Localization of the action of cholera toxin on adenyl cyclase in mucosal epithelial cells of rabbit intestine

Cellular localization of NKCC2 and its possible role in the Cl- absorption in the rat and human distal colonic epithelia

Recently, we demonstrated the expression of NKCC2, an absorptive isoform of NKCC specifically expressed in the kidney, in the rat gastrointestinal tract including the distal colonic mucosa. This study aims to investigate its localization in colonic epithelia and possible role in the colonic ion transport. Reverse transcription polymerase chain reaction (RT-PCR), Western blotting, and immunohistochemistry were used to investigate the expression and localization of NKCC2. The role of NKCC2 on the colonic ion transport was examined by mean of short-circuit current (I(SC)) monitoring. The results indicated that NKCC2 was expressed in the apical region of the epithelia in rat distal colon and human sigmoid colon. NKCC1, which is a secretive NKCC isoform, was localized predominantly in the basolateral membrane, which has been well documented. Serosal (basolateral) administration of bumetanide, an inhibitor of both NKCC1 and NKCC2, inhibited serosal forskolin-induced I(SC) increase by 66% but enhanced the luminal (apical) forskolin-induced I(SC) response by 63%. Furthermore, the blocking of epithelial Na(+) channels by apical addition of amiloride (10 μmol/L), K(+) channels by tetraethylammoniumion (TEA) (5 mmol/L), or glibenclimide (0.1 mmol/L) did not affect apical forskolin-induced I(SC) increase, excluding the involvement of cations, Na(+) and K(+), in the I(SC) response. The luminal forskolin-induced I(SC) increase was enhanced markedly by the apical pretreatment with bumetanide or the reduction of apical Cl(-) concentration by 114% and 198%, respectively, which were inhibited by apical addition of glibenclimide (1 mmol/L) by more than 60%. This finding suggests the involvement of an anion. Furthermore, the removal of basolateral HCO(3)(-) reduced apical forskolin-induced I(SC) by more than 75% indicated that the apical forskolin-induced I(SC) increase in rat distal colon was mediated by Cl(-) absorption and HCO(3)(-) secretion. In conclusion, NKCC2 is expressed widely in the colonic epithelium in rat distal colon and human sigmoid colon, especially in the apical membrane. It involves the process of colonic Cl(-) absorption coupled with HCO(3)(-) secretion.

Lactobacilli facilitate maintenance of intestinal membrane integrity during Shigella dysenteriae 1 infection in rats

OBJECTIVE

Lactobacilli are used in various dairy products and fermented foods for their potential health beneficial effects. Recently we reported the protective role of Lactobacillus rhamnosus and Lactobacillus acidophilus during Shigella dysenteriae 1 infection. Nevertheless, investigations on the membrane-stabilizing effect of L. rhamnosus and L. acidophilus have not been done. Hence, the present study evaluated the effect of L. rhamnosus and L. acidophilus on the maintenance of intestinal membrane integrity during S. dysenteriae 1-induced diarrhea in rats.

METHODS

Rats were divided into eight groups (n = 6 in each group). Induced rats received single oral dose of S. dysenteriae (12 x 10(8) colony-forming units [cfu]/mL). Treated rats received L. rhamnosus (1 x 10(7)cfu/mL) or L. acidophilus (1 x 10(7)cfu/mL) orally for 4 d, alone or in combination, followed by Shigella administration. At the end of the experimental period, animals were sacrificed and the assay of membrane-bound adenosine triphosphatases (Na(+)/K(+)-ATPase, Ca(2+)-ATPase, and total ATPase), immunoblot analysis of tight junctional proteins (claudin-1 and occludin), and transmission electron microscopic studies were performed.

RESULTS

Induced rats showed a significant (P < 0.05) reduction in the membrane-bound ATPases and reduced expression of tight junction proteins in the membrane, coupled with their increased expression in the cytosol, indicating membrane damage. Transmission electron microscopic studies correlated with biochemical parameters. Pretreatment with combination of L. rhamnosus and L. acidophilus significantly prevented these changes.

CONCLUSION

Lactobacillus rhamnosus and L. acidophilus synergistically offered better protection to the intestinal membrane when compared with individual treatments with these strains during S. dysenteriae 1 infection.

Regulation of active ion transport in the small intestine

The epithelium of the small intestine can both actively absorb and actively secrete electrolytes and water. Secretion can be elicited in vitro by adding cyclic AMP or a stimulator of intestinal mucosal adenylate cyclase (cholera and Escherichia coli enterotoxins, prostaglandins, vasoactive intestinal peptide) or an inhibitor of cyclic AMP phosphodiesterase (theophylline). Cyclic AMP appears to alter intestinal ion transport at two different loci: it inhibits a coupled influx process for Na+ and Cl- at the luminal border, thereby reducing active absorption of NaCl, and it also stimulates the active secretion of anion (or Na+ and anion). A variety of evidence suggests that these two effects of cyclic AMP reside in different types of cells, the former in villus cells and the latter in crypt cells. The latter process is Na+-dependent and is inhibited by low concentrations of ouabain and ethacrynic acid. Active ion absorption in vitro can be enhanced by (1) stimulating Na+-coupled organic solute absorption with glucose, amino acids and possibly also oligo peptides; (2) reducing the HCO3- concentration and/or pH of the serosal bathing solution; and (3) introducing an alpha-adrenergic agonist. Cholera toxin-induced fluid production in vivo can be diminished by the first of these manoeuvres. The in vivo efficacies of the other two have not been evaluated.

