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Summary1. Two forms of monoamine oxidase activity were differentiated in rat mesenteric and femoral artery by means of substrate and inhibitor specificities: one form deaminated tyramine, 5-hydroxytryptamine and noradrenaline and was highly sensitive to pargyline and clorgyline but resistant towards carbonyl reagents. This form resembled type A monoamine oxidase previously described. The other deaminated tyramine but not 5-hydroxytryptamine or noradrenaline and was inhibited by carbonyl reagents but not by clorgyline or pargyline. 2. About one third of the total monoamine oxidase in homogenates of rat mesenteric artery was recovered in a 105 g supernatant. Both forms were partially soluble, but relatively less of the type A activity was recovered in the soluble fraction. 3. Chemical sympathectomy with 6-hydroxydopamine resulted in a loss of 59% of monoamine oxidase activity in the mesenteric artery. There was a selective loss of type A activity, as revealed by the 70% decrease in 5-hydroxytryptamine deaminating ability and by the marked decrease in clorgyline sensitivity. The second monoamine oxidase species was resistant to 6-hydroxydopamine. The soluble activity was not affected by chemical sympathectomy. Most of the transmitter-specific monoamine oxidase of the arterial wall was localized within the adrenergic nerve endings. Our observations are consistent with the hypothesis that extraneuronal monoamine oxidase plays only a minor role in metabolizing noradrenaline in sympathetically innervated tissues. 4. Plasma amine oxidase might originate from the arterial wall since it has similar characteristics to that found in the mesenteric artery.

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