The synthesis of dopamine, from L-beta-3,4-dihydroxyphenylalanine (L-DOPA), in renal tissue of rats on either a normal sodium (NS) or high sodium (HS) diet for 1 week or 6 weeks was examined. Aromatic L-amino acid decarboxylase (AAAD) activity determined in kidney homogenates in "1 week HS" (Vmax = 11.5 +/- 1.6 nmol/mg protein/hr) and "6 weeks HS" rats (Vmax = 10.6 +/- 1.5 nmol/mg protein/hr) was greater (P < 0.02) than that in "NS" rats (Vmax = 7.7 +/- 0.8 nmol/mg protein/hr). Km (microM) values in "NS", in "1 week HS" and "6 weeks HS" rats were similar. The formation of dopamine in kidney slices loaded with 100 microM L-DOPA depended exponentially on the concentration of sodium in the medium (0-160 mM). In kidney slices obtained from "1 week HS" rats the decarboxylation of added L-DOPA was significantly greater (P < 0.01) than that observed in kidney slices obtained from "NS" and "6 weeks HS" rats; the rate constant of formation of dopamine as a function of sodium concentration in the incubation medium was, however, similar in "NS" rats to that in "1 week HS" and "6 weeks HS" rats. Ouabain produced a concentration dependent decrease in the synthesis of dopamine in all three experimental groups; the magnitude of the inhibitory effect of 1.0 mM ouabain was greater in "1 week HS" rats (77% reduction; P < 0.01) than in "NS" rats (59% reduction; P < 0.01) and in "6 weeks HS" rats (23% reduction; P = 0.08). Activation of protein kinase C by phorbol 12,13-dibutyrate (PDBu) and the calcium ionophore A23187 produced a concentration-dependent reduction in the formation of dopamine in rat kidney slices, but not in kidney homogenates; the magnitude of the inhibitory effect was greater in "1 week HS" rats than in "NS" and "6 weeks HS" rats. Submaximal concentrations of PDBu (10 nM) were synergistic with the inhibitory effect of A23187 on the formation of dopamine: again, this effect was more marked in "1 week HS" rats than in "NS" and "6 weeks HS" rats. The effects of PDBu and PDBu plus A23187, but not those of A23187 alone, were antagonized in a concentration-dependent manner by d-sphingosine, a protein kinase C inhibitor. It is concluded that the increased activity of AAAD in renal tissues of rats submitted to HS intake is accompanied in "1 week HS" but not in "6 weeks HS" rats by enhanced inhibition of dopamine formation during protein kinase C activation.