Interrelationships of Chloride, Bicarbonate, Sodium, and Hydrogen Transport in the Human Ileum

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1 Interrelationships of Chloride, Biarbonate, Sodium, and Hydrogen Transport in the Human Ileum LEsLE A. TURNBERG, FREDERICK A. BIEBERDORF, STEPHEN G. MORAWSKI, and JOHN S. FORDTRAN From the Department of Internal Mediine, The University of Texas Southwestern Medial Shool at Dallas, Texas A B S T R A C T Using a triple-lumen onstant perfusion system, the following observations were made in normal subjets. First, hloride, biarbonate, and sodium were found to exhibit net movement aross ileal muosa against eletrohemial gradients. Seond, during perfusion with a balaned eletrolyte solution simulating plasma, the ileum generally absorbed, but sometimes sereted fluid. A reiproal net movement of hloride and biarbonate was noted when sodium movement was zero. Inreasing rates of sodium absorption were assoiated with dereasing biarbonate seretion rates and finally biarbonate absorption. Even when biarbonate was absorbed ileal ontents were alkalinized (by ontration of luminal volume). Third, net hloride movement was found to be sensitive to biarbonate onentration in ileal fluid. For instane, hloride was absorbed from solutions ontaining 14 or 44 meq/liter of biarbonate, but was sereted when ileal fluid ontained 87 meq/liter of biarbonate. Fourth, when hloridefree (sulfate) solutions were infused, the ileum absorbed sodium biarbonate and the ileal ontents were aidified. Fifth, when plasma-like solutions were infused, the potential differene (PD) between skin and ileal lumen was near zero and did not hange when hloride was replaed by sulfate in the perfusion solution. These results suggest that ileal eletrolyte transport ours via a simultaneous double exhange, Cl/HCOs and Na/H. In this model neither the anion nor the ation exhange auses net ion movement; net movement results from the hemial reation between hydrogen and biarbonate. No other unitary model explains all of the following observations: (a) human ileal transport in vivo is essentially noneletrogeni even though Na, Cl, and HCO3 are transported against eletrohemial Dr. Turnberg's present address is Manhester Royal Infirmary, Oxford Road, Manhester 13, England. Reeived for publiation 3 February 1969 and in revised form 2 Otober gradients, (b) the ileum an serete as well as absorb, () ileal ontents are alkalinized during absorption of or during seretion into a plasma-like solution, and (d) the ileum aidifies its ontents when sulfate replaes hloride. Data obtained with a arboni anhydrase inhibitor support the proposed model. INTRODUCTION Current onepts of the mehanisms of ileal eletrolyte absorption (1, 2) are derived from both in vitro and in vivo experiments, although in reent years relatively few in vivo studies have been arried out. The most generally held view is that eletrolyte absorption in vitro is due to ative sodium transport and that the eletrial potential so generated is the main fore for anion absorption (1). In most speies that have been tested, eletrolyte absorption in vivo is not assoiated with a potential differene aross ileal muosa, and this has been attributed to simultaneous ative transport of sodium and hloride, both pumps eletrogeni but of the same magnitude so that no net potential differene is generated (1). In vivo, onentrations of hloride and biarbonate vary reiproally in ileal fluid with their sum equal to approximately 135 meq/liter; the biarbonate onentration is generally higher and the hloride onentration lower than their respetive plasma levels (1, 2). Buher, Flynn, and Robinson (3) were apparently the first to suggest that the appearane of biarbonate in ileal fluid is linked to the absorption of hloride (anion exhange), but it is important to note that reiproal net movements of hloride and biarbonate do not our during absorption of balaned eletrolyte test solutions but that hloride absorption is always muh higher than biarbonate seretion. Thus, as judged by the rate of biarbonate seretion, very little hloride ould be absorbed by anion exhange. The Journal of Clinial Investigation Volume

2 We have reently onduted a series of experiments in the human ileum in vivo whih are not ompatible with these onepts and whih have led us to propose a different mehanism for ileal ion transport. METHODS The subjets of this study were normal male and female volunteers, aged yr. The Ingelfinger triple-lumen perfusion tehnique was used exatly as desribed in detail in earlier publiations (4). The intestinal test segment was 3 m and the nonabsorbable marker was polyethylene glyol. Studies were done when the infusion tip of the triplelumen tube was 2-25 m from the teeth. Unless otherwise stated, the infusion rate was 1 ml/min. The mean onentration of solute in the test segment was alulated by averaging the onentration in fluid withdrawn from eah end of the test segment. Results are