CHARACTERIZATION OF CYCLOSPORINE-A UPTAKE IN HUMAN ERYTHROCYTES

More than 70% of cyclosporine A (CsA) is bound to erythrocytes at whol e blood concentrations of 50-1000 ng.ml(-1). Cytosolic CsA is bound to the erythrocyte peptidyl-prolyl cis-trans isomerase cyclophilin. Meas urements of serum CsA levels under clinical conditions are hampered by a temperature-dependent translocation of CsA into erythrocytes during cooling of the probes to room temperature. In order to characterize t he kinetics of CsA uptake and to find a specific uptake inhibitor, we developed a method to measure the velocity of uptake based on rapid co oling of the erythrocyte suspension. The total erythrocyte-binding cap acity for CsA amounted to 43.10(-5) nmol per 10(6) erythrocytes or 2.6 .10(5) molecules per erythrocyte. Whereas the erythrocyte-binding capa city of CsA was temperature-independent between 10 degrees C and 42 de grees C, uptake kinetics of CsA were temperature-dependent. The Arrhen ius plot for CsA uptake in human erythrocytes was linear and no transi tion temperature between 0 degrees C and 42 degrees C could be detecte d. Therefore the CsA uptake process in human erythrocytes did not fulf il the criteria of carrier-mediated transport. This indicates that CsA diffuses passively into human erythrocytes. Hence, erythrocyte CsA up take cannot be specifically inhibited.