The histamine H-3 receptor shows high degree of stereoselectivity for histamine analogues branched in the side chain. The hypothesis that gastroprotection by (R)-alpha-methylhistamine could be H-3 receptor-mediated was tested by comparing the effect: of (R)-alpha-methylhistamine and of (S)-alpha-methylhistamine on ethanol-induced histologic lesions in the mt gastric mucosa. Extensive damage was caused by 60 min exposure to absolute ethanol, 91% of the mucosa examined being damaged. Conversely only 23% of the mucose was damaged after pretreatment with (R)-alpha-methylhistamine (100 mg/kg i.g.). In the groups pretreated with (S)-alpha-methylhistamine (55.44 and 166.3 mg/kg i.g.) total damage ranged from 77 to 79%, though it was confined to the upper portion of the mucose. Morphometric analysis of stained intraepithelial mucosubstances revealed that (R)-alpha-methylhistamine pretreatment resulted in an increase in number and volume of surface mucous cells, not evident after (S)-alpha-methylhistamine pretreatment. Scanning electron microscopy confirmed light microscopy evaluations. The two isomers of alpha-methylhistamine differently affect the response of rat gastric mucose to absolute ethanol and they appear to differ in their influence on surface mucous cells. A basis for interpreting the effects of the two isomers of a-methylhistamine rests on the high degree of stereoselectivity of H-3 receptors and on the different affinities of the two isomers for these receptors.
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