the mechanisms of absorption the intestine

The Mechanisms of Sodium Absorption

in the Human Small Intestine

JOHNS. FORDTRAN,FLOYDC. RECTOR, JR., and NORMANW. CARTER

From the Department of Internal Medicine, The University of Texas SouthwesternMedical School at Dallas, Dallas, Texas

A B S T R A C T The present studies were designedto characterize sodium transport in the jejunumand ileum of humans with respect to the effects ofwater flow, sodium concentration, addition of glu-cose and galactose, and variations in anioniccomposition of luminal fluid. In the ileum, sodiumabsorption occurred against very steep electrochemi-cal gradients (110 mEq/liter, 5-15 mv), was un-affected by the rate or direction of water flow, andwas not stimulated by addition of glucose, galac-tose, or bicarbonate. These findings led to theconclusion that there is an efficiently active sodiumtransport across a membrane that is relatively im-permeable to sodium. In contrast, jejunal sodium(chloride) absorption can take place against onlythe modest concentration gradient of 13 mEq/liter, was dramatically influenced by water move-ment, and was stimulated by addition of glucose,galactose, and bicarbonate. The stimulatory effectof glucose and galactose was evident even whennet water movement was inhibited to zero by man-nitol. These observations led to the conclusionthat a small fraction of jejunal sodium absorptionwas mediated by active transport coupled eitherto active absorption of bicarbonate or active se-cretion of hydrogen ions. The major part of sodiumabsorption, i.e. sodium chloride absorption, ap-peared to be mediated by a process of bulk flow ofsolution along osmotic pressure gradients. Thestimulatory effect of glucose and galactose, evenat zero water flow, was explained by a model in

Address requests for reprints to Dr. John S. Fordtran,Department of Internal Medicine, The University ofTexas Southwestern Medical School, 5323 Harry HinesBoulevard, Dallas, Tex. 75235.

Received for publication 10 July 1967 and in revisedform 20 November 1967.

which the active transport of monosaccharide gen-erates a local osmotic force for the absorption ofsolution (NaCl and water) from the jejunal lu-men, which, in the presence of mannitol, is counter-balanced by a reverse flow of pure solvent (H20)through a parallel set of channels which are im-permeable to sodium. Support for the model wasobtained by the demonstration that glucose andbicarbonate stimulated the absorption of the non-actively transported solute urea even when netwater flow was maintained at zero by addition ofmannitol to luminal contents.

INTRODUCTIONIn the process of absorbing salt and water in thesmall intestine, sodium can be moved against bothan electrical and a chemical concentration gradient(1, 2). This has led to the conclusion that the pri-mary mechanism is the active pumping of sodiumfrom lumen to interstitium with secondary move-ment of the anion and water. The observation thatthe addition of actively transported sugars (in-cluding the nonmetabolizable 3-methyl-glucose)and amino acids accelerates sodium transport hasled to the conclusion that active monosaccharideand amino acid transport is coupled to and is ableto stimulate the sodium pump (3-5). The role ofpassive processes has not been clearly defined, butit is generally assumed that passive leak of so-dium tends to dissipate the gradients generatedby the sodium pump and thus reduce its efficiency.

The characteristics of the sodium absorptiveprocess do not appear to be the same in all seg-ments of the small intestine, at least in some ani-mal species. In early studies, Visscher found thatisotonic saline solutions were rapidly absorbed in

884 The Journal of Clinical Investigation Volume 47 1968

the ileum but not in the jejunum or duodenum ofthe dog (6). These differences were confirmed andextended by McHardy and Parsons (7) and byHindle and Code (8), but in man, isotonic salineis absorbed at approximately the same rate in thejejunum and ileum (9, 10). However, this find-ing that the absorption rate of sodium chloride inthe upper and lower human small intestine isthe same does not necessarily mean that the char-acteristics of the transport processes are also thesame. Previous studies in our laboratory on thepermeability of the jejunum and ileum suggest thatforces that influence passive movement of salt andwater might play a major role in the jejunum andonly a minor role in the ileum (11, 12).

The present studies were designed to character-ize sodium transport in the jejunum and ileum ofhumans with respect to: (a) bulk water flow,(b) sodium concentration gradients, (c) additionof glucose and galactose, and (d) variations inanionic composition of luminal fluid. In addition,the effect of glucose and bicarobonate on the po-tential difference between intestinal lumen andskin was determined, and the effect of lumen os-molality on net water movement was measured.

METHODSNormal male and female subjects between the ages of21 and 65 yr were studied by a perfusion technique de-veloped by Ingelfinger and co-workers. The small bowelwas intubated with a triple-lumen tube under fluoroscopiccontrol. Test solutions containing a nonabsorbablemarker, polyethylene glycol, were infused at a constantrate (8, 9, 12, or 16 ml/min, depending on the experi-ment) through the most proximal tube, and the perfusatewas collected through the two distal tubes 10 and 35 or40 cm beyond the infusion point. Net water and electro-lyte movement in the test segment (between the twocollecting-sites) were determined by methods previouslydescribed (11-13). Jejunal studies were carried out whenthe infusion point was at the ligament of Treitz, andileal studies when the infusion point was 225 cm from theincisor teeth. Mean solute concentration in the test seg-ment was computed by arithmetically averaging the con-centrations of solute in the fluid collected from each endof the segment. Practically identical results were ob-tained with logarithmic averages because the differencein solute concentration in fluid collected from the twocollection sites was generally small. Mean osmolality inthe test segment was calculated by plotting the osmolalityof fluid obtained from the proximal and distal ends ofthe test segment on semilogarithmic paper and taking themidpoint value (see reference 11 for a validation of thismethod).

In many of these experiments net water movement was

systematically varied in three consecutive test periodsby varying the amount of mannitol added to the infusedtest solutions. For instance, one test solution might con-sist of a slightly hypotonic (260 mOsm/kg) electrolytesolution without mannitol Rapid water absorption inthe test segment would occur with this solution. Thesecond solution might contain, in addition to electrolytes,50 mMmannitol, and under these circumstances net watermovement in the test segment would be near zero.Finally, the third solution would contain, in addition toelectrolytes, 150 mmmannitol, and this solution wouldelicit rapid secretion of water into the test segment.Whereas the magnitude and direction of net water flowwas easily controlled in this fashion, special precautionshad to be taken in order to insure equal mean flow rateswithin the test segment. If, for instance, the same electro-lyte solution was infused at the same rate once with andonce without added mannitol, mean flow rate in the testsegment would be higher with the mannitol-containingsolution which elicited secretion, than with the non-mannitol-containing solution which was absorbed. Thisproblem was solved by varying the rate at which the testsolution was infused into the intestine and by varying therate of collection from the proximal collecting tube.Specifically, the rate of infusion was slower, and the rateof collection via the proximal tube was higher when se-

cretion, rather than absorption, was anticipated within thetest segment. Another methodologic problem presented bysuch experiments was that of maintaining equal mean so-

dium concentrations within the test segment. If sodiumconcentrations were equal in two test solutions, one withand one without mannitol, the mean concentration withinthe test segment would be lower with the test solutionthat contained mannitol, rather than with the test solu-ion without mannitol, since with mannitol the volume ofintestinal contents increases and without mannitol thevolume of the intestinal contents decreases. This problemwas solved by varying the concentration of sodium in thetest solutions. Specifically, test solutions containing man-

nitol had a higher concentration of sodium than thosewithout mannitol.

