permeability characteristics of the

Journal of Clinical InvestigationVol. 44, No. 12, 1965

Permeability Characteristics of the Human Small Intestine *JOHNS. FORDTRAN,t FLOYDC. RECTOR,JR., MAYNARDF. EWTONt

NICHOLASSOTER,§ ANDJOHNKINNEY §(From the Department of Internal Medicine, The University of Texas Southwestern Medical

School, Dallas, Texas)

Relatively little information is available concern-ing the membrane structure of the mucosal cells ofthe small intestine. Hdber and Hober (1) andSchanker, Tocco, Brodie, and Hogben (2, 3)studied the absorption of a variety of substances inrats and found that absorption rate increased aslipid solubility increased. From this it has beendeduced that mucosal cell membranes are lipoidin nature and that lipid-soluble substances are ab-sorbed by dissolving in the cell membrane. How-ever, it has been known for many years that smallmolecules, although lipid insoluble, can also be ab-sorbed from the gastrointestinal tract. This hasled to the hypothesis that, although essentiallylipoidal, cell membranes are interspersed with wa-ter-filled pores, through which small molecules candiffuse. Hbber and Hdber (1) tested this hy-pothesis in the small intestine of the rat by corre-lating the absorption rate of nonlipid-soluble sub-stances with their molecular size. Their resultssupport the thesis that these molecules are ab-sorbed by diffusion through water-filled pores,since small molecules were absorbed more rapidlythan larger ones, and beyond a certain size (mo-lecular weight about 180, which corresponds to amolecular radius of about 4 A) no penetration oc-curred. Lindemann and Solomon's studies (4)are in close agreement, since they experimentallydetermined, by an independent method, the poreradius of the luminal surface of the rat jejunal cells

* Submitted for publication April 14, 1965, acceptedAugust 19, 1965.

Supported by research grant AM-06506 and traininggrant TI-AM-5395 from the National Institutes ofHealth.

t Address requests for reprints to Dr. John S. Ford-tran, Dept. of Internal Medicine, University of TexasSouthwestern Medical School, Dallas, Texas 75235.

t The work was done during the tenure of a postdoc-toral traineeship under grant TI-AM-5395.

§ The work was done during the tenure of a summerpredoctoral traineeship under grant TI-AM-5395.

to be approximately 4 A. This is the only avail-able estimate of intestinal pore size in any species.and no estimates at all are available for pore sizeat different levels of the small intestine.

The purpose of our studies, therefore, was toevaluate the permeability of the human intestinalmucosa by measuring the effective pore size at dif-ferent levels of the small intestine. The theoreticalbasis for the present studies rests on the demon-stration by Staverman (5) and Solomon (6) thatthe ability of a nonlipid-soluble solute to exert aneffective osmotic pressure gradient 1 across a mem-brane is a function of its molecular radius relativeto the radius of the water-filled pores in that mem-brane. Thus, the degree to which a solute ofknown molecular size is capable of exerting its fulltheoretic osmotic pressure gradient, which is de-fined as the reflection coefficient (a), can be usedto calculate the pore size of the membrane, which.in turn, determines the permeability of the mem-brane to nonlipid-soluble solutes.

To determine o- for a test solute, water move-ment into an intestinal segment perfused with ahypertonic solution of the test solute was com-pared with the water movement into the samesegment in response to hypertonic mannitol.Since mannitol was demonstrated not to penetratethe small intestinal mucosa to a measurable extent,its effective and theoretic osmotic pressure shouldbe equal, and it was therefore assumed to have a a

of 1.0. By these techniques we found that the

1 The effective osmotic pressure is the actual pressureexerted across a membrane separating two solutions ofdiffering concentration. If the membrane is perfectlysemipermeable, the effective and theoretic osmotic pres-sures are equal. However, for membranes that are notperfectly semipermeable, the effective osmotic pressurewill be less than the theoretic value. The effective os-motic pressure divided by the theoretic osmotic pressurehas been defined by Staverman (5) as the reflectioncoefficient.

1935

FORDTRAN,RECTOR, EWTON,SOTER, AND KINNEY

effective pore size in the upper small intestine isat least twice that in the lower small intestine.

Methods

Measurement of osmotic flow of water. The movement

of water into the intestinal lumen in response to hyper-tonic solutions of urea, erythritol, and sodium chloridewas compared in a given gut segment to that inducedby solutions of mannitol of the same tonicity; thesecomparative studies were performed at different levelsof the small intestine of 20 normal male and female sub-jects between the ages of 21 and 40. The net water move-

ment into the gut lumen was measured by the dilutionof a nonabsorbable marker, a technique that has beenpreviously validated (7-9).

