Six . . . pack balancing act : A conceptual model for the intestinal lining

The cells chat form the lining of the intestine belong to a class of cells termed 'polarized' or 'asymmetric.' The membranes surrounding these cells show functional differences at the luminal and contraluminal surfaces. The cells line up to form sheets and it is across these sheers that movement of fluids and solutes occurs. Such movement occurs in both directions across the lining and the occurrence of diarrhea or constipation depends to a considerable extent upon the net result of the absorptive and secretory mechanisms and their modulation by a host of factors such as neurotransmitters, bacterial products, drugs, etc. This review provides a simple framework for understanding the dynamics of the gut lining. Diamond's six-pack model of epithelia is modified to include the dynamic tension between absorptive and secretory mechanisms.

T l IE SI 11::.ET OF CELLS THAT LINES the intestinal tract stands at the i ntcrface lif two worlds -the outside world of the gut lumen and the inside world of the body.Materials that arc ingested remain external uncil they traverse chis sheet, enter the blooostream and arc incorporated into the form a nd suhsrance d the body.This lining which plays a crucial role in the bo<ly's economy issuhjeCLed toa harrage of influences from borh luminal and comraluminal aspects.Ir is the objective nf this review tn provide a conceptual framework for understanding the functions l)f chis li ning and the influences that impinge on it.No attempt wi ll he made to discuss these facwrs ex haust ively, and the reader is rcforred co several recent rev iews for details (1-8).
The clinician is faced with the cask of understand ing the fu nctions of ch is lining when he or she is confrontctl with a patien t suffering from d ia rrhea.Although a precise and universa lly acceptable definition of dia rrhea may be difficult, a functiona l definition can be given.Thus, Turnberg (9) commented chat from a funct ional point of view, dia rrhea can be defined as a "malahsorption of water."The task of the basic scie n tist lies in understanding the mechanisms responsible for this malabsorpti on and communicating that in-

TRANSPORT ACROSS SYMMETRICAL AND POLARIZED CELLS: A THOUGHT EXPERIMENT
To understand rhe mechan isms respo nsib le for the malabso rptio n o ( water, it is important Lo define the processes by wh ich water mo ves across the intestina l lining.T o begin we can ma ke the dogmalic as~ertion that in bio lugical systems the movement uf solu tes is primary with the movement of water being secondary.O r to put it simplywater follows salt.The problem thus 202 resolves itself into understanding the mech anisms by which solute movement occurs ac ross the intest ine.
Transpo rt physio logists divide ce lls into two broad categories -symmetrical and po la rized.The classical examp le of a sy mm e t r ica l ce l l wo uld be the e rythrocyte, the smooth musc le cell or the ne rve cell.The ente rocyte, the pa rietal cell, the cell lining the renal tubule a nd those lining the a irways are good examples of polarized cells.To unde rsta nd the d istinc tions between the two classes, it wo uld be instructive to begin with a simple 'tho ugh t experiment.' Imagine that a gigantic erythrocyte or ils equivalent is dropped into a beak e r conta in ing iso ton ic sodium chloride and asked to pump sod ium io ns from side A to side B (Figure la) .Since this is a tho ught experiment, we can endow the ce ll with the capacity to solve pro blems and call it 'ce ll gigant icus problem solvaticus.' Sym me trical cells a rc c harac t e rized b y a fa irl y uni fo rm distributio n of transpo rters.Thus, th e cell possesses entry and ex it slc ps for sodium io ns th a t a re dis-tributed symme trically as shown in Fig- ure la.If the en try step is passive and the exit step active, the sodi um iom entering the cell wou ld be pum ped out by a n energy-consuming process.G iven the distribut io n of ent ry and ex it steps, the cell would he un able to ach ieve it~ goa l of reduci ng the concent ra tion of sodium io ns on side A and inc reasi ng it o n side B.
Sin ce, however, the cell is capable of solv ing problems, it could a ttempt to reorganize the entry and exit steps in the fashio n shown in Figu re l b.Here the cell has lined up all the entry steps on o ne side and all the ex it steps on the ocher.N ow,sodium ions ente r from side A and are pumped l)Ut fro m side 8 .T he intentio ns of t his 'segregation strategy' may be la udable but would fa il in the lo ng run.S impl y put, the hackflu x of sod ium ions fro m side B to side A in the bat hing solution n ullifies the effect.
H owever, a simple addi tion to the segregation design ca n solve th e proble m.If t he sentient cell can find other likeminded cells, they can lin k up in such a way t hat entry and exi t steps are separated, and the junctions hetween the cells can ac t ro limit hackf1ux (Fig- ure le).In essence, this is the organization of the lining of the gut, the a irways , the rena l tubule, etc. Geome try, thus, plays a cruc ial role, a nd to understand the functio ning o f an epithelial ce ll, it is impo rtant ro understa nd the spatia l separatio n o f transpo mng elem ents ( ie, th e prope rti es o f th e a pi ca l and hasolatera l membran es) as we ll as the properties o f the junc tions.

