EXPERIMENTAL DIARRHEA

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1 EXPERIMENTAL DIARRHEA III. Bicarbonate transport in rat salmonella enterocolitis DON W. POWELL, M. D., LEIF I. SOLBERG, M.D., GERALD R. PLOTKIN, M. D., DON H. CATLIN, M.D., RONALD M. MAENZA, M.D., AND SAMUEL B. FORMAL, PH.D. Departments of Gastroenterology, Experimental Pathology, and Applied Immunology, Walter Reed Army Institute of Research, Washington, D. C. Bicarbonate is absorbed in the ileum of rats with salmonella enterocolitis rather than secreted as in normal animals. In this respect, the infected ileum resembles the normal rat jejunum. This similarity was confirmed by analysis of the relationship of Na to CI and HCOs transport in the normal and infected small intestine. HCOs absorption in the infected ileum was not due to alterations in systemic acid base balance. To explore the mechanism of the luminal disappearance of HCOs in the normal rat jejunum and infected rat ileum, changes in luminal solution ph and pc0 2 during intestinal transit were determined with a HCOa-free and HCOs-containing solutions. The pc0 2 of the HCOa-free solution was not altered during transit through the normal or infected intestine suggesting that the tissue pc0 2 w ~ s not elevated. However, during perfusion with HCOa-containing solutions, from which HCOa was being "absorbed," the solution ph decreased and pc0 2 increased significantly in the normal jejunum and in the infected ileum. This is evidence that H secretion was the mechanism of HCOa absorption in these segments. The normal rat jejunum was not capable of Na and H 2 0 absorption from a HCOs-free, 118 mm Na solution, a finding similar to that reported in the human jejunum where a HCO a-mediated Na transport mechanism (Na: H exchange) has been proposed. Recent studies of water and electrolyte transport in rat salmonellosis have demonstrated an abnormality of bicarbonate transport in the ileum. 1 Regardless of the stage or severity of the mucosal lesion, or Received September 15, Accepted January 25, Address requests for reprints to: Dr. Samuel B. Formal, Department of Applied Immunology, Walter Reed Army Institute of Research, Washington, D. C Dr. Powell's present address is: Department of Physiology, Yale University School of Medicine, New Haven, Connecticut Dr. Plotkin's present address is: Department of Medicine, Beth Israel Hospital, Boston, Massachusetts Dr. Maenza's present address is: The Health 1076 the presence or absence of diarrhea, infected animals absorbed HCOa in the ileum while normal animals secreted HCO s. Utilizing the techniques of Brodsky and Schlib 2 and Turnberg et ai., s we have examined HCOa transport in the intestine of normal rats and rats infected with Salmonella typhimurium. This study suggests that H secretion accounts for the observed abnormality of HCO s transport in rat salmonellosis. Center, University of Connecticut School of Medicine, Hartford, Connecticut The authors gratefully acknowledge the technical assistance of Smiley Austin, Thomas Pitts, William Hoffer, Robert Cass, and Richard Ciavarra.

