Effects of oral administration of PGE2 on gastric secretion and experimental peptic ulcerations

Inhibition of gastric mucosal damage by methylglyoxal pretreatment in rats

The effect of methylglyoxal pretreatment on gastric mucosal injuries caused by 80% ethanol, 25% NaCl and 0.2 M NaOH, was investigated in rats. The effects caused by pylorous ligation accumulated gastric acid secretions and ethanol-induced changes in gastric mucus secretions, levels of proteins, nucleic acid, malondialdehyde (MDA) and non-protein sulfhydryl groups were also investigated. Methylglyoxal pretreatment at oral doses of 50, 100 and 200 mg/kg body weight was found to provide a dose-dependent protection against the ulcerogenic effects of different necrotizing agents used. With the same dose regimen methylglyoxal offered significant protection against ethanol-induced damage on the parameters evaluated for histopathology. Furthermore, the pretreatment afforded a dose-dependent inhibition of pylorous ligated accumulation of gastric acid secretions and ethanol-induced depletion of stomach wall mucus, proteins, nucleic acids, NP-SH contents and an increase in the MDA levels in gastric tissue. The protective effect of methylglyoxal against ethanol-induced damage to the gastric wall mucosa may be mediated through its effect on mucous production, proteins, nucleic acids, NP-SH groups and its free-radical scavenging property under the influence of polyamines stimulated by ornithine decarboxylase activity (ODC).

Antisecretory and cytoprotective doses of enprostil do not alter gastric mucosal blood flow

Prostaglandins of the E series are antisecretory and cytoprotective. Cytoprotection occurs in the deep but not superficial gastric mucosa. It has been hypothesized that the mechanism of cytoprotection involves increased gastric mucosal blood flow (GMBF). However, basal and stimulated gastric mucosal blood flow is greater in the deep than superficial corpus mucosa. The purpose of this study was to investigate the effect of a prostaglandin E2 analog, enprostil, on GMBF in the deep mucosa, where cytoprotection is observed, in both antisecretory and cytoprotective doses. Gastric mucosal blood flow in the deep half of the mucosa was measured by the hydrogen gas clearance method before, during and after intragastric perfusion of enprostil, 0.1-100 micrograms/kg, in urethane anesthetized rats. Enprostil did not alter GMBF in any of the doses tested. Therefore, the cytoprotective action of enprostil is mediated by factors other than a primary increase in GMBF.

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.

Overview of gastroduodenal mucosal protection

The ability of the gastric mucosa to resist autodigestion has been recognized for over 200 years. Since these early observations, several components of gastroduodenal defense against injury from damaging luminal contents have been identified. The first line of defense is the thick layer of mucus gel into which bicarbonate is secreted by the underlying epithelial cells. The "mucus-bicarbonate" barrier sustains a pH gradient between the lumen and cell surface such that epithelial cells are maintained at pH 7 to 8, despite the presence of intraluminal acid. The epithelial cells form a second line of defense; since the pH gradient may be overwhelmed by physiologic concentrations of intraluminal acid, this mechanism may be important in maintaining mucosal integrity. The physical properties of the apical cell membrane and intercellular junctions and the presence of surface-active phospholipids on the membrane may be responsible for preventing hydrogen ions (H+) from diffusing into the mucosa by providing a physical barrier to their movement. Furthermore, epithelial cells are capable of rapid turnover and migration and may breach a defect in the epithelium within hours. The aftermath of mucosal damage may generate a further defense mechanism: a thick layer of mucus containing sloughed epithelial cells together with passive movement of bicarbonate-rich fluid from the damaged mucosa. This may prevent exposure of undamaged cell nests to acid and thus aid re-epithelialization. Finally, mucosal blood flow plays a vital role in maintaining epithelial integrity. Studies have shown that increasing or decreasing mucosal blood flow will, respectively, reduce or enhance susceptibility to damage. Although the precise physiologic control mechanisms for mucosal protection have not been defined, there is evidence that local endogenous prostaglandin metabolism may play an important role [4]. The release of neurotransmitters and hormones may also contribute to or modulate the defense mechanisms.

