Astrocytic inwardly rectifying potassium currents are dependent on external sodium ions

1. Two subtypes of astrocytes that expressed distinctly different ion channel complements were identified in primary cultures from rat spinal cord and hippocampus using whole cell patch-clamp techniques. One population of cells expressed voltage-activated Na+ currents and displayed outwardly rectifying I-V relationships; the other group of cells had no detectable Na+ currents and pronounced inwardly rectifying I-V curves. 2. Astrocytes expressing Na+ currents were hyperpolarized (by approximately 7 mV) upon removal of external sodium, suggesting a resting Na+ conductance in these cells. In contrast, cells expressing primarily inwardly rectifying K+ currents, Kir, depolarized (by approximately 4-6 mV) in low-sodium solutions. 3. Removal of external Na+ ions increased the input resistance (189% of control) and reduced the whole cell current amplitude (60% of control at -120 mV) of cells with Kir. The reduction in current amplitude was dose-dependent and became apparent after a 10% reduction of [Na+]0 in 7/7 cells tested. At -120 mV, the effect was near maximal in response to a 50% reduction of [Na+]0. 4. The outward potassium currents of cells expressing Na(+)-currents were unaffected by removal of bath Na+. 5. We conclude that the conductance of glial inwardly rectifying K+ channels is dependent on external sodium ions via a mechanism that does not involve sodium ion permeation or blockade of these channels.