THE ELECTRICAL POTENTIAL DIFFERENCEGENERATEDBYTHE LARGE INTESTINE: ITS RELATION TO ELEC-

TROLYTEANDWATERTRANSFER*

By I. L. COOPERSTEINAND STANLEY K. BROCKMAN

(From the National Heart Institute, National Institutes of Health, Bethesda, Md.)

(Submitted for publication July 27, 1958; accepted October 30, 1958)

Studies of the physicochemical forces involvedin the transfer of ions and water across biologicalmembranes have received great impetus from thework on the isolated frog skin by Ussing, Zerahnand Koefoed-Johnson (1, 2). This type of ap-proach would help to define the mechanisms in-volved in the transfer of fluid and electrolyteacross the intestine in vivo. A previous study onthe large intestine in vivo has stressed the strik-ing concentration gradients developed between thelumen and the plasma (3). Evaluation of thedriving forces for electrolyte and water transferacross the large intestine has not been possible,however, in the absence of measurements of trans-fer rates and electrical potential.

Recent studies on the isolated large intestine ofthe guinea pig and frog have indicated that theabsorption of sodium is active, and that of chlorideis passive (4, 5). The maintenance of an elec-trical potential gradient was found to be depend-ent upon active sodium transport. However,since the intestine can secrete (6) as well as ab-sorb, even these studies provide limited infor-mation.

In the experiments reported here, the electricalpotential difference across the large intestine of theanesthetized dog was measured. Simultaneouslythe net transfer of water, sodium, potassium, chlo-ride and bicarbonate between the lumen and theblood were determined. The unidirectional move-ments of sodium and chloride were calculatedfrom the rate of absorption of their radioisotopes.By correlating the electrical potential differencewith observed chemical events, it was possibleto obtain a more precise analysis of some of theforces influencing ionic and water transfer acrossthe large intestine. Information was obtainedwhich clearly demonstrates at least two active

*A preliminary report of this work was presented atthe National Meeting of the American Federation forClinical Research, Atlantic City, N. J., May, 1958.

transport systems: sodium absorption and bi-carbonate secretion.

METHODS

Thirteen healthy mongrel dogs weighing from 9 to 17Kg. were used. After anesthetization with 20 to 25 mg.per Kg. of pentobarbital sodium the abdomen was enteredthrough a small midline incision. A segment of largeintestine, 4 cm. distal to the caecum and 10 to 15 cm. inlength, was divided and flared cannulae carefully tiedinto each end. Care was taken to keep the blood supplyintact. The cannulae were exteriorized through twosmall stab wounds and the abdomen closed. The in-testinal segment was rinsed clean with isotonic salinewarmed to 370 C. An electrolyte solution from a reser-voir was recirculated through the intestinal segment us-ing plastic tubing and a Bowman infusion pump. Thesolution in the reservoir was maintained at 39° C. Theinitial total volume of perfusion solution varied from 100to 200 ml. depending on the size of the intestinal segment,and the flow rate ranged from 5 to 10 ml. per minute. Amodified Krebs-Henseleit solution was used with 100mg. per cent of glucose and 1.25 mEq. of calcium per L.(7). The concentrations of Na+, Cl-, K+ and HCO-, inthis solution were 138, 120, 5 and 22 mEq. per L., respec-tively. Penicillin (8 mg. per cent) and streptomycin (16mg. per cent) were added to minimize bacterial contami-nation. Phenol red and radioactive isotopes were addedto the perfusion fluid as described later.

The electrical potential difference across the wall oflarge intestine was detected with a pair of calomel elec-trodes and continuously recorded with a modified Brown-Minneapolis Honeywell recording potentiometer with aninput impedence of one megohm. Two pieces of largebore polyethylene tubing (PE 320) filled with 1 M KCland 3 per cent agar served as bridges from the calomelelectrodes to the luminal solution and the peritoneal sur-face of the intestinal segment. The junction potential wasless than 1 mV. in all instances. During the experimentan additional potential difference was measured betweenthe luminal solution and circulating venous blood in thevena cava. In many instances the potential differenceacross the intestinal wall showed rhythmical phasic vari-ations: The mean potential was then determined byplanimetry. These variations in electrical potential willbe the subject of a forthcoming report.

