Ion transport during cholera-induced ileal secretion in the dog

Gastrointestinal Function

This chapter describes the normal biochemical processes of intestinal secretion, digestion, and absorption. The digestive system is composed of the gastrointestinal (GI) tract, or the alimentary canal, salivary glands, the liver, and the exocrine pancreas. The principal functions of the gastrointestinal tract are to digest and absorb ingested nutrients, and to excrete waste products of digestion. Most nutrients are ingested in a form that is either too complex for absorption or insoluble, and therefore, indigestible or incapable of being digested. Within the GI tract, much of these substances are solubilized and further degraded enzymatically to simple molecules, sufficiently small in size, and in a form that permits absorption across the mucosal epithelium. This chapter explains in detail the mechanisms of salivary secretions, compositions of saliva, and the functions of saliva. The chapter also elaborates properties of bile as well as the synthesis of bile acids. The chapter explores the pathogenesis of the important gastrointestinal diseases of domestic animals, and the biochemical basis for their diagnosis and treatment. The chapter concludes with a discussion on disturbances of gastrointestinal function such as vomition, acute diarrheas, malabsorption, bacterial overgrowth, and ulcerative colitis.

Cyclic adenosine-3',5'-monophosphate production is greater in rabbit duodenal crypt than in villus cells

BACKGROUND

Duodenal surface epithelial cells secrete bicarbonate. Agonists of duodenal alkaline secretion (such as vasoactive intestinal polypeptide (VIP), prostaglandin E2 (PGE(2)), and forskolin) increase intracellular cyclic adenosine-3', 5-monophosphate (cAMP), and cAMP stimulates Cl-HCO(3)- exchange in duodenal brush border membrane vesicles. As intestinal villus and crypt cells differ in function, our aims were to contrast cAMP generation in duodenal villus versus crypt cells in response to VIP, PGE(2), and forskolin.

METHODS

Villus and crypt rabbit duodenal enterocytes were isolated by calcium chelation. To prevent the degradation of cAMP in vitro, phosphodiesterase activity was inhibited. cAMP production was quantitated in response to VIP (10(-10)-10(-5)M), PGE(2) (10(-10)-10(-4)M), and forskolin (10(-8)-10(-3)M).

RESULTS

In crypt cells cAMP generation was approximately 10-fold greater (P < 0.001) in response to VIP, PGE(2), and forskolin than to villus cells. The relative orders of potency (that is, D(50), VIP > PGE(2) > forskolin) and efficacy (that is, V max, forskolin > VIP and PGE(2)) were similar in villus and crypt cells.

CONCLUSION

cAMP production is greater in duodenal crypt than in villus enterocytes at rest and in response to forskolin, VIP, and PGE(2), suggesting that alkaline secretion may differ along the villus-to-crypt axis.

Water and solute absorption from a new hypotonic oral rehydration solution: evaluation in human and animal perfusion models

Controversy continues regarding the optimal composition of glucose electrolyte oral rehydration solutions for the treatment of acute diarrhoea. Four perfusion models (normal human jejunum, normal rat small intestine, cholera toxin treated secreting rat small intestine and rotavirus infected rat small intestine) have been developed and used to compare the efficacy of a hypotonic oral rehydration solution with standard United Kingdom British National formulary and developing world oral rehydration solutions (WHO). Despite obvious physiological and pathophysiological differences between these models there was general congruence in the water and solute absorption profiles of the different oral rehydration solutions. Hypotonic oral rehydration solution promoted significantly greater water absorption than other oral rehydration solutions in all rat models (p < 0.001) but apparently increased water absorption failed to achieve significance in human jejunum. British National Formulary-oral rehydration solution was unable to reverse net water secretion in both rotavirus and cholera toxin models. Net sodium absorption from hypotonic and WHO-oral rehydration solutions was significantly greater than from the low sodium British National Formulary-oral rehydration solutions (p < 0.001) except in the rotavirus model when absorption was similar to hypotonic-oral rehydration solutions. These findings show that there is agreement in the apparent efficacy of oral rehydration solutions in these animal and human perfusion models, and that improved water absorption with adequate sodium absorption may be achieved by reducing oral rehydration solution osmolality.

