IS LITHIUM A USEFUL TOOL IN STUDIES ON RENAL SODIUM HANDLING

The use of the renal clearance of lithium (C-Li) as a non-invasive mar ker of sodium and water delivery to the end of the proximal tubules as sumes that two conditions are fulfilled: (i) fractional reabsorption o f Li in the proximal tubule should be the same as that of sodium (and water); (ii) no net Li reabsorption (or secretion) should occur beyond the proximal tubule, Early evidence supporting the use of C-Li was ne cessarily circumstantial, but recently it has become possible to test the method more directly by using electrothermal atomic absorption spe ctrometry to measure tubular fluid Li concentrations in discrete nephr on segments (in experimental animals). The results of these micropunct ure investigations have challenged both of the underlying assumptions: most studies have indicated that Li reabsorption in the proximal tubu le lags slightly behind that of sodium (and water), and there is compe lling evidence for significant Li reabsorption in the loop of Henle. I n exceptional circumstances (severe sodium or potassium depletion) the re is clear evidence for Li reabsorption beyond the loop. Although it is possible that the conditions required for micropuncture studies ane sthesia and surgery) could influence the renal handling of Li, data fr om studies in humans strongly support the existence of a loop componen t to Li reabsorption. These findings lead us to conclude that C-Li sho uld not be used as a quantitative measure of end-proximal sodium and w ater delivery. However, it seems likely that the errors involved are r elatively small and to some extent offset one another; consequently, p rovided that appropriate precautions are taken, we believe that C-Li r emains a useful method for the qualitative assessment of proximal tubu lar function.