Mobilization of rat stomach ECL-cell histamine in response to short- or long-term treatment with omeprazole and/or YF 476 studied by gastric submucosal microdialysis in conscious rats

1. Mobilization of histamine from the ECL cells was monitored by gastric submucosal microdialysis in conscious rats. The ECL cells are known to operate under gastrin control and the purpose of the present study was to examine their in situ response to short-term (12 h) as well as long-term (28 days) hypergastrinaemia, induced by treatment with the proton pump inhibitor omeprazole. 2. Hypergastrinaemia promptly raised the histamine concentration in the microdialysate. The effect was prevented by CCK(2) receptor blockade (YF476). On day 7 of omeprazole treatment the microdialysate histamine concentration reached a peak, five times higher than before treatment. Subsequently (14 and 28 days), less histamine was mobilized. 3. Gastrin infusion (4 h) raised the microdialysate histamine concentration in a dose-dependent manner in fasted rats and freely fed rats and in rats treated with omeprazole for a week. However, while fasted and fed rats responded to low doses of gastrin, the omeprazole-treated rats required large doses of gastrin to respond. 4. When the amount of histamine mobilized was related to the serum gastrin concentration the following EC(50) values could be calculated: fasted rats 2.3 x 10(-10) M, freely fed rats 2.5 x 10(-10) M, omeprazole-treated rats 8.7 x 10(-10) M. The maximal histamine responses in the three groups were 18.4 pmol 4 h(-1)+/-0.8, 21.9 pmol 4 h(-1)+/-1.2 and 68.0 pmol 4 h(-1)+/-3.5, respectively. 5. The results suggest that ECL cells, exposed to a high gastrin concentration for a week, respond with a shift in the receptor-ligand binding affinity from high to low. Apparently, CCK(2) receptors of the ECL cells are subject to dynamic changes with respect to ligand-binding affinity.     

Pharmacological basis for the induction of gastric carcinoid tumours in the rat by loxtidine, an insurmountable histamine H2-receptor blocking drug.

The very late occurrence of gastric carcinoids in a life-span carcinogenicity study with loxtidine in the rat might have resulted from continuous achlorhydria induced by this long-acting unsurmountable histamine H2-antagonist. The nature of the anti-secretory activity of loxtidine was compared with that of ranitidine on histamine-induced acid secretion in the perfused stomach preparation of the rat and in the rat isolated gastric mucosa preparation. Ranitidine and loxtidine had qualitatively different inhibitory effects on acid secretion, ranitidine being a competitive antagonist of histamine even at high concentrations, whereas the effect of loxtidine on both preparations was unsurmountable at relatively low concentrations. These results support the hypothesis that the late formation of gastric carcinoids in rats receiving loxtidine is a consequence of persistent achlorhydria caused by unsurmountable blockade of parietal cell H2-receptors.     

Time course of inhibition of gastric acid secretion by omeprazole and ranitidine in gastric fistula rats

Basal, pentagastrin- and histamine-stimulated acid secretion were measured in gastric fistula rats treated with the H+/K(+)-ATPase inhibitor, omeprazole, and the H2-receptor antagonist, ranitidine. All doses of omeprazole (20, 30, 40, 80, 400 mumol/kg) and ranitidine (125, 187.5, 250, 375 mumol/kg) essentially abolished the basal acid output for various periods of time. Omeprazole, 80 mumol/kg, administered twice daily, reduced the 24-h basal acid secretion more effectively than did 400 mumol/kg given once daily. Four daily administrations of ranitidine reduced the 24-h basal acid output to a similar extent as omeprazole administered twice. Omeprazole (20, 80 mumol/kg) was more effective than ranitidine (125, 375 mumol/kg) in inhibiting acid secretion evoked by maximal doses of pentagastrin (650 nmol/kg per h) and histamine dihydrochloride (136 mumol/kg), whereas this difference was less pronounced for the inhibition of acid responses induced by a threshold dose (1.1 mumol/kg) of histamine. The inhibition evoked by omeprazole (80 mumol/kg x 2) and ranitidine (375 mumol/kg x 4) of basal and histamine (1.1 and 136 mumol/kg)-induced acid secretion was similar after 1 and 4 weeks of treatment. After the end of drug administration, the acid secretion induced by threshold doses of histamine was significantly elevated in the omeprazole-treated rats, whereas no significant hypersecretion of acid was seen during the recovery period in rats treated with ranitidine. Plasma gastrin concentrations were significantly elevated after 4 weeks of treatment with omeprazole but returned to pretreatment levels after 4 weeks of recovery.(ABSTRACT TRUNCATED AT 250 WORDS)     

