Colonic Compensation in Transmissible Gastroenteritis of Swine

Transcription

1 GASTROENTEROLOGY 1984;86;151-9 Clnic Cmpensatin in Transmissible Gastrenteritis f Swine R. A. ARGENZIO, H. W. MOON, 1. J. KEMENY, and S. C. WHIPP Natinal Animal Disease Center, Agricultural Research Service, U, S. Department f Agriculture, Ames, Iwa Absrptin f water and electrlytes by the small and large intestine was examined using a nnabsrbable marker technique in 3-day-ld and 3-wkld pigs. One-half f the pigs in each grup were rally infected with transmissible gastrenteritis virus; the remaining pigs served as cntrls. Threeday-ld cntrl pigs cncentrated the nnabsrbable fluid marker twelvefld alng the small and large intestine, indicating an efficiency f abut 95% in absrptin f the exgenus daily fluid lad presented t the intestine. In cntrast, the marker cncentratin in infected pigs shwed n change whatsever alng either the small r large intestine, indicating a cmplete absence f net fluid absrptin r secretin in these animals. Three-week-ld cntrl pigs cncentrated the marker similarly t the 3-day-ld grup, with the bulk f the fluid absrptin ccurring in the small intestine. Infected pigs in the 3-wk-ld grup had marked net fluid secretin in the prximal small intestine, s that abut twice the fluid lad was presented t the large intestine f the 3-wk-ld infected pigs as cmpared t the 3-day-ld infected grup. Hwever, in cntrast t the 3-day-ld infected grup, the large intestine f the 3-wk-ld infected pigs increased fluid absrptin sme six times ver the cntrl, and this cmpensatry respnse prevented diarrhea in these lder animals. Analysis f luminal cntents indicated that in the lder pigs, unabsrbed carbhydrate was almst cmpletely fermented t shrt-chain fatty acids in the cln, whereas in the yunger pigs the carbhydrate passed thrugh the cln unchanged. These Received March 5, Accepted January 5, Address requests fr reprints t; Dr. S. C. Whipp, Natinal Animal Disease Center, Agricultural Research Service, U.S. Department f Agriculture, P. O. Bx 7, Ames, Iwa 51. Dr. R. A. Argenzi's present address is; Department f Anatmy, Physilgical Sciences, and Radilgy, Schl f Veterinary Medicine, Nrth Carlina State University, Raleigh, Nrth Carlina by the American Gastrenterlgical Assciatin /84/$3. results demnstrate that develpment f micrbial digestin, tgether with rapid shrt-chain fatty acid absrptin, is a primary feature respnsible fr the clnic cmpensatin in the lder pigs with transmissible gastrenteritis. Transmissible gastrenteritis (TGE) is a crnavirus infectin f absrptive epithelial cells in the small intestine f pigs. The disease is characterized by atrphy f intestinal villi and acute diarrhea. Swine f all ages are susceptible t this infectin. The case fatality rate appraches 1% in pigs that becme infected during the first week after birth; hwever, it is cmparatively lw (:::;2%) in pigs that becme infected when they are lder (2:3 wk ld). Villus atrphy and diarrhea are less severe and f shrter duratin in lder pigs than in newbrn pigs. The reasn fr this age-dependent resistance is unclear; it has been ascribed, hwever, at least in part, t an accelerated rate f epithelial cell replacement in lder pigs (1-3). Nrmal newbrn pigs require three times as lng as nrmal 3-wk-ld pigs t replace villus absrptive cells (4). The cmparatively shrtlived villus absrptive cells f lder pigs als tend t prduce lwer titers f TGE virus than thse in newbrn pigs. Villus absrptive cells destryed by TGE virus are replaced by cells that migrate frm the crypts befre they are cmpletely differentiated. Crypt epithelium and the incmpletely differentiated cells cvering atrphic villi are resistant t attack by the TGE virus (2,3,5). Previus studies n the pathphysilgy f this disease have been limited t pigs >3 wk ld and t islated segments f small intestine r t in vitr mucsal sheets perfused with balanced electrlyte slutins (6-8). These studies have shwn that disaccharidase activity f the small bwel mucsa is reduced and that the peratin f the cupled Na- Abbreviatins used in this paper; PFU; plaque-frming units; SCFA; shrt-chain fatty acids; TGE; transmissible gastrenteritis.