Subjects were intubated with a triple-lumen polyvinyltube prepared by fusing single tubes with tetrahydro-furan. The opening of the tube used for infusion of thetest solutions was 10 and 30 cm proximal to that of thetwo collecting tubes. The internal diameter of the in-fusion tube was 0.7 mm; that of the collecting tubes was

1.8 mm. A 2-cm piece of 16F polyvinyl tubing contain-ing multiple small holes (no larger than 1.8 mm) was

attached to the opening of each collecting tube to facili-tate drainage. The location of the tube was determinedfluoroscopically after contrast material had been injectedinto the tube with the most distal opening, and was ex-

pressed as the length of tubing between the incisor teethand the infusion point when slack had been removed fromthe portion of the tube that was in the stomach. Ex-pressed in this way, the pylorus is approximately 60, theligament of Treitz 90, and the ileocecal valve 350 cm fromthe teeth.

The experimental protocol was as follows:1) All studies were performed after subjects had fasted

for 8 hours.2) The first study was performed when the infusion

point was 85 to 100 cm from the teeth, which is in thegeneral area of the ligament of Treitz.

3) Hypertonic mannitol was infused for 90 minutes.The first 30 minutes of this period was used for equili-bration; this was followed by a 1-hour test period duringwhich samples were collected from the proximal anddistal collecting sites.

4) Immediately thereafter, the material infused was

changed to a hypertonic solution of a penetrating solute(urea, erythritol, or sodium chloride), and after a 30-min-ute washout and equilibration period a second 1-hour col-lection was made from the proximal and distal collectingsites.

5) After completion of this initial study, subjects were

permitted to eat clear liquids, and the tube was allowedto pass into a lower segment of the small intestine.

6) After another 8-hour period, during which the sub-jects fasted, the lower segment was perfused sequentiallywith hypertonic solutions of mannitol and test solute, as

described in steps 3 and 4 above.

The sequence was repeated at three or four levels ofthe small intestine of each subject, so that an entire studymight last as long as 3 to 4 days. The order in which thehypertonic solutions were perfused was alternated amongsubjects.

All hypertonic solutions contained a nonabsorbablereference material, polyethylene glycol (PEG, molecularweight 4,000) (8), and were infused at a constant rateof 9 ml per minute via a Bowman infusion pump. Be-tween the infusion point and the proximal sampling site,the infused solution mixed to a variable extent with di-gestive secretions as well as with water and solutes,which move across the mucosa of the intestine into thehypertonic perfusate. After passing the proximal site,however, the osmolality of the perfusate can be alteredonly by water or solute moving across the small intestinalmucosal cells. To prevent water movement secondary tochemical gradients of sodium, all test solutions, excepthypertonic sodium chloride solutions, contained 140mmoles per L of sodium chloride. That the volumechange between the proximal and distal sampling sites isnot significantly influenced by absorption or secretion ofsodium salts and water is evident from a separate seriesof experiments (10) showing that, in the absence of glu-cose, j ejunal and ileal absorption rates of water fromisotonic electrolyte solutions are extremely small andnegligible compared to the rates of bulk water movementin response to osmotic pressure gradients. This obser-vation is in accord with experiments by Fisher (11),who found that in rats the absorption of isotonic Kreb'ssolution containing no glucose is "not significantly dif-ferent from zero," whereas even small amounts of glucosemarkedly stimulate water absorption. Therefore, dilu-tion of the solution between the proximal and distal col-lecting sites, as measured by the dilution of the nonab-sorbable PEG, is a measure of the volume of watermoving across the mucosal cells in the' 20-cm segment ofintestine in response to the hypertonic luminal contents.The equations used in this calculation were as follows:

I X [PEG]i SQE E[PEG]p a'

Q QEX[PEG]d , and

QN = QL - QE,

where I is the infusion rate in milliliters per minute; QEand QL are the flow rates, in milliliters per minute enter-ing and leaving the 20-cm test segment, respectively;[PEG] ,, [PEG],; and [PEG]d are, respectively, the PEGconcentrations of the infused solution, and the fluid ob-tained from the proximal and distal collecting sites; Sp isthe rate of sampling from the proximal collecting site inmilliliters per minute; and QN is the net movement ofwater across the mucosa of the 20-cm intestinal segmentin milliliters per minute.

In the upper intestine there was greater dilution of thematerial infused between the infusion point and the proxi-mal collecting site than in the lower intestine. Further-

1936

PERMEABILITY CHARACTERISTICSOF THE HUMANSMALL INTESTINE

more, the degree of dilution of the perfused fluid variedwith different hypertonic solutions. This poses two prob-lems: the- maintenance of equal mean flow rates and equalmean osmotic pressure gradients in the test segment. Tomaintain equal flow rates through the upper and lowerintestine with different hypertonic solutions, it was neces-sary to vary the rate of collection through the proximalcollecting tube. In the upper jejunum the rate of samplingfrom the proximal tube was 4 ml per minute for manni-tol, 2.5 ml per minute for erythritol, and 1.5 ml per min-ute for urea and sodium chloride solutions. In the mid-intestine, corresponding sampling rates were 2.5 for man-nitol and 1.5 for all other solutions. In the lower smallintestine, the sampling rate was 1.5 ml per minute re-gardless of solution. These sampling rates were selectedafter trial and error, because they yielded approximatelyequal mean perfusion rates through the 20-cm testsegments.

