adjacent structures that are anatomically linked but subserve different physiological roles. These differences are associated with changes in the motility patterns in progressively more distal segments including an increase in contractile frequency accompanied by an increase in the frequency of pacemaker potentials. A gradient in Em also exists across the thickness of the rectum with an apparent submucosal site of origin for slow waves. In the distal direction the electrical gradient becomes normalized and pacemaker potentials appear to emanate from throughout the muscle layer. The significance of these changes with regard to the mechanisms responsible for generating pacemaker potentials are discussed below.
Spontaneous phasic contractions and oscillations in Em occurred in all muscle strips isolated from the rectoanal region. However, characteristics of both changed significantly in the distal direction. The slow waves of the proximal rectum were very similar in time course to those previously described in the canine colon (17). And like colonic slow waves (24), rectal slow waves were not blocked by nifedipine. Several studies have suggested that slow waves in the canine colon arise from ICC located at the submucosal edge of the circular muscle layer (9, 12, 17, 22, 23, 25). These studies have shown that 1) a plexus of ICC exists at the submucosal edge; 2) slow waves can be recorded from ICC enzymatically dispersed from this edge by using the patch clamp technique; 3) in the intact tissue, slow waves are of greatest amplitude along the submucosal edge and decline almost to zero at the myenteric edge; and 4) slow waves are absent from the bulk smooth muscle when a thin strip of muscle containing the submucosal edge is removed from the tissue or is chemically damaged.
Our studies in the canine rectum also revealed a gradient in slow-wave amplitude across the thickness of the circular muscle layer. Furthermore, we found that when the muscle layer was divided into submucosal and myenteric halves that slow waves occurred only in the submucosal half. Thus rectal slow waves, like those of the colon, are likely to be generated by submucosal ICC. Rectal slow waves differ in that their amplitude is only ~50% of those in the colon. In more distal segments, the amplitude of pacemaker potentials declines even further. Despite this, the amplitude of spontaneous contractions throughout the rectoanal region is relatively constant (see Fig. 8). This situation may result because of the concurrent, reduced polarization of resting Em in the distal direction. A reduced resting Em would ensure that smaller amplitude oscillations still exceed the electrical threshold (19) for contraction. In contrast to rectal slow waves, there was no gradient in the amplitude of MPOs across the thickness of the IAS. Furthermore, when the IAS was subdivided (i.e., myenteric, interior, and submucosal sections) each subsection exhibited MPOs of equal amplitude. Thus unlike the rectum, no distinct site of origin was apparent for MPOs, but rather, they appeared to arise from pacemaker cells located throughout the circular muscle layer.
Because MPOs were abolished by the L-type calcium channel blocker nifedipine, one might argue that they must be of smooth muscle origin. Patch clamp studies in recent years suggest that the pacemaker currents of ICC are due to nonselective cation channels that are insensitive to dihydropyridines (6, 20). However, these studies involved the use of cultured ICC that may lead to downregulation of calcium channel activity. In fact, in one of the few patch clamp studies (10) completed using freshly dispersed ICC, it was reported that both L-type and T-type calcium channel currents were present. Interestingly, the MPOs recorded from the IAS in our study were indistinguishable in time course, amplitude, and frequency from the MPOs previously described for the myenteric edge of the canine colonic circular muscle layer (17). Previous studies have suggested that canine colonic MPOs arise from ICC located at the myenteric edge because: 1) a plexus of ICC is present at this edge (22), 2) colonic MPOs are of greatest amplitude in the vicinity of the ICC plexus and decline with distance away from this region (17), and 3) MPOs are absent from muscle strips that lack the myenteric edge (3). A further resemblance between colonic and sphincteric MPOs is that both are abolished by nifedipine (Ref. 3 and present study). Given these similarities it is tempting to speculate that the MPOs generated in both regions are due to a specific subset of ICC more highly dependent on L-type calcium channel activity for the generation and/or the conduction of pacemaker potentials to the adjacent smooth muscle. Because MPOs in the IAS are recorded throughout the muscle layer and in subsegments of this layer, we would predict that ICC are likely to be diffusely distributed rather than being restricted to a dense plexus region at one or the other edge.
Our study suggests that the transition from IAS to rectum is gradual rather than abrupt. This is true both anatomically in that one sees a gradual reduction in circular muscle thickness in the proximal direction as well as a gradual increased polarization of Em at the submucosal edge. On the other hand, one can loosely describe the sphincter and its transition into rectum as encompassing a 3-cm region of the distal GI tract, because over this distance, pacemaker potentials decline from 25 cpm at 0.5 cm to the rectal/colonic frequency of 5-6 cpm at 3 cm. In a related study, we have also found that motor innervation at 1 cm is exclusively sympathetic (i.e., abolished by guanethidine), whereas it constitutes ~80% of the neural response at 2 cm and 20% at 4 cm (21).
