Functional expression of Kir4.1 channels in spinal cord astrocytes

Spinal cord astrocytes (SCA) have a high permeability to K+ and hence have hyperpolarized resting membrane potentials. The underlying K+ channels are believed to participate in the uptake of neuronally released K+. These K+ channels have been studied extensively with regard to their biophysics and pharmacology, but their molecular identity in spinal cord is currently unknown. Using a combination of approaches, we demonstrate that channels composed of the K(ir)4.1 subunit are responsible for mediating the resting K+ conductance in SCA. Biophysical analysis demonstrates astrocytic K-ir currents as weakly rectifying, potentiated by increasing [K+](o), and inhibited by micromolar concentrations of Ba2+. These currents were insensitive to tolbutemide, a selective blocker of K(ir)6.x channels, and to tertiapin, a blocker for K(ir)1.1 and K(ir)3.1/3.4 channels. PCR and Western blot analysis show prominent expression of K(ir)4.1 in SCA, and immunocytochemistry shows localization K(ir)4.1 channels to the plasma membrane. K(ir)4.1 protein levels show a developmental upregulation in vivo that parallels an increase in currents recorded over the same time period. K(ir)4.1 is highly expressed throughout most areas of the gray matter in spinal cord in vivo and recordings from spinal cord slices show prominent K-ir currents. Electrophysiological recordings comparing SCA of wild-type mice with those of homozygote K(ir)4.1 knockout mice confirm a complete and selective absence of K-ir channels in the knockout mice, suggesting that K(ir)4.1 is the principle channel mediating the resting K+ conductance in SCA in vitro and in situ. (C) 2005 Wiley-Liss, Inc.