Mucosal cAMP Research Articles

Membrane trafficking produces distinct beta‐adrenergic signaling for Cl secretion and K secretion in guinea pig distal colonic mucosa

Epinephrine (epi) activated Cl and K secretion (sec) in isolated mucosa from guinea pig distal colon, measured as short‐circuit current. These components of the response were distinguished by inhibition of Cl sec with a β2 antagonist (ICI‐118551) that spared K sec; propranolol inhibited both. Interfering with endocytosis prolonged the transient Cl sec and inhibited sustained K sec. Both the dynamin inhibitor dynasore and the clathrin‐coated‐pit blocker mono‐dansylcadaverine produced this response. Dynasore also inhibited the sustained epi‐induced increase in mucosal cAMP, while leaving the transient cAMP increase intact. Manipulating the ubiquitylation cycle by inhibiting ubiquitin‐E1 ligase (UBEI‐41, BioGenova) also prolonged the epi‐induced Cl sec transient and slowed the onset of K sec. Conversely, inhibiting deubiquitylases (PR‐619, LifeSensors) alone stimulated K sec, while blunting Cl sec and producing a non‐additive increase in K sec with epi. Disruption of the actin (latrunculin B, cytochalasin D) and microtubule (nocodazole) cytoskeleton inhibited epi‐induced K sec without altering Cl sec. Together these results support an activation process involving internalization of β2‐adrenergic receptors that terminates signaling for Cl sec, and β‐adrenergic stimulation of K sec that requires receptor internalization prior to activation of the sustained cAMP signaling cascades. [NIH DK65845]

A Role for CagA/VacA in Helicobacter pylori Inhibition of Murine Duodenal Mucosal Bicarbonate Secretion

Duodenal mucosal bicarbonate secretion is diminished in patients with Helicobacter pylori (HP)-associated duodenal ulcer disease. We examined whether HP water extracts inhibit murine duodenal mucosal bicarbonate secretion in vitro, and the mechanisms involved. Murine duodenal mucosae were mounted in Ussing chambers. Short-circuit current and bicarbonate secretion was measured. CagA/VacA-positive HP water extract (HPWE+/+) markedly inhibited PGE2-, carbachol-, or the calcium ionophore A23187-stimulated bicarbonate secretion in a dose-dependent manner. While 3-isobutyl-1-methylxanthine-stimulated bicarbonate secretion was not affected by HPWE+/+, HPWE+/+ did diminish forskolin-stimulated bicarbonate secretion. HPWE+/+ markedly diminished PGE2-induced increases in duodenal mucosal cAMP. CagA/VacA of HP decreases Ca2+-mediated bicarbonate secretion downstream of increases in intracellular Ca2+. Dimunition of PGE2-stimulated bicarbonate secretion occurs, in part, by inhibition of adenylate cyclase, which leads to decreased cAMP levels. The ability of virulent HP strains to inhibit duodenal bicarbonate secretion through multiple intracellular pathways likely contributes to the pathogenesis of HP-associated duodenal ulcer disease.

Cyclic AMP Stimulates Fructose Transport in Neonatal Rat Small Intestine

Intestinal fructose transporter (GLUT5) expression normally increases significantly after completion of weaning in neonatal rats. Increases in GLUT5 mRNA, protein, and activity can be induced in early weaning pups by precocious consumption of dietary fructose or by perfusion of the small intestine with fructose solutions. Little is known about the signal transduction pathway of the dietary fructose-mediated increase in GLUT5 expression during early intestinal development. Recent microarray results indicate that key gluconeogenic enzymes modulated by cAMP are markedly upregulated by fructose perfusion; hence, we tested the hypothesis that cAMP plays an important role in regulating intestinal fructose absorption by simultaneously perfusing adenylyl cyclase, phosphodiesterase, or protein kinase A (PKA) inhibitors along with fructose. Intestinal fructose uptake rates increased by 100% in rat pups perfused with 8-bromo-cAMP. Simultaneous fructose and dideoxyadenosine (DDA; inhibitor of adenylyl cyclase) perfusion completely inhibited increases in fructose uptake rate induced by perfusion with fructose alone. Fructose perfusion increased intestinal mucosal cAMP concentrations by 27%, but simultaneous perfusion of fructose and DDA inhibited the fructose-induced increase in cAMP. However, GLUT5 and sodium-glucose cotransporter (SGLT1) mRNA abundance and glucose transport rates were each not significantly affected by 8-bromo-cAMP and DDA. Moreover, simultaneous perfusion of the small intestine with fructose and PKA inhibitor or N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamid. 2HCl, both inhibitors of PKA, did not prevent the fructose-induced increases in GLUT5 mRNA abundance and fructose uptake rate. Cyclic AMP appears to modulate fructose transport without affecting GLUT5 mRNA abundance, and without involving PKA.