Mechanism of intestinal secretion: effect of cyclic AMP on rabbit ileal crypt and villus cells

Cyclic AMP-dependent secretagogues such as cholera toxin inhibit the coupled absorption of Na+ and Cl- and stimulate the secretion of HCO3- and Cl- in the ileum. Aside from Cl- secretion, little is known about the mechanism of these cyclic AMP-mediated effects. We therefore determined the effect of forskolin, an agent known to increase intracellular cyclic AMP by stimulation of adenylyl cyclase, on Na+/H+ and Cl-/HCO3- exchange in isolated crypt and villus cells from rabbit ileum. Forskolin increased cyclic AMP in the villus cells and decreased intracellular pH. The effect of forskolin on pH in villus cells was HCO3- independent, Na+ dependent, and amiloride sensitive. Further, the rate of recovery from an acid load was decreased by forskolin. These data suggest that increasing cyclic AMP inhibits Na+/H+ exchange in villus cells. In crypt cells also, forskolin increased cyclic AMP; however, forskolin increased intracellular pH in these cells. The effect of forskolin in crypt cells was also HCO3- independent, Na+ dependent, and amiloride sensitive. However, the rate of recovery from an acid load was increased by forskolin, the opposite effect of that seen in villus cells. These data suggest that increasing cyclic AMP in crypt cells stimulates Na+/H+ exchange. Inhibition of Na+/H+ exchange on the brush border membrane in villus cells would be expected to inhibit coupled NaCl absorption (which occurs by coupling of Na+/H+ and Cl-/HCO3- exchange). Stimulation of Na+/H+ exchange in crypt cells, present only on the basolateral membrane, alkalinizes the cell, which would be expected to stimulate HCO3- secretion by stimulating the Cl-/HCO3- exchanger on the brush border membrane. Thus, these results provide a mechanism for some of the previously unexplained in vivo and in vitro effects of cyclic AMP on ileal electrolyte transport.

Studies of cholera toxin-induced changes of alkaline secretion and transepithelial potential difference in the rat intestine in vivo

A pH-stat technique was used to investigate the effects of cholera toxin (CT) on alkaline secretion from denervated intestines (jejunum, ileum, colon) in anaesthetized rats. Transepithelial potential difference (PD) was also followed in some experiments. CT, given intraluminally, caused a marked increase in jejunal alkaline secretion, whereas only a small effect was observed in the ileum and no apparent effect was noted in the proximal colon. The pronounced increase in jejunal alkaline secretion was found to be inhibited by 10-25% by hexamethonium (10 mg kg-1 body wt i.v.) and similarly by serosal application of lidocaine, whereas atropine (0.25 mg kg-1 body wt i.v.) had no effect. Thus the cholera toxin-induced alkaline secretion in the jejunum is attributed mainly to a non-nervous mechanism. The small effect of CT on ileal alkaline secretion observed in this study contrasts with the high ileal bicarbonate concentration reported in cholera by authors who estimated the concentration from the total carbon dioxide/bicarbonate contents. This discrepancy may be explained by a CT-evoked increased transport of the coupled Na+/H+ and Cl-/HCO3- exchangers, which cannot be measured with the pH-stat technique used in this study.