In designing a series of experiments, the compositionof test solutions and the infusion rates necessary to ac-

complish any particular mean sodium concentration andmean flow rate were calculated from the data on filtra-tion coefficients and reflection coefficients for sodium chlo-ride in the human intestine that were previously pub-lished from this laboratory (11). The experimental con-

ditions necessary to achieve the desired flow rates andsodium concentrations within the test segment were thenconfirmed by preliminary studies. For instance, in one

set of studies the effect of water movement on sodiumabsorption was studied. It was decided to keep mean so-

dium concentration in the test segment at approximately125 mEq/liter L and to have mean flow rates within thetest segment of about 11 ml/min. The test solutions, in-fusion rates, and the rate of sampling from the proximalcollecting tube in one subj ect are presented in TableI. It is evident that approximately equal mean flow ratesand mean sodium concentrations in the test segment

Mechanisms of Sodium Absorption in the Human Small Intestine 885

TABLE IExperiment 210. Example of Technique Used to Maintain Approximately Equal Flow Rate and Sodium

Concentration in Test Segment when the Rate and Direction of Water Flow are Changing

Rate of Flow rates Sodium concentrationsampling

Rate of proximal Entering Leaving Entering LeavingTest solution infused infusion tube segment segment Mean segment segment Mean

mil/min mil/min ml/min mEqlliterNaCI, 110 mmoles,'liter 16 1.9 12.6 10.1 11.3 117.5 127.7 123

NaCl, 140 mmoles/liierMannitol, 65 mmoles/liter 10 2.0 10.9 10.6 10.8 123.0 126.2 124

NaCl, 175 mmoles/literMannitol, 165 mmoles/liter 9 3.5 11.1 14.8 12.9 134.5 118.5 127

were obtained, even though the test solutions that wereactually infused and the infusion rates were quite dif-ferent. This is typical of these experiments, and theseresults were not the best of the group. In analyzing fivesuch triple studies, the average mean flow rate and theaverage sodium concentration for the three study pe-riods were almost identical.

Potential difference between skin and intestinal lu-men was measured with an intraenteric electrode that wasattached to the perfusion tube midway between theproximal and distal collection sites. The intraenteric elec-trode was constructed from a one-half inch piece of glasstubing which had an approximately 2-mm O.D. An asbestoswick was fused into one end of the glass; the glass tub-ing was then ground off to allow a constant but slowleak of its internal contents (3 M potassium chloride)through the asbestos wick. The resistance of the elec-trode was ultimately set at approximately 7-14,000 ohmsby alternately grinding and testing the resistance of suchan electrode against a calomel half-cell. A silver silverchloride wire was then cemented (with epoxy resin) intothe glass tubing, which previously had been filled with3 M potassium chloride. A 300 cm peiece of enameledcopper wire enclosed in polyvinyl tubing was solderedto the silver silver choride electrode. A portion of thepolyvinyl tubing which covered the copper wire wasslipped down over the glass tubing which contained theasbestos wick in such a way that only the end surfacecontaining the asbestos wick was exposed to enteric con-tents. The other end of the copper wire was attached tothe high impedance side of a Keithley 610A eletrometer(Keithley Instruments, Cleveland, Ohio). Before actualuse of this electrode system in an experiment, the po-tential between the glass electrode and the reference elec-trode (see below) was tested in a variety of solutions;only those glass electrodes which gave a potential differ-ence of less than 5 mv in normal saline, 0.5 M sodium bi-carbonate, and 5% glucose were used.

The reference for this system was a Beckman skinelectrode. Before applying the skin electrode, the skinwas vigorously scrubbed with alcohol to remove lipidmaterial, and three needle pricks were made in the skinat the site where the electrode was to be attached. Beck-man electrode paste was used to apply the skin electrode.

At the start of an experiment a small amount of pastewas allowed to extend from under the skin electrode, andthe glass-asbestos wick electrode was inserted into thispaste. Any potential difference between the two electrodeswas eliminated by means of a "bucking potential." Afterthis procedure, the subject then swallowed the tube as-sembly containing the asbestos glass electrode, and mea-surements of potential difference between skin and gutwere determined. Since the electrical resistance of theseelectrodes was low, it was not necessary to utilize acopper-screened room for these experiments. Fluctuationsin potential difference were very small (less than 1 mv).

Electrolyte, sugar, and polyethylene glycol concentra-tions were measured with previously described techniques(11, 12). Urea was measured by a urease technique usinga Conway microdiffusion apparatus (14).

When results of these experiments are given as a meanvalue, as in Figs. 3 and 4 and Tables II and III, thenumber of subj ects studied and 2 SE of the mean are indi-cated on the figures or in the tables. When each resultis plotted individually, as in Figs. 1, 2 (top part), 5, 6,7, and 9, each symbol represents a single observation inone subject. Since an attempt was made to study eachsubject with three different test solutions, the number ofdifferent subjects actually studied for a given experimentmay be roughly estimated by dividing the number of sym-bols by three. Occasionally a test was unsuccessful(wrong solution prepared, tube moved during the experi-ment, tube that became plugged with mucus and failedto drain, etc.), and hence the number of observations re-ported is not always an exact multiple of three. Whendata were collected to determine the effect of monosac-charides or bicarbonate on sodium, urea, and water trans-port (Figs. 4, 5, 7, and 9 and Tables II and III), theorder of study (with and without sugars or bicarbonate)among the different subjects was randomized. However,subjects did not always return for their second series oftests, and the number of subjects actually studied withand without glucose or bicarbonate was not necessarilythe same. The greatest discrepancy for this is shown inFig. 5A, wherein we attempted to study 16 patients withand without glucose. All 16 subjects were studied withglucose, however one of the three test solutions for onesubject was prepared incorrectly, therefore the total

886 J. S. Fordtran, F. C. Rector, Jr., and N. W. Carter

number of observations was 47. 13 of these same subjectswere sutdied without glucose, for a total of 39 observa-tions; three subjects, studied first with glucose, did notreturn for their second study.

RESULTS

Effect of water movement on sodium absorption.The effect of the magnitude and direction of watermovement (manipulated by mannitol concentrationin the test solution) on sodium (chloride) absorp-tion was compared in the jejunum and ileum whenmean sodium concentration and mean flow rate inthe test segment were approximately 110 mEq/liter and 10 ml/min, respectively. This concentra-tion of sodium was selected because it approxi-mates the sodium concentration in intestinal fluidafter normal meals (15). As shown in Fig. 1, so-dium absorption in the jejunum and ileum dif-fered markedly. When net water movement acrossthe intestinal wall was zero, sodium was secretedin the jejunum down a chemical concentrationgradient, whereas in the ileum sodium was ab-sorbed against its concentration gradient. Moreimportantly, however, bulk movement of waterhad a dramatic effect on sodium transport in thejejunum but not in the ileum. Sodium absorptionwas more or less constant in the ileum regardlessof whether water was absorbed or secreted intothe intestinal lumen. In the jejunum the movementof water into the intestinal lumen enhanced sodiumsecretion, whereas absorption of water diminishedsodium secretion and eventually promoted net so-dium absorption against a concentration gradient.

