The gastric antisecretory actions of prostaglandin E2 and stable prostacyclin analogues against different secretagogues in perfused whole-stomachs of rat or mouse in vitro

Suppression of histidine decarboxylase activity in rat oxyntic mucosa by beraprost sodium, a prostacyclin analogue

Prostaglandins (PGs) affect various aspects of gastric functions. In the present study the orally administered PGI2 derivative beraprost sodium (TRK-100.1 micrograms per kg body weight) decreased oxyntic histidine decarboxylase activity without changing serum gastrin levels. Antral pH increased 4 hr after treatment. Beraprost also decreased the pentagastrin-induced histidine decarboxylase activity at the same dose. Serum levels of secretin, somatostatin and glucose, and oxyntic mucosal levels of histamine and somatostatin, showed no significant change after treatment with beraprost. These results suggest that the response of oxyntic histidine decarboxylase to gastrin is modified by one or more prostanoids including PGI2. This mechanism might play a role in gastric mucosal protection.

Involvement of neuronal processes and nitric oxide in the inhibition by endotoxin of pentagastrin-stimulated gastric acid secretion

Administration of E. coli endotoxin (1 mg kg-1, i.v.) abolished the acid response induced by the i.v. infusion of pentagastrin (8 micrograms kg-1 h-1) in the continuously perfused stomach of the anaesthetized rat. Local serosal application of tetrodotoxin (36 ng per rat) completely restored acid responses to pentagastrin in endotoxin-treated rats. However, pretreatment with atropine (0.5 mg kg-1, s.c.), capsaicin (20, 30, and 50 mg kg-1, s.c. 2 weeks before the study) or guanethidine (16 mg kg-1, s.c. 3 and 16h before) did not influence the inhibitory effects of endotoxin. Continuous i.v. infusion with NG-nitro arginine methyl ester (L-NAME, 10 mg kg-1 h-1) restored the secretory responses to pentagastrin in endotoxin treated rats. The effects of L-NAME were reversed by L-arginine (100 mg kg-1 h-1, i.v.), but not by its enantiomer D-arginine (100 mg kg-1 h-1, i.v.). The secretory responses elicited by pentagastrin (10(-10)-10(-6) M) in the isolated lumen perfused stomach of the rat were not influenced by incubation (100 min) with endotoxin (10 micrograms ml-1). These observations with tetrodotoxin indicate that inhibition of acid secretion by endotoxin in vivo involves neuronal activity, while inhibition of NO synthesis had a comparable inhibitory action. Activation of a systemic non-adrenergic non-cholinergic neuronal pathway involving NO could thus mediate the acute acid inhibitory effects of endotoxin.

Involvement of endogenous nitric oxide in the inhibition by endotoxin and interleukin-1 beta of gastric acid secretion

Administration of Escherichia coli endotoxin abolished the acid secretory response induced by a bolus injection of pentagastrin in the continuously perfused stomach of the anaesthetized rat. Likewise, acid secretion stimulated by the continuous intravenous perfusion of pentagastrin was inhibited by administration of interleukin-1 beta (IL-1 beta). In both cases pretreatment with NG-nitro-L-arginine methyl ester (L-NAME) but not dexamethasone or indomethacin substantially restored the secretory responses to pentagastrin. The actions of L-NAME were reversed by the prior administration of L-arginine but not by its enantiomer D-arginine. Even though L-NAME increased blood pressure, this does not seem to be the mechanism by which endotoxin-induced acid inhibition was prevented, since similar systemic pressor responses induced by phenylephrine had no such effect. The secretory response elicited by pentagastrin in the isolated lumen perfused stomach of the rat was not influenced by incubation (100 min) with IL-1 beta. These observations suggest that the acute inhibition of acid responses to pentagastrin by endotoxin and IL-1 beta involves nitric oxide (NO) synthesis from L-arginine.