Specific labelling of gastric H+,K+-ATPase by omeprazole

Acid secretion is conducted by the parietal cell of the gastric mucosa. The H+,K+-ATPase has been shown to be specifically located to this cell and during recent years been recognized as the gastric proton pump. Omeprazole, a known inhibitor of acid secretion, administered in vivo was found to bind specifically to the H+,K+-ATPase of the rabbit gastric mucosa. A stoichiometry of 2.1 mol radiolabel per mol phosphoenzyme was calculated at total inhibition of the H+,K+-ATPase enzyme activity. In isolated gastric glands prepared from omeprazole-treated animals, the secretagogue-induced increase in oxygen consumption, related to acid secretion, was inhibited to the same level as the H+,K+-ATPase activity. Both the degree of acid secretion inhibition induced by omeprazole and the amount of inhibitor bound to the H+,K+-ATPase were found to be dependent on the stimulation state of the parietal cell. Inhibition of secretion by the H2-receptor blocker ranitidine prior to omeprazole treatment prevented both the inhibition of H+,K+-ATPase and oxygen consumption normally observed with omeprazole and, furthermore, reduced the binding levels of radiolabel to the enzyme. Inhibition of acid secretion by the H+,K+-ATPase inhibitor SCH 28080 totally prevented the binding of radiolabel to the H+,K+-ATPase. The inhibition by omeprazole could be fully reversed in gastric glands and H+,K+-ATPase isolated from omeprazole-treated animals by addition of beta-mercaptoethanol. The major product formed during reactivation was the reduced form of omeprazole, compound H 168/22. Neutralization of the gastric glands in vitro with imidazole totally prevented the inhibitory action of omeprazole. These experiments demonstrate the necessity of acid for the inhibition of gastric acid secretion by omeprazole and the binding of the inhibitor to the H+,K+-ATPase, both in vivo and in vitro, and also the specificity of omeprazole for the H+,K+-ATPase.     

Sustained Cholecystokinin–B/Gastrin Receptor Blockade Does Not Impair Basal or Histamine–Stimulated Acid Secretion in Chronic Gastric Fistula Rats

Abstract: Gastrin is a physiologically important secrelagogue. It is thought to stimulate parietal cells indirectly by mobilizing histamine from enterochromaffin–like (ECL) cells in the oxyntic mucosa. Gastrin stimulates the secretory activity and growth of the ECL cells via an action on cholecystokinin–B/gastrin receptors. Acute cholecystokinin–B/gastrin receptor blockade is known to inhibit gastrin–stimulated acid secretion but whether sustained cholecystokinin–B/gastrin receptor blockade will impair basal, gastrin– and histamine–stimulated acid secretion remains uncertain. The present study was designed to study the effect of long–term (4 weeks) cholecystokinin–B/gastrin receptor blockade on basal and stimulated acid secretion in conscious rats. The selective cholecystokinin–B/gastrin receptor antagonist YM022 (3 μmol kg^–1 hr^–l) was given to gastric fistula rats by continuous subcutaneous infusion via osmotic minipumps for various times from 2 hr to 4 weeks. Basal, gastrin– and histamine– stimulated acid secretion were examined during and after cessation of treatment. Basal and histamine–stimulated acid secretion was not affected by YM022 during the 4 week period of administration, whereas gastrin–induced acid secretion was inhibited. YM022 induced hypergastrinaemia in freely fed rats but did not affect the serum gastrin level in fasted rats. The serum gastrin concentration and gastrin–induced acid secretion returned to control levels 3–7 days after termination of YM022 administration.     

Changes in gastric ECL cells and parietal cells after long-term administration of high-dose omeprazole to patients with Barrett's esophagus

