CLEARANCE OF ENDOGENOUS LITHIUM IN HUMANS - ALTERED DIETARY SALT INTAKE AND COMPARISON WITH EXOGENOUS LITHIUM CLEARANCE

We compared endogenous with exogenous lithium clearance (C-Li) and stu died the effects of dietary salt intake on endogenous C-Li in healthy volunteers. Lithium was detectable within a narrow fourfold range in s erum and in urine in all 25 subjects studied [serum (n = 25), mean 0.2 7 +/- 0.02 mu mol/l, range 0.13-0.55 mu mol/l; urine (n = 20), range 1 .49-7.32, mean 4.09 +/- 0.36 mu mol/24 h] Mean clearance and fractiona l excretion of endogenous lithium were lower (15.2 +/- 2.0 ml/min and 16.4 +/- 2.1%, respectively) compared with results obtained using the exogenous C-Li technique (25.5 +/- 1.7 ml/min and 27.9 +/- 2.1%; P < 0 .01 and P < 0.05, respectively; n = 17). In a separate group of six no rmal subjects, absolute (8.7 +/- 2.9 vs. 20.7 +/- 3.8 ml/min) and frac tional excretion of lithium (8.3 +/- 2.9 vs. 18.0 +/- 5.1%) were signi ficantly lower on 5 days of low (31 +/- 10 mmol/day) vs. high sodium i ntake (357 +/- 78 mmol/day; P < 0.05). Use of endogenous C-Li preclude s the need for lithium tablets. This could be a particular advantage i n population studies and permits serial measurement of C-Li on differe nt days. Our results show that it is important to take dietary sodium intake into account in studies of endogenous C-Li. Lower values for en dogenous compared with exogenous C-Li could reflect differences in ren al handling depending on the plasma lithium concentration. This clearl y requires further study.