1. The present randomized, double-blind cross-over study compared endogenous and exogenous lithium clearance (CLi) for estimation of the effect of dopamine on tubular sodium reabsorption. Twelve normal, salt-repleted male subjects were investigated on three different occasions with either placebo or 450 mg or 600 mg of lithium given in random order at 22.00 hours. After an overnight fast, renal clearance studies were performed during a 1 h baseline period and subsequently during the second hour of an infusion of 3 μg min−1 kg−1 of dopamine.
2. Baseline values of endogenous CLi and fractional excretion of lithium (FELi) [27.0 (23.5–30.5) ml/min and 24.2 (203–28.2)% (means with 95% confidence interval)] were lower than exogenous values [lithium, 450 mg: 32.7 (29.9–35.4) ml/min (P < 0.05) and 27.4 (25.2–29.6)% (P < 0.05); lithium, 600 mg: 33.4 (29.2–37.6) ml/min (P < 0.05) and 28.6 (26.3–31.0)% (P < 0.01)]. Both test doses of lithium increased the baseline sodium clearance (CNa), but glomerular filtration rate and urine flow rate remained unchanged.
3. Dopamine increased CNa to similar values on the three study days. CLi increased to 40.9 (35.5–46.5) ml/min (endogenous lithium, P < 0.001), 43.2 (40.8–45.6) ml/min (450 mg of lithium, P < 0.01) and 44.9 (41.3–48.4) ml/min (600 mg of lithium, P < 0.001), respectively. FELi increased to 32.2 (27.5–37.0)% (P < 0.01), 35.4 (33.0–37.7)% (P < 0.01) and 35.9 (32.8–38.9)% (P < 0.01), respectively. Values during dopamine infusion did not differ significantly.
4. The lower baseline values of endogenous CLi and FELi compared with exogenous values suggest that CLi in humans depends on the plasma concentrations of lithium. However, the effect of dopamine on CLi and FELi was expressed to the same extent with endogenous and exogenous lithium, indicating that the two methods are interchangeable for estimation of dopamine-induced changes in tubular function.