Humancolonicsmoothmuscle: spontaneous ...

Gut, 1986, 27, 1006-1013

Human colonic smooth muscle: spontaneouscontractile activity and response to stretchRtC GILL, K R COTE, K L BOWES, AND Y J KINGMA

From the Departments ofSurgery and Electrical Engineering, University ofAlberta, Edmonton, Alberta,Canada

SUMMARY The length dependence of the spontaneous contractile activity of human colonicmuscle was assessed in vitro. Muscle obtained from the right colon was more distensible than thatof the left colon. This was true for all muscle layers. Maximum spontaneous active stress wasexerted by both circular and longitudinal muscle layers of the right colon at greater degrees ofstretch (p<0-001) than those of the left colon. The contractile frequency of longitudinallyoriented strips increased with length. The contractile frequency of intertaenial longitudinallyoriented strips from the right colon was lower (p<0001) than that of strips from the left colon.The contractile frequency of circularly-oriented strips from the right colon (6.25+±038 min) was

higher (p<0-001) than that of strips from the left colon (3-35±0-35 min). The human colonappears to consist of two distinct areas based on the mechanical behaviour of the smooth muscleduring spontaneous contraction.

The human large intestine may be subdivided intoright and left parts on the basis of differingembryology, blood supply, innervation andfunction.1 The right colon has a greater diameterthan that of the left, both on abdominalradiographs3 and post-mortem,4 and also appearsmore distensible than the left, being the area ofgreatest distension in the face of a distal colonicobstruction. Colonic pathology may also show aparticular pattern of localisation. Perforation of thecolon occurs most frequently in the caecum5 whilstdiverticular disease is most common in the descend-ing and sigmoid colon.6 Diverticular disease hasbeen associated with an abnormal response of thecolonic muscle to stretch. 8

In view of these observations, it would seemprobable that regional variations in colonic functionand pathology reflect an underlying difference in themechanical properties of muscle along the length ofthe human colon. Indeed, the spontaneous contrac-tions of human colonic muscle and their modulationby intrinsic neural discharge and stretch appearto be of considerable importance as colonic func-tion, albeit impaired, continues after extrinsicdenervation.9-11The spontaneous contractile activity of colonic

Address for correspondence: Richard C Gill, GI Science Research Unit, TheLondon Hospital Medical College, London El 2AJ.Received for publication 21 February 1986.

muscle obtained from several animal species hasbeen reported although little is known concerningthe dependence of these spontaneous contractionsupon muscle length. The majority of studies concern-ing the length dependence of colonic muscle havebeen conducted after the abolition of spontaneouscontractions in order to free the contractile elementand permit an analysis of the passive properties ofthe muscle.12-15 In these studies, the active contrac-tile properties of the muscle were assessed byexternal stimulation either electrically,12-14 by K+-depolarisation13 or by drugs such as acetylcholine orcarbachol.13 A recent in vitro study,16 however,established the length dependence of taenia fromguinea-pig caecum during spontaneous contraction.As far as human taenia coli is concerned, its

spontaneous contractile activity in vitro17-19 appearssimilar to that of the colon from the guinea pig,20pig2i and even the non-taeniated longitudinal musclelayer of the dog.22 The length dependence of humantaenia coli during spontaneous contraction is notknown. The spontaneous contractile activity of theintertaenial longitudinal muscle layer of the humancolon and its length-dependence have not beenestablished. Spontaneous contractile activity ofhuman colonic circular muscle layer in vitro is lessclearly defined than that of the cat,23 dog22 or pig.21Contractions of the circular muscle layer of thehuman colon at a frequency of 2-4 min have been

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Motility of human colon

reported by van Merwyk and Duthie19 who con-cluded that the mechanical response of this musclelayer to stretch was weak.

This present, in vitro, study was initiated toestablish the length dependence of the longitudinaland circular muscle layers of the human colonduring spontaneous contraction. Variations in thepatterns of spontaneous contraction and their mod-ulation by stretch have been assessed in musclestrips from different parts of the colon.