2 June 1971 EXPERIMENTAL DIARRHEA. III 1077 Methods The details of this disease model have been reported elsewhere,4 as have the experimental methods and the basic abnormalities of water and electrolyte transport." 5 In the present studies, experiments were performed on three groups of animals: control, 2-day infected, and 6-day infected (diarrhea). Under pentobarbital anesthesia, water and electrolyte transport was determined from segments of jejunum and ileum perfused simultaneously in situ at a rate of 0.5 ml per min with perfusion solutions maintained at 38 C. Six solutions were utilized, each having an osmolality of 295 to 305 milliosmoles per kg and containing polyethylene gly- TABLE 1. Perfusion solution composition" Solution Na K Cl HCO, 10, Man nitol mmofesl liter Mannitol electrolyte HCO.-free mannitol electrolyte Balanced electrolyte HCOs-free balanced electrolyte H CO,PO.-buffered electrolyte b Glucose electrolytee "All solutions contained polyethylene glycol, 600 mg per 100 m!. b Also contained 13 mm HPO. and 2 mm H,PO. C Also contained 56 mm glucose. col (PEG), 600 mg per 100 ml, as a nonabsorbable marker. These solutions (table 1) were designed to achieve a specific ionic status: (1) Low Na, HCOa (mannitol electrolyte); (2) low Na, HCO.-free (HCO.-free mannitol electrolyte); (3) normal Na, HCOa (balanced electrolyte); (4) normal Na, HCOa-free (HCOs-free balanced electrolyte); (5) normal Na, HCO. with PO. buffer (HCOaPO.-buffered electrolyte); and (6) low Na-glucose (glucose electrolyte). To determine changes in luminal solution ph and pc0 2, the jejunum and ileum of control and infected rats were perfused simultaneously with the balanced electrolyte solution or the HeO.-free balanced electrolyte solution followed by the HCO.PO.-buffered electrolyte solution. The ph and pc0 2 of these solutions were adjusted by bubbling the solution reservoir with 10% CO 2, To prevent CO 2 loss through the polyvinyl perfusion coils in the constant temperature water bath (used to maintain the perfusion solutions at 38 C), it was necessary to bubble the water bath with 10 or 15% CO 2, The ph and pc0 2 were determined on perfusion solution collected under paraffin for 20 min, both prior to and after perfusing each animal. Changes in perfusion solution ph and pc0 2 during intestinal transit were determined on a 20-min collection of perfusate following a 20-min steady state period. All ph and pc0 2 measurements were made 5 to 10 min following collection with an Instrumentation Laboratories, Inc. model 123-S1 blood gas apparatus after transferring the samples, without TABLE 2. Small intestinal water and electrolyte transport in control animals from solutions with varying Na concentration, with or without HCO, or glucose" HCOa-free mannitol electrolyte Mannitol electrolyte Balanced electrolyte Glucose electrolyte Jejunum: no. of animals H 2 O ± 4.4" 14.2 ± ± ± 3.2 Na ± 0.4" 3.7 ± ± ± 0.4 K ± 0.07" 0.14 ± ± ± 0.Q3 Cl ± 0.3" -0.9 ± ± ± 0.3 HCO, ± 0.2" 4.7 ± ± ± 0.3 Ileum : no. of animals H 2 O.., 7.4 ± ± ± ± 3.7 Na ± ± ± ± 0.4 K ± ± ± ± 0.02 Cl ± 0.8 e 6.0 ± ± ±.0.5 HCO, ± 0.4" -2.3 ± ± ± 0.3 a Expressed as /Lhters or /Lmoles per em per 30 mm. " P < 0.01 as compared with transport from the mannitol-electrolyte solution. c P < 0.05 as compared with transport from the mannitol-electrolyte solution.