Membrane distribution of sodium-hydrogen and chloride-bicarbonate exchangers in crypt and villus cell membranes from rabbit ileum

Present evidence suggests that in the small intestine, villus cells are primarily absorptive and crypt cells are primarily secretory. In order to further confirm that there are differences in transport properties between villus and crypt cells, we have separated villus from crypt cells, using calcium chelations techniques, and determined the distribution of Na:H and Cl:HCO3 exchange activity on brush border membrane and basolateral membrane preparations from these two cell populations. Separation of cells was determined utilizing alkaline phosphatase and maltase activity as a marker of villus cells and thymidine kinase activity as a marker of crypt cells. Utilizing these techniques, we were able to sequentially collect cells along the villus-crypt axis. Na-stimulated glucose and alanine uptake in brush border membrane vesicles diminished from the villus to the crypt region in the sequentially collected cells fractions, further suggesting separation of these cells. Brush border and basolateral membranes were then prepared from cells from the villus and crypt areas, utilizing a continuous sucrose gradient. In the villus cells, Na:H exchange activity was found associated with both the brush border and basolateral membrane, whereas, in crypt cells, Na:H exchange activity was only found on the basolateral membrane. Cl:HCO3 exchange activity was found only on the brush border membrane, in both villus and crypt cells. These studies suggest functional heterogeneity in ion transport between villus and crypt cells.

The effect of theophylline on intestinal bicarbonate transport measured by pH stat in Amphiuma

1. The influence of theophylline on the mucosa to serosa and serosa to mucosa fluxes of HCO3- were measured by the pH stat technique in isolated segments of proximal small intestine from Amphiuma maintained under short-circuited conditions. The mucosal or serosal fluid was exposed to media containing 25 mM-HCO3- (pH 7.4) while the pH of unbuffered media in the opposite compartment was maintained by addition of acid. 2. Theophylline significantly increased the secretory flux of HCO3- and significantly reduced the absorptive flux when measured in Cl- -free (SO4(2-)) media. 3. In normal media theophylline did not alter the secretory flux but significantly lowered the absorptive flux of HCO3-. 4. Acetazolamide (0.1 mM) inhibited the theophylline-stimulated secretory flux of HCO3- and reduced the effect of theophylline on the absorptive flux. 5. In normal intestine there was an inequality between the secretory or absorptive HCO3- flux and the short-circuit current (Isc) consistent with the presence of Cl- absorption. After addition of theophylline the Isc was more nearly equal to the net secretory or absorptive HCO3- flux. 6. Exogenous cyclic AMP had effects identical with theophylline. 7. The results provide strong evidence that elevation of cyclic AMP stimulates net HCO3- secretion in urodele small intestine and provide indirect evidence that Cl- absorption is simultaneously reduced.

Chloride transport and intracellular chloride activity in the presence of theophylline in Amphiuma small intestine

1. Simultaneous bidirectional tracer fluxes of 22Na and 36Cl were measured under short-circuited conditions in isolated stripped segments of Amphiuma duodenum incubated in media containing Cl- and HCO3-. 2. Theophylline (10 mM) increased the short-circuit current (Isc), eliminated net Cl- absorption and elevated the residual flux by more than 50%. 3. Subsequent addition to acetazolamide lowered the Isc and reduced the residual flux. 4. Using double-barreled Cl- specific micro-electrodes the intracellular activity of Cl- averaged 27.1 mM in the presence of theophylline, consistent with active accumulation of Cl-. 5. Theophylline depolarized the mucosal membrane of villus epithelial cells by an average of 6.9 mV. 6. The findings indicate that the inhibition of Cl- absorption by theophylline does not result from complete inhibition of mucosal Cl- uptake and provide evidence that HCO3- secretion is enhanced by theophylline.

Active chloride secretion in the normal human jejunum

To determine whether the small intestine normally secretes fluid, it would be necessary to reduce or inhibit the greater absorptive processes that would otherwise mask such secretion if present. To do this, we perfused bicarbonate-free solutions in the jejunum of normal subjects, because it has been shown that active absorption from this part of the human small intestine is dependent on luminal bicarbonate. We found that the jejunum did secrete sodium chloride and water when isotonic bicarbonate-free solutions were perfused. Further studies revealed that the sodium secretion was passive, but that chloride was secreted against an electrochemical gradient and that observed chloride flux ratios did not agree with the flux ratios calculated for passive chloride movement. We conclude, therefore, that the normal jejunum actively secretes chloride, but that this is masked by greater absorptive processes when balanced electrolyte solutions are perfused. The rate of this active chloride secretion may be one of the factors that regulate the rate of fluid absorption in the normal human intestine.

