Inactivation of Histamine and Occurrence of Smooth Muscle Stimulating Activity in Plasma of Sheep

Abstract

With incubation periods of 90 min., histamine inactivation in sheep plasma was nearly proportional to substrate concentration within the range of 2 to about 50 ,ug. histamine diphosphate/ml. plasma. Increasing the substrate concentration above 80 ,ug./ml. did not increase the histamine inactivation. At optimal substrate concentrations (80 pig./ml. plasma), a histamine inactivating capacity of 42-1±4-8 ,pg.histamine diphosphate/ml. plasma/hr. was found in seventeen sheep examined. About 80 per cent of the histamine inactivation was abolished by aminoguanidine, indicating that oxidative deamination is the major route of inactivation in sheep plasma in vitro.When plasma was incubated for more than 60 min. with an initial histamine concentration less than 20 ,ug./ml., smooth muscle stimulating activity interfering with the biological determination of histamine, was observed. Such activity developed rapidly when glass powder was added to plasma prepared with siliconized equipment and it is concluded that the interfering activity is due to substance(s) resembling bradykinin.