Methods. Subjects. Electrical Potential Difference

Transcription

1 GASTROENTEROLOGY 1982;83: Permeability Characteristics of Human Jejunum, Ileum, Proximal Colon and Distal Colon: Results of Potential Difference Measurements and Unidirectional Fluxes GLENN R. DAVIS, CAROL A. SANTA ANA, STEPHEN G. MORAWSKI, and JOHN S. FORDTRAN Department of Internal Medicine, Baylor University Medical Center, Dallas, Texas In order to assess the passive permeability characteristics of the human intestine in vivo, we measured potential difference in the jejunum, ileum, proximal colon, and distal colon during perfusion of various test solutions that were designed to establish chemical gradients for sodium or chloride, or both or neither. In addition, unidirectional fluxes of sodium and chloride were measured in 30-cm segments of the jejunum and ileum and entire colon during perfusion of balanced electrolyte solution. These studies indicate that there are marked differences in the pathways for passive ion movement in the areas of the intestine studied. In the jejunum, this pathway appears to be highly permeable to both sodium and chloride with modest cation selectivity. In the ileum this pathway is much more cation selective, predominantly because of a relative impermeability to chloride. In the colon, on the other hand, these passive pathways appear to be more anion than cation selective. The implication of these results for normal transport physiology are discussed. Measurement of potential difference (PD) across the intestine during perfusion of specially selected test solutions can give insight into the biologic properties of the intestinal mucosa. For example, during perfusion of the human ileum with a mannitol solution containing 75 mm of sodium chloride, there Received January 18, Accepted May 10, 1982 Address requests for reprints to: Glenn R. Davis, M.D., Department of Internal Medicine, Baylor University Medical Center, 3500 Gaston Avenue, Dallas, Texas This work was supported by United States Public Health Service Grant 1-R01-AM26794 from the National Institute of Arthritis, Metabolism and Digestive Diseases, and the Southwestern Medical Foundation's Abbie K. Dreyfuss Fund, Dallas, Texas by the American Gastroenterological Association /82/ $02.50 is a concentration gradient favoring passive diffusion of cations and anions from plasma to luminal fluid. Under these experimental conditions, potential difference across the mucosa is about 12 mv, lumenside positive (1); the fact that the lumen side of the mucosa becomes positively charged means that the major route for passive transepithelial diffusion of ions across the human ileal mucosa is more permeable to plasma cations than to plasma anions. Similar studies have not been conducted in the human colon, and at no site in the intestine has the technique been fully exploited, e.g., by perfusion of test solutions which create a chemical gradient for sodium but not for chloride, and vice versa. In the present studies we measured PD across four areas of the human intestine: jejunum, ileum, proximal colon, and distal colon. Two series of test solutions were perfused: (a) different concentrations of saline made isotonic with mannitol, and (b) sa line, mannitol, sodium sulfate, and choline chloride. For comparison with the results of these PD experi.. ments, we also measured unidirectional fluxes of Na and CI in the jejunum, ileum, and the entire colon during perfusion of a balanced electrolyte solution. Methods Subjects Normal volunteers were studied. This research was approved by an Institutional Human Research Review Committee on December 11, 1980, and all subjects gave informed written consent before becoming engaged in these experiments. Electrical Potential Difference Electrical potential difference (hereafter referred to as potential difference, PD) was measured between a