Intestinal filtration as a consequence of increased mucosal hydraulic permeability. A new concept for laxative action

Two mechanisms have been proposed to explain the secretory action of laxative compounds in the intestine: 1. increase of the intracellular amount of cyclic adenosine monophosphate due to stimulation of the adenylate cyclase system and 2. inhibition of intestinal transfer processes, in particular the Na,K-ATPase activated sodium absorption. In a set of in vivo and in vitro experiments in rat colon it could be demonstrated that dihydroxy bile acids (deoxycholate) and diphenolic laxatives (oxyphenisatin) enhance the hydraulic permeability of the mucosal tissue. The permeability changes take place--and there is good experimental evidence--at the zonulae occludentes which bind the epithelial cells together at their luminal borders. Due to laxative action the hydraulic permeability of the colonic mucosa increases to such an extent that according to the Starling forces the normal subepithelial hydrostatic pressure is a sufficient driving force to reverse net sodium, chloride, and water absorption into net secretion. A new concept of "intestinal filtration as a consequence of increased mucosal hydraulic permeability" is proposed to explain the laxative action of deoxycholate and oxyphenisatin in the colon. The question whether inhibition of Na,K-ATPase activity, cyclic AMP-mediated secretion or increased hydraulic permeability of the colonic mucosa are causatively linked to and quantitatively meaningful in intestinal secretion remains open.

Gastrointestinal Function

This chapter discusses the functions of gastrointestinal tract. The principal functions of the gastrointestinal tract are assimilation of nutrients and excretion of the waste products of digestion. Within the gastrointestinal tract, these substances are solubilized and degraded enzymatically to simple molecules, sufficiently small in size and in a form that permits absorption across the mucosal epithelium. The distribution of the different types of secretory cells in the salivary glands varies among species. The mandibular and sublingual glands are mixed salivary glands containing both mucous and serous types of cells, and produce a viscous secretion that contains large amounts of mucus. The cytoplasm of the secretory cells contains numerous zymogen granules that vary in size and number depending on the activity of the gland. These granules contain the precursors of the hydrolytic enzymes responsible for digestion of the major dietary components. The cells of the terminal ducts probably secrete the bicarbonate ion responsible for neutralizing hydrochloric acid that enters the duodenum from the stomach.

Contribution of villus and intervillus epithelium to intestinal transmural potential difference and response to theophylline and sugar

A chamber design is described which permits isolation of villus or intervillus epithelium from proximal segments of Amphiuma intestine and measurement of the transpithelial potential difference (psi ms) and short-circuit current (Isc) produced by each. In media containing Cl- and 10 mequiv./l HCO3- the villus generated a basal psi ms of 0.8 mV (serosa negative) and Isc of 12 microA/cm2 while the intervillus psi ms and Isc were not different from zero. Acetazolamide altered the villus psi ms by 1.2 mV; the intervillus psi ms by only 0.3 mV. Transepithelial gradients of HCO3- appeared to generate diffusion potentials across the intervillus but not the villus epithelium. The actively transported sugar galactose elevated psi ms by 0.6 +/- 0.1 mV in the intervillus epithelium and by 1.5 +/- 0.2 mV in the villus epithelium for a response ratio (0.6/1.5) = 0.4. The response ratio for valine was 0.3. In contrast, the response ratios for theophylline (0.7) and cyclic AMP (0.7) were significantly higher. These observations indicate that the entire epithelium is responsive to theophylline and cyclic AMP while Na+-dependent solute transport and the basal electrogenic ion transport processes are primarily functions of the cells lining the intestinal villus.