Fade and tachyphylaxis of gastric acid secretory response to pentagastrin in rat isolated gastric mucosa

1. Gastric acid secretory responses to pentagastrin were characterized in the rat isolated gastric mucosa. In particular, the mechanisms underlying fade, declining response upon continued stimulation, and tachyphylaxis, progressively reduced responses upon repeated stimulation, were investigated. 2. Pentagastrin, 10(-9)-10(-7) M, resulted in concentration-related increases in acid secretion, with a mean maximum of 2.65 mumol cm-2 h-1 in response to pentagastrin, 10(-7) M. Higher concentrations of pentagastrin produced sub-maximal secretory rates; we define this as auto-inhibition. The responses to all concentrations of pentagastrin demonstrated fade. The rate of fade was correlated with the maximum acid secretory rate, declining at about 36% of the peak over the first 16 min. 3. The PO2, PCO2, [HCO3-], pH, [glucose], [lactate], [Na+] and [K+] did not decline during the fade of the acid secretory response to pentagastrin, 10(-7) M. Addition of a second aliquot of pentagastrin was not able to reverse fade, but these tissues were responsive to histamine. Replacement of the serosal solution, before addition of a second aliquot of pentagastrin, increased the acid response from 3% to 24% of the first response. 4. Serosal solution from donor tissues, allowed to respond to pentagastrin and then the acid secretion to fade, was able to stimulate secretion in fresh recipient tissues, although at lower rates. 5. Acid secretory responses to a second dose of pentagastrin were not significantly different, whether the tissues were previously unstimulated, or stimulated with pentagastrin washed out after attaining its peak secretory response (after 10-20 min). The second response was significantly reduced if the first response was allowed to fade with the pentagastrin in contact for 100 min; i.e. fade significantly influenced the extent of tachyphylaxis. 6. Proglumide, 10(-2) M, a gastrin receptor antagonist, and omeprazole, 10(-5) M, an inhibitor of the gastric (H+ + K+)-ATPase, both inhibited pentagastrin-stimulated acid secretion to similar extents. The second response to pentagastrin after pentagastrin alone, or pentagastrin plus omeprazole were both reduced compared to responses after no stimulation or omeprazole alone, respectively. After pentagastrin plus proglumide, the second response to pentagastrin was not lower than after proglumide alone. Proglumide, but not omeprazole, therefore, prevented pentagastrin tachyphylaxis. 7. It is concluded that gastrin fade and tachyphylaxis are related phenomena. Part of the fade may be due to release of an inhibitor(s). The major proportion of tachyphylaxis is a result of specific interaction of gastrin with its receptors.

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.

Development of gastric mucosal protection against acid in the rat. Role of corticosteroids and prostaglandins

The ability of 16,16-dimethyl prostaglandin E2 (dmPGE2) to protect the gastric mucosa from acid-induced damage was assessed in suckling and adult rats. Suckling rats 9-11 days of age did not show protection with dmPGE2 at doses up to 100 micrograms/kg, whereas adult rats had an ED50 of approximately 2 micrograms/kg. The onset of responsiveness to dmPGE2 (5 micrograms/kg) occurred in the suckling rat at an age between 14 and 21 days. The "intrinsic" ability of the gastric mucosa to protect itself against acid-induced damage increased gradually between 7 and 21 days. The appearance of dmPGE2 sensitivity was unaffected by administration of hydrocortisone on day 7, although hydrocortisone alone induced a precocious development of the intrinsic protection against luminal acid. These data suggest that this mucosal protective property of the stomach appears during the end of the second week of life in the rat, just before the increase in acid secretory activity. This may be attributable to both the ontogenic appearance of a responsiveness to prostaglandins and an increase in circulating levels of adrenocorticosteroid hormones. Although the protective mechanisms responding to prostaglandins and the corticosteroids could be different, our results suggest that the two factors may act in concert to assure optimal protection against luminal acid.

Experimental use of prostaglandin E2 (Dinoprost) in the treatment of duodenal ulcer in humans

Oral administration of 20 mg PGE2/day/patient (5 mg every 4 hours, 4 times a day) promptly relieved pain in 10 patients suffering from duodenal ulcer. After 3 days of treatment, the ulceration--observed by radiologic and gastrofibroscopic means--diminished by 30% in dimensions, and after 10 days, it almost disappeared. The results were compared with those observed in a placebo-group, who received 20 mg lactose/day/patient and with those in another group treated by sodium bicarbonate.