expressed in milliliters of water or milliequivalents of eletrolyte absorbed or sereted per hour per 3 m of ileum. Usually three different test solutions were studied in the same individual on one morning. Potential differene (PD) between intestinal lumen and abraded skin was measured during perfusion with various test solutions by means of an intraluminal eletrode desribed in an earlier publiation from this laboratory (4). PD was measured during perfusion with the idential solutions used for measurement of ion movements and in the same subjets, although the tests were performed on different days. To validate the use of abraded skin as a referene site PD between skin and peritoneum was measured four times in patients undergoing peritoneal dialysis. With the skin as a referene, peritoneal PD + 2., +.2,., and -.2 mv in the four studies. Therefore, our results of PD measurements between skin and gut lumen should be approximately the same as between lumen and peritoneum. Eletrolyte and polyethylene glyol (PEG) onentrations were estimated with methods desribed previously (4). Total arbon dioxide was measured by the mirogasometri method using a Natelson mirogasometer (Model 6, Sientifi Industries, In., Springfield, Mass.). ph and PO2 were measured on an Ultramiro ph/po2 gas analyzer (Instrumentation Laboratories, In., Lexington, Mass.). The results are given as mean values ± 1 SE of the mean in the tables and figures. RESULTS Absorption and seretion rates Chloride. Ileal hloride absorption at varying hloride onentrations was assessed in nine subjets. Test solutions ontained, 5, or 1 meq/liter of hloride as the sodium salt. Sodium and potassium biarbonate and mannitol were added in amounts required to give eah solution a biarbonate onentration of 3 meq/ liter, a potassium onentration of 5 meq/liter, and an osmolality of 285 mosm/kg. As shown in Fig. 1, hloride absorption inreased linearly with inreasing mean luminal hloride onentration over the range of 1-94 meq/liter. Absorption ourred against a onsiderable lumen-to-plasma onentration gradient. A Chloride meq /hr/3 m mV "F (t5.2) -6h o -4 en -o u-4) (I) Ok +11.6mV (t 3.1) mv P D (t 2.5) CHLORIDE CONCENTRATION (meq /liter) FIGURE 1 Effet of luminal hloride onentration on net hloride movement. Sodium onentration was approximately 25 meq/liter higher than hloride; biarbonate onentration was 3 meq/liter in all studies. The potential differene (PD) between ileal lumen and skin at different sodium hloride onentrations is given at the top of the figure. The sign preeding a PD value indiates the orientation of the potential, + meaning lumen positive ompared to skin and - meaning lumen negative ompared to skin. The PD between skin and ileal lumen was measured in seven subjets during perfusion of idential solutions as in the above experiments and the mean values for these PDs are also shown in Fig. 1. As luminal hloride (and sodium) onentration was redued, the lumen beame inreasingly eletropositive with respet to the skin. Thus hloride absorption ours against both steep onentration and steep eletrial gradients. Sodium. Ileal sodium absorption, measured during perfusion of the test solutions just desribed, also ourred against steep onentration gradients, as previously desribed in detail (4). However, unlike hloride, sodium absorption ourred in the diretion of, and not against, an eletrial gradient. Utilizing the Nernst equation, it is possible to relate hemial and eletrial gradients and to infer whether sodium movement is passive or ative, at least as defined by this equation. For instane, in the present studies sodium absorption ontinued until sodium onentration was redued to approximately 27 meq/liter; at lower onentrations, sodium was sereted into the ileal lumen. By the Nernst equation, eletrial PD would have to be 43 mv to aount for this unequal distribution of sodium ions between ileal lumen and plasma. The observed PD was only approximately 22 mv, indiating that sodium was not distributed passively aross the ileal muosa and that ileal sodium absorption ours against eletrohemial gradients. 558 L. A. Turnberg, F. A. Bieberdorf, S. G. Morawski, and J. S. Fordtran

3 TABLE I Slow Perfusion Studies. Changes in Biarbonate Conentration Compared with Changes in PEG Conentration and Net Biarbonate Movement Inrease in Biarbonate biarbonate Inrease in PEG onentration onentration onentration Study at distal as- from proximal piration site to distal site from proximal to distal site A HC3* meqlliler % % meq/hr per 3 m Infused biarbonate onentration 3 meq/liter meq/liter T meq/liter * + sign indiates net seretion, - sign indiates net absorption. I Subjet in whom net water movement was into the lumen. Biarbonate. Preliminary studies revealed that net biarbonate movement' from balaned eletrolyte solutions was small and diffiult to measure aurately with the standard perfusion tehnique. Therefore, experiments were performed at a slow rate of perfusion, 