Esophageal mucosal prostaglandin E2 levels in health and in gastroesophageal reflux disease

In vivo prostaglandin E2 (PGE2) levels were measured in esophageal mucosa excised from 9 normal subjects, 11 patients with gastroesophageal reflux without esophagitis (GER) and 8 patients with reflux esophagitis (RE). Severity of GER was quantified by postcibal pH monitoring. A manometric study was also performed. No difference was found in PGE2 levels between healthy mucosa in controls (41.7 +/- 9.3 ng/g of wet tissue, at 15 cm above the lower esophageal sphincter (LES)) and healthy mucosa in GER (37.8 +/- 11.2 ng/g) or in RE (34.3 +/- 9.0 ng/l). However, PGE2 levels were significantly enhanced within the inflammatory mucosa in RE (290.4 +/- 45.7 ng/g). No difference was found in basal LES pressure between the 3 groups. These results suggest that PGE2 in the esophagus may be involved in pathogenesis of inflammation. Therefore PGE2 might not have the same cytoprotective function as in stomach or duodenum. No correlation was found between PGE2 levels in the esophagitis lesion or basal LES pressure. These data are not consistent with a possible relationship between LES pressure and the PGE2 content of the distal esophagus.

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.

Increased ex vivo synthesis of prostaglandin E2 by gastric tissue after hemorrhage in rats

Endogenously synthesized prostaglandins are potential mediators of gastrointestinal mucosal protection. Some data suggest that gastric ulceration caused by stressful stimuli is due to diminished mucosal synthesis of prostaglandins. To examine this hypothesis, we determined the effect of hemorrhage, an ulcerogenic stimulus, on ex vivo production of immunoreactive prostaglandin E2 by gastric tissue in the rat. Macroscopic gastric ulcers were reproducibly observed in Sprague-Dawley rats subjected to hemorrhage (3 ml/100 g body weight). The number of ulcers was linearly related to the duration of shock. Prostaglandin E2 synthesis was significantly increased during in vitro incubation of oxyntic and nonoxyntic stomach tissue excised from rats subjected to hemorrhage for 30 minutes (p less than 0.05). These results indicate that damage to the gastric mucosa in rats subjected to hemorrhage occurs despite augmented endogenous secretion of prostaglandin E2. Mechanisms other than impaired prostaglandin biosynthesis were probably responsible for mucosal injury in this model.

MDL-646, a new synthetic E1-prostaglandin with local protective effects on the gastric mucosa

The gastric protection, diarrheogenic and arterial hypotensive effects of MDL-646, a PGE1 derivative, have been studied in rats. The compound administered p.o. or i.v. was able to inhibit the macroscopic damage to gastric mucosa produced by noxious stimuli (ethanol and indomethacin). In the stomach perfusion test with the anesthetized rat, intravenously administered MDL-646 reduced histamine- or pentagastrin-stimulated gastric secretion. After intraduodenal administration (i.d.) doses at least 40-50 times greater were necessary for an antisecretory effect. In conscious rats with chronic gastric fistulas, intragastrically administered (i.g.) MDL-646 affected both acid concentration and volume of unstimulated gastric secretion. In experimental models for gastric lesions, MDL-646 was much more potent after oral (p.o.) (15-30 times) than after i.v. administration. (ED50 micrograms/kg: vs. alcohol lesions, 0.05 p.o. and 0.7 i.v.; vs. indomethacin ulcers, 7.0 p.o. and 195 i.v.). Our data would fit the hypothesis that it was a local effect on the gastric mucosa. The mechanism of this effect is not known. The supposed local activity coupled with the antisecretory effects and the good tolerability make it interesting to test MDL-646 as an anti-ulcer agent in man.