Effects of cholera toxin on villous tissue osmolality and fluid and electrolyte transport in the small intestine of the cat

The effects of cholera toxin on tissue osmolality and on net transport rates of water, sodium, chloride and potassium as well as on unidirectional fluxes of water and sodium were studied in vivo. In all experiments the toxin caused a net secretion of water, sodium, chloride and potassium. The unidirectional sodium transport from tissue to lumen was increased while the flux in the opposite direction was reduced 180 min after cholera toxin instillation. Cholera toxin produced only a small reduction in the villous tissue hyperosmolality, created by the intestinal countercurrent exchanger. This reduction was far too small to explain the observed net secretion of fluid and solutes induced by the cholera toxin. Other mechanisms underlying the cholera secretion are discussed.

Intestinal secretion induced by vasoactive intestinal polypeptide. A comparison with cholera toxin in the canine jejunum in vivo

The effect of vasoactive intestinal polypeptide (VIP) on intestinal water and electrolyte transport and transmucosal potential difference was investigated in the dog jejunum in vivo and compared to secretion induced by cholera toxin. Isolated jejunal loops were perfused with a plasma-like electrolyte solution. VIP (0.08 mug/kg per min) was administered directly into the superior mesenteric artery by continuous infusion over 1 h. From a dye dilution method, it was estimated that a mean plasma VIP concentration of 12,460 pg/ml reached the loops. VIP caused secretion of water and electrolytes; for example, chloride: control, 8 mueq/cm per h absorption; VIP, 92 mueq/cm per h secretion. A marked increase in transmucosal potential difference (control, -1.0 mV; VIP, -5.9 mV, lumen negative) occurred within 1 min after starting VIP infusion. Analysis of unidirectional fluxes showed increased plasma-to-lumen flux of sodium and chloride and decreased lumen-to-plasma flux of sodium. Chloride and bicarbonate were actively secreted against an electrochemical gradient. Although sodium secretion occurred down an electrochemical gradient, flux ratio analysis suggested a component of active sodium secretion. VIP caused a slight increase in protein output into the loops; light microscopy revealed capillary dilatation and closed intercellular spaces. The effect of VIP was readily reversible. Except for the delayed onset of secretion, the effect of cholera toxin was qualitatively similar to VIP; however, capillary dilatation and increased protein output were not noted with cholera toxin.

Effect of heat stable and heat labile Escherichia coli enterotoxins, cholera toxin and theophylline on unidirectional sodium and chloride fluxes in the proximal and distal jejunum of weanling swine

Acute, isolated loops of proximal and distal jejunum of weanling swine were exposed to either heat stable porcine Escherichia coli enterotoxin, heat labile porcine Escherichia coli enterotoxin, cholera toxin or theophylline. Unidirectional sodium fluxes in response to heat stable in the proximal jejunum were dependent on the length of time that the intestinal mucosae was exposed to the enterotoxin. Net water, sodium and chloride and unidirectional sodium and chloride flux measurements in the proximal jejunum in response to each agent uniformly indicated that net secretion of fluid and electrolytes was the result of increased unidirectional sodium secretion or blood-to-lumen flux and decreased unidirectional chloride absorption or lumen-to-blood flux. In addition heat stable cholera toxin and theophylline but not heat labile decreased unidirectional chloride secretion a small but significant amount in the proximal jejunum. Sodium and chloride flux measurements in the distal jejunum demonstrated that all four secretory agents could stimulate net secretion of water, sodium and chloride in that region. The response to these secretory agents as measured by sodium and chloride unidirectional flux rates was not similar to changes observed in the proximal jejunum. In the distal small intestine, whereas heat labile cholera toxin and theophylline induced similar qualitative changes in unidirectional sodium and chloride fluxes, that induced by heat stable differed.

