EXTRANEURONAL UPTAKE AND O-METHYLATION OF H-3 ADRENALINE IN THE RABBIT AORTA

The influence of uptake2 inhibitors on the O-methylation and accumulation of H-3-adrenaline by the isolated rabbit aorta was studied. Strips were incubated with 0.05 mumol/l H-3-(-)-adrenaline during 15 min. Monoamine oxidase and uptake, were inhibited and the 3H-adrenaline present in the tissue was measured as well as the metabolites found in the tissue and in the incubation fluid. In another series of experiments, monoamine oxidase, uptake1 and catechol-O-methyl transferase (COMT) were inhibited, and tritium accumulation was measured in the tissue. When COMT was inhibited, inhibitors of uptake2 produced a maximal reduction of H-3-adrenaline accumulation that did not exceed 50%. When COMT was intact, inhibitors of uptake2 diminished total H-3-removal and, more markedly, O-methylation and concomitantly increased the tissue content of H-3-adrenaline. Mineralocorticoids (corticosterone and deoxycorticosterone acetate) inhibited H-3-adrenaline uptake (when COMT was inhibited) and H-3-metanephrine formation (when COMT was functional) as effectively as did sexual steroids (17-beta-oestradiol, progesterone and testosterone); hydrocortisone (hemisuccinate or phosphate) had no effect (for concentrations up to 120 mumol/1). At the end of the incubation some strips were washed out with amine-free solution. Compartmental analysis of the efflux showed that the amine had distributed into three extraneuronal compartments (compartment I, II and III, with half times of 0.4, 4 and 15 min, respectively). Corticosterone (120 mumol/l) decreased the amount of H-3-adrenaline in compartment III and simultaneously increased the amount of the amine in compartment I (extracellular space). The extraneuronal accumulation of H-3-adrenaline in rabbit aorta can be only partially ascribed to uptake2, as already stated by other authors; our results clearly show that inhibition of uptake2 increases the amount of H-3-adrenaline in the extracellular space, i. e., uptake2 creates a concentration gradient for H-3-adrenaline in the extracellular space; elastin and collagen represent very probably the site of binding of H-3-adrenaline which is independent of uptake2.