Lithium activates mammalian Na⁺/H⁺ exchangers: isoform specificity and inhibition by genistein

Abstract.

Replacement of external NaCl with LiCl induced cytoplasmic alkalinization in CCL-39 cells and rat L6 myoblasts expressing the endogenous Na⁺/H⁺ exchanger isoform NHE1. This Li⁺-induced alkalinization is due to activation of the Na⁺/H⁺ exchanger because it was completely inhibited by 100 µM ethylisopropylamiloride (apparent K _d=1 µM) and because it did not occur in exchanger-deficient PS120 cells. The effect of Li⁺ was not mimicked by Na⁺, K⁺, Cs⁺ and choline⁺. Li⁺ caused cytoplasmic alkalinization in PS120 cells expressing NHE1 or NHE2, but not NHE3, when Li⁺ was added to cells at a concentration high enough to saturate their external transport sites as predicted from Li⁺ affinities. Li⁺ did not induce phosphatidylinositol (PI) turnover or intracellular Ca²⁺ mobilization. Li⁺-induced alkalinization was not affected by protein kinase C down-regulation, loss of glycogen synthase kinase 3β caused by antisense oligonucleotide treatment, or pretreatment with calphostin C, pertussis toxin, MEK inhibitor PD98059 and PI3-kinase inhibitor LY294002. However, it was markedly suppressed by the tyrosine kinase inhibitor genistein (10 µM). Thus, externally added Li⁺ activates NHE1 and NHE2 via a mechanism possibly involving a tyrosine kinase, causing an increase in cytoplasmic pH that could potentially affect various cell functions.