Relationship between changes in intraluminal pressure and transmural potential difference in the human and canine jejunum in vivo

Recordings of transmural potential difference (PD) across the jejunum of conscious man in situ are characterised by spontaneous fluctuations of up to 10 mV. In 25 of 31 subjects (comprising seven normal controls and 24 patients under investigation for malabsorption, six of whom had coeliac disease) we observed a clear association between these fluctuations and changes in intraluminal pressure recorded at the same site. The most frequent PD changes were associated with type III pressure waves. These consisted predominantly of large waver (3-1 +/- 0-1 mV; mean +/- SEM, n = 317) which reached maximal amplitude approximately 45 seconds after the pressure peak and had a duration of 120 +/- 3 s, but also included less frequent spikes (0-5 +/- 0-1 mV; n = 110) concurrent with the pressure wave with a duration of 5 +/- 1 s. Although by recording at two sites in the jejunum 10 cm apart we were able to demonstrate that type III pressure waves appeared to be propagated aborally at a median rate of 60 cm per minute, the apparent rates of propagation of the corresponding PD waves were much more variable. The largest PD changes (7-8 +/- 0-4 mV; n = 19), lasting several minutes, were found in association with runs of type I waves (basic rhythm) superimposed on a type III wave. Both pressure and PD activities were suppressed by intramuscular propantheline bromide. Intraluminal pilocarpine caused a transient rise in PD not always accompanied by a change in pressure. Distention of the jejunum by rapid injection of a bolus of isotonic sodium chloride produced a delayed rise in the PD which could be prevented by prior administration of propantheline bromide. Experiments using Thirty-Vella loops of proximal jejunum in conscious dogs confirmed the effect of jejunal distension on the PD and also demonstrated that spontaneous retching is preceded by an increase in the PD. Consideration of these results in conjunction with data from other workers suggests the hypothesis that the larger spontaneous fluctuations in transmural PD in the jejunum of conscious man are caused by changes in electrogenic secretion associated with intestinal motility and mediated by cholinergic mechanisms. The possible association of increased secretory activity with motility may have functions of lubrication as well as diluting and mixing the chyme for easier digestion and absorption.

Effects of cholera toxin on cellular and paracellular sodium fluxes in rabbit ileum

The diarrhea observed in patients with cholera is known to be related to secretion of water and electrolytes into the intestinal lumen. However, the exact mechanisms involved in these secretory processes have remained unclear. Although it is clear that purified toxin acts on epithelial cell metabolism, its activity on Na+ transport across intestinal mucosa is equivocal: reported either to prevent net Na+ absorption or to cause net secretion of Na+ from serosa to mucosa. Since total transmural Na+ fluxes across "leaky" epithelia involve very significant movement via a paracellular shunt pathway, we studied the effects of cholera toxin on the cellular and paracellular pathways of Na+ movement. Unidirectional Na+ fluxes were examined as functions of applied potential in control tissues and in tissues from the same animal treated with purified cholera toxin. Treatment of rabbit ileum in vitro with toxin simulated the cellular component of serosa-to-mucosa Na+ flux (from 2.41 +/- 0.49 muequiv./h per cm2 under control conditions to 4.71 +/- 0.43 muequiv./h per cm2 after treatment with toxin, P less than 0.01). The effect of cholera toxin on Na+ movement through the cells from mucosa to serosa appeared to be insignificant. Finally, a marked decrease in the Na+ permeability (P less than 0.01) and no detectable significant changes in transference number for Na+ of the paracellular shunt pathway were observed following treatment with cholera toxin. These results provide direct evidence for the hypothesis that purified cholera toxin stimulates active sodium secretion but has minimal effect on sodium absorption.

