Abstract
Lithium transport across the cell membrane is interesting in the light of general cell physiology and becau- se of its alteration during numerous human diseases. The mechanism of Li4 transfer has been studied mainly in erythrocytes with a slow kinetics of ion exchange and therefore under the unbalanced ion distribution. Prolife- rating cultured cells with a rapid ion exchange have not been used practically in study of Li4 transport. In pre- sent paper, the kinetics of Li4 uptake and exit as well as its balanced distribution across the plasma membrane of U937 cells were studied at minimal external Li+ concentrations and after the whole replacement of external Na+ for Li+. It has been found that a steady state Li+ distribution is attained at a high rate similar to that for Na+ and Cl- and that Li+/Na+ discrimination under the balanced ion distribution at 1-10 mM external Li+ keeps on 3 and drops to 1 following blocking of the Na,K-ATPase pump by ouabain. About of 80% of the total Li+ flux across the plasma membrane under the balanced Li+ distribution at 5 mM external Li+ accounts for the equiva- lent Li+/Li+ exchange. The most part of the Li+ flux into the cell down the electrochemical gradient is a flux through channels and its small part may account for the NC and NKCC cotransport influxes. The downhill Li+ influxes are balanced by the uphill Li+ efflux involved in Li+/Na+ exchange. The Na+ flux involved in the countertransport with the Li+ accounts for about 0.5% of the total Na+ flux across the plasma membrane. The study of Li+ transport is an important approach to understand the mechanism of the equivalent Li+/Li+/Na+/Na+ exchange because no blockers of this mode of ion transfer are known and it cannot be revealed by electrophysiological methods. Cells treated with the medium where Na+ is replaced for Li+ are recommended as an object for studying cells without the Na,K-ATPase pump and with very low intracellular Na+ and K+ concentration.
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