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

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 mumol/l), the oxidative deamination pathway predominated over the O-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.