Each experiment lasted four to seven hours and hourlysamples of 5.0 ml. were removed from the lumen. In

435

I. L. COOPERSTEINAND STANLEY K. BROCKMAN

order to maintain volume, each sample was replaced withfive ml. of the original solution and the resultant changein solute concentrations calculated. Chemical determina-tions of Na+, K+, Cl- and HCO3- were done on eachhourly sample. During each experiment, two samples ofplasma were analyzed similarily. Na4 and K+ were de-termined by flame photometry using lithium as an in-ternal standard, Cl- by the method of Cotlove, Tranthamand Bowman (8), and HCO3 by the method of Van Slykeand Neill. Osmolality of the plasma and luminal con-tent were determined by the freezing point depressionmethod (9).

Phenol red (0.004 per cent) was added to the originalsolution, and the optical density of each hourly samplewas determined at 550 m't after a 15-fold dilution with0.1 N NaOH (10). At the end of each experiment, theamount of phenol red left in the perfusate and salinewashings was determined spectrophotometrically. Re-covery of the dye was 97 ± 2 per cent, and this slight lossjustifies its use as an indicator of volume change. Thenet water flux was calculated with the equation:

v - Do°VoNet water flux = - D

where t is the time in hours, D. is the initial optical den-sity, D1 the optical density at the end of the hourly pe-riod, and V0 the volume of the initial solution.

The net flux (Mnet) of each ion was calculated by us-ing the equation:

Mnet - V0Co-ViC, 2)

where V0 and C0 are the total volume and concentrationat the beginning of any hourly period, and V1 and C, thevolume and concentration at the end of that period. Apositive net flux denotes movement from the lumen to theblood.

Trace amounts of Na' and Cl" were added to theoriginal perfusion solution and the influx of these ionsfrom lumen to blood determined -by the rate of disappear-ance of their radioisotopes. The initial sample wastaken one-half to one hour after perfusion was begun.The Na" disintegration rate was determined on eachsample in a well-type scintillation counter without in-terference from the beta particles of Cl` which were ab-sorbed by the glass wall of the vial containing the sample.The Na' was then allowed to decay and the Cl" distinte-gration rate counted with a Geiger-Muller tube after du-plicate 1 ml. samples were dried on filter paper bondedto copper discs. A minimum of 10,000 disintegrationswere counted and the activities were at least two tothree times background.

The influx (Mi) of Na4 and C17 from lumen to bloodwas calculated with the equation:

= roVo riVi-3MI~ C3

where r. is the absolute cpm per ml., V0 the volume at thebeginning of any hourly period, r, and V1 are these val-ues at the end of that period, the mean cpm per ml., andC the mean concentration of the ion in the perfusate.

In order to justify the use of Equation 3, the followingassumptions were made: First, the isotopic movementfrom lumen to blood' is unidirectional, since the activityof Na' and Cl3' in the blood was found to be indistin-guishable from background. Second, that the rate ofchange in concentration and isotopic activity of Na+ andCl- in the luminal fluid was linear with time. Theirarithmetic mean was used since calculations made on thebasis of an exponential decline gave similar results, andthe change in concentrations was less than 2 per cent andthe change in isotopic activity less than 10 per cent alur-ing each sampling period. Third, the kinetics of Na+ andCl- transfer across the intestinal epithelium can be de-scribed in terms of two compartments: lumen and plasma.

The outflux (M0) 'of Na+ and Cl- from blood to lu-men was obtained with the equation:

4)

The intestinal segment was weighed and measured atthe end of the experiment and the flux arbitrarily ex-pressed as microequivalents (,uEq.) per Gm. of wet tis-sue per hour. Water movement was expressed as ml.per Gm. of tissue per hour.