THE SIX-PACK MODEL OF EPITHELIAL ORGANIZATION
A simple heurist ic mode l fo r unde rstanding epithe lial o rga ni zati on was proposed by Dia mo nd (1 2).The 111l)Jel has been modified and prese m ed with suitable C anad ian content in Figure 2.This mo del proroses t hat epithelial cells arc hasical ly organized as six-racks of beer extending in two dime nsio ns.The beer c ans co rrespo nd to t h e epithelial celb with the r o p-top end o( the beer cans corresponding Lo rhe apical cell membranes, t he wall:, o f the cans to the la teral ce ll mem branes and the bottoms of the cans LO the basa l cell membranes.The sraces hetween the cells correspo nd to the lateral irue rcellular spaces and the pl astic of the s ixpack to the tigh t junctio ns th.i t encircle the apical end of each cell and serarate the lumina l solutio n from the lateral mterccllular sr<1ces. The analogy is n m , howe ver, exac t, since there a rc gap junc t ions and de:,mosomes t ha t serve to interconnec t adjacent cells a~ well.Thus rhe s ix-rack, unlike the e pith e lial sh ee t , ca n be flexed.G iven this organizat ional pattern, there are two r ossible route~ for permeatio n acrns~ the epithelial sheet.Th ese a re t e rm e d 'trnn sc e l lul a r' (through the cells) and 'paraccllu lar' (between t he ccl b ).
The cruc ial rol e nf geometry and sidedness in epithelial func tio n cannot be overes tim a ted .The m ech anis m~ respons ible fo r the ma inte na nce of 'sidedness' h,ivc been studi ed us ing c ulrnred epithelia l cells ( l3 ).C cll -w -cell and cell-to-substratum contact appear to play a cruc ial rol e.When epithelial cells arc isolated I hey appea r to lose their po lar ity; thi s rrnpe rty is very quickl y regained if rhc b n lated cclb