3 1078 POWELL ET AL. Vol. 60, No.6 exposure to air, from the collection cylinders to well fitting glass syringes. The differences in perfusion solution and perfusate ph and peo 2 were analyzed by the paired Student's t-test. Results Electrolyte transport in control animals. In table 2 are shown water and electrolyte transport rates from the RCOa-free mannitol-electrolyte solution and three RCOacontaining solutions in which the Na concentration was varied or glucose was present. Examination of this data reveals three important points. First, the jejunum was not capable of water and Na absorption from the RCOa-free mannitol-electrolyte solution, but the ileum was. Second, RCO a was absorbed in the jejunum (except from the RCOs-free solution where RCOa transport was essentially zero) and was secreted in the ileum. Third, excluding the RCO a - free solution, RCO a absorption in the jejunum and secretion in the ileum was fairly constant in spite of wide variations in Na and R 2 0 transport. Therefore, CI was the major anion accompanying an increase in Na absorption in both segments. This relationship between Na transport and anion transport is better demonstrated in figures 1 and 2, where the regressions of RCO a and CI transport on Na transport from the mannitol-electrolyte and glucoseelectrolyte solutions are depicted. Both solutions contained Na, 118; CI, 93; and RCO a, 30 mmoles. In the jejunum, at low rates of Na absorption, CI was secreted while RCO a made up anion absorption in its entirety. At higher Na absorption rates, RCOa absorption remained essentially constant while CI absorption increased and eventually exceeded RCO a absorption. In the ileum (fig. 2), the correlation between Na and RCOa transport y = 0.21 ± ± r = P < No :tl C I No U Hea 3 ME 6 GE I- a: 0 0 (/) C z E <l 0 a: I- '" ~ o E u U -. I ;:, "0 "- " o B.O o -----;----- y = 0.82 ± x ±O_30S r = P < B.O No TRANSPORT I'M l em 130mln. FIG. 1. The relationship of Na transport to CI and HCO. transport in the normal rat jejunum from mannitolelectrolyte (0,.) and glucose-electrolyte (t.,.) solutions. Na versus CI is depicted by clear symbols; Na versus HCO. by solid symbols. Both solutions contained Na, 118; K, 5.0; CI, 93; and HCO" 30 mmmoies per liter and were 305 milliosmoles per kg of H 2 0. The glucose-electrolyte solution contained 56 mm glucose and 16 mm mannitol, while the mannitol-electrolyte solution contained only 72 mm mannitol. Na transport, as given in table 2, clustered around 3.7 and 11.4!lmoles per cm per 30 min. Therefore, data from a pilot study with these solutions, which had intermediate Na absorption rates, were included in order to obtain more points with which to calculate the regressions.

4 June 1971 EXPERIMENTAL DIARRHEA. III No II C! No ~ HC0 3 ME t:. GE f a: o Q. c' ~ 'E <t 0 a:,, f- ~ E o u U "' :I: :::; "C. c: c Ao o o sf» 0 0 A y::::ie 0.71 ± ~ ± r = p < o ~ - ~ ~ r y = 0.23 ± ± r = p < ~ No TRANspaRT I'M / em. /3.0 min. FIG. 2. The relatianship.of Na transpart ta Cl and HCO, transpart in the narmal rat ileum fram the mannital-electralyte and glucose-electralyte salutians as shawn in table 2. Symbols and perfusion salutians are the same as in legend to figure 1. was not as good as in the Jejunum, although the regression suggests that HCO a secretion was less at maximal Na absorption. Extrapolating these regression lines to zero Na transport in both the jejunum and ileum reveals no significant difference in their Y intercepts in a given segment except for algebraic sign. In the jejunum (fig. 3) the slopes of the regression lines at zero Na transport, 3.39 ± 0.3 ~ m o of l e s are essentially parallel, so that at higher CI would be secreted while 3.57 ± 0.3 ~ o l of e s HCO a are absorbed. In the ileum at zero Na transport 3.06 ± 0.4 ~ o lof ehco s a would be secreted while 3.55 ± 0.5 ~ o lof eci s are absorbed. Electrolyte transport in infected animals. The abnormality of HCO a transport in the infected animals is shown in table 3. In the infected jejunum, HCO a absorption was somewhat variable but was essentially the same as in the control jejunum. Na transport in the infected jejunum ranged between 4.5 and 7.9 ~ m o l e s. In the infected ileum, HCO s was absorbed instead of secreted as in the control ileum. This HCO s absorption occurred in the face of Na and H 2 0 secretion in the 6-day infected animals. The relationship of Na to anion transport in the 2- and 6-day infected animals perfused with the mannitol-electrolyte