The activation of adenylate cyclase by cholera toxin: possible interaction with the nucleotide regulatory site

The application of cholera toxin to intact cells causes a stimulation of adenylate cyclase activity. The effect is characterized by a lag period followed by a progressive rise in enzyme activity over several hours. Only a few minutes' exposure to the toxin is required to produce effects lasting over several days. Stimulation of adenylate cyclase by cholera toxin in broken cell preparations requires the presence of nicotinamide-adenine dinucleotide (NAD) and an unidentified component of the cytosol. Guanyl nucleotides and certain non-hydrolysable analogues of guanosine triphosphate also stimulate adenylate cyclase. Stimulation by the analogues results in a highly activated enzyme which has characterisitcs similar to those of adenylate cyclase after stimulation by cholera toxin. Thus the stimulation is irreversible, the enzyme may be "solubilized" by non-ionic detergents in the activated state, and responses to certain hormones are enhanced. Therefore the possibility exists that cholera toxin acts on the guanyl nucleotide regulatory protein of the adenylate cyclase complex. In exploring this possibility it was found pretreatment with cholera toxin not only blocked the stimulatory effect of subsequently added guanylylimidodi-phosphate (GppNHp) but that the latter reduced the stimulation by toxin. Similarly, pretreatment with GppNHp blocked the effect of cholera toxin. The similarities in the effects of cholera toxin and GppNHp, together with the mutual interference of their activities, suggests that cholera toxin acts at the same regulatory site at which guanyl nucleotides exert their effects on adenylate cyclase.

The nature and action of cholera toxin

Cholera diarrhoea is due to the action of a toxin that acts on all animal cells by stimulating the enzyme adenylate cyclase, which catalyses the production oc cyclic AMP from ATP. In intestinal brush border cells raised cyclic AMP levels result in increased secretion of chloride ions, leading to fluid accumulation in the gut. Escherichia coli produces a similar toxin. The receptor for cholera toxin on the cell membrane appears to be a complex containing the ganglioside GGnSLC (or GM1). Cholera toxin is a protein composed of two different kinds of subunits linked non-covalently. Each toxin molecule has one subunit A and four or more subunits B. Subunit B is inactive but binds to the ganglioside GGnSLC on the cell surface. Subunit A does not bind to cell membranes or gangioside and is slightly toxic to intact cells but strongly and instantly active in lysed cells. The binding of whole toxin through the B subunit to the cell is followed by a lag before subunit A penetrates the cell membrane (leaving subunit B on the surface) and stimulates the adenylate cyclase. The stimulation of adenylate cyclase depends on the presence of NAD and other co-factors present in the cell sap.

Identification of a candidate membrane protein for the basolateral peptide transporter of rat small intestine

A candidate protein for the basolateral peptide transporter of rat jejunum is described. Vascular perfusion of the photoaffinity label, [4-azido-D-phe]-L-ala (2.5mM), had no effect on the transepithelial transport of the non-hydrolysable dipeptide D-phe-L-gln (1mM) from the lumen, its mucosal accumulation or wash-out into the vascular perfusate. When the label was perfused luminally, the transepithelial transport of D-phe-L-gln was inhibited by 38% (P<0.001) and accumulation increased by 62% (P<0.05). These data are consistent with those of a basolateral transporter that is strongly asymmetric in its substrate binding and transport properties. Labelling of basolateral membrane vesicles with [4-azido-3,5-3H-D-phe]-L-ala revealed that the majority of label was incorporated into a single protein of M(r)112+/-2 kDa and pI 6.5. MALDI-TOF analysis of tryptic digests of the protein followed by database searches established that this protein was novel with no obvious similarity to PepT1, the apical membrane transporter.

Oral absorption of cephalosporins is quantitatively predicted from in vitro uptake into intestinal brush border membrane vesicles

In order to establish a method to predict oral absorption of drugs, which are absorbed by the oligopeptide transporter (PepT1), fraction absorbed (F) of cephalosporin antibiotics was predicted from in vitro uptake into rat intestinal brush border membrane vesicles (BBMV). Using in vitro uptake data, F values of cephalosporins in humans were predicted using the equation derived from the complete radial mixing (CRM) model, which was proposed by Amidon et al. (Amidon et al., J. Pharm. Sci. 69 (1980) 1369). In the present study, uptake into BBMV was measured at 25 and 4 degrees C in the presence of an H+ -gradient, and the uptake clearance (CLuptake) was calculated. Clearance for the uptake mediated by PepT1 (DeltaCLuptake) was then calculated as CLuptake at 25 degrees C minus that at 4 degrees C. When DeltaCLuptake and F values were analyzed according to the present equation, fairly good correlation between DeltaCLuptake and F was observed. It was further demonstrated that the present method may be able to quantitatively predict F values of cephalosporins by using several cephalosporins as standards.

Effects of Russell's viper venom on renal lysosomal functions in experimental mice

The lysosome-enriched fraction of mice kidney was isolated by homogenization and differential centrifugation. Lysosomal functions, namely lysosomal enzyme activities and membrane integrities were investigated in (a) the renal lysosome-enriched fraction, incubated with different concentrations of Russell's viper venom (RVV) for various time intervals (in vitro test) and (b) the kidney homogenate of mice, which had been envenomed with different dosages of RVV and been sacrificed after various time intervals post-envenomation (in vivo test). Three typical marker enzymes for lysosome were used, namely N-acetyl-beta-D-glucosaminidase (NAG), cathepsin D and acid phosphatase. It was found that, with increasing dosages of RVV and increasing time intervals after RVV treatment, the activities of all lysosomal enzymes generally increased and the lysosomal membrane integrities apparently reduced in in vitro and in vivo conditions, respectively. Among the three typical marker enzymes, NAG was found to be the most specific, sensitive and informative marker enzyme for the study of lysosomal functions in the kidney of mice treated with RVV.

GNAZ in human fetal cochlea: expression, localization, and potential role in inner ear function

Dissociation of an activated alpha-subunit from the beta-gamma complex directly regulates secondary messenger proteins. To address the potential role of G proteins expressed in human fetal cochlea, degenerate oligonucleotide primers corresponding to the 3'-end of the conserved region of alpha-subunits were used for polymerase chain reaction amplification of reverse-transcribed total human fetal cochlear mRNAs; GNAZ and GNAQ were isolated. These two G proteins are unique among the G-protein family because they lack a typical pertussis modification site. GNAZ is expressed in high levels in neural tissue while GNAQ is ubiquitously expressed. We characterized GNAZ expression using Northern blots, tissue in-situ hybridization and immunohistochemistry techniques to elucidate the potential role of this protein in inner ear function. Our data suggest that GNAZ may play a role in maintaining the ionic balance of perilymphatic and endolymphatic cochlear fluids.