RESULTS

Electrical

The electrical potential difference across theepithelium of the large intestine ranged in dif-ferent experiments from 10 to 40 mV. In allinstances the lumen was negative with respect tothe peritoneal surface. The potential differencebetween the luminal solution and circulatingblood was the same when the KCl agar bridge onthe peritoneal surface of the intestine was placed inthe vena cava. In some instances, rhythmicalphasic variations in the potential were noted' tooccur with a periodicity of 3 to 5 minutes, dura-tion of 15 seconds to 2 minutes, and amplitudeof + 1 to' + 20 mV. In one dog with a Thiry-Vella loop of colon, the potential between thelumen and the venous blood was noted to varyfrom 20 to 50 mV. over a six month period. The'potential difference across the wall of the smallintestine and gall bladder was measured by thetechnique described, using the modified Krebs-Henseleit solution. The absence of an appreciablepotential difference across; these organs empha-sizes the significance of the'observed potential inthe colon.

436

MO= Mi M.t.

ION TRANSFERAND POTENTIALS ACROSSTHE COLON

TABLE I

Net rates of H20 and electrolyte transfer during absorption *

H20 HC03-t K+t Na-t Cl-t P.D.

mi/Gm./hr.+0.42T +6.0 -2.5$ +61.0 +53 19 mV.

(40.11)§ (:13.6) (i1.2) (+14.5) (±12.7)

* Eleven dogs, 55 one hour sampling periods.t Values for HC03-, K+, Na+ and Cl- expressed as /AEq.

per Gm. of intestinal segment per hour.$ (+) denotes movement from lumen to blood; (-)

denotes movement from blood to lumen.§ = standard deviation.

Chemical

Two patterns of electrolyte and water transferwere observed. The first pattern (Tables I andII) was observed in 11 dogs and was characterizedby the net absorption of water, sodium and chlo-ride into the blood. The concentration of sodiumin the luminal fluid was always less than thatobserved in the plasma. Thus sodium was ab-sorbed against both a concentration and an elec-

trical gradient. The absorption of chloride intothe blood was in the direction of its electricalgradient but against its concentration gradient.The net transfer of potassium was from blood tolumen. Like chloride it moved with the electricalbut against its chemical gradient. In the originalperfusion solution the ratio of Cl- to HCO3- was

6:1, but these ions were absorbed in the ratio of9:1 (range, 6:1 to 20:1). The net effect of theionic movements was an alteration of the electro-lyte pattern of the luminal fluid, with sodium andchloride concentrations decreasing, and potassiumand bicarbonate increasing. This concentrationpattern has been observed previously (3).

The second pattern was observed in the two

remaining dogs (Table III), and was character-ized by a profuse bicarbonate secretion into thelumen with an accompanying net secretion ofwater, sodium and potassium. In this instance,bicarbonate was the predominant ion being trans-

ported against a chemical and electrical potential

TABLE II

A representative experiment showing net rates of H20 and electrolyte transfer during absorption

Time H20 HC03-* K+* Na+* C1-* P.D.

hr. ml./Gm./hr.t juEq./Gm./hr. mEq.1L. juEq./Gm./hr. mEq.IL. AEq./Gm./hr. mEq.IL. gEq./Gm./hr. mEq.1L. mV.0 22.1 6.0 138 120 160-1 +0.85t + 7.1 23.9 - 3.3t 7.4 + 99.5 139 +109 119 161-2 +0.43 + 3.8 24.9 - 0.9 8.1 + 97.0 133 + 57.0 118 172-3 +0.64 0 25.4 - 2.8 9.5 + 86.0 132 + 82.0 117 203-4 +0.57 + 9.5 27.8 - 6.6 11.7 + 81.0 131.5 + 73.0 116 224-5 +0.58 + 4.2 30.6 - 4.7 13.6 + 64.0 131 + 62.0 115 245-6 +0.53 + 9.0 31.3 - 4.3 15.8 + 80.0 129 + 69.0 113 26

+3.40 +33.6 -22.6 +507.5 +452.0

* Plasma values of HCO3-, K+, Na+ and C1- were 22.0, 4.1, 149 and 118 mEq. per L., respectively.t Net weight of tissue, 21.2 Gm.; length, 12.2 cm.t (+) denotes movement from lumen to blood; (-) denotes movement into lumen.