1.7 ml/min, over a long (4 hr) period of time. This slow perfusion aentuates ioni and PEG onentration hanges and markedly enhanes the auray of any given experimental result. The long perfusion period, whih was preeded by 1 hr equilibration, preluded more than a single experiment on a given day. Perfused test solutions ontained 3, 35, or 4 meq/ liter of biarbonate, 15 or 11 meq of hloride, 135 or 14 meq of sodium, and 5 meq/liter of potassium. Results of 13 slow perfusion studies are shown in Table I. 1 Diffiulties arise in deiding whih of several mehanisms is responsible for biarbonate movement. Removal of biarbonate from the lumen ould be ahieved by absorption of biarbonate ions or by seretion of hydrogen ions. The idential effet to hydrogen seretion ould be produed by hydroxyl ion absorption from water whih would leave an exess of hydrogen ion in the lumen. Biarbonate aumulation ould our by reversal of any of these proesses. Unless otherwise speified the terms biarbonate "absorption" and "seretion" are used in the loose sense to indiate removal from or aumulation within the lumen ahieved by one of the above proesses. In every study biarbonate onentration inreased as fluid passed from proximal to distal aspiration site. Sine water absorption ourred as shown by the rise in PEG onentration, some of the inrease in biarbonate onentration was due to a ontration of luminal volume. However, as revealed in the table, per ent rise in biarbonate onentration between proximal and distal aspiration sites was higher than the per ent rise in PEG onentration in 1 of 13 studies. In these experiments biarbonate seretion was at least partly responsible for the high onentrations ahieved in the ileum. In the three experiments in whih biarbonate absorption was noted, the rise in biarbonate onentration was due entirely to a shrinkage of ileal volume due to rapid water absorption. PD was measured four times during slow perfusion of the same test solution. Mean PD was +.5 ±3.3 mv. Relation between water and ion absorption rates during perfusion with a balaned eletrolyte solution Beause of the high degree of auray of the slow perfusion studies, these experiments were hosen to examine the relation between sodium and anion movements in individual perfusion periods. These results are Real Eletrolyte Transport 559

4 E W, on * r w E -8 r I NaoI II /" r II ~U U S HCO; t Abssorption I Seretion SECRETION ABSORPTION A WATER ml /h r/3 m FIGuRE 2 Relation of ion and water movements in slow (1.7 ml/min) perfusion studies. Regression lines were drawn by method of least squares. P values for differene between hloride and sodium and sodium and biarbonate movements at -zero water flow were <.1 and <.1, respetively. A Chloride meq /hr/3m -4F shown in Fig. 2, where sodium, hloride, and biarbonate movements are related to water movement. Mean luminal sodium onentration was 137 (range ), mean potassium onentration was 4.9 (4.5-6.), and mean hloride onentration was 97 (87-18) meq/ liter. Mean biarbonate onentration varied between 32.2 and 52.6 meq/liter, depending on the rate of water absorption and biarbonate seretion. Several onlusions an be drawn from the data and regression lines shown in Fig. 2. First, it is lear that zero sodium absorption is assoiated with zero water movement whih is ompatible with the thesis that water movement is a passive onsequene of net solute movement (1). Seond, when water and sodium absorption are zero, biarbonate is sereted and hloride is absorbed at approximately equal rates. This is suggestive of an. mo a I F 14) Cf) BICARBONATE CONCENTRATION (meq /liter) FIGURE 3 Effet of luminal biarbonate onentration on net hloride movement. Chloride and sodium onentrations were onstant at 4 and 135 meq/liter, respetively; sulfate was added for ioni balane, and all test solutions were isotoni. 56 L. A. Turnberg, F. A. Bieberdorf, S. G. Morawski, and J. S. Fordtran

5 E C. w E z w w z -6 1HC No O _ No AHCO3 CI "sno /1 HC3 OC 3 '-Cl N<o HCO3 N l w z.4 1 1,1 I 1 1 BICARBONATE CONCENTRATION IN INFUSED TEST SOLUTION FIGURE 4 Ion movement at different luminal biarbonate onentrations. The infusion rate for these studies was 5 ml/min. Atual mean onentrations in the test segment were sodium 136 and 137, hloride 42 and 6 and biarbonate 13 and 81 meq/liter, respetively, for the low and high biarbonate solution. Negative values indiate absorption, positive values indiate seretion. anion exhange. Third, inreasing sodium absorption is assoiated with dereasing biarbonate seretion, until, at an absorptive rate for sodium of 3. meq/hr per 3 m, net absorption of biarbonate ours. Finally, in one study all ions and water were sereted; thus, the normal ileum does not always absorb but has the apaity to serete as well. As already mentioned, PD during perfusion with the test solutions used in these experiments was +.5 ±3.3 mv. Effet of biarbonate on hloride movement If, as suggested by the data presented in Fig. 2, hloride/biarbonate