Mechanism of cholera toxin action on a polarized human intestinal epithelial cell line: role of vesicular traffic

The massive secretion of salt and water in cholera-induced diarrhea involves binding of cholera toxin (CT) to ganglioside GM1 in the apical membrane of intestinal epithelial cells, translocation of the enzymatically active A1-peptide across the membrane, and subsequent activation of adenylate cyclase located on the cytoplasmic surface of the basolateral membrane. Studies on nonpolarized cells show that CT is internalized by receptor-mediated endocytosis, and that the A1-subunit may remain membrane associated. To test the hypothesis that toxin action in polarized cells may involve intracellular movement of toxin-containing membranes, monolayers of the polarized intestinal epithelial cell line T84 were mounted in modified Ussing chambers and the response to CT was examined. Apical CT at 37 degrees C elicited a short circuit current (Isc: 48 +/- 2.1 microA/cm2; half-maximal effective dose, ED50 integral of 0.5 nM) after a lag of 33 +/- 2 min which bidirectional 22Na+ and 36Cl- flux studies showed to be due to electrogenic Cl- secretion. The time course of the CT-induced Isc response paralleled the time course of cAMP generation. The dose response to basolateral toxin at 37 degrees C was identical to that of apical CT but lag times (24 +/- 2 min) and initial rates were significantly less. At 20 degrees C, the Isc response to apical CT was more strongly inhibited (30-50%) than the response to basolateral CT, even though translocation occurred in both cases as evidenced by the formation of A1-peptide. A functional rhodamine-labeled CT-analogue applied apically or basolaterally at 20 degrees C was visualized only within endocytic vesicles close to apical or basolateral membranes, whereas movement into deeper apical structures was detected at 37 degrees C. At 15 degrees C, in contrast, reduction to the A1-peptide was completely inhibited and both apical and basolateral CT failed to stimulate Isc although Isc responses to 1 nM vasoactive intestinal peptide, 10 microM forskolin, and 3 mM 8Br-cAMP were intact. Re-warming above 32 degrees C restored CT-induced Isc. Preincubating monolayers for 30 min at 37 degrees C before cooling to 15 degrees C overcame the temperature block of basolateral CT but the response to apical toxin remained completely inhibited. These results identify a temperature-sensitive step essential to apical toxin action on polarized epithelial cells. We suggest that this event involves vesicular transport of toxin-containing membranes beyond the apical endosomal compartment.

Effect of enterotoxigenic Escherichia coli heat stable toxin on intestinal lysosomal enzymes in the suckling rat

A preliminary study on 9 suckling Wistar rats, which received E. coli stable toxin, and on 12 sham-operated controls showed that acid phosphatase, the marker enzyme for lysosome, was significantly increased in the infected group whereas alkaline phosphatase, glucose 6-phosphatase, succinic dehydrogenase, and proteinase, the marker enzymes for brush border, microsome, mitochondria, and the soluble fraction, respectively, remained unaffected. The results suggest that lysosome, the subcellular organelle responsible for intracellular digestion could be modified by E. coli stable toxin. In another set of experiments, where 7 infected suckling rats and 7 sham-operated controls were used, the maximal activities of lysosomal enzymes (released by Triton X-100) were found to be increased in the infected group confirming the results obtained in the preliminary experiment. The values of the ratio between maximal and basal activity (an expression of the degree of retention of enzymes to lysosome) of acid phosphatase and cathepsin D were also significantly increased, indicating that lysosomal membrane may also be stabilized during the infection. The increased activities of lysosomal enzymes and the increased lysosomal membrane stability suggest that intracellular digestion by lysosome could be increased during E. coli stable toxin infection.

Intestinal brush border membranes contain regulatory subunits of adenylyl cyclase

Cholera toxin alters intestinal function by stimulation of adenylyl cyclase [ATP pyrophosphate-lyase (cyclizing) or adenylate cyclase, EC 4.6.1.1]. The mechanism of this activation is unknown and particularly puzzling because adenylyl cyclase is confined to the basal lateral membrane of enterocytes, whereas it is the brush border membrane that binds the toxin and contains proteins that undergo cholera toxin-catalyzed ADP ribosylation. It is shown that cholate extracts from cholera toxin-treated brush border membranes can efficiently reconstitute adenylyl cyclase activity in the guanine nucleotide-binding regulatory component (Gs)-deficient cyc- variant of the S49 mouse lymphoma cell line (cyc- cells lack the alpha subunit of Gs needed to activate the catalytic subunit of adenylyl cyclase). Moreover, NaF (in the presence of Al3+) and guanyl-5'-yl imidodiphosphate mediate strong activation of cyc- adenylyl cyclase provided the cholate extracts of brush border membranes are also present. Therefore, it appears that brush border membranes contain high levels of regulatory subunits of adenylyl cyclase in the absence of catalytic subunits. This represents a previously unrecognized feature of this transduction system that presumably plays an important role in the derangement of intestinal cell function by cholera toxin.

A novel imino-acid carrier in the enterocyte basolateral membrane

Basolateral membrane vesicles prepared from rat small intestinal epithelial cells were used to study the sodium-independent transport of L-proline. The uptake of L-proline was unaffected by the presence of sodium and showed saturation kinetics (Kt = 0.5 mM and Vmax = 23.3 pmol/mg protein per s). Competition experiments indicated that other amino acids had an affinity for the carrier system with L-leucine greater than L-alanine greater than sarcosine greater than glycine greater than L-lysine greater than OH-proline greater than taurine greater than beta-alanine greater than D-alanine greater than D-proline greater than L-serine greater than phenylalanine greater than valine greater than D-serine greater than phenylalanine greater than valine greater than D-serine greater than MeAIB greater than methionine greater than threonine. This pathway does not resemble those previously described either in the brush-border membrane of intestinal epithelial cells or the plasma membrane of other cell types. The lack of effect of methionine and threonine indicate that proline is not using the L-type system, while the very low affinity for MeAIB and the Na+ independence suggest that this is a novel system for imino acids. The relatively high capacity of this system and its low Kt, which is almost identical to the proline system in the brush-border membrane, strongly suggest that this is an important pathway in the final step for proline absorption by the small intestine.