TABLE III

An experiment showing net rates of H20 and electrolyte transfer during secretion

Time H20 HC03-* K+* Na+* C1-* P.D.

hr. inl./Gm./hr.t IAEq./Gtn./hr. mEq./L. uEq./Gm./hr. mEq./L. uEq./Gm./hr. mnEq.1L. pEq./Gm./hr. mEq./L. mV.0 25.8 5.9 149 129 250-1 -0.42t - 62.0 32.9 - 7.0 6.6 - 32 146 + 71 122 251-2 -0.57 - 49.0 36.2 - 7.2 7.0 - 43 145 - 3 117 232-3 -0.13 - 23.0 39.1 - 7.2 7.9 - 23 145 + 7 115 203-4 -0.05 - 20.0 42.9 - 5.5 8.9 - 20 142 - 1 114 22

-1.17 -154.0 -26.9 -118 +10

* Plasma levels of HCO3-, K+, Na+ and Cl- were 24.1, 3.8, 150 and 116 mEq. per L., respectively.t (+) denotes movement from lumen to blood; (-) denotes movement from blood to lumen.I Net weight of tissue, 23.5 Gm.; length, 12.0 cm.

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I. L. COOPERSTEINAND STANLEY K. BROCKMAN

TABLE IV

Chloride flux ratios

No. of one hour _C__P__sm_ Ml* Observedt Calculatedt Observed 100Expt. flux periods ECl-lumen] Mo ratio ratio Calculated

1 7 113 103 3.3 2.7 123118 30

2 6 120 44 5.0 2.4 200113 9

3 5 114 63 3.1 2.2 140127 1 8

4$ 6 116 52 1.2 2.4 50129 46

5t 4 115 28 1.3 2.5 50117 22

* M1 is influx (lumen to blood); Mo is outflux (blood to lumen). Flux expressed as /AEq. per Gm. per hour.

t Observed ratio calculated from Mi. [CIl-18.m]]; calculated ratio from antilog E(mV.)Mo [Clhume] '60t Experiments 4 and 5 characterized by net secretion of water into the lumen, Experiments 1, 2 and 3 by net

absorption.

gradient. Again chloride was absorbed in thedirection of its electrical gradient. Both sodiumand potassium entered the lumen along their elec-trical gradient, but the ratio of sodium to potas-sium secreted into the lumen was 5 :1 as comparedto a much higher ratio in the plasma. This demon-strates that even with secretion of bicarbonate intothe lumen, the concentration pattern in the luminal

fluid is still that of a decrease in sodium and chlo-ride and an increase in potassium and bicarbonate.

In the five experiments presented in Table IVthe fluxes of chloride into the blood (MI) and intothe lumen (Mo) are given. Since net chloridemovement was from the lumen to the blood, theratio Mj :Mo is greater than 1.0.

In Table V the unidirectional sodium fluxes are

TABLE V

Sodium flux ratios

No. of one hour [Na+plasma] Mi* Observedt Calculatedt ObservedExpt. flux periods [Na+lumen] MO ratio ratio Calculated

1 5 143 81 3.3 0.55 600135 24

2 6 150 93 2.7 0.40 670135 34'

3 6 145 55 2.1 0.50 420L41 26

4t 6 146 30.7 0.80 0.40 200138 38.7

St 4 150 40 0.64 0.42 147142 63.8

* Mi is influx (lumen to blood); Mo is outflux (blood to lumen). Flux expressed as ,AEq. per Gm. per hour.

t Observed ratio for sodium calculated from iL* [Na+pl, ]; calculated ratio from 1/antilog E (mV.)Mo [Na~jumen] ;cluaeraifrm1ntog 60$ Experiments 4 and 5 characterized by net secretion of water into the lumen, Experiments 1, 2 and 3 by net ab-

sorption.

