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PubMed1997

Novel insights into histamine H2 receptor biology.

Histamine exerts multiple biological actions through one of three receptor subtypes (H1, H2, and H3). This review focuses on new developments regarding the structure and function of the H2 receptor. In addition to the important role this receptor plays in stimulating gastric acid secretion, recent studies have demonstrated that it is also involved in regulating gastrointestinal motility and intestinal secretion. The potential role of the H2 receptor in regulating cell growth and differentiation has also been added to the list of actions this biogenic amine may exert in both normal and transformed tissues. Molecular cloning of the gene indicates that it has the structural characteristics of a heptahelical G protein-linked receptor. Site-directed mutagenesis studies of this receptor reveal the presence of key amino acids within the third and fifth transmembrane domains that are critical for ligand recognition. Molecular approaches have also shed light on the structural components of the H2 receptor important in regulating desensitization and internalization. Although the H2 receptor was classically thought to couple to the adenylate cyclase pathway, recent work with the cloned receptor indicates that it can also activate the phosphoinositide signaling cascade through an independent G protein-dependent mechanism. The novel observation that histamine may stimulate c-fos gene expression lends further support to the possible role of this receptor in regulating cell growth and differentiation.

Amino Acid Sequence↗
PubMed2000

Correlations between antibody immune responses at different mucosal effector sites are controlled by antigen type and dosage.

Monitoring specific secretory immunoglobulin A (IgA) responses in the intestines after mucosal immunization or infection is impeded by the fact that sampling of small intestinal secretions requires invasive methods not feasible for routine diagnostics. Since IgA plasma cells generated after intragastric immunization are known to populate remote mucosal sites as well, secretory IgA responses at other mucosal surfaces may correlate to those in the intestines and could serve as proxy measures for IgA secretion in the gut. To evaluate the practicability of this approach, mice were immunized intragastrically with 0.2, 2, and 20 mg of ovalbumin plus 10 microg of cholera toxin, and the antigen-specific local secretory IgA responses in duodenal, ileal, jejunal, rectal, and vaginal secretions, saliva, urine, and feces, as well as serum IgG and IgA responses were analyzed by enzyme-linked immunosorbent assay. Correlation analysis revealed significant relationships between serum IgG and IgA, urinary IgA, salivary IgA, and secretory IgA in duodenal, jejunal, ileal, and rectal secretions for the 0.2-mg but not for the 20-mg ovalbumin dose. Fecal samples were poor predictors for intestinal antiovalbumin IgA responses, and no correlations could be established for cholera toxin, neither between local anti-cholera toxin levels nor to the antiovalbumin responses. Thus, specific IgA in serum, saliva, or urine can serve as a predictor of the release of specific IgA at intestinal surfaces after intragastric immunization, but the lack of correlations for high ovalbumin doses and for cholera toxin indicates a strong dependency on antigen type and dosage for these relationships.

Animals↗
PubMed1995

Comparison of the mechanisms of action of cholera toxin and the heat-stable enterotoxins of Escherichia coli.