Effect of sodium concentration. Since net wa-ter movement has such a marked influence on so-dium absorption in the jejunum, the concentrationgradient against which sodium transport can oc-cur in this area of the gut must be measured undercarefully controlled conditions with respect to wa-

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-25 -50 -75 -100ABSORPTION

ter movement. It is impossible to measure this lim-iting concentration gradient simply by studyingsolutions with progressively lower sodium concen-trations (keeping osmolality constant by someinert solute such as mannitol), since these dif-ferent test solutions evoke different rates and di-rections of water movement, which, in turn, affectsodium absorption. In order to establish the chem-ical concentration gradient against which sodiumcan be absorbed in the jejunum, the rate of sodiummovement when water movement was zero wasdetermined for a number of different luminal so-dium concentrations. Sodium movement (positivevalues = secretion or gain to lumen, negative val-ues = absorption or loss from lumen) at zero wa-ter flow was + 4.8 mEq/hr per 30 cm when so-dium concentration was 112 mEq/liter (Fig. 1,corrected from 4.0 mEq/hr per 25 cm), + 1.2mEq/hr per 30 cm when sodium concentrationwas 123 mEq/liter (Fig. 5), - 0.2 mEq/hr per 30cm when sodium concentration was 127 mEq/liter (Fig. 2), and - 3.5 mEq/hr per 30 cm whenluminal sodium concentration was 140 mEq/liter(Fig. 2). As shown in Fig. 2, by plotting thesemeasured values for sodium movement at zero wa-ter flow against luminal sodium concentration, itwas determined that sodium movement was zeroat zero water flow when luminal sodium concen-tration was 127 mEq/liter. When luminal sodiumconcentration was less than this value, sodium wassecreted, and when luminal sodium concentrationwas higher than 127 mEq/liter, sodium was ab-sorbed. Thus, sodium as the chloride salt can beabsorbed against a gradient of only 13 mEq/literin the human jejunum, assuming plasma sodiumconcentration to be 140 mEq/liter. In addition toestablishing the gradient against which sodiumcan be absorbed in the jejunum, these data demon-

FIGURE 1 Effect of water flow on sodium (chloride)movement in jejunum and ileum. The sodium con-centration in the test segment was 112 mm± 2.3 inthe jejunal and 110 mm+ 1.5 in the ileal studies. Re-gression lines are calculated by the method of leastsquares. Positive values indicate net secretion (gainto the lumen) and negative values indicate absorption(loss from the lumen).


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MEAN SODIUMCONCENTRATION

mEq/liter

strate that sodium as the chloride salt can be ab-sorbed from the jejunum in the absence of waterflow, glucose, or bicarbonate, but only when lu-minal concentration is nearly equal to that ofplasma.

Measurement of the gradient against which so-dium can be absorbed in the ileum was relativelysimple, because sodium and mannitol concentra-tions can be reciprocally varied without muchchange in water movement, and because watermovement exerts a relatively small effect on so-dium movement, as shown in Fig. 1. Sodium ab-sorption was therefore measured when mean so-dium chloride concentration in the test segmentwas 124 (nine studies), 75 (11 studies), 36 (13studies), and 16 mEq/liter (five studies). Alltest solutions were made isotonic to plasma byadding appropriate amounts of mannitol and allcontained 30 mmglucose (see below for effect ofglucose).

The results of these ileal experiments are shownin Fig. 3, and it is evident that ileal sodium ab-sorption does not cease until the luminal concen-

I

FIGURE 2 Sodium movement at zero wa-ter flow for different luminal sodium(chloride) concentrations. The studies inthe upper part of the diagram were car-ried out with a luminal sodium concentra-

' tion of 127 and 140 mEq/liter. For each-200 concentration the rate of sodium move-

ABSORPTION ment is plotted against the rate of watermovement. Regression lines are by themethod of least squares. The rate anddirection of sodium movement at zerowater flow is calculated (dotted lines),

0- and plotted in the lower part of the dia-gram as a function of luminal sodiumconcentration. The other two points arecalculated from the data in Figs. 1 (for112 mEq/liter) and 5 (for 123 mEq/liter). Note that sodium absorption is zeroat zero water flow when luminal sodium(chloride) concentration is 127 mEq/liter.

140

tration is less than 30 mEq/liter, i.e., the ilealmechanism for sodium absorption can operateagainst a gradient of about 110 mEq/liter. Sodiumabsorption reached near maximum rates when itsconcentration was 75 mEq/liter, and increasedonly slightly when concentration was raised abovethis level. Thus, the sodium absorptive mechanismexhibits some evidence of saturation kinetics.Chloride absorption was also measured in theseexperiments and, as shown in Fig. 3, chloridewas absorbed even when mean chloride concentra-tion in the test segment was as low as 16 mEq/liter, indicating that this anion can be absorbedagainst a gradient of at least 84 mEq/liter (as-suming normal plasma chloride levels of 100 mEq/liter). Furthermore, the absorption of chloride isa linear function of chloride concentration, withno evidence of saturation phenomena within therange of chloride concentrations studied.

Effect of glucose and galactose on sodium ab-sorption. Previous in vitro and in vivo studiesin animals have shown that glucose stimulates so-dium absorption; it has also been shown that glu-


-

NET IONMOVEMENT

mEq/hr/30 cm

Cl'

n=5 n:13

20 40Na Cl

n-11

FIGURE 3 Effect of luminal NaCl concen-

tration on sodium and chloride movement inn:9 the ileum. The number of studies at each

concentration is indicated.60 80 100 120

CONCENTRATIONmEq/1 iter

cose in luminal fluid stimulates sodium absorp-tion in the human jejunum (2). The number ofstudies carried out in the human ileum has beentoo few to determine whether or not glucose stimu-lates sodium absorption in this area, althoughmost animal studies showing an effect of glucosewere actually carried out in the lower small bowel.

In the present experiments, mean sodium andchloride concentrations in the test segment were

maintained at approximately 110 mEq/liter. In-fused test solutions contained either zero, 25, or

105 mmglucose in the jejunal studies, and eitherzero, 15, or 90 mmglucose in the ileal studies.Mannitol was added in amounts necessary to keepthe sum of glucose and mannitol concentrationsequal in each solution. The effect of mean glucoseconcentration in the test segment and of glucoseabsorption rate on sodium movement in the je-junum and ileum is shown in Fig. 4. In the je-junum, glucose absorption had a markedly stim-ulatory effect on sodium absorption, whereas inthe ileum the glucose concentration in the test seg-

ment and the rate of glucose absorption had no

significant effect on sodium absorption. Althoughdata on water movement are not recorded in Fig.4, glucose stimulated water absorption in both theupper and lower small bowel.

The mechanism of the stimulatory effect ofglucose on sodium absorption in the jejunum isnot clear from these data. Two major possibilities

are that glucose stimulates active sodium absorp-tion per se, or that the active transport of glucosestimulates water movement, which, in turn, pro-motes the passive absorption of sodium. In orderto evaluate these two possibilities, the absorptionrate of sodium in the presence and absence of glu-cose was studied at varying rates and directionsof water movement, while sodium concentrationwas maintained at a constant level. In this way,

two regression lines could be obtained, one with

and one without glucose. Since sodium concentra-tion was constant, the effect of glucose on sodiumabsorption, independent of water movement, could

be ascertained. Subjects were studied with threetest solutions, designed to produce different ratesof water movement, on 2 separate days, 1 day with

z

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0 IoI 1C

0 20 40 60 0 5 10 15GLUCOSE CONCENTRATION GLUCOSE ABSORPTION

IN TEST SEGMENT RATEmmoles/llter mmoles/hr per 30 cm

FIGURE 4 Effect.of glucose concentration and absorptionrate on sodium movement when sodium (chloride) con-

centration was 110 mM+ 2.6.

