Guaran Effect on Rat Intestinal Absorption

Transcription

1 GASTRONTROLOGY 1984;86: Guaran ffet on Rat ntestinal Absorption A Perfusion Study BRND LSNHANS DTR ZNKR and WOLFGANG F. CASPARY with the tehnial assistane of ROLAND BLUM Division of Gastroenterology and Metabolism Department of Mediine. University of G6ttingen D-34 G6ttingen Federal Republi of Germany Among the omponents of dietary fiber the soluble polysaharides primarily guaran and petin have been found to impair intestinal absorption. Little is known however about the mehanism of this effet. The diret ation of guaran on small intestinal absorption was evaluated by a single-pass perfusion tehnique. Cuaran in the perfusate (1-7 gil) inhibited small intestinal absorption of atively transported ompounds suh as a-methyl-d-gluoside yloleuine and tauroholate and also of the passively permeating solutes 2-deoxY-D-gluose and urea. Visosity-related inhibition by guaran was found to depend on the rate of perfusion and was only detetable at perfusion rates below.4-.5 mllmin. Higher perfusion rates abolished and even reversed the inhibitory effet. The observed alterations of absorption rates aused by guaran were ompletely reversible after swithing to a guaran-free perfusate. The onentration-dependent absorption of D-gluose and a-methyl-d-gluoside at perfusion rates of.4 and.2 mllmin respetively revealed an inrease in the transport onstant and essentially unaltered maximal transport apaity in the presene of guaran. Additionally net water absorption hanged to seretion upon addition of guaran. When Reeived Deember Aepted Otober Address requests for reprints to: Bernd lsenhans Ph.D.. nstitut fur Pharmakologie und Toxikologie. Ludwig-Maximilians Universitiit. Nussbaumstrasse 26 D-8 Miinhen 2 Federal Republi of Germany. Prof. Dr. Caspary's present address is Stadtkrankenhaus. Leimenstrasse 2. D-645 Hanau Federal Republi of Germany. This investigation was supported by grants Ca 719 and 7111 from the Deutshe Forshungsgemeinshaft. A preliminary report of this work was presented at the X nternational Congress of Gastroenterology 198. Hamburg Federal Republi of Germany (Hepato-Gastroenterology 198;Suppl:abstrat 44.6) by the Amerian Gastroenterologial Assoiation $3. petin and arrageenan were used in solutions of omparable visosity their effet was similar to that of guaran. The results suggest a general mehanism by whih soluble visosity-enhaning polysaharides influene the intestinal absorption of nutrients. The most likely explanation appears to be an inrease in the unstirred layer resistane to diffusion. Under our experimental onditions this ourred at low perfusion rates but was inreasingly ounterated by raising the rate of perfusion. Several reports (1-7) have emphasized that the addition of dietary fiber substanes suh as guaran and petin to test meals redues postprandial blood gluose and insulin onentrations in healthy ontrols and diabetis and improves blood gluose profiles or glyosuria or both in diabetis. Dietary fiber onsists of various ompounds that differ in hemial and physial harateristis. Nonstarh polysaharides are the main onstituents of dietary fiber and may be divided into two groups: skeletal and reserve food polysaharides. The former often omplexed with lignin tend to be rather insoluble in water and swell little on immersion in water. Most polysaharides of the seond group inluding plant gums and muilages are soluble an imbibe water readily may swell and form visous solutions. t appears that arbohydrate gelling agents in partiular being representative of the soluble omponent of dietary fiber retard intestinal nutrient absorption by inreasing the visosity of an ingested meal (89). Several mehanisms are oneivable by whih arbohydrate gelling agents might interfere with the absorptive and digestive funtions of the gastrointes- Abbreviations used in this paper: KH. Krebs-Henseleit; PG. polyethylene glyol.