Effects of indomethacin, piroxicam and selected prostanoids on gastric acid secretion by the rat isolated gastric mucosa

The effects of the cyclo-oxygenase inhibitors indomethacin and piroxicam have been investigated on histamine- and dibutyryl cyclic AMP-induced acid secretion in the rat isolated gastric mucosa. The relative potencies of a number of prostanoids as inhibitors of histamine-induced acid secretion were determined in an attempt to classify the prostaglandin receptor mediating this response. Indomethacin (8 X 10(-9) - 2.7 X 10(-6) M) and piroxicam (3 X 10(-6) M) potentiated the secretory responses elicited by histamine. This effect might be due to inhibition of the biosynthesis of antisecretory prostanoids. Indomethacin (2.7 X 10(-6) M) and piroxicam (3 X 10(-6) M) also potentiated the secretory response to dibutyryl cyclic AMP, but since prostaglandin E2 (PGE2, 10(-5) M) did not inhibit this secretory response, the mechanism of the potentiation may differ from that of histamine. The potency of the thromboxane mimetic U-46619 as an inhibitor of histamine-induced acid secretion was markedly reduced in the presence of indomethacin, suggesting that U-46619 may release endogenous antisecretory prostanoids. In the presence of indomethacin (2.7 X 10(-6) M) all the prostanoids tested produced concentration-related inhibitions of histamine-induced gastric acid secretion. PGE-analogues were the most potent compounds, the rank order of potency being 16, 16 dimethyl PGE2 greater than PGE2 greater than PGF2 alpha greater than U-46619 greater than PGD2 greater than PGI2. This order of potency is very similar to that obtained in smooth muscle preparations containing 'EP' receptors, suggesting that this receptor type also mediates inhibition of histamine-induced acid secretion in the rat.

Possible involvement of non-steroidal anti-inflammatory drugs in vagal-mediated gastric acid secretion in rats

Effects of several non-steroidal anti-inflammatory drugs (NSAIDs) such as aspirin (ASA), indomethacin (IM), flurbiprofen (FP), ibuprofen (IP), phenylbutazone (PBZ) and flufenamic acid (FA) were studied on the gastric ulceration and gastric acid secretion induced by restraint and water-immersion stress (RWIS) or various secretagogues in rats. These drugs significantly increased ulcer formation. IM (1, 3 and 10 mg/kg, s.c.) reduced gastric mucosal prostaglandin (PG) content dose-dependently. There was an appreciable correlation between this decrease in the PG content of gastric tissue and associated ulceration. The gastric acid secretion induced by the peripheral secretagogues, methacholine, gastrin and histamine, was not significantly influenced by IM pretreatment. In contrast, the gastric acid secretion induced by the vagal mediated secretagogues, insulin, 2-deoxy-D-glucose (2-D-G) and RWIS, was markedly increased by IM pretreatment. These effects were not observed in vagotomized rats. By intracerebroventricular (i.c.v.) injection of IM, no influence was observed on the gastric acid secretion and ulcer formation induced by 2-D-G or RWIS. These results suggest that acidic NSAIDs potentiate the gastric acid output induced by stimulation of vagus nerve activity, and prostaglandins (PGs) may influence gastric acid output by regulating vagus nerve activity.

Evaluation of the protection of rat gastric mucosa by a prostaglandin analogue using cellular enzyme marker and histologic techniques

Damage to the rat gastric mucosa after oral administration of ethanol and the effect of pretreatment with a prostaglandin analogue has been evaluated using histologic and enzyme-marker techniques. Rat whole stomachs were incubated in vitro and the intraluminal release of the cytoplasmic enzyme lactate dehydrogenase and the lysosomal enzymes acid phosphatase and beta-glucuronidase was determined by spectrophotometric techniques. Ethanol irrigation in vivo for 10 min significantly elevated the subsequent intraluminal release of both cytoplasmic and lysosomal enzymes in vitro. Pretreatment with 16,16-dimethyl prostaglandin E2 (0.1-1.25 microgram/kg p.o.) in doses that substantially inhibited the formation of macroscopically apparent necrotic lesions failed to prevent enzyme release. However, higher doses of the prostaglandin analogue (2.5-20 micrograms/kg) did significantly reduce the intraluminal release of the cellular enzymes, with the lysosomal enzymes being more readily inhibited. Histologic studies confirmed that the lower doses of the prostanoid prevented deep tissue necrosis and vasocongestion, yet did not protect surface epithelial cells from ethanol-induced damage. However, with the highest dose of 16,16-dimethyl prostaglandin E2 (20 micrograms/kg) a significant reduction in the extent of damage to the superficial epithelial cells was observed, suggesting a correlation with the findings using enzyme markers of cell damage. The apparent protective mechanisms of this prostanoid under the present conditions may involve mucus and fluid effusion that could allow restitution of the surface epithelial layer.