GENERAL ORGAN IZATION OF INTESTIN AL TRANSPORT : TERMS USED
Across the intestina l e pi the lium, t ransport of ions and water occurs in bmh directions, ie, fro m lumen 10 blood and vice versa .T wo terms arc used to defi ne these movemen ts.A bsorption refers Lo the move men t of solme and w;:i rcr from lumen to blood, whereas secretion defi nes moveme n t in t he oppositedi rection.ln the intestine, movement in e ithe r d irection occurs alo ng t he en ti re le ngth of the gut and "i1 is useful l\l th in k of the func tion of t he mucosa as va rying along a conti n uum from a maxi ma l absorpt ion lO a max imal sec retory state" (2 ).T he fi na l result is the algehraic sum of all of the absorpt ion and secret ion of fl uid that occurs a long the cm ire length of th e gut (Figure 3) .
In genera l, the intesti ne receives 9 L of f1 u id per day -I. 5 to 2 L fro m ingested food and liljuids and the rest fro m e ndogeno us secrerio ns (sali vary, gastri c. , pa ncrea tic, bil iary an d inte:,t ina l).Eigh ty-four re r cen t of the fl uid is absorbed in the small bowe l wit h 1hc colo n absorbing 11 %, leadi ng to a net fecal output of 150 to 200 mL per day.The gut lining: A framework h ydrn lys 1s of the hound (~Tl".The nudcn11Jc, 111 turn, acts o n its own tM-gL't cn:ymcs the prorem k111asL's.These enzymes cons ist of catalyllL s ub unm wh1d1 a rl' mh1h1ted hy regulatory subuni ts.BindingofLAMPhy the regulatory ,uhun 11 relc,!',c, this 111hih1t1lm, which then .,Ilows thl' catalyt1L suhu nn to phnsphorylatc a variet y nf proteins.Some of the phnsphorylated prote1m can form e n her part of l he t ransporters m thei r regul.11orydomams.T his L'X • trcmcly compkx systL'm pm\ldes amplificau nn at e,1ch ~tep and .iffordsthl' possihtl111es tit f1m• tuning.In th1' ,ensL' 1t resembles the complex ,L•ries 1if even ts l hat cnnstitull' the clott 111g C:1'• cade and serves to underscore Narure's penchant for repeated variat ions on n t heme.
Calcium ions: A lrera uons 111 mtrncel lular u1lci um ion concentr,H II m um serve tn alte r membrane transpnri processes.In most cells, d,e level of 111traccllular Lalu um 1s determmcJ hy three mainr mel.hanisms -t he entry of u 1lc ium ions from the ex trace llulnr ,pace mto the l ytnsol .1cross the plasma memhrnne; tlw extrusion of calcium ions from t ht• eel I hy an energy-dependent process; ,ind the release or sequestration of caluum ions hy intracellular 11rgane lies sue h as the endoplasm iL reciculum or m1 ruc hondna.lncrcases 111 intracellular calci um c,111 he hrought ahout hy promotion of the mward movement of calci um ions th rough ,pec 1fiL caluum 10n L hannels, or hy relca~e of hound caluum.T his la 1 ter effect can be ach ieved hy a lteration of t h e levels nf other mediators such ns cAMP or innwnl phospha t es.Thus acetylchnlml' aumg nn musumn1c re ceprors mcreascs the turnover of phnsphoi nositides and the production of   The numerous roles played hy these second messenge rs h ave been elucidated in classical reduct io nist fashion by the isolation of mdiv idua l clements for ease of analysis.lt must be emphasized that the situation in vivo may be quite complex and not easy to predict.
The a rguments to t his ~rage can be summarized as follows.Epithelial cells are polarized and organized into sheers.The transport c haracteristics of the apical membranes are distinct from those of the basolateral me mbran es.The cel ls are organized in to sheets to produce n e t movement of solutes and writer across the entire shee t.In the intestine, movement of solutes and water nccur in bnth direction s; the direction of net movement could be either from lumen to blood (net absorption) or from blood tn lumen (net secretion) (5).Constira-tion or diarrhea can he re lated to shifts in balance towa rds net absorption o r net secretion .
Given that the ep ithelial cells exhibit a 'sidedness,' it can be a ntic ipated that the tra nsport properties ca n he modulated from either lumina l o r contra lumina l side.S uc h mo dulatio ns could shift the balance cowards net ahsorption or net secretion, and thus have consequences for th e total func tioning of the gut.Some factors that can modulate the transport properties of the gut a re shown in Figure 5.

CONTRALUMINAL FACTORS
Neuroactive substances: Th is group of substances includes both neurotransmitt e rs a nd e nteroe nd oc rin e s ubsta n ces.The former are present in nerve end ings and are released on neural sti mulation, whereas the latter are present in paracrine or e ndocrine cells in the intestina l epithelium or lamina propria.In seve ra l instances, the same substances ( vasoactive intesunal peptide and serotonin) a re prese nt in both locations.
The inte~tines are densely innerva ted .A spec ia l subse t of the a utonomic n ervo us system, the e nteric nervous system appears to constitute v irtually a ' little brain.'There are a rpmximatcly 10 8 neurons in the enteric nervou~ systems; as m an y as arc in the spinal cord.The ac u viry of thi s 'little brain' can be modu lated by extrinsic projections fro m the cen tral nervous system, allowmg the 'h1g brain' w exerc ise overall imegrau ve control (7).A large number of ne urotransmitters have been identified.These include acctylcholine and catecholammcs, as well ;.is a list of peptides ( vasmicrivc imestinal p e ptide, d yn o rphin, su b s tan ce P, galan in, somatostarin and ncu ropeptide Y) .The ph ysiological :md pathnph ysiological implications of this vast number of n e uro transmitters are only n ow being explored (6).
The m esse nge rs present in t he enteroendocrine cells inc lude, among others, mmilin, neuro temin , secretin, vasoactive intestinal peptide and serotonin.The messengers discharged from these cells ca n tunction as true hormon es, hei ng carried w distant sites in the bloodstream, or can 11ct in a parac rine fashion co modulate functions of a djacent ce lls.In instances where tumours of the ente mendocrin e celh occur, the products can have rrofounJ effects on e ncerocytes.The c lassical example is that of 'wate ry diarrhea syn• drome,' where vasoact ive 1ntest111al peptide appears to be the major c ulprit.
Infla mmatory mediators (6): Acute or c hroni c inflammation is associatcJ with the production of a va riety ot mediators that inc lude, among others, histamine, seroto nin, k1nins, arach,donic acid me tabolites and lymphok incs.These s ubs tan ces ca n alter enterocyte function direc tly hy occupying receptors, or indi rectly by altering the activity of enteric n erves.Alterati o n s in th e production nf these medi a tors in diverse rnflamm awry states could have profo und consequences for the absorptive and sec retory functi um of the intestine.