and glucose-electrolyte solutions is shown in figures 3 and 4. In the infected jejunum Na transport rates, a greater proportion of anion transport was made up of HCO s than was the case with the control jejunum (fig. 1). Anion transport rates at zero Na transport (the Y intercept) were less than the control jejunum, but are again equal in magnitude but opposite in direction. In the infected ileum (fig. 4) the slopes of the regression lines are similar to the control ileum (fig. 2). However, the relationship between Na and anion transport at zero Na absorption was both qualitatively and quantitatively altered. At zero N a transport, CI was secreted rather than absorbed as in the control ileum. Con-

5 1080 POWELL ET AL. Vol. 60, No.6 TABLE 3. Small intestinal Na and HC0 3 transport in control and infected animals Segment and solution Infected animals Control animals 2-Day 6-Day No. N. HCO, No. N. HCO, No. Na HCO, J1InQJes/cm /30 min,.mole.,/cm /30 m in,.mo/es/cm /30 min Jejunum Mannitol electrolyte ± ± ± ± ± ± 0.1 Balanced electrolyte ± ± ± ± ± ± 0.5 Glucose electrolyte ± ± ± ± ± ± 0.7 Ileum Mannitol electrolyte ± ± ± ± ± ± 0.2 Balanced electrolyte ± ± ± ± ± ± 0.6 Glucose electrolyte ± ± ± ± ± ± y = 0.47 ± ± 0. 4 ~ r = p <_ No.xl. CI No U HC0 3 a ME A GE t- 0:: 0 Cl. c: (/) Z E <l 0 0:: t- '" "- E 0'" U U " I ::;: y = 0.53 ± ± 0.49 r = P < " :I, No TRANSPORT,.M/em/30min. FIG. 3. The relationship of Na transport to Cl and HCO. transport from the mannitol electrolyte and glucose electrolyte solutions in the jejunum of rats infected with Salmonella typhimurium. Symbols and perfusion solutions as in legend to figure 1. versely, HCO a was absorbed in the infected ileum, but correlated poorly with Na transport (r = 0.355, P < 0.02). Mechanism of BeO a absorption in control and infected animals. To rule out systemic alterations in acid base balance as a cause for the luminal disappearance of HC0 3 in the infected ileum, acid base studies were done on arterial blood of control and infected animals 30 to 60 mm following anesthetization and sham perfusion. (Intestinal segments were cannulated but not perfused). The results are shown in table 4. There was no difference in blood ph and, except for the 6-day infected animal, in pc0 2 or plasma total CO 2 between the control and infected animals.

6 June 1971 EXPERIMENTAL DIARRHEA. IIJ I- 0:: (L c: (f) z <l E 0:: l- 0 t<> 6.0 "- 0 '" u E 4.0 I u 0 "- t. (; ::< :I. U t. t. t. t. 6.0 " t. 8.0 No ~ CI No II HC0 3 ME.0. GE y = 0.85 ± ± 023 r =.918 p < y = 0.12 ± ± 0.20 r = P < Na TRANSPORT -4.0 I'M / em / 30min -6.0 FIG. 4. The relationship of Na transport to Cl and HCO. transport from the mannitol-electrolyte and glucose-electrolyte solutions in the ileum of rats with salmonellosis. Symbols and perfusion solutions as in legend to figure 1. TABLE 4. Systemic acid base status of sham perfused control and infected animals" ExpeTimental group' Blood ph Blood pco, Plasma total CO, Control mmoles/iiter (12) 7.50 ± ± ± 1.2 Infected 2-Day (10) 7.51 ± ± ± Day (7) 7.52 ± ± 2.0c 17.1 ± 0.9 c "Mean ± SE. mm Hg b Number in parentheses, number of animals studied. C Significance of difference from control animals, P < To explore further the mechanism of luminal HCOa disappearance in the control jejunum and infected ileum, changes in perfusion solution ph and pc0 2 during intestinal transit were determined. To be sure that these changes were related to HCOa transport and not to equilibration with tissue ph and pc0 2, control and infected animals were perfused first with the HCOa-free balanced electrolyte solution. The results are shown in table 5. There was no significant change in solution pc0 2, although the solution became slightly but significantly more alkaline. When control animals (fig. 5) were perfused with HCOa-containing solutions, the pc0 2 of the jejunal perfusate increased significantly by 6.9 ± 0.7 mm