A decreased tubular uptake of dopa results in defective renal dopamine production in aged rats.

A major proportion of urinary dopamine derives from the renal decarboxylation of circulating dopa. This study evaluates the effects of aging on renal production of dopamine using 3- and 12-mo-old male Wistar rats. Urinary excretion of Na+, norepinephrine (NE), 3,4-dihydroxyphenylglycol, and dopa were similar in the two groups. Urinary dopamine and 3,4-dihydroxyphenylacetic acid (DOPAC) were lower in older animals (dopamine, 20 +/- 6 vs. 47 +/- 7 nmol/24 h, P < 0.001; DOPAC, 142 +/- 36 vs. 304 +/- 56 nmol/24 h, P < 0.03). Urinary 3-O-methyldopa (OM-dopa) was higher in 12-mo-old rats (6.2 +/- 2.0 vs. 3.3 +/- 0.20 nmol/24 h, P < 0.03). Levels of dopa and NE in renal cortex from 12-mo-old rats were higher (P < 0.001) than in younger animals. Dopamine content in renal cortex from 3-mo-old rats was 295 +/- 64 pmol/g, whereas it was undetectable in 12-mo-old animals. Aromatic-L-amino-acid decarboxylase and monoamine oxidase activities were higher (P < 0.001) in renal cortex from 12-mo-old animals. Catechol-O-methyltransferase activity was similar in both groups. The uptake of dopa by the luminal membrane was explored using brush-border membrane vesicles. The Na(+)-gradient-driven (100 mM) uptake of dopa into vesicles from 3-mo-old animals showed at 10 s an overshoot threefold greater than the equilibrium uptake. The overshoot was blunted in 12-mo-old rats.(ABSTRACT TRUNCATED AT 250 WORDS)