exhange does our, net movement of one anion might be influened by the luminal onentration of the other. A series of perfusion experiments was therefore performed in whih hloride onentration was maintained at 4 meq/liter while biarbonate onentration was varied between 14 and 87 meq/liter. Sodium onentration was 135 meq/liter in eah solution, the missing anions being provided by TABLE I I PD in Four Subjets in Whom Sodium Conentration Was Constant While Anion Conentrations Were Varied. In These Four and One Other Subjet the Effet of Gluose on PD is Shown in the Last Column. Solutions Containing Sulfate Also Contained Suffiient Mannitol to Make Them Isotoni with Plasma Na+ = 135 Na+ = 135 Na+ = 135 Na+ = 135 C1- = 4 CV- = 4 C1- = 11 CI- = 11 Infused HC3- = 1 HCOs- = 1 HC3- = 3 HCO3- = 3 solutions* S4K = 45 So4- = S4- = Gluose = 5 (n = 4) (n = 4) (n = 4) (n = 5) Mean PD 1I SE ±t ± ±1.7 * Eah solution ontained 5 meq/liter of potassium. Heal Eletrolyte Transport 561

6 A Biarbonate meq /hr/3m Cb.. 4 in U) CHLORIDE CONCENTRATION (meq /liter) +3.7mV (±2.5) FIGURE 5 Effet of luminal hloride onentration on biarbonate movement. Luminal sodium paralleled the hloride onentration but was approximately 25 meq/liter higher at eah of the three points. Luminal biarbonate onentration was onstant ( meq/liter), regardless of sodium hloride onentration. sulfate. Isotoniity was maintained with mannitol. 1 subjets were studied with eah of the three test solutions. As shown in Fig. 3, luminal biarbonate onentration influened hloride movement despite the maintenane of a onstant hloride onentration. When the mean luminal biarbonate onentration was 14 meq/liter, hloride was absorbed against a onentration gradient. However, hloride absorption was redued at a biarbonate onentration of 44 meq/liter; and when mean biarbonate onentration was 87 meq/liter, hloride was sereted. Five additional studies were performed to examine more losely the net movements of hloride and biarbonate as biarbonate onentration was varied. In order to inrease auray the perfusion rate was redued from the standard rate of 1 ml/min to 5 ml/min, and eah study period lasted 2 hr instead of 1 hr. Test solutions had a hloride onentration of 4 meq/liter and biarbonate onentration was either 1 or 1 1 PD meq/liter. Sodium onentration was 135 meq/liter in both solutions with the missing anion being provided by sulfate. As before, isotoniity was maintained with mannitol. As shown in Fig. 4, raising the biarbonate onentration of the infused solution from 1 to 1 meq/ liter aused hloride absorption to be redued or hloride seretion to be indued, and the hange in net biarbonate movement was approximately the same as the hange in hloride movement, exept in those subjets in whom sodium absorption was different in the two studies. These experiments onfirm a reiproal net movement of hloride and biarbonate as luminal biarbonate onentration is varied. Measurements of PD were made during perfusion of solutions with varying anion onentrations while sodium onentration was maintained at 135 meq/liter. Table II demonstrates that PD does not hange signifiantly when hloride and biarbonate onentrations are varied reiproally or when both are largely replaed by sulfate. Therefore, the hanges' in hloride movement indued by altering luminal biarbonate onentration our in the absene of hange in PD. Effet of hloride onentration on biarbonate movement The effet of varying (sodium) hloride onentration on ileal biarbonate movement was assessed in 1 subjets. Luminal ontents had a onstant biarbonate onentration ( meq/liter) and osmolality was made onstant at 285 mosm/kg by addition of appropriate amounts of mannitol to the test solutions. As shown in Fig. 5, biarbonate was sereted at a slow rate when luminal hloride onentration was 94 meq/ liter. However, biarbonate was absorbed when hloride onentration was lowered to 5 and 1 meq/liter. PD was mv (lumen positive) when lumen hloride onentration was 1 meq/liter, and +3.7 mv when hloride onentration was 94 meq/liter. Thus ab- TABLE II I Ion Movement in the Presene and Absene of Chloride (Mean Values ± 1 SE). Conentrations Refer to Mean Conentration in Test Segment AH2 [Na] ANa [K] AK [C1] ACI [HCOa- AHCO3 Infusion Proximal Distal Chloride free solution t n = 6 Chloride ontaining solution n = 6 PCo2 562 L. A. Turnberg, F. A. Bieberdorf, S. G. Morawski, and J. S. Fordtran