Disturbances of colonic ion secretion in inflammation: role of the enteric nervous system and cAMP.

We used the trinitrobenzenesulphonic acid (TNBS) rat model of experimental colitis to study the alterations in electrogenic ion transport in the inflamed distal colon. The distal colon exhibited decreased basal transport and reduced short-circuit current responses to carbachol and isobutylmethylxanthine (IBMX). The concentration/response curve for IBMX was also shifted to the right. Ion substitution experiments indicated that electrogenic transport was attributable chiefly to Cl(-) secretion. The mucosal layer of the inflamed distal colon (devoid of the submucosa) exhibited normal maximal responses to carbachol and IBMX, although the concentration/response curve for the latter was again shifted to the right. Tetrodotoxin markedly increased the response of the normal distal colon to both secretagogues and nullified the inhibition of the response to carbachol, but not that to IBMX, in the inflamed colon. The response of the mucosal preparation to 8-bromoadenosine 3',5'-cyclic monophosphate was similar in the normal and inflamed intestine, while the G protein activator NaF had a greater effect in the latter. The expression of the cystic fibrosis transmembrane conductance regulator (CFTR), as assessed by Northern blotting, was unchanged. cAMP levels in isolated colonocytes were markedly reduced by inflammation. We conclude that colonic inflammation produces disturbances of the enteric nervous system resulting in defective mucosal cAMP production and inhibition of ionic secretion.

Effects of short chain fatty acids on colonic Na + absorption and enzyme activity

Short chain fatty acids (SCFA) stimulate colonic Na + absorption and inhibit cAMP and cGMP-mediated Cl − secretion. It is uncertain whether SCFA have equivalent effects on absorption and whether SCFA inhibition of Cl − secretion involves effects on mucosal enzymes. Unidirectional Na + fluxes were measured across stripped colonic segments in the Ussing chamber. Enzyme activity was measured in cell fractions of scraped colonic mucosa. Mucosal 50 mM acetate, propionate, butyrate and poorly metabolized isobutyrate stimulated proximal colon Na + absorption equally (300%). Neither 2-bromo-octanoate, an inhibitor of β-oxidation, nor carbonic anhydrase inhibition affected this stimulation. All SCFA except acetate stimulated distal colon Na + absorption 200%. Only one SCFA affected proximal colon cGMP phosphodiesterase (PDE) (18% inhibition by 50 mM butyrate). All SCFA at 50 mM stimulated distal colon cAMP PDE (24–43%) and decreased forskolin-stimulated mucosal cAMP content. None of the SCFA affected forskolin-stimulated adenylyl cyclase in distal colon or ST a-stimulated guanylyl cyclase in proximal colon. Na +–K +-ATPase in distal colon was inhibited 23–51% by the SCFA at 50 mM. We conclude that all SCFA (except acetate in distal colon) stimulate colonic Na + absorption equally, and the mechanism does not involve mucosal SCFA metabolism or carbonic anhydrase. SCFA inhibition of cAMP-mediated secretion may involve SCFA stimulation of PDE and inhibition of Na +–K +-ATPase.

Stimulation of sodium chloride absorption from secreting rat colon by short-chain fatty acids.