In the IAS, a complex pattern of spontaneous contractile activity was observed consisting of slow fluctuations in tone with superimposed contractions occurring at the MPO frequency. Intracellular recordings throughout the muscle layer revealed MPO activity. A previous study (8) of this region reported that each rapid frequency contraction was associated with an MPO; however, no specific electrical event was described for slow fluctuations in tone. In our study, electrical and contractile activity were recorded from different preparations; thus we cannot be certain as to the origin of the slow fluctuations in tone. It is likely that MPOs produce summed contractions as well as high-frequency phasic contractions. The tendency of the muscle to produce summed contractions is greatly enhanced with low frequency sympathetic nerve stimulation, e.g., even 1-Hz nerve stimulation of the canine IAS generates a tonic contracture (21). Spontaneous contractile amplitude of IAS muscle strips is far below what the muscle is capable of generating (i.e., 12.5% of maximum at 1 cm, see Fig. 8). In contrast, 20 Hz nerve stimulation produces an 85% maximum contraction of the same muscle strips (21). Thus it is not surprising that removal of sympathetic neural input to the IAS in vivo leads to a substantial decline in sphincteric tone (e.g., see Refs. 1 and 13). In summary, the function of the IAS (i.e., to generate tone to aid in maintaining continence) is likely to be achieved through the combined actions of sympathetic nerves in concert with the inherent tendency of this tissue to produce ongoing high-frequency electrical oscillations.
The pattern of spontaneous contractions in the canine rectum and the underlying slow waves are very similar to the contractile and electrical activity of the canine colon. However, the amplitude of spontaneous rectal contractions is considerably larger, i.e., rectal muscles generate spontaneous contractions that are 10 and 7% of maximum at 4 and 8 cm, respectively, whereas spontaneous contractions of the canine colon are generally < 1% of maximum (5). Whereas the colon has a reabsorptive role, the rectum usually remains empty via nonpropulsive segmentation. Greater spontaneous contractile activity in this region would aid in this function.
The development of an electrical gradient across the thickness of the circular muscle layer from IAS to rectum is both novel and interesting. This transition occurs because the submucosal edge becomes progressively more negative, whereas the myenteric edge remains at the same potential. Associated with the development of an electrical gradient is the transformation of pacemaker potentials from MPOs to slow waves along with an apparent change in the site of origin of pacemaker potentials. Previous studies (11) of the proximal colon suggest that submucosal ICC are responsible for drawing the Em in this region down to more negative levels. Thus the appearance of slow waves in the rectum is likely to be correlated with the appearance of a discrete submucosal plexus of ICC. In Ref. 2 the anatomic characteristics of ICC and their relationship to smooth muscle and nerves in the canine rectoanal region are explored.
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ACKNOWLEDGEMENTS |
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This work was supported by National Institute of Diabetes and Digestive and Kidney Diseases Grant RO1-DK-54490 (to K. D. Keef and S. M. Ward).
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FOOTNOTES |
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Address for reprint requests and other correspondence: K. D. Keef, Dept. of Physiology and Cell Biology, MS 352, Univ. of Nevada, Reno, NV 89557 (E-mail: Kathy{at}physio.unr.edu).
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. Section 1734 solely to indicate this fact.
First published January 22, 2003;10.1152/ajpgi.00295.2002
Received 22 July 2002; accepted in final form 13 January 2003.
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REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
1.
Carlstedt, A,
Fasth S,
Hulten L,
and
Nordgren S.
The sympathetic innervation of the internal anal sphincter and rectum in the cat.
Acta Physiol Scand
133:
423-431,
1988[ISI][Medline].
2.
Horiguchi, K,
Keef KD,
and
Ward SM.
Distribution of interstitial cells of Cajal in the tunica muscularis of the canine rectoanal region.
Am J Physiol Gastrointest Liver Physiol
284:
G756-G767,
2003.
3.
Keef, KD,
Anderson U,
O'Driscoll K,
Ward SM,
and
Sanders KM.
Electrical activity induced by nitric oxide in canine colonic circular muscle.
Am J Physiol Gastrointest Liver Physiol
282:
G123-G129,
2002
4.
Keef, KD,
Du C,
Ward SM,
McGregor B,
and
Sanders KM.
Enteric inhibitory neural regulation of human colonic circular muscle: role of nitric oxide.
Gastroenterology
105:
1009-1016,
1993[ISI][Medline].
5.
Keef, KD,
Ward SM,
Stevens RJ,
Frey BW,
and
Sanders KM.
Electrical and mechanical effects of acetylcholine and substance P in subregions of canine colon.
Am J Physiol Gastrointest Liver Physiol
262:
G298-G307,
1992
6.