It was also found by trial and error that approximatelyequal osmotic pressure gradients in the upper and lowerintestinal segments were achieved when the osmolality ofthe infused material was varied as follows: upper in-testine, 800 mOsmper kg, and lower intestine, 550 mOsmper kg. Infusion of solutions with these osmolalities re-sulted in mean osmotic gradients between luminal con-tents and blood of 95 to 300 mOsmper kg.

In all studies, an intravenous infusion of 3.5% glucosesolution in water was given at a constant rate to preventdehydration. The rate of infusion was 600 ml per hourin jejunal studies and 300 ml per hour for mid- and lowerintestinal studies. Blood was collected at the beginningand end of each perfusion; in no case did serum osmo-lality exceed 300 mOsmper kg, nor did dilution occur.To minimize variation in the level of circulating anti-diuretic hormone, 200 mUper hour of aqueous Pitressinwas given intravenously.

Analytical methods. PEGwas determined in duplicateby the method of Hyden (12). In our hands this methodis reproducible within ±2% on duplicate testing. Man-nitol was analyzed by the method of Corcoran and Page(13), sodium by flame photometer, and osmolality byfreezing point depression in a Fiske osmometer.

Results

Determination of filtration coefficients for bulkflow of water at different levels of the small in-testine. To determine accurately the filtrationcoefficient for bulk flow of water (i.e., the quantityof water that will be moved across a given lengthof intestinal mucosa under the influence of knowneffective osmotic pressure), it is necessary to estab-lish the concentration gradient with a substancethat does not penetrate the membrane and istherefore capable of exerting its full theoretic os-motic pressure. Mannitol was chosen because inmost animal studies it has been found not to be ab-

[PEG]Sample to[PEG] Infused

.8

.6

0 ~~~~04 . A

X / Jej/unumr/*laum

L IA

.6 .6 1.0 1.2 1.4[Mannitoll Sample[1Mannitoll Infused

FIG. 1. ABSORPTIONOF MANNITOL. Solutions contain-ing mannitol and polyethylene glycol (PEG) were in-fused into the jejunum (ten studies) and ileum (six stud-ies) of normal subjects. The relative dilution or concen-tration of these solutes after traversing 20-cm segmentsof intestine is shown. A line of perfect agreement is in-dicated. The dilution or concentration of mannitol wasapproximately the same as that of PEG, indicating thatmannitol is not absorbed to a measurable extent within20-cm segments of human small intestine.

sorbed (3, 14) or at the most, absorbed at a verylow rate (15). As shown in Figure 1, mannitolabsorption did not occur in 20-cm segments ofhuman jejunum (ten studies) and ileum (sixstudies) of normal human subjects. It shouldtherefore exert its full theoretic osmotic pressure.

When solutions made hypertonic by the addi-tion of mannitol. were perfused through the smallintestine, there was rapid movement of water intothe lumen of the test segment, which decreased theosmolality of the luminal contents. Consequently,the osmotic pressure gradient between plasma andluminal contents was not constant, but fell progres-sively from proximal to distal points in the testsegment. Four studies were performed to deter-mine the profile of the osmotic pressure gradientalong the length of a perfused segment. In thesestudies intestinal contents were sampled at foursites, 10 cm apart, beyond the point of constant in-fusion. The osmolality of plasma was measuredat the beginning and end of each perfusion and influid collected continuously for 1 hour from eachof the four collecting tubes. The plasma osmolalitywas subtracted from the osmolality of fluid col-lected from the four sampling sites to obtain the

1937

1.4

!.2


Pylorus

FIG. 2. THE VOLUMEOF WATERMOVEMENTPPER MILLIOSMOLE GRADIENT IN 20-CM SEGMENTSINTESTINE IN RESPONSETO HYPERTONICMANNII

nitol is assumed to exert its full theoretic osrsure (see text), and results therefore are a meafiltration coefficient of the intestinal mucosa.

was fitted visually. Lg. Tr. = ligament of Trei

theoretic osmotic pressure gradient at eiIn all four studies a straight-line relatiottained when the logarithm of the osmoticgradient was plotted against distancesubsequent studies the mean osmoticgradient in the test segment was deterplotting the proximal and distal osmoticgradients on semilogarithmic paper andmidpoint value.

By dividing the calculated volume chduced by hypertonic mannitol by the mea

pressure gradient between plasma an(fluid, the filtration coefficient at differof the small intestine was obtained. Tare shown in Figure 2. There was a pfall in water movement from proximalsegments of the small intestine; in the a

ligament of Treitz approximataely 0.0'minute of water moved into the 20-cmment per mOsmgradient, whereas inileum the corresponding value was 0.1conclude, therefore, that the upper intesmuch higher permeability to the bulk floithan does the lower small intestine.