Effect of hyperglycemia on D-glucose transport across the brush-border and basolateral membrane of rat small intestine

Experimental hyperglycemia leads to an increase in the capacity of the rat small intestine to absorb glucose. This effect occurs within hours from the onset of hyperglycemia and is thought to involve an induction of glucose transport in the brush-border and/or basolateral membrane of the intestinal epithelium. We devised a protocol for the simultaneous preparation of brush-border vesicles and basolateral vesicles from rat small intestine to determine the locus for the induction of glucose transporter in hyperglycemic rats. A 6 h period of intravenous infusion with a 30% glucose solution had no effect on the initial rate of glucose uptake across jejunal or ileal brush-border vesicles when measured in the absence of a Na+ gradient, suggesting that enhanced glucose uptake is not dependent on an increase in the number of Na+-dependent secondary active glucose transporters in the brush-border. Hyperglycemia did not effect the rate of glucose uptake across ileal basolateral vesicles but did cause a 78% increase in the initial rate of carrier-mediated D-glucose uptake across jejunal basolateral vesicles. The induction of glucose transport in the jejunal basolateral membrane was characterized by a rapid rate of glucose equilibration across the vesicles (t 1/2 = 46 s sorbitol infused controls, 18 s hyperglycemia) and a 75% increase in the Vmax for carrier-mediated glucose uptake with no significant change in Kt. When the rats were pretreated with cycloheximide prior to intravenous infusion, the initial rate of D-glucose uptake dropped to 13% of that seen in jejunal basolateral vesicles prepared from untreated rats. These results suggest a rapid turnover rate for the Na+-independent glucose transporter in the basolateral membrane of the enterocyte. An increase in the number of functioning glucose transporters in the basolateral membrane may play an important role in the short-term induction of glucose absorption by the jejunum of the hyperglycemic animal.

Adenylate cyclase from rabbit small intestine: activation by cholera toxin and interaction with calcium

The stimulation of adenylate cyclase in various fractions of plasma membranes from rabbit small intestinal epithelium has been studied. In crude plasma membranes cholera toxin activated 5-fold at 10 micrograms/ml; vasoactive intestinal peptide (VIP) activated at concentration from 10(-8) to 10(-7) M, the maximal stimulation being 6-fold. Fluoride activated 10-fold at 10 mM. VIP-stimulated enzyme was inhibited by Ca2+ concentrations in the micromolar range. In the presence of calmodulin a biphasic response was obtained. At low Ca2+ concentration (4 x 10(-9)-6 x 10(-8) M) the enzyme was activated. As the Ca2+ concentration was increased the enzyme was concomitantly inhibited. We have investigated the mechanism by which cholera toxin activates intestinal adenylate cyclase. We have found that cholera toxin catalyzed incorporation of 32P into proteins located in the brush-border membrane whose molecular weights are in the range of 40-45kDa. These membranes bind [3H]GTP with a Kd of 1.8 x 10(-7) M. In contrast, basal lateral membranes do not contain any protein which becomes labeled in a toxin-dependent manner when incubated with cholera toxin and [32P]NAD. The modification of brush-border membrane protein occurred in spite of the absence of adenylate cyclase in these membranes. Adenylate cyclase in basal lateral membranes was poorly activated by cholera toxin as compared to crude plasma membranes. On the other hand, the ability of VIP and fluoride to activate the enzyme was enhanced in basal lateral membranes with respect to crude membranes. The results are discussed in relation to the mechanism by which cholera toxin activates adenylate cyclase in intact intestinal cells.

The activation of adenylate cyclase from small intestinal epithelium by cholera toxin

ADP-ribosylation of membrane proteins from rabbit small intestinal epithelium was investigated following incubation of membranes with [32P]NAD and cholera toxin. Cholera toxin catalyzes incorporation of 32P into three proteins of 40 kDA, 45 kDa and 47 kDa located in the brush-border membrane. In contrast, basal lateral membranes do not contain any protein which becomes labeled in a toxin-dependent manner when incubated with cholera toxin and [32P]NAD. The modification of membrane proteins from brush border occurred in spite of the virtual absence in these membranes of adenylate cyclase activatable either by cholera toxin, vasoactive intestinal peptide (VIP) or fluoride. The three agents activated adenylate cyclase when crude plasma membrane were used. Cholera toxin activated fivefold at 10 micrograms/ml. Vasoactive intestinal peptide activated at concentrations from 10-300 nM, the maximal stimulation being sixfold. Fluoride activated 10-fold at 10 mM. When basal lateral membranes were assayed for adenylate cyclase it was found that, with respect to the crude membranes, the specific activity of fluoride-activated enzyme was 3.3-fold higher, VIP stimulated enzyme was maintained while cholera-toxin-stimulated enzyme showed half specific activity. Moreover, while fluoride stimulated ninefold and VIP stimulated fivefold, cholera toxin only stimulated twofold at the highest concentration. The results suggest that the activation by cholera toxin of adenylate cyclase located at the basal lateral membrane requires ADPribosylation of proteins in the brush border membrane.

In vitro absorption of [14C]leucine during inflammation and the effect of antiinflammatory drugs in the jejunum of rats

1. Inflammation was induced in the hindlegs of rats by formalin injection and the in vitro absorption of [14C]leucine was studied. 2. Treatment of rats with formalin caused a reduction in the in vitro absorption of leucine from the mucosa of jejunum. 3. Oral administration of oxyphenbutazone or a herbal anti-inflammatory drug (Withania somnifera) prior to formalin injection resulted in no alteration in the jejunal absorption of [14C]leucine.

