ENDOGENOUS VERSUS EXOGENOUS LITHIUM CLEARANCE FOR EVALUATION OF DOPAMINE-INDUCED CHANGES IN RENAL TUBULAR FUNCTION

1. The present randomized, double-blind cross-over study compared endo genous and exogenous lithium clearance (C-Li) for estimation of the ef fect of dopamine on tubular sodium reabsorption. Twelve normal, salt-r epleted male subjects were investigated on three different occasions w ith either placebo or 450 mg or 600 mg of lithium given in random orde r at 22.00 hours. After an overnight fast, renal clearance studies wer e performed during a 1h baseline period and subsequently during the se cond hour of an infusion of 3 mu g min(-1) kg(-1) of dopamine. 2. Base line values of endogenous C-Li and fractional excretion of lithium (FE (Li)) [27.0 (23.5-30.5) ml/min and 24.2 (20.3-28.2)% (means with 95% c onfidence 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); lit hium, 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 cle arance (C-Na), but glomerular filtration rate and urine flow rate rema ined unchanged, 3. Dopamine increased C-Na to similar values on the th ree study days. C-Li 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), respectivel y, FE(Li) 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 do pamine infusion did not differ significantly. 4. The lower baseline va lues of endogenous C-Li and FE(Li) compared with exogenous values sugg est that C-Li in humans depends on the plasma concentrations of lithiu m. However, the effect of dopamine on C-Li and FE(Li) was expressed to the same extent with endogenous and exogenous lithium, indicating tha t the two methods are interchangeable for estimation of dopamine-induc ed changes in tubular function.