Effects of prostaglandin E2, 16,16-dimethyl prostaglandin E2 and a prostaglandin endoperoxide analogue (U-46619) on gastric secretory volume, [H+] and mucus synthesis and secretion in the rat

Protective Effects of Genipin on Gastrointestinal Disorders

Genipin, an aglycone of geniposide, is a major component of gardeniae fructus, and has been used to treat jaundice, various inflammatory disorders, and liver disease, and has also been used as a natural cross-linking agent. The authors conducted several experiments to evaluate the protective effects of genipin on gastrointestinal disorders, such as, gastritis and gastric ulcers. Genipin showed inhibitory effects against HCl·ethanol-induced acute gastritis and indomethacin-induced gastric ulcers in rats and increased prostaglandin E₂ (PGE₂) in AGS gastric cancer cell. Genipin had significant effects on aggressive factors, acid-neutralization, and gastric secretion, and inhibited H⁺/K⁺-ATPase (a proton pump), which secretes gastric acid. The results obtained indicate that genipin has significant gastroprotective effects and might be useful for treating and preventing gastric lesions.

Intestinal epithelial function: the case for immunophysiological regulation. Implications for disease (2)

Substantial amounts of data have been reported showing a role for immunomodulation of epithelial function (particularly ion secretion and permeability) using animal models of anaphylactic reactions. In part one of this review we outlined the main immune cell types and mediators/cytokines that are currently known to influence epithelial physiology either directly, or indirectly via an intermediate cell type. Here we will expand on the significance of these studies and show how antigenic activation of the mucosal immune system can evoke changes in epithelial function that may be beneficial to the host by mediating loss/inactivation of the antigen. However, a continued and inappropriate immune stimulation can lead to pathophysiological reactions and disease. Thus, we will present data on immune regulation of epithelial function with direct applicability to understanding the mechanism underlying human intestinal inflammatory and secretory disease. Finally, we highlight key strategic points in the cascade of immune events that can control epithelial function and thus may be of relevance in the formulation of new therapeutic approaches to intestinal inflammation.

The effect of 16,16-dimethyl prostaglandin E2 on proliferation of an intestinal goblet cell line and its synthesis and secretion of mucin glycoproteins

The effect of 16,16'-dimethyl prostaglandin E2 (dmPGE2) on the human colonic adenocarcinoma derived mucus-secreting goblet cell line HT29-18N2 was investigated. The proliferation rate of HT29-18N2 was increased by exposure to 10 or 100 microM dmPGE2. Exposure to 10 or 100 microM dmPGE2 caused a significant decrease in the rate of radiolabeled glucosamine incorporation into newly synthesized glycoproteins during an 8 or 24 h exposure. At concentrations as low as 1 microM, dmPGE2 accelerated the secretion of mucin glycoproteins as assessed by the release of newly synthesized radiolabeled glycoproteins, a mucin-specific enzyme-linked immunoassay and a whole-mount immunofluorescence assay. A 1 h exposure to dmPGE2 did not, however, result in a morphometrically detectable decrease in intracellular mucous granule stores or elicit any other readily detectable morphological change. The experimental results suggest elevated levels of PGs may contribute to the previously recognized decreases in intracellular mucin stores and shifts in the types of mucins species present at sites of mucosal inflammation in ulcerative colitis patients.

A new method of separation and quantitation of mucus glycoprotein in rat gastric mucus gel layer and its application to mucus secretion induced by 16,16-dimethyl PGE2

A method was established for recovering the mucus gel layer of rat gastric mucosa without damage to underlying surface epithelium. The mucus gel was solubilized by stirring the gastric mucosa in a solution of N-acetylcysteine (NAC), a mucolytic agent. Optimal mucus gel solubilization was possible by treatment with 2% NAC for 5 minutes at room temperature. Mucus glycoprotein was quantitatively extracted and measured from the mucus gel sample obtained by the NAC treatment. This treatment caused no damage to surface epithelial cells, as observed by a light microscope. Besides NAC, pronase solution was also adequate for solubilizing the mucus gel layer without any damage to the surface epithelium. However, extraction and measurement of mucus glycoprotein from the pronase-treated mucus gel sample was not possible due to contamination by high molecular hexose-containing substances which were eluted along with the mucus glycoprotein from the column of Bio-Gel A-1.5m. This NAC method was used to examine changes in mucus glycoprotein content in the mucus gel at one hour following the oral administration of 16,16-dimethyl prostaglandin E2. A significant increase in mucus glycoprotein of the gel was brought about by the prostaglandin treatment. Thus, the present method was suitable for estimating the amount of mucus secreted in to the mucus gel layer.