2 October 1982 PERMEABILITY CHARACTERISTICS OF INTESTINE 845 perfused test solution, which served as a flowing intraluminal electrode, and a subcutaneous reference electrode. This method has recently been described in detail (2). The electrodes were connected via 3 M KCI agar bridges and calomel half-cells to the input terminals of a batteryoperated electrometer (Keithley Instruments, Inc., Cleveland, Ohio), and the output was displayed on a chart recorder (Rikadenki, Tokyo, Japan). The test solutions were perfused at a rate of 10 ml/min. After a 30-min equilibration, PO was measured continuously for 30 min with each solution. Jejunal studies were carried out when the infusion opening of a polyvinyl tube was at the ligament of Treitz, ileal studies when the tube was located in the ileum ( cm from the incisors), proximal colon studies when the opening of the tube was in the ascending colon, and distal studies when it was in the rectosigmoid (10 cm above the anal verge). The tube was passed trans orally for jejunal, ileal, and proximal colon studies, and transanally for studies in the distal colon. Tube position was'always confirmed by fluoroscopy in the jejunal, ileal. and proximal colon experiments. For proximal colon studies, the colon was cleansed of fecal material 24 h before PO was measured, by rapid oral ingestion of a special nonabsorbable saline lavage solution (3). For distal colon studies, the subject was cleansed with a 750-cm 3 enema, which consisted of the solution to be tested; after this was evacuated, 750 cm 3 of the test solution was infused into the lower colon over a 15-min period, and a continuous infusion of the test solution was then instituted at a rate of 10 mllmin for PO measurement. Several test solutions were studied in each area of the intestine, and their composition is given in Table i. The order of perfusion was randomized. Unidirectional Fluxes of Sodium and Chloride Unidirectional fluxes and net water and electrolyte movement were determined by using the steady-state, triple-lumen perfusion technique, as previously described in detail for the small intestine (2,4) and colon (5,6). Test solutions were perfused at a rate of 10 mllmin in the small intestine and 20 mllmin in the colon studies. The colon was cleansed of fecal material 24 h before colon perfusion, as described above for PO studies in the ascending colon. Equilibration periods were 50 min in the small bowel, after which samples were collected continuously for 1 h, with a 10-min delay between proximal and distal tubes. In the colon studies, when the rectal effluent was watery clear, 50 mg of sulfobromophthalein (BSP) was injected via the proximal tube. When BSP was no longer detectable in the fluid obtained at the rectal collection site, samples from proximal (located in the cecum) and distal (rectum) sites were continuously collected for 2 h. Based on previously determined transit times, there was a 15-min delay between collections at the proximal and distal sites (7). The test solution consisted of a balanced electrolyte solution of plasmalike composition (see Table 1) and, in addition, contained 0.5 /LCi of 24Na and 36Cl and polyethylene glycol (PEG), 2 gil, as a nonabsorbable volume marker. Table 1. Composition of Test Solutions Solution Balanced electrolyte solution 145 rom NaCI 100 mm NllCI 50 tum NaCI Mannitol Sodium sulfate Choline chloride Composition 100 mm NaCl, 40 mm NaHC0 3, 4 rom KCl 145 mm NaCl, 4 mm KCl 100 mm NaCl, 4 mm KCl, 80 mm mannitol 50 mm NaCl, 4 mm KCl, 180 mm mannitol 280 mm mannitol, 4 mm KCl 72.5 mm Na2S04, 4 mm KC!, 100 mm mannitol 105 mm choline chloride, 90 mm mannitol Infused test solutions and aspirated samples were analyzed for PEG by the Hyden method (8), and electrolytes were determined by standard techniques (2). 36CI and 24Na were assayed in a Packard 2425 liquid scintillation spectrometer (Downer's Grove, Ill.). Net water and electrolyte movement were calculated by standard n o m ~ b s o r b a b marker equations (9). Unidirectional fluxes were calculated by the equation of Berger and Steele (10). Results Potential Difference Balanced electrolyte solution. Figure 1 shows PD during perfusion of this solution in the four areas of the intestine. Potential difference averaged -3 mv in the jejunum, -6 mv in the ileum, -17 mv in the proximal colon, and -35 mv in the distal colon. All differences were statistically significant (p < 0.001). Although such detailed comparisons in different regions of the human intestine have :;-60 e ~ W-50 (,) z w ex:: -40 W LL. LL J c:{ ~ -20 z W ~ %- ~!!!.E-;r o JEJUNUM SMALL INTESTINE I L ~ U M L I.. +!: :+ PROXIMAL DISTAL COLON Figure 1. Potential difference (PD) in the jejunum (h = 56), ileum (n = 25), proximal colon (n = 17), and distal colon (n = 15) during perfusion of a balanced electrolyte solution, The PDs in each area of the intestine were significantly different statistically (p < 0,001).

