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Abstract
Background: The histamine H3 receptor has been shown to inhibit pentagastrin-induced gastric acid secretion in dogs. Since pentagastrin releases histamine in dogs, we have now assessed whether the effects of H3-receptor ligands may be indirectly mediated by changes in gastric histamine release. Methods: Pentagastrin infusions (1 or 6 μg/kg/h), alone or together with the H3-receptor agonist (R)α-methylhistamine (1.2 μmol/kg/h) or the antagonist thioperamide (0.1 μmol/kg/h), were performed in dogs. One group (anaesthetized) was used for enzyme immunoassays of plasma histamine and, when required, (R)α-methylhistamine in the gastrosplenic vein, and another group (non-anaesthetized) for measurement of gastric acid secretion. Results: Histamine levels were increased five-and eight-fold after 1 and 6 μg/kg/h pentagastrin, respectively, whereas acid output was nearly maximal at the lower dosage. (R)α-methylhistamine, at a plasma concentration of 0.15 μM, inhibited histamine release by 78% (P < 0.007) and 37% (not significant) and the total acid output by 44% (P < 0.05) and 19% (not significant) after infusion of 1 and 6 μg/kg/h pentagastrin, respectively. Thioperamide, together with pentagastrin in low dose, significantly increased histamine release by 212% (P < 0.05), whereas acid output increased by 34% (not significant). Conclusions: The histamine H3 receptor mediates a negative feedback control of pentagastrin-induced release of gastric histamine. It is tonically activated by endogenous histamine after pentagastrin in low dosage. The control of acid secretion by the H3 receptor seems to involve modulation of endogenous histamine release, possibly by means of enterochromaffin-like cells.