Cation transport by the neuronal K(+)-Cl(-) cotransporter KCC2: thermodynamics and kinetics of alternate transport modes

10.1152/ajpcell.00005. 2004.—Both Cs and NH4 alter neuronal Cl homeostasis, yet the mechanisms have not been clearly elucidated. We hypothesized that these two cations altered the operation of the neuronal K-Cl cotransporter (KCC2). Using exogenously expressed KCC2 protein, we first examined the interaction of cations at the transport site of KCC2 by monitoring furosemide-sensitive 86Rb influx as a function of external Rb concentration at different fixed external cation con-centrations (Na, Li, K, Cs, and NH4). Neither Na nor Li affected furosemide-sensitive 86Rb influx, indicating their inability to interact at the cation translocation site of KCC2. As expected for an enzyme that accepts Rb and K as alternate substrates, K was a competitive inhibitor of Rb transport by KCC2. Like K, both Cs and NH4 behaved as competitive inhibitors of Rb transport by KCC2, indicating their potential as transport substrates. Using io