Histamine H2-receptors and gastric secretion

Evidence against the temperature-dependent interconversion of histamine H1- and H2-receptors in the guinea-pig ileum

1 The possible temperature-dependent interconversion of histamine H1- and H2-receptors in the guinea-pig ileum suggested from previous studies was re-investigated by use of new and selective H2-receptor agonists and antagonists. 2 Chlorpheniramine, and H1-blocker, caused a rightward shift of the cumulative histamine dose-response curve at both 37 degrees C and 12 degrees C. Conversely cimetidine and tiotidine, two H2-receptor blockers, were ineffective at both temperatures. Metiamide behaved as a non competitive antagonist at 12 degrees C but only in very high concentrations. 3 Dimaprit and impromidine, two selective H2-receptor agonists, were inactive at both 37 degrees C and 12 degrees C when given alone, whereas at both temperatures they elicited the already described relaxation of the contractions induced by histamine. 4 Similar results were obtained on the guinea-pig whole ileum and on the longitudinal muscle strip: this indicates a lack of interference of the circular smooth muscle. 5 Our results allow us to conclude that no temperature-dependent interconversion of histamine H1- and H2-receptors occurs in the guinea-pig ileum.

Action of histamine receptor agonists and antagonists on the rat uterus

1 Histamine and a series of compounds acting selectively on H1- 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 greater than histamine greater than N'-N'-dimethylhistamine greater than 5-methylhistamine greater than 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.