Influence of extra-intestinal inflammation on the in vitro absorption of 14C-glucose and the effects of anti-inflammatory drugs in the jejunum of rats

Inflammation was induced in the hind legs of rats by formalin injection and the in vitro jejunal absorption of 14C-glucose was studied. Treatment of rats with formalin caused a reduction in the in vitro absorption of glucose from the jejunum. Oral administration of oxyphenbutazone or a herbal anti-inflammatory drug (Withania somnifera) prior to formalin injection, resulted in no alteration in the jejunal absorption of glucose.

The importance of the enteric nervous system for the bile-salt-induced secretion in the small intestine of the rat

We have investigated the possible involvement of the enteric nervous system in the intestinal secretion induced by sodium deoxycholate. Hexamethonium, lidocaine, and tetrodotoxin significantly inhibited the fluid secretion in extrinsically innervated and denervated rat jejunal segments. Atropine had no effect. We conclude that the sodium-deoxycholate-induced intestinal secretion is partly caused by the activation of local nervous reflexes.

Preferential binding of vasoactive intestinal polypeptide to basolateral membrane of rat and rabbit enterocytes

Binding of radioiodinated vasoactive intestinal polypeptide (VIP) to intestinal cell membranes of the rabbit ileum and rat jejunum was investigated. Specific binding of 125I-labeled VIP could be demonstrated only on the basolateral membrane and not on the brush border membrane. This corresponded with the lack of an effect on ion transport when VIP was applied to the mucosal side of an in vitro preparation of rabbit ileum. VIP altered ion transport only when it was applied to the serosal side. The binding of 125I-VIP was specific and dependent upon incubation temperature. There was a close correlation between the potency of VIP for inhibition of 125I-VIP binding and that for increasing adenylate cyclase activity. These observations demonstrate that VIP receptors are located on the basolateral membrane.

Ricinoleate and deoxycholate are calcium ionophores in jejunal brush border vesicles

The intestinal secretagogues ricinoleate and deoxycholate have been tested for a capacity to form complexes with Ca2+ ions and to affect the passive equilibration of Ca2+ ions across the jejunal brush border membrane. Both of these agents formed butanol-soluble Ca2+ complexes in a model phase distribution system. They also promote the passive uptake and efflux of Ca2+ across brush border vesicles in a concentration-dependent manner. The levels of ricinoleate and deoxycholate that increase the rate of transvesicular Ca2+ movement are in the 100 to 300 microM range. Concentrations as high as 1.0 mM had no significant detergent effects in vesicles as measured by release of entrapped sorbitol. The kinetics of Ca2+ uptake and efflux are similar in brush border vesicles treated with A23187, ricinoleate, or deoxycholate. The influx rates observed in this study were high enough to cause the collapse of a Ca2+ gradient, which had been generated by Ca--Mg ATPase enzyme activity in the brush border membrane. Ricinoleate did not affect Ca--Mg ATPase activity at concentrations used in this study, but deoxycholate was inhibitory, indicating two potential modes for elevation of intracellular Ca2+ content by deoxycholate. When compared with the effects of the Ca2+ ionophore, A23187, it appears that both ricinoleate and deoxycholate could have significant intestinal secretory activity due to this Ca2+ ionophore property. It is also noteworthy that, at least in this model system, potential secretory effects are expressed at concentrations significantly below levels that have been associated with detergent effects or altered epithelial morphology.

Controlled trial of chlorpromazine as antisecretory agent in patients with cholera hydrated intravenously

A randomised controlled trial was conducted to investigate the ability of chlorpromazine to reduce intestinal secretion in cholera. Chlorpromazine had reduced loss of intestinal fluid in animals with diarrhoea induced by cholera toxin, and in a preliminary study the drug had reduced purging in patients with cholera. Forty-six adults with cholera were included in the randomised trial. Of these, 34 were treated with chlorpromazine (1 mg/kg or 4 mg/kg either by mouth or intramuscularly) and 12 served as controls. After treatment with the drug there was a significantly greater reduction in the rate of fluid loss in the treated patients than in the controls during the first (p less than 0.005), second (p less than 0.05), and fourth (p less than 0.01) eight-hour periods, but not during the third eight-hour period; the dose of 4 mg/kg was only marginally more effective than 1 mg/kg. The effect of chlorpromazine was strikingly biphasic, with one peak during the first eight hours and another 24-32 hours after administration. Chlorpromazine also significantly reduced the duration of diarrhoea, frequency of vomiting, and amount of intravenous fluid required. The drug induced mild sedation and no hypotension in these well-hydrated patients. These findings confirm the effectiveness of chlorpromazine in reducing fluid loss in cholera. A sedative effect, however, especially in children, may limit its usefulness and requires further study.

Ultracytochemistry of cholera-toxin binding sites in ciliary processes

Cholera toxin reduces the rate of aqueous humor in concentrations (10-11M) that do not disturb the morphology of the aqueous-humor forming epithelial cells of the ciliary processes of the rabbit eye. The search for an endogenous mediator of aqueous-humor formation comparable to cholera toxin in its mode of operation prompted us to map the distribution of cell surface receptors for cholera toxin in the ciliary processes of the eyes of rabbits. Cytochemical studies were carried out with the use of conjugates of cholera toxin to fluorescein isothiocyanate (CT-FITC) and to horseradish peroxidase (CT-HRP), and of the B subunit of cholera toxin to horseradish peroxidase (B-HRP). Multiple fluorescent CT-FITC binding sites were observed on the outer nonpigmented epithelial layer near the crests of the processes. Processes incubated with CT-HRP in vitro showed surface staining of 30-40% of the nonpigmented epithelial cells. A prominent reaction product was observed along the basal and lateral plasma membranes of these cells. In vivo studies carried out after arterial infusion of B-HRP showed a reproducible dense reaction product between the apical surfaces of the pigmented epithelium (PE) and of the nonpigmented epithelium (NPE) facing each other. Aggregations of reaction product were observed with the electron microscope in the extracellular space between the apices of PE and NPE. The apical plasma membrane of the endothelium of the blood vessels near the crests of the ciliary processes was stained after either in vivo or in vitro exposure to peroxidase conjugates. These findings indicate that the cell-surface receptors which mediate the action of cholera toxin on aqueous humor formation are very likely localized in the apical plasma membranes of the epithelium of the ciliary processes.

