HISTAMINE-RECEPTORS IN THE GUINEA-PIG ILEUM

Histamine and some related compounds acting selectively on H2- or H1-receptors were tested for their ability to contract the guinea pig ileum, in the usual whole ileum preparation and in the longitudinal muscle preparation. The concentrations elicited by histamine in both kinds of preparations were not potentiated by cimetidine or metiamide and were not inhibited by administration of H2 receptor selective agonists in doses which were subthreshold for contracting the guinea pig ileum; higher doses of the H2 agonists could actually potentiate the effect of histamine. Apparently, H2 receptors with relaxing effect do not occur in the guinea pig ileum, or at least they are not involved in the contraction of the longitudinal muscle layers. The possibility that a sub-type of H2 receptors with properties different from those of the classical H2 receptors exists in the guinea pig ileum cannot be excluded.