Pharmacological analysis of the CCKB/gastrin receptors mediating pentagastrin-stimulated gastric acid secretion in the isolated stomach of the immature rat.

1. The CCKB/gastrin receptors mediating pentagastrin stimulation of gastric acid secretion by histamine release and by direct stimulation of oxyntic cells have been characterized in the immature rat isolated stomach assay. This was achieved by estimating antagonist affinity values for competitive antagonists from three distinct chemical classes (L-365,260, PD134,308 and JB93190) in the absence and presence of a high concentration of the histamine H2-receptor antagonist, famotidine (30 microM). 2. Pentagastrin produced concentration-dependent stimulation of gastric acid secretion in the absence and presence of famotidine. Famotidine depressed the maximum secretory response to pentagastrin although the degree of depression varied between experimental replicates (25-60%). This variation was attributed to the histamine-release mediated component of acid secretion, as judged by the consistency of the maximum responses obtained in the presence, but not absence, of famotidine. 3. All three CCKB/gastrin receptor antagonists behaved as surmountable antagonists in the absence and presence of famotidine. JB93190 (pKB approximately 9.1, approximately 8.9, in the absence and presence of famotidine, respectively) was approximately 30 fold more potent than either L-365,260 (pKB approximately 7.4, approximately 7.1) or PD134,308 (pKB approximately 7.6, approximately 7.4). 4. It was assumed that the famotidine treatment converted pentagastrin-stimulated acid secretion from a combination of an indirect action due to the release of histamine and a direct action on the oxyntic cell to solely a direct action on the oxyntic cell. A simple mathematical model of this two-receptor system was developed. The direct and indirect components were assumed to sum to produce the total response to pentagastrin obtained in the absence of famotidine. It was found that this model could account quantitatively for the behaviour of the three antagonists without invoking a difference in antagonist affinity for the CCKB/gastrin receptors mediating the direct and indirect actions of pentagastrin. However, a conclusion of receptor homogeneity has to be qualified because the model was also used to generate simulations which indicated that the analysis could only detect antagonist affinity differences of greater than one log-unit between enterochromaffin-like (ECL) and oxyntic cell CCKB/gastrin receptor populations.     

Inhibition of gastrin- and histamine-stimulated gastric acid secretion by gastrin and cholecystokinin antagonists in the rat

The gastric acid inhibitory activities of a peptide-like gastrin receptor antagonist, Boc-beta Ala-Trp-Leu-Asp-O(CH2)2-Ph-4-F (CH-486), a nonpeptide gastrin/CCK-B antagonist (L-365,260), and a CCK-A antagonist (L-364,718), were investigated in the gastric lumen-perfused anaesthetized rat. A single i.v. injection of CH-486, 100 mumol/kg, reduced acid secretion stimulated by pentagastrin, 15 micrograms kg/h, to unstimulated levels, with no recovery within 50 min. Histamine-, 0.1 mumol kg/min, and carbamylcholine-, 0.1 mg kg/h, stimulated acid secretion were also reduced to unstimulated levels by CH-486, 100 mumol/kg, although with these latter two stimulants the inhibition was transient. L-365,260 and L-364,718, 10 mumol/kg, significantly inhibited both pentagastrin- and histamine-stimulated acid secretion, the latter again transiently. We conclude that the non-selective nature of the gastric acid inhibitory activity of gastrin antagonists might allow novel approaches to control gastric acid secretion in peptic ulcer disease.     

Inhibitory effect of intravenous GABA antagonists on gastric acid secretion stimulated by secretagogues in rats

Effect of intravenous administration of GABA antagonists on gastric acid secretion in perfused stomachs was studied in rats anesthetized with urethane. Bethanechol (BeCh)-stimulated acid secretion was definitely inhibited by bicuculline, a GABA antagonist, and strychnine, a glycine antagonist, but not by picrotoxin and pentylenetetrazol, GABA antagonists. The inhibitory effect of bicuculline and strychnine was accompanied by vigorous convulsions. Only the bicuculline-induced inhibition was still seen in d-tubocurarine paralyzed rats, and it was abolished in spinal rats. 2-Deoxy-D-glucose (2-DG)-stimulated acid secretion was apparently depressed by all the GABA antagonists of bicuculline, picrotoxin and pentylenetetrazol. The inhibitory effect of picrotoxin, but not bicuculline, on the 2-DG stimulation was still elicited in spinal rats. Inhibition of acid secretion stimulated by pentobarbital, a centrally acting secretagogue, was produced by picrotoxin and pentylenetetrazol in spinal rats. These findings suggest that bicuculline acts centrally to inhibit acid secretion stimulated both peripherally by BeCh and centrally by 2-DG, besides nonspecific mechanisms due to convulsions, and the action would be directed to centers which are implicated in stimulation of the sympatho-adrenomedullary system; picrotoxin and pentylenetetrazol also act centrally to inhibit 2-DG- or pentobarbital-stimulated acid secretion through depression of the central vagal tone which leads to inhibition of gastric acid secretion.     

Effects of GABA antagonists and structural GABA analogues on baclofen stimulated gastric acid secretion in the rat

The effects of GABA receptor antagonists (bicuculline and phaclofen) and structural GABA-analogues on baclofen stimulated gastric acid secretion were studied in standardized perfused rat stomach preparations. Pretreatment with bicuculline, a GABAA-receptor antagonist, in the doses of 1 and 3 mg/kg, subcutaneously, had no influence on the gastric acid response to baclofen. In addition, phaclofen, a GABAB antagonist, in the doses of 3 to 30 mg/kg, intravenously, was found to have no significant effect on the acid response to baclofen. However, GABA-analogues (MOPS and ABA; 10-30 mg/kg, i.v.) and lipophilic GABA derivatives structurally related to beta-amino acids (APPA and APHA; 30 mg/kg, i.v.) significantly counteracted the secretagogue action of baclofen. Further experiments on APPA action showed that the antisecretory effect of APPA could be overcome by higher doses of baclofen. APPA did not affect bethanechol stimulated acid secretion. These results suggest that the secretagogue action of baclofen is independent to GABAA- and GABAB-receptors and that APPA may interact with baclofen in regulation mechanisms of acid secretion, although further investigations are necessary to define the mode of action of APPA on the GABA-ergic system.     

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.     

The effect of atropine on acid secretion stimulated by acetylcholine, histamine and gastrin in the isolated whole stomach of the rat.

1 An isolated stomach preparation from immature rats has been used to study the effect of atropine on gastric acid secretion. 2 The acid secretory response to acetylcholine was not inhibited by atropine at a concentration of 0.3 micrometer. Concentrations of atropine of 1 to 3 micrometer produced a measurable inhibition of acid secretion, and a concentration of atropine of 10 micrometer caused a complete block of acid secretion which could not be surmounted by high concentrations of acetylcholine. 3 The acid secretory response to histamine was not inhibited by concentrations of atropine of up to 1 mM. 4 Concentrations of atropine of 1 micrometer and 10 micrometer did not inhibit gastrin-stimulated acid secretion, although a significant inhibition of acid output was observed with atropine at concentrations of 0.1 mM and 1 mM. 5 These findings are discussed in relation to the role of cholinergic mechanisms in the control of gastric acid secretion.     

The effect of cimetidine on basal gastric acid secretion in the rat.

1. The effect of cimetidine on the basal gastric acid secretion of the rat has been investigated in an anaesthetized lumen-perfused preparation. 2. Six rats previously given large doses of cimetidine orally showed no significant difference in basal gastric acid secretion when compared with six control rats. 3. Intravenous administration of 1 mg and 8 mg of cimetidine failed to inhibit significantly basal gastric acid secretion. 4. Although rats with a basal gastric acid secretion above 2.5 micro Eq/10 min. showed a consistent small reduction in basal gastric acid secretion after intravenous cimetidine, this was not seen in rats with a basal gastric acid secretion below 2.5 micro Eq/10 min. 5. THese results contrast sharply with the pronounced inhibition of basal gastric acid secretion by cimetidine in man and the possible reasons for this are discussed. 6. The results are also contrasted with previous work on gastric fistula rats which showed higher basal gastric acid secretion and significant inhibition by cimetidine.     

The effect of atropine, propantheline and poldine on the vagally stimulated gastric motility and the histamine-stimulated acid gastric secretion in the rat

Histamine-induced acid gastric secretion in the anaesthetized rat was not diminished by poldine in a dose which reduced vagally stimulated gastric contractions by approximately 75%. A dose of atropine, twice as large as the dose which reduced gastric contractions by 75%, had no apparent effect on the histamine-stimulated acid gastric secretion up to 2 hr after the injection. Only when more than 40 times as much atropine was injected did a slight inhibition of the acid secretion occur in 80 to 120 min. Propantheline, in a dose which inhibited gastric contractions by approximately 75%, slightly diminished acid secretion in 40 to 80 min. This effect was not increased by a further dose of propantheline. It was concluded that, in so far as any inhibition of acid gastric secretion had occurred, this could not be interpreted as an anti-muscarine or a direct toxic effect, but rather as an indirect effect possibly due to interference with the blood flow through the stomach wall.     

The effect of hexamethonium on gastric acid secretion in the conscious rat

Evidence against a role for histamine in pentagastrin-stimulated acid secretion has been obtained previously using hexamethonium in gastric fistula rats. This possibility has been re-examined in conscious rats provided with gastric fistulae or Heidenhain pouches. Hexamethonium (20 mg/kg s.c.) inhibited basal acid secretion and acid secretion stimulated by histamine and pentagastrin in gastric fistula rats. The same dose of hexamethonium failed to produce a significant inhibition of acid secretion stimulated by bethanechol, pentagastrin or histamine in the presence of a low dose of bethanechol in Heidenhain pouch rats. These results provide no evidence to oppose the view that pentagastrin-stimulated acid secretion in the rat is mediated at least in part through the mobilization of gastric mucosal histamine. The inhibition of secretagogue induced acid secretion in the gastric fistula rat is mainly the result of a reduction in the basal acid output.     

The effect of phloxin on bethanechol and histamine stimulated gastric acid secretion in rats

Summary The effect of phloxin (Na salt of tetrabromo-tetrachlorofluorescein) on histamine- and bethanechol-stimulated gastric acid secretion was studied in anaesthetized rats. Concentrations of phloxin, equimolar with those of the stimulants, depressed the secretory response to histamine and had no influence on the bethanechol effect. The results suggest that histamine is not involved in the events triggered by bethanechol which stimulate gastric acid secretion in rats.Supported by the Deutsche Forschungsgemeinschaft and the Gesellschaft der Freunde der Universität Tübingen.     

The effect cystamine on gastric secretion in the rat.

The effects of cystamine on gastric secretion were studied in conscious and anaesthetized rat preparations. In the conscious gastric fistula rat cystamine inhibited the basal acid output but increased pepsin output. This pepsinogogue action was inhibited by both atropine and metiamide. In the anaesthetized rat cystamine stimulated gastric acid output, an effect blocked by cimetidine which had an inhibitory E.D. 50 which was not significantly different from that obtained against histamine-stimulated secretion in this preparation. Atropine at high doses failed to inhibit the response. Depletion of mast cell histamine by compound 48/80 the secretory response to cystamine. In the light of these results possible mechanisms of action for the secretagogue effects of cystamine are discussed.     

Inhibitory effect of the cannabinoid receptor agonist WIN 55,212-2 on pentagastrin-induced gastric acid secretion in the anaesthetized rat

The effect of the cannabinoid (CB) receptor agonist WIN 55,212-2 on gastric acid secretion was studied in the anaesthetized rat after stimulation with pentagastrin. WIN 55,212-2 (0.5-2 mg/kg, i.v.) was inactive on basal secretion but caused a marked inhibition (80%) of the acid secretion stimulated by pentagastrin (10 microg/kg, i.v.). The enantiomer WIN 55,212-3 (1-3 mg/kg, i.v.) did not significantly modify basal or pentagastrin-induced acid secretion. The inhibitory effect of WIN 55,212-2 against pentagastrin was prevented by the administration of the selective cannabinoid CB1 receptor antagonists SR141716A (1 mg/kg, i.v.) and LY320135 (1 mg/kg, i.v.); by contrast, the CB2 receptor antagonist SR144528 (0.3-1 mg/kg, i.v.) was without effect. The selective CB2 receptor agonist JWH-015 (0.1-10 mg/kg, i.v.) was inactive on the increase of acid output stimulated by pentagastrin. These results suggest that the inhibitory effect of WIN 55,212-2 on pentagastrin-stimulated acid secretion in the anaesthetized rat is mediated by specific cannabinoid receptors. Moreover, the antagonism of WIN 55,212-2-induced effects by the selective CB1 receptor antagonists SR141716A and LY320135 together with the ineffectiveness of both the CB2 receptor agonist JWH-015 and the CB2 receptor antagonist SR144528 indicate that CB1 receptor subtypes are predominantly involved in the antisecretory effect of WIN 55,212-2.     

The effect of brocresine (NSD-1055) on basal and stimulated gastric acid secretion in chickens.

The effect of the histidine decarboxylase inhibitor 4-bromo-3-hydroxybenzyloxyamine-dihydrogenphosphate (brocresine, NSD-1055) on basal and histamine-, pentagastrin- and carbachol-stimulated gastric acid secretion was investigated in unanaesthetized gastric fistula chickens. Basal secretion was initially reduced by brocresine for 45 min and then slightly stimulated for 90 min. The histamine stimulated gastric secretion was competitively inhibited by brocresine. The acid response to low doses of pentagastrin and carbachol was enhanced, that to high doses unchanged. The results can be explained in the light of the assumption that a metabolic product of brocresine, 3-hydroxy-4-bromo-benzyl-alcohol stimulates gastric acid secretion.     

Analysis of the behaviour of selected CCKB/gastrin receptor antagonists in radioligand binding assays performed in mouse and rat cerebral cortex

1. The previously described complex behaviour of the CCKB/gastrin receptor antagonist, L-365,260, in radioligand binding assays could be explained by a variable population of two binding sites. We have investigated whether other CCKB/gastrin receptor ligands (PD134,308, PD140,376, YM022 and JB93182) can distinguish between these sites. 2. In the mouse cortex assay, Hill slopes were not different from unity and the ligand pKI values did not differ when either [125I]-BH-CCK-8S or [3H]-PD140,376 was used as label as expected for a single site (G2). 3. In the rat cortex, where previous analysis of replicate (n=48) L-365,260 data indicated the presence of two CCKB/gastrin sites (G1 and G2), the competition data for PD134,308, PD140,376, YM022 and JB93182 could be explained by a homogeneous population of CCKB/gastrin sites because the Hill slope estimates were not significantly different from unity. However, the estimated affinity values for JB93182 and YM022 were significantly higher and that for PD134,308 was significantly lower than those obtained in the mouse cortex when the same radioligand was used. In view of our previous data obtained with L-365,260, the rat cortex data were also interpreted using a two-site model. In this analysis, SR27897 expressed approximately 9 fold, PD134,308 approximately 13 fold and PD140,376 approximately 11 fold selectivity for the G2 site. In contrast, JB93182 expressed approximately 23 fold and YM022 approximately 4 fold selectivity for the G1 site. If the two-site interpretation of the data is valid then, because of its reverse selectivity to L-365,260, JB93182 has been identified as a compound which if radiolabelled could provide a test of this receptor subdivision.     

Inhibitory effect of isoprenaline on gastric acid secretion in the rat. The role of endogenous histamine

In dogs beta-adrenoreceptor agonists inhibit gastric acid secretion stimulated by exogenous gastrin to a much greater extent than acid secretion stimulated by exogenous histamine. One possible explanation for this observation is that endogenous histamine is important in gastrin-mediated acid secretion and that isoprenaline and related beta-adrenoreceptor agonists block gastric mucosal histamine release. This possibility was tested in the present study in gastric lumen-perfused anaesthetized rats. Intravenous infusion of isoprenaline (12 microgram kg-1 h-1) inhibited maximal, pentagastrin-stimulated acid output by 50-70% (P less than 0.01), but had no significant inhibitory effect on the maximal acid secretory response to histamine. In contrast to its inhibitory effect on gastrin-stimulated acid output, isoproterenol had no effect on gastric histamine output during pentagastrin infusion. We conclude that isoprenaline selectively inhibits gastrin-stimulated acid secretion in the rat, as in the dog, and by a mechanism other than inhibiting gastric histamine release.     

The effect of ciprofibrate on gastric secretion in the rat

The potential of ciprofibrate to inhibit gastric secretion has been investigated in the rat. A significant gastric antisecretory effect was observed following a single oral administration of 300 and 500 mg kg-1 and following a single intraduodenal dose of 100, 300 and 500 mg kg-1. The toxicological significance of this finding is discussed in the light of a spate of recent publications linking changes in gastric morphology with hypergastrinaemia produced as a secondary effect of inhibition of acid secretion.     

Utilization of the perfused stomach in anaesthetized rats to study the inhibitory effect of somatostatin in gastric acid secretion.

The perfused stomach in the anaesthetized rat was used to investigate the action of somatostatin on the gastric acid secretion stimulated by gastrin, histamine and carbamylcholine. Evidence is produced that somatostatin competitively inhibits gastrin-stimulated acid secretion whereas it inhibits carbamylcholine-stimulated acid secretion by a mechanism which is non-competitive in nature and it has no action on histamine-stimulated secretion. The model of the persued stomach in the anaesthetized rat seems suitable to study the inhibition caused by drug on stimulated acid secretion.