Inhibition of electroneutral NaCl absorption by cyclic adenosine monophosphate (cAMP) results in fluid malabsorption in cholera. Short-chain fatty acids (SCFA) stimulate electroneutral NaCl absorption from the colon. The present study investigated effects of elevated cAMP on SCFA-stimulated NaCl absorption in rat distal colon. The effect of SCFA on fluxes of 22Na and 36Cl was studied under voltage-clamp conditions, in the presence and absence of secretagogues inducing mucosal cAMP elevation [ie, theophylline, cholera toxin (CT) and forskolin]. The effect of butyrate concentration on Na absorption in CT- and theophylline-treated mucosa was compared with control normal mucosa. cAMP was measured in isolated colonocytes in the presence of secretagogues with and without SCFA using a radioassay method. All secretagogues were noted to inhibit net Na absorption and to induce net Cl secretion. In the presence of SCFA, net Na absorption was normal, and net Cl secretion was partly reversed. The flux data indicated that NaCl absorption from secreting colon was stimulated by SCFA and that Cl secretion was partially inhibited. The effects of SCFA on NaCl absorption were similar regardless of the secretagogue used. The kinetics of butyrate-stimulated Na absorption were altered by theophylline and CT, which decreased Km (6.87 and 7.17, respectively, compared to 10.75 mM for control) and increased Vmax (4.55 and 8.33 compared to 3.45 mM/eq/cm2/hr for control). cAMP production by colonocytes in response to secretory stimuli was significantly reduced (34.4%) by butyrate but not by acetate or propionate. In conclusion, SCFA-stimulated Na absorption is up-regulated by cAMP and may be an absorptive pathway that can be utilized in the therapy of cholera. Effects of butyrate on cAMP generation are also likely to be useful in secretory diarrhea.

A possible molecular basis for the effect of gastric anti-ulcerogenic drugs.

The exact mechanism of action of the anti-ulcerogenic drugs is still under debate. According to the literature, under normal conditions the cAMP:cGMP ratio in the rat gastric mucosa is approximately 8:10. Following prostacyclin administration, this ratio transiently decreases but later shows a strong elevation, indicating profound changes in the intracellular cyclic nucleotide balance. There is evidence that this elevation or 'shift' in the cAMP:cGMP ratio is linked, on a cellular or molecular level, to the anti-ulcerogenic, cytoprotective processes in the stomach. Cimetidine and ranitidine (widely used H2 receptor-blocking drugs) administered at doses that are too low to interfere with gastric acid secretion, cause an elevation in the cAMP:cGMP ratio, an effect that is also observed with other prostaglandin derivatives and anti-ulcerogenic drugs. In this article, Gabor A. Balint discusses these data and how the elevation of the gastric mucosal cAMP:cGMP ratio is a useful molecular marker that could provide insights into the effects of anti-ulcerogenic drugs.

Clinical and experimental study on gastric mucosal pathology, gene expression, cAMP and trace elements of pixu patients

Clinical and experimental study on gastric mucosal pathology, gene expression, cAMP and trace elements of pixu patients

Nitecapone effect on the synthesis and secretion of gastric sulfomucin

1. 1. The effect of a new antiulcer agent, nitecapone, on the synthesis and secretion of sulfomucin in gastric mucosa was investigated using mucosal segments incubated in the presence of [ 3H]proline, [ 3H]glucosamine and [ 35S]sulfate. 2. 2. The drug, while showing no discernible effect on the apomucin synthesis, evoked a dose-dependent increase in mucin glycosylation and sulfation, which at 225 μM nitecapone, attained its maximum of 1.8 and 2.2-fold stimulation, respectively. 3. 3. The analysis of mucin secretory responses revealed that nitecapone caused a concentration-dependent enhancement in sulfomucin secretion attaining maximum increase of 1.5-fold at 150 μM nitecapone. 4. 4. The stimulatory effect of nitecapone on sulfomucin secretion was accompanied by 1.4-fold increase in mucosal cAMP level, and showed sensitivity to protein kinase A inhibitors, thus pointing towards the involvement of protein kinase A in mediation of gastric sulfomucin secretory responses to nitecapone. 5. 5. The ability of nitecapone to enhance sulfomucin synthesis and secretion could be of importance to the gastroprotective action of this agent.

Effects of aspirin on 1,2-dimethylhydrazine-induced colonic carcinogenesis.