PURPOSE: Long-term administration of PPI causes hyperplastic changes of the gastric parietal cells; however, the detailed mechanism remains to be clarified. We administered high-dose omeprazole to patients with Barrett's esophagus for 2 years, and investigated changes in gastric ECL (Enterochromaffin-like) cells using endoscopic biopsy specimens to clarify the etiology of hyperplasia of the parietal cells. METHODS: The subjects were 69 patients who were diagnosed as having Barrett's esophagus (39 males, 30 females). We established two groups, an omeprazole-treated group and a ranitidine-treated group. Upper digestive tract endoscopy was performed before administration, and 12 and 24 months after the start of administration. Biopsy was performed in the greater curvature of the gastric body. The ECL/parietal cell counts and the grade of hyperplasia of the gastric mucosa were determined under a microscope. In addition, the fasting serum gastrin level was measured, and statistical analysis was performed. RESULTS: In the omeprazole-treated group, the ECL cell count was markedly increased 12 months after the start of administration, but was lower than the pretreatment value 24 months after the start of administration. The parietal and ECL cell counts significantly increased. Furthermore, there were no changes in mucosa thickness. The fasting serum gastrin level significantly increased. In the ranitidine-treated group, there was no increase in the ECL cell count, and the parietal cell count was decreased. There was no significant increase in mucosa thickness. The fasting serum gastrin level increased, although the rate of increase was markedly smaller than that in the omeprazole-treated group. CONCLUSION: Not the direct pharmacological actions of PPI but hypergastrinemia-associated secondary changes may be etiologically involved in hyperplasia of the parietal cells related to long-term administration of PPI.     

The cellular localization of the cholecystokinin 2 (gastrin) receptor in the stomach

The role of the gastric acid secretagogues acetylcholine, gastrin and histamine has been debated for decades. Initially, the mast cell was considered the source of acid stimulatory histamine. Later, H?kanson & Owman (1969) showed that the entero-chromaffinlike (ECL) cell produces and stores histamine in several species, including rat and man. Kahlson et al. (1964) showed that food and gastrin stimulated oxyntic mucosal histamine synthesis and release, Berglindh et at. (1976) that histamine and cholinergics but not gastrin induced acid secretion in isolated oxyntic glands and parietal cells, and Rangachari (1995) that acetylcholine or gastrin released histamine in isolated mucosa. These findings suggested that gastrin stimulates acid secretion through release of ECL cell histamine. Studying simultaneous histamine release and acid secretion in isolated oxyntic mucosal cells, we found that gastrin stimulated acid secretion only in preparations releasing histamine. Moreover, in the isolated rat stomach, gastrin stimulated both histamine release and acid secretion. Maximal acid output was higher with histamine than with gastrin, and augmented by acetylcholine but not by gastrin. These findings strongly suggested that gastrin acts by releasing histamine. Finally, a fluorescein-labelled gastrin analogue bound to the ECL cell, not to the parietal or stem cell regions. This is interesting, recalling that gastrin has a potent and specific trophic effect on the ECL cell and only a general effect on all other oxyntic cell types. In conclusion, physiological observations are best explained by localising the CCK2 receptor only to the ECL cell, the other effects of gastrin on the gastric mucosa being secondary to the release of mediators from the ECL cell.     

Review: antisecretory drugs: cellular mechanisms of action

Parietal cell secretory function may be inhibited by three mechanisms. (1) Receptors for gastrin, histamine and acetylcholine are present on the canine parietal cell, and parietal cell function may be directly inhibited by specific antagonists for each of these receptors. (2) Receptor activation of parietal cell function is mediated by cyclic AMP-dependent (histamine) and calcium-dependent (cholinergic agents and gastrin) mechanisms. The antisecretory action of prostaglandins reflect interference with histamine activation of adenylate cyclase. The current generations of calcium channel blockers have only weak antisecretory actions in vivo and are unlikely to be useful in clinical practice. (3) A third mechanism of inhibition is blockade of H+/K(+)-ATPase by substituted benzimidazoles, such as omeprazole. Each of these three mechanism provides modalities of potential clinical usefulness for treating acid-peptic disease. Gastrin and acetylcholine receptors are present on other fundic cells, in addition to the parietal cell. These other cells include the somatostatin cell in the dog fundic mucosa and the histamine-containing enterochromaffin-like (ECL) cell present in the fundic mucosa of several species. The relative impact of these receptors on different cell types on the regulation of acid secretion remains uncertain, and is probably variable among different species. One gastrin receptor of considerable importance is the gastrin receptor that exerts a trophic effect on the ECL cell in the fundic mucosa. Sustained hypergastrinaemia in response to profound hypochlorhydria is associated with hyperplasia of this cell type; the elucidation of the conditions that promote this hyperplasia and the clinical consequences of this association are pressing challenges.     