7 sorption of biarbonate ourred against a PD of 22.3 mv at a hloride onentration of 1 meq/liter, while at the highest hloride onentration (94 meq/liter), biarbonate movement (seretion) was in the diretion favored by the small PD gradient (3.7 mv). These experiments show that net biarbonate movement is influened by luminal sodium hloride onentration and that biarbonate an be absorbed against eletrohemial gradients (see footnote 1). Ion transport during sodium sulfate perfusion Sine the studies reorded in Fig. 5 indiated that biarbonate an be absorbed against eletrohemial gradients when luminal hloride onentration is low, it was deided to study the effet of hloride removal in greater detail, espeially in regard to its effet on Na and K movements and on ph and Po2 of luminal fluid. To make these observations, the ileum was perfused with an eletrolyte solution ontaining 14 meq/liter Na, 5 meq/liter K, 25 meq/liter HCOs, and no Cl. The missing anion was sulfate, and mannitol was added to bring the final osmolality to 29 mosm/kg. A ontrol solution had the same biarbonate, sodium, and potassium onentrations but ontained 1 meq/liter of hloride rather than sulfate. The infused test solutions were bubbled with 5% C2 for 3 min before and during the intestinal perfusions. ph 7.6 F 7.4 H 7.3 F p p Infusion solution Proximal site Distal site FIGURE 6 Effet of hloride and hloride-free (sulfate) solutions on ph of infused test solution and fluid olleted from the proximal and distal olletion sites of the threelumen tube. Test solutions were bubbled with 5%o C2 before infusion. Sodium, potassium, and biarbonate onentrations of both test solutions were 14, 5, and 25 meq/liter, respetively. oe a _ -2 _ -lo o.- Cl-free solution -FIO--I soluton A TIME IN MINUTES FIGURE 7 Potential differene (lumen to skin) during perfusion with hloride and hloride-free (sulfate) test solutions. All test solutions ontained 14, 5, and 25 meq/liter of sodium, potassium, and biarbonate, respetively. A positive PD indiates lumen positive with respet to skin; a negative potential means lumen negative with respet to skin. Table III shows the mean onentration of ions in the test segment and their movements for the two infusion solutions. Substitution of sulfate for hloride was assoiated with a redution in sodium absorption from 3.5 to 1.3 meq/hr. Potassium movement was the same in the presene or absene of hloride. Biarbonate was sereted in the presene of hloride and absorbed in its absene. Fig. 6 shows ph of the infusion solutions and fluid olleted at proximal and distal sites under a layer of oil. The ph values of both infusion solutions ranged from 7.3 to 7.4. Whereas the hloride-ontaining solutions either beame more alkaline or did not show a ph hange, the ph values of the hloride-free solutions fell. At the proximal olleting site, the ph had fallen to values between 7.9 and Between the proximal and distal sites, the ph ontinued to fall to values between 6.94 and As shown in Table III, luminal Po2 was approximately the same during perfusion of the two test solutions. As shown in Fig. 7, substitution of sulfate for hloride had no demonstrable effet on PD. Three other studies gave similar results. Carboni anhydrase inhibitor. The effets of aetazolamide' (2 mg/liter of perfused fluid) on sodium, hloride, and biarbonate movements were tested during perfusion of a balaned eletrolyte solution. As shown in Fig. 8, absorption of sodium and hloride was markedly inhibited by aetazolamide. 2 Diamox, Amerian Cyanamid Co., Lederle Laboratories Div., Pearl River, N. Y. Heal Eletrolyte Transport 563

8 There was no onsistent effet on net biarbonate movement whih was near zero with and without aetazolamide. Sodium diffusion potential As shown in Table II and in Fig. 7, the PD between ileal lumen and skin is not affeted by reiproal variation of hloride and biarbonate or hloride and sulfate. By ontrast, progressive lowering of luminal sodium onentration is assoiated with a progressive inrease in PD with the lumen positive to skin. These results, illustrated in Fig. 9, are ompatible with a sodium diffusion potential. Similar results were obtained in the rat by Wright (5) and this onlusion is ompatible with PD measurements in man reported by Gustke, Whalen, Geenen, and Soergel (6). As shown in Table II, the average PD aross ileal muosa is lose to zero when the ileum is perfused with an eletrolyte solution simulating plasma; when gluose is added to suh solutions, the PD beomes substantially negative, as previously reported (4). DISCUSSION These experiments were designed primarily to eluidate three aspets of ileal transport. First, we measured ion movement and PD as ion onentrations varied in order to establish whih ions equilibrate passively and whih must be subjeted to some speial transport proess. Seond, we determined the interrelationship between the movement of various ions by arefully measuring Na, K, Cl, and HCO3 movements during perfusion of a balaned eletrolyte solution and by assessing the effet of the onentration of one ion on the movement of the others. Finally, by measuring potential differene under. 