Sodium Transporters Are Involved in Lithium Influx in Brain Endothelial Cells.

Variability in drug response to lithium (Li+) is poorly understood and significant, as only 40% of patients with bipolar disorder highly respond to Li+. Li+ can be transported by sodium (Na+) transporters in kidney tubules or red blood cells, but its transport has not been investigated at the blood-brain barrier (BBB). Inhibition and/or transcriptomic strategies for Na+ transporters such as NHE (SLC9), NBC (SLC4), and NKCC (SLC12) were used to assess their role on Li+ transport in human brain endothelial cells. Na+-free buffer was also used to examine Na+/Li+ countertransport (NLCT) activity. The BBB permeability of Li+ evaluated in the rat was 2% that of diazepam, a high passive diffusion lipophilic compound. Gene expression of several Na+ transporters was determined in hCMEC/D3 cells, human hematopoietic stem-cell-derived BBB models (HBLEC), and human primary brain microvascular endothelial cells (hPBMECs) and showed the following rank order with close expression profile: NHE1 > NKCC1 > NHE5 > NBCn1, while NHE2-4, NBCn2, and NBCe1-2 were barely detected. Li+ influx in hCMEC/D3 cells was increased in Na+-free buffer by 3.3-fold, while depletion of chloride or bicarbonate had no effect. DMA (NHE inhibitor), DIDS (anionic carriers inhibitor), and bumetanide (NKCC inhibitor) decreased Li+ uptake significantly in hCMEC/D3 by 52, 51, and 47%, respectively, while S0859 (NBC inhibitor) increased Li+ influx 2.3-fold. Zoniporide (NHE1 inhibitor) and siRNA against NHE1 had no effect on Li+ influx in hCMEC/D3 cells. Our study shows that NHE1 and/or NHE5, NBCn1, and NKCC1 may play a significant role in the transport of Li+ through the plasma membrane of brain endothelial cells.