3 846 OA VIS ET AL. GASTROENTEROLOGY Vol. 83, No.4 not been previously reported, the findings are consistent with previous studies in the small bowel, where the PD in the ileum is more negative than the jejunum (11); and in the colon where the PD distally is more negative than proximally (12). Although the studies shown in Figure 1 reveal clear-cut average differences for the four areas of the intestine, it is evident that there is substantial variation among different subjects in any given area, and that overlap exists when measurements made at one site are compared with another. Different concentrations of NaC1. As shown in Figure 2, lowering NaCI concentration in the perfused test solution caused PD to become lumenside positive in the jejunum (p < 0.05) and ileum (p < 0.05), caused no change in the proximal colon (p > 0.4), and in the distal colon caused PD to become slightly more negative with the 50 mm NaCI solution. Normal saline, mannitol, sodium sulfate, and choline chloride. In contrast to the data shown in Figure 2, where different groups of normal subjects were studied in each of the four gut segments, the results shown in Table 2 were all obtained from 7-50 > -40.s.-./ w /Distal Colon () -30,Proximal ~. - " Colon ' ~ ffi -20 I.L.. I.L J «i= z 0 w b a ~ ~ - ~ J ; J ~ ~ " ; ' - /Ileum Table 2. Potential Difference a in the Jejunum, Ileum, Proximal Colon, and Distal Colon During Perfusion of Various Test Solutions b Sodium Choline 145 mm NaCI Mannitol sulfate chloride Jejunum -1 ± 1 +9 ± l c -5 ± l c +20 ± 3 c Ileum -8 ± ± 2 c -5 ± ± 2 C Proximal -12 ± 2-24 ± 2 c -49 ± 7 c +1 ± 4 C colon Distal -31 ± 3-44 ± 2 c -62 ± 5 c -21 ± 3 C colon n = 7. a (-) Lumen-negative; (+) lumen-positive; in units of millivolts. b See Table 1 for composition. c p < 0.05 by Student's t-test when compared with 145 mm NaC!. normal subjects. Studies in one area of the gut were separated from studies in another by at least 24 h. Saline. As with perfusion of the balanced electrolyte solution (Figure 1), perfusion with isotonic saline was associated with more highly negative PD as the recording site moved distally through the gut. Mannitol. Perfusion of the isotonic mannitol solution (plus 4 mm KCI to act as a conductant) caused the PD to become lumen-side positive in the jejunum and ileum; the change in the ileum was greater than that in the jejunum. On the other hand, in both areas of the colon the PD was more negative during mannitol perfusion than during perfusion of saline. Sodium sulfate. Potential difference during perfusion of the sodium sulfate solution was similar to that noted with saline in the ileum. In the jejunum, sodium sulfate caused the PD to become slightly more negative, compared with the PD noted with saline. In both regions of the colon, the PD was much more negative with sodium sulfate than with saline. Choline chloride. In the jejunum and ileum, PD became markedly positive with choline chloride, while in both areas of the colon the PD became less negative (compared with saline). In the proximal colon, the PD with choline chloride was near zero. Figure 2. Potential difference (PO) in the jejunum (n = 15). ileum (n = 6). proximal colon (n = 8), and distal colon (n = 5) during perfusion of solutions containing 145 mm, 100 mm, and 50 mm NaCI (see Table 1 for composition). Potential difference in the jejunum, ileum, proximal colon, and distal colon with the 145 mm NaCI were all significantly different (p < 0.05) as was the change in PO in the jejunum and ileum during perfusion of the 100 and 50 mm NaCI when compared with 145 mm NaC!. The changes in the proximal colon PO were not significantly different statistically. In the distal colon the PO with the 100 mm and 50 mm NaCl solutions was significantly different (p < 0.05). while none of the other differences were statistically significant. Unidirectional Fluxes of Sodium and Chloride Table 3 shows net and unidirectional fluxes of sodium and chloride in the jejunum, ileum, and colon. In the jejunum and ileum, sodium lumen-toplasma (L ~ P) and sodium plasma-to-iumen (P ~ L) fluxes were significantly greater than those for chloride. Plasma-to-Iumen flux of chloride in the ileum was remarkably low when compared with other areas of the intestine. On the other hand, in the colon the chloride P ~ L flux was greater than the sodium P ~ L flux. This occurred despite the fact that the PD

