The influence of inhibition of catechol-O-methyl transferase or of monoamine oxidase on the extraneuronal metabolism of 3H-(-)-noradrenaline in the rat heart.

The extraneuronal metabolism of 3H-(-)-noradrenaline (1 nmol/l) was determined in rat hearts obtained from reserpine-pretreated animals (in the presence of 30 mumol/l cocaine). Inhibition of monoamine oxidase (MAO) (by pretreatment of the animals with pargyline) increased the formation of O-methylated metabolites by nearly that amount by which the formation of deaminated metabolites declined; hence, catechol-O-methyl transferase (COMT) seemed to be able to nearly fully compensate for the loss of MAO activity. However, when COMT was inhibited (by the presence of either 1 or 10 mumol/l U-O521), the increase in the formation of deaminated metabolites was smaller than the decrease in the formation of O-methylated metabolites; hence, MAO seemed to be unable to fully compensate for the loss of COMT activity. These results are discussed with regard to the hypothesis that the two extraneuronal enzymes co-exist in one compartment. As inhibition of COMT causes a much greater increase in the steady-state tissue/medium ratio for 3H-(-)-noradrenaline than does inhibition of MAO, it is suggested that it is this increase in the intracellular concentration of 3H-(-)-noradrenaline which - by promoting an efflux of the unchanged amine that is proportional to the tissue/medium ratio - actually decreases the net removal of 3H-(-)-noradrenaline from the perfusion fluid. The results are compatible with (but no evidence for) the hypothesis that the two enzymes co-exist in the same extraneuronal compartment.