Hyperpolarization-activated ion currents in cultured rat cortical and spinal cord astrocytes

Hyperpolarization-activated currents were recorded f r om r a t b r a i n c o r t i cal and spinal cord astrocytes maintained i n culture. Spinal cord astrocytes expressed pr i m a r i l y an i n w a r d rectifier potassium current characterized by time-dependent inactivation, a strong dependence on extracellular N a + and insensitivity to i n t r a c e l l u l a r GTP- -y-S ( 0 . 2 mM). I n cortical astrocytes voltage clamp protocols aimed to elicit currents activated at, or negative to cell membrane potentials led to the development of two distinct ion currents. The most prominent current resembled the inwar d rectifier potassium current. This component was sensitive to blockade by extracellular cesium and was greatly reduced d u r i n g recordings performed w i t h GTP-7-S ( 0 . 2 Mm) added to the pipette solutions. The remaining current component was similar to the endothelial I h a current. I h a conductance was enhanced by extracellular potassium and the current reversal potential behaved as expected for a mixed cation, N a 7 K + current. I h a was nearly abolished after removal of extracellular Na". These results are consistent w i t h the expression of a novel mixed cation conductance i n g l i a l cells, possibly involved i n extracell u l a r potassium buffering