Some effects of histamine in the depolarized rat uterus

Abstract

1 The relaxant effect of histamine in the isolated rat uterus remained after the preparation was depolarized in a potassium Ringer. 2 The effect was abolished by the calcium-chelating agent, ethyleneglycol bis-aminoethyl ether-tetraacetic acid (EGTA). 3 Histamine caused relaxation during calcium-induced contractures in a depolarized uterus treated with EGTA, but did not produce relaxation during barium-induced contractures. 4 While responses of a normally polarized rat uterus to acetylcholine were inhibited by histamine, those of a depolarized uterus were enhanced by histamine. The inhibitory effects of isoprenaline and papaverine on acetylcholine responses were maintained in depolarized preparations. 5 Both the enhancing effect of histamine on responses to acetylcholine and the relaxation of calcium-induced contractures were abolished by burimamide, indicating that H₂-receptors mediate the effects of histamine in the depolarized tissue. 6 A reduction in the rate of exchange of calcium across the depolarized cell membrane was demonstrated with high concentrations of histamine. 7 The results are consistent with the hypothesis that in the rat uterus, the stimulation of H₂-receptors by histamine is accompanied by a reduction in calcium exchange across the membrane which may result in a decrease in the concentration of free intracellular calcium available to stimulate contraction of the myofilaments. Histamine may act by increasing the binding of calcium within the cell.