Hg, and ph decreased by 0.22 ± 0.01 unit. Na and HCO a absorption were 3.4 ± 0.4 and 3.3 ± 0.4 /lmoles per cm per 20 min in this group. In the control ileum 2.2 ± 0.3!lilloles of Na were absorbed and 1.1 ± 0.2 /lmoles of HCOa were secreted, but the ph and pc0 2 of the HC0 3 -containing solutions were unchanged during transit. When the infected animals (fig. 6) were perfused with the HCOa-containing solutions, jejunal perfusate ph decreased significantly by 0.14 ± 0.02 unit. The increase in pc0 2, 3.4 ± 1.5 mm Hg, although less than in the control jejunum, was significant (P < 0.05) by the paired t-test. Na absorption of 2.6 ± 0.5!lilloles and HCO a absorption of 2.4 ± 0.3!lilloles were measured. The infected ileum now appeared to behave as the control jeju-

7 1082 POWELL ET AL. Vol. 60, No.6 TABLE 5. Changes in ph and pco, of the HCO.-free balanced electrolyte solution after transit in the jejunum and ileum of control and infected animals a Segment b Entering perfusion solution p H "'ph Entering perfusion solution pco, "'pco, HCO. Transport mmhg mmhg wno/es/cm/20 min Control animals Jejunum (5) 6.37 ± ± ± ± ± 0.1 ileum (5) 6.08 ± ± ± ± ± 0.2 Infected animals Jejunum (5) 6.09 ± ± ± ± ± 0.2 ileum (5) 6.00 ± ± ± ± ± 0.2 a Mean ± SE. Numbers in parentheses, number of animals studied. 7.7 JEJUNUM ILEUM S ph ph L--'------l------L-----'--J7.2 ENTERING EXITING ENTERING EXITING pc ILEUM pcoz pcoz mm Hg mm Hg ENTERING EXITING ENTERING EXITING FIG. 5. Changes in perfusion solution ph and pco, after transit in the control jejunum or ileum. The balanced electrolyte solution is shown in clear symbols (0); the HCO,PO.-buffered electrolyte solution by solid symbols (e). The combined mean ± SE entering ph was 7.57 ± 0.01 in the jejunum and 7.55 ± 0.02 in the ileum with ~ ph's of ± 0.01 and 0.01 ± 0.02 respectively. The mean entering pco, was 36.6 ± 1.1 mm Hg in the jejunum and 38.3 ± 1.3 in the ileum, with ~ pco,'s of 6.9 ± 0.7 and 0.9 ± 1.3 respectively.

8 June 1971 EXPERIMENTAL DIARRHEA. III 1083 E.!:! JEJUNUM ILEUM ph ph ENTERING EXITING ENTERING EXITING pc JEJUNUM ILEUM pc02 pc02 mmhg mm Hg ENTERING EXITING ENTERING EXITING FIG. 6. Changes in perfusion solution ph and pco, after transit in the infected jejunum and ileum. Symbols are the same as in legend to figure 5. The combined mean entering ph was 7.55 ± 0.02 in both the jejunum and ileum with ~ ph's of ± 0.02 and ± 0.02 respectively. The mean entering pco, was 36.2 ± 1.0 mm Hg in the jejunum and 35.9 ± 0.9 in the ileum, with ~ pco,'s of 3.4 ± 1.5 and 6.8 ± 0.9 respectively. num. The ph of the HC0 3 -containing solution was reduced by 0.10 ± 0.02 unit, and the pc0 2 increased by 6.8 ± 0.9 mm Hg as 0.4 ± 0.3 ILmole of Na was secreted and 0.9 ± 0.2 1Lffi0ie of HCO a was absorbed. Discussion The mucosal or serosal presence of HCO a appears to increase Na absorption in a number of epithelia 6-10 including the human jejunum. 3, 11, 12 In the human jejunum this effect is seen with HCO a in the mucosal solution, and is not due to differences in solution ph,l1 Fordtran et al. 12 have shown that the human jejunum is not capable of Na absorption from a low Na solution (127 mm) in the absence of luminal HCO a The failure of Na and H 2 0 absorption in the rat jejunum from a 118- mm solution in which HC0 3 was replaced by iodate indicates a similar phenomenon in the rat. (Iodate was chosen to replace HCO a because Clarkson et al.13, have shown that this anion is not transported in the same manner as HC0 3 in the rat intestine. We have, however, repeated these experiments replacing HC0 3 with