Prevention and reversal of cholera enterotoxin-induced intestinal secretion by methylprednisolone induction of Na+-K+-ATPase

The relationship of the mucosal enzyme systems Na+-K+-activated adenosine triphophatase (Na-K-ATPase) and adenylate cyclase and their associated intestinal transport processes was studied in the rat ileum. Two ileal loops were constructed in each anesthetized rat; one loop was inoculated with saline, the other loop with choleragen. Net transport of water and electrolytes was measured in vivo after which enzyme activity was measured in the mucosa of the perfused loops. All doses of choleragen between 5 and 150 mug decreased water movement as early as 3 1/2 h after inoculation. A linear relationship between the dose of choleragen and the level of net water and electrolyte secretion was observed when choleragen doses between 5 and 150 mug were incubated in ileal loops for 4 h. Adenylate cyclase activity was always increased in secreting intestinal loops, whereas Na-K-ATPase was unaffected by choleragen. In animals pretreated with methylprednisolone acetate, 3 mg/100 g per day for 3 days before loop inoculation, saline loops had enhanced mucosal Na-K-ATPase activity had increased net water and electrolyte absorption; choleragen-exposed loops had increased adenylate cyclase and Na-K-ATPase activities, and net absorption of water and electrolytes 4 h after inoculation. These effects of methylprednisolone acetate were still present 19 1/2 h after inoculation. When a single injection of methylprednisolone acetate was given 3 1/2 h after choleragen inoculation, both adenylate cyclase and Na-K-ATPase were activated, and net intestinal absorption of water and electrolytes was observed 19 1/2 h after inoculation. These results suggest that methylprednisolone can prevent and reverse the secretory effects of choleragen by selectively stimulating a coexisting absorptive process.

Effects of cholera toxin on cochlear endolymph production: model for endolymphatic hydrops

To evaluate a possible role for adenylate cyclase [ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1] and adenosine 3':5'-cyclic monophosphate in the secretion of endolymph, we studied the effect of an intra-scala media injection of purified cholera toxin (an adenylate cyclase stimulant) on cochlear endolymph volume, endolymphatic potential, and endolymphatic Na and K concentrations.

Effect of Klebsiella pneumoniae enterotoxin on intestinal transport in the rat

The effects on intestinal transport of either a semipurified preparation of enterotoxin elaborated by Klebsiella pneumoniae or similaryly prepared control material were tested by marker perfusion studies in the small intestine of rats. At a concentration of 2 mg/ml, the enterotoxin produced net secretion of water, Na, and Cl in both jejunal and ileal segments; HCO3 transport was not affected. Net secretion was evident within 30 min after intorduction of the toxin and was maximal after 90 min. The addition of 56 mM glucose to the enterotoxin-containing perfusion fluid resulted in reversal of water and Na transport to net absorption in both intestinal segments. The enterotoxin also produced a significant depression of xylose absorption in both the jejunum and ileum but did not affect the absorption of either glucose or L-leucine. Intestinal structure was not altered after perfusion of the toxin but insillation of approximately one-quarter of the total perfusion dose into a ligated jejunal loop for 18 h produced fluid secretion and structural abnormalities. These observations confirm the fact that other species of coliform bacteria in addition to tescherichia coli are capable of elaborating an enterotoxin. Such species commonly contaminate the small intestine of persons with tropical sprue and it is suggested that chronic exposure of the intestinal mucosa to the enterotoxin elaborated by these bacteria may be a factor in the pathogenesis of intestinal abnormalities in thid disorder.

Effects of cholera toxin on cyclic adenosine 3',5'-monophosphate concentration and secretory processes in the exocrine pancreas