4 a. - Nao -4 1 I I I CON- ACET. CON- ACET. TROL TROL FIGURE 8 Effet of aetazolamide on sodium and hloride movements during perfusion with a balaned eletrolyte solution. l ' +2 _ +15.O- > E +1 X L. A. Turnberg, F. A. Bieberdorf, S. G. Morawski, and J. S. Fordtran SODIUM CONCENTRATION (meq /liter) FIGURE 9 Effet of luminal sodium onentration on potential differene (mean ±+1 SE) between ileal lumen and skin. Orientation of PD is the same as in Fig. 1. a variety of experimental onditions, we attempted to determine whether or not ion transport in the ileum in vivo is eletrogeni. Ion movement against eletrohemial gradients The results presented in Fig. 1 provide lear evidene for absorption of hloride against a steep eletrohemial gradient. Similarly, the data in Table I suggest that biarbonate seretion usually ours against an eletrohemial gradient when luminal hloride onentration is approximately 1 meq/liter. However, when luminal hloride onentration is low, biarbonate movement in the reverse diretion, i.e. absorption, ours against eletrohemial gradients (Fig. 5, Table III). Thus, under different onditions there is evidene for movement of biarbonate against an eletrohemial gradient in either diretion aross the membrane. In addition to the apparently ative nature of these transport proesses, it is lear that the onentration of one anion markedly influenes movement of the other. The present results onfirm previous investigations reported from this laboratory suggesting that sodium is atively transported in the human ileum (4). In the present studies the orientation of the PD developed aross the ileal membrane (lumen positive) favored sodium absorption from ileal solutions having a low sodium onentration; however, the magnitude of this PD ould not alone aount for the observed onentration gradient against whih sodium was absorbed. Furthermore, when mm saline solutions plus gluose are perfused, the ileal lumen beomes eletronegative (Table II and referene 4); sodium absorption ontinues under these onditions demonstrating onlu-

9 sively that sodium an be absorbed against eletrohemial gradients. We. found no evidene for potassium movement against an eletrohemial gradient in the normal human ileum. Interrelation between ion movements Variation of normal ileal funtion during balaned eletrolyte perfusion. In previous ileal perfusion experiments, wide variation in the rates of sodium and water absorption from day to day in the same subjet and between different normal subjets has been noted. Seretion rather than absorption is oasionally observed (unpublished observations of the authors). Similar findings were seen in the slow perfusion studies reported in the present paper, and the high auray of these studies preludes experimental error as an explanation for this variation in ileal behavior. Seretion of sodium, hloride, biarbonate, and water was observed at one end of the sale and absorption of all these at the other. This marked diversity of ileal ativity ourred in response to fators not defined in the present study; unknown hormonal or irulatory phenomena may have played a part. The interrelation between different ion movements at varying rates of absorption or seretion was also of interest. Only when sodium movement was zero did an equimolar anion exhange beome obvious. Sodium absorption was assoiated with a derease in biarbonate seretion and at high sodium absorption rates, biarbonate was absorbed; sodium seretion, on the other hand, was assoiated with enhaned biarbonate seretion. Regardless of the rate or diretion of fluid movement, ileal fluid beomes alkaline. During fluid seretion, alkalinity results from biarbonate seretion; during absorption, alkalinity results from water absorption (whih auses luminal biarbonate onentration to rise) and in many instanes also from biarbonate seretion. Effet of biarbonate onentration on hloride movement. The data presented in Figs. 3 and 4 learly demonstrate that hloride movement is dependent upon luminal biarbonate onentration. This effet was not mediated by a hange in PD, as shown in Table II. Although these observations, like the reiproal anion movement at zero sodium movement during balaned eletrolyte perfusion, suggest a hloride/biarbonate exhange, it is lear that net hloride absorption from physiologi solutions is muh greater than net biarbonate aumulation within the lumen. For instane, when luminal ontents were similar in ioni omposition to plasma, hloride was absorbed at a rate of 6.5 meq/ hr per 3 m, mainly as sodium hloride, while biarbonate was sereted at a rate of only.1 meq/hr per 3 m (ompare Figs. 1 and 5). Thus, that portion of hloride absorption normally mediated via an exhange must be exeedingly small if judged by the net biarbonate aumulation rate. However, this ignores the possibility that some of the exhanged, i.e. sereted, biarbonate may have been removed by a separate proess, suh as hydrogen seretion. Real ion movements during perfusion of hloride-free solutions. The studies desribed in Table III and Fig. 6 demonstrate that when sodium plus a nonabsorbable anion (sulfate instead of hloride) is perfused, biarbonate is absorbed and hydrogen ions aumulate in the ileal lumen. Several mehanisms might explain this observation. Ative sodium absorption in the absene of an absorbable anion might ause the intraluminal PD