Monoamine Oxidase Activity in the Cerebral Vasculature: Comparison between Fresh Microvessels from Different Structures and Cell Cultures Derived from Microvessels

Monoamine oxidase (MAO) activity was studied in various preparations of porcine brain microvessels to explore further the role of this enzyme in the blood–brain barrier to catecholamines. No difference was noted (V_m and K_m) between microvessels isolated from three structures (caudate nucleus, thalamus, and cerebral cortex) in which the responses to circulating catecholamines in vivo are markedly different. Large and small microvessels from the caudate nucleus and the thalamus presented the same specific activity. Cell cultures obtained from small microvessels were rich in endothelial cells as identified by the presence of Factor VIII–related antigen. These preparations displayed an MAO activity about ninefold less than freshly isolated microvessels, although their prostaglandin synthetase activity appeared normal. These results suggest that MAO activity is not the main factor determining the regional differences in the cerebrovascular reactions to catecholamines, that MAO is not specifically localized in the endothelium but must be also present in the smooth muscle, and that the MAO activity is greatly decreased during cell culture.