Water movement produced by othcompared to that induced by mannitol.nary studies showed that when intestina

were perfused with solutions made hypertonic byurea, erythritol, or NaCl, the osmotic pressuregradient between intestinal lumen and blood fellexponentially as a function of length in a fashionsimilar to that observed with mannitol. Therefore,mean osmotic pressure gradient and net watermovement per unit osmotic pressure gradient werecalculated as described for mannitol in the previoussection.

At a given level in the small intestine in a givenindividual the rate of fall in osmolality was ap-proximately the same for hypertonic solutions of

350 mannitol and urea (18 studies), mannitol andlleo ecaf erythritol (18 studies), and mannitol and sodiumvalve chloride (13 studies). This phenomenon is prob-

'ER MINUTE ably due to the fact that fall in osmolality results, OF SMALL from two different but reciprocally related factors:TOLt. Man- penetration of the membrane by the solute andsure ofth bulk flow of water into the intestinal lumen. For

The line large molecules that are unable to penetrate, theitz. fall in osmolality is due entirely to bulk water flow.

For smaller molecules, both mechanisms are oper-ach point. ative but in a reciprocal fashion: the greater then was ob- solute penetration, the less the bulk flow of water,

pressure and vice versa.In all The results of studies comparing water move-

pressure ment induced by mannitol and urea, mannitol andmined by erythritol, and mannitol and sodium chloride are

pressure shown in Figures 3, 4, and 5. On the left side oftaking the each Figure water movement per minute per mil-

liosmole gradient for each pair of solutes is shownianges in- at different levels of the small intestine. In thein osmotic upper intestine mannitol produced more waterd luminal movement than did the other solutes, but in the*ent levels lower intestine the other solutes pulled almost as'he results much water as mannitol. This can be appreciatedrogressive more clearly in the right side of each Figure, which

to distal plots the ratio of water movement caused by urea,rea of the erythritol, or sodium chloride divided by that pro-44 ml per duced by mannitol in the same test segment. Thel test seg- ratio was low in the upper intestine and rose pro-the lower gressively, approaching unity in the lower ileum.

0005. We Average values for the osmotic ratio in the uppertine has a intestine (80 to 200 cm from the teeth) were 0.48w of water for urea, 0.64 for erythritol, and 0.58 for sodium

chloride. Corresponding values for the lower in-er solutes testine (200 to 300 cm from the teeth) were 0.89

Prelimi- for urea, 0.98 for erythritol, and 1.0 for sodium1 segments chloride. In the sodium chloride studies in the

1938


50 100 150 200cm from Teeth

250 300

1.o0

.80

.? .60

.4

.2

50 100 150 200 250 300cm from Teeth

FIG. 3. WATERMOVEMENTIN 20-cm SEGMENTSOF SMALL INTESTINE IN RESPONSETO

OSMOTICGRADIENTSEXERTEDBY UREAANDMANNITOL (LEFT SIDE) AND THERATIO OF WATER

MOVEMENTPRODUCEDBY UREA DIVIDED BY THAT PRODUCEDBY MANNITOL (RIGHT SIDE).

lower intestine one value for the osmotic ratio was

unreasonably high (1.50). If this value is ex-

cluded, the mean osmotic ratio for sodium chloridein the lower small intestine was 0.95.

Concentration of sodium in filtrate. To deter-mine whether flow of water into the intestine inresponse to osmotic pressure gradients can alsopull plasma sodium salts into the lumen, the con-

centration of sodium in the filtrate was calculated.From the concentration of PEG, net movement ofwater into the 20-cm test segment was known.In 46 of the hypertonic mannitol perfusion studies,sodium concentration in fluid obtained from eachcollecting site was measured, and net sodium move-

ment into the test segment was determined. Know-ing net water and sodium movements, we calcu-lated the sodium concentration in water movingacross the mucosal cells in response to osmoticgradients.

The concentration of sodium in the filtrates av-

eraged 60 15.9 (SD) mEq per L in the upper

jejunum (80 to 125 cm from the teeth), 34 19.3mEq per L in the mid-intestine (125 to 200 cm),and 10 25.7 mEqper L in the lower ileum (200to 300 cm).

Discussion

The present studies demonstrate several im-portant characteristics of the osmotic flow of wateracross the mucosa of the small intestine. First,

the flow of water induced by known effective os-motic pressure gradients (i.e., filtration coefficient)is approximately ninefold greater in the first partof the jejunum than in the terminal ileum. Sec-ond, the ability of a given concentration gradientto promote the movement of water is not constantbut varies with the molecular size of the substanceused. For instance, in the jejunum the flow ofwater per minute per milliosmole gradient wasover two times as great with mannitol as withurea. Finally, the variation in the effective os-

motic pressure exerted per unit concentrationgradient by different solutes was more marked inthe upper than in the lower small intestine. In thejejunum a given concentration gradient of urea

was approximately 48%o as effective as was man-

nitol, whereas in the ileum the effective osmoticpressure with similar concentration gradients ofurea and mannitol was approximately equal.

