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This study has examined the influence of sodium (0, 20, 40, 80, 120, and 160 mM) and ouabain (100, 500, and 1,000 microM), 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 microM 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 microM. The accumulation of newly formed dopamine and DOPAC in kidney slices loaded with L-DOPA (50 and 100 microM) 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 microM 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.(ABSTRACT TRUNCATED AT 250 WORDS)

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