43;8

ION TRANSFERAND POTENTIALS ACROSSTHE COLON

1001

'H20(Mi.)

75 1Salin

0

50' a -a * *-

Monnito,25 -

1 2 3 4 5TIME (hr.)

FIG. 1. VOLUMEOF PERFUSATEAT EACH SAMPLINGHOUR IS PLOTTED AGAINST TIME

See text.

presented. In the first three experiments therewas a net transfer of sodium from lumen to bloodand the ratio of sodium influx (MI) to the sodiumoutflux (Mo) is greater than 1.0. The ratio ofsodium fluxes was less than 1.0 in the remainingtwo experiments since there was net sodiummovement into the lumen.

For electroneutrality the sum of the charges ofthe ions being transferred must equal zero. Thiswas not observed in each sampling period due toslight errors in the chemical results used in calcu-lating net transfer, but the average of the nettransfer of ions in each experiment, as well as themean for all experiments, was found to approachzero.

WaterThe data from two experiments presented in

Figure 1 show that there was no significant ab-sorption of water without net absorption of solute.In one a saline solution was recirculated throughthe lumen and in the other isotonic mannitol wasused. Changes in water content were quite smallin the latter experiment because of the slight ab-sorption of mannitol. However, in the two ex-periments in which there was extensive bicar-bonate secretion, net water transfer was quitevariable as indicated by the experiment repre-sented in Table III. The osmolality of the luminalcontents remained essentially constant, indicatingthat absorption or secretion was isotonic.

DISCUSSION

The preceding results confirm the previous ob-servations on the isolated large intestine which

showed that sodium was absorbed against itselectrical and chemical gradient (4, 5). This nettransfer requires energy and can be defined asactive. In two experiments the secretion of bi-carbonate was observed to be against its electricaland chemical gradient; this secretion also must beactive. Since chloride absorption and potassiumsecretion were observed to be in the direction oftheir electrical gradients and opposite to theirchemical gradients additional information is neces-sary to describe their means of transfer more fully.

Ussing (11) has developed the following equa-tion to describe the passive movement of an ionacross a semi-permeable membrane:

or

Mi .e(zF/RT).EMo ai

log -M1I= RFT E E .V)*at 370° C.

5)

where Mi and MOare the influx and outflux of theion; a1 and a,, respectively, the chemical activity ofthe ion in plasma and lumen; z, F, R, T., respec-tively, the charge of the ion, the Faraday, the gasconstant and absolute temperature; and E(mV.),the electrical potential difference across the mem-brane in question.

The relationship between passive ionic transferand electrical and chemical potential gradients isevident from Equation 5. Any significant devia-tion from this equation must be explained interms of an energy expenditure or other factorsnot considered. One of these factors, "solventdrag," will be discussed with relation to the trans-fer of chloride.

Since sodium is actively transported, the ratio ofits unidirectional fluxes does not follow the aboveequation. However, as noted in two experiments(Table V, Experiments 4 and 5), net sodiummovement was from a higher to a lower electro-chemical potential and accompanied bicarbonateand water into the lumen. At the observed po-tential the observed flux ratio for sodium is largerthan the calculated value. One possible explana-tion for this finding is that sodium is still beingactively transported even though this "activetransport" is being overwhelmed by the otherforces acting on the sodium ion.

Flux rates of sodium across the isolated largeintestine of the frog have been expressed as p.Eq.per unit area (12), but recalculation of values on

439

I. L. COOPERSTEINAND STANLEY K. BROCKMAN

a weight basis gave net sodium fluxes of 30 to 60,uEq. per Gm. of tissue per hour. Thus the rateof sodium absorption across the isolated frog colonat 25° C. is of the same order of magnitude asthe rate of sodium absorption in the dog.

