1. Using the renal clearance of lithium (CLi) as an index of proximal tubular outflow of sodium and water, together with simultaneous measurements of effective renal plasma flow, glomerular filtration rate (GFR) and sodium clearance (CNa), renal function and the tubular segmental reabsorption rates of sodium and water during dopamine infusion (3 μg min−1 kg−1) were estimated in 12 normal volunteers.
2. CNa increased by 128% (P <0.001). Effective renal plasma flow and GFR increased by 43% (P <0.001) and 9% (P <0.01), respectively. CLi increased in all subjects by, on average, 44% (P <0.001). Fractional proximal reabsorption [1-(CLi/GFR)] decreased by 13% after dopamine infusion (P <0.001), and estimated absolute proximal reabsorption rate (GFR–CLi) decreased by 8% (P <0.01). Absolute distal sodium reabsorption rate [(CLi–CNa) × PNa, where PNa is plasma sodium concentration] increased (P <0.001), and fractional distal sodium reabsorption [(CLi–CNa)/CLi] decreased (P <0.001).
3. It is concluded that natriuresis during low-dose dopamine infusion is caused by an increased outflow of sodium from the proximal tubules that is not fully compensated for in the distal tubules.