Methods

TISSUEStrips of human colonic muscle obtained at surgerywere studied in vitro. Their contractile force andfrequency in response to stretch were assessed.

ACQUISITIONTissue was obtained from 22 patients undergoingpartial colectomy for carcinoma. After resection ofthe diseased portion, a ring of colon (approximately2-5 cm long) was removed from either the proximalor distal margin of the future anastomotic site; theblood supply of the segment was maintained untilthe moment of excision. Specimens were im-mediately placed in oxygenated Krebs' Ringersolution. Six specimens from ascending, five fromtransverse, four from descending and seven fromsigmoid colon were obtained.

PREPARATIONThe colonic segments were opened along the borderof a taenia, carefully cleaned to prevent soiling ofthe muscle layers with faecal content, and pinned tothe surface of a dissecting dish filled with oxygen-ated Krebs' Ringer solution. The mucosa andsubmucosa were cut off. Strips of muscle, approxi-mately 4 mm wide, were cut with the long axis lyingin the direction of the muscle fibres. Longitudinally-oriented strips (10-15 mm long) were obtained bothfrom the taenia and the region between taeniae.Circularly oriented strips (8-10 mm long) weretaken from the intertaenial region. Strips werepinned at one end to the Sylgard surface of ahorizontal tissue chamber maintained at 37± 1°Cand continuously superfused with oxygenatedKrebs' Ringer solution. Care was taken during thisprocedure to avoid stretching the preparations.

RECORDING TECHNIQUEContractile activity of the muscle strips was re-corded with a force displacement transducer (GrassFT03). Each muscle strip was attached to a forcetransducer with surgical 4-0 silk thread. Recordingswere made of the contractile activity on a BeckmanR511A polygraph.

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SOLUTIONSThe superfusate used was a modified Krebs' Ringersolution containing the following (mM): Na+, 139-2;K+, 5-4; Ca2 , 2-5; Mg2+, 1-2; Cl-, 125-1; HCO3,22-0; H2PO4, 1-2; glucose, 10-1. When equilibratedwith 95% 02, 5% CO2 gas mixture at 37TC, thissolution had a pH of 7.3-7-4.

EXPERIMENTAL PROTOCOLAt, the start of each experiment, strips weremounted slack (<005 mN force) in the tissuechamber and allowed to equilibrate for a period of30 min. The length of the strips was then measured.Subsequently, the length of the strips was increased,stepwise, by increments of 1 0 mm each 15 min. Theexperiment was concluded either when the musclestrip ceased to contract spontaneously or the upperlimit of the force transducer was reached (approxi-mately 250 mN). After the experiment, the stripswere removed from the tissue chamber, firmlyblotted without crushing, and weighed on a balance(Mettler B5) to the nearest 0.1 mg.

CROSS SECTIONAL AREAThe cross sectional area (A) of each muscle strip wascalculated from the equation A=(M/pL). fT. M isthe mass (mg), p is the density (mg mm ) and L isthe length (mm) of the strip determined at rest. Thedensity of the tissue was taken as 1-056 mg mn- .

fT is the proportion of the muscle strip contributedby the longitudinal (taenial and intertaenial longi-tudinally oriented strips) or circular (intertaenialcircularly oriented strips) muscle layer. A value forfT was obtained from both histological and non-fixedtransverse sections of unstretched muscle from allparts of the colon.

ANALYSIS OF DATARecordings of contractile activity were analysedvisually. Three variables were assessed at eachincrement of length increase for each of the strips;active force, baseline force and contractile frequency.Active tension was taken to be the differencebetween maximum and minimum force during acycle of spontaneous contraction. Data wereobtained only from the last 10 min recording at eachincrement of length increase to allow a period of fiveminutes for the strip to accommodate the increase inlength. This was found to be necessary as once thebaseline tension of the strip was greater than zero,further increases in length produced an initialincrease in baseline tension which decayed curvi-linearly to a constant value. The major part of thisdecay was complete within two minutes. The lengthof the strip at which an increase in baseline tensionwas first recorded was measured and referred to as



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the initial length, Li. Subsequent changes in thelength of the strip were normalised with respect tothis initial length.The number of spontaneous contractions during

the 10 minute recording period were counted andthe contractile frequency determined.The active and baseline stresses in the longitu-

dinal axis of the muscle strip were calculated byexpressing the active and baseline tensions, respec-tively, per unit cross sectional area of the appropri-ate muscle layer.