The influence of aspirin (ASA) on 1,2-dimethylhydrazine (1,2-DMH)-induced colonic carcinogenesis was examined in weanling Sprague-Dawley rats. The incidence of adenocarcinomas in response to a single dose of 1,2-DMH was reduced 60% in rats receiving ASA for 1 week before and after the carcinogen. However, ASA had no effect on tumor incidence when initiated 4 weeks after a single dose of 1,2-DMH and continuing until the animals were killed at 36 weeks. The doses of ASA employed suppressed by 95% or more ex vivo colonic prostaglandin E2 (PGE2) production and reduced colonic mucosal cAMP levels in both rats exposed to 1,2-DMH and in age-matched controls. Proliferative activity of colonic mucosa as assessed from tritiated thymidine ([3H]dThd) incorporation into mucosal DNA was increased at 1 week but suppressed by 36 weeks after 1,2-DMH exposure. ASA significantly increased colonic mucosal DNA synthesis, suppressed colonic PGE2 production and reduced mucosal cAMP levels at both 1 and 36 weeks in rats given the 1,2-DMH vehicle. However, ASA failed to alter the enhanced mucosal DNA synthesis observed at 1 week or the suppressed DNA synthesis observed at 36 weeks after a single dose of 1,2-DMH, despite significant inhibition of colonic PGE2 production and reduction in mucosal cAMP levels by ASA. Treatment of rats for 1 week with ASA significantly inhibited basal and arachidonate stimulated decomposition of the 1,2-DMH intermediary metabolite methylazoxy-methanol, assessed ex vivo in colonic mucosal homogenates. Thus, while other mechanisms are not excluded, suppression of 1,2-DMH induced colonic carcinoma by concurrent administration of ASA may be linked in part to altered metabolic activation of this carcinogen via cyclooxygenase-dependent co-oxidation. By contrast, the previously reported suppression of the promotional phase of colonic carcinogenesis in rats by the delayed introduction of cyclooxygenase inhibitors may not be linked to inhibition of local colonic prostanoid production, since (i) inhibition of colonic prostanoid synthesis by ASA did not mimic this antipromotional effect, and (ii) the doses of non-steroidal anti-inflammatory drugs employed in some earlier studies may not significantly inhibit colonic prostanoid synthesis.

Zur Behandlung mit Clonidin bei kurzdarmsyndrombedingter therapierefraktärer Diarrhö

A patient with refractory diarrhoea (up to 10 l/d) following colectomy and ileostomy was treated with clonidine, after loperamide, tinctura opii, cholestyramine and somatostatin had failed to reduce stool volume to less than 6 l/d. Under combined treatment with clonidine (1200 micrograms/d) and somatostatin (6 mg/d), which was well tolerated, stool weights were normalised within 24 hours. This case report on the successful anti-diarrhoeic effect of clonidine is completed by experimental data from rat jejunal and duodenal segments. In the presence of the adenylate cyclase-stimulating agent forskolin, clonidine normalised both mucosal cAMP content and cAMP-induced hypersecretion in rat intestine. This suggests that the anti-diarrhoeic effect of clonidine in-vitro results from an alpha 2-receptor mediated inhibition of the stimulated adenylate cyclase. Case report and experimental data therefore support the theory that therapeutical application of clonidine in diarrhoea may be successful.

Influence on intestinal secretion of eicosanoids

Eicosanoids have been shown to be important modulators of intestinal secretion. In cholera, cAMP is often regarded as the sole mediator, but recent data suggest that 5-hydroxytryptamine (5-HT) and prostaglandin (PG) E2 also play important roles. Thus cholera toxin (CT) increases their release from the rat jejunum in vivo, and human cholera is associated with an increased luminal 'overflow' of PGE2. In vitro evidence of secretion can be obtained with PG concentrations 100- to 1000-fold lower than those required for activation of the adenylate cyclase. Furthermore, 5-HT induces secretion associated with increased 'overflow' of PGE2, but without a change in mucosal cAMP. CT-induced release of PGE2 and fluid secretion can be decreased by indomethacin or by the 5-HT2-receptor antagonist, ketanserin, whereas the release of 5-HT and cAMP is not affected by either substance. Secretion caused by vasoactive intestinal polypeptide (VIP) is associated with increased mucosal cAMP levels, without a change in PGE2 release, and is unaffected by indomethacin and ketanserin. These results suggest that CT stimulates the release of 5-HT, which in turn causes the release of PGE2. The latter substances probably act via a local intramural reflex and contribute to secretion by mechanisms that are independent of cAMP.

Correlation of fecal cAMP with fluid loss in cholera

Adenosine 3′,5′-cyclic monophosphate (cAMP) has been suggested to mediate in the intestinal secretory mechanism in cholera (1–3). cAMP appears to affect intestinal ion transport processes at two separate sites, namely villus cells and crypt cells (4). In villus cells, cAMP inhibits coupled NaCl exchange across the brush border and in crypt cells, cAMP stimulates active anion secretion (4). Fecal sodium excretion has been shown to increase with the rate of fluid loss as measured by fecal volume in milliliters per hour (5). Changes in fluid and electrolyte movement are associated with increased mucosal permeability in in vivo situations (6,7) and thus may allow cAMP to be excreted into the feces. The present study is intended to investigate the correlation of fecal cAMP excretion with the rate of fluid loss in cholera patients with a view to establish a probable model for interpreting intestinal secretory response without measuring mucosal cAMP.