to beome negative with the result that hydrogen and potassium would be sereted passively. This possibility is unlikely sine PD was the same when the ileum was perfused with hloride-ontaining or hloride-free solutions and sine sodium absorption from hloride-free solutions did not stimulate potassium seretion. At least two possible explanations remain. First, a neutral sodium biarbonate pump might beome ativated when hloride is removed from the ileal lumen. Seond, a sodium-hydrogen exhange arrier ould effet sodium biarbonate absorption without a hange in PD. Our data do not, unfortunately, provide a lear indiation that either of the last two possibilities is orret. However, two fats are lear. The ileum ontinues to absorb sodium in the absene of luminal hloride, and sodium absorption under these onditions is assoiated with biarbonate absorption and aidifiation of ileal ontents. Noneletrogeni nature of ileal ion transport in man The experiments reported here suggest that ion transport in the normal human ileum in vivo does not generate an eletrial potential aross ileal muosa. This statement is based on the following piees of evidene: (a) when balaned eletrolyte solutions are perfused through the human ileum, the average PD between skin and ileal lumen (whih should approximate lumen/peritoneum PD as disussed in Methods) is near zero; the lumen is slightly positive as often as it is slightly negative. This observation has also been made in ileum of experimental animals in vivo, although the interpretation was that hloride and sodium were atively absorbed at the same rates by separate (and negating) eletrogeni pumps (1). (b) When the human ileum is perfused with a balaned eletrolyte solution in whih hloride has been replaed by poorly absorbed sulfate, the PD between skin and lumen is the same as when a hloride-ontaining solution is perfused. This suggests that an eletrogeni hloride pump is not negating the potential of an eletrogeni sodium pump when hloride-ontaining solleal Eletrolyte Transport 565

10 li, _ Cl ILEAL LUMEN PLASMA FIGURE 1 A double exhange model that best explains our observations on the relation of different ion movements in the ileum. lutions are perfused.3 Sodium (biarbonate) absorption from a hloride-free solution is not aompanied by potassium seretion whih is further evidene against a hange in PD sine potassium probably equilibrates passively aross ileal muosa (1, 2). () In a low resistane epithelium in whih net ion fluxes are small ompared to unidiretional fluxes, sodium transport might be eletrogeni but produe a low potential that ould not be distinguished experimentally from zero. (We probably ould not pik up a PD hange less than 2-3 mv by our system.) Fortunately, however, we have some idea of the magnitude of PD that an eletrogeni sodium pump would have to generate in order to aount for the biarbonate onentration gradient that is developed aross ileal muosa during perfusion of hloride-free solutions. As shown in Table III, the mean biarbonate onentration in the study segment fell to 15.5 meq/liter. The average onentration of biarbonate at the end of the study seg- 3 This analysis assumes equal passive permeability of the ileum to sulfate and hloride. Only if the ileum were more permeable to sulfate than to hloride would the observed results be ompatible with eletrogeni sodium transport (sulfate diffusion potential anels sodium transport potential). Had the lumen beome negative under these experimental onditions, this might indiate either eletrogeni sodium transport or a higher ileal permeability to hloride than sulfate (hloride diffusion potential). 566 L. A. Turnberg, F. A. Bieberdorf, S. G. Morawski, and J. S. Fordtran ment was 13 meq/liter. To aount for this hemial gradient on an eletrial basis would require a PD of 17 mv, assuming a plasma biarbonate onentration of 25 meq/liter. A PD hange (as hloride is replaed by sulfate) of this magnitude would be easily detetable. Sine no PD hange at all ould be observed, an eletrially neutral mehanism must be postulated for sodium biarbonate absorption under these experimental onditions. The PD hange observed when the sodium hloride onentration of ileal ontents was progressively lowered was learly related to sodium and not to hloride onentration, i.e., a sodium diffusion potential. These results suggest a muh higher permeability of ileal muosa to sodium than to anions and are onsistent with the finding that the maximum eletrohemial gradients against whih sodium an be absorbed is muh smaller than that against whih hloride an be absorbed. This may also help explain why gluose added to eletrolyte solutions perfusing the human ileum auses the lumen to beome eletrially negative even though sodium absorption is not inreased (4). Gluose enhanes ileal water absorption, and its effet on PD might be due to a streaming potential (7). This possible mehanism for the gluose effet has not, to our knowledge, been ruled out in vitro.

