Endogenous dopamine synthesis and dopa-decarboxylase activity in rat renal cortex

The dopamine content of either cortical slices or isolated glomeruli prepared from rat kidneys and of their incubation medium was measured at different times both under basal conditions and in the presence of L-DOPA. Production of dopamine from L-DOPA by purified cytosolic proteins of the whole cortex was also measured. Dopamine synthesized by these 3 renal preparations accumulated linearly with time over 60 min. Dopamine produced by the glomeruli was more rapidly released into the incubation medium than that produced by the cortical slices. The dopamine synthetic rate was very low in the absence of L-DOPA but increased rapidly when L-DOPA was added to the incubation medium. No plateau was reached in the range of concentrations studied (0-100 microM) when cortical slices or cytosolic proteins were studied whereas dopamine production by isolated glomeruli reached an equilibrium above 10 microM L-DOPA. Dopamine synthesis in the presence of 100 microM L-DOPA was linearly related to the amount of renal protein. The synthetic rates were 43, 2.2 and 0.2 nmoles . h-1 . mg -1 for the cytosolic proteins, the cortical slices and the isolated glomeruli respectively. Dopamine synthesis by the cortical slices in the presence of 100 microM L-DOPA was progressively inhibited by increasing concentrations of alpha-methyl-DOPA. Cortical slices prepared from rats treated by benserazide, an inhibitor of L-DOPA decarboxylase, synthesized much less dopamine than those from control rats. These results show that rat renal cortex deprived of neuronal supply can synthesize dopamine in vitro from extracellular L-DOPA.