Involvement of tubular sodium in the formation of dopamine in the human renal cortex

Abstract

This study has examined the influence of sodium (0, 20, 40, 80, 120, and 160 mM) and ouabain (100, 500, and 1,000 μM), an inhibitor of the enzyme Na+-K+ ATPase, on the synthesis of dopamine in slices of human renal cortex loaded with exogenous L-dihydroxyphenylalanine (L-DOPA). The deamination of newly formed dopamine into 3,4-dihydroxyphenylacetic acid (DOPAC) was also examined. The formation of dopamine and its deamination to DOPAC in slices and homogenates of human renal cortex closely depended on the concentration of L-DOPA added to the medium; in homogenates of renal cortex, the production of dopamine was found to be 74% of that occurring in the renal medulla. Decarboxylation of L-DOPA was found saturable at 1,000 μM L-DOPA, which had Vmax and Km values for L-amino acid decarboxylase activity of, respectively, 5.8 ± 0.6 nmol/mg of protein per hour and 62 ± 8 μM. The accumulation of newly formed dopamine and DOPAC in kidney slices loaded with L-DOPA (50 and 100 μM) was found to be partially dependent on the concentration of sodium in the medium; at 0 mM sodium, the synthesis of dopamine from L-DOPA was found to be half of that occurring at 160 mM sodium. A similar picture could be observed for DOPAC. The fractional rate of accumulation (k; mM sodium-1) at 50 and 100 μM L-DOPA was, respectively, 0.0016 ± 0.0002 and 0.0016 ± 0.0005 for dopamine and 0.0018 ± 0.0002 and 0.0021 ± 0.0005 for DOPAC. In experiments performed in the presence of 120 and 160 mM sodium, ouabain (100, 500, and 1,000 μM) was found to inhibit the accumulation of newly formed dopamine and DOPAC (12 to 63% reduction; P < 0.05) in slices loaded with 50 μM L-DOPA. When the experiments were performed in the presence of 20 mM sodium in the incubation medium, the renal production of dopamine and DOPAC was reduced by 29 and 37%, respectively, (P < 0.05) and ouabain (500 and 1,000 μM) was no longer able to reduce the accumulation of either newly formed dopamine or DOPAC. In conclusion, the results presented here show that the formation of dopamine in kidney slices loaded with L-DOPA is partially dependent on extracellular sodium and partially sensitive to ouabain, suggesting the involvement of a cotransport system of sodium and L-DOPA into the tubular epithelial cell.