Transport of amino acids into rat erythrocytes and its potential physiological role

Previous studies have proposed the possibility that erythrocytes (RBC) are involved in the interorgan transport of amino acids; however, this role has not been confirmed. In order to study the likelihood that erythrocytes are involved in the interorgan transport, rates of influx and efflux of glycine, threonine, lysine, histidine and leucine were measured in rat red blood cells. Time course of influx of leucine, a large neutral amino acid, was very rapid (319 mumoles/L RBC. min), and a steady state was reached between 5 to 10 min of incubation, whereas glycine influx was very slow (5.04 mumoles/L RBC. min). Threonine influx was similar to leucine although the rate was slower (41.4 mumoles/L RBC. min); however, the steady state was reached in 30 minutes and its uptake showed less capacity. Histidine and lysine showed a continuous influx, and did not reach a steady state after 60 min of incubation. Efflux of leucine was extremely rapid indicating a rapid equilibration between the incubation medium and the intracellular space of the erythrocytes. Threonine efflux had a half life (t1/2) of between two to three min, independently of the medium used. Histidine showed a t1/2 of around six min, whereas for the small neutral amino acid glycine it was of 14 to 17 min. These results indicate that some large neutral amino acids are not involved in the potential interorgan transport by red blood cells due to the rapid equilibration of the concentration of amino acid between cells and the medium.