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Abteilung für Allgemeine Zoologie, FB Biologie, Universität Kaiserslautern, Germany.

1. The intracellular pH (pHi) was measured in voltage-clamped, giant neuropile glial cells in isolated segmental ganglia of the leech Hirudo medicinalis, using double-barrelled, pH-sensitive microelectrodes and a slow, two-electrode voltage-clamp system. The potential sensitivity of the pHi regulation in these glial cells was found to be due to an electrogenic Na(+)-HCO3- cotransporter (Deitmer & Szatkowski, 1990). 2. In the presence of 5% CO2 and 24 mM HCO3- (pH 7.4), pHi shifted by 1 pH unit per 110 mV, corresponding to a stoichiometry of 2HCO3-: 1 Na+ of the cotransporter, while in Hepes-buffered CO2-HCO3(-)-free saline (pH 7.4), pHi changed by 1 pH unit per 274 mV. The potential sensitivity of pHi decreased at lower pHo, being 1 pH unit per 216 mV at external pH (pHo) 7.0. 3. Changing pHo between 7.8 and 6.6 induced pHi shifts with a slope of 0.72 pHi units per pHo unit in non-clamped, and of 0.80 pHi units per pHo unit in voltage-clamped cells, indicating that pHi largely followed pHo. The electrochemical gradient of H(+)-HCO3- across the glial membrane was around 56 mV, and remained almost constant over this pHo range. 4. The membrane potential-dependent and pHo-sensitive shifts of pHi were unaffected by amiloride, an inhibitor of Na(+)-H+ exchange. 5. The intracellular acidification upon lowering pHo could be reversed by depolarizing the membrane as predicted from a cotransporter, whose equilibrium follows the membrane potential by resetting pHi. 6. The results indicate that the pHi of leech glial cells is dominated by the electrogenic Na(+)-HCO3- cotransporter, and is hence a function of the membrane potential, and the Na+ and H(+)-HCO3- gradients, across the cell membrane.

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