The distribution of potassium conductance over the surface of freshly dissociated salamander astrocytes was determined by monitoring cell depolarizations evoked by focal increases in the extracellular potassium concentration. The specific potassium conductance of the endfoot processes of these cells was approximately tenfold higher than the conductance of other cell regions. This dramatically nonuniform conductance distribution may play an important role in the regulation of extracellular potassium levels by glia in the brain.Although astrocytes outnumber all other types of glial cells and neurons in the vertebrate brain, their membrane properties and functions are not well understood (1,2). Müller cells, the principal glial cells of the retina, resemble astrocytes in many respects. In Müller cells, 95 percent of the total cell membrane conductance is localized to the cell's endfoot process (3, 4). It is not known whether this striking membrane specialization occurs only in these specialized retinal cells or whether membrane conductance is also distributed nonuniformly across the surface of all astrocytes throughout the brain. I have addressed this issue by determining the distribution of potassium conductance over the surface of freshly dissociated astrocytes.Astrocytes were isolated from the optic nerve of the salamander Ambystoma tigrinum. Optic nerves were excised with the meninges intact and partially teased apart before incubation in papain [0.6 mg/ml in Hepes-buffered Ringer solution; see (4) for composition of Ringer solutions] for 30 minutes at 28°C. They were then washed several times in Hepes-Ringer and maintained at 0°C for 3 to 4 hours before the cells were dissociated by gentle trituration. Isolated cells were placed in a recording chamber and perfused with a bicarbonate-buffered Ringer solution (containing 2.5 mM K + ) maintained at 15°C. Experiments were performed on cells within 2 hours of dissociation.The cells isolated with this dissociation procedure all had a similar appearance. They had several stout, branching processes radiating from a prominent soma. These cells were provisionally identified as astrocytes on the basis of their appearance and because astrocytes are the principal cell type found in the salamander optic nerve. [Amphibian optic nerves contain no intrinsic neurons and few myelinated fibers or oligodendrocytes (5,6).] The radial processes of these cells terminated in bulbous enlargements. These were the endfeet, which lie at the ends of astrocyte processes in the intact brain and optic nerve. Cells had from 3 to 17 endfeet each, although the exact number was sometimes difficult to determine (mean = 8.7; n= 30). Some radial processes lacked endfeet, probably because they were broken off during dissociation. Dissociated cells were characterized by immunofluorescent labeling of glial fibrillary acidic protein (GFAP), a characteristic cytoplasmic marker of astrocytes (7). All dissociated cells were labeled heavily with monoclonal antibodies to GFAP (Fig. 1), identifying th...