Sodium-lithium exchange and sodium-potassium cotransport in human erythrocytes. Part 1: Evaluation of a simple uptake test to assess the activity of the two transport systems.

A reliable and simple uptake test is proposed to assess the activity of the sodium lithium (Na+-Li+) countertransport and the sodium-potassium (Na+-K+) cotransport system of human erythrocytes. The test consists in measurement of phloretin-sensitive Li+ uptake from magnesium (Mg2+) media containing 2 mM Li+ and of furosemide-sensitive rubidium (Rb+) uptake from Na+ media containing 5 mM Rb+. Both Li+ and Rb+ are determined by atomic absorption spectrophotometry. The particular experimental conditions of the uptake assay are analyzed with respect to parameters that potentially affect the results, such as pH, cell volume, internal Na+, external Na+ and Rb+, presence of other ions, and concentration and protein-binding of furosemide, respectively. It is found that the activity of the two transport systems varies more than threefold among normotensive individuals, the activities in erythrocytes of each individual donor remaining stable within +/- 10% over more than two red cell life spans. The Li+ and Rb+ uptake rates are highly correlated to Li+ and K+ or Na+ release rates as determined by the methods of Canessa et al. and Garay et al. The test can thus be applied for routine screening in the search of "abnormalities" of red cell cation transport in hypertensive patients.