11 A model for ion transport in the ileum Any omplete model for ileal transport must take into aount the ability of the ileum to serete as well as absorb, the interrelations of ioni movements at different rates of absorption or seretion, the effet of biarbonate onentration on hloride movement, the biarbonate absorption and aidifiation of luminal ontents when the ileum is perfused with sodium plus a nonabsorbable anion, and the noneletrogeni nature of ion transport. The simplest model whih fits our data and observations is one in whih a hloride-biarbonate exhange is linked with a sodium-hydrogen exhange. Aording to this double exhange hypothesis, illustrated in Fig. 1, neither the anion nor the ation exhange brings about net ion movement with the result that an eletrial potential differene is not generated. Net movement observed by measuring the disappearane or aumulation of solute is the onsequene of the hemial reation between hydrogen and biarbonate to form water and C2. If the rate of the anion and ation exhange is exatly equal, the model funtions as a NaCl pump and observed biarbonate movement is zero; on the other hand, if the rate of the two exhanges is not equal, net biarbonate movement will be observed. For instane, when hloride absorption exeeds sodium absorption, more biarbonate than hydrogen would enter the lumen resulting in net biarbonate seretion. Conversely, when sodium absorption exeeds hloride absorption, more hydrogen than biarbonate enters the lumen resulting in net biarbonate absorption. Ileal seretion an our by reversing the usual diretion of the two exhange mehanisms.' Sine the rate of movement of sodium and hloride is often different and sine sodium is absorbed even when hloride is ompletely replaed by sulfate, it is lear that the two exhange proesses an operate independently of eah other. However, independent ation or anion exhange would ause rapid development of steep ion onentration gradients aross ileal muosa and this presumably reates a passive funtional link between the two exhanges so that ativity in one exhange proess tends to be followed by similar ativity in the other. The inhibition of sodium and hloride absorption by the arboni anhydrase inhibitor, aetazolamide, also lends support to the model proposed in Fig. 1. Inhibition of arboni anhydrase, whih is known to be present in the ileum of laboratory animals (8, 9), would be ex- Alternately, seretion might be explained by a separate double exhange arrier system, oriented to serete sodium and hloride in exhange for absorbed hydrogen and biarbonate. This separate system might be loated in the same or in different ells of the intestinal muosa. Seretion would result when the magnitude of transport in this seond system was greater than that of the NaCl absorption oriented double exhange. peted to inhibit sodium and hloride absorption if, in the proposed model, hydrogen and/or biarbonate exhanged for sodium and hloride are derived from arbon dioxide and water under the influene of arboni anhydrase. These experiments annot be used as onlusive proof for the model, however, sine it is possible that aetazolamide exerts its effet diretly on sodium and hloride absorption and not via inhibition of arboni anhydrase (1). On the basis of our data, it is not possible or neessary to speify the loation of the exhange proesses; these ould reside on one or more of the several membranes that separate luminal fluid from plastha. Although we are well aware that we have not presented definitive proof for this hypothesis, it is worth noting that we and many others have been unable to design or suggest an alternate unitary model that satisfatorily explains our experimental observations. ACKNOWLEDGMENTS Dr. Floyd Retor helped us in many ways and this paper ould not have been written without his generous support. We are also grateful to Dr. Norman Carter who supervised the PD measurements between skin and peritoneal avity. This work was supported under U. S. Publi Health Servie Training Grant 5 TO1 AM 549 and Researh Grant 5 RO1 AM 656. REFERENCES 1. Shultz, S. G., and P. F. Curran Intestinal absorption of sodium hloride and water. Handbook of Physiology. Amerian Physiologial Soiety, Washington, D. C. 3(Setion 6): Fordtran, J. S., and F. J. Ingelfinger Absorption of water, eletrolytes, and sugars from the human gut. Handbook of Physiology. Amerian Physiologial Soiety, Washington, D. C. 3 (Setion 6): Buher, G. R., J. C. Flynn, and C. S. Robinson The ation of the human small intestine in altering the omposition of physiologial saline. J. Biol. Chemn. 155: Fordtran, J. S., F. C. Retor, and N. W. Carter The mehanisms of sodium absorption in the human small intestine. J. Clin. Invest. 47: Wright, E. M Diffusion potentials aross the small intestine. Nature (London). 212: Gustke, R. F., G. E. Whalen, J. E. Geenen, and K. H. Soergel Muosal potential differene in the intat human small intestine. Gastroenterology. 52: (Abstr.). 7. Dietshy, J. M Water and solute movement aross the wall of the everted rabbit gallbladder. Gastroenterology. 47: Maren, T. M Carboni anhydrase: hemistry, physiology and inhibition. Physiol. Rev. 47: Carter, M. J., and D. S. Parsons Carboni anhydrase ativity of muosa of small intestine and olon. Nature (London). 219: Kitahara, S., K. R. Fox, and C. A. M. Hogben Depression of hloride transport by arboni anhydrase inhibitors in the absene of arboni anhydrase. Nature (London). 214: 836. Heal Eletrolyte Transport 567

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