Effects of SCH 32651 on resting and stimulated acid secretion in guinea-pig isolated fundic mucosa

Effects of SCH 32651, a novel antisecretory and cytoprotective agent, on resting and stimulated acid secretion by the guinea-pig isolated fundic mucosa were studied. SCH 32651 inhibited resting acid secretion in proportion to concentrations in serosal solution (0.1-10 microM), the IC50 being 4.4 microM. Cimetidine and atropine at concentrations up to 100 microM were inactive. Serosal application of SCH 32651 inhibited acid secretory responses to histamine (10 microM), methacholine (1 microM) or dibutyryl cyclic AMP (0.5 mM) plus theophylline (1 mM) in a concentration-dependent manner. The IC50S against histamine, methacholine and db cyclic AMP plus theophylline were 4.2 microM, 0.71 microM and 2.9 microM, respectively. In contrast, atropine and cimetidine each at 100 microM, a concentration that entirely abolished responses to methacholine and histamine, respectively, did not affect acid responses to db cyclic AMP plus theophylline. The inhibitory effects of SCH 32651 on resting and histamine-stimulated acid secretion were readily reversible upon washing. SCH 32651 0.1 mM in the mucosal solution also greatly suppressed the resting and stimulated acid secretion. In the presence of histamine treatment, SCH 32651 concomitantly caused a marked rise in K+ entry into the mucosal solution in parallel to a decline in the appearance of H+ in the same solution. The various events demonstrated by SCH 32651 in the present study are shared by omeprazole, a potent antisecretory agent working through inhibition of gastric H+/K+-ATPase. We conclude that SCH 32651 as a potent antisecretory agent seems to act directly on the parietal cell, near or at the site of H+/K+-ATPase which is a final step in the acid secretory process triggered by various stimuli.

Effect of hyperosmotic xylitol on gastric secretion in the rat: lack of influence of cyclooxygenase inhibitors

The effects of hyperosmotic solutions of xylitol were tested on acid and bicarbonate outputs from the rat stomach as well as on acid back-diffusion. Hyperosmotic solutions were instilled into anesthetized rats by an esophageal cannula and drained through a duodenal catheter. Spontaneously secreting and histamine-stimulated rats were used in different experiments. Hyperosmotic xylitol solutions at concentrations of 18.4% or higher produced graded inhibitions of the histamine-induced acid secretion. A 34.5% xylitol solution also inhibited spontaneous acid secretion. The same solution also caused a certain degree of acid back-diffusion and increased output of bicarbonate. Neither the inhibition acid secretion nor stimulation of bicarbonate output were affected by cyclooxygenase inhibitors indomethacin or flufenamic acid. It is concluded that hyperosmotic xylitol reduces gastric acidity by three mechanisms namely inhibition of acid secretion, increased bicarbonate output and increased back-diffusion of acid. None of these mechanisms seem to depend on prostaglandin biosynthesis.

Vascular responses of the isolated perfused stomach of rabbit and rat to histamine

Gastric vascular responses to histamine and its selective H1- and H2-agonists in vitro were investigated in the isolated vascular-perfused stomach of rabbit and rat. In the rabbit stomach under resting conditions bolus injection of histamine (5-80 nmol) caused a small increase in perfusion pressure (PP). However, during infusion of noradrenaline (1 microM), which elevated vascular tone, histamine (5-80 nmol) caused a dose-dependent biphasic vascular response, an initial increase followed by a fall in PP. Under similar conditions, the H1-agonist 2-pyridyl ethylamine, (2-PE; 50-400 nmol) elevated PP, whereas the H2-agonist dimaprit (80-640 nmol) reduced PP. The H1-antagonist mepyramine (0.25 microM) converted the biphasic response of histamine (5-50 nmol) to a monophasic vasodilation whereas the H2-receptor antagonist, cimetidine (1 microM) converted the biphasic response to a vasoconstriction. Similar responses were observed during conditions of elevated vascular tone induced by vasopressin or angiotensin II. In the rat stomach, histamine and both dimaprit and 2-PE reduced PP during conditions of elevated vascular tone. These findings support the presence of both H1- and H2 receptors in the gastric vasculature of both rat and rabbit; in both species, the H2-response is vasodilation whereas the nature of the H1-response is species dependent.

