A study on the renal synthesis of dopamine in aged rats

The present study has examined the synthesis of dopamine from L-DOPA in kidney slices and in kidney homogenates of 3- and 24-month-old rats. The deamination of newly-formed dopamine into 3,4-dihydroxyphenylacetic acid (DOPAC) was also studied. The assay of L-DOPA, dopamine, noradrenaline and DOPAC was performed by high pressure liquid chromatography with electrochemical detection. Incubation of renal slices and homogenates of whole kidney with exogenous L-DOPA (0.1-100 microM) resulted in a concentration-dependent formation of both dopamine and DOPAC. At 50 and 100 microM L-DOPA, but not at lower concentrations (10 and 25 microM), the total amounts of dopamine and DOPAC formed were significantly greater in kidney slices obtained from 3-month-old rats. By contrast, the total amount of dopamine and DOPAC formed was greater in homogenates of renal tissues from aged rats than from young animals; this was particularly evident at 5.0 and 10.0 microM L-DOPA in the incubation medium. However, the DOPAC/dopamine ratios, both in kidney slices and kidney homogenates, were found to be higher in young rats than in old rats. The present results suggest an impairment in the formation of dopamine and of its deamination into DOPAC in renal tissues of aged rats; however, the reduced synthesis of dopamine does not appear to be the result of a decreased activity of the enzyme aromatic L-amino acid decarboxylase.