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Abteilung für Allgemeine Zoologie, Universität Kaiserslautern, Germany.

1. The role of the giant neuropile glial cells in the buffering of activity-related extracellular pH changes was studied in segmental ganglia of the leech Hirudo medicinalis L. using pH-sensitive microelectrodes and a slow, two-electrode voltage-clamp system. Neuronal activity was induced by electrical stimulation of a ganglionic side nerve (20 Hz, 1 min). 2. In CO2-HCO3(-)-buffered saline the glial cells were depolarized by 6.5 +/- 2.3 mV and alkalinized by 0.024 +/- 0.006 pH units (mean +/- SD) during the stimulation. The stimulation induced an acidification of 0.032 +/- 0.006 pH units in the extracellular spaces (ECS). 3. Voltage clamping the glial cells suppressed the stimulus-induced glial depolarization and turned the intraglial alkalinization into an acidification of 0.045 +/- 0.021 pH units (n = 6) that closely resembled the acidification observed in the presence of the anion transport blocker DIDS (4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid, 0.5 mM), and in CO2-HCO(3-)-free saline. 4. Voltage clamping the glial cell resulted in the appearance of a distinct stimulus-induced extracellular alkalinization of 0.024 +/- 0.013 pH units at the onset of the stimulation, as also observed during DIDS application and in the absence of CO2-HCO3-. 5. The results suggest that glial uptake of bicarbonate is mediated by depolarization-induced activation of the electrogenic Na(+)-HCO3- cotransport, which suppresses the profound alkalinization of the ECS during neuronal activity. This is the first direct evidence the glial cells actively modulate extracellular pH changes in a voltage-dependent manner.