Potentiation by Aminoguanidine of the Sensitivity of Sheep to Histamine Given by Mouth. Effect of Aminoguanidine on the Urinary Excretion of Endogenous Histamine

The sensitivity of sheep to orally administered histamine was greatly increased by aminoguanidine, a substance which inhibits histaminase (diamine oxidase, E.C. 1.4.3.6.). The increment in urinary free histamine after giving histamine by mouth (1·0–10·0 g. histamine diphosphate) was about 22 times larger in aminoguanidine‐treated sheep than in untreated ones. Furthermore, toxic manifestations were observed when 1 g. of histamine diphosphate or more was given to aminoguanidine‐treated sheep, whereas doses as high as 10 g. only had a slight effect on untreated animals. When aminoguanidine was given, and oxidative deamination thus largely prevented, methylation of histamine seemed to increase. Aminoguanidine increased the urinary excretion of endogenous free histamine on average by a factor of about 12. The above mentioned results indicate that oxidative deamination of histamine is more important in sheep than in species previously examined.