Staverman (5) has pointed out that the abilityof a nonlipid-soluble solute to generate effectiveosmotic pressure is inversely related to its abilityto permeate the membrane, which in turn is de-pendent upon molecular size of the substance inquestion relative to the size of the water-filledpores that penetrate the membrane (6). To makethis relationship more explicit, the term "reflec-tion coefficient" has been introduced, which is de-fined as the ratio of the effective to the theoreticosmotic pressure (see footnote 1). Thus, for a

solute that is as large as or larger than the water-

.05

E .

0

E.03

E

E .02

* Monnitolo Urea

I 1 l

0

UreaMonnitol

0

o @o 00

0 0 0

0

0 0 0a

00

0

1939

,AL

'°/1-

FORDTRAN,RECTOR, EWTON, SOTER, AND KINNEY

cm from Teeth

12k

1.0to

60.8-22080E 06

0.4

02F

FIG. 4! WATERMOVEMENTIN 20-CM SEGMENTSOF SMALL INTESTINE IN RESPONSETO

OSMOTICGRADIENTS EXERTEDBY ERYTHRITOL AND MANNITOL (LEFT SIDE) AND THE RATIOOF WATERMOVEMENTPRODUCEDBY ERYTHRITOL DIVIDED BY THAT PRODUCEDBY MANNITOL

(RIGHT SIDE).

filled pores, the effective and theoretic osmoticpressures will be equal, and the reflection coeffi-cient will be 1. In contrast, for solutes that are

smaller than the pore the effective osmotic pres-

sure will be less than the theoretic value, and thereflection coefficient will fall between 0 and 1.0.

The data in the present studies can be used inestimating the reflection coefficients of urea, eryth-ritol, and sodium chloride in the small intestine.Since mannitol was shown not to be absorbed, we

assumed that it exerts its full theoretic osmoticpressure and has a reflection coefficient of 1.0.Therefore, ax = Qx/Qm, where Qmand Qx rep-

resent the volume of water flow induced by knowntheoretic osmotic gradients of mannitol and soluteX, respectively, and ax is the reflection coefficientof solute X.

The ratio of the osmotic flows for urea, erythri-tol, and sodium chloride relative to mannitol is,therefore, a measure of the reflection coefficients ofthese solutes. These values have been calculatedfor each experiment and are presented in the right-hand side of Figures 3, 4, and 5. Since the re-

flection coefficients for all of these solutes were

higher in the ileum than in the jejunum, one may

conclude that the lower small intestine is less per-meable than the upper small intestine.

Although differences in the filtration coefficientsin the upper and lower intestine might be partlyexplained on the basis of differences in surface area

or blood flow, the markedly different reflectioncoefficients for urea, erythritol, and sodium chlo-ride indicate a basic difference in the intrinsic per-

meability of the jejunum and ileum. There isgood evidence that bulk flow of water (14, 16) andsmall lipid-insoluble solutes such as urea and eryth-ritol (1, 6) penetrate cell membranes through wa-

ter-filled channels or pores. The over-all perme-

ability of the cell membrane to water and water-soluble solutes, therefore, is determined to a largeextent by the number and characteristics of thesechannels. Since, however, the reflection coeffi-cients of the various solutes are independent ofboth the number of channels and their length, itseems reasonable to assume that the differencesin both the filtration coefficients and the reflectioncoefficients in the jejunum and ileum are due todifferences in the effective cross-sectional area ofeach channel.

Several factors make it difficult to assess thecross-sectional area of these channels in any givenarea of the intestine. First, there are at least threemembranes in series (luminal and basal epithelialcell membranes and capillary membrane of the

ErythritolMannitol 0 0

0

0 0

0~~~~~~0 00~~~~~0

0

0~~~~

0

0o

I

50 100 150 200 250cm from Teeth

300

1940


0 * Mannitolo NoCI

0

I I le

'.5

1.2

1L..212 .8

F- .6

.4

.2

i 0

ANOCI 0Mannitol

o 0

0

_ oO~~~~~~ 0

O

50 /00 150 200 250 300 50 100 /50 200 250 300cm from Teeth cm from Teeth

FIG. 5. WATERMOVEMENTIN 20-CM SEGMENTSOF SMALL INTESTINE IN RESPONSETO

OSMOTICGRADIENTS EXERTEDBY SODIUM CHLORIDE AD MANNITOL (LEFr SIDE) AND THE

RATIO OF WATERMOVEMENTPRODUCEDBY SODIUM CHLORIDE DIVIDED BY THAT PRODUCED

BY MANNITOL (RIGHT SIDE).

venous capillaries) that determine over-all perme-

ability. Second, the channels in each of these dif-ferent membranes might differ widely in size.Finally, the channels in any given membrane prob-ably do not have uniform dimensions. For pur-

poses of mathematical analysis, and for inter-preting our results, however, it is useful to con-

sider the intestinal barrier as a single membranepenetrated by right-cylindrical pores of uniformdimensions. In this way an effective pore radiusof the intestine can be calculated with concepts ofRenkin (17) and Solomon (6). It should bestressed that the calculated effective pore radiusdoes not apply to any single membrane separatingthe intestinal lumen from blood; rather, the calcu-lated values indicate the size that right-cylindricalpores of uniform size would have to be in a singlemembrane to have given the experimentally de-termined reflection coefficients for urea and eryth-ritol that we found in the human small intestine.