9 1084 POWELL ET AL. Vol. 60, No.6 phosphate with identical results.) In our studies, the lack of absorption from the HC0 3 -free solution was not likely due to differences in ph (7.3 to 8.2) in the ungassed HC0 3 -free and HC0 3 -containing solutions, as McHardy and Parsons 14 have shown only a minimal effect on Na and water absorption in the rat jejunum with ph differences in this range. An additional similarity between jejunal electrolyte transport in the rat and in man is the relationship between Na and anion transport as shown in figure 1.12 Since apparent HC0 3 absorption can come about via H, OH, or HC0 3 transport itself, deciding which mechanism may be operative in a system where HC0 3 disappears may be difficult. Brodsky and Schlib 2 suggested that an increase in mucosal pc02 accompanied by a decrease in ph would be indicative of H secretion, while a decrease in both pc02 and ph would indicate HC0 3 absorption. These parameters have been investigated in studies of the mechanism of HC0 3 transport by the turtle urinary bladder. 15,16 With regard to the intestine, several investigators have found a high jejunal fluid pc02, and have suggested that this represented H secretion with the resulting titration and disappearance of luminal HC0 3.17'20 However, the intestinal mucosa may at times have a high tissue pc02, and because of the free tissue diffusibility of this gas, any changes in luminal solution pc02 could represent equilibration with the tissue content. 21 Turnberg et ai. 3 resolved this by demonstrating that the increment in luminal pc02 was greater when the human jejunum was perfused with a HC0 3 -containing solution, from which HC03 was being "absorbed," than when the perfusion was carried out with a HC0 3 -free solution. Thus they presented compelling evidence that H secretion is the mechanism of jejunal HC0 3 absorption in man. We have repeated these experiments in the rat jejunum with essentially the same results. There was no change in solution pc02 in the normal rat jejunum during perfusion with the HC0 3 -free balanced electrolyte solution (table 5), but a significant increase in pc02 and a decrease in ph were found during perfusion with the balanced electrolyte and HC03-P04 buffered electrolyte solutions (fig. 5). The reciprocal relationship between Na and anion transport in the rat ileum (fig. 2) is also similar to that reported in the human ileum. 22 The Y intercepts of the CI and HC0 3 regression lines suggest some form of CI: HC03 exchange, as do the findings of HubeP3 and Turnberg et al,22 that the magnitude of HC0 3 secretion in the ileum is directly related to the intraluminal CI concentration. However, further studies by these investigators tend to rule out a simple one for one exchange In addition, although the guinea pig ileum in vivo exhibits a reciprocal Na to anion relationship similar to that in figure 2,25 recent in vitro studies in this species have shown that CI absorption, although intricately related to HC0 3 transport, is not accomplished by simple anion exchange (Powell et ai., unpublished observations). Fordtran and his associates 3 have proposed that the only active Na transport in the jejunum is that mediated by HC0 3 (Na: H exchange) and that a double exchange mechanism involving Na: H, CI: HC0 3 will explain the interrelationship of the transport of these ions in the ileum. 22 We have not systematically explored these possibilities and cannot evaluate their hypotheses for the rat. However, the marked similarities of intestinal electrolyte transport in the rat and man suggest another model in which these phenomona might be profitably investigated. The ileum of the rat with salmonella enterocolitis transports CI and HC0 3 in a pattern similar to the normal jejunum. HC0 3 is absorbed rather than secreted (table 3), and the relationship between Na and anion transport is altered so that it resembles the jejunum (figs. 1 and 4). The most obvious cause for HC0 3 absorption by the infected ileum would be systemic metabolic acidosis which would produce Hand HC0 3 blood to lumen gradients