Stimulated gastric prostaglandin output, and the effect of inhibition of prostaglandin synthetase, in the conscious cat

1. In the conscious gastric fistula cat there was no correlation between the outputs of gastric acid and PGE secreted in response to incremental doses of pentagastrin and histamine acid phosphate. 2. PGE secreted in response to pentagastrin and histamine was not dose dependent. 3. Flurbiprofen significantly inhibited the gastric output of PGE but did not influence acid or pepsin secretion. Inhibition of PGE secretion was accompanied by evidence of gastric mucosal haemorrhage. 4. It is concluded that gastric juice PGE is unlikely to be involved in the physiological control of acid secretion.

Effects of 16, 16-dimethyl prostaglandin E2 methyl ester on aspirin-and indomethacin-induced gastrointestinal lesions in dogs

The pathogenesis of aspirin- or indomethacin-induced gastric and/or intestinal lesions was studied in dogs. 16,16-Dimethyl PGE2 methyl ester (16-DMPGE2) at 2 or 10 microgram/kg in two divided doses given intramuscularly markedly inhibited gastric lesions produced by orally ingested aspirin at 200 mg/kg/day given twice daily for 1 or 5 days. While 16-DMPGE2 at the same dose also inhibited gastric lesions induced by a single oral administration of indomethacin at 20 mg/kg, gastric lesions including deep antral ulcers, produced by repeated administration of indomethacin, were not affected. Intestinal lesions produced by indomethacin given once, or for 5 or 10 days, were not affected. These results suggest that the lack of engoenous prostaglandins may be involved in the pathogenesis of gastric lesions produced by aspirin and indomethacin given once but may not be involved in the pathogenesis of indomethacin-induced deep lesions in the stomach and intestine.

Prostaglandins and serotonin: nonpeptide diarrheogenic hormones

Prostaglandins and serotonin are vasoactive compounds with profound effects on the gastrointestinal tract. Both cause inhibition of gastric acid secretion (although serotonin stimulates gastric pepsin secretion), stimulation of intestinal motility, and conversion of small intestinal mucosa from absorption to secretion of water and electrolytes. Their effects on pancreatic and biliary function are still not clear. Although prostaglandins appear to elicit their effects primarily by a paracrine mode of action, and serotonin is primarily a neurotransmitter (neurocrine), it is clear that even under normal conditions both can function as humoral agents. For example, we have shown that serotonin plays a physiologic role as a humoral inhibitor of gastric acid secretion. However, the effects of these agents become more pronounced in patients with humorally mediated diarrheogenic syndromes. Serotonin (and related indoles, particularly 5-hydroxytryptophan) has been firmly implicated as a cause of diarrhea in patients with carcinoid syndrome; our recent studies suggest that the diagnosis can be more effectively made by measuring circulating immunoreactive serotonin concentrations than urinary excretion of 5-HIAA; that some circulating serotonin escapes hepatic inactivation and, thus, large intestinal tumors can cause carcinoid syndrome in the absence of hepatic metastases; and that large amounts of serotonin are produced by some noncarcinoid diarrheogenic tumors, including medullary carcinomas of the thyroid and tumors associated with the WDHA syndrome. A large number of tumors of probable neural crest origin, including medullary thyroid carcinoma, carcinoids, and tumors associated with the WDHA syndrome, secrete large amounts of prostaglandins, particularly PGE2. The clinical response of at least some of the patients harboring these tumors to inhibitors of prostaglandin synthesis (particularly indomethacin) suggests that prostaglandins play a role in the etiology of these diarrheogenic syndromes.

Endogenous prostaglandins in peptic ulcer disease

Plasma concentrations of prostaglandins E and F have been measured by radioimmunoassay in patients undergoing diagnostic upper intestinal endoscopy. The results fail to support a previously reported deficiency of plasma PGE in duodenal ulcer patients. Plasma prostaglandin concentrations failed to correlate with the parameters of gastric secretion studied; and were unaffected by histamine H2-receptor blockade or the activity of duodenal ulceration. During combined pentagastrin and insulin secretory studies there was a significant correlation between the outputs of PGE and acid into gastric juice.

