Molecular origin of Na+/Li+ exchanger: Evidence against the involvement of major cloned erythrocyte transporters

Numerous studies have demonstrated heightened Na+/Li+ countertransport (NLCT) activity in erythrocytes of patients with essential hypertension or diabetic nephropathy. The same carrier also contributes to the therapeutic action of lithium salt, widely used in the treatment of psychiatric disorders. However, the molecular origin of NLCT remains unknown. This study examined the role of major ion transporters in NLCT by comparative analysis of its activity and that of ion transporters providing inwardly directed 86Rb, 22Na and 32P fluxes. NLCT was below the detection limit in rat erythrocytes and ∼50-fold higher in rabbits compared to humans. Unlike NLCT, the activities of Na+,K+-ATPase, Na+,K+,2Cl− cotransporter and anion exchanger were somewhat similar in the erythrocytes of these species, whereas Na+,Pi cotransport was in 1:2:6 proportion in rats, humans and rabbits, respectively. Loading of erythrocytes with Li+ for NLCT measurement did not affect the activity of Na+,Pi cotransporter. Keeping in mind that NLCT is much higher in rabbits vs humans and rats, we compared the set of membrane proteins in these species using 2-dimensional gel electrophoresis. This approach revealed 174 common spots, whereas 132 proteins were detected only in human and rabbit erythrocyte membranes. Among these proteins, we found 17 spots whose expression was higher by more than 5-fold in rabbit compared to human erythrocytes. Thus, our results argue against the involvement of major ion transporters in NLCT. They also show that comparative proteomics is a potent tool to identify the molecular origin of this carrier.