4 October 1982 PERMEABILITY CHARACTERISTICS OF INTESTINE 847 Table 3. Sodium and Chloride Unidirectional Fluxes in Jejunum, Ileum, and Colon During Perfusion of a Balanced Electrolyte Solution a Sodium b Chloride b Unidirectional fluxes Unidirectional fluxes Net Net absorption L-.P P-.L absorption L-.P P-.L Jejunum " 24.5 c (n = 26) ±1.3 ±3.8 ±3.3 ±1.2 ±2.2 ±1.5 Ileum c 4.9 c (n = 15) ±1.7 ±2.4 ±2.1 ±1.4 ±1.5 ±0.8 Colon c 14.5" (n = 10) ±3.2 ±4.0 ±1.1 ±3.1 ±4.9 ±2.3 a See Table 1 for composition. b In units of meq/h in colon, meq/h. 30 cm in ileum and jejunum; L -. P = lumen-to-plasma, P -. L = plasma-to-lumen. c p < when compared with the sodium flux in the same area of the intestine. (lumen-negative) and the concentrations of ions in the plasma (Na 140, CI 100) would favor higher sodium than chloride diffusion. Discussion Jejunum and Ileum Previous studies have shown that during perfusion with a balanced electrolyte solution, the PD in the jejunum is slightly negative (lumen-side negative) and that the PD in the ileum is also small but somewhat more negative than in the jejunum (11). Our current studies confirm this (Figure 1), and also reveal similar PD values during perfusion of isotonic sodium chloride (Figure 2, Table 2). This PD is generally attributed to electrogenic sodium pumping by the basolateral membranes of the mucosal cells; the epithelium is considered to be "leaky," and chloride absorption or sodium backleak, or both, via the "shunt" pathway prevents the development of a large potential difference. In both areas of the small intestine, perfusion of saline-depleted mannitol solution was associated with a lumen-side positive PD. This is consistent with the cation selectivity of the channels for passive diffusion of ions that was noted in the Introduction. That the PD in the ileum becomes so much more positive than in the jejunum (+38 vs. +9 my, Table 2) was not known before; this suggests a more marked discrepancy between cation and anion diffllsion in the ileum than in the jejunum. The perfusion studies (Table 3) revealed this to be the case. Although P ~ L fluxes of both sodium and chloride were lower in the ileum than in the jejunum (perhaps, in part, because of reduced surface area per unit length of intestine), the P ~ L flux of chloride in the ileum was especially low (Na:CI P ~ L ratio = 1.6 in the jejunum and 3.5 in the ileum). There are two possible explanations for this latter observation. First, the ileum may be intrinsically less permeable to chloride than the jejunum. Second, the more lumen-negative PD in the ileum (see Table 2) might retard chloride diffusion and increase sodium diffusion from plasma to lumen. During perfusion of both areas of the small bowel with choline chloride, the PD became positive ( my). Under these conditions, there is no gradient for chloride diffusion but a steep gradient favoring passive movement of sodium from plasma to lumen and a steep gradient favoring choline diffusion from lumen to plasma. The fact that the PD becomes lumen-side positive means that the small bowel mucosa is much more permeable to sodium than to choline. It was previously reported (1) and confirmed here (Table 2) that perfusion of the ileum with a sodium sulfate solution did not alter PD, compared with the PD recorded with a balanced electrolyte solution. There are several possible reasons (acting singly or together) for the failure of PD to change as sulfate is substituted for chloride in the ileal perfusates. Included in these are neutral entry of sodium chloride across the brush border membrane, a leaky shunt pathway so that sodium ions transported across the basolateral membrane readily diffuse back into the lumen, and approximately equal ileal permeability to chloride and sulfate, so that the anion diffusion potentials cancel out during perfusion of the lumen with sodium sulfate. The present