2 646 LSNHANS T AL. GASTRONTROLOGY Vol. 86. No.4 tinal trat. t has reently been shown that petin and guaran indue a delay in gastri emptying (91). Dietary fiber may also affet small intestinal transit time (811) and motility (11) in different ways depending on the type. substane administered. The fat that petin auses a derease in postprandial hyperglyemia in patients with gastri resetion (12) would suggest that arbohydrate gelling agents affet intestinal absorption diretly. n fat reent in vitro studies (13-15) demonstrated diret inhibition of absorptive and digestive funtions by various arbohydrate gelling agents in the rat small intestine. n these experiments the visosity-dependent inhibition of sugar and amino aid absorption (1315) and of disaharide and dipeptide hydrolysis (14) was attributed to inreased unstirred layer resistane or thikness. The aute effet of arbohydrate gelling agents on rat small intestinal absorption was investigated using a single-pass perfusion tehnique in order to eluidate whether this mehanism is detetable in an in vivo system. Sine guaran represents one of the most widely used arbohydrate gelling agents in the field of dietary fiber researh most experiments were performed with this polysaharide. Guaran is a galatomannan a polysaharide that onsists of a linear hain of mannose units with single galatose unit side hains. Due to this long straight-hain onfiguration guaran forms highly visous solutions at relatively low onentrations (16). Materials and Methods Materials n-[u- 14 C]Gluose (sp. at. 23 mcimmol) 2- deoxy-n-[1-3 HJgluose (sp. at. 2 Cimmol) and 1-aminoylopentane-1-[ 14 CJarboxyli aid (yloleuine; sp. at. 45 mcimmol) were obtained from Amersham Buhler GmbH (Braunshweig Germany); a-methyl-n-[u- 14Clgluoside (sp. at. 32 mcimmol) n-[l- 14 C]mannitol (sp. at. 5 mcimmol) [24-14 C]tauroholi aid (sp. at. 53 mcimmol) [14C]polyethylene glyol 4 (sp. at..6 mcig) [3HJpolyethylene glyol 4 (sp. at. 1.5 mcig) and [3HJpolyethylene glyol 9 (sp. at. 3.5 mcig) from NN Chemials GmbH (Dreieih Germany). Guaran was purhased from Roeper (Hamburg Germany) petin from Serva (Heidelberg Germany) and arrageenan type from Sigma (Taufkirhen Germany); other materials were obtained from standard ommerial soures. Tehnique Nonfasting female Wi star rats weighing between 15 and 2 g were anesthetized intraperitoneally with 7 mgkg sodium pentobarbital. The abdominal avity was opened and a segment of small intestine was annulated distally and proximally to provide a perfusion segment of -15 m of jejunum; 1-m segments of ileum were used for tauroholate absorption studies. The proximal end of the jejunal segments was -15 m distal to the duodenojejunal flexure and the distal end of the ileal segments was -5 m proximal to the ileoeal valve. The segments were rinsed with saline at 37 C and then replaed. Care was taken not to disturb the blood supply. The edges of the abdominal wall were lamped together and overed with a gauze pad soaked with saline. Rats were perfused while lying on their baks and were maintained at 37 C by means of a heat lamp while monitoring retal temperature. The openings of the inflow and outflow atheters were kept at the same level slightly below the level of the intestinal segments. stimations of intraluminal pressure during perfusion were arried out in a series of separate experiments by measuring the height of a water olumn in a vertial tube. This was attahed to a T-shaped tube that replaed the inflow atheter of the intestinal segment in these experiments. The data therefore reflet not.emly the intraluminal pressure but also the pressure required to overome frition in the outflow atheter. The absorptive area per entimeter serosal length was estimated under the various perfusion onditions. During perfusion both ends of the small intestinal segment were lamped simultaneously and the fluid volume ontained in the segment was measured. An apparent surfae area was alulated by assuming the perfused segment to represent a ylindrial tube. n general 1-12 rats were perfused simultaneously. When appropriate they were divided into two groups one perfused with the polysaharide solution the other with the orresponding ontrol buffer. n other studies the protool was altered and three or four groups were perfused: at