Effects of parathyroid hormone on cyclic-AMP concentrations of in vitro Necturus maculosus gastric antrum

The effects of bovine parathyroid hormone (bPTH 1–84) on the stimulation of intracellular cyclic-AMP [cAMP] were investigated in an in vitro preparation of Necturus maculosus antral mucosa. When the antrum was exposed to 1, 5, 10, or 100 n M bPTH 1–84, there was an ∼2-fold nonlinear increase in tissue [cAMP] over basal values. The pretreatment of the antral mucosa with 1 m M isobutylmethylxanthine (IBMX, a phosphodiesterase inhibitor) increased with detectability of mucosal [cAMP]. The addition of 1, 5, 10, or 100 n M bPTH 1–84 to tissues pretreated with IBMX resulted in an ∼3.5-fold linear increase in mucosal [cAMP] over basal values. The time course of the generation of mucosal cAMP to 10 n M bPTH 1–84 resulted in a small but significant transient increase at 2.5 min after the addition of bPTH 1–84 but no change in the medium [cAMP]. In tissues pretreated with 1 m M IBMX the response to 10 n M bPTH 1–84 was a large biphasic increase of [cAMP] at 2.5 min that progressively declined to near basal values by 15 min. There was also a significant sustained increase in the [cAMP] in the bathing medium at 2.5 min of tissues pretreated with IBMX followed by 10 n M bPTH 1–84. These results suggest the presence ofan adenylate cyclase that can be activated by a mammalian bPTH 1–84 in elevating intracellular cAMP levels in the N. maculosus antral mucosa.

Sucralfate is protective against indomethacin-induced intestinal ulceration in the rat.

The therapeutic effects of sucralfate on ulcerated gastric and duodenal mucosa is well known. There is, however, very little information about its effect on the mucosa of the small intestine. We studied the possible protective effect of sucralfate against indomethacin-induced intestinal ulceration in the rat. Sucralfate was found to possess a marked protective effect on the intestinal mucosa (ulcer index 23.16 +/- 6.58 vs. 225 +/- 36.37; p less than 0.001). Sucralfate elevated basal mucosal prostaglandin E2 generation (p less than 0.001), and partially overcame the inhibition of prostaglandin E2 synthesis caused by indomethacin (p less than 0.03), but had no effect on mucosal cAMP level. The effect of sucralfate on prostaglandin E2 content might partially explain its protective effect on the intestinal mucosa.

Effects of auranofin and myochrysine on intestinal transport and morphology in the rat.

Auranofin (SKF-D 39162) is an oral gold preparation for the treatment of rheumatoid arthritis. One of its major side effects is diarrhoea. To determine one possible mechanism for this we compared the effects of auranofin and myochrysine on intestinal water and solute transport in the rat. Jejunal perfusion with 2 mM auranofin (n = 5) induced fluid and electrolyte secretion and inhibited glucose absorption (p less than 0.01). Auranofin (0.2 mM) induced fluid secretion in the jejunum (n = 5; p less than 0.01) and colon (n = 6; p less than 0.01). In contrast, 2 mM myochrysine enhanced jejunal water and electrolyte absorption (n = 6; p less than 0.02). Both compounds enhanced absorption of mannitol (p less than 0.01). Perfusion of 0.2 mM auranofin for two hours had no significant effect on mucosal c-AMP levels (n = 4). After perfusion for two hours with 2 mM auranofin the jejunal mucosa showed severe injury by light and scanning electronmicroscopy while myochrysine had no apparent effect. The damage after perfusion with 0.2 mM auranofin for two hours was less severe. Auranofin was more rapidly absorbed than myochrysine (p less than 0.05). These effects provide an explanation for the diarrhoea associated with auranofin therapy.

Site and mechanisms of action of kinins in rat ileal mucosa.