Enhancement of murine susceptibility to oral lactate dehydrogenase-elevating virus infection by non-steroidal anti-inflammatory agents, and antagonism by misoprostol

The murine lactate dehydrogenase-elevating virus (LDV) was used to study the effects of prostaglandin-acting agents on mucosal resistance to virus infection. Mice treated with non-steroidal anti-inflammatory drugs (NSAIDs) prior to oral exposure to LDV demonstrated a reduction in the mucosal barrier to LDV infection. Histological studies indicated that these NSAID effects were not a result of gross or microscopic tissue damage. The effects of two NSAIDs, indomethacin and diclofenac, were inhibited by co-treatment of mice with misoprostol, a synthetic PGE1 analog. The ability of misoprostol to modulate NSAID effects was not due to direct antiviral activity or to actions on LDV-permissive macrophages. These results show that the mammalian mucosal barrier to virus infection is prostaglandin-sensitive, and provide a model for the study of resistance to viral infection.

Effects of PGE2, carbenoxolone, cholecystokinin, and the thromboxane-mimetic U46619 on protein and glycoprotein production by isolated pig gastric mucosal cells

We studied the effects of prostaglandin (PG) E2, carbenoxolone, cholecystokinin-octapeptide (CCK-OP), and the thromboxane-mimetic U46619, all known to stimulate gastric mucus secretion in vivo, on protein and glycoprotein synthesis in and release from isolated and enriched pig gastric mucous cells, as measured by the incorporation of [3H]L-leucine and N-acetyl-[14C]D-glucosamine respectively into cellular and released acid insoluble material. PGE2 stimulated glycoprotein and protein synthesis (EC50 7 and 30 nmol/L, respectively) and release (EC50 50 and 140 nmol/L, respectively) in a concentration-dependent manner, whereas carbenoxolone, CCK-OP and U46619 failed to enhance the incorporation of the tracers. We conclude that stimulation of mucus secretion by PGE2 is related to direct effects on protein and glycoprotein production of gastric non-parietal cells, whereas indirect effects may be involved in the stimulation by carbenoxolone, CCK-OP, and U46619.

Effective use of rabbit gastric antral mucosal slices in prostaglandin synthesis and metabolism studies

Intact slice preparations of rabbit stomach (antral mucosa, corporal mucosa, antral muscle and corporal muscle) were incubated and the released prostaglandins (PGs) were measured by reverse-phase high-performance liquid chromatography using 9-anthryldiazomethane for derivatization. With respect to total PG production, the highest amounts were generated by antral mucosal slices. Antral mucosal slices produced PGE2, 6-keto PGF1 alpha, thromboxane B2, PGF2 alpha and PGD2 (in descending order of magnitude) and possessed a high capacity for producing 13,14-dihydro-15-keto derivatives of both PGE2 and PGF2 alpha. Studies utilizing aspirin, EGTA or Ca2+ revealed that PG release by antral mucosal slices in the present in vitro system reflects a composite of the activities of phospholipase A3, PG cyclooxygenase and PG-metabolizing enzymes. These results show that antral mucosal slices will be useful in physiological and pharmacological studies on PG synthesis and metabolism of the stomach.

