The effect of inhibitors of extraneuronal uptake on the distribution of 3H-(+/-)noradrenaline in nerve-free rabbit aortic strips.

1. Nerve-free rabbit aortic strips were exposed to 1.18muM 3H-(+/-)noradrenaline for 30 min. When either MAO or COMT was inhibited, far more O-methylated (MAO inhibited) than deaminated metabolites (COMT inhibited) were formed during the incubation. The accumulation of unchanged amine in the extraneuronal stores was inversely related to the rate of metabolism. 2. After inhibition of both metabolizing enzymes, nerve-free strips were first incubated with the amine and then washed out with amine free solution. Compartmental analysis of the efflux curves showed that two extraneuronal compartments were involved in accumulation (with half times of efflux of 3 and 11 min, respectively). 3. 86muM corticosterone or 30muM phenoxybenzamine greatly decreased the accumulation of noradrenaline in these two compartments. 4. When corticosterone or phenoxybenzamine was added to the wash out solution only, the half time of the efflux from both compartments was greatly increased. However, this effect was seen only after inhibition of COMT and not after inhibition of MAO only (the filling of the extraneuronal stores with unchanged noradrenaline being better after inhibition of COMT than when this enzyme was intact). The effect of corticosterone appeared to be reversible, that of phenoxybenzamine irreversible. 5. Analysis of the efflux of metabolites (in experiments in which only one enzyme was inhibited) indicated that corticosterone affected the efflux of noradrenaline but not that of the metabolites. 6. When either COMT or MAO was inhibited throughout the experiment, very little or no metabolism of noradrenaline occured during prolonged wash out. On the other hand, dis-inhibition of COMT during wash out (by the omission of U-0521 from the wash out solution after it had been present during the initial incubation) revealed that noradrenaline, stored extraneuronally during the initial incubation, is quickly O-methylated during wash out, especially when the efflux of the parent amine is inhibited by corticosterone. 7. The results show that COMT is the major extraneuronal noradrenaline-metabolizing enzyme of rabbit aorta, that inhibition of COMT is a pre-requisite for any corticosterone-sensitive accumulation of noradrenaline, that there are two important extraneuronal compartments (compartments III and IV; Henseling et al., 1976a), and that inhibitors of extraneuronal uptake inhibit both, influx and efflux of noradrenaline.