1. Clearance of inulin and p-aminohippurate and excretion of water and sodium were measured for eight to 11 clearance periods of 20 min duration in anaesthetized, 3% volume-expanded rats, before and after intravenous infusions of the amino acids l-dopa (l-3,4-dihydroxyphenylalanine) and 5-hydroxytryptophan. During the final two clearance periods, the peripheral decarboxylase inhibitor, carbidopa (S-α-hydrazino-3,4-dihydroxy-α-methylbenzenepropanoic acid monohydrate), was infused additionally.

2. Renal formation of dopamine (3,4-dihydroxyphenethylamine) and 5-hydroxytryptamine was demonstrated during infusions of l-dopa and 5-hydroxytryptophan, respectively; carbidopa blocked the renal formation of these biogenic amines.

3. During infusion of dopa, a diuresis and a natriuresis were observed; during the infusion of 5-hydroxytryptophan, slight reductions in clearances of inulin and p-aminohippurate, but significant reductions in sodium and water excretion, were measured.

4. The addition of carbidopa diminished diuretic and natriuretic responses to dopa as renal dopamine excretion decreased; the infusion of carbidopa also ameliorated the antinatriuretic and antidiuretic effects of 5-hydroxytryptophan, as 5-hydroxytryptamine excretion decreased.

5. Although dopa and 5-hydroxytryptophan are substrates for the same enzyme, aromatic l-amino-acid decarboxylase, simultaneous infusions of both amino acids at comparable rates gave no evidence of competitive inhibition of amine synthesis. However, the infusion of dopa, after 5-hydroxytryptophan, decreased its antinatriuretic and antidiuretic effects.

6. These data raise the possibility that dopamine and 5-hydroxytryptamine are formed as reciprocal intrarenal hormones by the identical enzyme, aromatic l-amino-acid decarboxylase, which is located within cells of the renal tubule.

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