An inwardly directed electrogenic sodium‐bicarbonate co‐transport in leech glial cells.

Abstract

SUMMARY1. 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 ofthe glial cells increased from 6-85+0-06 to 7418+0-13 (mean+S.D., n = 25).3. This C02-HC03--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 a' 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 ofC02-HC03--buffered saline. This C02-HC03--dependent rise in a' a 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 aia were inhibited by the stilbene 4,4-diisothiocyanostilbene-2,2'-disulphonic acid (DIDS, 05-1 0 mm).6. Removal of external Cl-and depletion of intracellular Cl-did not inhibit the C02-HC03--dependent alkalinization.7. The C02-HC03--dependent alkalinization was unaffected by inhibitors of the carbonic anhydrase, acetazolamide (0-2 mM) or ethoxzolamide (2 ,UM).8. The membrane potential became more negative by 3-20 mV upon addition of C02-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 C02-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 HC03-: 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 C02-HC03-an inwardly directed electrogenic Na+-HCO3-co-transport is stimulated across the glial membrane, which J. W. DEITMER AND W.-R. SCHLUE greatly determines the pHi and thereby affects the intracellular buffering power of the glial cells.

Sodium‐bicarbonate cotransport current in identified leech glial cells.

“…Addition of 5 % C02-24 mm HC03-did not markedly alter the course of the membrane potential and al5, and decreased pHi by about 0 04 pH units. This indicates that external Na+ is needed to produce the potential and ion changes described previously upon addition of C02-HC03-(see also Deitmer & Schlue, 1989;Deitmer, 1992b). Changes of intracellular Na+ (a'a, lower trace), intracellular pH (pH,, middle trace) and membrane potential (Em, upper trace) during brief re-addition of Na+ to a Na+-free saline; Na+ was replaced by N-methyl-D-glucamine and records were made simultaneously with a triple-barrelled ion-sensitive microelectrode.…”

“…1, and Deitmer & Schlue, 1989). When the glial membrane potential is clamped near its resting level (-65 to -75 mV), an outward current is recorded upon addition of C02-HC03- (Fig.…”

“…We have recently shown that shift of acid-base equivalents across the glial membrane is a glial cells actively regulate their intracellular pH (Deitmer & Na+-HCO3-cotransport, which has been identified in epi- Schlue, 1987Schlue, , 1989 and also participate in pH regulation of thelial tissue (see Boron & Boulpaep, 1989), in glial cells of extracellular spaces in the nervous tissue by uptake and invertebrates (Deitmer & Schlue, 1987, 1989 and vertebrates secretion of bicarbonate (Deitmer, 1991). The latter may (Astion & Orkand, 1988;Kettenmann & Schlue, 1988; increase the effective buffering capacity in these extracellular Newman, 1991;Chow, Yen-Chow, White & Woodbury, 1991), spaces and thereby counteract extracellular acidosis or and in cardiac tissue (Dart & Vaughan-Jones, 1992; Lagadicalkalosis (Deitmer, 1992a Scharschmidt, 1992).…”

Sodium-Coupled Bicarbonate Transporters

“…In 1989, Deitmer and Schlue described an inwardly d irected electrogenic Na/HCO 3 cotransporter in leech glial cells [19]. Based on known va lues of [ Na + ] i , pH i and V m , they concluded that the stoichiometry must be 1:2.…”

Triple-quantum filtered NMR imaging of sodium in the human brain

“…Ischemia, lack of blood flow, leading to lack of oxygen has been shown to produce 4 mM increases in the intracellular 23Na concentration of cells [12]. Lack of oxygen in a cell forces it to produce lactic acid which decreases the intracellular pH.…”