Influence of the route of administration of prostaglandin E1 on rat gastric secretion

Changes in gastric secretion induced by the subcutaneous, intraduodenal or intragastric administration of prostaglandin E 1 (PGE1) were evaluated in pylorus-ligated rats. Subcutaneous and intraduodenal injections produced a dose-related inhibition in both total acid and volume of gastric secretion. Dose-response curves for inhibition obtained by these routes were parallel, although PGE1 was more potent when given subcutaneously. Gastric administration produced a dose-related decrease in acid and an increase in volume. The slope of the dose-response curve for acid inhibition with this route was flatter than with subcutaneous or intraduodenal administrations. The present results suggest that PGE1 inhibits gastric secretion by the same mechanism of action when given subcutaneously or into the duodenum, while the effects observed after gastric administration are consequences of local actions. The difference in potency of PGE1 given subcutaneously and in the duodenum would seem to be due to differences in absorption from the site of administration and/or to a greater metabolism of PGE1 during its absorption from the intestines.

SC-29333: a potent inhibitor of canin gastric secretion

The gastric antisecretory effects of SC-29333, a novel prostglandin E1 analogs, were compared to the reference standard PGE1 methyl ester (PGE1ME) in gsstric fistula and Heidenhain pouch dogs. Secretion was stimulated submaximally by continuous intravenous infusion of either histamine or pentagastrin. Meal-stimulated gastric secretory studies were also conducted. SC-29333 effectively inhibited volume, acid output, and papsin secretion, in a dose-dependent manner. Intravenously (i.v.), SC-29333 was found to be approximately 30 times more potent than PGE1ME. The ranges of active i.v. bolus doses for SC-29333 and PGE1ME were 0.3-3.0 and 10-30 mug/kg, respectively. Unlike PGE1ME, SC-29333 was orally effective at doses of 10-30 mug/kg and was considerably better tolerated and longer acting than PGE1ME at active antisecretory doses. It is concluded, therefore, that SC-29333 is a potent, long-acting, orally effective inhibitor of gastric secretion in the dog.

Relationship between the prevention of rat gastric erosions and the inhibition of acid secretion by prostaglandins

The formation of gastric mucosal erosions induced by indomethacin in the rat was inhibited in a time- and dose-dependent manner by antisecretory prostaglandins, the methyl analogues of PGE2 being 400 times as active as the parent prostaglandin. PGA2, a methyl analogue of PGF2alpha and the H2-receptor antagonist metiamide, also inhibited erosion formation. There was a variable relationship between the doses required to inhibit erosions and to inhibit gastric acid secretion. In the anaesthetised rat, the low incidence of erosions with indomethacin was markedly increased by concurrent gastric perfusion with acid saline and taurocholate. This mucosal damage was inhibited by the methyl analogues of PGE2, suggesting protective actions on the mucosa other than inhibition of acid secretion.

Gastric antisecretory effects of E prostaglandins in Rhesus monkeys

The gastric antisecretory actions of prostaglandin E1 methyl ester (PGE1ME) and prostaglandin E2 (PGE2) were evaluated in unanesthetized gastric fistula rhesus monkeys. Basal and stimulated gastric secretory studies were conducted. Multiple subcutaneous injections of either histamine or pentagastrin were given hourly for four consecutive hours. When a constant plateau of gastric secretion was reached, the PGs were administered as a single intravenous bolus at doses of 10-100 mug/kg. PGE1ME inhibited basal, histamine- and pentagastrin-stimulated gastric secretion. PGE2 was found to inhibit the histamine- stimulated gastric secretion. The PGs showed greater sensitivity to the inhibition of acid concentration rather than the volume of secretion. The PGs signfiicantly altered gastric juice concentration of hydrogen and sodium ion inversely, while potassium and chloride concentration were not altered. These experiments suggest that the rhesus monkey is a useful species for studying the gastric antisecretory effects of E prostaglandins.