Mechanisms of Sodium Absorption in the Human Small Intestine

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glucose, and 1 day without. The order of test days(with or without glucose) was randomized.

In one set (five subjects) of studies 50 mmglu-cose was added to the test solution on the test daythat glucose was present. Absorption of glucosewas rapid, so that the mean glucose concentrationin the test segment was only 7.7 mmoles/liter. Theregression line with glucose was slightly higherwith than without, but the differences were notsignificantly different statistically.

A second set of studies was carried out usingfrom 70 to 90 mmglucose in the test solutions(depending on the infusion rate, etc., as describedin Methods). Under these conditions, the glucoseconcentration in fluid aspirated from the proximaltube averaged 40 mmoles/liter, and mean glucoseconcentration in the test segment averaged 24.2.As shown in Fig. 5A, glucose clearly raised theregression line, so that for any given rate of netwater movement, glucose stimulated sodium ab-

sorption in the jejunum. For instance, when waterflow was zero, sodium was secreted at a rate of 1.8mEq/hr per 30 cm without glucose, but absorbedat a rate of 4 mEq/hr per 30 cm in the presence ofglucose. Similar studies were carried out in theileum, and as shown in Fig. 5D, glucose does notsignificantly enhance sodium absorption when wa-ter flow is zero.

To see if an actively transported, but poorly me-tabolized, sugar would have a similar effect onjejunal sodium absorption, studies with and with-out galactose were carried out. As shown in 5B,galactose also stimulates jejunal sodium absorp-tion at any given rate of water movement. Whennet water movement was zero, sodium movementwas approximately zero in the absence of galac-tose and absorbed at a rate of approximately 4.5mEq/hr per 30 cm in the presence of galactose.In the ileum, galactose had no statistically sig-nificant effect on sodium transport (P > 0.3),

A JEJUNUMGLUCOSEI_ *o

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A/: 3 9 * Glu. 24

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D ILEUM GLUCOSE

e-- nvyoles/- *- liter

e Gl". 22_ e H. 114

Gl,. 0Ha Ile

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+100 0 -100 -200 +100 0 -100 -200A H20 ml /hr per 30 cm

FIGURE 5 Effect of glucose, galactose, and bicarbonate on sodium movementin the jejunum, and effect of glucose on sodium movement in the ileum, atvarying rates of water movement. The sodium and glucose concentration andmean flow rate in the test segment were the same regardless of the rate or di-rection of water movement (see Methods). The rate of sugar absorption inthe studies with sugar was 19.3, 18.2, and 9.3 mmoles/hr per 30 cm in A, B, andD, respectively. The rate of bicarbonate absorption from the solutions containingbicarbonate in C was 12.4 mmoles/hr per 30 cm. The P values are calculatedat zero water flow.


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+10

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* * mon- * liter

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TABLE I IEffect of Galactose on Sodium and Water Movements in the

Ileum (Results are Mean ± 2 SE of 12 Studies at eachGalactose Concentration)

Galactose Meanconcentra- [Na+] in

tion in Galactose test seg- Na+ ab- H20 ab-test segment absorption ment sorption sorption

mmoles/liter mmoles/hr mEqi mEq/hr ml/hrper 30 cm liter per 30 cm per 30 cm

0 0 1084:1.4 2.542.2 1311635.1 1:4.0 9.9 42.2 108 ±0.6 3.1±i1.4 28±+ 1357.6±+6.6 14.4±-3.2 108±-1.2 3.8 ± 1.8 364±11

although this sugar stimulates water absorptionrate in the ileum, as shown in Table II.

Effect of Anion. Data in rats by McHardy andParsons (7) and in humans by Malawer1 haveshown that jejunal sodium absorption is higherfrom solutions containing bicarbonate than frompure sodium chloride solutions, but the effect ofthese anions on sodium absorption has apparentlynot been studied in the human ileum.

Measurements of jejunal sodium absorptionfrom test solutions containing 140 mEq/liter ofsodium, 5 mEq/liter of potassium, and no glucoseare shown in Table III. When infused test solu-tions contained chloride as the only anion, themean bicarbonate concentration in the test seg-ment was 2.1 mEq/liter, and sodium absorptionwas 3.4 mEq/hr per 30 cm. When infused testsolutions contained 30 mEq/liter of bicarbonate,mean bicarbonate concentration in the test seg-ment was 15.6 mEq/liter and sodium absorptionincreased to 4.9 mEq/hr per 30 cm. Finally, when

1 Malawer, S. J. Unpublished observations.

infused test solutions contained 70 mEq/liter ofbicarbonate, mean luminal bicarbonate concen-tration was 29.8 mEqhliter and sodium absorptionrose to 9.7 mEq/hr per 30 cm. These studies there-fore confirm the observation that bicarbonate injejunal contents stimulates sodium absorption inthis area of the small bowel. Water absorptionwas also stimulated by bicarbonate. In contrast,sodium and water absorption in the ileum wereunaffected by bicarbonate concentration or the di-rection and rate of bicarbonate and chloride move-ment, as also shown in Table III.

These experiments, in which chloride and bi-carbonate concentrations in the test solutions werereciprocally varied, also demonstrate that bicar-bonate is preferentially absorbed in the jejunum,whereas chloride is preferentially absorbed in theileum. For instance, in the jejunum, bicarbonateabsorption exceeded that of chloride even whenluminal bicarbonate concentration was less thanthat of plasma and chloride concentration exceededthat of plasma (Table III, row 2). Furthermore,bicarbonate, but not chloride, is absorbed againsta concentration gradient in the jejunum. On theother hand, in the ileum, chloride is absorbedagainst steep concentration gradients (Table IIIand Fig. 3). Although not demonstrated here,previous studies have suggested that in associa-tion with ileal chloride absorption, there is a secre-tion of bicarbonate (16).

In order to study the stimulatory effect of bi-carbonate on jejunal sodium absorption in greaterdetail, the effect of bicarbonate on jejunal sodiumtransport at different rates of water flow wasmeasured. Seven subjects were studied with three

TABLE II IEffect of Variations in Bicarbonate and Chloride Concentrations (mEqiliter) on Electrolyte Movement (mEq/hr

per 30 cm) and Water Absorption (ml/hr per 30 cm) in the Jejunum and Ileum (Mean Values 4 2 SE ofMean are Given. A Minus Sign Denotes Absorption, a Positive Sign Secretion)

Mean [HCO3-] Mean [CI-] Mean [Na+]in test HCO3- in test C1- in test Na+ H20

segment movement segment movement segment movement movement

Jejunial studies, n =182.1 ±0.4 +0.4+0.2 137 ±1.4 -4.9±41.4 138i0.7 -3.4±1.3 -31±-19

15.6±+2.2 -4.7 ±2.1 124 ±0.2 -2.7 ±1.8 137±4-0.5 -4.9 ±1.6 -38 ±2429.8 ±3.6 -12.5±t1.8 104 ±5.3 +0.7±43.3 138±-0.6 -9.7 ±2.4 -76±-36

Ileal studies, n = 128.2±1.3 +2.5±0.5 100 ±2.0 -6.4±1.2 111±-2.3 -4.6±1.2 -48±15

48.3±41.8 -2.9 ±1.1 61.2 ±1.8 -1.3 ±0.6 112±0.9 -5.0±i1.6 -52 ±20


-200

-15-A a

1-

F

F-

+15k0

+200 +150 +100SECRETI

+50 0 -e)N

A H20ml/hr/30 cm

50 -1OO -150 -200ABSORPTION

FIGuRE 6 Effect of water flow on so-dium, chloride, and bicarbonate move-ment in the jejunum when luminal con-centration of these ions was constant atthe specified value, regardless of the rateor direction of water movement.

test solutions on 2 separate days, 1 day whenmean bicarbonate concentration in the test segmentwas 35 mmoles/liter, the other day with no bi-carbonate. Water movement was manipulated bythe amount of mannitol added to the three testsolutions. Results are shown in Fig. 5C. At everylevel of water movement, sodium absorption wasgreater in the presence of bicarbonate than in itsabsence. Thus, the addition of bicarbonate to je-junal contents had much the same effect on sodiumabsorption as glucose: both enhance net sodiumabsorption independent of any effect on over-allnet water movement.