Cholecystokinin-B/Gastrin Receptor Blockade Suppresses the Activity of Rat Stomach ECL Cells

Abstract: Gastrin controls the histamine- and chromogranin A- producing enterochromaffinn-like (ECL) cells, the predominant endocrine cell population in the acid-producing part of the rat stomach. They are responsible for most of the circulating pancreastatin, a chromogranin A-derived peptide. The present study examines the ability of two potent and highly selective cholecystokinin-B/gastrin receptor antagonists. RP73870 and YM022, to incapacitate the ECL cells. The two antagonists were given by continuous subcutaneous infusion to otherwise untreated rats and to hypergastrinaemic rats treated with gastrin-17 (continuous subcutaneous infusion) or omeprazole (orally) for 7 days. Several parameters reflecting ECL cell activity were measured: The oxyntic mucosal histidine decarboxylase activity, the histamine concentration, the histidine decarboxylase mRNA and chromogranin A mRNA concentrations, and the serum pancreastatin concentration. In addition, the serum gastrin concentration was measured. RP73870 and YM022 greatly lowered the oxyntic mucosal histidine decarboxylase activity and the histidine decarboxylase mRNA and chromogranin A mRNA concentrations, and also reduced the oxyntic mucosal histamine concentration and the serum pancreastatin concentration. Moreover, they raised the serum gastrin concentration. With respect to blockade of histidine decarboxylase activity, 1.0 μmol· g^?1 · hr^?1 was an almost maximally effective dose for both RP73870 and YM022. The corresponding ID₅₀ values were 0.04 and 0.05 μmol- kg^?1 · hr^?1. RP73870 and YM022 inhibited the hypergastrinaemia-evoked rise in all ECL-cell parameters. The results suggest that sustained cholecystokinin-B/gastrin receptor blockade causes lasting deactivation of the ECL cells.     

Effect of omeprazole on gastric secretion in H+,K+-ATPase and in pepsinogen-rich cell fractions from rabbit gastric mucosa

In order to study the effects of the substituted benzimidazole omeprazole on gastric secretory functions, parietal cells and chief cells from rabbit gastric mucosa were separated and enriched by density gradient centrifugation in Percoll. H+,K+-ATPase activity, as well as a 100,000 dalton protein, was found to copurify with a cell fraction morphologically characterized as mainly parietal cells (purity approximately 65%), while pepsinogen copurified with a cell fraction morphologically characterized as chief cells (purity approximately 90%). A spontaneous pepsinogen release (9.9 micrograms/mg cell dry wt X 2 hr), unaffected by both atropine and omeprazole, was found in the chief cell fraction. The release was approximately doubled by both carbacholine (4 X 10(-5)M) and dibutyryl cAMP (db-cAMP, 10(-3)M). The cholinergic stimulation was selectively blocked by atropine, while omeprazole had no effect on pepsinogen release induced by either of the secretagogues. On the other hand, omeprazole inhibited both db-cAMP- and histamine-stimulated acid secretion quantified as [14C]aminopyrine (AP) accumulation in the parietal cell fraction. Cimetidine counteracted only acid secretion induced by histamine. These findings indicate that omeprazole has a specific effect on acid secretion, and are consonant with the hypothesis that the effect is due to H+,K+-ATPase inhibition.     

Omeprazole. An updated review of its pharmacology and therapeutic use in acid-related disorders.

Omeprazole is the first of a new class of drugs, the acid pump inhibitors, which control gastric acid secretion at the final stage of the acid secretory pathway and thus reduce basal and stimulated acid secretion irrespective of the stimulus. In patients with duodenal or gastric ulcers, omeprazole as a single 20 mg daily dose provides more rapid and complete healing compared with ranitidine 150 mg twice daily or 300 mg at nighttime, or cimetidine 800 or 1000 mg/day. Patients poorly responsive to treatment with histamine H2-receptor antagonists respond well to omeprazole--most ulcers healed within 4 to 8 weeks of omeprazole 40 mg/day therapy. Omeprazole 20 or 40 mg/day has been administered as maintenance therapy for peptic ulcer disease for up to 5.5 years with very few ulcer recurrences. In patients with erosive or ulcerative oesophagitis, omeprazole 20 or 40 mg/day produces healing in about 80% of patients after 4 weeks, and is superior to ranitidine with respect to both healing and symptom relief. Healing rates of greater than 80% are achieved after 8 weeks in patients with severe reflux oesophagitis unresponsive to H2-receptor antagonists. Maintenance therapy with a daily 20 mg dose prevents relapse in about 80% of patients over a 12-month period. Omeprazole is considered to be the best pharmacological option for controlling gastric acid secretion in patients with Zollinger-Ellison syndrome. Daily dosages of 20 to 360 (median 60 to 70 mg successfully reduce basal acid output to target levels (less than 10 mmol/h or less than 5 mmol/h in patients with severe oesophagitis or partial gastrectomy) during treatment for up to 4 years. Omeprazole is well tolerated in short term studies (up to 12 weeks); the reported incidence of serious side effects (about 1%) being similar to that seen in patients treated with an histamine H2-receptor antagonist. The longer term tolerability of omeprazole has been investigated in patients treated for up to 5.5 years. Slight hyperplasia, but no evidence of enterochromaffin-like (ECL) cell dysplasia or neoplasia or ECL cell carcinoids has been reported. ECL cell carcinoids have been observed in rats after life-long treatment with high doses of omeprazole or ranitidine, or in rats with partial corpectomy; the weight of experimental evidence indicates that this is a result of prolonged hypergastrinaemia.(ABSTRACT TRUNCATED AT 400 WORDS)     