Characterization of the prostanoid receptor profile of enprostil and isomers in smooth muscle and platelets in vitro

1. Enprostil is composed, in approximately equal proportions, of 4 allenic isomers which are prostanoids structurally related to prostaglandin E2 (PGE2). The isomers are denoted as RS-86505-007, RS-86812-007 which are in the 'natural' R and S configuration (with respect to PGE2) and RS-86505-008 and RS-86812-008 which are in the 'unnatural' R and S configuration. In the present study we have characterized their activity at prostanoid receptors, in vitro. 2. Enprostil acted as a highly potent (-log EC50 = 8.30 +/- 0.08; mean +/- s.e.mean, n = 6) EP3 receptor agonist in the guinea-pig vas deferens, although no activity was observed at guinea-pig tracheal EP2 receptors at concentrations up to and including 10 microM. Attempts to study the action of enprostil at EP1 receptors were complicated by a general increase in the spontaneous activity of the guinea-pig isolated ileum. This response was stereospecific (i.e. observed, with the 'natural' R and S isomers only) and was not mediated through EP1, FP or TP receptors. 3. Enprostil also exhibited a potent agonist effect at FP and TP receptors in the rat colon and guinea-pig aorta (-log EC50 values = 7.34 +/- 0.11 and 6.54 +/- 0.07, mean +/- s.e. mean, n = 4-8 respectively). No activity at concentrations up to and including 10 microM was observed at DP or IP receptors in the guinea-pig platelet mediating inhibition of ADP-induced aggregation. 4. A similar profile was observed with the 'natural' R and S allenic isomers of enprostil (RS-86505- 007 and RS-86812-007, respectively); RS-86505-007 was between 4 and 10 fold more potent than the racemic enprostil. The 'unnatural' allenic R and S isomers of enprostil were much less potent than enprostil, with the latter being virtually inactive. 5. Enprostil and the 'natural' R and S isomers, therefore, were EP3, FP and TP agonists, being most potent at the EP3 receptor. The preferred configurations for these receptors appears to be the R, and to a lesser extent the S, form of the natural allenic isomer. The effect of enprostil at EP1 receptors was not characterized in view of the presence of excitatory EP3 receptors in the guineapig ileum. These data were in accordance with the pharmacological activity of enprostil, including inhibition of gastric acid secretion (possibly EP3) and diaorrhea (possibly TP).

Effects of endogenous and exogenous prostaglandins on glycoprotein synthesis and secretion in isolated rabbit gastric mucosa

We studied gastric glycoprotein synthesis and secretion in organ culture before and during cyclooxygenase inhibition and replacement with exogenous prostaglandins (16,16-dimethyl prostaglandin E2 and prostaglandin F2 alpha). Isolated rabbit antral and fundic mucosal explants incorporated [14C]N-acetylglucosamine and [3H]leucine in a linear fashion and steadily secreted labeled proteins and glycoproteins during the 24-h incubation period. On sepharose 4B, greater than 90% of the secreted protein-bound [14C]N-acetylglucosamine was found in the high molecular weight peak. Incorporation of tracer was not influenced by cyclooxygenase inhibition with indomethacin or the addition of exogenous prostaglandins. Secretion of newly formed glycoprotein, however, was significantly inhibited by indomethacin and stimulated by both tested prostaglandins in a concentration-dependent manner. 16,16-Dimethyl prostaglandin E2 caused significant stimulation in concentrations that are well in the physiologic range for endogenous prostaglandin E2, whereas prostaglandin F2 alpha stimulated in 100 times higher concentrations. We conclude that in the isolated gastric mucosa both endogenous and exogenous prostaglandins stimulate mucus secretion. For prostaglandin E2, but not prostaglandin F2 alpha, a role in the physiologic regulation of gastric mucus secretion is probable.

U46619 and carbocyclic thromboxane A2-induced increases in tracheal mucous gel layer thickness

U46619 or carbocyclic thromboxane A2 (CTA2) administered intravenously (IV) to rats produced dose-related increases of tracheal mucous gel layer thickness. Significant gel thickening was observed at doses ranging from 3.5 pg to 35 ng and from 500 pg to 50 ng for U46619 and CTA2, respectively. Intravenous treatment with pinane thromboxane A2 (PTA2), a thromboxane antagonist, prior to injection of U46619 or CTA2 attenuated the mucous gel layer response. The effect of PTA2 on U46619 and CTA2 was dose-dependent over the dosage range of PTA2 tested (1.0-31.6 micrograms/kg). PTA2 was equiactive against U46619 and CTA2 stimulated increases in gel thickness (ED50's 6.64 and 6.43 micrograms/kg respectively) suggesting a similar site of action for U46619 and CTA2. Slow reacting substance (SRS) injected IV into rats stimulated mucous gel layer thickening that was also inhibited by pretreatment with PTA2. These findings lend further support for the role of thromboxane in pathophysiologic conditions in which bronchorrhea contributes to the symptomatology.

