AJP - Renal Physiology, Vol 252, Issue 3 382-F386, Copyright © 1987 by American Physiological Society
Lithium clearance in mineralocorticoid escape in humans
W. H. Boer, H. A. Koomans and E. J. Dorhout Mees
Lithium clearance (CLi) has been advanced as an indicator of Na delivery
from the proximal tubules. We studied CLi in eight healthy males before and
after mineralocorticoid escape, a maneuver that may induce suppression of
fractional proximal Na reabsorption (FPRNa). FPRNa was also estimated from
changes in maximal free water clearance (CH2O). According to the latter
method, FPRNa dropped from 85.7 +/- 2.2 to 81.3 +/- 3.4%, whereas inulin
clearance rose from 125 +/- 11 to 149 +/- 15 ml/min. The changes in CLi
were surprisingly large, from 40.6 +/- 7.6 to 75.8 +/- 18.4 ml/min. If
lithium is a valid marker of Na handling in the proximal tubule in humans,
this change would imply a fall in FPRNa from 67.5 +/- 3.9 to 49.7 +/- 9.0%,
suggesting a much larger shift in tubular Na reabsorption in escape than
hitherto suspected. In addition, it would suggest that the inevitable back
diffusion of a part of the solute-free water in the distal nephron, and
thus overestimation of FPRNa by the CH2O method, increases importantly
during escape. Alternately, lithium may not be a good marker of proximal
tubular Na handling. For instance, both lithium reabsorption and escape may
take place beyond the proximal tubule, or lithium may be excreted in the
distal nephron in certain conditions. Present methods do not permit further
analysis of these options in the human model.
