The actions of histamine on the separated layers of the guinea-pig uterus: the influence of ovarian steroids

Effect of histamine on the longitudinal and circular muscle of the oestrogen dominated rat uterus

The response of the longitudinal and circular myometrial strips to histamine was studied in oestrogen-treated rats. Histamine produced a dose-related inhibitory response in KCl-contracted longitudinal and circular uterine strips. Histamine was equipotent in producing the relaxant response but the maximal effect achieved in the longitudinal muscle was higher than the circular one. Ranitidine antagonized the histamine-induced relaxation with a similar dose ratio in both longitudinal and circular strips. Clemizole and reserpine treatment did not produce any modification of the dose-response curve to histamine. In the longitudinal and circular strips which were not preconstricted by KCl, neither histamine nor 2-pyridylethylamine, even in strips pretreated with ranitidine, produced any effect when added to the organ bath. Our results show that the response of histamine in both longitudinal and circular uterine layers of the oestrogen-treated rats are mediated exclusively by histamine H2-receptors.

Influence of hormonal treatment on the response of the rat isolated uterus to histamine and histamine receptor agonists

The response of the isolated uterus to histamine and histamine agonists was investigated in progesterone- and oestrogen-treated rats. The uterine inhibitory responses to histamine and 4-methylhistamine (a histamine H2 receptor agonist) were similar in KCl-contracted uteri from progesterone- and oestrogen-treated rats. The histamine H1 receptor agonist, 2-pyridyl-ethylamine, produced a relaxant response only in progesterone dominant uterus. This was inhibited by the histamine H1 receptor antagonist. In the rat isolated uterus which was not preconstricted by KCl, neither histamine, 4-methylhistamine, nor 2-pyridyl-ethylamine produced any effect in the presence or absence of ranitidine. Ranitidine competitively antagonized the histamine-relaxant uterine response in oestrogen-treated rats (pA2 = 7.21 (6.83-7.58)), but not in progesterone-treated rats, except in the presence of clemizole (10(-7) M) when the pA2 value of ranitidine against histamine was similar to that obtained in oestrogen-treated rats (pA2 = 6.74 (6.64-6.85)). These results indicate that treatment with ovarian steroids influences responses mediated by the histamine receptors of the isolated rat uterus. Both histamine H2 and H1 receptors contribute to the uterine inhibitory effect of histamine in progesterone-treated rats.