Chloride is transferred from a higher to a lowerelectrochemical potential. The electrical potentialdifference between lumen and blood is the majordriving force. With net water secretion into thelumen there is still net chloride transfer fromlumen to blood (Table III). In comparing thedog and frog colon with respect to the trans-epithelial fluxes of chloride the following dif-ferences occur: One, the net chloride flux duringabsorption in the dog (Table I) is greater thanthe net flux of 10 to 20 JuEq. per Gm. of tissueper hour in the frog. Two, unlike the observationson the frog in vitro, the data presented here do notsuggest appreciable exchange diffusion of chloride.

In Table IV the calculated and observed ratiosof chloride are compared in five experiments, andthey do deviate from the values calculated forpassive diffusion. These deviations could be ex-plained by the following factors: 1) Althoughchloride is transferred predominately by passivediffusion there may be a component of activechloride transport or exchange diffusion.1 2) Inthe presence of rhythmical potential variations thepotential difference as determined by planimetrymay not be the true mean potential. 3) It is likelythat the last factor, "solvent drag," is the mostimportant for the following reasons:

"Solvent drag" is the physical effect of netwater flow exerted upon solute and is a functionof membrane structure as defined by Ussing andAndersen (4). This flow reduces solute move-ment in the opposite direction, while enhancing itin the same direction. Net water flow into theblood (Table IV, Experiments 1 through 3)would increase chloride influx and/or decreaseoutflux. Net water flow into the lumen wouldhave the opposite effect (Table IV, Experiments4 and 5). Such an effect was observed on thechloride flux ratios: The chloride ratio is lower

1 Chloride transfer across the isolated large intestineof the frog during absorption involves a component ofexchange diffusion. If exchange diffusion were opera-tive in the dog the chloride flux ratios would be sig-nificantly less than and not greater than the values cal-culated for passive diffusion (12).

than predicted by Equation 5 when net watersecretion occurs and higher than predicted whennet water absorption takes place. On the otherhand, the observed sodium flux ratio (Table V,Experiments 4 and 5) is larger than the calculatedvalue for passive diffusion despite net water trans-fer into the lumen. This indicates that activesodium absorption was still operative during netsodium transfer into the intestine.

The limitations of Equation 5 as presented herefor the in vivo intestine have been predicted byUssing and he has developed an equation whichtakes "solvent drag" into account (13). A roughestimate of the "solvent drag" force can be ob-tained from the flux ratio of water which was notmeasured. However, Berger has recently meas-ured D2O fluxes across the colonic mucosa of theunanesthetized dog (14). In experiments char-acterized by net water absorption, the water fluxratios were 1.23 to 1.32. If one uses this figure torecalculate the chloride ratios (Table IV, Experi-ments 1 through 3) there is a closer agreementbetween the observed and calculated results. Thissuggests that chloride transfer is influenced by"solvent drag" as well as by the electrochemicalgradient.

The net flux of potassium from blood to lumenwas in the proper direction for passive transfer(Tables I through III). This does not rule outan active secretion of potassium, or a sodium-potassium exchange at one surface of the epethelialcell, a mechanism postulated for the kidney andfrog skin (15, 16). The unidirectional transfersof potassium were not measured and the nature ofthe experiment did not permit the attainment ofthe steady state where Mi = M0 in order to testthe relationship:

E = 60 log K+JUMenKplasma,

Subsequent studies have revealed that, in someinstances, potassium secretion may be active, andthis will be the subject of a forthcoming report.