Using formulas derived by Renkin (17), Solo-mon (6) has pointed out that curves can be drawnrelating the reflection coefficient of a nonlipid-soluble solute to the pore radius of a membrane.In Figure 6, three such curves are shown, one forurea (molecular radius, 2.3 A), one for erythritol(molecular radius, 3.2 A), and one for mannitol

(molecular radius, 4.0 A). These radii were de-termined from molecular models by Schultz andSolomon (18) and are the same as those used byLindemann and Solomon in calculating the equiva-lent pore radius of the luminal surface of the ratjejunum (4).

By using the curves in Figure 6, the effectivepore radius in any area of the intestine can bedetermined from the reflection coefficients forurea and erythritol obtained in that region. Forexample, in the jejunum the average reflectioncoefficient for urea of 0.48 intercepts the urea curve

at an effective pore radius of 6.7 A, whereas thereflection coefficient of erythritol intercepts itscurve at a pore radius of 8.8 A. In striking con-

trast, the corresponding intercept points in theileum indicate values of 3.0 and 3.8 A, respectively.Thus, the effective pore radius in the jejunum isapproximately twice the size of the effective poreradius in the ileum. Since there was a progressiverise in the reflection coefficients of urea and eryth-ritol from upper jejunum to lower ileum, one can

conclude that there is a progressive decrease ineffective pore size in going from the upper to thelower small intestine.

Theoretically, mannitol. which has a molecularradius of 4.0 A, should be able to penetrate the

.06F

E 0(0

0.04N\Ec

E

1-_

.02 _

.01_

1941

..05r


sr

3 5 7 9 11 13 15Pore Radius A

FIG. 6. THEORETIC CURVES DEPICTING THE RELATION-

SHIP BETWEEN THE REFLECTION COEFFICIENT (a) AND

MEMBRANEPORE RADIUS FOR THE NONLIPID, NONELECTRO-

LYTE SOLUTESUSEDIN THE PRESENTSTUDY. These curves

were calculated by computer from Renkin's equations(17) as used by Solomon (6). The reflection coefficientsof urea and erythritol in jejunum and ileum are indicated.

upper small intestine (effective pore radius of 7to 8.5 A) and therefore should have a reflectioncoefficient of less than 1.0. Our failure to demon-strate absorption of mannitol in the jejunum (Fig-ure 1) most likely is because molecules of 4 Ahave a theoretic reflection coefficient of 0.82 to0.92 when related to a membrane with pores thesize estimated for the jejunum. With this highreflection coefficient, the absorption rate of man-

nitol would be extremely slow and perhaps unde-tectable by our method. Our assumption that thereflection coefficient of mannitol is 1.0 might,therefore, possibly result in an error when calcu-lating the pore radius of the upper small intestine.This error, however, would be in the direction ofoverestimating the reflection coefficients for urea

and erythritol in the jejunum (but not in theileum, where the reflection coefficient of mannitolwould be expected to be 1) and therefore under-estimating the effective jejunal pore size. Thus,the difference in effective pore radius between theupper and lower small intestine may be slightlygreater than our calculations indicate.

The variation in effective pore size affords a

reasonable explanation for some of the differencesin absorptive characteristics of the upper and lowersmall intestine. For example, Fordtran, Clodi,

Soergel, and Ingelfinger (19) found that xylosewas absorbed rapidly in the jejunum, but that ab-sorption decreased progressively in the more dis-tal segments, and almost none was absorbed in thelower ileum. Since pentoses have a molecularradius of approximately 3.5 A (18), it is reason-able to suggest that their absorption profile is de-termined primarily by the gradient of effectivepore size in the intestine.

Of greater physiologic importance is the effectof variations in effective pore size on the absorp-tion of water and electrolytes. As shown in Fig-ure 2, the filtration of water induced by known os-motic pressure gradients was ninefold greater inthe upper jejunum than in the lower ileum. Thisdifference in filtration coefficient parallels thechange in pore size and is therefore at least par-tially explainable on the basis of Poiseuille's law,which states that the flow of water through cylin-drical pores is proportional to the fourth power ofthe radius (20). Bulk water flow is, therefore,very sensitive to pore size, and a twofold differencein pore size could easily account for a ninefold dif-ference in filtration coefficients;