Arterial walls are protected against deposition of platelet thrombi by a substance (prostaglandin X) which they make from prostaglandin endoperoxides

Prostaglandin (PG) endoperoxides (PGG2 and PGH2) contract arterial smooth muscle and cause platelet aggregation. Microsomes from pig aorta, pig mesenteric arteries, rabbit aorta and rat stomach fundus enzymically transform PG endoperoxides to an unstable product (PGX) which relaxes arterial strips and prevents platelet aggregation. Microsomes from rat stomach corpus, rat liver, rabbit lungs, rabbit spleen, rabbit brain, rabbit kidney medulla, ram seminal vesicles as well as particulate fractions of rat skin homogenates transform PG endoperoxides to PGE- and PGF- rather than to PGX-like activity. PGX differs from the products of enzymic transformation of prostaglandin endoperoxides so far identified, including PGE2, F2alpha, D2, thromboxane A2 and their metabolites. PGX is less active in contracting rat fundic strip, chick rectum, guinea pig ileum and guinea pig trachea than are PGG2 and PGH2. PGX does not contract the rat colon. PGX is unstable in aqueous solution and its antiaggregating activity disappears within 0.25 min on boiling or within 10 min at 37degrees C. As an inhibitor of human platelet aggregation induced in vitro by arachidonic acid PGX was 30 times more potent than PGE1. The enzymic formation of PGX is inhibited by 15-hydroperoxy arachidonic acid (IC50 = 0.48 mug/ml), by spontaneously oxidised arachidonic acid (IC 50 less than 100 mug/ml) and by tranylcypromine (IC50 = 160 mug/ml). We conclude that a balance between formation by arterial walls of PGX which prevents platelet aggregation and release by blood platelets of prostaglandin endoperoxides which induce aggregation is of the utmost importance for the control of thrombus formation in vessels.

Gastric antisecretory actions of (15S)-15-methyl prostaglandin E2 methyl ester and natural prostaglandin E2 in rhesus monkeys

The gastric antisecretory actions of (15S)-15-methyl prostaglandin E2 methyl ester (Me-PGE2) and Prostaglandin E2 (PGE2) were evaluated in the unanesthetized gastric fistula rhesus monkey. Secretion was submaximally stimulated by multiple subcutaneous injections of histamine acid phosphate given every hour for four consecutive hours. When a steady-state plateau of gastric secretion was reached, the PG's were administered as a single bolus dose either intravenously (i.v.) or intragastrically (i.g.). Both PG's inhibited histamine-stimulated gastric secretion. The PG's showed greater sensitivity in inhibiting acid concentration while not affecting volume output. Active i.v. and i.g. antisecretory doses of ME-PGE2 ranged from 3 to 10 mug/kg, while PGE2 showed significant antisecretory activity at i.v. bolus doses of 30-100 mug/kg and i.g. bolus dose of 1.0 mg/kg. Thus, Me-PGE2 is estimated to be at least 10 and 300 times more potent than PGE2 by the i.v. and i.g. administration routes, respectively. These findings indicate that the rhesus monkey shows some similarities to man in responsiveness to gastric secretory inhibition by E-prostaglandins.

Bile-induced acute erosive gastritis. Its prevention by antacid, cholestyramine, and prostaglandin E2

Intragastric administration of whole bile-produced acute erosive gastritis in rats. Concomitant intragastric administration of prostaglandin E2, Cholestyramine and an antacid (Maalox) was effective in preventing this type of acute erosive gastritis. The protective rffect of prostaglandin E - 2 in this regard was significantly more marked as compared to Cholestyramine and Maalox.

