Characterization of histamine H3 receptors inhibiting 5-HT release from porcine enterochromaffin cells: further evidence for H3 receptor heterogeneity.
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)-alpha-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)-alpha-methyl-histamine to the right (pKB value 8.38). The inhibitory effect of 1 mumol/l (R)-alpha-methyl-histamine was antagonized in a concentration-dependent manner by thioperamide (IC50: 65 nmol/l) and dimaprit (IC50: 8.6 mumol/l); however, the effect of (R)-alpha-methyl-histamine was weakly antagonized by burimamide (by 38% at 100 mumol/l) and not significantly affected by other H3 receptor antagonists, such as impromidine, betahistine and phenyl-butanoyl-histamine (each up to 100 mumol/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.
Schwörer
H
H
Department of Medicine, University of Göttingen, Germany.
Reimann
A
A
Ramadori
G
G
Racké
K
K
D000818:Animals; D002851:Chromatography, High Pressure Liquid; D004347:Drug Interactions; D004759:Enterochromaffin Cells / Q000187:drug effects* / Q000378:metabolism; D005260:Female; D017442:Histamine Agonists / Q000378:metabolism / Q000494:pharmacology; D006633:Histamine Antagonists / Q000494:pharmacology; D006897:Hydroxyindoleacetic Acid / Q000378:metabolism; D007421:Intestine, Small / Q000187:drug effects / Q000378:metabolism; D008297:Male; D018100:Receptors, Histamine H3 / Q000187:drug effects / Q000378:metabolism*; D012701:Serotonin / Q000378:metabolism*; D013552:Swine
D017442:Histamine Agonists; D006633:Histamine Antagonists; D018100:Receptors, Histamine H3; D012701:Serotonin; D006897:Hydroxyindoleacetic Acid