The mechanisms which enable cholera toxin (CT) and the Escherichia coli heat-stable enterotoxins (STa and STb) to stimulate intestinal secretion of water and electrolytes are only partially understood. CT evokes the synthesis of 3',5'-cyclic AMP (cAMP), and STa is known to elevate intestinal levels of 3',5'-cyclic GMP (cGMP). Neither of these recognized second messengers appears to mediate E. coli STb responses. We compared the secretory effects of CT, STa, and STb using the pig intestinal loop model and also measured the effects of toxin challenge on the synthesis of cAMP, cGMP, and prostaglandins (e.g., prostaglandin E2 [PGE2]), as well as on the release of 5-hydroxytryptamine (5-HT) from intestinal enterochromaffin cells. All three enterotoxins elicited fluid accumulation within a 2-h observation period. A combination of maximal doses of STa with STb yielded additive effects on fluid accumulation, which suggested different mechanisms of action for these toxins. Similarly, challenge of pig intestinal loops with a combination of CT and STb resulted in additive effects on fluid accumulation and luminal release of 5-HT. Unlike its effect on intestinal tissues from other animals, CT did not appear to elicit a dose-dependent cAMP response measurable in mucosal extracts from pig small intestine. In contrast, luminal fluid from CT-challenged pig intestinal loops contained dose-related amounts of cAMP and PGE2 that had been secreted from the mucosa. cAMP responses to STa or STb could not be demonstrated in either mucosal tissue or luminal fluid. In contrast, cGMP levels were increased in the intestinal fluid of loops challenged with STa but not in those challenged with STb. While the mechanisms of action of CT and STa are thought to involve impulse transmission via the enteric nervous system, we demonstrated significant stimulation of PGE2 synthesis and 5-HT release for CT and STb but very little for STa. We conclude from these data that the mechanisms of action of STa, STb, and CT are distinct, although the mode of action of STb may have some similarity to that of CT. Since STb stimulated the release of both PGE2 and 5-HT from the intestinal mucosa, the data suggested the potential for an effect of STb on the enteric nervous system.

Animals↗
PubMed1991

ApoA-I secretion by rabbit intestinal mucosa cell cultures.

Lipid and apolipoprotein (apo) A-I concentrations in different density fractions of New Zealand White (NZW) and Watanabe (WHHL) rabbit plasma were studied. Aside from the low plasma apoA-I and high density lipoprotein (HDL) cholesterol levels in WHHL rabbits, the distribution of apoA-I was also different between the two rabbits. ApoA-I was concentrated in both the HDL2 and HDL3 fractions of NZW rabbits but was found primarily in the HDL3 fraction of WHHL rabbits. ApoA-I secretion in these two rabbits was further studied in vitro by using intestinal and hepatocyte cell cultures. ApoA-I secretion was highest from cultures of the duodenum and the proximal end of the jejunum; whereas, cell cultures of the distal end of the small intestine secreted very little apoA-I into the medium. Intestinal cell cultures from WHHL rabbits secreted less, but significant amounts of, apoA-I compared to that of NZW rabbits. ApoA-I was most concentrated in the density range of 1.12-1.21 (HDL3) fraction in medium containing 10% fetal calf serum (FCS). Serum-free medium promoted apoA-I secretion by intestinal cell cultures that was mostly found in the d greater than 1.21 (lipoprotein-deficient) fraction. Hepatocytes isolated from the same rabbits by collagenase perfusion secreted little apoA-I, and it was found only in the d greater than 1.21 fraction. The addition of oleic acid into the culture medium with 10% FCS decreased the secretion of total apoA-I and HDL by intestinal cell cultures and increased the secretion of very low density lipoprotein (VLDL) and intermediate density lipoproteins (IDL).(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1983

Speculations on the role of motility in the pathogenesis and treatment of diarrhoea.

Comparatively little attention has been given to the role of gastrointestinal motility in the pathogenesis and treatment of diarrhoea. Here the relationship between motor activity and absorption in the small intestine has been assessed, as has that between small intestinal secretion and motility, and between colonic salvage and motility. It is suggested that diarrhoea should be considered as a disturbance of intestinal flow, involving disturbances in both motility and transport. The role of antimotility agents (particularly opiate-like agents such as loperamide) is reviewed. The most successful antidiarrhoeal agents are those that combine a reversal of epithelial secretion with an action on motility.

Animals↗
PubMed1994

Immunization with live aroA recombinant Salmonella typhimurium producing invasin inhibits intestinal translocation of Yersinia pseudotuberculosis.