STATISTICAL ANALYSISResults are expressed as the mean±SD, and analy-sed statistically using Student's t test.

Results

LENGTH STRESS CHARACTERISTICSSpontaneous contractile activity was recorded fromall the 46 longitudinally-oriented strips of taenia(Fig. la), 40 inter-taenial longitudinally-orientedstrips (Fig. lb) and 32 inter-taenial circularly-oriented strips (Fig. Ic, d).

In general, the spontaneous active force gener-ated increased with length up to a maximum.Further increases in length were associated with adecrease, or abolition, of active force generated anda marked increase in baseline stress (Fig. 2).

Differences were observed in the response of eachmuscle layer to stretch.

(a) Taenial longitudinally-oriented stripsMaximum active force generated occurred at2-40±0.15 Li in taenial longitudinally-oriented strips

Gill, Cote, Bowes, and Kingma

Table 1 Maximum active stress, and length at which thiswas observed, for longitudinal and circular muscle layers ofhuman colon in vitro

Colon region

AscendingT L-O*

I-T L-OtI-T C-Of

TransverseT L-O

I-T L-OI-T C-O

DescendingT L-O

I-T L-OI-T C-O

SigmoidT L-O

I-T L-OI-T C-O

Maximumactive stress(mN mm-2)

43±1191±1755+1 3

Length atmaximum activestress (L)

2-40±0-152-35±0-153-60±0-20

16±6 2-35±0-20118±29 2-35±0-156-4±0-8 3-60±0-40

22±7 1-80±0-20120±23 1*90±0-155-5±0 6 2-20±0-15

23±9128±1811-8±2-0

1*80+0-101*90±0-102-15+0-10

Values are mean±SD.*Taenial longitudinal muscle layer.tlntertaenial longitudinal muscle layer.tIntertaenial circular muscle layer.

obtained from the ascending and transverse - that is,right, colon, this being at a greater length (p<0-001)than that of strips obtained from the descending andsigmoid - that is, left, colon (1.80±0415 Li). Themaximum active force generated by the taenia of theascending colon was greater (p<0.001) than that ofthe taenia obtained from the transverse, descendingor sigmoid colon (Table).

mN O .

1 mn

mN10]

mN 5] ~ ~0

1min

0mN5]

1 min

Fig. 1 Spontaneous contractile activity recordedfromtaenial (A) and intertaenial (B) longitudinal strips obtainedfrom the transverse colon at 1-15 Li. Note the differentpattern ofspontaneous contractions recordedfromintertaenial circular strips obtainedfrom the left (C) andright (D) transverse colon at 2-2 Li.

(b) Inter-taenial longitudinally oriented stripsMaximum active stress was recorded at 2 35±0 15 Liin inter-taenial longitudinally-oriented strips fromthe ascending and transverse colon, this being agreater length (p<0.001) than that of strips from thedescending and sigmoid colon (1.90±0-15 Li).These values are not significantly different(p> 0-1) from those of taenia obtained fromcorresponding regions of the human colon. Themaximum active stress exerted by the inter-taeniallongitudinal muscle layer did not vary significantly(p> 0-25) between strips obtained from differentparts of the human colon but was, in all cases,greater (p<0-001) than that of the taenia (Table).

(c) Circular stripsMaximum active stress was recorded at 3 60±0.30 Liin inter-taenial circular strips obtained from theascending and transverse colon, this being at a



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Longitudinal(Taenial )

I;-A

xi4

150-

1001

501

Longitudinal( Inter-taenial )

2 3 4

150-

100

50-

42 3 4

1501

2 3 4

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30

2 3 4

I.E201

3C

2C

707~~~~~~-

3 4

1 2 3 4 5

I -.

