HUMAN RED CELLS FROM PRE (HEPATO SPLENIC-LATE FETAL) AND POSTNATAL (BONE MARROW-ADULT') STAGES OF HAEMOPOIESIS: NA⁺/LI⁺ EXCHANGE KINETIC

Abstract

The kinetic parameters of Na⁺-Li⁺ exchange were studied in both neonatal and adult' red blood cells in order to evaluate if the asymmetry for Li⁺ fluxes (Na⁺-contralateral-dependent) is expressed in both types of cells. Maximum velocities (Vmax) and Km (half-activation constant) were measured for Li⁺ fluxes in both types of cells. In human neonatal red blood cells (nRBC), extracellular Na⁺-dependent Li⁺ efflux was shown to be a saturable function of external sodium concentration with high affinity (apparent Km: 2.1 mM) and low capacity (maximal velocity Vmax < 0.3 mmoles Li⁺ l'''1 cells h⁻¹). The Vmax and apparent Km for cellular Na⁺-dependent Li⁺ influx were higher (Km < 12.9 mM; Vmax < 1.07 mmoles Li⁺ l⁻¹ cells h⁻¹). The results provide evidence for intrinsic functional asymmetry in this transport system, as the transporter is more prevalent and stable in the inward-facing conformation. These kinetic observations may be explained in terms of the simple carrier transport model. Similar findings were found for this system in human adult' red cells. These results point to the asymmetry pattern (related to Na⁺ activation of Li⁺ flux) as developed in red cells from late prenatal stages of hemopoiesis.