A COMPARISON BETWEEN ENDOGENOUS AND EXOGENOUS LITHIUM CLEARANCE IN THE ANESTHETIZED RAT

The present study tests whether the endogenous renal lithium clearance (C-Li) may be used as an alternative to the conventional method in or der to avoid the adverse effects of exogenous lithium on renal functio n. Endogenous C-Li) was measured by flameless atomic absorption spectr ometry (AAS) in halothane anaesthetized, laparotomized rats and compar ed with values obtained after subsequent intravenous (i.v.) administra tion of LiCl. Endogenous plasma lithium concentration (P-Li) was 0.30 +/- 0.06 mu mol l(-1). When measured by both conventional flame emissi on AAS and flameless AAS on the same samples, values for P-Li did not differ significantly. Endogenous C-Li/GFR was 0.195 +/- 0.01 (means +/ - SEM). Amiloride caused no significant change in either the endogenou s C-Li or Cr-51-EDTA clearance, a measure of glomerular filtration rat e (GFR), but it caused a significant increase in C-Na. Administration of exogenous Li+ had no significant effect on C-Li (230 + 33 vs. 234 /- 35 mu l min(-1) gKW(-1)); both urine flow and C-Na increased signif icantly (P < 0.01) (5.4 +/- 0.4 vs. 9.6 +/- 1.3, and 2.4 +/- 0.5 vs. 1 0.7 +/- 2.4 mu l min(-1) gKW(-1), respectively). GFR and calculated pr oximal tubular reabsorption rate (APR) both decreased by similar to 10 0 mu l min(-1) gKW(-1) (both P less than or equal to 0.025), suggestin g a moderate inhibition of APR after the acute elevation of plasma Li concentration. It is concluded that the endogenous C-Li may be used a s an alternative to the conventional method.