least one with the ontrol buffer and the others with solutions either of different polysaharides or different visosities. For perfusion rates of.2.5 and.9 mllmin different equilibration periods of usually 7 55 and 4 min respetively were used. After reahing a steady state for water and solute absorption three to five 1-min olletions were obtained for the duration of the experiment. The animals were killed by ardiotomy after the perfusion and the test segments were stripped from the mesentery and ut at the inflow and outflow atheters. The segment length was measured on filter paper without tension. Perfusion Solutions The ontrol perfusion medium was a Krebs-Henseleit phosphate (KH) buffer ph 7.3; osmolarity 29 mosmolll. Test solutes were added at the expense of sodium hloride if their onentration exeeded 1 mm. Polysaharides were added under vigorous stirring and solutions were boiled for 5 min in order to ahieve omplete dissolution and hydration. Addition of guaran did not influene ph or osmolarity. With petin and arrageenan however the ph had to be readjusted by adding NaOH thereby slightly inreasing the osmolarity. For petin (18 gil) and arrageenan (6 gil) these inreases were 8 and 1 mosmolll respetively. The relative visosity of the polysaharide solutions was determined at 37 C using a

3 April 1984 GUARAN AND NTSTNAL ABSORPTON u " 5 C>o.'.8 LJ '" 1! a:: 5 vf-t C A'. X a.'.8.'.8 Rat e f per f u s ion mllmin Figure 1. ffet of guaran on small intestinal absorption of various organi solutes at different perfusion rates. Solutes and their onentrations were A: yloleuine 1 mm; B: a-methyl-d-gluoside 1 mm; C: tauroholate 1 mm; and D: 2-deoxY-D-gluose 1 mm. A single-pass perfusion tehnique was used in the midjejunum (ABD) or the distal ileum (C). Solid lines and losed irles represent the results obtained with Krebs-Henseleit phosphate buffer alone (ontrol). and broken lines and open irles the results with guaran (7 gil). Mean ± SD n = 5; * P <.5 signifianes relate to the differene in absorption rate between ontrol and guaran-ontaining medium at a partiular perfusion rate. 1 a B _ J..'.8 j-----l!!-- apillary visosimeter; the visosity of the KH buffer was arbitrarily taken as one. Aording to the radioative label of the test solute [14C]polyethylene glyol (PG) 4 or [3H] PG 4 was used as the nonabsorbable volume marker. Reovery experiments using single-pass perfusion onditions (.5 mllmin) revealed PG reoveries of 98.5 ± 1.9% for KH buffer and of 97.8 ± 2.2% for guaran (5 gil) solutions (mean ± 1 SD n = 1). An ultraentrifugation study was performed in order to detet possible interations between PG and the polysaharides in solution. Polysaharide solutions (guaran 7 gil; arrageenan 6 gil; petin 18 gil) and KH buffer (ontrol) ontaining [ 14 C]mannitol and [3H]PG 4 in one and [3H]PG 9 and [ 14 C]PG 4 in another experiment were entrifuged at 175 g for 65 h [Bekman ultraentrifuge L5-65 rotor type 5Ti Bekman nstruments n. Cedar Grove N.J.]. The onditions were suffiient to sediment 97% of the polysaharides. There was no differene in the reoveries of the radioative marker moleules (95%-1%) from the supernatant of the different solutions. The development of a slight onentration gradient of the markers in the polysaharide solutions during entrifugation did not differ from the gradient observed in the KH ontrol solution. These results demonstrate the suitability of PG 4 as a freely distributable volume marker for the perfusion of homogeneous polysaharide solutions. Analyti Methods and Calulation of Results Radioativity was assayed with a Searle liquidsintillation system (Mark ) by using an automati quenh orretion and a ounting program for doublelabeled samples. Radioativity was measured in eah 1- min sample and the mean value was alulated for eah study period. With the exeption of the surfae area measurements (see Results) the data given in this study represent the mean ± 1 SD of groups of rats. Solute transport rates were alulated using standard formulas (17) and expressed as nanomoles per entimeter per minute. The unpaired Student's t-test was used to determine whether signifiant differenes existed between groups. Reently it was suggested (18) that the logarithmi mean of the in- and outflow onentration of a solute provides the best value