Differential distribution of Kir4.1 in spinal cord astrocytes suggests regional differences in K+ homeostasis.

Neuronal activity in the spinal cord results in extracellular potassium accumulation that is significantly higher in the dorsal horn than in the ventral horn. This is suggestive of differences in K(+) clearance, widely thought to involve diffusional K(+) uptake by astrocytes. We previously identified the inward rectifying K(+) channel Kir4.1 as the major K(+) conductance in spinal cord astrocytes in situ and hence hypothesized that different expression levels of Kir4.1 may account for the observed differences in potassium dynamics in spinal cord. Our results with immunohistochemical staining demonstrated highest Kir4.1 channel expression in the ventral horn and very low levels of Kir4.1 in the apex of the dorsal horn. Western blots from tissue of these two regions similarly confirmed much lower levels of Kir4.1 in the apex of the dorsal horn. Whole cell patch-clamp recordings from astrocytes in rat spinal cord slices also showed a difference in inwardly rectifying currents in these two regions. However, no statistical difference in either fast-inactivating (Ka) or delayed rectifying potassium currents (Kd) was observed, suggesting these differences were specific to Kir currents. Importantly, when astrocytes in each region were challenged with high [K(+)](o), astrocytes from the dorsal horn showed significantly smaller (60%) K(+) uptake currents than astrocytes from the ventral horn. Taken together, these data support the conclusion that regional differences in astrocytic expression of Kir4.1 channels result in marked changes in potassium clearance rates in these two regions of the spinal cord.