In all but two experiments there was net ab-sorption of bicarbonate along its electrical gradi-ent. Although the original perfusate contained6 times as much chloride as bicarbonate, up to 20times as much chloride was reabsorbed in 11 ex-periments. Thus despite net absorption of bi-carbonate, the concentration of bicarbonate in the

440

ION TRANSFERAND POTENTIALS ACROSSTHE COLON

lumen increased. This suggests that the epitheliumof the large intestine is less permeable to bicarbo-nate than chloride, or there is active secretion ofbicarbonate into the lumen. The latter mechanismcan occur, as shown in Table III, where bicarbo-nate secretion into the lumen was opposite to itselectrochemical gradient and was accompanied bywater, sodium and potassium. The ratio of bi-carbonate to water in the net secretion was 130mEq. per L. in these latter instances. This isnot unreasonable when compared to pancreaticsecretions containing 110 mEq. per L. of bicarbo-nate or more, and gastric secretions as high as 150mEq. per L. of hydrogen ions. The variations inactive bicarbonate secretion may be due to changesin neurohumoral influences, since stimulation ofthe pelvic nerves and administration of cholinergicdrugs induced alkaline secretion by the colon (6).

In this discussion the term bicarbonate secretionhas been used for brevity. However, these experi-ments do not distinguish among bicarbonate se-cretion, hydroxyl ion secretion or hydrogen ionabsorption. Previous studies on the isolated froglarge intestine suggested that in addition to sodiumthere was another ion being actively transported,although this ion was not identified (12). If ac-tive bicarbonate secretion is under the influence ofa neurohumoral mechanism it is not surprising thatit could not be demonstrated in the in vitro system,but was clearly demonstrated and found to haveconsiderable variability in vivo.

Driving forces for net water flow are the gradi-ents of hydrostatic and osmotic pressure. Thelatter is created by the two ion transport mecha-nisms: sodium absorption and bicarbonate secre-tion. It was noted that there was no significantnet transfer of water without solute. In additionto electrical and chemical forces, this net watermovement can be a third driving force for otherdiffusing solutes ( 11 ). The effect of this "solventdrag" has been discussed previously with regardto the transfer of chloride.

In the preceding discussion the assumption hasbeen made that the measured potential differencerepresents the transmucosal potential. This as-sumption seems justified in view of the followingobservations: First, the isolated large intestinewhen stripped of its outer muscular coat can gen-erate a potential difference and actively transportsodium (4). Second, Rehm has presented evi-

dence that no significant potential difference canbe measured across the muscularis and peritonealsurface of the stomach in the presence of a sig-nificant electromotive force across the stomachwall (17). Third, the peritoneal cavity and cir-culating venous blood were found to be equipo-tential in the present study.

Transepithelial potential differences have beenconsidered to be due directly to the unidirectionalactive transport of an ion (1), e.g., sodium ab-sorption and/or bicarbonate secretion. However,recent studies indicate that such bioelectrical po-tentials may be diffusion potentials resulting fromactive ion-exchange mechanisms involved in ac-tive transport (16). It is interesting that stro-phanthidin, a known inhibitor of cation transport(18), causes a drop of at least 50 per cent in theobserved potential and partial inhibition of the so-dium and bicarbonate active transport mechanismsacross the large intestinal epithelium (19).

SUMMARY

An electrical potential difference of 10 to 40mV. (the lumen being negative with respect to theblood) was measured across the large intestineof 13 anesthetized dogs. Correlation of this po-tential difference with chemical events demon-strated two active transport mechanisms: sodiumabsorption and bicarbonate secretion. Net waterflow occurred, and its direction and magnitude weredependent on the degree of active transport of so-dium and/or bicarbonate. The absorption ofchloride was in the direction of its electrochemicalpotential gradient, probably modified by net watermovement. Potassium was transferred from theblood to the lumen, in the direction of its electricalgradient but opposite to its chemical gradient.

ACKNOWLEDGMENT

The authors wish to thank Drs. C. A. M. Hogben andE. Cotlove for suggestions and critical review of themanuscript.

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17. Rehm, W. S. Evidence that the major portion of thegastric potential originates between submucosa andmucosa. Amer. J. Physiol. 1946, 147, 69.

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