10 June 1971 EXPERIMENTAL DIARRHEA. III 1085 conducive for the luminal disappearance of HC0 3 However, acid base studies (table 4) have shown that all of these animals are in a state of respiratory alkalosis owing to hyperventilation induced in some manner by the operative and perfusion procedure. Thus the mechanism of HC0 3 disappearance in the infected ileum resides at a local tissue level and can be explored in the same way as the normal rat. The lack of an increase in solution pc0 2 during perfusion of the infected intestine with the HC0 3 -free balanced electrolyte solution (table 5) indicates that the mucosal pc0 2 of these animals was not elevated in response to the infection. If the 0.5 ml per min perfusion rate allowed adequate tissue-luminal fluid CO 2 equilibration, the solution pc0 2 in the HC0 3 -free studies should reflect the mucosal pc0 2 It appears that the mucosal pc0 2 in the infected and control animals was 23 to 25 mm Hg in both the jejunum and ileum. This low value is in keeping with the respiratory alkalosis noted in these animals. The pc0 2 increment in the infected jejunum (3.4 ± 1.5 mm Hg) perfused with the HC0 3 -containing solutions was not as large as that in the normal jejunum (6.9 ± 0.7). This was due to no measurable pc0 2 increase in 4 of the 5 animals perfused with the HC0 3 P0 4 -buffered electrolyte solution (fig. 6). When the animals perfused with the balanced electrolyte solution are considered separately, the Ll pc0 2 was 6.2 ± 0.8 mm Hg. We cannot be certain whether these results were due to experimental factors, or whether they represent an altered HC0 3 absorption mechanism in the infected jejunum. However, the resufts in the infected ileum are clear. The solution ph decreased and the pc0 2 increased significantly during perfusion with the HC0 3 - containing solutions, evidence that H secretion was responsible for the luminal HC0 3 disappearance in this segment. There may be a relationship between metabolism, H production, HC0 3 absorption, and Na transport in the normal jejunum and infected ileum. Wilson has shown that H derived from the conversion of glucose to lactate is preferentially extruded across the luminal cell border in the rat jejunum. 26 Gilman and Koelle 27 have shown that the rat jejunum is dependent on glycolysis, which would produce lactate and H, for Na transport. Conversely, the normal rat ileum, which does not secrete H, utilizes aerobic metabolism and Krebs cycle intermediates for Na transport. 27 The development of H secretion by the infected ileum could come about by a shift from aerobic to anaerobic metabolism in the inflamed mucosa, producing H ion which diffuses into the lumen. Alternatively, H ion secretion in the infected ileum may represent some direct effect on the ion transport mechanisms involved. Turnberg and associates'22 double exchange mechanism could be altered to secrete H ion. However, it is difficult to see how this mechanism, as postulated, would account for both the Na and H secretion that was observed in the infected 6-day animals with ileal secretion. In summary, the apparent transport of HC0 3 is quite different in the normal rat jejunum and ileum. Bicarbonate is absorbed in the jejunum, most likely by way of H secretion, but is secreted in the ileum. It remains to be determined whether the ileal secretion is accomplished through HC0 3 transport or H absorption. The luminal presence of HC0 3 stimulates Na absorption in the jejunum but not in the ileum. Other investigators have suggested that this occurs through Na: H exchange. There is an intricate relationship between HC0 3 and CI transport in both segments, although available evidence is against a simple one for one exchange. Bicarbonate absorption in the jejunum infected with S. typhimurium is altered somewhat from normal, but is probably accomplished by the same mechanisms. Finally, in the infected ileum there is a reversal of HC0 3 transport from secretion to absorption, and this is brought about by H secretion as in the normal and infected jejunum.