2 152 ARGENZIO ET AL. GASTROENTEROLOGY Vl. 86, N.6 glucse transprt mechanism is impaired. Secretry functin f the small bwel, hwever, appears t be intact. These results crrelate well with the villus atrphy and crypt cell hyperplasia bserved histlgically. Althugh such studies can elucidate abnrmalities in epithelial in transprt mechanisms r mucsal enzyme activity, they d nt accunt fr the effect f digestive cntents that may have a cnsiderable impact n transmural in and water transprt in malabsrptive diseases. Fr example, it is assumed that unabsrbed carbhydrate, besides exerting an appreciable effective smtic pressure in the small bwel, is rapidly fermented t shrt-chain fatty acids (SCF A) in the cln, thereby increasing further the smtic driving frce fr net fluid secretin (9). Alternatively, the cln may be cmpensating fr small bwel malabsrptin t sme degree by rapidly absrbing these SCF A, as bserved in human jejunileal bypass patients (1). In TGE, neither mrphlgical changes nr viral infectin is bserved in the cln and, thus, functinal changes in the cln (if any) may be slely related t the abnrmal cntents presented t it frm the small bwel. Therefre, the present study was undertaken t determine the significance f clnic functin in this disease and if clnic functin cntributes t the age-dependent resistance bserved bth clinically and experimentally. A fluid marker technique was used which allwed the estimatin f the effect f nrmal digestive cmpnents n net water mvements in bth the small and the large intestine. Materials and Methds Pigs Thirty-tw hysterectmy-derived, clstrum-deprived, new brn pigs were randmly assigned t tw equal grups. One grup was designated 3-day-ld pigs and the ther designated 3-wk-ld pigs. Eight pigs were selected randmly frm each grup t be infected. All pigs were raised in the labratry in islatin t prevent inadvertent expsure t the virus. Pigs were fed 1 ml f sterilized cw's milk [supplemented with vitamins, minerals, and eggs (11)] twice a day int which was incrprated the water-sluble marker plyethylene glycl-4 (PEG) at a cncentratin f 5 gil. Pigs maintained fr 3 wk were als given 15 ml f a grund, grain-based pig starter ratin twice a day beginning at week 2, and this was increased t 3 ml by week 3. Feed intake was restricted t insure cmplete cnsumptin f the meal. Bth cntrl and infected pigs readily cnsumed the ttal amunt f feed given at each feeding. Steady-state marker cnditins were apprximated as described by Hamiltn and Re (12). On the day f necrpsy, the daily feedings were divided int eight equal prtins and fed at hurly intervals fr 3 h and at 3-min intervals fr a remaining 2 h. Althugh Hamiltn and Re (11) had established that a relatively cnstant cncentratin f PEG was btained by 5 h with this schedule by means f a fistula near the ligament f Treitz, it was questined whether r nt steady-state marker cnditins wuld be btained thrughut the digestive tract, especially in the mre distal segments. We define the steady state as a cnstant perfusin f PEG in amunt (CV) per unit time (t) thrughut the digestive tract, where C is PEG cncentratin and Vis vlume flw. Then, CjV/t = CValt, where i and refer t the input and utput, respectively, t any segment f the digestive tract. Therefre, the flw rate past a sampling pint is Vlt = Cj V/Ct. These cnditins imply that in the steady state, C and V must be cnstant in any given segment as a functin f time. In rder t test the assumptin that relatively stable PEG cncentratins were btained in each segment, preliminary experiments were cnducted with cntrl pigs f each age grup. These pigs were fed their meals accrding t the abve schedule and were necrpsied in grups f three at 3, 4, 5, and 6 h fllwing the initial mrning feeding. This prcedure permitted examinatin f the change in marker cncentratins with time in varius segments f the bwel. These studies shwed that relatively cnstant marker cncentratins were established by 5 h in all segments f the tract examined (see Results), and, therefre, 5 h appeared sufficient t apprximate steady-state cnditins in the main experiment. The cnstancy f V is nly assumed, but appears reasnable in view f the small, frequent feedings. Inculum The virus preparatin was frm the same stck as used previusly (3). Pigs in the 3-day-ld grup were infected intragastrically n day 3 with 5 ml f a 1: 2, dilutin f the stck virus suspensin cntaining 1.5 x 1 6 plaque-frming units (PFU)/ml. Pigs in the 3-week-ld grup were given 12 ml f the same dilutin f virus n day 21 t cmpensate fr their greater bdy weight. Necrpsy The 3-day-ld pigs were randmly assigned t necrpsy at days 5 and 8 fr cntrls and days 6 and 7 fr infected animals. Three-week-ld pigs were