In the jejunal studies just reported, on the testday when luminal contents contained 35 mmbi-carbonate, the concentrations of bicarbonate, so-dium, and chloride were constant no matter whatthe direction or rate of water movement. Conse-quently, these studies also demonstrated the rela-tive degree to which the jejunal movement ofthese ions is affected by water flow. It is evident inFig. 6 that whereas sodium and chloride move-ment are flow-dependent, bicarbonate absorptionis relatively independent of water movement. Bi-carbonate absorption was almost as high whenwater absorption was rapid as when net secretionof water into the jejunal lumen was occurring.It is also evident in Fig. 6 that at every level

of water flow the absorption of sodium wasgreater than that of chloride; the difference repre-sents that sodium absorbed in association withbicarbonate. The relatively constant difference be-tween sodium and chloride at different waterflows indicates that the sodium absorbed as sodiumbicarbonate is flow-independent, and only thatportion absorbed as sodium chloride is influencedby water movement.

Effect of glucose and bicarbonate on urea ab-sorption. The data just presented have demon-strated that both glucose and bicarbonate enhancethe absorption rate of sodium from the jejunum,even when water movement is zero. To examinethe specificity of this enhancement, the effect ofglucose and bicarbonate on the movement of apassively absorbed, noncharged solute was inves-tigated. Wechose to study urea, since our previousexperiments showed that urea and sodium chloridehave approximately the same reflection coefficientin the jejunum (11) and since all the data so farreported on urea absorption suggest that there isno active intestinal transport for this solute (17).

Our results with urea are shown in Fig. 7. Inthe jejunum we found that both glucose and bi-carbonate stimulate urea absorption. For instance,as shown in Fig. 7A, at zero water movement inthe absence of glucose there was a modest secre-


z

° -100.

IC

_ 0

I - -

-

to

Z E o

0+10InJ

C,r)

.

A JEJUNUMGLUCOSE B JEJUNUMBICARBONATE

* @0

0

Urea .* 0 0 -200

I10 -0 0

t 2

* *Gi. 2

Ur 4.6L P=.3

1 UI a

+100 0 -200

AH20 ml/hr per30cm

FIGURE 7 Effect of glucose on urea movement in the jejunum (A) and ileum (C)at varying rates of water movement, and of bicarbonate on urea movement in thejejunum (B). Mean plasma urea was 4.9 mmoles/liter in these subjects. The Pvalues are calculated at zero water flow.

tion of urea into the lumen, whereas in the pres-ence of glucose there was a significant absorptionof urea. Bicarbonate had a similar effect, as shownin Fig. 7B. Since the concentration of urea in theluminal fluid and plasma were the same in theseexperiments, this net absorption of urea at zerowater movement cannot be attributed to diffusiondown a concentration gradient.

JEJUNUM

FPedw ofSNMeh

In the ileum, glucose did not significantly en-hance urea absorption (P = 0.3), as shown inFig. 7C.

Potential difference (PD). Results of a typicalexperiment are shown in Fig. 8. The top part ofthe figure gives results obtained in the jejunum,the lower part the results in the ileum. In theesophagus the PD was approximately -2 mv, in

F.SALINE-+GLUCOSE+- SAL -+GLU -o14-SAL.+HC93-.

F-ndas ofStomach

.-- SALINE------I-o---GLUCOSE ----I-p-SALINE4-q HCO3-1 W

ILEUM II I

II

Esophagus

+10I *--

0 20 40 60 80 100 120 140 160 180 200 220TIME (Minutes)

FIGURE 8 Potential difference (lumen to skin)during perfusion with saline, saline with glu-cose, and saline-bicarbonate solutions. Positiveand negative values are in reference to the in-testinal lumen.

Mechanisms of Sodium Absorption in the Human SmaU Intestine

E

0.

0 400

S.E4 -200

4:40

> +10

0. -40

-301-

-20-

-10F

0

C ILEUM GLUCOSE

)I

893

TABLE IVPotential Difference between Intestinal Lumen and Skin (Mean of Five Studies)

Jejunum Ileum

PD PD

Solution perfused* Mean Range Solution perfused* Mean Range

120 mMNaCi 110 mMNaCi+0.6 [(+10) -(-10)] -5 [( 0) -( -8)]

20 mMMannitol 60 mMMannitol

140 mMNaCi 110 mmNaCi-4 [( +4)-(-15)] -15 [(-9)-(-22)]

70 mmGlucose 60 mmGlucose

50 mMNaCi 50 mmNaCi70 mmNaHCOa +1 [(+11)-( -9)] 60 mMNaHCO% -5 [(+1)-( -8)]35 mmMannitol 60 mmMannitol

* All solutions had 5 mMKCl and were infused at a rate of 12 ml/min.

the stomach fundus it was -34 mv, and in thestomach antrum it was from -8 to -6 mv.Upon entering the duodenum the PD fell to ap-proximately -6 mv and during perfusion of thejejunum with a sodium chloride solution (TableIV) the PD fell to -1 mv. Addition of glucosecaused the PD to become more negative, increas-ing to -6 mv. By alternating saline and saline-glucose solutions it was demonstrated that thesePD changes were reproducible. Finally, the sub-stitution of bicarbonate for chloride had no effecton PD. As shown in the bottom part of Fig. 8,the PD in the ileum during saline infusion was-8 mv. The addition of glucose had a definite andsustained effect, increasing the PD to approxi-mately -22 mv. As in the jejunum, bicarbonatehad no effect on PD. When the electrode wasremoved, the PD was -39 mv in the fundus ofthe stomach and zero in the esophagus.

Four other similar studies were performed, withthe perfusion solutions given in Table IV. Thesesolutions were chosen so that water absorptionrate would be approximately zero and sodiumconcentration was constant at the midpoint of thetest segment (where PD was measured). All ofthe infused solutions contained 5 mEq/liter ofpotassium. By using these solutions, diffusionpotentials and streaming potentials were mini-mized. The results of the PD measurements aresummarized in Table IV. In the jejunum, theaverage PD was +0.6 mv during saline perfusionand -4 mv in the presence of glucose. Substitution

of bicarbonate for chloride in the saline perfusionshad no effect on PD. In the ileum the average PDduring saline perfusion was -5 mv, and glucosecaused the PD to become more negative, the aver-age value being -15 mv. Substitution of bicarbon-ate for chloride had no effect on ileal PD.