1. The effect of purified cholera toxin on secretory processes of exocrine pancreas has been studied in the isolated, saline-perfused cat pancreas and in incubated pieces of rat pancreas. 2. The toxin evoked a biphasic secretory response from the perfused cat pancreas. An initial small phase, which began within minutes of toxin application, was an artefact due to the presence of NaN3 in the cholera toxin preparation as supplied; it could be entirely reproduced by NaN3 at the concentration expected during toxin stimulation. A second, sustained phase of secretion, due to the action of the toxin proper, began within 30-60 min, increasing in magnitude for many hours and persisting in the absence of toxin. It was accompanied by a parellel rise in tissue cyclic AMP concentration, and could be potentiated by theophylline. 3. The composition of the secretion stimulated by cholera toxin resembled that evoked by secretin; e.g. it contained a high concentration of bicarbonate and only basal amounts of digestive enzymes. 4. Similarly, cholera toxin did not stimulate enzyme secretion by incubated rat pancreas, despite large rises in tissue cyclic AMP concentration. 5. Because cholera toxin has thus far been shown to have no other effect than that of stimulating adenylate cyclase, these observations support the conclusion that cyclic AMP does mediate the electrolyte secretory response of the pancreas to secretin, but offers no evidence that cyclic AMP plays a similar role in the regulation of pancreatic enzyme secretion stimulated by cholecystokinin-pancreozymin or acetylcholine.

Effect of bile acids on electrical properties of rat colon: evaluation of an in-vitro model for secretion

A preparation of rat colon mucosa has been evaluated as a useful model to study effects of bile acids on the changes in short-circuit current and transmural electrical potential difference (pd) which others have associated with alterations in electrolyte transport. Using this preparation, it was found that bile acids were effective in increasing short-circuit current and pd when applied to the serosal, but not the mucosal, surface of the tissue. Furthermore, taurine-conjugated cholic acid, which has no demonstrable effect on the colon in vivo, was found to increase short-circuit current and pd significantly in the in-vitro preparation. These data indicate the limitations of the in-vitro model in studying the mechanism of bile acid-stimulated intestinal secretion.

Effect of cholera enterotoxin on ion transport across isolated ileal mucosa

The effects of cholera enterotoxin on intestinal ion transport were examined in vitro. Addition of dialyzed filtrate of Vibrio cholerae (crude toxin) to the luminal side of isolated rabbit ileal mucosa caused a delayed and gradually progressive increase in transmural electric potential difference (PD) and shortcircuit current (SCC). A similar pattern was observed upon addition of a highly purified preparation of cholera toxin, although the changes in PD and SCC were smaller. Na and Cl fluxes across the short-circuited mucosa were determined with radioisotopes 3-4 hr after addition of crude toxin or at a comparable time in control tissues. The toxin caused a net secretory flux of Cl and reduced to zero the net absorptive flux of Na. Similar flux changes were observed when either crude or purified toxin was added in vivo and tissues were mounted in vitro 3-4 hr later. Additon of D-glucose to the luminal side of toxin-treated mucosa produced a large net absorptive flux of Na without altering the net Cl and residual ion fluxes. Adenosine 3',5'-cyclic phosphate (cyclic AMP) and theophylline had previously been shown to cause a rapid increase in SCC and ion flux changes similar to those induced by cholera toxin. Pretreatment of ileal mucosa with either crude or purified cholera toxin greatly reduced the SCC response to theophylline and dibutyryl cyclic AMP, which, together with the flux data, suggest that both cyclic AMP and cholera toxin stimulate active secretion by a common pathway. Inhibition of the SCC response to theophylline was observed after luminal but not after serosal addition of toxin. In vitro effects of cholera toxin correlated closely with in vivo effects: heating toxin destroyed both; two V. cholerae filtrates which were inactive in vivo proved also to be inactive in vitro; PD and volume flow measurements in isolated, in vivo ileal loops of rabbit revealed that the PD pattern after addition of toxin is similar to that seen in vitro and also correlates closely with changes in fluid movement. The results suggest that stimulation by cholera toxin of a cyclic AMP-dependent active secretory process of the intestinal epithelial cells is a major cause of fluid loss in cholera.

Failure of cholera enterotoxin to alter cyclic 3',5'-adenosine monophosphate-mediated responses in toad urinary bladder

Cholera enterotoxin, which stimulates mammalian adenyl cyclase, failed to affect cyclic 3,5'-adenosine monophosphate-mediated functions in toad bladder. In addition, cyclase-mediated responsiveness to vasopressin was maintained.