LUMINAL FACTORS
I ntest inal microflora (18,19): Earlier, a comparison was made between the renal wbule <1nd the gut.Although such compari~lms arc va lid from a t ransport perspccnve, th ey arc clearly m adequare when other fa ctors a re con s idered.Whi le the conte nt s of the re nal tuhull' are bacteriologicall y sterile under normal conditions, the lumen of the gut b These include acrohcs, focultauve anaerobes and an aerobe,.The numbers mcrease towards the d1st.1lsegments, ~actcrial dcn sit y increasing sharpl y in 1he colon where the average feca l hac-1mal coun t may be as h igh as 10 9 /g.The metabol1C cap,Kll)' of colonic tlnra 1s quire di verse and can he modulated significantly by va riations in diet lir Jrugs.This extensive capacny has ro1h therapeutic and tox igcnic po tenual.
The wxigcniL po1enual of intesunal flora 1s high.Reduct ion of .1:0compounds by gut flora oft en pmdut L's amines that arc mnre wxic than the parent compmmd.This happens wirh fond coluurings suc.h as Brown FK, Red 2G, Rt•d JOB, Orange Ci and Orange RN.Aminl),K 1Jscan he metabolized hy Jecarhnxy lation to produce phanrn1LO• log1callr act1\'L' monoamines and d1a min es s u c h as hi sta mine fro m h1st1d111c, tryptaminl' from tryptophan, and ryramme from tyrosine.These compo unds ca n evoke severe reactions.It is thus possible that patients ,, hl) cnmpla111 of a llergies to ce rtai n fond -Muffs are suffering the con sequences of metaboli tes produced hy their gu t flora rathl'r than 1rue a llergies m the 11nmunological sense.Pathogenic organisms (5): Path oge ns t hat a lter entcmcyte funcuon from thl' lummal s1Je can d{l so ei ther hy 111, ading and damaging the intcsuna l mucosa, hy producing wx1ns that ,1lter .1hsorp11ve,mJ seLretnry funcuom nf the mucosa, or hy a comh111a tion o f hot h .lnv,1si vc mga111sms I hat damage the mucos.1 include v irusl", (Norwa lk, rotav irus A large number of drugs can produce either diarrhea or constipation as a recognized side effect.Thus 40% of patients treated with the gold com-pound auranofin for rheumatoid arthritis exr erience a variety of g::istrointestinal side effe cts.These range from loose sLools to frank diarrhea.The mechanisms responsible include bo th inhibition of absorption and activation of secretion.The enteric nerves appeared to be activated as well.In the case of auranofin, it appears that there is permeation of the drug from the luminal side (20).
It must be emphasized that a number of drugs affect encerocyte function from the conrraluminal side.These include drugs given as antidiarrheals, such as loperamidc or clonidine, as well as those given for other purposes that may have intestinal side effects (3 ).

