Inhibitors of histidine decarboxylase decrease basal gastric acid secretion in the rat

Gastric ECL-cell hyperplasia produces enhanced basal and stimulated gastric acid output but not gastric erosion formation in the rat

1. The purpose of this study was to examine the change in gastric acid output and gastric erosion formation produced by inducing gastric enterochromaffin-like (ECL) cell hyperplasia in female rats. 2. Rats were treated with vehicle or ranitidine (1,200 mumol/kg/day x 4 wks) administered via SC Alzet minipumps. Experiments were performed 24 hours after removing the minipump, when the inhibitory effect of ranitidine on gastric acid secretion had been lost. 3. Basal gastric acid secretion was 7-fold higher in chronic ranitidine animals than in sham control. 4. Both total and net gastric acid secretions stimulated by carbachol/pentagastrin infusion or histamine injection were significantly higher in the chronic ranitidine animals than in controls. 5. By contrast, intracisternal injection of the chemical vagal stimulant RX77368 (100 ng) resulted in no net increase in acid output of recovered ranitidine-pretreated group. 6. No significant changes in gastric erosions produced experimentally by cold exposure plus restraint or indomethacin pretreatment were noted in recovered chronic ranitidine animals compared to sham controls. 7. These findings suggest that achlorhydria-induced ECL cell hyperplasia augments both basal and stimulated gastric acid secretory function. The histamine results implicate an enhanced parietal cell mass, upregulation of H2 receptors, and/or second-messenger events at the parietal cell as the mechanism for the enhanced gastric secretory response.

Pharmacology of alpha-fluoromethylhistidine, a specific inhibitor of histidine decarboxylase

alpha-Fluoromethyl-[S]-histidine (FMH) is a specific and potent inhibitor of histidine decarboxylase, which forms histamine from histidine. It acts selectively and irreversibly by formation of a covalent linkage, possibly with the serine residue in the active site of the enzyme. A single administration of FMH decreases the histamine content only of non-mast cells in the brain and stomach of rodents, but repeated administration gradually decreases the histamine content of mast cells in all tissues. Thus, FMH can be used to deplete histamine in pharmacological studies. As no marked side-effects have been observed during administration of FMH, it may be useful in pathological conditions, such as some allergic diseases, peptic ulcers and mastocytosis, in which excess production of histamine is involved.