The inhibitory effects of 5-hydroxytryptamine on gastric acid secretion by the rat isolated stomach

1 The effect of 5-hydroxytryptamine (5-HT) on acid secretion by a rat isolated stomach preparation has been studied. 2 5-HT at 10(-5)M in the serosal bathing fluid produced significant inhibition of the acid secretory responses to histamine, pentagastrin and isoprenaline but was without effect on basal secretion or that due to bethanechol, dibutryl cyclic adenosine 3',5'-monophosphate (db cyclic AMP) or phosphodiesterase inhibition with ICI63197. Increasing the concentration of 5-HT to 5 x 10(-5) M did not change this pattern of response whilst 5-HT at 10(-6) M did not cause consistent inhibition. 3 The inhibitory action of 5-HT could be prevented by the antagonist methysergide (2.5 x 10(-5) M). This concentration of methysergide alone did not affect responses to secretagogues or basal acid output. 4 Neither propranolol (2.5 x 10(-5) M) nor tetrodotoxin (10(-6) M) antagonized the inhibitory action of 5-HT. 5 Both indomethacin (2.8 x 10(-5) M) and ibuprofen (2.4 x 10(-4) M) antagonized the action of 5-HT. Indomethacin alone had no effect upon secretagogue responses. 6 5-HT at 10(-5) M had no inhibitory action when applied to the mucosal side of the preparation. 7 The results indicate that 5-HT can act directly on the stomach of the rat to produce inhibition of acid output. This inhibition is selective and may involve the products of cyclo-oxygenase activity.

The rat isolated stomach sheet; an in vitro model for the study of the physiology and pharmacology of gastric acid secretion

The development of an isolated stomach preparation from immature rats is described. Acid secretion was measured by titration of mucosal samples taken at 15 min intervals. Nonstimulated acid secretion was generally in the range 60-180 mumol H+g-1 wet weight of mucosa hr-1 and was maintained for up to 5.5 hr. A small proportion of stomachs (20%) had a higher level of spontaneous secretion which fell to the more usual low level within 1.5 hr. Pentagastrin (1.3 X 10(-8) M-1.3 X 10(-7) M), bethanechol (2 X 10(-6) M-3 X 10(-5) M), histamine (5 X 10(-6) M-5 X 10(-4) M) and theophylline (2 X 10(-4) M-1 X 10(-3) M) induced concentration-dependent stimulation of acid secretion. Linear log-dose response relationships were obtained for each of the four stimulants. The regression coefficients from each drug study, calculated over a similar increase in concentration, were not significantly different from one another. The maximal secretory rates recorded with the four secretagogues were comparable, range 152.5-226.0 mumol H+g-1hr-1. The methodology and results are compared with other mammalian in vitro stomach preparations currently used to study the physiology and pharmacology of gastric acid secretion.

Investigation of the vascular actions of arachidonate lipoxygenase and cyclo-oxygenase products on the isolated perfused stomach of rat and rabbit

The vascular actions of several prostanoids and arachidonate lipoxygenase products were investigated on the gastric circulation of rat and rabbit in vitro perfused with Krebs' solution. Under resting conditions, prostacyclin and PGE2 produced small decreases in perfusion pressure with prostacyclin being the more potent. During vasoconstriction induced by infusion of noradrenaline, vasopressin or angiotensin II, prostacyclin was 20-40 times as active as PGE2 as a gastric vasodilator in rat or rabbit stomach. PGF2 alpha was a less potent vasoconstrictor than noradrenaline, while the epoxy-methano endoperoxide analogue produced a long-lasting vasoconstriction. The putative metabolite, 6-oxo-PGE1 was less active than prostacyclin as a vasodilator, having comparable activity to PGE1, whereas 6-oxo-PGF1 alpha had very little activity. The endoperoxide, PGH2 reduced perfusion pressure, this effect being inhibited by concurrent infusion of 15-HPETE. The vasodilation induced by arachidonic acid was likewise reduced by 15-HPETE, and abolished by indomethacin infusion. The arachidonate lipoxygenase hydroperoxides were vasodilator in the gastric circulation, the rank order of potency being 12-HPETE greater than 11-HPETE greater than 5-HPETE greater than 15-HPETE in both rat and rabbit stomach. It is possible that such vasoactive lipoxygenase products, may play modulator roles in the gastric mucosa.