Assay of sodium-lithium countertransport rate by direct measurement of erythrocyte lithium concentrations.
A method is described for the in-vitro measurement of erythrocytic sodium-lithium countertransport rate (SLC) by direct assay of intra-erythrocytic lithium (Li) concentrations in Li loaded cells during controlled Li efflux. Bicarbonate stimulated erythrocytic Li loading to an approximate intracellular concentration of 3 mmol/l can be achieved within 4 min. Maximum SLC rates are achieved at intra-erythrocytic concentrations greater than 2.5 mmol/l. Lithium efflux is linear over 0-60 min, and independent of pH change. The within-batch coefficient of variation for positive displacement pipetting of Li loaded erythrocytes is 2.8%. Storage of erythrocytes in isosmotic MgCl2 for 24 h had no effect on the SLC. Intra-individual coefficient of variation for SLC was less than 6%. The reference range for Caucasian subjects (n = 43) was 0.14-0.49 mmol Li +/l erythrocytes/h.