Dose-response, meal-stimulated gastric antisecretory study of prostaglandin E1 analog, misoprostol, in man

In a randomized, double-blind, crossover trial, the effects of 50-, 100-, and 200-micrograms doses of misoprostol on meal-stimulated gastric acid secretion were compared with placebo in 16 healthy male subjects. Compared with placebo, the 100- and 200-microgram doses produced significant reductions in acid output for 2 and 3 hr, respectively, following the test meal (P = 0.05). Misoprostol did not influence either the fasting or postprandial serum gastrin levels as compared with placebo. No adverse experiences were reported by any of the subjects. One subject experienced a transient rise in SGPT as compared with baseline, which may have been due to ethanol intake. This study provides a scientific rationale on which to base additional trials of misoprostol in patients with disease related to gastric acid production.

Close-arterial administration of the thromboxane mimetic U-46619 induces damage to the rat gastric mucosa

The pro-ulcerogenic actions of the thromboxane mimetic, U-46619 on the rat gastric mucosa have been investigated, utilizing a novel technique which allows administration directly into the left gastric artery. Local intra-arterial infusion of U-46619 (100-500 ng/kg/min for 10 min) induced dose-dependent macroscopic damage in both the corpus and antral regions, characterized as vasocongestion, disruption and haemorrhage, with deep penetrating ulcers in the antral mucosa. Vascular congestion, epithelial cell and glandular disruption was observed histologically in both corpus and antral regions. Local intra-arterial infusion of lower doses of U-46619 (25-100 ng/kg/min) significantly disrupted the mucosa in the presence of 10% ethanol in a concentration which itself did not induce macroscopic damage. The damaging actions of U-46619 were substantially reduced by pretreatment with the thromboxane-receptor antagonist, BM 13,177 (5mg/kg i.v.) or 16,16-dimethyl PGE2 (5 micrograms/kg s.c.). These findings support the role of endogenous thromboxane A2 as a local mediator of gastric injury.

Thromboxane receptors can modulate gastric acid secretion in the rat

The effects of PGE2 and the thromboxane A2 mimetic, U-46619, have been investigated on gastric secretion in the rat isolated gastric mucosa. Both compounds produced concentration-related inhibitions of histamine-induced secretion whereas only U-46619 inhibited methacholine-stimulated and basal secretion, and neither compound had any effect on the secretory response to dbcAMP. Indomethacin had no effect on the antisecretory activity of PGE2 but markedly reduced the potency of U-46619 suggesting that endogenous prostaglandins play a role in the U-46619 responses. However, direct inhibitory effects of U-46619 were seen at high concentrations. The thromboxane receptor antagonist AH23848, at concentrations selective for thromboxane receptors, had no effect on responses to PGE2 but markedly inhibited the effects of U-46619. We conclude that the antisecretory profile of U-46619 differs from that of PGE2. U-46619 has both direct and indirect antisecretory effects and these are mediated via thromboxane receptors in the rat gastric mucosa.

The effect of prostaglandins on gastric parietal and non-parietal secretion in the anaesthetized rat

The effects of prostaglandin E2 (PGE2), 16,16-dimethyl prostaglandin E2 (dmPGE2) and prostaglandin F2 alpha (PGF2 alpha) on gastric secretion have been examined in the anaesthetized rat. The prostaglandins stimulated the secretion of a non-parietal juice which was rich in Na+ and Cl- with smaller amounts of K+ and HCO3-. Doses of the prostaglandins that stimulated gastric non-parietal secretion also inhibited histamine stimulated gastric acid secretion, and thus the same rank order of potency was obtained for the prostaglandins on the two secretory mechanisms, viz. dmPGE2 greater than PGE2 greater than PGF2 alpha. During the secretion of endogenous gastric acid the secretagogue effect of the prostaglandins on non-parietal secretion was diminished, and this effect appeared to be due to the presence of intraluminal acid since it was mimicked by application of exogenous HCl to the gastric mucosal surface. Thus, the present results show that prostaglandins affect both the parietal and non-parietal secretions in the rat stomach and that these mechanisms show some interdependence.

