HISTAMINE H-1 AND H-2 VASODILATOR MECHANISMS IN THE GUINEA-PIG ILEUM

Background/Aims: Histamine dilates gastrointestinal blood vessels. Whether this is caused by direct activation of vascular histamine receptors or by activation of enteric neurons is not known. The aim of this study was to determine which of these pathways is activated by histamine and to examine the cellular mechanisms involved. Methods: The effects of histamine were studied in in vitro submucosaI preparations from the guinea pig ileum using videomicroscopy to monitor changes in submucosal arteriolar diameter. Results: Histamine caused a tetrodotoxin-insensitive dose-dependent dilation (median effective concentration [EC(50)], 1 mu mol/L), showing direct activation of vascular histamine receptors. The H-1 antagonist pyrilamine, but not the H-2 blocker ranitidine, competitively inhibited the histamine dilatation. The nitric oxide synthase inhibitor N-G-monomethyl-L-arginine (L-NMMA) inhibited histamine vasodilations by 66%. Indomethacin alone did not alter histamine vasodilations but, when combined with L-NMMA, caused a significantly greater inhibition of the histamine response compared with L-NMMA alone. L-Arginine prevented the actions of L-NMMA. In the presence of both H-1 and H-2 antagonists, periarteriolar stimulation of sympathetic nerves evoked a tetrodetoxin-sensitive vasoconstriction, which was inhibited by histamine (EC(50), 0.8 mu mol/L). This histamine action was blocked by the H-3 antagonist thioperamide. Conclusions: Histamine can produce vasodilation of submucosal arterioles by two distinct mechanisms: activation of vascular H-1 receptors resulting in release of nitric oxide from endothelium and activation of H-3 receptors on sympathetic nerve terminals resulting in presynaptic inhibition of vasoconstrictor tone.