for relating absorption rates to solute onentrations. However evaluation of our data aording to this method did not reveal any qualitative improvement. Quantitative hanges were of the order of 1% (see Figure 5 Results). The alulation of absorption rates based on inflow solute onentrations therefore appear adequate. Results Different perfusion rates were tested at a onstant guaran onentration. The results were ompared with those obtained with the low-visosity KH buffer. The onentration of guaran in the perfusate was 7 gil resulting in a visosity of 25 entipoise. As depited in Figure 1 all solutes tested showed a similar pattern. At low perfusion rates e.g..2 mllmin guaran aused a derease in the absorption rate of all solutes of 5%; this was signifiant for the atively transported ompounds but not for 2-deoxy D-gluose. As the perfusion rate was raised the rate of absorption inreased for eah solute tested. This effet was insignifiant and negligible with the KH buffer ompared with the dramati hanges obtained with perfusion of the guaran-ontaining solutions. These results led us to examine the interrelationships between guaran onentration or visosity perfusion rate and rate of absorption. Using 1 mm yloleuine and 1 mm a-methyl-d-gluoside several guaran onentrations (up to 7 gil) were tested at three different perfusion rates (Figures 2A and 2B). At the low perfusion rate of.2 mllmin (urve 1) inhibition of absorption was detetable at guaran

4 648 LSNHANS T AL. GASTRONTROLOGY Vol. 86 No A. 3 t u. Ul " o o Ul. < u 6 oil. " o Q o oil. < 2 2 Guaran onentration 6 gil} 2 6 Guaran onentration gl 2 1 Relative 5 25 visosity entipoise} Re lative visosity (ent ipoise) Figure 2. Changes in the response of the absorption rate to the perfusion rate by inreasing the onentration of guaran in the perfusion medium i.e. inreasing its visosity. Solutes and their onentrations were A: yloleuine 1 mm ; B: a-methyl-d-gluoside 1 mm. Three different perfusion rates were used:.2 mlmin (1).5 mlmin (2) and.9 mlfmin (3). Mean ± SO n = 5; * P <.5 signifianes relate to the orresponding ontrol values obtained with Krebs-Henseleit phosphate buffer the visosity of whih was arbitrarily taken as unity (at 37 C). onentrations of 1 gil and ontinued up to onentrations of 7 gil. At the intermediate perfusion rate of.5 mllmin (urve 2) the slight inhibition of absorption seen with low guaran onentrations was abolished or even reversed when the polysaharide onentration exeeded 5 gil. At the high perfusion rate of.9 mllmin (urve 3) no inhibition ourred at low guaran onentrations and an inrease was observed at higher onentrations. As Figure 2 (A and B) demonstrates there was a similar response in Table 1. ffet of Guaran on Jejunal Absorption of [14C}Urea n Vivo at Two Different Perfusion Rates Rate of absorption (nmol. em -. min- ) Rate of Guaran perfusion Control (mlfmin) (KH buffer) (2 gil) (7 gil) ±.5 2. ±:.2" 1.7 ±.2" :!:: ±: ±.9" Urea onentration was 1 mm; eah value represents the mean of a group of 1 rats (mean ± SO). (l P <.5. the absorption rate of the two test solutes to hanges in perfusate visosity at all three perfusion rates. Another omparable experiment revealed essentially the same pattern for the absorption of urea (Table 1). The absorptive surfae area per entimeter of serosal length was then estimated in order to larify these observations. The experimental onditions for these measurements were the same as for the preeding experiments. The apparent surfae area (Figure 3) inreased with inreasing guaran onentration up to 7 gil. At the low perfusion rate of.2 mllmin this inrease was 1% (p >.5). At.5 mllmin a signifiant inrease of 2% was obtained whih further inreased to 4% when the perfusion rate was raised to.9 mllmin. Petin (18 gil) at a perfusion rate of.9 mllmin aused an inrease in the apparent surfae area of 2% (1.35 ±.14 m 2!m of length n = 5). The following experiments were performed in order to determine whether these effets were due to irreversible alterations within the small intestine. A test perfusion period with the guaran-ontaining perfusate was preeded and followed by ontrol