The influence of omeprazole on the protective effects of secretagogues against ethanol-induced gastric ulceration in rats

The influence of omeprazole on the antiulcer effects of histamine, methacholine and pentagastrin on ethanol-induced gastric ulceration was studied. Histamine 5 mg/kg, methacholine 500 micrograms/kg or pentagastrin 100 micrograms/kg pretreatment s.c. markedly reduced ethanol (50%, 10 ml/kg p.o.)-induced gastric ulceration in pylorus-ligated conscious rats, but significantly elevated the gastric secretory volume and acid output. Omeprazole pretreatment 50 mg/kg s.c. reduced gastric secretion and abolished the ulcer-protecting effects of the three secretagogues. The same doses of histamine, methacholine or pentagastrin did not significantly alter the gastric secretory volume but increased the total acid output of ex-vivo stomach chamber preparations in anaesthetised animals. Histamine worsened ethanol-evoked ulceration whereas methacholine and pentagastrin had no effect on the lesions. Omeprazole pretreatment did not prevent ulcer aggravation by histamine. It is concluded that the antiulcer effects of the three secretagogues in conscious animals are probably due to the increased secretory volume which lessens the ulcerogenic action of ethanol by its dilution. Omeprazole reduces the secretory volume and thus prevents the antiulcer effect of these secretagogues. The findings with histamine and omeprazole in the ex-vivo stomach experiments support the suggestion that it is unlikely that increased gastric acid secretion affects the ulcerogenicity of ethanol in rat stomach.     

Effects of topical application of acidified omeprazole on acid secretion and transmucosal potential difference in anesthetized rat stomachs

Effects of topical application of omeprazole on transmucosal potential difference (PD), luminal pH and histamine-stimulated acid secretion were examined in anesthetized rat stomachs, and they were compared with those of systemic administration. Omeprazole was suspended in 1% CMC with NaHCO3 (pH 9.0) or dissolved in 0.1 N HCl (pH 1.0). Both omeprazole (30 mg/kg, pH 9.0) and cimetidine (100 mg/kg), given i.d., increased the pH and inhibited acid secretion induced by histamine (8 mg/kg/hr, i.v.), while basal gastric PD was markedly elevated only by the former. Similar responses in PD, pH and acid output were obtained dose-dependently after brief exposure of the stomach (10 min) to omeprazole (0.3-30 mg/kg), even in acidic conditions, but the effects of acidified omeprazole disappeared depending upon the latency period in 0.1 N HCl; there was no effect when applied at more than 30 min after dissolution. Of interest, subsequent exposure of the stomach to a mercaptane compound (cysteine, 100 mg/kg) for 30 min significantly reversed the antisecretory effect of omeprazole (both i.d. and i.g.) but not of cimetidine. These results suggest that omeprazole has a local antisecretory action even in acidic stomachs, probably through an inhibition of the H+/K+ATPase activity, and the increase of PD caused by omeprazole may be a characteristic phenomenon seen after the blockade of H+/K+ ATPase, but is not associated with acid inhibition itself.     

