H1- and H2-receptors in the smooth muscle of the ruminant stomach.

1 The effects of histamine on the longitudinal and circular smooth muscle from the rumen and reticulum of the bovine stomach have been analyzed by the use of H1- and H2-receptor antagonists. 2 Histamine caused three different types of response of the smooth muscle preparations: a contraction, contraction followed by relaxation, and a relaxation. These responses were resistant to the effect of tetrodotoxin, atropine, combined treatment with alpha- and beta- adrenoceptor blocking agents, hexamethonium and also to guanethidine. 3 Mepyramine antagonized the contractile responses and metiamide antagonized the relaxation responses in a highly selective and competitive manner. 4 It is concluded that histamine-induced excitatory and inhibitory responses of the smooth muscles from the bovine forestomach are mediated by histamine H1- and H2-receptors, respectively.