necrpsied n days 23 and 26 fr cntrls and days 24 and fr infected animals. Pigs were killed with pentbarbital Na and the stmach and intestinal sites were immediately ligated. These sites included the stmach, tw equal lengths f small intestine, the cecum plus the first lp f spiral cln, and the remainder f the cln. Tw 1-cm segments f small intestine were fixed in situ by injecting a 1% frmalin slutin intraluminally. These segments were frm sites lcated 1 m distal t the ligament f Treitz and 1 m prximal t the ilececal junctin. Segments f cecum and cln were als fixed in 1% frmalin. Frmalin-fixed segments were embedded in paraffin, cut int sectins 7 Mm thick, and examined with a light micrscpe equipped

3 June 1984 COLONIC FUNCTION IN TGE 153 with an cular micrmeter. Five well-riented villi frm each segment f small intestine were measured t determine mean villus height fr the jejunum and ileum in each pig. Muscsal depth was measured in the cecum and cln f all pigs (five measurements per site per pig). Intestinal cntents were cllected frm each ligated segment and the ph was measured immediately. A prtin f cntents was centrifuged at 2, rpm and the supernatant was cllected, diluted 1: 5 with distilled water, and frzen. The remainder was frzen immediately and was pled frm all pigs in the grup. The small intestine was grund and virus titer was determined, as previusly described (3). Chemical Analysis Diluted samples were analyzed fr PEG by the methd f Hyden (13), smlality by freezing-pint depressin, Na and K by flame phtmetry, and CI by the methd f Schales and Schales (14). Pled samples were thawed, mixed, centrifuged at 4 C, and analyzed fr ttal carbhydrate (15) and fr vlatile fatty acids by gas chrmatgraphy (16). Results Virlgy Small intestinal tissue frm all cntrl pigs was negative fr TGE virus. Pigs expsed t virus at day 3 and necrpsied 3 and 4 days after being infected had virus titers f 4.7 ± 2.7 (SE) x 1 5 PFU/ml f intestinal hmgenate. Three-week-ld pigs als necrpsied 3 and 4 days after being infected displayed significantly lwer virus titers f 4.45 ± 2.6 x 1 4 PFU/ml f hmgenate, even thugh these pigs were inculated with 2.5 times the dse given t the 3-day-ld grup. Histpathlgy Results f the histpathlgic studies f intestinal mucsa, expressed as villus length (jejunum, ileum) r mucsal depth (cecum, cln), are shwn in Figure 1. Three-day-ld pigs expsed t the virus had marked villus atrphy f the jejunal and ileal mucsa with a mean villus length f <% f the cntrl tissues (Figures 1 and 2). The degree f villus atrphy was remarkably cnstant with little variatin amng pigs. The mucsal depth f the cecum and cln f expsed pigs, hwever, was unaffected, nr were there any ther mrphlgical alteratins seen in these tissues when cmpared t the cntrls. In cntrast, the height f jejunal and ileal villi in the 3-wk-ld infected grup was nt significantly different frm that in the cntrl grup (p >.1). Large variatins were present in the jejunum f these =. 5 :z:: e:; 4...J 3 <t ~ 2 u :::;) :::Ii 1 a: ~ en :::;) 6 :::l 5 :> CONTROL 3 DAYS INFECTED 3 WEEKS Ce C Ce C SEGMENT Figure 1. Villus height r mucsal depth f small and large intestine frm 3-day-ld and 3-wk-ld cntrl r infected pigs. 51" prximal small intestine; SI" distal small intestine; Ce, cecum; C, cln. Mean ± 5E, n = 8. lder infected pigs, as shwn by the relative size f the standard errr (Figure 1). Three f the 8 pigs had a nrmal jejunal villus length f 637 ± 99 ]Lm, and the remaining 5 pigs had a mean jejunal villus length f 222 ± 4 ]Lm (Figure 2). Such variatin in lder pigs has been demnstrated previusly and was expected. Hwever, n crrelatin between the degree f villus atrphy and virus titer was present. As in the 3-day-ld grup, there were n structural alteratins f the cecum and cln in the lder pigs. Net Water Mvement Preliminary experiments. The PEG cncentratin in the varius segments f the digestive tract examined as a functin f time are shwn in Figure 3. Althugh large individual variatin was present, relatively stable PEG cncentratins were btained fr the yung pigs by 5 h after the initial mrning feeding. Large individual variatin was als present in the lder pigs; hwever, changes in PEG cncentratin with time fr each site were nnsignificant. Inasmuch as large changes in PEG cncentratin between certain segments were btained, it appeared reasnable t expect that changes in varius segments f the bwel as a result f the infectin culd be established with cnfidence using the 5-h necrpsy schedule. Using this single time perid als

4 154 ARGENZIO ET AL. GASTROENTEROLOGY Vl. 86, N. 6 b c d Figure 2. Sectins f jejunum.. Nrmal mucsa in a cntrl pig frm the 3-day-ld grup. b. Atrphic villi with irregular cubidal epithelium and synechial frmatin in an infected pig frm the 3-day-ld grup. c. Nrmal mucsa in a cntrl pig frm the 3- wk-ld grup. Cmpare with. By 3 wk, crypts are deeper, villi are shrter, and absrptive cells are n lnger vaculated in nrmal pigs. d. Villi are atrphic and sme have undergne synechial frmatin in an infected pig frm the 3-wk-ld grup. Only 5 f the 8 infected pigs in this age grup develped this lesin. allwed the number f pigs t be increased in each grup frm 3 t 8, thus reducing the errr due t individual variatin. Hwever, it shuld be emphasized that large errrs are pssible with the present methd. The verall variatin in PEG cncentratin amng the pigs in a given segment apprached ± 34% and was as high as ± 6% in the distal cln. Cupled with this, a variatin as high as ±2% culd be expected frm nn-steady-state cnditins, leading t an verall uncertainty in estimating the net flw f water in cntrl segments f ±4%. We als assume a relatively cnstant marker release frm the stmach and a regular prpulsin f marker thrugh the small intestine; cnditins which may very well be altered in infected animals. Nevertheless, the use f this methd allwed a nninvasive and mre physilgic examinatin f net water mvement in the entire digestive tract and, thus, wuld be mre

5 June 1984 COLONIC FUNCTION IN TGE 155 5,"",,---'"' 3 DAY DLD ~ l ---*-~---;f ' E t WEEK DLD ~ ~ ~---- ~ -----~ J[ IO~ TlME(h) Figure 3. Cncentratin f PEG in gut segments frm 3-day-ld and 3-wk-ld col1trl pigs as a functin f time after initial AM feeding. Segments are designated as fllws: stmach,.; jejunum, ; ileum, D; cecum, 6; and cln,... Mean ± SE, n = 8. suitable fr assessing the effect f nrmal digestive cmpnents n net water mvement than the cnventinal perfusin methds. Cntrl and infected pigs. The cncentratin f PEG in the varius segments f the gastrintestinal tract is shwn in Figure 4. The marker was cncentrated fivefld by the time ingesta reached the ileum in 3-day-ld cntrl pigs, and was further cncentrated in the large intestine t a value nine times that fund in gastric cntents. In striking cntrast, n change in PEG cncentratin was bserved alng the entire gastrintestinal tract in the 3-day-ld pigs with TGE. Similar data recrded fr the 3-wk-ld grup are shwn in the lwer prtin f Figure 4. Again, the cntrl pigs cncentrated the marker severalfld during passage alng the small and large intestine. The infected pigs were unable t cncentrate the marker in the small intestine t the same degree; hwever, unlike the 3-day-ld grup, the large intestine f these 3-wk-ld infected pigs did cncentrate the nnabl3rbable marker. Examinatin f the relative PEG cncentratins alng the gastrintestinal tract may be quantitatively misleading, because a small change in marker cn~ centratin in the upper tract, where the flw rate is high, indicates relatively large vlume changes. Cnversely, althugh the marker is cncentrated severalfld in the distal cln, very little net mvement f water may actually be present. Therefre, the data are repltted in Figure 5 as the percentage f fluid intake passing the midpint f each f the gastrintestinal segments listed. In expressing the data in this manner, we must assume that a steady state in regard t feed and PEG intake is present. Althugh we are aware that we have nt prven this assumptin, the large differences in marker cncentratin bserved between individual segments and between cntrl and infected pigs shuld allw an estimate f the relative quantitative cntributin f each f the segments t net water mvement. Several imprtant aspects are revealed in Figure 5 which wuld nt be evident frm an examinatin f PEG cncentratins alne. This is especially true fr the large intestine. Fr example, the upper prtin f Figure 5 shws that in the 3-day-ld cntrl pigs, the actual quantitative cntributin f the large intestine t net water absrptin is minimal. Hwever, the actual percentage f fluid intake entering the large intestine is nly ~1%-12%. Similarly, the percentage f fluid intake in the 3-wk-ld cntrl pigs, shwn in the lwer prtin f Figure 5, has been reduced t ~15%-2% by the time the ingesta enter the large intestine. Thus, these figures wuld indicate that in nrmal pigs at bth f these ages, the small intestine accunts fr the majrity f net water absrptin. The mst striking differences in the infected pigs were bserved in the large intestine. The clns f e.... "" <.!:> W a CONTROL 3 DAYS INFECTED 3 WEEKS Ce C Ce C SEGMENT Figure 4. Cncentratin f PEG in gut segments frm 3-day-ld and 3-wk-ld cntrl r infected pigs. St, stmach; SI " prximal ne-half f small intestine; SI 2 distal nehalf f small intestine; Ce, cecum and prximal lp f spiral cln; C, distal cln. Mean ± SE, n = 8.

6 156 ARGENZIO ET AL. GASTROENTEROLOGY Vl. 86, l\: UJ :..: ~ 2 ~ 9 2 ~ ~ 2 u... 1 't!fl SI SII SI2Ce C 3 DAYS 3 WEEKS SEGMENT SI SII SI2 Ce C Figure 5. Flw rate f gastrintestinal cntents passing the midpint f each f the segments listed in 3-day-ld and 3- wk-ld cntrl r infected pigs. Values fr the 3-wk-ld infected grup are shwn fr all 8 pigs (shaded bars) and fr nly the 5 pigs with villus atrphy (pen bars). Calculated frm the PEG cncentratins (Figure 4) and frm the daily PEG intake given in milk (see the text). St, stmach; SIl prximal ne-half small intestine; SI 2, distal ne-half small intestine; Ce, cecum and prximal lp f spiral cln; C, distal cln. Mean ± SE, n = 8. the 3-day-ld grup were unable t absrb a significant vlume even when presented with six times the lad seen in the cntrls. This is clearly nt the case in the lder infected pigs. These data are shwn fr bth the entire grup f 8 lder pigs (shaded bars) and fr nly the 5 lder pigs with villus atrphy (pen bars). This separatin is justified n the basis f the histlgic results given abve; the crrelatin between villus height and PEG cncentratin in the jejunum f these infected 3-wk-ld pigs was.96. These results indicate that actual net luminal accumulatin f water was present in the upper ne-half f the small intestine in the 5 pigs with villus atrphy. They als shw that the lwer small intestine and large intestine were capable f cmpensating fr this large increase in flw. Electrlyte Changes The ph and smtic pressure f the gastrintestinal cntents fr all grups f pigs are shwn in Figure 6. The ph was lwer in the large intestinal cntents f infected animals and this was especially true in the 3-day-ld grup. The cecal ph f bth cntrl and infected 3-wk-ld pigs was markedly lwer than that seen in the 3-day-ld pigs. The smlality f gastrintestinal cntents fr 3- day-ld pigs varied little thrughut the tract and was similar t the milk smlality. Cnsiderably higher values were present in gastric cntents f 3- wk-ld pigs. These values decreased markedly in the small intestine f infected pigs, whereas in the cntrls, a marked decrease ccurred in the cln. The cncentratins f Na, K, and CI are shwn in Figure 7. The cncentratin f Na increased alng the small intestine t a greater degree in the 3-dayld cntrl grup than in the infected pigs, and similarly decreased in the cln t a greater degree in the cntrl pigs. This apparently reflects a decrease in surface area fr diffusin in infected pigs; hwever, n apparent differences in Na cncentratin were nted in the lder pigs. The reasns fr these differences in age are unclear, but it shuld als be nted that the substantial net secretin present in the jejunum f the lder infected pigs may have cntributed t their luminal Na cncentratin. Small differences in the cncentratin f CI between cntrl and infected pigs were nted; hwever, gastric cntents f lder pigs had CI cncentratins that were abut twice the values shwn in the yunger :x:: 6... ~ ?4... ~4 ~ 3.- >- 35 HAY."J... =.'..... CONTROL -- INFECTEO ~3 --l ~3 en 2 ~ WEEK, ~r 51 51, 51 2 Ce C SI Ce C SEGMENT Figure 6. ph and smlality in gut segments frm 3-day-ld and 3-wk-ld cntrl and infected pigs. St, stmach; SI " prximal ne-half small intestine; SI 2, distal ne-half small intestine; Ce, cecum and prximal lp f spiral cln; C, distal cln. Mean ± SE, n = 8.

7 June 1984 COLONIC FUNCTION IN TGE 157 m qll 15 1.,,1 z 5 1 U 5 ::.:: 5,. / 'J', 3 DAYS 3 WEEKS...,.-.' \.' '1.. ~... ". ~.. CONTROL ~ - INFECTED 'i[ ~.. :.-,... ~.' ~ SI $1, S12 Ce C SI SIt S1 2 Ce C Figure 7. Cncentratins f Na, Cl, and K in gut segments frm 3- day-ld and 3-wk-ld cntrl and infected pigs. St, stpmach; SI" prximal ne-half small intestine; SI 2, distal ne-half small intestine; Ce, cecum and prximal lp f spiral cln; C, distal cln. Mean ± SE, n = 8. pigs. These cncentratins were reduced substantially in the lwer small bwel. Ptassium cncentratins displayed a reciprcal relatinship with Na, but n marked differences were seen amng the different grups. The cmpsite results f SCF A and ttal carbhydrate cncentratins in the cecum and cln cntents f these pigs are shwn in Table 1. Cncentratins f SCF A were much lwer in the 3-day-ld infected grup than in the cntrl grup, whereas in the lder pigs, similar and higher ttal SCF A cncentratins were present in bth grups. An increase in lactate cncentratin was nted in the lder infected grup f pigs. In cntrast, ttal carbhydrate cncentratins were much higher in the 3-day-ld infected grup than in the cntrls. In fact, the cncentratins f carbhydrate in the cln f the infected pigs wuld accunt fr >4% f the ttal smlality f the cntents (d. Figure 6 and Table 1) if all f the cntents were in the frm f lactse. Hwever, in the lder pigs, similar lw cncentratins f ttal carbhydrate were seen in bth grups, and the carbhydrate cncentratin in the clns f the 3-wk-ld infected pigs was nly abut ne-sixth that f the yung infected grup. Althugh the much lwer SCF A cncentratins in 3-day-ld pigs suggested undev.elped micrbial ppulatins in the large bwel, it was questined whether this bservatin wuld als apply t cnventinally reared pigs. Fr example, the hysterectmy-derived animals maintained in a labratry envirnment may nt btain a lush micrbial ppulatin as quickly as pigs reared n the sw. Therefre, 3 cnventinal pigs were remved frm the sw n day 5 f their life and cntents frm the cecum and cln were cllected and analyzed fr SCF A, as described abve. These results shwed that SCF A cncentratins ranged frm 51.8 t 132 mm in the prximal cln and frm 31.8 t 6.6 mm in the distal cln. Thus, these cncentratins ranged frm thse bserved in the same age grup f hysterectmy-derived cntrl pigs t nearly as high as thse in the 3-wk-ld cqntrl grup. Therefre, it is prbable that the artificial experimental cnditins unifrmly delayed the develpment f a micrbial ppulatin t sme degree. Table 1. Shrt-Chain Fatty Acids (SCFA) and Ttal Carbhydrate Cncentratins (CHO) Grup/ SCFA (meq/l) Ttal CHO segment C, Cz C C. Lactate Ttal" (g/1 ml) 3-Day cntrl Cecum Cln q.o Day infected Cecum Cln Wk cntrl Cecum Cln ;; Wk infected Cecum Cln a Ttal SCFA includes 14 mn- and dicarbxylic acids (15). Only the majr SCFA have been listed individually. Values are frm pled samples f intestinal cntents frm each grup f 8 pigs. C,-C. are frmate, acetate, prpinate, and butyrate, respectively. Lactate includes bth D and L ismers.

8 158 ARGENZIO ET AL. GASTROENTEROLOGY Vl. 86. N. 6 Discussin Villus atrphy, assciated with decreased levels f mucsal disaccharidases, has been unifrmly demnstrated in studies f TGE (1,6). These changes have suggested that unabsrbed carbhydrate may be respnsible fr smtic retentin f water in the small bwel lumen r even the inductin f net secretin because f the increase in effective smtic pressure f the luminal cntents (17). The present results cncerning the small intestine f infected pigs demnstrated zer net fluid mvement, r even net fluid secretin int the jejunum in the case f the lder pigs. This latter effect appeared t be the result f dilutin f the hypertnic gastric cntents with a hyptnic fluid, the smlality f which can be calculated t be ~185 msmlil. Thus, these results in the small intestine are cnsistent with the hypthesis f small bwel malabsrptin and fluid accumulatin as a result f smtic frces; hwever, an active secretry prcess by the small intestine cannt be entirely ruled ut. The respnse f the large intestine t TGE infectin differed markedly in the tw age grups f pigs. The cln f the 3-day-ld pigs was incapable f net fluid absrptin, whereas the 3-wk-ld cln increased absrptin sme sevenfld ver the cntrl and this cmpensatry respnse prevented a significant increase in the fecal utput f water. Several pssible reasns fr this change in clnic functin shuld be cnsidered. First, it is pssible that in the yung pigs there may be a defect in clnic transprt mechanisms caused directly by the virus. Hwever, this seems unlikely because the virus des nt invade epithelial cells r cause histlgic changes in the large intestine. Furthermre, a significant fractin f the smtic activity entering and leaving the cln f these yung pigs was in the frm f carbhydrate (presumably lactse) which, as such, cannt be absrbed by the clnic mucsa. Secnd, pigs f this age grup may have undevelped clnic transprt mechanisms r clnic absrptive capacity (e.g., surface area). There is nw evidence that, in fact, active Na absrptin by in vitr pig clnic mucsa reaches very high and near maximal rates by day 1 f life due t high aldsterne secretin rates at this time (18,19). Therefre, it is unlikely that the transprt capacity f this tissue represents a limiting factr in clnic absrptin. Hwever, the cln f these yunger pigs is prprtinally much smaller in size than in lder pigs and the pssibility f ttal surface area r transit time as a limiting factr needs t be explred. Nevertheless, the same argument can be invked as with the first alternative; namely, a majr prprtin f the cn- tents was in the frm f carbhydrate which cannt be absrbed by clnic mucsa regardless f the surface area invlved. Finally, a mst likely explanatin f the differences in functin cncerns carbhydrate metablism by clnic bacteria. In fact, it is precisely an veractive bacterial fermentatin f carbhydrate t shrtchain rganic acids that has been pstulated as being respnsible fr luminal acidificatin and smtic diarrhea invlving the cln; i.e., fermentative diarrhea (9). Fermentatin f carbhydrate did prduce a degree f acidificatin and high levels f rganic acids in the lder pigs, and in the case f the lder infected pigs, a prprtin f this was in the frm f D-and L-lactic acid. If the feed intake had nt been restricted, it is likely that large amunts f lactic acid and acidificatin f the cntents wuld have resulted in an smtic diarrhea. Hwever, under these experimental cnditins, clnic absrptin in the lder pigs was unimpaired. Thus, the evidence suggests the alternative hypthesis; namely, that the fermentatin prcess itself is the rate-limiting factr in reducing the smtic lad in the yunger pigs. A similar cnclusin was reached in human studies invlving carbhydrate infusins int the intact cln (2). Figure 8 summarizes the flw f carbhydrate in grams per day int and ut f the large intestine tgether with the amunt f carbhydrate disappearing frm the cln, presumably fermented t SCF A. Als shwn in parentheses is the number f :millismles per day entering and leaving the large intestine that were nt accunted fr by the prduct (Na + K) x 2. This smtic fractin wuld presumably include carbhydrate and peptides. This fractin was calculated fr individual pigs instead f the pled result fr carbhydrate and, thus, may be a mre accurate descriptin f unabsrbed carbhydrate. In additin, these values are shwn fr nly the 5 pigs with villus atrphy in the 3-wk-ld infected grup, whereas the pled carbhydrate sample includes all pigs. Clearly, the cln f the lder infected pigs was capable f effectively dispsing f the unabsrbed carbhydrate, whereas in the yunger infected pigs, nearly all f the dietary carbhydrate passed thrugh the cln unchanged. It is nw well established that SCF A are rapidly absrbed by clnic mucsa and utilized as a surce f energy. Therefre, the micrbial prcess cnverts unabsrbable and, therefre, smtically active material t rapidly absrbed SCF A, thereby reducing the effective smtic pressure f the clnic cntents, and salvaging calries which wuld therwise be unavailable. In additin, these SCF A, at cncentratins bserved in the 3-wkld pigs, have been shwn t augment Na and water

