VALIDATION OF RED-CELL SODIUM-LITHIUM COUNTERTRANSPORT MEASUREMENT - INFLUENCE OF DIFFERENT LOADING CONDITIONS

Increased sodium-lithium countertransport in erythrocytes from patient s with long-standing type I (insulin-dependent) diabetes mellitus has been considered as an early marker of nephropathy. Since the activity and kinetics of the sodium-lithium countertransport may critically dep end on loading conditions, this study was aimed at determining sodium- lithium countertransport activity, Michaelis constant K-m and maximum velocity V-max in erythrocytes loaded in two different Li+ solutions. Sodium-lithium countertransport activity was determined in erythrocyte s in 8 healthy control subjects after loading with 150 mmol/l LiCl com pared with those loaded with 150 mmol/l LiHCO3. Sodium-lithium counter transport activity was similar for both loading procedures, although t he erythrocyte lithium content did significantly differ (mean +/- SEM, 7.0 +/- 0.5 for LICl and 8.9 +/- 0.5 mmol/l of cells for 150 mmol/l L iHCO3). There were no significant changes in the K-m and V-max. Increa se of osmolality in efflux media containing 200 and 250 mmol/l NaCl re sulted in a negligible shrinking of the red blood cells, not exceeding 2.2%. The main advantage is the short loading time of 15 min for LiHC O3 compared with 3 hours for LiCl. Under these conditions saturating i ntracellular Li+ concentrations can be obtained much more rapidly than with LiCl loading, thereby minimising alterations of the cell membran es. LiHCO3 loading shortens the experimental time considerably and ena bles a greater number of samples to be screened from larger population cohorts.