The Yersinia pseudotuberculosis inv gene encodes invasin, a 103-kDa outer membrane protein that allows bacteria to enter mammalian cells. The gene was subcloned into the attenuated aroA mutant of Salmonella typhimurium SL3261. Invasin was produced by the recombinant Salmonella strain and increased the ability of microorganisms to translocate from the intestinal lumen to the mesenteric lymph nodes. Specific antibodies for invasin were detected in sera and intestinal secretions of mice following oral immunization with the live Inv+ Salmonella strain. The immunization strongly inhibited intestinal translocation of Y. pseudotuberculosis when this pathogen was inoculated to mice but failed to prevent Yersinia dissemination from the gut lymphoid tissue.

3-Phosphoshikimate 1-Carboxyvinyltransferase↗
PubMed1986

Current-noise analysis of the basolateral route for K+ ions across a K+-secreting insect midgut epithelium (Manduca sexta).

The isolated midgut of a lepidopteran larva (Manduca sexta), 5th instar was investigated with voltage-clamp and fluctuation analysis techniques. With high K+ insect saline on both sides the outward-directed short-circuit current (Isc) was carried by K+ (IK) from serosal to mucosal compartment. IK could be blocked, in a dose-dependent manner by serosal Ba2+ ions. There was no current with serosal Na+. Noise analysis of IK revealed a Lorentzian component in the power spectrum when Ba2+ was present in the serosal solution. The Ba2+/receptor kinetics show pseudo-first order characteristics only at low [Ba2+]s. For [Ba2+]s greater than KBa, the apparent Ba2+ association rate decreases with a hyperbolic course as a function of serosal [Ba2+] which could indicate some "substrate-inhibition"-like interaction of Ba2+ at its receptor site. It is concluded that the serosal membranes of the K+-secreting intestinal cells contain the common type of Ba2+-blockable K+ channel which provides the serosal pathway for K+ during secretion which is ultimately driven by the mucosally-located electrogenic K+-ATPase.

Animals↗
PubMed1990

Human duodenal mucosal bicarbonate secretion--physiological and clinical aspects.

Over the past few years, we have addressed some of the components that regulate human duodenal bicarbonate secretion (DBS) using a technique for the isolation of a 4-cm duodenal segment from gastric, pancreaticobiliary and distal intestinal secretions. Our observations are summarized as follows. Resting DBS is, to a large extent, maintained by vagal innervation; atropine decreases basal bicarbonate secretion by approximately 80%. Luminal acidification with HCI (H+) is a major physiological stimulus of DBS, resulting in a prompt and significant increase of 3- to 4-fold. H(+)-stimulated DBS is likely to be mediated, at least in part, by prostaglandins of the E class, and by VIP. Each of the latter stimulates DBS independently in a dose-related manner. Since atropine is without effect on the response induced by luminal H+ and sham feeding, in both cases DBS is probably stimulated via a non-cholinergic pathway(s). Moreover, since basal, H(+)- and PGE2-stimulated proximal DBS are unaltered when the plasma-to-lumen bicarbonate concentration gradient is abolished, it may be concluded that DBS in humans involves active transport processes. It is significant that compared to normal subjects, patients with duodenal ulcers show markedly diminished basal and H(+)-stimulated proximal DBS. Furthermore, there is surprisingly little overlap between duodenal ulcer (DU) patients and normal subjects. These findings suggest that an intrinsic cellular or subcellular defect in proximal duodenal mucosal bicarbonate secretion is present in patients with duodenal ulcer disease.

Acid-Base Equilibrium↗
PubMed1996

Lanreotide inhibits human jejunal secretion induced by prostaglandin E1 in healthy volunteers.

