Characterization of histamine H3 receptors inhibiting 5-HT release from porcine enterochromaffin cells: Further evidence for H3 receptor heterogeneity

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Abstract
The nature of the histamine receptor mediating inhibition of 5-HT release was investigated in strips of the porcine small intestine by investigating the effects of histamine ligands on the overflow of endogenous 5-HT and its metabolite 5-hydroxyindoleacetic acid (5-HIAA). The overflow was measured by HPLC, combined with electrochemical detection and represents calcium-sensitive 5-HT release from enterochromaffin cells, as reported previously. The histamine H3 receptor selective agonists (R)-α-methyl-histamine and imetit inhibited the overflow of 5-HT maximally by 50–60%, with EC50 values of 48 and 3.2 nmol/l, respectively. Effects on 5-HT overflow were always accompanied by similar effects on the overflow of 5-HIAA. Thioperamide (100 nmol/l) shifted the concentration response curve of (R)-α-methyl-histamine to the right (pKB value 8.38). The inhibitory effect of 1 μmol/l (R)-α-methyl-histamine was antagonized in a concentration-dependent manner by thioperamide (IC50: 65 nmol/l) and dimaprit (IC50: 8.6 μmol/l); however, the effect of (R)-α-methyl-histamine was weakly antagonized by burimamide (by 38% at 100 μmol/l) and not significantly affected by other H3 receptor antagonists, such as impromidine, betahistine and phenylbutanoyl-histamine (each up to 100 μmol/l). In conclusion, H3 receptors mediating inhibition of 5-HT release from porcine enterochromaffin cells have a particular pharmacological profile indicating that heterogeneity of H3 receptors may exist. The data suggest that histamine H3 receptors modulating 5-HT release in pig small intestine do not belong to either H3A or H3B receptors as defined in rat tissue.