Anaesthetic agents inhibit gastrin-stimulated but not basal histamine release from rat stomach ECL cells

By mobilizing histamine in response to gastrin, the ECL cells in the oxyntic mucosa play a key role in the control of the parietal cells and hence of gastric acid secretion. General anaesthesia suppresses basal and gastrin- and histamine-stimulated acid secretion. The present study examines if the effect of anaesthesia on basal and gastrin-stimulated acid secretion is associated with suppressed ECL-cell histamine secretion. A microdialysis probe was implanted in the submucosa of the ventral aspect of the acid-producing part of the stomach (32 rats). Three days later, ECL-cell histamine mobilization was monitored 2 h before and 4 h after the start of intravenous infusion of gastrin (5 nmol kg(-1) h(-1)). The rats were either conscious or anaesthetized. Four commonly used anaesthetic agents were given 1 h before the start of the experiments by intraperitoneal injection: chloral hydrate (300 mg kg(-1)), pentobarbitone (40 mg kg(-1)), urethane (1.5 g kg(-1)) and a mixture of fluanisone/fentanyl/midazolam (15/0.5/7.5 mg kg(-1)). In a parallel series of experiments, basal- and gastrin-induced acid secretion was monitored in six conscious and 25 anaesthetized (see above) chronic gastric fistula rats. All anaesthetic agents lowered gastrin-stimulated acid secretion; also the basal acid output was reduced (fluanisone/fentanyl/midazolam was an exception). Anaesthesia reduced gastrin-stimulated but not basal histamine release by 55 - 80%. The reduction in gastrin-induced acid response (70 - 95%) was strongly correlated to the reduction in gastrin-induced histamine mobilization. The correlation is in line with the view that the reduced acid response to gastrin reflects impaired histamine mobilization. Rat stomach ECL cells were purified by counter-flow elutriation. Gastrin-evoked histamine mobilization from the isolated ECL cells was determined in the absence or presence of anaesthetic agents in the medium. With the exception of urethane, they inhibited gastrin-evoked histamine secretion dose-dependently, indicating a direct effect on the ECL cells. Anaesthetized rats are widely used to study acid secretion and ECL-cell histamine release. The present results illustrate the short-comings of such an approach in that a number of anaesthetic agents were found to impair not only acid secretion but also the secretion of ECL-cell histamine - some acting in a direct manner.     

Omeprazole in peptic ulcers resistant to histamine H2-receptor antagonists

Eighteen patients with duodenal, gastric or jejunal ulcers, resistant to at least 3 months treatment with histamine H2-receptor antagonists, singly or in combination with other anti-ulcer drugs, were treated with 40 mg omeprazole once daily for up to 8 weeks. All ulcers healed, the majority within two weeks. After ulcer healing patients were given maintenance therapy with high doses of cimetidine or ranitidine. Of 15 patients on maintenance therapy with H2-receptor antagonists, 12 (80%) developed a relapse after a period ranging from 3 to 52 weeks. Two patients were lost to follow-up. After re-healing on 40 mg omeprazole, two patients were given 20 mg omeprazole daily as maintenance therapy but relapses occurred again after 14 and 26 weeks respectively. After re-healing on 40 mg omeprazole, these two patients and one additional patient received maintenance therapy with 40 mg omeprazole daily. At present these three patients have been relapse-free for periods varying from 16 to 52 weeks. No side effects were registered during treatment with omeprazole. It is therefore concluded that omeprazole is highly effective in healing refractory peptic ulcers and that omeprazole maintenance therapy may be useful for prevention of relapse. Patients are sometimes seen with peptic ulceration which appears resistant to therapy with histamine H2-receptor antagonists, colloidal bismuth subcitrate, sucralfate or pirenzepine, either given as monotherapy for a prolonged period of time or as combination therapy. Usually the reason for such therapeutic failure remains obscure. Whether virtually total abolition of acid secretion will allow ulcer healing in these circumstances is unknown.(ABSTRACT TRUNCATED AT 250 WORDS)     

Review: the enterochromaffin-like cell, intragastric acidity and the trophic effect of plasma gastrin

Although enterochromaffin-like (ECL) cells form the major endocrine cell population of the non-antral stomach, they have been largely overlooked in the study of gastric disease. In the human, their product and functions are unknown, but they are associated with histamine secretion in rodents. The cells are controlled by neural and hormonal factors, the most significant of the latter being gastrin. Interest in ECL cells has been stimulated by the observation that hyperplasia of these cells, sometimes leading to formation of gastric carcinoid tumours, occurs in conditions of persistent hypergastrinaemia - for example, in response to the achlorhydria of individuals with pernicious anaemia. The advent of new highly potent inhibitors of gastric acid secretion is allowing more information to be obtained on the physiology and functions of the ECL cell. However, there is clearly a great deal more to be discovered about this enigmatic endocrine cell type.     