Kinin-induced secretion in the intestine is accompanied by marked increases in mucosal adenosine 3', 5'-cyclic monophosphate (cAMP) and prostanoids that undoubtedly contribute to the overall secretory response. The cellular site at which these effects are initiated is unclear although a recent hypothesis has suggested the epithelial cell as the target for kinin action (6). We have investigated the effects of kallidin on the phospholipase-prostanoid-cAMP pathway in whole ileal mucosa and in epithelial cells isolated from the same tissue in the rat. Kallidin (1 microM) stimulated a marked rise (24 to 30-fold) in PG (prostaglandin) E2 release from the serosal surface of stripped ileal mucosa within 1-2 min, which correlated closely with the rise in mucosal short-circuit current. Mucosal cAMP levels were also increased two to threefold by kallidin. However, kinins were unable to elicit effects under the same conditions in suspensions of viable epithelial cells. PGE2 release was unaffected by kallidin or bradykinin at concentrations up to 100 microM, whereas cAMP levels could be stimulated by forskolin and PGE2 but not by kinin. Studies of intestinal phospholipase A2 (PLA2) activity also suggest a nonepithelial site for kinin action. This enzyme is responsible for liberating arachidonic acid from cellular phospholipids and has been shown to be activated by kinins in several tissues. In the intestine, PLA2 activity was found to be concentrated within the subepithelium with significantly lower levels in the epithelium itself. In addition, kallidin was unable to influence phospholipid labeling (an indirect measure of PLA2 activity) in cells incubated with [14C]arachidonic acid.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibitory effect of somatostatin on cholera toxin-induced diarrhea and glycoenzyme secretion in rat intestine.

The effects of somatostatin on cholera toxin-induced secretory diarrhea and the appearance of glycoenzymes in the intestinal lumen and intestinal lymph were investigated in rat small intestine. After exposure to cholera toxin, marked fluid accumulation in the small intestinal tract and elevation of the jejunal mucosal cyclic AMP (cAMP) concentration were observed. The activity of alkaline phosphatase, aminopeptidase and sucrase increased in the intestinal lumen after toxin exposure. In intestinal lymph, alkaline phosphatase activity was increased after cholera toxin administration, while aminopeptidase activity remained unchanged. Somatostatin suppressed cholera toxin-induced secretory diarrhea, but it did not affect the elevated mucosal cAMP concentration. This peptide also inhibited the appearance of glycoenzymes in the intestinal lumen and lymph induced by cholera toxin administration. These results suggest that somatostatin exerts its inhibitory effects on cholera toxin-induced secretory diarrhea and on the appearance of glycoenzymes in the intestinal lumen and lymph by affecting processes beyond cAMP formation.

Mast cells in the regulation of intestinal electrolyte transport

Experiments were performed to determine the ionic basis and physiological messengers of transepithelial ion transport alterations (short-circuit current, Isc) measured during the induction of intestinal anaphylaxis in an Ussing chamber. Antigens derived from Trichinella spiralis, an intestinal parasite, were used to challenge jejunal tissue from guinea pigs immunized by infection with the parasite. Histamine (10(-4) M) caused an increased in Isc that was similar to that induced by antigen. Diphenhydramine (10(-5) M) inhibited the epithelial electrical responses to histamine by 100% and to antigen by 60-70%. Indomethacin (10(-5) M), in combination with diphenhydramine, completely inhibited the antigen-induced rise in Isc. Furosemide (10(-4) M) caused 50-60% inhibition of the increase in Isc induced by antigen and histamine. Antigen challenge of isolated enterocytes did not alter intracellular cAMP content. However, antigen challenge of jejunal segments in which epithelial cells were in contact with sensitized mast cells increased mucosal cAMP content. These results suggest that electrogenic Cl- secretion, mediated in part by cAMP, contributes to antigen-induced jejunal ion transport changes and that histamine and prostaglandins are involved in eliciting these epithelial responses.

Inhibition by loperamide of deoxycholic acid induced intestinal secretion.

The effect of loperamide on deoxycholic acid (DOC)-induced secretion was studied in ligated loops of the rat jejunum and colon in vivo. In controls, loperamide slightly augmented fluid absorption. 3 mmol DOC caused net fluid secretion. Loperamide reduced this secretion in the colon and reversed it to absorption in the jejunum. Na-K-ATPase specific activity and cAMP levels were measured in the jejunum, and [14C]erythritol clearance as an index of mucosal permeability in the colon. In the jejunum this opiate analogue affected neither basal or DOC-depressed Na-K-ATPase, nor mucosal cAMP. In the colon it reduced a large increase of the erythritol clearance caused by DOC. It is suggested that loperamide interferes with DOC-induced intestinal secretion in part by lowering mucosal permeability.