Antisecretory and mucosal protective actions of misoprostol. Potential role in the treatment of peptic ulcer disease

Misoprostol, a synthetic methyl ester analogue of prostaglandin E1, inhibits basal, nocturnal, and stimulated gastric acid secretion. In doses of 400 to 1,200 micrograms daily, misoprostol accelerates the healing of duodenal and gastric ulcers in humans. In addition to its antisecretory actions, misoprostol has gastroduodenal mucosal protective (cytoprotective) effects in animals and in humans. In humans, these cytoprotective actions have been demonstrated in acid-dependent studies using non-antisecretory doses and in acid-independent studies using antisecretory doses. Patients with peptic ulcer disease may have a relative deficiency of mucosal prostaglandin synthesis as compared with nonulcer control subjects. In addition, patients who consume nonsteroidal anti-inflammatory drugs and those who are cigarette smokers may also have depressed gastric mucosal prostaglandin synthesis. There is some evidence that misoprostol reverses the deleterious effect of smoking on duodenal ulcer healing and that it is effective in treating and preventing mucosal damage induced by nonsteroidal anti-inflammatory drugs and alcohol.

The effects of the stable thromboxane A2-mimetic, U46619, on gastric mucosal damage and gastric non-parietal secretion in the rat

The effects of the thromboxane A2-mimetic, U46619, and the thromboxane receptor antagonist, AH23848, on ethanol-induced gastric mucosal damage and gastric non-parietal secretion have been examined in the rat. Oral dosing with U46619 or AH23848 produced a dose-related inhibition of ethanol-induced gastric mucosal damage in the conscious rat, and these effects were partially blocked by indomethacin treatment. Intragastric application of U46619 or AH23848 to the stomach of the anaesthetized rat stimulated the gastric secretion of a juice which consisted principally of Na+ and Cl- ions. These secretagogue effects of both compounds were blocked by indomethacin treatment. These results show that U46619 and AH23848 induce secretory and protective effects in the stomach of the rat, although these responses probably do not involve thromboxane receptors and are mediated, at least in part, by endogenous prostaglandins. The results are discussed in relation to the role of endogenous thromboxane A2 in gastric mucosal protection, and of the possible protective function of non-parietal secretion in the stomach.

Prostaglandin synthesis along the gastrointestinal tract of the rabbit: differences in total synthesis and profile

In the present study we systematically investigated the synthesis of prostaglandins in the mucosa and the muscle layer along the length of the rabbit gut. Homogenates of mucosa and muscle layer were incubated with (14C)-labelled arachidonic acid, and prostaglandin formation was determined using thin-layer chromatography. With respect to total prostaglandin synthesis the highest values in the mucosa were measured in fundus, antrum and colon, whereas the prostaglandin synthesis in the muscle layer was maximal in the small bowel, particularly the ileum. In the mucosa, the prostaglandins E2 and F2a predominated, and there were minor differences along the gastrointestinal tract. In the muscle layer of the stomach, high amounts of 6-keto prostaglandin F1a, the stable degradation product of prostacyclin were produced, while small and large bowel homogenates synthesized mostly F2a. Consistently the prostaglandins A2/B2 were a major product in most locations. In addition, PG E2 catabolism to 15-keto PG E2 and 13,14-dihydro-15-keto PG E2 in the absence of NAD was slow. No significant changes in total prostaglandin synthesis and prostaglandin profile were detected between 24 hrs fasted and normally fed rabbits at any part of the gastrointestinal tract.

