Depolarization-induced acid secretion in gliotic hippocampal slices

Gliotic hippocampal slices were used to study glial acid secretion in a tissue largely devoid of neural elements. Rat hippocampal slices were prepared 10-28 days after sterotaxic injection of kainate. Cresyl Violet staining and immunohistochemistry for glial fibrillary acidic protein demonstrated a loss of neurons and a proliferation of reactive astrocytes in area CA3. Extracellular pH and K+ shifts were recorded in CA3 in response to K+ iontophoresis. Elevation of K+ evoked an extracellular acid shift that was two- to three-fold larger in gliotic versus unlesioned tissue. Ba2+ caused a slow extracellular acidification, and blocked both the depolarizing responses of the glial cells and the acid shifts evoked by K+. The K(+)-evoked acid shifts were abolished in Na(+)-free media, and diminished in HEPES-buffered solutions. Inhibition of extracellular carbonic anhydrase caused a reversible enhancement of the K(+)-evoked acid shifts, an effect that could be mimicked during H+ iontophoresis in agarose gels. Gliotic acid shifts were unaffected by amiloride or its analogs, stilbenes, zero Cl- media, zero or elevated glucose, lactate transport inhibitors, zero Ca2+ or Cd2+. Smaller acid shifts could be evoked in normal slices which were also enhanced by benzolamide, and blocked by Ba2+ and zero Na+ media. It is concluded that acid secretion by reactive astrocytes is Na+ and HCO3(-)-dependent and is triggered by depolarization. The similar pharmacological and ionic sensitivity of the acid shifts in non-gliotic tissue suggest that these properties are shared by normal astrocytes. These characteristics are consistent with the operation of an electrogenic Na(+)-HCO3- co-transporter. However, the enhancement of the acid shifts by inhibitors of extracellular carbonic anhydrase suggests that CO3(2-), rather than HCO3-, is the transported acid equivalent.