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The role of monoamine oxidase in the metabolism of exogenous noradrenaline by the human saphenous vein *

Summary— Human saphenous vein segments were obtained from patients subjected to coronary bypass surgery. As determined by HPLC‐ED, the veins had a relatively low content of noradrenaline and high content of the deaminated metabolites, dihydroxyphenylglycol (DOPEG) and dihydroxymandelic acid (DOMA). In vein segments which had been incubated with 3H‐noradrenaline (0.1 μmol/l), the oxidative deamination pathway predominated over the 0‐methylating one. Deamination occurred both at the neuronal and extraneuronal level; DOPEG appearing to be a good index of intraneuronal deamination, whereas DOMA and O‐methylated and deaminated metabolites were mainly formed extraneuronally. Both MAO type A and MAO type B selective inhibitors reduced the deamination of noradrenaline; deamination was also found to be partially sensitive to semicarbazide. Inhibition of neuronal uptake or of deamination increased O‐methylation. The human saphenous vein thus metabolizes exogenous noradrenaline following a pattern which substantially differs from that shown to occur in various blood vessels from other animal species.