Stimulatory effect of enprostil, an anti-ulcer prostaglandin, on gastric mucus secretion

Enprostil, a synthetic analogue of prostaglandin E2, is known to be a potent inhibitor of gastric acid secretion, and has marked anti-ulcer activity in rodents. Enprostil was administered in doses ranging from 15 to 250 micrograms/kg to rats prepared using the Shay procedure. Three hours later, the rats' stomachs were removed and processed either for the chemical determination of mucus, or for scanning electron microscopy. For the chemical determination, the secreted gastric juice was removed and the adherent gastric mucus was eluted with 2 M sodium chloride. The anthrone method was used to determine the mucus present. Enprostil was found to significantly increase gastric mucus at a dose of 60 micrograms/kg when measured by the anthrone test. Enprostil administered by the oral route was most effective in stimulating mucus secretion, suggesting a local or topical action of enprostil on mucus-secreting cells. Scanning electron microscopy of rat fundic mucosa after enprostil administration (50 to 100 micrograms/kg) revealed the presence of thin veil-like layers covering the epithelial surface, which was interpreted as an increase in mucus secretion. Higher magnifications (X 2,000) clearly showed the layers of mucus covering the surface epithelial cells. Enprostil's apparent increase of gastric mucus secretion may contribute to its anti-ulcer activity and may promote gastric healing.

Effects of misoprostol on gastric acid and mucus secretion in man

Misoprostol, a synthetic prostaglandin E1 analog, at doses of 200, 400, or 800 micrograms, and placebo were administered orally in random fashion to eight healthy male volunteers. The effects on basal and pentagastrin (0.6 microgram/kg/hr)-stimulated acid and mucus (N-acetylneuraminic acid measurement) secretion were then determined. Misoprostol in 200-, 400-, and 800-micrograms doses reduced basal acid secretion by 91%, 93%, and 93%, respectively. Mean 2-hr acid secretion was reduced by 27%, 33% (P less than 0.01), and 51% (P less than 0.01), respectively. Reductions in secretory volumes paralleled acid changes. Mucus secretion increased by 37%, 82%, and 95% during the basal period following misoprostol doses of 200, 400, and 800 micrograms, respectively. Increase in mucus of 27%, 31%, and 38% was observed during maximal acid inhibition (1-30 min) by misoprostol in 200-, 400-, and 800-micrograms doses. The concentration of gastric juice mucus was significantly increased. Subjects experienced no significant side effects during the study, and there were no significant changes in hematological or chemical blood studies. Misoprostol, a potent inhibitor of gastric acid secretion, also stimulates mucus secretion. This mucogenic effect may be important in the mucosal protective action of misoprostol and its antiulcer efficacy in man.

Therapeutic aspects of prostaglandins in the treatment of peptic ulcer disease

Studies of antisecretory compounds such as the H2-receptor antagonists have altered therapy and enhanced the understanding of peptic ulcer disease (PUD). While it is agreed that the dictum "no acid, no ulcer" is valid, acid hypersecretion does not appear to be the major determinant in a significant number of ulcer patients. More careful consideration of mucosal integrity in the pathogenesis of PUD is therefore necessary. A small but significant number of patients (5-15%) do not heal, despite the use of effective antiulcer drugs. Moreover, the posthealing recurrence rate may reach 75% after treatment is discontinued, and nearly one third of cigarette-smoking patients on maintenance therapy may suffer recurrences. Prostaglandins (PGs) are particularly important as potent antisecretory and effective antiulcer agents. In addition, recognition of their cytoprotective effects has stimulated research into the understanding and importance of mucosal protection and mucosal defense mechanisms. Animal studies show that PGs at nonantisecretory dosages prevent the development of gastric ulcers caused by virtually any insult. In humans, PGs prevent the mucosal damage caused by aspirin and ethanol. In some studies, the maintenance of normal mucosal integrity has been linked to normal mucosal production of PGs. Therefore, it is possible that exogenous PGs may be effective in patients whose ulcers do not heal with conventional therapy. They may reduce the recurrence of ulcers, particularly in those patients whose defect in mucosal integrity appears to be the major problem. Furthermore, in those patients subjected to the toxic effects of alcohol ingestion, nonsteroidal antiinflammatory drugs, antineoplastic drugs, and stress, exogenous PGs may prevent mucosal lesions. As such, PGs could be the ideal antiulcer drug.