Effects of IY-81149, a newly developed proton pump inhibitor, on gastric acid secretion in vitro and in vivo

The inhibitory effects of IY-81149 (2-[[(4-methoxy-3-methyl)-2- pyridinyl]methyl-sulfinyl]-5-(1H-pyrol-1-yl)-1H-benzimidazole, CAS 172152-36-2), a newly developed proton pump inhibitor (PPI) on gastric acid secretion were investigated in vitro and in vivo. In rabbit parietal cell preparation, IY-81149 irreversibly inhibited H+/K(+)-ATPase in dose-dependent manner with an IC50 of pump inhibitory activity of 6.0 x 10(-6) mol/l and that of omeprazole (CAS 73590-58-6) was 1 x 10(-4) mol/l at pH 7.4. On cumulation of 14C-aminopyrine in histamine stimulated parietal cells, the IC50 of IY-81149 was 9.0 x 10(-9) mol/l and that of omeprazole was 1.9 x 10(-8) mol/l. The inhibition rates of IY-81149 and omeprazole at a concentration of 1 x 10(-9) mol/l in human parietal cells were 137% and 64%, respectively. In pylorus-ligated rats, IY-81149 showed a 2-3 times stronger inhibitory activity than omeprazole against gastric acid secretion. The ED50 of IY-81149 and omeprazole administered intraduodenally was 1.6 mg/kg and 3.8 mg/kg. In the case of oral administration, the ED50 of IY-81149 and omeprazole was 1.94 mg/kg and 5.64 mg/kg, respectively. But after 24 h administration, the anti-secretory activity of IY-81149 was lower than that of omeprazole at all doses tested. In anesthetized rats, IY-81149 dose-dependently increased gastric pH which was lowered by histamine infusion. In the case of i.v. injection, the ED50 of IY-81149 and omeprazole was 1.2 and 1.4 mg/kg and in the case of i.d. administration, the ED50 of IY-81149 and omeprazole was 3.9 and 4.1 mg/kg, respectively. IY-81149 also significantly inhibited pentagastrin-stimulated gastric secretion. Its ED50 was 2.1 mg/kg and that of omeprazole was 3.5 mg/kg with i.d. administration. In the case of i.v. injection, IY-81149 was equipotent to omeprazole. IY-81149 also inhibited gastric acid secretion strongly in fistular rats. The ED50 of IY-81149 administered intraduodenally was 0.43 mg/kg and that of omeprazole was 0.68 mg/kg. In Heidenhain pouch dogs, the acid output was completely blocked at 0.3 mg/kg, 135 min after i.v. administration. Omeprazole showed a similar effect as IY-81149. The histamine induced increase of acid output in the Heidenhain pouch dog was blocked by 71% 150 min after oral administration of enteric-coated IY-81149 at a dose of 3 mg/kg, and omeprazole showed similar effects. In conclusion, IY-81149 revealed the characteristics as a strong proton pump inhibitor, and its potency against gastric acid secretion was superior to that of the reference drug, omeprazole.     

Gastric phenotypic abnormality in cholecystokinin 2 receptor null mice

Gastrin, released from antral G-cells, plays an important role in the regulation of gastric acid secretion and is trophic for the stomach. The cholecystokinin type 2 (CCK)2 receptor (previously referred to as CCK-B/gastrin receptors) is expressed in both parietal cells and ECL cells in the oxyntic mucosa of stomach. Gastric phenotypic abnormality has been observed in CCK2 receptor null (gene knock-out) mice. Such mice displayed markedly impaired gastric acid secretion, atrophy of the oxyntic mucosa and hypergastrinaemia. The impaired acid secretion may be the result of a reduced parietal cell mass, a reduced proportion of actively secreting parietal cells (with secretory canaliculi), and a replacement of ECL cells by histamine-free ECL-like cells. The ECL-like cells, observed in the CCK2 receptor null mice, lacked the hallmark features of wild-type ECL cells, i.e. histamine and cytoplasmic secretory vesicles. However, they had the features of endocrine cells, such as the content of pancreastatin (a fragment of chromogranin A), with cytoplasmic small dense-core granules and microvesicles. We propose that the replacement of ECL cells by ECL-like cells in the mutant mice reflects an altered differentiation of the same precursors that develop into ECL cells in wild-type mice. Thus, studies of CCK2 receptor null mice demonstrate the importance of the receptor in the regulation of gastric acid secretion and in the differentiation of ECL cells in the oxyntic mucosa of stomach.     