11 1086 POWELL ET AL. Vol. 60, No.6 It is not known whether such alterations in HCO a transport as described in this disease model occur in the human ileum or large intestine in diarrheal conditions. This would result in metabolic alkalosis through loss of Hand Cl in the stool. It is of interest that metabolic alkalosis has been noted in several Instances of severe diarrhea in man. 28 REFERENCES 1. Powell DW, Plotkin GR, Maenza RM, et al: Experimental diarrhea. 1. Intestinal water and electrolyte transport in rat salmonella enterocolitis. Gastroenterology 60: , Brodsky WA, Schlib TP: Mechanism of acidification in turtle bladder. Fed Proc 26: , Turnberg LA, Fordtran JS, Carter NW, et al: Mechanism of bicarbonate absorption and its relationship to sodium transport in the human jejunum. J Clin Invest 49: , Maenza RM, Powell DW, Plotkin GR, et al: Experimental diarrhea: salmonella entercolitis in the rat. J Infect Dis 121: , Powell DW, Plotkin GR, Solberg LI, et al: Experimental diarrhea. II. Glucose-stimulated sodium and water transport in rat salmonella enterocolitis. Gastroenterology 60: , Funder J, Ussing HH, Wieth JO: The effect of CO, and hydrogen ions on active Na transport in the isolated frog skin. Acta Physiol Scand 71:65-76, Rumrich G, Ullrich KJ: The minimum requirements for the maintenance of sodium chloride reabsorption in the proximal convolution of the mammalian kidney. J Physiol (London) 197:69P- 70P, Gonzales CF, Shamoo YE, Brodsky WA: The accelerating effect of serosal HCO, on Na + transport in short-circuited turtle bladders. Biochim Biophys Acta 193: , Machen TE, Diamond JM: An estimate of the salt concentration in the lateral intercellular spaces of rabbit gallbladder during maximal fluid transport. J Membrane BioI 1: , Cooperstein IL, Hogben CA: Ionic transfer across the isolated frog large intestine. J Gen Physiol 42: , Sladen GE, Dawson AM: Effect of bicarbonate on sodium absorption by the human jejunum. Nature (London) 218: , Fordtran JS, Rector FC, Carter NW: The mechanism of sodium absorption in the human small intestine. J Clin Invest 47: , Clarkson TW, Rothstein A, Cross A: Transport of monovalent anions by isolated small intestine of the rat. Amer J Physiol 200: , McHardy GJ, Parsons DS: The absorption of water and salt from the small intestine of the rat. Quart J Exp Physiol 42:33-48, Schlib TP, Brodsky W A: Acidification of mucosal fluid by transport of bicarbonate ion in turtle bladders. Amer J Physiol 210: , Green HH, Steinmetz PR; Frazier HS: Evidence for proton transport by turtle bladder in presence of ambient bicarbonate. Amer J Physiol 218: , McGee LC, Hastings AB: The carbon dioxide tension and acid-base balance of jejunal secretions in man. J BioI Chern 142: , Robinson CS, Luckey H, Mills H: Factors affecting the hydrogen ion concentration of the contents of the small intestine. J BioI Chern 147: , Parsons DS: The absorption of bicarbonate-saline solutions by the small intestine and colon of the white rat. Quart J Exp PhysioI41: , Swallow JH, Code CF: Intestinal transmucosal fluxes of bicarbonate. Amer J Physiol 212: , Hamilton JD, Dawson AM, Webb JP: Observations upon small gut "mucosal" po, and pco, in anesthetized dogs. Gastroenterology 55:52-60, Turnberg LA, Bieberdorf FA, Morowski SG, et al: Interrelationships of chloride, bicarbonate, sodium and hydrogen transport in the human ileum. J Clin Invest 49: , Hubel KA: Bicarbonate secretion in rat ileum and its dependence on intraluminal chloride. Arner J Physiol 213: , Hubel KA: Effect of luminal chloride concentration on bicarbonate secretion in rat ileum. Amer J Physiol 217 :40-45, Powell DW, Malawer SJ, Plotkin GR: Secretion of electrolytes and water by the guinea pig small intestine in vivo. Amer J Physiol 215: , Wilson TH: Concentration gradients of lactate, hydrogen and some other ions across the intestine in vitro. Biochem J 56: , Gilman A, Koelle ES: Substrate requirements for ion transport by rat intestine studies in vitro. Amer J Physiol 199: , Diarrhea and acid-base disturbances. Lancet 1: , 1966