Functional expression of K ir 4.1 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 Ba 2+ . 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. © 2005 Wiley‐Liss, Inc.