Urinary dopamine excretion in healthy volunteers: effect of sodium diet and acute water load.

The present study was designed to investigate, in human subjects, urinary dopamine excretion under different conditions of sodium and water homeostasis. In a cross-over trial, ten healthy volunteers were subjected to low-salt (LS; dietary salt restriction, sodium chloride (NaCl) intake <5 g per day), normal-salt (NS; normal food ad libitum), and high-salt (HS; normal food plus NaCl 100 mg/kg per day) regimens for 8 days in a randomized order. On day 7, urine was collected for 24 h. The variations in urinary sodium excretion reflected the dietary salt intake (LS: 16.3+/-4.7; NS: 144.1+/-18.2; HS: 221.9+/-12.9 mmol 24 h(-1) 1.73 m(-2)), but were not accompanied by significant changes in urinary dopamine excretion. On day 8, clearance studies showed that an acute oral water load of 1500 ml did not alter glomerular filtration rate or renal plasma flow but significantly increased urinary flow rate without affecting dopamine excretion. Assuming that excreted dopamine is not metabolized or reabsorbed during the tubular passage, both the unchanged urinary dopamine output in spite of 14-fold variations in sodium excretion and its independence of an acute water load argue against the hypothesis that dopamine in the tubular lumen acts as a natriuretic and/or diuretic factor in humans.