Effects of E prostaglandins, diphenoxylate and morphine on intestinal motility in vivo

The mechanism of the gastrointestinal motility effects of diphenoxylate and morphine in preventing E prostaglandin (PG) diarrhea was investigated. Duodenal motility studies were conducted in the anesthetized dog. Two contractile force transducers were oriented to record contractions from both the circular and longitudinal muscles. In some experiments the basic electrical rhythm (BER) was also recorded. Blood pressure was monitored from the femoral artery and drug injections were made in the femoral vein. Diphenoxylate shared with morphine the capacity to stimulate circular muscle contractions which correlated with the appearance of spike potentials on the BER. Prostaglandin E1 methyl ester (PGE1ME) showed marked relaxation of the circular muscle and abolishment of spike potentials. PGE1ME also blocked the stimulatory effects of diphenoxylate and morphine on the circular muscle. PGE1ME and PGE2 were found to be equally potent in producing diarrhea in mice. Diphenoxylate and morphine were found to be equally potent in inhibiting PG's diarrhea. These studies suggest that the constipating actions of diphenoxylate and morphine are a consequence of the increased circular muscle activity of the intestine.

Influence of the position of the side chain hydroxy group on the gastric antisecretory and antiulcer actions of E1 prostaglandin analogs

The influence of transposing the C-15 hydroxy group of prostaglandin E1 methyl ester (PGE2ME) on gastric antisecretory and antiulcer actions was investigated. The compound (+/-)15-deoxy- 16alpha, beta-hydroxy PGE1ME (SC-28904) was equipotent to the reference standard PGE1ME in suppressing histamine-stimulated gastric secretion in the Heidenhain pouch (HP) dog. In contrast to PGE1ME, SC-28904 was longer acting when administered intravenously and also showed significant oral activity in the histamine-stimulated gastric fistula dog. SC-28904 was also equipotent to PGE1ME (range of active doses of 0.5 to 5.0 mg/kg, s.c.) in inhibiting forced-exertion gastric ulceration in rats. The compound (+/-)15-deocy-17alpha, beta-hydroxy PGE1ME (SC-30963) was an inactive antisecretory agent in the dog at the 1.0 mg/kg i.v. bolus dose. This dose was 100 times greater than the active antisecretory dose of PGE1ME. Likewise, SC-30693, when administered subcutaneously at a 5.0 mg/kg dose, was also totally inactive in preventing gastric ulcers induced by forced exertion in rats. The important implications of this work are that some of the receptor sites for the PGE1 molecule could easily accomodate the side chain hydroxy group either in the C-15 or C-16 position. Moreover, the hydroxy group in the latter position significantly improved the biological activity of PGE1ME.

Comparative gastric antisecretory and antiulcer effects of prostaglandin E1 and its methyl ester in animals

The influence of methyl esterification of the carboxyl group of PGE1 on the gastric antisecretory and antiulcer activities were studied. The gastric antisecretory effects of PGE1 free acid and PGE1 methyl ester (PGE1ME) were studied in the Heidenhain pouch dog. Secretion was stimulated with constant intravenous infusion of histamine dihydrochloride. When a steady-state plateau of gastric secretion had been reached, the prostaglandins were administered either by a single intravenous bolus (10.0 mug/kg) or by continuous infusion (1.0 mug/kg/min). PGE1ME was found to be slightly more potent and longer-acting than PFE1 when administered by a single i.v. bolus. PGE1ME was also shown to be more potent than PGE1 when infused intravenously for a two-hour period. PGE1ME caused a significant alteration in gastric juice concentration of hydrogen and sodium ions in an inverse relationship. Potassium and chloride concentration were not altered from pre-existing steady-state values following administration of either form of prostaglandin. Similarly, PGE1ME was also found to possess significantly greater antiulcer activity in the rat forced-exertion ulcer test. These findings support the hypothesis that methyl esterification of the prostaglandin molecule will increase some of the biological actions of PGE1 through inhibition of metabolic beta-oxidation of the carboxylic side chain.