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Acid/base transport across the leech giant glial cell membrane at low external bicarbonate concentration
Author(s) -
Deitmer Joachim W.,
Schneider HansPeter
Publication year - 1998
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1111/j.1469-7793.1998.459be.x
Subject(s) - intracellular ph , chemistry , cotransporter , biophysics , bicarbonate , sodium , alkali metal , intracellular , base (topology) , membrane , membrane potential , acid–base homeostasis , acid–base reaction , hirudo medicinalis , inorganic chemistry , leech , biochemistry , biology , mathematical analysis , mathematics , organic chemistry , world wide web , computer science
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 % CO 2 and 24 m m HCO 3 − , the intracellular pH (pH i ) rapidly changes due to a potent, reversible Na + ‐HCO 3 − cotransport across the glial membrane. We have now investigated the transport mechanism which leads to pH i changes in the nominal absence of CO 2 /HCO 3 − , where the HCO 3 − concentration is expected to be below 1 m m . 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 m m 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 m m ), 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 m m ) 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 HCO 3 − . After adding the membrane‐permeable carbonic anhydrase inhibitor ethoxyzolamide (EZA, 2 μ m ) to the saline, the acid loading, but not the alkali loading, was significantly reduced (by 25 %, P < 0.01 ), while lowering the residual CO 2 /HCO 3 − concentration in the saline by O 2 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 m m had no significant effect on acid/base loading. 7 It is concluded that a residual HCO 3 − concentration of less than 1 m m in nominally CO 2 /HCO 3 − ‐free salines and HCO 3 − produced endogenously in the glial cells support alkali and acid loading across the glial cell membrane, presumably by activation of the reversible Na + ‐HCO 3 − cotransporter. The results suggest a very high selectivity and affinity of this cotransporter for HCO 3 − ; they imply that HCO 3 − ‐dependent processes may not be negligible even in the nominal absence of CO 2 /HCO 3 − , when the HCO 3 − concentration is expected to be in the submillimolar range.