Koh, SD,
Sanders KM,
and
Ward SM.
Spontaneous electrical rhythmicity in cultured interstitial cells of Cajal from the murine small intestine.
J Physiol
513:
203-213,
1998
7.
Krier, J.
Motor function of anorectum and pelvic floor musculature.
In: Handbook of Physiology. The Gastrointestinal System. Motility and Circulation. Bethesda, MD: Am Physiol Soc, 1989, sect. 6, vol. I, pt. 1, chapt. 27, p. 1025-1053.
8.
Kubota, M,
Suita S,
and
Szurszewski JH.
Membrane properties and the neuro-effector transmission of smooth muscle cells in the canine internal anal sphincter.
J Smooth Muscle Res
34:
173-184,
1998[Medline].
9.
Langton, P,
Ward SM,
Carl A,
Norell MA,
and
Sanders KM.
Spontaneous electrical activity of interstitial cells of Cajal isolated from canine proximal colon.
Proc Natl Acad Sci USA
86:
7280-7284,
1989[Abstract].
10.
Lee, HK,
and
Sanders KM.
Comparison of ionic currents from interstitial cells and smooth muscle cells of canine colon.
J Physiol
460:
135-152,
1993[Abstract].
11.
Liu, LW,
and
Huizinga JD.
Electrical coupling of circular muscle to longitudinal muscle and interstitial cells of Cajal in canine colon.
J Physiol
470:
445-461,
1993[Abstract].
12.
Liu, LW,
Thuneberg L,
and
Huizinga JD.
Selective lesioning of interstitial cells of Cajal by methylene blue and light leads to loss of slow waves.
Am J Physiol Gastrointest Liver Physiol
266:
G485-G496,
1994
13.
Penninckx, F,
Lestar B,
and
Kerremans R.
The internal anal sphincter: mechanisms of control and its role in maintaining anal continence.
Baillieres Clin Gastroenterol
6:
193-214,
1992[ISI][Medline].
14.
Sanders, KM.
A case for interstitial cells of Cajal as pacemakers and mediators of neurotransmission in the gastrointestinal tract.
Gastroenterology
111:
492-515,
1996[ISI][Medline].
15.
Sanders, KM,
and
Smith TK.
Motoneurones of the submucous plexus regulate electrical activity of the circular muscle of canine proximal colon.
J Physiol
380:
293-310,
1986[Abstract].
16.
Sangwan, YP,
and
Solla JA.
Internal anal sphincter: advances and insights.
Dis Colon Rectum
41:
1297-1311,
1998[ISI][Medline].
17.
Smith, TK,
Reed JB,
and
Sanders KM.
Origin and propagation of electrical slow waves in circular muscle of canine proximal colon.
Am J Physiol Cell Physiol
252:
C215-C224,
1987
18.
Speakman, CT.
Pharmacology of the internal anal sphincter and abnormalities in faecal incontinence.
Eur J Gastroenterol Hepatol
9:
442-446,
1997[ISI][Medline].
19.
Szurszewski, JH.
Electrical basis for gastrointestinal motility.
In: Physiology of the Gastrointestinal Tract, edited by Johnson LR.. New York: Raven, 1987, p. 383-423.
20.
Thomsen, L,
Robinson TL,
Lee JC,
Farraway LA,
Hughes MJ,
Andrews DW,
and
Huizinga JD.
Interstitial cells of Cajal generate a rhythmic pacemaker current.
Nat Med
4:
848-851,
1998[ISI][Medline].
21.
Tichenor, SD,
Buxton IL,
Johnson P,
O'Driscoll K,
and
Keef KD.
Excitatory motor innervation in the canine rectoanal region: role of changing receptor populations.
Br J Pharmacol
137:
1321-1329,
2002
22.
Torihashi, S,
Gerthoffer WT,
Kabayashi S,
and
Sanders KM.
Identification and classification of interstitial cells in the canine proximal colon by ultrastructure and immunocytochemistry.
Histochemistry
101:
169-183,
1994[ISI][Medline].
23.
Ward, SM,
Burke EP,
and
Sanders KM.
Use of rhodamine 123 to label and lesion interstitial cells of Cajal in canine colonic circular muscle.
Anat Embryol (Berl)
182:
215-224,
1990[ISI][Medline].
24.
Ward, SM,
and
Sanders KM.
Dependence of electrical slow waves of canine colonic smooth muscle on calcium gradient.
J Physiol
455:
307-319,
1992[Abstract].
25.
Xue, C,
Ward SM,
Shuttleworth CW,
and
Sanders KM.
Identification of interstitial cells in canine proximal colon using NADH diaphorase histochemistry.
Histochemistry
99:
373-384,
1993[ISI][Medline].