Effect of osmolality on water movement, withand without glucose and bicarbonate. Many ofthe studies carried out in these experiments in-volved manipulation of water movement in thepresence or absence of glucose or bicarbonate,while sodium concentration was maintained at aconstant level. As already explained, the watermovement was varied by manipulating the osmo-lality (with mannitol) within the test segment.It was possible therefore from these studies toascertain the effect of osmolality on water flow,in the presence or absence of glucose or bicar-bonate while sodium concentration was constant.The results in the jejunum with and without glu-cose are shown in Fig. 9A and demonstrate severalpoints. First, water movement is linearly relatedto osmolality within the test segment. Second,when sodium concentration is approximately 120mmoles/liter, the osmolality of fluid within thetest segment that is associated with zero waterflow is 290 mOsm/kg when glucose is absent,, and320 mOsm/kg in the presence of 24.2 mmglucose.Thus, in the absence of actively transported sugarsthe jejunum cannot absorb water from 120 mmsodium chloride solutions against an osmotic pres-sure gradient, whereas in the presence of 24 mM


C JEJUNUMBICARBONATE

* *moIs.,

N." ""24

*N *NOor

250 300 350

MEANOSMOLALITY

B JEJUNUM ISOTONIC SALINEI ivl.

*lCa.=f,

0

--L A.. FIGURE 9 Effect of osmolality on jejunal andI_____________ileal water movement. In A the effect of glu-

cose with 120 mmsodium chloride in the je-D ILEUM GLUCOSE junum is shown; in B the effect of osmolality

on jejunal water flow is shown for a luminal., n solution of 140 mmsodium chloride; in C the

effect of bicarbonate is shown; and in D the ef-PI ,II: fect of glucose with 114 mmsodium chloride

.p ;qin the ileum is shown. The P values are calcu-lated at zero osmotic pressure gradient, assumed

,_________________ ,to be 290 mOsm/kg.250 300 350

IN TEST SEGMENT

glucose, water is absorbed from such solutionsagainst an osmotic pressure gradient of 30 mOsm/kg (assuming plasma osmolality of 290 mOsm/kg). Data presented in Fig. 9B demonstrate thatwhen the jejunal contents contain 140 mmsodiumchloride, zero water flow is associated with a

lumen osmolality of 290 mOsm/kg. Thus, even

when luminal contents contain sodium (chloride)in the same concentration as plasma, the jejunumcannot absorb water against an osmotic pressuregradient in the absence of glucose.

As shown in Fig. 9C, when the jejunal test seg-ment contained 34.8 mmbicarbonate, the osmo-

lality of luminal fluid associated with zero waterflow was 297 mOsm/kg. The sodium chloridecontrol studies for this series of observations re-

vealed an osmolality of 283 mOsm/kg at zero

water flow, and this value is significantly differentfrom the value with bicarbonate. Thus, althoughthe difference from control studies was small, thesubstitution of bicarbonate for chloride does ap-pear to promote water absorption against a slightosmotic pressure gradient.

As shown in Fig. 9D, the osmolality of ileal con-

tents associated with zero water flow when luminalfluid contained 114-118 mEq/liter of sodiumchloride was 312 mOsm/kg. The addition of glu-cose did not change the gradient against whichthe ileum absorbed water.

DISCUSSION

Previous studies have shown that the rate ofabsorption of sodium and water from isotonic

saline solutions is approximately the same in thehuman jejunum and ileum. Although this suggests

that the absorptive processes are similar, the pres-

ent studies show marked differences in four im-portant respects: sodium absorption in the jeju-num can occur against only slight concentrationgradients, is markedly influenced by net waterflow, and is stimulated by the addition of activelytransported sugars and by bicarbonate; whereasin the ileum, sodium absorption occurs againststeep concentration gradients, and is not influ-enced by water flow, nor by the addition of sugars

or bicarbonate.Ileum. In the ileum sodium can be absorbed

against large concentration gradients and againsta potential difference of 5-15 mv, which suggestsan active transport process for sodium. The abilityof the ileum to transport against such steep gra-

dients suggests also that the membrane has a lowpermeability for sodium chloride with little backleak. This is substantiated by the fact that sodiummovement in the ileum is not influenced by bulkwater flow, and by our previous studies showinga high reflection coefficient for sodium chloride inthe ileum (11 ).

The finding that reciprocal variations in chlorideand bicarbonate concentrations do not influencethe rate of sodium absorption or the potentialdifference in the ileum may appear surprising in

view of the fact that chloride is absorbed in pref-erence to bicarbonate in this part of the small in-testine. It might have been expected, therefore, thatsodium chloride absorption would proceed more


E0

0c14x -200

-100

0

+100

rapidly than sodium bicarbonate absorption. How-ever, the fact that chloride was absorbed morerapidly than sodium suggests that much of theileal chloride absorption may have occurred via anion exchange mechanism with bicarbonate, and, assuch, be unrelated to net movement of sodium.Although our data do not establish the presenceof such an anion carrier, a number of previousobservations are compatible with such a mecha-nism. For instance, Bucher, Flynn, and Robinsonshowed that saline test solutions instilled into theballoon-isolated ileum equilibrated with an alkalinepH, a high concentration of bicarbonate, and a lowchloride concentration (16). Furthermore, ilealcontents after a normal meal have a lower chlorideconcentration and a higher pH than plasma (15).In addition, a preliminary study by Hubel in therat ileum has shown that bicarbonate secretionoccurs only if chloride is present in ileal contents(18); this dependency of bicarbonate secretion onluminal chloride is also consistent with an anionexchange carrier in ileal membranes. These obser-vations, plus those reported in the present paper,are therefore compatible with the hypothesis thatchloride is absorbed in the ileum in exchange forsecreted bicarbonate in a manner unrelated tosodium transport.

The addition of glucose or galactose to luminalcontents did not stimulate the rate of sodiumabsorption in the ileum, although glucose did in-crease the negative potential difference betweenintestinal lumen and skin. An increase in potential(tifference without a change in sodium absorptionrate might be explained by an alteration of mucosalpermeability. Another possible explanation is thatsodium transport might be stimulated by glucosebut causes no increase in net sodium absorption,because the potential difference increases andthereby increases the back leak of sodium. Thelatter suggestion, i.e. a coupling of sodium andglucose transport, would be most compatible withthe conclusions of other workers who have usedin vitro preparations (3-5).

A final observation about ileal absorption is thatwater from 110 mm sodium chloride solutionscan be absorbed against an adverse osmotic pres-sure gradient of approximately 20 mOsm/kg.Similar observations have previously been madein experimental animals, and the most generallyaccepted explanation of this phenomenon is that

proposed by Curran in which sodium is activelytransported into some small poorly mixed thirdcompartment (1). The fact that glucose did notenhance either the osmotic gradient against whichwater absorption occurs or net sodium absorptionis compatible with the hypothesis that activesodium transport is the main driving force forestablishing a hypertonic solution within the mid-dle compartment of the Curran model.

Jejunum. In marked contrast to the ileum, thejejunum is capable of absorbing sodium chlorideagainst only a slight electrochemical gradient (13mEq/liter, 0 mv). Moreover, bulk water flow hasa marked effect on sodium movement, and absorp-tion of sodium and the gradient against whichsodium absorption can occur is enhanced by sub-stitution of bicarbonate for chloride and by addi-tion of glucose or galactose to luminal contents(Fig. 5).