ECL Cell Histamine Mobilization Studied byGastric Submucosal Microdialysis in Awake Rats:Methodological Considerations

Abstract: The ECL cells are endocrine/paracrine cells in the acid‐producing part of the stomach. They secrete histamine in response to circulating gastrin. Gastric submucosal microdialysis has been used to study ECL‐cell histamine mobilization in awake rats. In the present study we assess the usefulness and limitations of the technique. Microdialysis probes were implanted in the gastric submucosa. Histological analysis of the stomach wall around the probe revealed a moderate, local inflammatory reaction 1–2 days after implantation; the inflammation persisted for at least 10 days. Experiments were conducted 3 days after the implantation. The “true” submucosal histamine concentration was determined by perfusing at different rates (the zero flow method) or with different concentrations of histamine at a constant rate (the no‐net‐flux method): in fasted rats it was calculated to be 87±5 (means±S.E.M.) nmol/l and 76±9 nmol/l, respectively. The corresponding histamine concentrations in fed rats were 93±5 and 102±8 nmol/l, respectively. With a perfusion rate of 74 μl/hr the recovery of submucosal histamine was 49%, at 34 μl/hr the recovery increased to 83%. At a perfusion rate below 20 μl/hr the microdialysate histamine concentration was close to the actual concentration in the submucosa. The ECL‐cell histamine mobilization was independent of the concentrations of Ca²⁺ in the perfusion medium (0–3.4 mmol/l Ca²⁺). In one experiment, histamine mobilization in response to gastrin (10 nmol/kg/hr subcutaneously) was monitored in rats pretreated with prednisolone (60 mg/kg) or indomethacin (15 mg/kg). The two antiinflammatory agents failed to affect the concentration of histamine in the microdialysate either before or during the gastrin challenge, which was in accord with the observation that the inflammatory reaction was modest and that inflammatory cells were relatively few around the probe and in the wall of the probe. In another experiment, rats were given aminoguanidine (10 mg/kg) or metoprine (10 mg/kg) 4 hr before the start of gastrin infusion (5 nmol/kg/hr intravenously). Metoprine (inhibitor of histamine N‐methyl transferase) did not affect the microdialysate histamine concentration, while aminoguanidine (inhibitor of diamine oxidase) raised both basal and gastrin‐stimulated histamine concentrations. We conclude that microdialysis can be used to monitor changes in the concentration of histamine in the submucosa of the stomach, and that the inflammatory reaction to the probe is moderate and does not affect the submucosal histamine mobilization.     

Submucosal microinfusion of endothelin and adrenaline mobilizes ECL-cell histamine in rat stomach, and causes mucosal damage: a microdialysis study

Rat stomach ECL cells release histamine in response to gastrin. Submucosal microinfusion of endothelin or adrenaline, known to cause vasoconstriction and gastric lesions, mobilized striking amounts of histamine. While the histamine response to gastrin is sustainable for hours, that to endothelin and adrenaline was characteristically short-lasting (1-2 h). The aims of this study were to identify the cellular source of histamine mobilized by endothelin and adrenaline, and examine the differences between the histamine-mobilizing effects of gastrin, and of endothelin and adrenaline. Endothelin, adrenaline or gastrin were administered by submucosal microinfusion. Gastric histamine mobilization was monitored by microdialysis. Local pretreatment with the H1-receptor antagonist mepyramine and the H2-receptor antagonist ranitidine did not prevent endothelin- or adrenaline-induced mucosal damage. Submucosal microinfusion of histamine did not cause damage. Acid blockade by ranitidine or omeprazole prevented the damage, suggesting that acid back diffusion contributes. Gastrin raised histidine decarboxylase (HDC) activity close to the probe, without affecting the histamine concentration. Endothelin and adrenaline lowered histamine by 50-70%, without activating HDC. Histamine mobilization declined upon repeated administration. Endothelin reduced the number of histamine-immunoreactive ECL cells locally, and reduced the number of secretory vesicles. Thus, unlike gastrin, endothelin (and adrenaline) is capable of exhausting ECL-cell histamine. Microinfusion of alpha-fluoromethylhistidine (known to deplete ECL cells but not mast cells of histamine) reduced the histamine-mobilizing effect of endothelin by 80%, while 1-week pretreatment with omeprazole enhanced it, supporting the involvement of ECL cells. Somatostatin or the prostanoid misoprostol inhibited gastrin-, but not endothelin-stimulated histamine release, suggesting that endothelin and gastrin mobilize histamine via different mechanisms. While gastrin effectively mobilized histamine from ECL cells in primary culture, endothelin had no effect, and adrenaline, a modest effect. Hence, the striking effects of endothelin and adrenaline on ECL cells in situ are probably indirect, possibly a consequence of ischemia.