The effect of hydrocortisone on the transport of water, sodium, and glucose in the jejunum. Perfusion studies in normal subjects and patients with coeliac disease

During perfusion of an isotonic solution containing 56 mmol/l glucose and 122 mmol/l sodium chloride, net movements of water and sodium and glucose absorption were measured in the absence and presence of intraluminal hydrocortisone along 20-cm segments of proximal jejunum in five normal subjects and five patients with active coeliac disease. In normal subjects, the mean net absorption of water and sodium was significantly increased in the presence of hydrocortisone decreased. In the coeliac patients, the mean net secretion of water and sodium was significantly decreased. In neither group was the effect of hydrocortisone associated with a significant increase in glucose absorption. Similar changes in net movements of water and sodium occurred in both groups, even though absorption of hydrocortisone in coeliac patients was 44% of the control value. These results suggest that hydrocortisone may have exerted a local effect on the mucosal transport of water and sodium.

Altered intestinal fluid movement in response to Trichinella spiralis in immunized rats

Net intestinal fluid movement was measured in immunized and non-immunized rats infected with the enteric stages of the nematode, Trichinella spiralis. Animals were studied 30 min, 5 days and 30 days after receiving infective larvae. Net water movement across the mucosal surface of the gut was measured in vivo by perfusing a cannulated segment (approximately 30 cm) of proximal small intestine with an isotonic solution containing a nonabsorbable marker, 14C polyethylene glycol, at a rate of 0.5 ml/min. Uninfected rats showed net absorption of water, 25 microliter/h per cm of intestine. This response was unaltered when rats were infected (7 x 10(3) larvae/rat) and examined 30 minutes later. Five days after primary infection net tissue-to-lumen fluid movement, ie. net secretion, occurred at a rate of 45 microliter/h per cm. When rats were studied 30 days after primary infection, net absorption equivalent to the preinfection level was observed once again. Previously infected (immunized) rats, when challenged (secondary infection, 7 x 10(3) larvae/rat) and examined within 30 min showed a significant decrease in net absorption rate as compared with non-immunized rats. Absorption returned to the preinfection level at both 5 and 30 days post challenge. Results support the conclusion that T. spiralis caused a decrease in net lumen-to-tissue fluid movement during primary and secondary infections. The response was initiated faster in previously infected hosts. The rapid induction of net fluid movement in the direction of secretion upon secondary contact with the parasite is associated temporarily with prevention of worm establishment.

Effect of chlorpromazine on the secretory activity of Escherichia coli heat-stable enterotoxin

The effect of chlorpromazine on the net intestinal accumulation of fluid induced by Escherichia coli heat-stable (ST) enterotoxin in an infant mouse model was examined. Chlorpromazine, when administered with ST enterotoxin, caused a highly significant decrease in net intestinal fluid accumulation. The inhibition of ST activity was dose dependent with various concentrations of chlorpromazine (P less than 0.001). A significant inhibition of toxic activity was also observed when chlorpromazine was administered before (P less than 0.02) or after (P less than 0.05) ST enterotoxin challenge. No significant differences in fluid accumulation were observed between control mice treated with buffer alone and those treated with only chlorpromazine. These data indicate that chlorpromazine markedly decreases the net intestinal fluid accumulation induced by E. coli ST enterotoxin. Further studies on the potential use of chlorpromazine in both the prophylaxis and the treatment of diarrheal diseases appear warranted.

Large scale, analytical method for isolating basal lateral plasma membranes from rat duodenum

A procedure is described for obtaining large amounts of basal lateral plasma membranes from the rat duodenal epithelium. The yield is approximately 50%, and the purification factor is 18; preparations from 25 rats routinely contain 100 mg of protein. The procedure depends on mild homogenization with a nitrogen cavitation bomb, followed by removal of brush borders by sedimentation in a weak centrifugal field. Basal lateral membranes in the resulting supernatant are partially purified by differential centrifugation in a medium which approximates their equilibrium density, and then further purified by equilibrium density gradient centrifugation in a high capacity zonal rotor. Brush border membranes may be isolated from the 450 x g pellet. Since both brush border and basal lateral membranes may be isolated from the same homogenate, this preparative procedure is suitable for such analytical purposes as determinations of distribution of enzyme activities between the two surfaces of the epithelium. The large scale of the isolation procedure makes it an appropriate starting point for purification of specific basal lateral membrane components.

The effect of cholera toxin and heat labile and heat stable Escherichia coli enterotoxin on cyclic AMP concentrations in small intestinal mucosa of pig and rabbit

The effect of cholera toxin, heat labile and heat stable Escherichia coli enterotoxin on mucosal cyclic AMP concentrations was determined on the proximal jejunum of weanling pigs and young rabbits. Ligated loops were injected with solutions containing no enterotoxin for control and either cholera toxin, heat labile or heat stable E. coli enterotoxin. The loops were drained after either two, four or six hours incubation at which time accumulated fluid was recorded and mucosal samples removed for determination of cyclic AMP concentration. In the rabbit, cholera toxin and heat labile, but not heat stable E. coli enterotoxin stimulated intestinal secretion while in the pig all three enterotoxins induced net fluid accumulation. Cholera toxin and heat labile, but not heat stable E. coli enterotoxin elevated rabbit mucosal cyclic AMP concentrations. In the pig these enterotoxins had no significant effect on mucosal cyclic AMP concentrations. The results are inconsistent with the hypothesis that the adenyl cyclase system is an essential step for enterotoxin induced intestinal secretion. The activation of intestinal adenyl cyclase by bacterial enterotoxins may only be an associated and not a necessary event for the stimulation of intestinal secretion.