The finding that water flow has a markedeffect on sodium movement in the jejunum isconsistent with our previous studies on the reflec-tion coefficient of sodium chloride in the smallintestine. Using the equations of Kedem andKatchalsky (19), it can be shown that the abilityof bulk water flow to influence solute movementis directly related to the reflection coefficient ofthe solute in question. The quantity of solute (J.)movement induced by bulk flow of water (J,) isgiven by the expression: is = C(1 - o)J, inwhich C is the mean concentration of the soluteacross the membrane and ur is the Stavermanreflection coefficient. It is seen from this equationthat, the lower the reflection coefficient the greaterthe effect of bulk water flow on solute movement.Previously we reported that the reflection coeffi-cient of sodium chloride in the jejunum was 0.5,whereas in the ileum it approached 1.0 (11).These values are therefore consistent with thepresent studies showing that water flow has amajor effect on sodium transport in the jejunumand a negligible effect on sodium transport in theileum. An independent measurement of the reflec-tion coefficient of sodium chloride from the presentdata yields a value of 0.92 for the ileum (Figs. Iand 5D) and an average value of 0.4 for thejejunum (Figs. 1, 2, 5, 6, and 7).2 The finding

2 The slopes of the curves in Figs. 1, 2, 5, 6, and 7 givethe value of J,/J,. Dividing this value by the mean so-


that bulk water flow has such a pronounced effecton jejunal sodium movement suggests that passiveprocesses for sodium absorption may be veryimportant in the jejunum.

The observation that substitution of bicarbonatefor chloride enhances sodium absorption suggestsseveral possibilities. If sodium were absorbed byan active process while its associated anion movedpassively along electrochemical gradients, then theabsorption rate of sodium would be dependent onthe permeance of the anions in luminal fluid. Onepossibility, therefore, is that the jejunal mucosais more permeable to bicarbonate than to chloride.However, the direction and rate of water move-ment influence the movement of sodium chloridemuch more than the movement of sodium bicar-bonate (Fig. 6). This indicates that the reflectioncoefficient for sodium bicarbonate is much higherthan that for sodium chloride, and excludes freerpermeability of bicarbonate as the mechanism bywhich bicarbonate enhances sodium absorption. Asa matter of fact, it seems highly unlikely that bicar-bonate is absorbed in the jejunum by passiveprocesses. The steep concentration gradient againstwhich bicarbonate is -absorbed (down to luminalconcentrations of about 4 mEq/liter, Table IV)cannot be explained by transmucosal potential dif-ference, since this would require a potential differ-ence of approximately -50 mv. Potentials of thismagnitude have never been reported across thesmall intestine of any animal species (2), and arecent study in Soergel's laboratory (20) and ourown results (Table IV) indicate that the maxi-mumpotential difference across the jejunal mu-cosa is of the order of from -4 to -16 mv. Theseobservations therefore suggest that bicarbonate ab-sorption is mediated by some active process. Bi-carbonate absorption by an active process couldstimulate sodium absorption in several ways. Forinstance, active absorption of bicarbonate mightincrease sodium absorption by creating favorableor decreasing unfavorable electrical gradientsacross the mucosa, and thereby decrease theelectrochemical gradient against which sodium isabsorbed at any given luminal concentration ofsodium. Our PD measurements are against sucha mechanism, since the PD did not change whenbicarbonate was added to saline solutions. Alterna-

dium concentration across the intestinal wall gives( 1- a).

tively, active hydrogen secretion (which wouldreact with bicarbonate to form carbonic acid, whichin turn would decompose to H2O and CO2 withdiffusion of the latter across the mucosa) mightinfluence sodium absorption electrically as justdescribed, or by a direct coupling of hydrogensecretion and sodium absorption, which previouslyhas been suggested in the rat jejunum by Parsons(21). The failure of the PD to change suggestsdirect coupling between hydrogen secretion andsodium absorption.3 If the rate of hydrogen secre-tion were affected by the hydrogen ion concen-tration of jejunal fluid, raising the bicarbonateconcentration of luminal contents would increasehydrogen secretion, and, thereby, increase the rateof sodium reabsorption.

The marked influence of water movement onsodium movement and the ability to absorb sodiumagainst only small electrochemical gradients raisethe possibility that only that portion of sodiumtransport linked to hydrogen secretion is activelyabsorbed (as the bicarbonate salt), whereas thegreater fraction of sodium is absorbed passivelyas sodium chloride. The small sodium concentra-tion gradient sustained across the jejunal mucosa(13 mEq/liter, Fig. 2), according to this view,would be a consequence of sodium-hydrogen ex-change, which would maintain a low luminalbicarbonate concentration and keep total anionconcentration in luminal fluid lower than that inplasma. The absorption of sodium from isotonicsodium chloride solutions containing no bicar-bonate would simply represent diffusion of sodiumchloride down chemical concentration gradients(lumen chloride 140, plasma 100). Since sodiumchloride has a lower reflection coefficient thansodium bicarbonate, it would be more sensitiveto the movement of water than sodium bicarbonate.

The addition of either glucose or the poorlymetabolized but actively transported galactosestimulated net sodium absorption in the jejunumand increased the transmucosal PD slightly. Thisis in contrast to the ileum where these sugars hadno effect on net sodium transport, but did causea significant increase in PD. It has been suggestedthat the stimulatory effect of the actively trans-ported sugars and amino acids is due to a directcoupling of their active transport process to the

3 A neutral pump transporting sodium and bicarbonateis an alternative possibility.


sodium pump (3-5). If this were the mechanismof action, it would have been expected that theaddition of glucose and galactose, which weretransported rapidly in both the jejunum andileum, would have stimulated sodium absorptionin both areas of the small intestine. In view of thefact that glucose and galactose stimulate sodiumabsorption only in those areas of the intestinewhere bulk water flow influences sodium move-ment, it is attractive to propose that the activetransport of these sugars does not stimulate an ac-tive sodium pump directly, but, instead, generatesan osmotic flow of water which, in turn, enhancessodium absorption. Two observations are, how-ever, difficult to fit into this model. First, in theexperiments shown in Fig. 5, the addition of eitherglucose or galactose stimulates sodium absorptionat any level of water flow; even when net watermovement was zero, a stimulatory effect on sodiumabsorption was noted. Second, these experimentswere designed with a small limiting sodium con-centration gradient of approximately 15 mEq/liter, so that in the absence of sugar there would belittle net sodium movement at zero water move-ment. Thus, the ability of added sugars to stimu-late sodium absorption at zero water absorptionindicates that the ability to absorb sodium againstconcentration gradients has been enhanced.

It is still possible, however, to explain the stim-ulatory effect of sugars on jejunal sodium absorp-tion, even at zero water movement, by a primaryeffect on water flow, without invoking a directcoupling of sugar transport to an active sodiumpump. The model for such a mechanism involvesa fluid circuit in which salt solution moves throughone set of aqueous channels, while pure solventmoves in a reverse parallel fashion through a sec-ond set of channels. This is similar to the fluidcircuit theory originally proposed by Peters andVisscher (22) and more recently by Krugerand Bresler (23). The principal feature of such asystem to explain our observations is that activeglucose absorption generates a local osmotic gradi-ent which results in bulk flow of salt and wateracross a permeable membrane. Now, in order tocounterbalance the water movement generated byglucose transport, it is necessary to add mannitolto the luminal fluid. The mannitol, however, ex-erts its osmotic effect, not only across the perme-able membrane, but also across a second set ofaqueous channels which are visualized as being

permeable only to water, so that reentry of sodiuminto the gut lumen is restricted. When mannitolhas been raised sufficiently to reduce water flowto zero, there is still a positive absorption of saltand water through the permeable portion of themembrane, balanced by a backflow of water intothe lumen through the impermeable portion of themembrane-in other words, a fluid circuit. Insuch a model, therefore, glucose would stimulatesodium absorption at zero net water movement,and, in addition, would increase the concentrationgradient against which sodium can be absorbed.