studies reveal that, in contrast to the ileum, jejunal PD becomes slightly more negative with sodium sulfate than with sodium chloride. This may be explained by postulating that the ionic diffusion pathway in the jejunum is less permeable to sulfate than to chloride. If this were the case, sodium pumping would generate higher PD values during perfusion of the sulfate solution since passive sulfate absorption could not so readily mitigate the sodium transport potential as would passive chloride absorption during perfusion with sodium chloride. In addition, the chloride diffusion potential (P ~ L) would be greater than the sulfate diffusion

5 848 DAVIS ET AL. GASTROENTEROLOGY Vol. 83, No.4 potential (L ~ P), and this would also cause the lumen side of the jejunal mucosa to be more negative during sodium sulfate perfusion. Colon Previous work has shown that sodium is absorbed by the human colon against steep electrochemical gradients (13-16). The process that mediates this transport (presumably active sodium pumping by basolateral membranes) is believed to be responsible for the lumen-side negative PD that is observed in the colon in humans (16-18). The reason for the higher PD in the colon than in the small intestine (during perfusion of a balanced electrolyte solution) is thought to be related to the tighter nature of its passive permeability pathway, i.e., less backleak of sodium (and perhaps reduced chloride permeability) allows higher PD values for a given rate of active sodium pumping. Our current experiments reveal additional but previqusly unrecognized differences between the human small and large intestine; these were brought out by perfusion of different test solutions. We first assessed colonic permeability by progressively lowering the NaCI concentration of luminal contents. As shown in Figure 2, colon PD changed little, if at all, as luminal NaCI concentration was reduced from 145 to 50 mm. (This is in sharp contrast to findings in the jejunum and ileum, where PD moved in a positive direction.) Curran and Schwartz made a similar observation in the rat colon (19). These workers found the PD to be -7.3 mv when luminal NaCl concentration was 150 meq/l, -9.8 mv when NaCI concentration was 55 meq/l, and mv when NaCI concentration was 50 meqll. It is not possible to tell from their paper whether or not these differences were statistically significant, and it is not known what part of the rat colon they studied; nevertheless, their data in rats and ours in humans make it clear that the colon PD remains lumen-side negative as NaCI concentration is reduced to meq/l. This finding is, however, difficult to interpret because it is not known to what extent active sodium transport contributes to the lumen-negative PD when NaCI concentration in the colonic lumen is meq/l. More definitive information could be obtained if active sodium absorption were eliminated, and this can be accomplished by perfusing a sodium chloride-free solution, such as isotonic mannitol. If active sodium pumping across basolateral membranes were the only factor that caused the colon to have a negative PD, one would expect the PD to be zero when the luminal sodium chloride concentration is zero. If the colonic mucosa were more permeable to cations than to anions, as would be expected on the basis of the permeability characteristics of the small intestine (see above), the PD should become positive as the sodium chloride concentration is lowered to near zero. This would be because of more rapid sodium than chloride diffusion from plasma to lumen, at a time when active sodium absorption was nil because of the absence of sodium within the colonic lumen. However, our studies show that PD in the colon did not become zero or positive, but actually became more negative as luminal NaCl concentration approached zero during perfusion of isotonic mannitol solution (Table 2). Accentuation of the lumen-negative PD under these conditions strongly suggests that passive permeability channels in the colonic mucosa are more permeable to chloride than to sodium. During choline chloride perfusion there is no chemical gradiimt for