1 4 5

3i4

2 3 ~~4

Fig. 2 Active (solid circles) and passive (solid squares) stress ofspontaneously contracting longitudinal and circularmuscle layers ofhuman colon as a function of their initial length (Ld). Standard deviations ofthe mean values are shown asbars except when smaller than the symbol.

30Circular

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Gill, Cote, Bowes, and Kingma

greater length (p<0-001) than that of strips from thedescending and sigmoid colon (2-20±0-10 Li). Themaximum active stress exerted by the circularmuscle of the sigmoid colon was greater (p<0001)than that of the ascending, transverse or descendingcolon. In all parts of the colon, the active stressexerted by the circular muscle was less (p<0001)than that of either the taenial or inter-taeniallongitudinal muscle (Table).

LENGTH-FREQUENCY CHARACTERISTICSIn order to compare the effect of increasing lengthon the contractile frequency of strips obtained fromdifferent parts of the colon, the contractile fre-quency has been expressed in relation to the lengthat which the maximum active stress was recorded,referred to as Lmax.

(a) Longitudinal stripsThe spontaneous contractile frequency of taenialand intertaenial longitudinal muscle obtained fromall colonic regions increased with length between0-5-10 Lmax (Fig. 3a, b). The contractile fre-quencies of strips obtained from the ascending andtransverse, ie right, colon were not significantlydifferent (p> 0-4); these data are thus combined.Similarly, no significant difference (p> 0-4) wasobserved in the descending and sigmoid, ie left,colon, and these data are also combined. Noconsistent difference (0-05<p> 0.4) was observedbetween the contractile frequencies of taenial stripsobtained from the right or left colon between0-5-1 0 Lmax. Further, the contractile frequencies ofintertaenial longitudinal strips obtained from the leftcolon were not significantly different (p> 0.1)from those of taenial longitudinal strips obtainedfrom either the right or left colon at lengths of0-5-1-0 Lmax. The contractile frequencies of inter-taenial longitudinal strips from the right colon were,however, lower (p<0-001) than those of eitherintertaenial strips from the left colon or taenia fromright and left colon at all lengths studied.

(b) Circular stripsThe contractile frequency of intertaenial circularstrips obtained from all parts of the colon did notvary significantly (p> 0.4) with increases in lengthbetween 0-3-1-0 Lmax (Fig. 3c). The contractilefrequency of strips obtained from the right and leftparts of the colon showed marked regional variation(Fig. 4).Above Lmax, the contractile frequency of the

intertaenial circularly-oriented strips increased. Thisincrease in frequency could not be accuratelyquantified as contractions were observed at fre-quencies of 3-7 min, 8-15 min and 14-27 min; on

-

,c

E-

V-

1.0 15

8

6

4.

2-

UI

0D

0 0.5L max

10

Fig. 3 Spontaneous contractile frequency of taenial (A)and intertaenial (B) longitudinal strips as a function oflength. Shaded areas represent the mean±I SD ofdataobtainedfrom ascending and transverse (J) and descendingand sigmoid (0) colon respectively. The spontaneouscontractile of intertaenial circular strips (C) obtainedfromthe ascending and right-transverse (upper panel 0), lefttransverse (lower panel O) and descending and sigmoid(1) colon are similarly expressed.

0D

0i10 15

1()1(



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occasions these contractions summated and gaverise to contractions of prolonged duration whichsometimes recurred at a frequency of 1-3 min (Fig.5).

c 6-E

o 4-

3-

2Rt Lt

Asc Trans Desc Sig

Fig. 4 Contractile frequency of circular strips obtainedfrom different parts of the human colon at strip lengths< Lmax,

1 05 Lmax100 _

30 s

1 05 Lmcix

mN1000\inN]0

30s

1 25 Lmax

100,

mN0]30 s

Fig. 5 Spontaneous contractile activity of intertaenialcircular strip from the ascending colon at lengths exceedingLmax Note contractile activity atfrequencies of8-15 minand 14-27 min; these could summate and give rise tocontractions oflong duration.