The data presented in this paper on urea ab-sorption argue strongly for the presence of a fluidcircuit across the jejunal mucosa. First, glucosewas shown to enhance urea absorption at zeronet water flow. This is in favor of the fluid circuitmodel, but could conceivably be explained bycoupling of active glucose and active urea trans-port (even though all the data so far reported onurea absorption suggest that this solute is absorbedpassively). However, bicarbonate also stimulatesurea absorption at zero net water movement, andit is extremely difficult to visualize how both glu-cose and bicarbonate transport could be coupled toactive urea transport. We believe, therefore, thatthe enhancement of urea absorption by glucose andbicarbonate can best be explained by the presenceof a fluid circuit mechanism in jejunal mucosa.

Although we consider these urea experiments tostrongly suggest the presence of a fluid circuitacross the jejunal mucosa that is activated by glu-cose transport in the presence of a poorly ab-sorbable solute (such as mannitol), our data donot necessarily prove that this is the mechanism bywhich glucose stimulates sodium absorption. It ispossible that active sodium transport stimulated byglucose, rather than glucose transport itself, isgenerating the fluid circuit postulated to explainthe urea data. On the other hand, it should be re-called that the definition of active transport ismovement of a substance that cannot be explainedon the basis of electrochemical gradients or sol-vent drag. Ordinarily, solvent drag is consideredinoperative if net water flow is zero, but in thepresence of a fluid circuit system as describedabove, solvent drag force may be operative evenwhen net water flow is zero. Thus, enhancementof sodium absorption in the jejunum against aconcentration gradient at zero water flow by glu-cose cannot be used as a criterion for stimulation


of active sodium transport. This does not, ofcourse, mean that active transport does not exist,but it does mean that the criteria for active trans-port in the human jejunum cannot be met.

A model which explains many features of je-junal transport is illustrated in Fig. 10. In thismodel a fraction of luminal sodium is absorbed bybeing coupled in some way with active bicarbonatetransport (probably via a sodium-hydrogen ex-change). In addition, glucose is actively trans-ported across the jejunal mucosa. Either theactively transported glucose or bicarbonate is de-posited into the intercellular space generating lo-cal osmotic gradients. The tight junction betweenthe cells is assumed to be highly permeable to wa-ter, sodium chloride, and urea, but relatively im-permeable to glucose and to sodium bicarbonate.(Whereas anatomical studies have shown that thisjunction is completely fused [24], it is thoughtto have a low electrical resistance in some epi-thelial membranes [25], and has been suggestedas the site of a shunt pathway for sodium ions inthe frog skin [26] ). Consequently, a solution ofsodium chloride and urea flows across the tightjunction from lumen into the intercellular spacein response to osmotic gradients created by glu-cose and bicarbonate transport. In contrast to thetight junction, a part of the cell membraneproper is assumed to be relatively impermeableto sodium chloride and urea, as well as to glucoseand mannitol. Therefore, when mannitol is addedto luminal fluid to counteract the stimulatory ef-fect of glucose and bicarbonate on bulk flow, its

BulkFlow

Lumen Glucose r NoHC03 Mannitol

Cell jJHCO3I

H20NaCI UUrea H2°

Interstitium

FIGURE 10 A model to illustrate how a fluid circuitsystem might enhance sodium and urea absorption atzero net water flow. In order to achieve reverse flow ofH20, it is necessary that one of the compartments acrosswhich mannital exerts its effect must be isolated osmati-cally from the compartments in which glucose and sodiumbicarbonate are accumulated. This may be in differentregions of the same cell, as shown in this figure, or indifferent cells of the mucosa.

effect will be exerted not only across the tightjunction, but also across the cell membrane, induc-ing a flow of water from interstitial space into thelumen. Thus, at zero net water flow there is stilla positive absorption of water (and sodium andurea) through the permeable membrane, balancedby a backflow of water into the lumen through theimpermeable membrane.

It should be emphasized that we have no datato support this specific type of fluid circuit system.Another possibility is that the movement of solu-tion and water occurs across membranes of twodifferent cell types, rather than across two differ-ent membranes of the same cell.

An additional feature of jejunal absorptionwhich is explained by a fluid circuit system of thetype described in Fig. 10 is the movement of wateragainst an osmotic pressure gradient. In the ab-sence of glucose or bicarbonate there is no waterabsorption against an osmotic pressure gradient.However, in the presence of glucose, water can beabsorbed against osmotic pressure gradients ofapproximately 30 mOsm/kg, and in the presenceof bicarbonate against a gradient of about 10mOsm/kg. These results suggest that the activetransport of glucose and bicarbonate into thepoorly mixed third compartment of this modifiedCurran model is the mechanism responsible forabsorption of water against osmotic pressuregradients.

ACKNOWLEDGMENTS

The authors express their appreciation to StephenMorawski, Charlotte Douglas, Mary Jane Benton, andCarolyn Arnold for their contribution to these studiesand to the preparation of the manuscript.

This paper was supported by research grant AM-06506from the National Institutes of Health.

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7. McHardy, G. J. R., and D. S. Parsons. 1957. Theabsorption of water and salt from the small intestineof the rat. Quart. J. Exptl. Physiol. 42: 33.

8. Hindle, W., and C. F. Code. 1962. Some differencesbetween duodenal and ileal sorption. Am. J. Physiol.203: 215.

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12. Fordtran, J. S., F. C. Rector, T. W. Locklear, andM. F. Ewton. 1967. Water and solute movement in thesmall intestine of patients with sprue. J. Clin. Invest.46: 287.

13. Fordtran, J. S. 1966. Marker perfusion techniques formeasuring intestinal absorption in man. Gastroenter-ology. 51: 1089.

14. Conway, E. J. 1957. Microdiffusion Analysis andVolumetric Error. C. Lockwood, London.

15. Fordtran, J. S., and T. W. Locklear. 1966. Ionicconstituents and osmolality of gastric and small-intestinal fluids after eating. Am. J. Digest. Diseases.11: 503.

16. Bucher, G. R., J. C. Flynn, and C. S. Robinson. 1944.The action of the human small intestine in altering thecomposition of physiologic saline. J. Biol. Chem. 155:305.

17. Wilson, T. H. 1962. Intestinal Absorption. W. B.Saunders Co., Philadelphia.

18. Hubel, K. A. 1966. Ileal bicarbonate secretion and itsdependency on intraluminal chloride. Proc. CentralSoc. Clin. Res. 39: 37. (Abstr.)

19. Kedem, O., and A. Katchalsky. 1961. A physicalinterpretation of the phenomenological coefficients ofmembrane permeability. J. Gen. Physiol. 45: 143.

20. Gustke, R. F., G. E. Whalen, J. E. Geenen, and K.H. Soergel. 1967. Mucosal potential difference in theintact human small intestine. Gastroenterology. 52:1134. (Abstr.)

21. Parsons, D. S. 1956. The absorption of bicarbonate-saline solutions by the small intestine of the whiterat. Quart. J. Exptl. Physiol. 41: 410.

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