chloride diffusion, there is a steep concentration gradient favoring P ~ L diffusion of sodium, a reverse gradient favoring choline diffusion from lumen to plasma, and active sodium pumping is nil since only small amounts of sodium are present in the lumen. Under these conditions, the colon PD became less negative (rather than more negative as with mannitol) than during perfusion with isotonic saline. These findings support the contention that a chloride diffusion potential accounts for maintenance or accentuation of lumenside negative potential difference during perfusion of test solutions of mannitol with a NaCl concentration less than that of plasma. The fact that PD in the proximal colon was near zero during perfusion of choline chloride suggests that this region is approximately equally permeable to choline and to sodium. The fact that the PD was still lumen-side negative (-21 my, Table 2) during choline chloride perfusion in the distal colon suggests that the lower colon may be more permeable to choline than sodium. If so, this would indicate that the distal colon has a lower permeability to sodium than does the proximal colon. This could then explain the fact that distal colon PD is more lumen-side negative than the proximal colon during perfusion of a balanced electrolyte solution (Figure 1) or of isotonic saline (Figure 2, Table 2). Although previous studies in the ileum revealed no change inpd as the perfusion fluid was changed from isotonic sodium chloride to a sodium sulfate solution (see above), the hypothesis for the colon described in the previous paragraphs would predict that colon PD would become more negative during sodium sulfate perfusion. This is because luminal sodium concentration would be high (allowing active electrogenic sodium absorption to continue Unabated), and because the luminal chloride concentra-

6 October 1982 PERMEABILITY CHARACTERISTICS OF INTESTINE 849 tion would be near zero and thus set the stage for a chloride diffusion potential. We used this experiment as a test of the hypothesis, and found that PO increased from -12 to -49 mv in the proximal colon and from - 31 to - 62 m V in the distal colon. Thus, this experiment lends support to the hypothesis. To obtain more information about the relative permeability of the colon to chloride and to sodium, we measured unidirectional flux rates of these ions during perfusion of a balanced electrolyte solution. For this purpose, it is most important to examine P ~ L fluxes, since they are largely independent of active transport processes and are thus determined primarily by passive forces. Because it is extremely difficult to accurately measure fluxes in isolated segments of the colon, we studied the entire colon as a unit. Since the proximal and distal colon are qualitatively similar in their PD profile as different test solutions are perfused, our lack of ion flux data in specific segments of the colon does not appear to be a serious disadvantage. In the colon, the P ~ L flux of chloride was higher than the P ~ L flux of sodium (even though electrochemical gradients were more favorable to sodium diffusion). This is consistent with our estimates from PD studies. Although direct comparison between the various areas of the intestine are difficult because of differences in mucosal surface area, the colon appears to have a relatively high chloride permeability and a relatively low sodium permeability when compared with the ileum, while the jejunum is highly permeable to both ions (see Table 3). Conclusions These studies have revealed major differences in ionic permeability between different regions of the human intestine. The major characteristic of the jejunum is high permeability to both sodium and chloride (Na > CI; CI > S04)' It is ideally designed for passive absorption or passive secretion of NaC!. The major characteristic of the human ileum is its very low permeability to chloride (Na ~ CI; CI = S04)' This markedly restricts passive absorption or secretion of NaC!, and makes chloride absorption from the ileum dependent on some active or facilitated process. Functionally, the extremely low colonic