5 April 1984 GUARAN AND NTSTNAL ABSORPTON s:;. :u 1.3 Q. N g 1.2 (;.. g 'S Vl o Guaran onentration (gil) Relative visosity (entipoise) Figure 3. Changes in the apparent absorptive surfae area per entimeter of serosal length. The experimental onditions were those of the experiments shown in Figure 2. Three different perfusion rates were used:.2 mllmin (1).5 mllmin (2) and.9 mllmin (3); mean ± SM n = 6; * P <.5 p-value refers to omparisons with the ontrol (zero guaran onentration) at a given perfusion rate. periods during whih KH buffer alone was perfused. At a perfusion rate of.9 mllmin gluose absorption inreased in the presene of guaran (7 gil) but returned to the initial values (Figure 4A) during the subsequent ontrol period. At a perfusion rate of.2 mllmin (Figure 4B) the redution in the absorption rate was gradually reversed when guaran (2 gil) was omitted and KH buffer was reinfused; the absorption returned to the initial ontrol level after 2 h. nhibition persisted longer with progressively higher guaran onentrations. Thus more than 3 h were required to attain ontrol values after perfusing a 5- gil guaran solution during the test period. The time for restoring the initial absorption rates during the seond ontrol period ould be shortened by an intermediate perfusion of KH buffer at a muh higher rate (>1. mllmin). A rough analysis of the samples from the seond ontrol period showed that the deline in the residual inhibition was aompanied by a derease in anthrone-reative material i.e. the approximate sum of mono- and polysaharides (unpublished data). n order to further eluidate the mehanism of the inhibitory effet of guaran at low perfusion rates the onentration-dependent absorption of D-gluose was measured at a perfusion rate of.4 mllmin (Figure 5A). The data were analyzed aording to published methods (192) in order to differentiate between passive diffusion and ative transport. After subtrating the omponent of diffusion represented by the hathed straight lines in Figure 5A from the total absorption rate the resulting urves (Figure 5B) revealed that guaran did not affet the maximal transport apaity but did inrease the apparent Mihaelis onstant of the ative transport system (Km Krn'). The onentration-dependent absorption of a-methyl-d-gluoside (Table 2) was similar to that of D-gluose. Sine arbohydrate gelling agents may influene water absorption by their water holding properties (21) the perfusion experiment depited in Figure 5 was analyzed for the orresponding net water movements. As shown in Table 3 the addition of guaran to the perfusate aused a signifiant hange in net water transport at all gluose onentrations. Whereas water absorption in KH buffer inreased with higher gluose onentrations an analogous effet ' 2 ' u ' 1 a.. '" -; 2 C> - a:: A B ii' i o PerfUSion period (min) Figure 4. Reversibility of the effet of guaran on the absorption ()f D-gluose (1 mm) at two different perfusion rates A:.9 mllmin and B:.2 mllmin. For eah perfusion rate a group of 6 rats underwent a perfusion period of go min with a guaran-ontaining medium [hathed arrows) preeded and followed by a ontrol period with Krebs- Henseleit phosphate buffer (open arrows). Conentration of guaran as follows: A 7 gil B 2 gil. ah point represents the mean value from 6 animals [mean ± SD).

6 65 LSNHANS T AL. GASTRONTROLOGY Va\. 86 No. 4.. u ;;..1 A Table 3. ffet of Guaran on the Rate of Water Absorption in Rat Jejunum at Different Gluose Conentrations Gluose onentration (mm) Control (KH buffer) +.9 ± ± ± ± ±.7 Rate of water absorption (-Ll m- i. min-i) + Guaran (5 gil) -.6 ±.4" -.4 ±.2" -.4 ± ± ±.7 Data were obtained from the experiments shown in Figure 4. Mean ± SD (n = 5). P <.5.. '" " o W M n_nn n: ClJO.1tt= z::- o Km Km' Gluose onentration (mm) Figure 5. ffet of guaran in the perfusate on the onentrationdependent jejunal absorption of a-gluose. A: Rate of absorption in the presene (- - -) or absene (e- - - e) of guaran (5 gil). The triangles represent the orresponding absorption rates alulated on the basis of the logarithmi mean of the in- and outflow onentration of gluose. Perfusion rate:.4 mlmin; eah point represents the mean value from 5 animals (mean ± SD); signifianes are given for the differenes between the ontrol and the guaran experiment; * p <.1 ** P <.5. B: "Ative transport" after subtrating the apparent diffusion omponent indiated by the broken straight lines in A from total transport (for referenes see Results). Apparent Mihaelis onstant of the transport in the ontrol: Km; with guaran added: Km' was not observed for guaran solutions; rather water seretion remained essentially onstant. Two other polysaharides petin and arrageenan were also tested in order to determine whether Table 2. Conentration-Dependent Absorption of a Methyl-D-Gluoside Q a-methyl-dgluoside onentration (mm) Control (KH buffer) 5.7 ± ± ± ± 33 :. Rate of absorption (nmol m - i. min-i) + Guaran (5 gil) 2.8 ± OA b 34.3 ± 5A b 85 ± 16 b 11 ± 31 b The rate of perfusion was.2 mllmin. ah value represents the mean of a group of 5 rats (mean ± SD). " ffet of guaran during perfusion of rat jejunum. b p <.5. the effets observed with guaran were representative for visosity-enhaning polysaharides. The experimental onditions were essentially those used in the experiments shown in Figure 1 (A and B). The onentration of petin and arrageenan was hosen to yield a visosity similar to that of the guaran solution (-25 entipoise). The results in Table 4 learly demonstrate an inhibitory effet at low perfusion rates for both polysaharides. On raising the rate of perfusion however slightly different patterns were observed. Whereas the results with arrageenan resembled those with guaran (Figure 1) the addition of petin to the perfusate did not ause a similar inrease in the rate of absorption at higher perfusion rates. stimations of intraluminal pressure were arried out during jejunal perfusion of solutions with and without guaran (Table 5). Whereas intraluminal pressure with KH buffer alone was quite onstant at all perfusion rates addition of guaran aused signifiant inreases with rising rates of perfusion. At a perfusion rate of.9 mllmin petin (18 gil) inreased intraluminal pressure to 2.4 ± 4.1 mh 2 Table 4. ffet of Petin and Carrageenan on Jejunal Absorption of Cyloleuine (1 rnm) and a Methyl-D-Gluoside (1 mm) in Rats U Rate of absorption (nmol. em-i. min-) Rate of perfusion Control Petin Carrageenan (mllminj (KH buffer) (18 gl) (6 gil) Cyloleuine.2 5. ± ±.5 b 3.1 ±.5 b ± ± 1.2 b 6.9 ± ± ±.6 b 12.4 ± 2.1 b a-methyl-d-gluoside ± ± 4.gb 29.4 ± 1.9 b ± ± 8.5 b 41.1 ± 3.4 b ± ± 5. b 71.4 ± 18. ah value represents the mean of a group of 5 rats (mean ± SD). a Shows influene of different perfusion rates. b p <.5.

7 April 1984 GUARAN AND NTSTNAL ABSORPTON 651 Table 5. ntraluminal Pressure During Jejunal Perfusion of Krebs-Henseleit Phosphate Buffer With and Without Guaran ntraluminal pressure (mh 2 O) Rate of perfusion Control + Guaran (mllmin) (KH buffer) (2 gil) (5 gil) (7 gil) ± ± ± ± ± ± ± ± ± ± ± ± 5.4 Determinations were arried out on 6 rats for eah group (ontrol and three guaran onentrations). The rate of perfusion was raised sequentially (for further details see Methods); mean ± SD. p <.5. (n = 5). This pressure was signifiantly lower than the one obtained with a guaran solution under omparable onditions. Disussion The present in vivo perfusion study learly demonstrates that arbohydrate gelling agents like guaran petin or arrageenan redue small intestinal absorption of organi solutes. Most strikingly the inhibition of absorption by polysaharide solutions was very sensitive to hanges in the rate of perfusion whereas buffer alone did not ause suh a response. At least two opposing effets were observed with guaran: depending upon polysaharide onentration and perfusion rate inhibition of intestinal absorption ould be gradually replaed by enhanement. As high perfusion rates inreased intestinal absorption from guaran solutions i.e. solutions ausing a higher unstirred layer resistane than that aused by the KH buffer hanges in the dimensions of the unstirred layer are not the reason for these observations. Rather the higher intraluminal pressure indued by the visosity of the polysaharide solutions intensified by inreasing perfusion rates is likely to inrease the absorptive surfae area thus leading to higher absorption rates of passively and atively transported solutes (22). This view is supported by the present results regarding jejunal urea absorption a ompound that may serve as a type of intestinal surfae area marker (22). Similar to the absorption of other solutes that of urea in the presene of guaran was redued at low perfusion rates and enhaned at high perfusion rates. This phenomenon may be interpreted as the effet of an inreased unstirred layer resistane at low perfusion rates and that of an inreased absorptive surfae area at high perfusion rates. This interpretation agrees with findings of inreased urea uptake at high perfu- sion rates (22) and dereased absorption in the presene of an unstirred layer (23). The present findings of hanges in the apparent surfae area during perfusions with guaran are onsistent with these onlusions. The slight inrease in surfae area with guaran at low perfusion rates may prevent a progressive derease in absorption with inreasing guaran onentrations. Reent in vitro investigations (1315) with various arbohydrate gelling agents demonstrated inreased unstirred layer resistane to be the mehanism by whih intestinal transport was impaired. The main features of the inhibition by polysaharide solutions were shown to be reversibility and an inrease in the transport onstant KD> ombined with unaltered maximal transport apaity. These harateristis have been interpreted in terms of an unstirred layer effet on the basis of theoreti;al onsiderations (24) and experimental findings (2526). Unstirred layer effets are not readily demonstrated in vivo beause of the high perfusion rates (22) or the segmented flow onditions (2327) needed to redue its apparent thikness with aqueous solutions of low visosity. nreasing the visosity of the perfusion medium however should reveal inreases in the effet of the unstirred layer. Although transport onstants measured in vitro by tissue uptake are different from those obtained in vivo the inrease in Km observed in vivo with visosity-enhaning polysaharides oneivably reflets an inrease in unstirred layer resistane (23). A tentative analysis of the onentration-dependent absorption of - gluose under onditions where guaran exerted a signifiant inhibition indiates that guaran auses an inrease in the apparent transport Km without hanging the alulated maximal transport apaity. Although not graphially analyzed beause solute onentrations above 6 mm were not used onentration-dependent absorption of a-methyl-o-gluoside in the presene of guaran appears to support this finding. n addition analysis of the onentration-dependent absorption of o-gluose reveals a redution of the apparent passive diffusion of gluose in the presene of guaran. This is quite ompatible with an unstirred layer effet as it was shown previously that the apparent passive permeability oeffiients of sugars are diminished in the rabbit jejunum under unstirred onditions i.e. in the presene of a high unstirred layer resistane (28). The net water absorption with the ontrol solution hanged to seretion with addition of guaran. This might be due to the general water-holding properties of polysaharides (21). A reent in vivo study (29) showed that preperfusion of rat intestine with a guaran solution redues the net water absorption during a subsequent guaran-free perfusion. t was

8 652 LSN HANS T AL. GASTRONTROLOGY Vol. 86. No.4 suggested that hanges in the osmolarity of the fluid layer diretly adjaent to the muosal surfae might be responsible for this observation. The exat mehanism remains to be eluidated. mpairment and enhanement of small intestinal absorption by guaran solutions were both ompletely reversible although inhibition at low perfusion rates delined only gradually when guaran was omitted and KH buffer reinfused. This was probably due to a slow washout of the polysaharide. t is possible that the improved gluose tolerane observed 4 h after ingestion of guaran with gluose may be attributed to a similar phenomenon (3). This view is supported by reent findings (29) ona protrated inhibitory effet of guaran on the in vivo absorption of gluose in rat small intestine. The effets observed in this study may apply to a variety of solutes and to the entire length of the small intestine as suggested by the results of the ileal perfusions with tauroholi aid. Bile aid sequestration by binding to fiber as found for miellar tauroholi aid (31) annot aount for the inhibition of tauroholate absorption by guaran at low perfusion rates sine higher rates would not have abolished or even reversed this effet. The present results obtained with guaran may reflet a widely appliable priniple by whih