1. Somatostatin inhibits hormonal secretions in the gastrointestinal tract. Somatostatin analogues are used in the treatment of VIPome-related watery diarrhoea. In addition, more than 10% of patients with AIDS suffer from diarrhoea likely due to the increased intestinal secretion of water and ions. However, the direct effect of somatostatin on the flux of water and ions in the intestine has not been, so far, analyzed in vivo. The aim of the present study was to evaluate the effect of lanreotide, a somatostatin analogue, on the movements of water and ions in the jejunum in man. 2. Accordingly, 10 healthy volunteers (age 18-35 years, mean 27) and two patients with AIDS (26 and 33 years) suffering from water diarrhoea (> 800 ml day-1) underwent intestinal perfusion using a four lumen tube with proximal occluding balloon. The segment tested was 25 cm long. The jejunum was infused by an isotonic control saline solution containing polyethylene glycol (PEG) as nonabsorbable marker. Basal jejunal secretions were measured in all subjects. Prostaglandin E1 (PGE1) was administered intraluminally to stimulate jejunal secretion in healthy volunteers. The effect of intravenous lanreotide on the jejunal PGE1-induced secretions of water and electrolytes was analysed in healthy subjects and on the basal secretions in AIDS patients. Each period was analyzed on the basis of three (10 min) successive intestinal juice collections after 20-30 min equilibration time. The antisecretory effect of lanreotide was evaluated in each subject as the difference between fluxes compared to the control period. 3. In healthy volunteers, PGE1 induced secretion of H2O, Na+, K+ and Cl- in the jejunum and lanreotide reduced significantly PGE1-induced response. In both AIDS patients basal fluxes of water and ions were reduced by lanreotide in a dose-dependent manner. 4. Somatostatin can reduce stimulated-jejunal secretion of ions and water in normal subjects and may improve water diarrhoea in AIDS patients.

Acquired Immunodeficiency Syndrome↗
PubMed1985

Choice of anesthetic for intestinal absorption and secretion experiments using rats.

The net rates of glucose and water absorption from, and secretion of water into, the lumen of the rat small intestine were compared using alpha-chloralose, halothane, pentobarbitone, and urethane as anesthetics. Water and glucose transport were determined by recirculation of perfusion fluid through the lumen of the jejunum. Net fluid secretion was stimulated by intraarterial (i.a.) infusion of prostaglandin E1 (PGE1) and vasoactive intestinal peptide (VIP). The results show that the anesthetics used exert only minimal influence on the physiological rates of glucose and water absorption and there is no correlation between the small differences in glucose and water absorption among the anesthetics. However, when the fluid secretory mechanism of the mucosa is stimulated with either PGE1 or VIP, pentobarbitone allows the greatest rates of secretion, but halothane prevents secretion. It is suggested that of the anesthetics used, pentobarbitone is the most appropriate for intestinal secretion experiments.

Alprostadil↗
PubMed2007

Serotonin and its role in colonic function and in gastrointestinal disorders.

Serotonin (5-HT) is most commonly thought of as a neurotransmitter in the central nervous system. However, the predominant site of serotonin synthesis, storage, and release is the enterochromaffin cells of the intestinal mucosa. Within the intestinal mucosa, serotonin released from EC cells activates neural reflexes associated with intestinal secretion, motility, and sensation. Two important receptors for serotonin that are located in the neural circuitry of the intestines are the 5-HT(3) and 5-HT(4) receptors; these are the targets of drugs designed to treat gastrointestinal disorders. 5-HT(3) receptor antagonists are used to treat nausea and emesis associated with chemotherapy and for functional disorders associated with diarrhea. 5-HT(4) receptor agonists are used as promotility agents to promote gastric emptying and to alleviate constipation. Because of the importance of serotonin in normal gut function and sensation, a number of studies have investigated potential changes in mucosal serotonin signaling in pathologic conditions. Despite the inconsistencies in the current literature, changes in serotonin signaling have now been demonstrated in inflammatory bowel disease, irritable bowel syndrome, postinfectious irritable bowel syndrome, and idiopathic constipation. Emerging evidence has led to many contradictory theories regarding serotonin signaling and its roles in the pathology of gut disorders. This review summarizes the current medications affecting serotonin signaling and provides an overview of our current knowledge of the changes in serotonin that occur in pathologic conditions.

Animals↗
PubMed1981

Pharmacological actions of calcitonin on the gastrointestinal tract and their therapeutical implications.

