Effect of external K+, Ca2+, and Ba2+ on membrane potential and ionic conductance in rat astrocytes

1. The purpose of this study was (a) to identify if astrocytes show a similar non-Nernstian depolarization in low K+ or low Ca2+ solutions as previously found in human glial and glioma cells, and (b) to analyze the influence of the K+ conductance on the membrane potential of astrocytes. 2. The membrane potential (Em) and the ionic conductance were studied with whole-cell patch-clamp technique in neonatal rat astrocytes (5-9 days in culture) and in human glioma cells (U-251MG). 3. In 3.0 mM K+ Em was -75 +/- 1.0 mV (mean +/- SEM, n = 39) in rat astrocytes and -79 +/- 0.7 mV (n = 5) in U-251MG cells. In both cell types Em changed linearly to the logarithm of [K+]0 between 3.0 and 160 mM K+ free medium caused astrocytes to hyperpolarize to -93 +/- 2.7 mV (n = 21) and U-251MG cells to depolarize to -27 +/- 2.1 mV (n = 3). 4. The I-E curve did not show inward rectification in astrocytes at this developmental stage. The slope conductance (g) exhibited only a small decrease (-19%) in K+ free solution and no significant change in 160 mM K+. 5. Ba2+ (1.0 mM) depolarized astrocytes to -45 +/- 2.9 mV (n = 11), decreasing the slope conductance (g) by 42.4 +/- 8.3% (n = 11). Ca2+ free solution depolarized astrocytes to -53 +/- 3.4 mV (n = 12) and resulted in a positive shift of the I-E curve, increasing g by 15.3 +/- 8.2% (n = 8). 6. Calculations indicated that a block of K+ channels explains the depolarizing effect of Ba2+. The effects of K+ free or Ca2+ free solutions on Em can be explained by a transformation of K+ channels to non-specific leakage channels. That astrocytes show a different reaction to low K+ than glioma cells can be related to the lack of inwardly rectifying K+ channels in astrocytes at this developmental stage.