Effect of heat stable and heat labile Escherichia coli enterotoxins and cholera toxin in combination with theophylline on unidirectional sodium and chloride flux in the small intestine of weanling swine

The effect of heat stable and heat labile Escherichia coli enterotoxins or cholera toxin in combination with theophylline on net water, sodium and chloride and unidirectional sodium and chloride fluxes was examined in acute isolated loops of jejunum of weanling swine. The effect of heat stable enterotoxin in combination with theophylline was determined in loops located in the proximal jejunum, while combinations of theophylline and either heat labile enterotoxin or cholera toxin were studied in the distal jejunum. In each situation the addition of theophylline resulted in an additive rather than a synergistic increment of intestinal secretory activity. This study implies that the intestinal adenyl cyclase system and enterotoxin induced intestinal secretion may not be directly related in the swine small intestine.

Studies on the time course and rate-limiting steps in the activation of adenylate cyclase in rat liver by cholera toxin

Cholera toxin stimulates adenylate cyclase in rat liver after intravenous injection. The stimulation follows a short latent period of 10min, and maximum stimulation was attained at 120min. Half-maximal stimulation was achieved at 35min. In contrast with this lengthy time course in the intact cell, adenylate cyclase in broken-cell preparations of rat liver in vitro were maximally stimulated by cholera toxin (in the presence of NAD+) in 20min with half-maximal stimulation in 8min. Binding of cholera toxin to cell membranes by the B subunits is followed by translocation of the A subunit into the cell or cell membrane, and separation of the A1 polypeptide chain from the A2 chain by disulphide-bond reduction, and finally activation of adenylate cyclase by the A1 chain and NAD+. As the binding of cholera toxin is rapid, two possible rate-limiting steps could be the determinants of the long time course of action. These are translocation of the A1 chain from the outside of the cell membrane to its site of action (this includes the time required for separation from the whole toxin) or the availability of NAD+ for activation. When NAD+ concentrations in rat liver were elevated 4-fold, by the administration of nicotinamide, no change in the rate of activation of adenylate cyclase by cholera toxin was observed. Thus the intracellular concentration of NAD+ is not rate-limiting and the major rate-limiting determinant in intact cells must be between the time of toxin binding to the cell membrane and the appearance of subunit A1 at the enzyme site.

Characterization of bile acid binding to rat intestinal brush border membranes

Studies were performed to characterize the binding1 of bile acids to intestinal brush border membranes. Total 14C-taurodeoxycholate binding was: 1) similar for brush borders prepared from jejunum and ileum, 2) linear with respect to monomer concentration, 3) uninhibited by a structural analog, and 4) not depressed by boiling or trypsin. A linear relationship existed between binding and the number of hydrogen bonds formed by a bile acid and the slope of the line corresponded to delta deltaF of 300 cal/mol. The binding of bile acids to the 105,000 x g supernatant fraction of sonicated brush borders was similar to the binding of phospholipid liposomes using gel chromatography. These data suggest that: 1) the kinetics and characteristics of binding of bile acid to ileal brush borders do not reflect the kinetics and characteristics of active ileal transport previously obtained in whole tissue preparations, but instead reflect the kinetics and characteristics of passive jejunal transport; 2) a determinant of binding is hydrogen bonding with water; 3) isolated intact brush borders are relatively polar membranes; and 4) binding to solubilized brush borders may represent partitioning between the aqueous phase and membrane lipid.

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.

The surface membrane of the small intestinal epithelial cell. I. Localization of adenyl cyclase

The subcellular distribution of adenyl cyclase was investigated in small intestinal epithelial cells. Enterocytes were isolated, disrupted and the resulting membranes fractionated by differential and sucrose gradient centrifugation. Separation of luminal (brush border) and contra-luminal (basolateral) plasma membrane was achieved on a discontinuous sucrose gradient. The activity of adenyl cyclase was followed during fractionation in relation to other enzymes, notably those considered as markers for luminal and contraluminal plasma membrane. The luminal membrane was identified by the membrane-bound enzymes sucrase and alkaline phosphatase and the basolateral region by (Na+ + K+)-ATPase. Enrichment of the former two enzymes in purified luminal plasma membrane was 8-fold over cells and that of (Na+ + K+)-ATPase in purified bisolateral plasma membranes was 13-fold. F--activated adenyl cyclase co-purified with (Na+ + K+)-ATPase, suggesting a common localization on the plasma membrane. The distribution of K+-stimulated phosphatase and 5'-nucleotidase also followed (Na+ + K+)-ATPase during fractionation.

Structure and function of cholera toxin and hormone receptors

The enterotoxin from Vibrio cholerae is a protein of 100,000 mol wt which stimulates adenylate cyclase activity ubiquitously. The binding of biologically active 125I-labeled choleragen to cell membranes is of extraordinary affinity and specificity. The binding may be restricted to membrane-bound ganglioside GM1. This ganglioside can be inserted into membranes from exogenous sources, and the increased toxin binding in such cells can be reflected by an increased sensitivity to the biological effects of the toxin. Features of the toxin-activated adenylate cyclase, including conversion of the enzyne to a GTP-sensitive state, and the increased sensitivity of activation by hormones, suggest analogies between the basic mechanism of action of choleragen and the events following binding of hormones to their receptors. The action of the toxin is probably not mediated through intermediary cytoplasmic events, suggesting that its effects are entirely due to processes involving the plasma membrane. The kinetics of activation of adenylate cyclase in erythrocytes from various species as well as in rat adipocytes suggest a direct interaction between toxin and the cyclase enzyme which is difficult to reconcile with catalytic mechanisms of adenylate cyclase activation. Direct evidence for this can be obtained from the comigration of toxin radioactivity with adenylate cyclase activity when toxin-activated membranes are dissolved in detergents and chromatographed on gel filtration columns. Agarose derivatives containing the "active" subunit of the toxin can specifically absorb adenylate cyclase activity, and specific antibodies against the choleragen can be used for selective immunoprecipitation of adenylate cyclase activity from detergent-solubilized preparations of activated membranes. It is proposed that toxin action involves the initial formation of an inactive toxin-ganglioside complex which subsequently migrates and is somehow transformed into an active species which involves relocation within the two-dimensional structure of the membrane with direct perturbation of adenylate cyclase molecules (virtually irreversibly). These studies suggest new insights into the normal mechanisms by which hormone receptors modify membrane functions.

