Evidence for glial control of extracellular pH in the leech central nervous system

Double-barrelled pH-sensitive microelectrodes were used to measure the intracellular pH (pHi) of neuropil glial cells and the pH of extracellular spaces (pH(e)) within isolated, intact ganglia of the leech Hirudo medicinalis. By application of a weak acid (propionate, 40 mM) or a weak base (ammonium, 20 mM) the total buffer capacity was estimated by changes in glial pHi and in pH(e). The buffering power of glial cells and in the extracellular spaces was increased by up to threefold in the presence of CO2/bicarbonate. The anion exchange inhibitor 4,4-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS, 0.3-0.5 mM) reversed this increase in buffering power both in the glial cells and in the extracellular spaces. Inhibitors of the carbonic anhydrases reduced the CO2/bicarbonate-dependent increase in extracellular buffering power. The results suggest that extracellular H+ buffering dependent upon the availability of bicarbonate is linked to DIDS-sensitive bicarbonate transport across the glial membrane.