Altered expression of renal acid-base transporters in rats with lithium-induced NDI

Prolonged lithium treatment of humans and rodents often results in hyperchloremic metabolic acidosis. This is thought to be caused by diminished net H⁺ secretion and/or excessive back-diffusion of acid equivalents. To explore whether lithium treatment is associated with changes in the expression of key renal acid-base transporters, semiquantitative immunoblotting and immunocytochemistry were performed using kidneys from lithium-treated (n = 6) and control (n = 6) rats. Rats treated with lithium for 28 days showed decreased urine pH, whereas no significant differences in blood pH and plasma HCO₃^- levels were observed. Immunoblot analysis revealed that lithium treatment induced a significant increase in the expression of the H⁺-ATPase (B₁-subunit) in cortex (190 ± 18%) and inner stripe of the outer medulla (190 ± 9%), and a dramatic increase in inner medulla (900 ± 104%) in parallel to an increase in the expression of type 1 anion exchanger (400 ± 40%). This was confirmed by immunocytochemistry and immunoelectron microscopy, which also revealed increased density of intercalated cells. Moreover, immunoblotting and immunocytochemistry revealed a significant increase in the expression of the type 1 electrogenic Na⁺-HCO₃^- cotransporter (NBC) in cortex (200 ± 23%) and of the electroneutral NBCn1 in inner stripe of the outer medulla (250 ± 54%). In contrast, there were no changes in the expression of Na⁺/H⁺ exchanger-3 or of the Cl^-/HCO₃^- exchanger pendrin. These results demonstrate that the expression of specific renal acid-base transporters is markedly altered in response to long-term lithium treatment. This is likely to represent direct or compensatory effects to increase the capacity for HCO ₃^- reabsorption, NH₄⁺ reabsorption, and proton secretion to prevent the development of systemic metabolic acidosis.