The marked differences in the rate of bulk flowof water in the jejunum and ileum are in strikingcontrast to the relatively slight differences in therates of unidirectional diffusion of water previ-ously reported by Fordtran and his colleagues (7)and by Whalen, Harris, and Soergel (21). Wehave, furthermore, verified this apparent discrep-ancy in the same intestinal segments of jejunumand ileum of three normal subjects. This disparitybetween diffusion and bulk flow is not, however,unique to the small intestine. For instance, An-dersen and Ussing (16) found that enlarging thepore size of the frog skin by antidiuretic hormonehas a marked effect on the bulk flow of water (in-creased three- to fourfold) but only a slight effecton the diffusion of water (increased less than 1.5-fold). One reason for this discrepancy, as previ-ously pointed out by others, is that diffusion of wa-ter is proportional to the second, but bulk flow tothe fourth, power of the radius of the pore. Asmall change in pore radius, therefore, will have amuch greater effect on bulk flow than on diffusionof water. Berliner (22) has suggested that anadditional factor might be changes in the shape ofa pore in such a way that resistance to bulk flowis altered without influencing the total pore area

1942

PERMEABILITY CHARACTERISTICS OF THE HUMANSMALL INTESTINE

available for diffusion of water. Since in prac-tically every instance the force driving net move-ment of water across a living membrane is eitherhydrostatic or osmotic pressure, the factors gov-erning the bulk flow of water, rather than dif-fusional flow, are of primary importance. The fail-ure of isotopic diffusion to depict the true differ-ences in the upper and lower intestine illustratesthe futility in using isotopic techniques alone tocharacterize water movement across biologicmembranes.

Water absorption in the small intestine is gen-erally considered to occur passively as a conse-quence of the active absorption of glucose, sodium,and so on. The transport of solute is believed tocreate a local osmotic pressure on the blood sideof the intestinal cell membrane that promotes thebulk flow of water through pores. Just as bulkflow of water into the intestinal lumen in responseto perfusion with a hypertonic solution was muchless in the ileum, net water absorption secondaryto active solute transport will be relatively re-stricted in the ileum compared to the jejunum.Stated in another way, in order to achieve thesame rate of water absorption in the two areas ahigher local osmotic gradient would be requiredin the ileum than in the jejunum. This is theprobable explanation of Visscher's observation thatduring absorption of initially isotonic solutions,jejunal solutions remain isotonic, whereas ilealsolutions become slightly but definitely hypotonicto plasma (23).

As a final point, the present studies provide ameans of evaluating the extent to which the ab-sorption of water from the intestine may facilitatethe movement of solutes across the intestinal mu-cosa. In this process, which has been termed"solvent drag," the solutes in effect are caught inthe moving stream of water and swept throughthe pores of the intestinal cells. The extent towhich solvent flow across a membrane will pro-mote the net movement of a given solute, however,is directly related to the reflection coefficient ofthat solute, as discussed by Kedem and Katchalsky(24). This relationship is explicitly stated by theequation, J. = C8(1 - o8)J,, where J8 is the rateof solute movement due to solvent drag, J, the rateof solvent flow, and C8 the concentration of thesubstance in the parent fluid. Stated in anotherway, the concentration of the substance in the fil-

trate or absorbate (J./J,) is equal to its concen-tration in the original fluid times (1 - a). If,therefore, the reflection coefficient for a given sub-stance is very low, solvent drag will have a largeeffect on its movement, whereas if the a is veryhigh, solvent flow will have little or no effect.Thus, in the upper jejunum where the reflectioncoefficient for NaCl is relatively small (0.45), thebulk flow of water should have a significant effecton its transport rate; NaCl concentration in a fil-trate or absorbate should be roughly 55% of itsconcentration in the parent fluid. In the lowersmall intestine, where the reflection coefficient forNaCl approaches 1.0, solvent drag should havelittle or no effect on its movement. The effect ofsolvent drag on the movement of sodium salts isclearly demonstrated in studies with hypertonicmannitol. In the upper jejunum the concentra-tion of sodium salts in the filtrate pulled fromplasma into the lumen was approximately 60 mEqper L, or approximately 40% of plasma concen-tration; this figure is in reasonable agreement withthat predicted from the reflection coefficient. Incontrast, in the lower ileum the concentration ofsodium in the filtrate was low (average, 10 mEqper L). These different concentrations of sodiumin the filtrates in the upper and lower small intes-tine serve as independent confirmation of the dif-ferent values for aNaCl in these two areas of theintestine.

Summary

Water movement into the small intestine in re-sponse to osmotic gradients created by nonlipid-soluble solutes of graded molecular size (urea, 2.3A; erythritol, 3.2 A; and mannitol, 4.0 A) and bysodium chloride was measured in normal subjects.Mannitol was not absorbed to a measurable extent,and this solute was therefore assumed to be capableof exerting its full theoretic osmotic pressure.Water movement in response to osmotic gradientscreated by mannitol was, therefore, a reasonablyaccurate measure of the filtration coefficient of thesmall intestinal mucosa. It was found that therewas a progressive fall in bulk water movementfrom proximal to distal segments of the small in-testine in response to hypertonic mannitol, theupper intestine showing a ninefold higher perme-ability to water flow than the lower small intestine.

