ACTION OF HISTAMINE RECEPTOR AGONISTS AND ANTAGONISTS ON THE RAT UTERUS

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Abstract
1 Histamine and a series of compounds acting selectively on Hr and H2-receptors were tested on the isolated oestrous uterus of the rat. 2 Histamine had a dose-dependent inhibitory effect on the contractions elicited by acetylcholine. This action was unaffected by H1-blockers but was competitively inhibited by H2-blockers. The H1-selective agonist, 2-(2-aminoethyl)thiazole was ineffective at doses 100 times greater than those of histamine. Conversely, all the H2-agonists showed activity in the order of potency: N'-methylhistamine > histamine > N'-N'-dimethylhistamine > 5-methylhistamine > 5-methyl-N'-methylhistamine. Among the non-imidazole compounds, dimaprit had an activity identical to that of histamine, but all the dimaprit-like compounds showed negligible activity. 3 The data obtained suggest that in the rat uterus, (a) the activation of H2-receptors is responsible for the inhibitory effect of histamine and its analogues; (b) the integrity of the histamine molecule seems to be less crucial than that of the dimaprit molecule for the maintenance of the H2-activity, since changes in its structure modify but do not abolish the biological activity as they do in the case of dimaprit; (c) the order of activity of the various H2-receptor agonists is different from that observed in other tissues.