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Acid/base transport across the leech giant glial cell membrane at low external bicarbonate concentration

Joachim W. Deitmer and Hans-Peter Schneider

1. We have studied acid/base transport across the cell membrane of the giant neuropile glial cell in the leech (Hirudo medicinalis) central nervous system induced by changing the external pH (pH_o), using double-barrelled, pH-sensitive microelectrodes. In the presence of 5 % CO₂ and 24 mM HCO₃^-, the intracellular pH (pH_i) rapidly changes due to a potent, reversible Na⁺-HCO₃^- cotransport across the glial membrane. We have now investigated the transport mechanism which leads to pH_i changes in the nominal absence of CO₂/HCO₃^-, where the HCO₃^- concentration is expected to be below 1 mM.

2. The intracellular pH increased and then decreased when pH_o was altered from 7·4 to 7·8 and then 7·0 with a rate of increase of +0·026 ± 0·008 and a rate of decrease of -0·028 ± 0·009 pH units min^-1 (± s.d., n = 49), indicating an acid/base flux rate of 0·64 and 0·71 mM min^-1 across the glial membrane, respectively.

3. In the absence of external sodium (Na⁺replaced by N-methyl-D-glucamine), pH_i slowly decreased, and the rate of alkali and acid loading was reduced to 19 and 28 %, respectively, (n = 12). Amiloride (2 mM), which inhibits Na⁺-H⁺ exchange, had no effect on the alkali/acid loading (n = 6).

4. The alkali and acid loading were not impaired after the removal of external chloride (Cl^-_o, replaced by gluconate; n = 11), but were significantly reduced by the anion transport inhibitor 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS, 0·5 mM) to 23 and 16 %, respectively, of the control (P < 0·001; n = 5).

5. Alkali and acid loading were affected differently by manipulating the availability of residual HCO₃^-. After adding the membrane-permeable carbonic anhydrase inhibitor ethoxyzolamide (EZA, 2 µM) to the saline, the acid loading, but not the alkali loading, was significantly reduced (by 25 %, P < 0·01), while lowering the residual CO₂/HCO₃^- concentration in the saline by O₂ bubbling significantly reduced the alkali loading (by 59 %, P < 0·02), but not the acid loading.

6. Changing the membrane holding potential in voltage-clamped glial cells or raising the external K⁺ concentration to 30 mM had no significant effect on acid/base loading.

7. It is concluded that a residual HCO₃^- concentration of less than 1 mM in nominally CO₂/HCO₃^--free salines and HCO₃^- produced endogenously in the glial cells support alkali and acid loading across the glial cell membrane, presumably by activation of the reversible Na⁺-HCO₃^- cotransporter. The results suggest a very high selectivity and affinity of this cotransporter for HCO₃^-; they imply that HCO₃^--dependent processes may not be negligible even in the nominal absence of CO₂/HCO₃^-, when the HCO₃^- concentration is expected to be in the submillimolar range.

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