Uptake and metabolism of histamine by the rat tapeworm Hymenolepis diminuta: an in vitro study

Abstract

The uptake of histamine by the rat tapeworm Hymenolepis diminuta was examined in intact worms and in worms denuded of tegument. Uptake was by a sodium-independent mechanism and was not inhibited by compounds that inhibit active transport of amines. The uptake of histamine was linear over all concentrations examined, including concentrations to which the worms are exposed in vivo. Q10 was consistent with uptake by simple diffusion. Histamine uptake was inhibited at pH 5.9 but not at higher pH values. The only metabolite of histamine found was imidazole acetic acid. Production of this metabolite was inhibited by heat treatment of the worms and by the presence of known diamine oxidase inhibitors, suggesting the occurrence of diamine oxidase. This enzymatic activity was found in the tissues of the worm. In contrast, H. diminuta did not synthesize histamine from histidine. Collectively the data suggest that histamine in the tissues of H. diminuta is of exogenous origin, entering the tissues by diffusion, where it is metabolized by an endogenous diamine-like oxidase to imidazole acetic acid.