11o1

DiscussionSpontaneous contractions of both circular and longi-tudinal muscle layers of the human colon were, ingeneral, only observed when the baseline stressexceeded zero at lengths equal to or slightly greaterthan the initial length L*. This suggests that somedegree of stretch is required in order to elicitspontaneous contractile events from human colonicisolated muscle.The active and baseline stress curves of both

circular and longitudinal muscle layers of the right(ascending and transverse) colon were less affectedby length changes than were the left (descendingand sigmoid) colonic muscles. This would beconsistent with the right colon being more dis-tensible than the left and functioning as an accom-modative reservoir in which the temporary storage,absorption and bacterial fermentation of contentcould proceed.2 This marked difference between theright and left regions may be related, to theirdiffering innervation, blood supply and embryo-logical origin.' A further distinction could be madebetween the contractile properties of the right andleft colon. Circular strips obtained from the rightcolon contracted at nearly twice the frequency ofstrips from the left colon at lengths less than Lm,,x. Asimilar observation has been made in vivo.24 Thisprovides some evidence of a right-to-left colonicgradient.The spontaneous active stress exerted by both

circular and longitudinal muscle layers increasedwith length until an optimal length, Lrn,x wasreached, and further length increases decreased theforce generated. This is similar to findings reportedby others in caecum17 2s blood vessels, 6 "7 2xurinary bladder,29 small intestine3 uterus31 andcolonic' 31 smooth muscle preparations in whichspontaneous contractile events have been arrested.The marked increase in baseline stress beyond L,,11xmay serve, physiologically, to prevent overdisten-sion of the colon and subsequently ensure that thecontractile elements are not stretched beyond thepoint at which they can exert maximum active stress.This might be of importance in certain pathologicalconditions which result in over distension of theviscus, particularly as others have shown thatstretching smooth muscle preparations to lengthsgreater than Lmax is associated with an irreversibleimpairment in the ability to contract.32 33 Wedefined the baseline stress as the minimum stressexerted by the smooth muscle during a cycle ofspontaneous contraction. This definition is similar tothat of Price et al. 17 who concluded that theminimum stress exerted by the guinea pig taeniacaecum during a cycle of spontaneous contractionwas comparable to the passive stress exerted by a

f



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1012 Gill, Cote, Bowes, and Kingmapreparation in which the spontaneous contractionshad been arrested with adrenaline. We did notdetermine the passive properties of human colonicmuscle after the abolition of spontaneouscontractions and cannot exclude the possibility thatthe contractile element, as well as connective tissueand non-contractile elements, contributed to thebaseline stress.The absolute values for the maximum active stress

of spontaneously-contracting human colonic musclereported in this paper are not likely to reflect themaximum active stress that this muscle is capable ofgenerating. Gabella34 has shown that the activecontractile stress exerted by guinea-pig taeniacaecum in response to carbachol is greater than theactive stress of spontaneous contraction at the samemuscle length. Thus, we cannot assume that all thecontractile elements of human colonic muscle arecompletely, or even partially, activated duringspontaneous contraction. The maximum activestress exerted by the intertaenial longitudinal musclelayer was considerably greater than that of thetaenia. This may reflect either a greater activation ofthe inter-taenial longitudinal muscle layer duringspontaneous contraction or a higher muscle-to-connective tissue ratio in the inter-taenial region;histological assessment of the relative proportion ofsmooth muscle fibres to collagen and elastin isneeded.

In conclusion, this study shows that spontaneouscontractions of human colonic muscle are dependentupon length. The human colon appears to consist oftwo distinct areas, based on the mechanical be-haviour of the muscle during spontaneous contrac-tion in vitro, which may be related to their differingembryological origin and function.

The authors are indebted to the surgeons attachedto the Department of Surgery, University of AlbertaHospital for the acquisition of colonic tissue. Wethank Ms K Clarke for typing tgie manuscript.

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