1. Dihydroxyphenylalanine is the precursor of all endogenous catecholamines. In laboratory animals, renal uptake and decarboxylation of circulating dihydroxyphenylalanine accounts for most of dopamine in urine. Dopamine is natriuretic, and in rats, dietary salt loading increases renal dihydroxyphenylalanine uptake by increasing the rate of entry (spillover) of dihydroxyphenylalanine into arterial plasma. In experimental animals and in humans, dietary salt loading increases urinary excretion of dihydroxyphenylalanine and dopamine. The present study examined in humans the extent to which circulating dihydroxyphenylalanine is the source of urinary dopamine and of the dopamine metabolite dihydroxyphenylacetic acid, and whether, as in animals, dietary salt loading affects dihydroxyphenylalanine spillover.

2. L-Dihydroxyphenylalanine (0.33 μg min−1 kg−1) was infused intravenously for 300 min after 7 days of a low-salt (mean 41 mmol/day) or a high-salt (mean 341 mmol/day) diet in 12 healthy subjects. Concentrations of dihydroxyphenylalanine, dopamine and dihydroxyphenylacetic acid were measured in urine and in antecubital venous plasma. Infusion of L-dihydroxyphenylalanine produced a steady-state mean dihydroxyphenylalanine level about 10 times the endogenous level. About 30% of infused dihydroxyphenylalanine estimated to be delivered to the kidneys via the arterial plasma was excreted as dopamine, and about 30% was excreted as dihydroxyphenylacetic acid.

3. Dietary salt loading increased urinary excretion rates of dihydroxyphenylalanine [from 0.08 ± (SEM) 0.01 to 0.14 ± 0.03 nmol/min, t = 2.80, P <0.02] and dopamine (from 1.03 ± 0.19 to 1.30 ± 0.28 nmol/min, t = 2.35, P <0.05), whereas dihydroxyphenylalanine spillover appeared to be unchanged.

4. Renal uptake and decarboxylation of circulating dihydroxyphenylalanine accounted for virtually all the urinary excretion of endogenous dopamine, but for only a minor portion of the excreted endogenous dihydroxyphenylacetic acid.

5. We conclude that in humans: (1) circulating dihydroxyphenylalanine is the main source of urinary dopamine but only a minor source of urinary dihydroxyphenylacetic acid; and (2) increased spillover of endogenous dihydroxyphenylalanine does not account for the increased excretion of these compounds during salt loading.

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