Functional characterization of the neuronal-specific K-Cl cotransporter: implications for [K+](0) regulation

The neuronal K-Cl cotransporter isoform (KCC2) was functionally expressed in human embryonic kidney (HEK-293) cell lines. Two stably transfected HEK-293 cell lines were prepared: one expressing an epitope-tagged KCC2 (KCC2-22T) and another expressing the unaltered KCC2 (KCC2-9). The KCC2-22T cells produced a glycoprotein of similar to 150 kDa that was absent from HEK-293 control cells. The Rb-86 influx in both cell lines was significantly greater than untransfected control HEK-293 cells. The KCC2-9 cells displayed a constitutively active Rb-86 influx that could be increased further by 1 mM N-ethylmaleimide (NEM) but not by cell swelling. Both furosemide [inhibition constant (K-i) similar to 25 mu M] and bumetanide (K-i similar to 55 mu M) inhibited the NEM-stimulated Rb-86 influx in the KCC2-9 cells. This diuretic-sensitive Rb-86 influx in the KCC2-9 cells, operationally defined as KCC2 mediated, required external Cl- but not external Na+ and exhibited a high apparent affinity for external Rb+(K+) [Michaelis constant (K-m) = 5.2 +/- 0.9 (SE) mM; n = 5] but a low apparent affinity for external Cl- (K-m >50 mM). On the basis of thermodynamic considerations as well as the unique kinetic properties of the KCC2 isoform, it is hypothesized that KCC2 may serve a dual function in neurons: 1) the maintenance of low intracellular Cl- concentration so as to allow Cl- influx via ligand-gated Cl- channels and 2) the buffering of external K+ concentration ([K+](o)) in the brain.