Comparison of the rate of absorption and proteolysis of [14C]choleragen and [14C]bovine serum albumin in the rat jejunum

[14C]choleragen was used to study the rate of disappearance of choleragen enterotoxin from the jejunum of rats. [14C]bovine serum albumin (BSA) was studied in a similar manner. Almost one-third of the labeled toxin had disappeared from the intestine after 6 h. Its rate of disappearance was the same in germfree rats as in conventional rats. The rate of proteolysis of [14C]choleragen and [14C]BSA by intestinal mucodal lysosomal enzymes was also studied. Neither was significantly degraded by neutral proteases; however, heat-inactivated toxin was. They were all degraded by acid proteases; however, the rate of BSA proteolysis was only one-third of that of toxin. Soybean trypsin inhibitor had no effect on the in vivo disappearance of toxin nor on the acid proteases. It did inhibit the neutral protease digestion of heat-treated toxin. Aprotinin and protamine inhibited disappearance in loops of gut but had no effect to inhibit degradation rates. Gangliosides inhibited both rates of disappearance and proolysis of toxin. These agents had some different effects on disappearance rates and proteolysis of BSA. The data indicate that cholera enterotoxin is absorbed by intestinal mucosal cells and is degraded by acid proteases in the cells.

Na+-K+-activated adenosine triphosphatase and intestinal electrolyte transport. Effect of adrenal steroids

Sodium-potassium-activated adenosine triphosphatase (Na-K-ATPase) is associated with electrolyte transport in many tissues. To help delineate its role in intestinal transport, changes in rat intestinal electrolyte and water transport induced by injecting methylprednisolone acetate 3 mg/100 g or deoxycorticosterone acetate (DOCA) 0.5 mg/100 g per day for 3 days were correlated with changes in Na-K-ATPase activity. Methylprednisolone increased sodium and water absorption, potassium secretion, transmural potential difference, and Na-K-ATPase activity in the jejunum, ileum, and colon. Examination of isolated epithelial cells demonstrated that the jejunal and ileal increase in Na-K-ATPase occurred in both the villus tip and crypermeability, Mg-ATPase, and adenylate cyclase activities were unchanged by methylprednisolone. DOCA increased sodium and water absorption, potassium secretion, transmural potential difference, and Na-K-ATPase activity in the colon alone. Colonic Mg-ATPase and adenylate cyclase activities were unaffected. Jejunal and ileal enzyme activity, electrolyte transport, and permeability were unchanged by DOCA. Methylprednisolone and DOCA were not additive in their effect on colonic Na-K-ATPase activity. Methylprednisolone and DOCA increased electrolyte and water transport and Na-K-ATPase activity concomitantly in specific segments of small intestine and colon. These data are consistent with an important role for Na-K-ATPase in intestinal electrolyte and water transport.

Immunohistochemical localization of 3': 5'-cyclic AMP and 3': 5'-cyclic GMP in rat liver, intestine, and testis

Cyclic GMP and cyclic AMP have been localized in rat liver, small intestine, and testis by a fluorescent immunocytochemical procedure. In liver, cyclic AMP is distributed along sinusoids predominantly, and increased fluorescence is seen sinusoidal areas after glucagon administration. Cyclic GMP is located in nuclear elements and on the plasma membranes of hepatocytes. In jejunum, cyclic AMP is found predominantly at the basal and lateral sides of brush border cells and in the lamina propria, while cyclic GMP is located to the brush border membrane, smooth muscle, and nuclear elements. In testis, cyclic AMP is found in cytoplasm of cells at the perimeter of the seminiferrous tubules and in interstitial cells, while cyclic AMP is visualized on the plasma membrane of the cells lining the tubules. Cyclic GMP is also seen on chromosomes of premeiotic spermatocytes and in sperm. These data provide histological evidence implicating diverse roles for the nucleotides in these tissues. The nuclear localization of cyclic GMP in all of these tissues suggests a role for the nucleotide in nucleus-directed events.

A comparative study on the effects of different bile salts on mucosal ATPase and transport in the rat jejunum in vivo

The effects of deoxycholate, taurocholate and cholate on transport and mucosal ATPase activity have been investigated in the rat jejunum in vivo using closed-loop and perfusion techniques. In the closed-loops, 5 mM deoxycholate selectively inactivated (Na+ + K+)-ATPase, and net secretion of Na+ induced by 2.5 mM deoxycholate was due to reduced lumen to plasma flux of the ion; deoxycholate (2.5 mM) produced marked inhibition of 3-0-methylglucose transport. Luminal disappearance rates of deoxycholate (60.5 plus or minus 2.9% per g wet st of gut) greatly exceeded those of taurocholate (4.3 plus or minus 1.0). In the perfusion studies 1 mM deoxycholate induced net secretion of water, Na+ and C1-, and inhibited active glucose transport; concomitantly "total" ATPase, (Na+ + K+)-ATPase, and Mg-2+-ATPase were inhibited. At higher concentrations (5 mM) deoxycholate stimulated Mg-2+-ATPase activity. Taurocholate and cholate at 1mM had no effect on transport of (Na+ + K+)-ATPase. Mucosal lactase, sucrase and maltase activities were not affected by 1 mM deoxycholate, taurocholate or cholate. These results suggest that deoxycholate inhibits sodium-coupled glucose transport by inhibition of (Na+ + K+)-ATPase at the lateral and basal membranes of the epithelial cell, rather than from an effect at the brush-border membrane level.