Histamine receptors in isolated guinea pig duodenal muscle: H3 receptors inhibit cholinergic neurotransmission.

A series of histamine H3 receptor agonists and the H3 receptor antagonist thioperamide were tested in the isolated guinea pig duodenum, to investigate the role of this new receptor subtype in the intestinal contractility. At the same time the selectivity of the different compounds for the various histamine receptor subtypes was investigated. In the presence of famotidine (10(-6) M) and thioperamide (10(-5) M), histamine, N alpha-methylhistamine (NMH) and (R)-alpha-methylhistamine (alpha-MH) exerted a concentration-dependent contractile effect through activation of H1 receptors; the ratio of potency was histamine = NMH greater than alpha-MH (this last compound was approximately 500 times less potent). In the presence of pyrilamine (10(-6) M) and thioperamide (10(-5) M), histamine, dimaprit and impromidine caused a slight contractile effect, showing a high degree of tachyphylaxis; this effect was abolished by tetrodotoxin (10(-6) M) and by famotidine (10(-6) M). alpha-MH was ineffective up to 10(-4) M. The H2 receptor agonists dimaprit (10(-4) to 10(-3) M) and impromidine (10(-6) to 10(-5) M) caused a relaxant effect on the contraction elicited by acetylcholine (ACh), BaCl2 and electrical stimulation. This effect, which was unaffected by famotidine, was not mimicked by alpha-MH and not reversed by thioperamide (10(-5) M). In the presence of pyrilamine (109-6) M) and famotidine (10(-6) M), histamine, NMH and alpha-MH inhibited the twitch responses to electrical stimulation, with EC50 values of 1.17 x 10(-7), 6.76 x 10(-8) and 2.45 x 10(-8) M, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)