The effect of partial inhibition of monoamine oxidase on the steady-state rate of deamination of 3H-catecholamines in two metabolizing systems

Two different "deaminating systems" were compared (i.e., intact tissues in which an uptake process translocates the 3H-catecholamine from the extracellular space to the intracellular MAO): the adrenergic nerve endings of the rat vas deferens exposed to 10 nmol/l 3H-(-)-noradrenaline, and the extraneuronal deaminating system of the rat heart perfused with 50 nmol/l 3H-(-)-adrenaline. Vesicular uptake and COMT were inhibited. In both systems MAO was partially inhibited by pargyline, and the steady-state tissue content of the 3H-catecholamine was determined as well as the steady-state rate of deamination. Rat vas deferens (preincubated with 10-40 nmol/l pargyline for 30 min). Inhibition of neuronal MAO caused not more than a moderate decrease of the steady-state rate of deamination of 3H-(-)-noradrenaline, but the steady-state tissue content was greatly increased. Determinations of the activity of MAO in homogenates of vasa deferentia showed that preincubation with 10 and 20 nmol/l pargyline inhibited the enzyme by 80 to 95%. Rat heart (of animals pretreated with 1 to 30 mg/kg pargyline). Inhibition of extraneuronal MAO caused a steep decline of the steady-state rate of deamination of 3H-(-)-adrenaline, but only a small rise in the steady-state tissue content. The decisive difference between the two deaminating systems lies in the fact that the ratio "kmao/kout" (where the two k-values characterize the activity of the unsaturated intracellular MAO and the ability of the 3H-catecholamine to leave the relevant cells, respectively) is much higher for the neuronal deaminating system exposed to 3H-(-)-noradrenaline than for the extraneuronal deaminating system exposed to 3H-(-)-adrenaline.(ABSTRACT TRUNCATED AT 250 WORDS)