Evidence for an extraneuronal location of monoamine oxidase in renal tissues
1. The endogenous dopamine (DA) and noradrenaline (NA) tissue levels in three different areas of the dog kidney, and their modification by 6-hydroxydopamine (6-OHDA), pargyline plus 6-OHDA, pargyline or reserpine were studied by means of high pressure liquid chromatography with electrochemical detection. 2. 6-OHDA alone or in combination with pargyline induced parallel decreases of DA and NA contents in the inner cortex. In the outer cortex and the medulla, 6-OHDA selectively reduced NA without a parallel decrease in DA tissue content. Previous treatment with pargyline abolished this selectivity in the outer cortex but not in the medulla. Five days after administration of pargyline alone, DA and NA tissue content was not different from that observed in controls. 3. Reserpine caused a marked decrease in the DA and NA tissue content in all the three renal areas studied, though in the medulla a reserpine-resistant DA-component was found. 4. On the basis that small dense cored vesicles (SDCV) in noradrenergic neurones are more susceptible to 6-OHDA than large dense cored vesicles (LDCV), and that this difference is abolished by previous administration of pargyline, our findings suggest that the 6-OHDA-insensitive store of DA in the outer cortex of the dog kidney is located in a neuronal compartment different from that which contains NA, but most probably in noradrenergic neurones.