The role of calcitonin (CT) in the regulation of the calcium homeostasis in humans is doubtful, while the therapeutic use in various bone diseases gains increasing interest. Numerous investigations during the last eight years have indicated that CT affects a variety of gastrointestinal organs when CT is administered in pharmacological high doses: CT inhibits gastric acid and pepsin secretion, gastrin release, pancreatic enzyme secretion as well as the hormonally stimulated contraction of the lower esophageal sphincter and of the gallbladder. CT increases intestinal secretion. The therapeutic use of CT in gastric hypersecretory states appears to be inferior and less practicable compared to the histamine-H2-receptor antagonists. The benefit of CT in the clinical course of acute pancreatitis was observed in two controlled double blind studies but CT did not lower the mortality rate. CT does not influence increases in serum-amylase and -lipase occurring after ERCP.

Acute Disease↗
PubMed1994

Effect of 5-hydroxytryptamine antagonists on cholera toxin-induced secretion in the human jejunum.

In rats, the combined administration of the 5-HT2 antagonist ketanserin and the 5-HT3 antagonist tropisetron inhibits cholera toxin-induced intestinal secretion. We investigated whether these agents and the 5-HT3 antagonist ondansetron can inhibit cholera toxin-induced secretion in the human jejunum using a segmental perfusion technique. In a first control period the subjects' jejunums were perfused continuously with a plasma-like electrolyte solution. In a second control period they either received a combination of tropisetron plus ketanserin, or tropisetron or ondansetron alone. Cholera toxin 6.25 micrograms was then administered intrajejunally and the experiments were continued for 4 h. Net water movements during the 4th hour after CT administration minus net water movement during the first control period was used for further calculation and was referred to as net luminal gain. In perfusion studies with tropisetron plus ketanserin resp. ondansetron the net luminal gain of water (+ 161 +/- 26 resp. 189 +/- 28 ml 30 cm-1 h-1, mean +/- SEM) was significantly higher compared to perfusion studies with cholera toxin alone (+ 94 +/- 30). Treatment with tropisetron did not change the CT-induced net luminal gain of water (+ 108 +/- 41). Movements of sodium, chloride, bicarbonate and potassium paralleled the movement of water. In agreement with these observations we found a deterioration of clinical parameters after the end of the perfusion studies in four of five subjects treated with CT 25 micrograms plus ketanserin and tropisetron.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1994

Palliation of malignant intestinal obstruction using octreotide.

Vomiting due to malignant intestinal obstruction is an unpleasant terminal event in many cancer patients, which responds poorly to conventional therapies. Somatostatin and its long-acting analogues reduce intestinal secretion. For this reason, octreotide was used in a phase I/II study of patients with intractable vomiting secondary to intestinal obstruction due to malignant disease. Vomiting was controlled or the volume of nasogastric aspirate was markedly reduced in 18 of 24 (75%) patients receiving a subcutaneous infusion of octreotide (median initial dose 300, range 100-600 micrograms/day) for a median of 9.4 (range 1-38) days. A further 2 patients had partial relief of their symptoms. Octreotide is an effective treatment of nausea and vomiting due to malignant bowel obstruction.

Adult↗
PubMed2000

Intestinal transport during fasting and malnutrition.

Fasting or malnutrition (FM) has dramatic effects on small intestinal mucosal structure and transport function. Intestinal secretion of ions and fluid is increased by FM both under basal conditions and in response to secretory agonists. Intestinal permeability to ions and macromolecules may also be elevated by FM, which increases the potential for fluid and electrolyte losses and for anaphylactic responses to luminal antigens. Mucosal atrophy induced by FM reduces total intestinal absorption of nutrients, but nutrient absorption normalized to mucosal mass may actually be enhanced by a variety of mechanisms, including increased transporter gene expression, electrochemical gradients, and ratio of mature to immature cells. These observations underscore the value of enteral feeding during health and disease.

Biological Transport↗
PubMed1981

Study of immunisation regimes for the stimulation of local immunity in the pig intestine.

