The uptake of histamine by the rat tapeworm Hymenolepis diminuta was e
xamined in intact worms and in worms denuded of tegument. Uptake was b
y a sodium-independent mechanism and was not inhibited by compounds th
at inhibit active transport of amines. The uptake of histamine was lin
ear over all concentrations examined. including concentrations to whic
h the worms are exposed in vivo. Q10 was consistent with uptake by sim
ple diffusion. Histamine uptake was inhibited at pH 5.9 but not at hig
her pH values. The only metabolite of histamine found was imidazole ac
etic acid. Production of this metabolite was inhibited by heat treatme
nt of the worms and by the presence of known diamine oxidase inhibitor
s, suggesting the occurrence of diamine oxidase. This enzymatic activi
ty 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 endog
enous diamine-like oxidase to imidazole acetic acid.