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The ability of urea, erythritol, and sodium chlo-ride to exert osmotic pressure, relative to that ex-erted by mannitol, was found to vary at differentlevels of the small intestine. In the jejunum, urea,erythritol, and NaCl were only 45 to 60%o as ef-fective as mannitol in inducing bulk water flow,whereas in the ileum they were almost as effectiveas mannitol. These data indicate a basic differencein the permeability of the upper and lower smallintestine that cannot be explained by differencesin surface area or blood flow. The observationscan be explained by differences in the radius ofpores in the membranes of the small intestine.With the formulas and concepts of Renkin andSolomon the effective pore radius of the jejunumwas calculated to be at least two times larger thanthat of the ileum.

References1. Hbber, R., and J. Hbber. Experiments on the ab-

sorption of organic solutes in the small intestineof rats. J. cell. comp. Physiol. 1937, 10, 401.

2. Schanker, L. S., D. J. Tocco, B. B. Brodie, and C. A.M. Hogben. Absorption of drugs from the ratsmall intestine. J. Pharmacol. exp. Ther. 1958,123, 81.

3. Hogben, C. A. M., D. J. Tocco, B. B. Brodie, andL. S. Schanker. On the mechanism of intestit alabsorption of drugs. J. Pharmacol. exp. Ther.1959, 125, 275.

4. Lindemann, B., and A. K. Solomon. Permeabilityof luminal surfaces of intestinal mucosal cells.J. gen. Physiol. 1962, 45, 801.

5. Staverman, A. J. The theory of measurement ofosmotic pressure. Recueil 1951, 70, 344.

6. Solomon, A. K. Measurement of the equivalent poreradius in cell membranes in Membrane Transportand Metabolism, A. Kleinzeller and A. Kotyk, Eds.New York, Academic Press, 1960, p. 94.

7. Fordtran, J. S., R. Levitan, V. Bikerman, B. A. Bur-rows, and F. J. Ingelfinger. The kinetics of wa-ter absorption in the human intestine. Trans.Ass. Amer. Phycns 1961, 74, 195.

8. Jacobson, E. D., D. C. Bondy, S. A. Broitman, andJ. S. Fordtran. Validity of polyethylene glycolin estimating intestinal water volume. Gastroen-terology 1963, 44, 761.

9. Cooper, H., R. Levitan, J. S. Fordtran, and F. J.Ingelfinger. A method for studying absorption ofwater and solute from the human small intestine.In preparation.

10. Malawer, S. J., M. Ewton, J. S. Fordtran, andF. J. Ingelfinger. Interrelation between jejunalabsorption of sodium, glucose and water in man.J. clin. Invest. 1965, 44, 1072.

11. Fisher, R. B. The absorption of water and of somesmall solute molecules from the isolated small in-testine of the rat J. Physiol. (Lond.) 1955, 130,655.

12. Hyden, S. A turbidometric method for the deter-mination of higher polyethylene glycols in biologicmaterials. Annals of the Agricultural College ofSweden 1955, 22, 139.

13. Corcoran, A. C., and I. H. Page. A method for thedetermination of mannitol in plasma and urine. J.biol. Chem. 1947, 170, 165.

14. Curran, P. F. Na, Cl and water transport by ratileum in *itro. J. gen. Physiol. 1960, 43, 1137.

15. Hindle, W., and C. F. Code. Some differences be-tween duodenal and ileal sorption. Amer. J.Physiol. 1962, 203, 215.

16. Andersen, B., and H. H. Ussing. Solvent drag onnon-electrolytes during osmotic flow through iso-lated toad skin and its response to antidiuretichormone. Acta physiol. scand. 1957, 39, 228.

17. Renkin, E. M. Filtration, diffusion, and molecularsieving through porous cellulose membranes. J.gen. Physiol. 1954, 38, 225.

18. Schultz, S. G., and A. K. Solomon. Determination ofthe effective hydrodynamic radii of small moleculesby viscometry. J. gen. Physiol. 1961, 44, 1189.

19. Fordtran, J. S., P. F. Clodi, K H. Soergel, andF. J. Ingelfinger. Sugar absorption tests, withspecial reference to 3-O-methyl-d-glucose andd-xylose. Ann. intern. Med. 1962, 57, 883.

20. Pappenheimer, J. R., E. M. Renkin, and L. M. Bor-rero. Filtration, diffusion and molecular sievingthrough peripheral capillary membranes. A con-tribution to the pore theory of capillary permeabil-ity. Amer. J. Physiol. 1951, 167, 13.

21. Whalen, G., J. Harris, and K. Soergel. Bidirectionalflux of sodium and water in the human small in-testine (abstract). Gastroenterology 1965, 48, 859.

22. Berliner, R. W. Membrane transport Rev. mod.Physics 1959, 31, 342.

23. Visscher, M. B. Transport of water and electrolyteacross intestinal epithelia in Mctabolic Aspects ofTransport across Cell Membranes, Q. R. Murphy,Ed. Madison, University of Wisconsin Press,1957, p. 57.

24. Kedem, O., and A. Katchalsky. A physical inter-pretation of the phenomenological coefficients ofmembrane permeability. J. gen. Physiol. 1961, 45,143.

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permeability characteristics of the

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