gel-forming and visosity-enhaning polysaharides impair small intestinal absorption sine all three arbohydrate gelling agents inhibited absorption at low perfusion rates. nterestingly however they did not equally enhane absorption at higher perfusion rates. The diverging effet of petin may result from a different effet on intraluminal pressure and hene on the effetive absorptive area. n fat the observed hanges in pressure and surfae area were less with petin ompared with those obtained with guaran under omparable onditions (perfusion rate.9 mllmin). These differenes are probably due to different rheologial properties of the polysaharides at higher flow veloities (for properties of polysaharides see referene 32). Reent findings (33) on the lak of inhibition of intestinal gluose absorption by petin (1 gil) at a perfusion rate of 1 mllmin do not ontradit the present work. As our experiments learly show the nature of the polysaharide its onentration in solution and the rate of perfusion may result in inhibitory and enhaning effets that ould anel eah other. The effet of guaran may also depend on the intestinal preparation used. This is refleted by reent studies (34) where guaran solutions instilled into tied duodenojejunalloops in rats aused inhibition of absorption at low sugar onenhations. We onlude that one of the most important points in attempts at reproduing physiologi onditions in animals and humans is the seletion of the orret perfusion rate. n the light of a reent in vivo investigation in rats (35) on the relationship between disaharide hydrolysis uptake of the monosaharides and perfusion rates it appears that a perfusion rate of 1 mllmin is probably too high to be physiologially relevant. f therefore the physiologi state is better represented by lower perfusion rates arbohydrate gelling agents may be effetive in retarding small intestinal absorption by an inrease in unstirred layer diffusion resistane. Referenes 1. Gassull MA. Goff DV. Haisman p. et al. The effet of unavailable arbohydrate gelling agents in reduing the post-prandial glyaemia in normal volunteers and diabetis. J. Physiol (Lon d) 1976;259:52P-3P. 2. Jenkins DJA. Goff DV. Leeds AR. et al. Unabsorbable arbohydrates and diabetes: dereased post-prandial hyperglyaemia. Lanet 1976;ii: Jenkins DJA. Leeds AR. Gassull MA. e t al. Derease in postprandial insulin and gluose onentrations by guar and petin. Ann ntern Med 1977;86: Jenkins DJA. Hokaday TDR. Howarth R. et al. Treatment of diabetes with gua gum. Redution of urinary gluose loss in diabetis. Lanet 1977;ii: Miranda PM. Horwitz DL. High-fiber diets in the treatment of diabetes mellitus. Ann ntern Med 1978;88: Morgan LM. Gouldner T1. Tsiolakis D. et al. The effet of unabsorbable arbohydrate on gut hormones. Modifiation of postprandial GP seretion by guar. Diabetologia 1979;17: Caspary WF. lsenhans B. Siifke U. et al. ffet of dietary fiber on absorption and motility. Front Horm Res 198;7: Jenkins DJA. Wolever TMS. Leeds AR. et al. Dietary fibres. fibre analogues. and gluose tolerane: importane of visosity. Br Med J 1978;1: Leeds AR. Bolster NR. Andrews R. Truswell AS. Meal visosity. gastri emptying and gluose absorption in the rat (abstr.). Pro Nutr So 1979;38:44A. 1. Holt S. Heading RC. Carter DC. et al. ffet of gel fibre on gastri emptying and absorption of gluose and paraetamol. Lanet 1979;i: Bueno L. Praddaude F. Fioramonti 1. Rukebush Y. ffet of dietary fiber on gastrointestinal motility and jejunal transit time in dogs. Gastroenterology 1981;8: Jenkins DJA. Gassull MA. Leeds AR. et al. ffet of dietary fiber on ompliations of gastri surgery: prevention of postprandial hypoglyemia by petin. Gastroenterology 1977; 73: lsenhans B Siifke U Blume R Caspary WF. The influene of arbohydrate gelling agents on rat intestinal transport of monosaharides and neutral amino aids in vitro. Clin Si 198;59: lsenhans B Siifke U Blume R Caspary WF. n vitro inhibition of rat intestinal surfae hydrolysis of disaharides and dipeptides by guaran. Digestion 1981;21: Johnson T Gee JM ffet of gel-forming gums on the intestinal unstirred layer and sugar transport in vitro. Gut 1981;22: Goldstein AM Alter N Seaman JK. Guar gum. n: Whistler RL BeMiller N eds. ndustrial gums. 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