9 June 1984 COLONIC FUNCTION IN TGE 159 LARGE I NTESTI NE GM CHO/DAY -.4 t CONTROL (3.2) (1.9) -2. t TGE (28.1) (26.1) -2.4 t CONTROL (7.4) (.2) TGE t (33.2) (4.) J:, J:" Figure 8. Grams f ttal carbhydrate (CHO) entering and leaving large intestine frm 3-day-ld and 3-wk-ld cntrl r infected pigs. Calculated frm pled samples frm all pigs. Values in parentheses are millismles entering r leaving the large intestine which were calculated frm the ttal measured smlality minus the prduct (Na + K) x 2. These latter values were btained frm individual pigs and are shwn fr nly the 5 pigs with villus atrphy in the 3-wk-ld infected grup. absrptin frm the cln f several animal species, including humans (21,22). The develpment f micrbial digestin, therefre, appears t playa central rle in the changes in clnic functin bserved. It is necessary t pint ut, hwever, that the artificial experimental cnditins may have delayed the develpment f micrbial digestin in the yung pigs. The range f SCF A cncentratins bserved in the cnventinal pigs f this same age suggests a mre rapid, but wide, variatin in the rate f develpment f the prcess under thse cnditins. Thus" the envirnment and diet f the animal may be equally r even mre critical a factr than age in the inculatin f the bwel r the ability f the resident bacteria t ferment carbhydrate. Further study will be necessary t quantitatively assess the individual cntributin f these factrs t the develpment f clnic functin. References 1. Mn HW, Nrman JO, Lambert G. Age-dependent resistance t transmissible gastrenteritis f swine (TGE). 1. Clinical signs and sme mucsal dimensins in small intestine. Can J Cmp Med 1973;37: Thake DC, Mn HW, Lambert G. Epithelial cell dynamics in transmissible gastrenteritis f nenatal pigs. Vet Pathl 1973;1: Mn HW, Kemeny LJ, Lambert G, et al. Age-dependent resistance t transmissible gastrenteritis f swine. III. Effects f epithelial cell kinetics n crnavirus prductin and n atrphy f intestinal villi. Vet Pathl 19;12: Mn HW. Epithelial cell migratin in the alimentary mucsa f the suckling pig. Prc Sc Exp Bii Med 1971;137: Pensaert M, Haelterman EO, Burnstein T. Transmissible gastrenteritis f swine: virus-intestinal cell interactins. 1. Immunflurescence. histpathlgy and virus prductin in the small intestine thrugh the curse f infectin. Arch Gesamte Virusfrsch 197;31: Butler DG, Gall DG, Kelly MH, et al. Mechanisms respnsible fr diarrhea in an acute viral enteritis in piglets. J Clin Invest 1974;53: Kerzner B, Kelly MH, Gall DG, et al. Transmissible gastrenteritis: sdium transprt and the intestinal epithelium during the curse f viral enteritis. Gastrenterlgy 1977;72: Shephard RW, Gall DG, Butler DG, et al. Determinants f diarrhea in viral enteritis. The rle f in transprt and epithelial changes in the ileum in transmissible gastrenteritis in piglets. Gastrenterlgy 1979;76: Phillips SF. Diarrhea: a current view f the pathphysilgy. Gastrenterlgy 1972;63: Bnd JH, Currier BE, Buchwald H, et al. Clnic cnservatin f malabsrbed carbhydrate. Gastrenterlgy 198;78: Amtwer WC, Calhd JR. A beta-prprilactne sterilized milk frmula fr specific pathgen-free pigs. Lab Anim Care 1964;14: Hamiltn DL, Re WE. Electrlyte levels and net fluid and electrlyte mvements in the gastrintestinal tract f weanling swine. Can J Cmp Med 1977;41: Hyden S. A turbidmetric methd fr the determinatin f higher plyethylene glycls in bilgical materials. K Lantbrukshgsk Ann 1956;22: Schales, Schales SS. A simple and accurate methd fr the determinatin f chlride in bilgical fluids. J Bii Chern 1941;14: Mrris DL. Quantitative determinatin f carbhydrates with Dreywd's anthrne reagent. Science 1948;17: Salanitr JP, Muirhead PA. Quantitative methd fr the gas chrmatgraphic analysis f shrt-chain mncarbxylic and dicarbxylic acids in fermentatin media. Appl Micrbil 19;29: Frdtran JS, Ingelfinger FJ. Absrptin f water, electrlytes, and sugars frm the human gut. In: Cde CF, Heidel W, eds. Handbk f physilgy. Sectin 6, Alimentary canal. Vl. III, Washingtn DC: American Physilgical Sciety, 1968, 74: Bentley PI. Smith MW. Transprt f electrlytes acrss the helicidal cln f the newbrn pig. J Physil 19;249: Fergusn DR, James PS, Patersn JYF, et al. Aldsterne induced changes in clnic sdium transprt ccurring naturally during develpment in the nenatal pig. J Physil 1979;292: Saunders DR, Wiggins HS. Hw d single dses f carbhydrates such as lactulse cause diarrhea (abstr). Gastrenterlgy 1981;8: Argenzi RA. Shrt-chain fatty acids and the cln. Digest Dis Sci 1981;26: Ruppin H, Bar-Meir S, Sergel KH, et al. Absrptin f shrtchain fatty acids by the cln. Gastrenterlgy 198; 78:15-7.