W. Besch et al., VALIDATION OF RED-CELL SODIUM-LITHIUM COUNTERTRANSPORT MEASUREMENT - INFLUENCE OF DIFFERENT LOADING CONDITIONS, European journal of clinical chemistry and clinical biochemistry, 33(10), 1995, pp. 715-719
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.