permeability to sodium helps make the colon a highly efficient sodium absorbing organ, whereas the higher chloride permeability would allow passive chloride absorption and cause the lumen-negative PO to be maintained even when lumen sodium concentration is markedly reduced (as it is under normal conditions) (20). Maintenance of lumen-side negative PO might act as a deterrent to bicarbonate secretion and as a mechanism to facilitate organic anion absorption. The distal colon is even more impermeable to sodium than' the proximal colon; this probably accounts for the fact that distal colon PD is more negative than proximal colon PO during perfusion of a plasmalike solution or isotonic saline. Otherwise, our studies revealed no major difference in the permeability characteristics of these two regions of the large intestine. The anatomic or chemical factors, or both, that are responsible for the colonic mucosa being more permeable to chloride than sodium (whereas the reverse is true in the small intestine) are unknown. Presumably, it might involve a different alignment of dipoles within the tight junctions of the paracellular pathway, or perhaps passive transepithelial ion movement in the colon occurs by a different more anion-selective route, i.e., transcellularly instead of paracellularly. References 1. Turnberg LA, Bieberdorf FA, Morawski SG, et al. Interrelationship of chloride, bicarbonate, sodium and hydrogen transport in the human ileum. J Clin Invest 1970;49: Davis GR, Santa Ana CA, Morawski S, et al. Active chloride secretion in the normal human jejunum. J Clin Invest 1980;66: Davis GR, Santa Ana CA, Morawski SG, et al. Development of a lavage solution associated with minimal water and electrolyte absorption or secretion. Gastroenterology 1980;78: Cooper H, Levitan R, Fordtran JS, et al. A method for studying absorption of water and solute from the human intestine. Gastroenterology 1966;50: Levitan R, Fordtran JS, Burrows BA, et al. Water and salt adsorption in the human colon. J Clin Invest 1962;41: Krejs GJ, Walsh JH, Morawski SG, et al. Intractable diarrhea: intestinal perfusion studies and plasma VIP concentrations in patients with pancreatic cholera syndrome and surreptitious ingestion of laxatives and diuretics. Am J Dig Dis 1977; 22: Schiller LR, Davis GR, Santa Ana CA, et al. Mechanism of the antidiarrheal action of codeine (abstr). Gastroenterology 1981;80: Hyden S. A turbidometric method for determination of higher polyethylene glycols in biological material. Ann Agr ColI lsweden) 1955;22: Fordtran JS, Rector FC, Ewton MF, et al. Permeability characteristics of the human small intestine. J Clin Invest 1965; 44: Berger EY, Steele JM. The calculation of transfer rates in two compartment systems not in dynamic equilibration. J Gen Physiol 1958;41: Fordtran JS, Rector FC, Carter NW. The mechanisms of sodium absorption in the human small intestine. J Clin Invest 1968;47: Rask-Madsen J. Simultaneous measurement of electrical polarization and electrolyte transport by the entire normal and inflamed human colon during in vivo perfusion. Scand J GastroenteroI1973;8: Devroede GJ, Phillips SF. Conservation of sodium, chloride and water by the human colon. Gastroenterology 1969; 56:101-9.

7 850 DAVIS ET AL. GASTROENTEROLOGY Vol. 83, No Grady GF, Duhamel RC, Moore EW. Active transport of sodium by human colon in vitro. Gastroenterology 1970; 59: Hawker PC. Mashiter KE, Turnberg LA. Mechanisms of transport of Na, Cl and K in the human colon. Gastroenterology 1978;74: Geall MG, Spencer RJ, Phillips SF. Transmural electrical potential differences of the human colon. Gut 1969;10: Edmonds q, Godfrey RC. Measurement of electrical poten- tials of the human rectum and pelvic colon in normal and aldosterone-treated patients. Gut 1970;11: Giller J, Phillips SF. Electrolyte absorption and secretion in the human colon. Dig Dis Sci 1972;17: Curran PF, Schwartz GF. Na, CI and water transport by the rat colon. J Gen Physiol 1960;43: Wrong 0, Metcalfe-Gibson A, Morrison RBI. In vivo dialysis of faeces as a method of stool analysis. I. Technique and results in normal subjects. Clin Sci 1965;28: