The localization of a histamine H2-receptor adenylate cyclase system in canine parietal cells and its inhibition by prostaglandins

A method is described for the preparation of parietal cell-enriched suspensions from dog gastric mucosa. Histamine and E type prostaglandins produce an elevation of cyclic AMP concentration in mixed cell preparations. Parietal cell-rich fractions respond to histamine but only weakly to prostaglandins whilst in fractions virtually free from parietal cells the converse is observed. Prostaglandins which are good antisecretory agents, PGE₁, PGE₂ and 16,16 dimethyl PGE₂ are potent inhibitors of the histamine elevation of cyclic AMP in parietal cell-rich fractions, whilst PGF₂ shows 1% of their potency. The experiments described support the view that histamine stimulates gastric acid secretion by excitation of an H₂-receptor adenylate cyclase system in the plasma membrane of the parietal cell and that acid secretory inhibition by prostaglandins is a result of inhibition of that system.     

Inhibition of gastric acid secretion in the dog by the histamine H2-receptor antagonists ranitidine and cimetidine

The H₂-receptor antagonists ranitidine and cimetidine inhibit gastric acid secretion elicited by a test meal in the dog. Ranitidine is approximately 10 times more potent than cimetidine in this respect.     

Binding of [3H]ICIA 5165, an H2-receptor antagonist to guinea pig gastric mucosa

ICIA 5165, 2-guanidino-4-[4-(2-cyano-3-methylguanidino)butyl] thiazole, a selective histamine H₂-receptor antagonist was radiolabelled with tritium to a specific activity of 50.8 Ci/mmoll for use in binding studies. Radiolabelling did not impair bioactivity.Binding characteristics of [³H]ICIA 5165 to guinea pig gastric mucosa were determined. Ligand binding was rapid, reaching equilibrium within five minutes at 0°C, reversible and saturable. Specific [³H]ICIA 5165 binding had an equilibrium dissociation constant of 1.29×10^–8 M, determined by Scatchard plot analysis, and of 1.02×10^–8 M, calculated from the ratio of the dissociation to association rate constants. A Hill number, n_H, of 1.02 was determined for the specific binding component. Specific binding of [³H]ICIA 5165 to gastric mucosal supernatant was not inhibited by methapyrilene, diphenhydramine, mepyramine, d-chlorpheniramine or I-chlorpheniramine (all at 10^–7 M), or by atropine or propranolol (both at 10^–6 M). Specific [³H]ICIA 5165 binding was inhibited in a concentration dependent manner by non-radioactive ICIA 5165 and tiotidine, as well as by a variety of other agents, with H₂ agonist or H₂ antagonist properties. In competition experiments, however, difficulties encountered in accurately defining the degree of specific binding indicate some reservation should be observed in interpreting these results.     

Protein kinase C inhibits stimulation of adenylate cyclase by the histamine H2 receptor in rat parietal cells

The action of protein kinase C on the stimulation of adenylate cyclase activity by the histamine H₂ receptor was investigated in rat parietal cells. Protein kinase C was activated by preincubating cells with 12-O-tetradecanoylphorbol 13-acetate (TPA), and adenylate cyclase activity was measured in sonicated extracts. TPA (100 nM) inhibited adenylate cyclase activity stimulated by histamine (100 nM-500 M). This effect was related to the concentration of TPA. TPA (100 nM) enhanced the stimulation of adenylate cyclase activity by forskolin (100 M) but had no effect on the stimulation by NaF (10 mM). In conclusion, protein kinase C inhibits stimulation of adenylate cyclase by the histamine H₂ receptor. This action could be mediated by changes in the number or affinity of histamine H₂ receptors or in the coupling of the receptor to the stimulatory guanine nucleotide regulatory subunit Gs.     

Evaluation of novel compounds interacting with H2-histamine receptors: Effect on histamine-sensitive adenylate cyclase activity in guinea-pig gastric mucosa

The activity of a number of compounds belonging to the novel class ofN-imidazolylphenyl-N'-alkyl-formamidines on histamine-sensitive adenylate cyclase was evaluated. All substances inhibited histamine-dependent adenylate cyclase activation. The compounds which were investigated in a wider concentration range, i.e. DA 4360, DA 4577, and DA 4626, behaved as simple competitive antagonists, yielding apparentK _B values comparable with those estimated in conventional H₂-receptor assays. These results provide further evidence for the highly selective H₂-receptor antagonism of these new molecules, and confirm the suitability of the histamine-stimulated adenylate cyclase assay in guinea-pig gastric cells as a functionally reduced system for the study of H₂ antagonists.     

Structure-activity relationship of histamine H2-receptor agonists

Conclusions As demonstrated by the results the active C-5(4) substituted histamines and N-methyl-histamines are to various extents selective H₂-receptor agonists.In contrast to the ileal H₁-receptors the atrial H₂-receptors appear to be noticeably stereoselective.     

Studies on the mode of action of histamine H1- and H2-receptor antagonists on gastric histamine methyltransferase

Histamine methyltransferase from pig antrum mucosa was inhibited by 33 H₁-receptor antagonists, by the H₂-receptor antagonists burimamide and metiamide and by the burimamide analogue 5-methylburimamide, which did neither act on H₁-nor on H₂-receptors. Whereas all H₁-receptor blocking agents as well as metiamide and 5-methylburimamide inhibited the enzyme in a competitive manner, the type of inhibition found for burimamide was a mixture of non-competitive and uncompetitive with respect to histamine, which is similar to that one observed with 1-methylhistamine, the product of the histamine methylation reaction. Furthermore, all compounds tested — with the only exception of burimamide — activated the gastric histamine methyltransferase in lower concentrations, the most potent activator being piprinhydrinate (180% increase of the enzyme activity). This enhancement of 1-methylhistamine formation by antihistaminic drugs was not due to a true activation of the enzyme by increasingV _max, but was caused by partially abolishing the inhibition of histamine methyltransferase by so-called optimum concentrations of histamine. Two explanations were given for the different mode of action of burimamide compared with that of metiamide and the other antihistaminic drugs: (1) The change in the type of inhibition from burimamide to metiamide seemed to be due to the introduction of a methylgroup into position 5 of the imidazole ring. (2) Burimamide and 1-, 2- and 3-methylhistamine were the only compounds tested in which the imidazole nucleus was substituted at position 4, but not at position 5, and which thus probably produced substrate or product inhibition.supported by a grant of the Deutsche Forschungsgemeinschaft (Lo 199/3).     

Characterization of the VIP- and secretin-stimulated adenylate cyclase system from human lung

The response of a crude particulate adenylate cyclase preparation from surgically removed human lung to guanine nucleotides, sodium fluoride, -adrenergic agonists, prostaglandins, vasoactive intestinal peptide (VIP), secretin, and [Val⁵]secretin was investigated. The enzyme activity increased 5, 10, and 9-fold, respectively, with GTP, Gpp(NH)p, and sodium fluoride. This activity was stimulated (in the presence as well as in the absence of added GTP) byd,l-isoproterenol,l-epinephrine andl-norepinephrine, the relative potency of these agonists being compatible with the existence of -adrenoceptors of the -adrenoceptors of the ₂ subtype. Prostaglandins E₁ and E₂, but not PGF₁ and PGF₂, stimulated the enzyme, PGE₁ being at least 10 times more potent than PGE₂. The biphasic pattern of stimulation of the same adenylate cyclase activity by VIP suggested the presence of high- and low-affinity VIP receptors coupled to the enzyme. This stimulation by VIP was not inhibited by secretin-(7–27). The stimulation of adenylate cyclase by secretin and [Val⁵]secretin was also biphasic, suggesting the coexistence of high- and low-affinity secretin receptors. Secretin-(7–27) was able to inhibit completely the secretin stimulation acting through high-affinity secretin receptors but exerted no effect on the stimulation operating through low-affinity secretin receptors, which might indicate that the latter receptors were in fact VIP-preferring receptors. [Val⁵]secretin was also used to differentiate these peptide receptors, since its properties were more VIP-like than those of secretin.Abbreviations VIP vasoactive intestinal peptide - Sn secretin - cyclic AMP cyclic adenosine 3:5-monophosphate - Gpp(NH)p guanosine 5-O-(2-3-imido)triphosphate - EGTA ethylene glycol bis (2-aminoethyl ether)-N,N,N,N-tetraacetic acid     

Characterization of vascular histamine H1-receptors: Multiple affinity states of aortic histamine H1-receptor

We have shown that [³H]mepyramine labels histamine H₁-receptor-binding sites in bovine aortic membranes. Further characterization of H₁-receptors in this tissue was done by the interaction of an unlabelled histamine receptor agonist or antagonist, with the radioantagonist [³H]mepyramine-binding sites. The competition-binding assays have uncovered differences in the characteristics of the agonist/receptor interaction not shared by antagonists. Agonists interact in the heterogeneous manner with the radioantagonist-labelled sites, showing shallow competition curves with then _H 0.50–0.72, whereas antagonists were devoid of this effect (steeper slopes of the inhibition curvesn _H1). The results suggest the presence in this tissue of multiple affinity states of histamine H₁-receptor, differentiated by high and low affinity for agonists and the same affinity for antagonists.     

Development of gastric carcinoids inmastomys during histamine2-receptor blockade

We have investigated the effect of histamine₂-receptor blockade on gastric carcinoid formation inMastomys, a rodent prone to develop gastric carcinoids. During long-term treatment (8–24 weeks) with loxtidine, a 3–5 fold increase in plasma gastrin levels was observed. Oxyntic mucosal histamine and histidine decarboxylase activity were increased 2 times and 4–10 times respectively in loxitidine-treated animals, as compared to controls. An increase in oxyntic mucosal thickness was also noted in all treated animals, while gross tumors were only observed in animals treated for 24 weeks. Immunocytochemical analysis of treated animals revealed a marked proliferation of chromogranin-positive cells in the oxyntic mucosa. These cells were identified as ECL cells because they were labeled by histamine antibodies, but not by gastrin-, somatostatin-or serotonin-antibodies. Hyperplasia of endocrine cells was noted after 8 weeks of treatment, while dysplastic lesions were seen after 16 weeks and the first micro- or macrocarcinoids after 24 weeks of treatment. No tumors, or hyperplastic changes, were observed in control animals. The results demonstrate that histamine₂-receptor blockade significantly enhances the development of gastric carcinoids inMastomys and suggest that hypergastrinemia may be important for the development of these tumors.     

Characterization of coexistent histamine H1- and H2-receptor binding sites in the purified guinea pig myocardial membranes from ventricles

In the present work, we identified and characterised histamine H1-and H2-receptors in highly purified myocardial membranes isolated from female guinea pig ventricles. We determined the binding parameters for the interactions of³H-mepyramine with the histamine H1-receptor binding site and³H-tiotidine with the histamine H2-receptor binding site. Binding of both ligands in our study was saturable, reversible and of high affinity. Scatchard's analysis of the specific³H-mepyramine binding revealed the existence of high and low affinity binding sites with apparent K_D values of 0.4 nM and 4.5 nM, respectively. The density of binding sites (B_max) was 100 fmol/mg protein for the high and 466 fmol/mg protein for the low affinity binding site.³H-tiotidine binds to a single population of binding sites with a K_D of 1.0 nM and a B_max of 27 fmol/mg protein. These data suggest that both histamine H1-and H2-receptors coexist in the guinea pig myocardium with a significantly higher prevalence of the histamine H1-receptor population.     

A species comparison of the histamine H2-receptor on mast cells and basophils

The histamine H₂-agonist dimaprit was employed in experiments to investigate the role of H₂-receptors in mediator release systems from the rat, guinea-pig and man. Whereas inhibition of histamine release by dimaprit was observed in all 3 species, this effect appeared not always to be related to H₂-receptor occupancy. Although the data in general support previous evidence for the presence of H₂-receptors on human basophils and guinea-pig mast cells, the use of dimaprit as a pure H₂-agonist in these studies is questioned. No evidence for H₂-receptors on rat mast cells was obtained.     

Quantification of H2-agonism by clonidine and dimaprit in an adenylate cyclase assay

In homogenates of guinea-pig ventricle clonidine and dimaprit both stimulate adenylate cyclase and exhibit bell-shapedE/[A] curves. The two properties (stimulatory and inhibitory) could be resolved using a theoretical model assuming a down line auto-inhibitory mechanism. In the case of clonidine a further depressive property could be seen in the presence of high concentrations of the selective H₂-receptor antagonist tiotidine which is not explicable in terms of this model.The results suggest that clonidine has a direct agonistic effect on H₂-receptors in guinea-pig heart. However, like dimaprit, clonidine appears to be a partial agonist because its expression is confounded by a secondary inhibitory property(ies).     

Accelerated mobilization and formation of histamine in the gastric mucosa evoked by vagal excitation

1. The changes in the rate of histamine formation and in the histamine content of the parietal cell containing region of the gastric mucosa have been studied in rats under the influence of agents which evoke or abolish vagal excitation.2. The hypoglycaemia producing agents, insulin and 2-deoxyglucose (2-DG), raised the mucosal histamine-forming capacity (HFC) in a way similar to that previously observed on re-feeding, gastrin injection, and distension of the stomach wall.3. In cats, insulin injection elicited an elevation of mucosal HFC similar to the corresponding effect of insulin in rats.4. Hoechst 9980, which inhibits post-ganglionic cholinergic transmission, counteracted the elevation of mucosal HFC following vagal excitation, but did not inhibit changes produced by gastrin, thus indicating the absence of a cholinergic intermediary link between gastrin and changes in mucosal histamine.5. It is emphasized that although re-feeding, vagus excitation, gastrin and distension all produce similar changes in mucosal histamine, the clarification of the precise role of histamine as a natural stimulant for the parietal cells may require a fresh kind of approach.     

Pharmacological control of the histamine H2 receptor-adenylate cyclase system by famotidine and ranitidine in normal and cancerous human gastric epithelia

In human fundic glands, famotidine was about 17 times more potent than ranitidine as an inhibitor of histamine — stimulated cAMP generation. This H₂-receptor antagonist had no effect on the receptoradenylate cyclase systems sensitive to PGE₂, isoproterenol (₂), VIP and on forskolin-induced activation of the Gs/catalytic units of the membrane-bound enzyme prepared from human fundic glands. In the HGT-1 human gastric cancer cell line, famotidine and ranitidine showed long lasting, irreversible actions probably related to a slow rate of dissociation from the histamine H₂-receptor.