Depolarization-induced alkalinization of astrocytes in gliotic hippocampal slices

Depolarization-induced, intracellular alkaline shifts were studied in reactive astrocytes within slices of gliotic hippocampus. Slices were prepared 10-28 days after sterotaxic injection of kainic acid into the hippocampus of anesthetized rats. Astrocytes in gliotic CA3 were impaled with double-barreled pH sensitive microelectrodes and depolarized by iontophoresis of K+ from an adjacent micropipette. Elevation of extracellular K+ produced an intracellular alkalinization that grew with increasing membrane depolarization, ranging from approximately 0.10 to 0.30 pH units. Exposure to Ba2+ depolarized the cells and produced a similar alkalinization. In the presence of Ba2+, the K(+)-induced depolarization and the associated alkaline shift were abolished. The depolarization-induced alkaline shifts were partially inhibited (40 +/- 8.9%) in Na(+)-free media and were enhanced in bicarbonate versus HEPES-buffered saline. The alkalinizations were unaffected by incubation in chloride-free media, or by the stilbene 4,4'-dinitrostilbene-2,2'-disulfonic acid. It is concluded that the depolarization-induced alkaline shift of reactive astrocytes is mediated in part by a Na+ and HCO3(-)-dependent mechanism that is insensitive to stilbenes. These characteristics correspond well with the properties of depolarization-induced acid secretion in the gliotic tissue. In addition, a separate, Na(+)-independent mechanism contributes to the depolarization-induced alkalinization. In view of the absolute Na+ dependence of acid secretion in the gliotic slices, we propose that the latter mechanism does not extrude acid across the plasma membrane.