Experiments are described which demonstrate that intraperitoneal injection of a soluble protein antigen (ovalbumin) in Freund's complete adjuvant (FCA) followed 14 days later by seven days of repeated infusions of an ovalbumin/DEAE-dextran mixture resulted in the appearance of IgA-specific antiovalbumin-containing cells (AOCC) in the intestinal lamina propria and IgA associated specific antibody in intestinal secretion of pigs. Neither intraperitoneal injection alone nor infusion of the antigen mixture alone resulted in a significant local intestinal immune response. In intraperitoneal primed animals the repeated infusion of ovalbumin without DEAE-dextran or a single intraduodenal injection of ovalbumin both resulted in a similar AOCC response in the intestine but, in the absence of DEAE-dextran, IgA was not the predominant class specificity of AOCC. Evidence is also presented for a depression of the intestinal immune response when intraperitoneal immunisation was performed using a large dose of antigen in FCA.

Animals↗
PubMed1976

Alterations in levels and interrelations of plasma apolipoproteins induced by diet.

Diets high in carbohydrates increase the rates of hepatic secretion of VLDL in man. VLDL particles are secreted in greater numbers; they also tend to contain more triglycerides and to be larger in size. Comparable changes in intestinal secretion of chylomicrons and VLDL accompany fat absorption. We studied the metabolism of the ApoC apoproteins under these circumstances because in vitro studies suggest that these apoproteins may play important roles in the catabolism of VLDL. ApoA-I and ApoB were also examined because of their importance in HDL and VLDL structure, respectively. The fasting plasmas of 16 normolipemic young adults were examined before and after they had been fed high carbohydrate diets for 4-5 days. ApoA-I and ApoB were measured by radioimmunoassay. The relative proportions of ApoC-II and ApoC-III in VLDL were determined by disc gel electrophoresis. Lipids in plasma and in individual lipoproteins were determined chemically. Lipoproteins were isolated by ultracentrifugal and precipitation methods. VLDL-TG and VLDL-chol and VLDL-protein rose by factors of 2.4, 1.67, and 1.88, respectively. (Final value divided by initial value.) Since TG rose more than the others, VLDL became enriched by TG. LDL-chol fell by a factor of 0.78 while LDL-TG and LDL-ApoB remained constant, i.e., LDL became relatively enriched in TG and ApoB. HDL-TG rose by 1.42 and HDL-chol fell by 0.74, i.e., HDL too became TG enriched. Plasma Apo-A-I fell by 0.84. Thus, there were alterations in both the levels and compositions of all lipoproteins. The proportion of ApoC-II relative to ApoC-III in VLDL increased in each subject on the carbohydrate diet (mean rise was 1.55-fold). The increases in ApoC were detectable after as little as 48 hr of diet, and the same results were obtained in two people who were each tested twice on two separate occasions several months apart. Thus, the changes in ApoC were reproducible. Proportions of ApoC-II were greater in SF greater than 400 than in SF 20-400 subfractions of VLDL on ad lib diets. Proportions of ApoC-II rose in both density subfractions on carbohydrate diets. The changes in both subfractions suggest that rises of ApoC-II in total VLDL (SF greater than 20) fraction were not due solely to the accumulation of SF greater than 400 particles of unaltered ApoC composition in postdiet plasma. The changes following carbohydrate could have been due to secretion of altered lipoproteins or due to diet induced alterations in apoprotein catabolism. Three subjects were given 150 gm of corn oil to drink. Lipoproteins were analyzed, as above, at 0 and 3-12 hr after the drink. In spite of several-fold rises in the SF greater than 20 TG and proteins, the relative proportions of ApoC-II and ApoC-III remained constant. Thus, an acute dietary fat load and 2-4 days of carbohydrate diets provoked different responses. The fluctuations in the levels and relative proportions of ApoC support the notion that the ApoC group may have importance in vivo in the metabolism of VLDL.

Adolescent↗