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An inwardly directed electrogenic sodium‐bicarbonate co‐transport in leech glial cells.
Author(s) -
Deitmer J W,
Schlue W R
Publication year - 1989
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 - 10.1113/jphysiol.1989.sp017567
Subject(s) - dids , amiloride , chemistry , bicarbonate , intracellular ph , acetazolamide , carbonic anhydrase , biophysics , sodium , ion transporter , hepes , intracellular , steady state (chemistry) , biochemistry , medicine , biology , membrane , enzyme , organic chemistry
1. We have used double‐barrelled ion‐sensitive microelectrodes to measure the intracellular pH, pHi, the intracellular Na+ activity, aiNa, and the membrane potential in identified glial cells of the central nervous system of the leech Hirudo medicinalis to study the effect of CO2‐HCO3‐. 2. When a HEPES‐buffered saline was exchanged for a saline buffered with 2% CO2 + 11 mM‐HCO3‐, keeping the pH constant at 7.4, the mean steady‐state pHi of the glial cells increased from 6.85 +/‐ 0.06 to 7.18 +/‐ 0.13 (mean +/‐ S.D., n = 25). 3. This CO2‐HCO3‐ ‐dependent alkalinization was inhibited in the absence of external Na+ (exchanged by N‐methyl‐D‐glucamine), but was unaffected by the inhibitor of Na+‐H+ exchange, amiloride (2 mM). 4. The aiNa of the glial cells increased by 2‐4 mM from a mean steady state of 7.2 +/‐ 2 mM (mean +/‐ S.D., n = 6) upon introduction of CO2‐HCO3‐ ‐buffered saline. This CO2‐HCO3‐ ‐dependent rise in aiNa increased to about double when the pHi had been decreased by acid loading the cells (addition and subsequent removal of NH4+). 5. The CO2‐HCO3‐ ‐dependent increases of pHi and aiNa were inhibited by the stilbene 4,4‐diisothiocyanostilbene‐2,2'‐disulphonic acid (DIDS, 0.5‐1.0 mM). 6. Removal of external Cl‐ and depletion of intracellular Cl‐ did not inhibit the CO2‐HCO3‐ ‐dependent alkalinization. 7. The CO2‐HCO3‐ ‐dependent alkalinization was unaffected by inhibitors of the carbonic anhydrase, acetazolamide (0.2 mM) or ethoxzolamide (2 microM). 8. The membrane potential became more negative by 3‐20 mV upon addition of CO2‐HCO3‐. This hyperpolarization was even further enlarged in the presence of Ba2+ (which reduces the K+ permeability) or at increased external K+ concentration (which depolarizes the membrane and brings the membrane potential to the K+ equilibrium potential). The CO2‐HCO3‐ ‐induced membrane hyperpolarization was inhibited in Na+‐free saline and in the presence of DIDS. Ouabain (0.5 mM) sometimes reduced, but never abolished, the hyperpolarization. 9. The stoichiometry of the co‐transport is suggested to be 2 HCO3‐:1 Na+ with an equilibrium potential of ‐90 mV calculated for this coupling ratio in the steady state. 10. It is concluded that in the presence of CO2‐HCO3‐ an inwardly directed electrogenic Na+‐HCO3‐ co‐transport is stimulated across the glial membrane, which greatly determines the pHi and thereby affects the intracellular buffering power of the glial cells.