a
Cell

Figure
Figure l) A 'thot1ght ex/1ern11cnr ' w dcmonsrrme the differences between symmerrical and J10larized cells.a An imaginary cell (cell giganricm Jm1hlcm solvaricus) is drn/1/1ed inw a beaker of isoronic sodium chloride and asked w transport sodmm ions so as to reduce the concenrrarwn of mdrum ions rm srde A and increase iL on .1icleB. The cell , being symmcrrical, has a fairly 1mifom1 distribution of enrry and exir sreps; it is assumed thw r/1c entn ste/1~ ar, passive and the exit ste/1s are active ( solid circles re/n'cse111 active exit steps) .Such an organization /nevents the cell from transpornn.~ sodr wn wn.1 acro,s che cell.b Here the cell h(IS atrempced w solve the prohbn by a segregation stratei..ry.T he e111ry and exit sre/1s are separated as shown Now, .\(/drummTL1 entering the cell from side A are /> um/1ed ou t inro the solu mm facing side B. Th rs maneuvre is nor entirely successful , since the sodium ions /nm1/1cd om on side B quickly diffuse throuf{h rhe bathing medit1m w side A. c Herc two cells have linked up in such a way that che enrry and exic seeps are se{lmaced Back-diffus ion of sodium wns is resrricred by the 'seals' becween che cells.If a large number of cell;; join together , an epithelial sheet is funned.Sodrwn ions can be rransfened across che entire sheet.The symmetrical cell~ are now polarized with the /Jroperties of rhe membrane on one side being different from those 011 rhe other
Figure 6 summarizes the diverse factors that can modulate enrerocytc function and ti lt the inrcsrine.In: AnJreola TE, l loffman JF, Fanestil DD, Schultz SG, eds.Phys1<1logy of Membmne Disordm.New York: Plenum Press, 1986:559-94.7. Cooke I lJ.Role of the 'little brain' in the gut in water and electrolyte homeostasis.FASEB J 1989;3: 12 7-38.8. Hubel KA.Neura l control of intestinal electrolyte control.In: Davison JS, Shaffer EA, eds.G:mroincestinal and I lepatic Secretions: Mechanism and Control.Calgary: University of Calgary Press, 1988: I 7 5-80.9. T urnbcrg LS.Pathophysiology of Jiarrhoea.In: Misciewicz JJ, Pounder RF, Venable CW, eds.Diseases of the Gut an<l Pancreas.Oxford, Boston: Blackwell Scientific Publishers, 1987:41 -58.I 0. Snyder J. Too many dea ths from diarrhea?JAMA I 988;260:3329.11.Ho MS, Glass RI, Pinsky PF, ct al. Diarrheal deaths in American ch ildren.Are they preventable?JAMA l 988;260:328 1-5.12. Diamond JR.The epithelial junction: Bridge, gate and fence.Physiologist 1977;20: l 0-8.l 1. Rodriguez-Bou Ian E. Nelson WJ.Morphogenesis of the polarised epithelial ce ll phenotype.Science th e balance towa rd e ither net absorption o r net ~ecreti on .CONCLUSION The objective of this article has been to provide a simple conceptual framework for the imesLinal lining.OiamonJ's six-pack model emphasi:i:s the crucia l role of geometry m epithelial function.Placing th e six-pack on a fu le rum se r ves to und erscore the dynamic tension heL ween secretion and absorption.Though admitted ly ~1mplistic, it is h oped tha t this framework wi ll prove useful to busy clinicians a1 they a tt e mpt to comprehend rhe diverse influences that impinge on chat marvell ous ltnmg, the mal func tion of which leads pauents LO their offices with embarrassing compl ainc,.l 989;245:718-2 5. 14.StewarrCP, Turnherg LA.A microclectrodc study ,lf rc,pomc, w secretagogucs hy epithelial crll s un villus and crypt of rar sm:111 intestine.Am J Physiol (G,1stro111tcst Liver Physinl 20) 1989;257:GB4-43.I 5. Brown OR, Chandan R, Quito FL, Seybold VS.Receptor rcgul atinn or 10n a An imaginary cell (cell giganricm Jm1hlcm solvaricus) is drn/1/1ed inw a beaker of isoronic sodium chloride and asked w transport sodmm ions so as to reduce the concenrrarwn of mdrum ions rm srde A and increase iL on .1icleB. The cell , being symmcrrical, has a fairly 1mifom1 distribution of enrry and exir sreps; it is assumed thw r/1c entn ste/1~ ar, passive and the exit ste/1s are active ( solid circles re/n'cse111 active exit steps) .Such an organization /nevents the cell from transpornn.~ sodr wn wn.1 acro,s che cell.b Here the cell h(IS atrempced w solve the prohbn by a segregation stratei..ry.T he e111ry and exit sre/1s are separated as shown Now, .\(/drummTL1 entering the cell from side A are /> um/1ed ou t inro the solu mm facing side B. Th rs maneuvre is nor entirely successful , since the sodium ions /nm1/1cd om on side B quickly diffuse throuf{h rhe bathing medit1m w side A. c Herc two cells have linked up in such a way that che enrry and exic seeps are se{lmaced Back-diffus ion of sodium wns is resrricred by the 'seals' becween che cells.If a large number of cell;; join together , an epithelial sheet is funned. Sodrwn ions can be rransfened across che entire sheet.The symmetrical cell~ are now polarized with the /Jroperties of rhe membrane on one side being different from those 011 rhe other formation LO the busy clinic ian.T hat this is not a trivia, task is emphas ized by a recent edito ri al comment by S nyder (1 0) entitled 'Too many dea ths fro m diarrhea.'The comment was provoked by a study t hat sho wed tha t dunng the years 197 3 lo 1983, there we re 500 deaths per year due to diarrhea in the United Stares.Ir was disturbing that not only did these deaths occur in an advanced industria lized natio n rat he r than in the Third Wo rld , but 50% o f the deaths occurred afte r the chi ld had reached a medical fa c il ity ( 11 ).