ELECTROGENIC SODIUM-DEPENDENT BICARBONATE SECRETION BY GLIAL-CELLS OF THE LEECH CENTRAL-NERVOUS-SYSTEM

The ability to move acid/base equivalents across the membrane of identified glial cells was investigated in isolated segmental ganglia of the leech Hirudo medicinalis. The intracellular pH (pH(i)) of the glial cells was measured with double-barreled, neutral-ligand, ion-sensitive microelectrodes during step changes of the external pH (pH(o)) 7.4-7.0). The rate of intracellular acidification after the decrease in extracellular pH (pH(o)) was taken as a measure of the rate of acid/base transport across the glial membrane. Taking into account the total intracellular buffering power, the maximum rate of acid/base flux was 0.4 mM/min in CO2/HCO3--free saline, and 3.92 mM/min in the presence of 5% CO2/10 mM HCO3-, suggesting that the acid/base flux was dependent upon HCO3-. The rate of acid influx/base efflux increased both with the external HCO3- concentration and with increasing pH(i) (and hence HCO3-i). This suggested that the decrease in pH(i) was due to HCO3- efflux. The rapid decrease of pH(i) was accompanied by a HCO3- -dependent depolarization of the glial membrane from -74 +/- 5 mV (n = 20) to - 54 +/- 7 mV (n = 13). Both this depolarization and the rate of intracellular acidification were greatly reduced by the anion exchange inhibitor 4,4-diisothiocy-anatostilbene-2,2'-disulfonic acid (DIDS; 0.3-0.5 mM), but were not affected by the removal of external Cl-. Reduction of the external Na+ concentration to one-tenth normal affected the rate of intracellular acidification only in the presence of CO2/HCO3-: the rate increased within the first 3-5 min after lowering external Na+; after longer exposures in low external Na+ the rate decreased, presumably due to depletion of intracellular Na+. Amiloride (1 mM), which inhibits the Na+-H+ exchange in these cells, had no effect on the rate of intracellular acidification. The intracellular Na activity (aNa(i)) of the glial cells was measured to be 5.2 +/- 1.0 mM (n = 8) in CO2/HCO3--free saline; aNa(i) increased to 7.3 +/- 2.2 mM (n = 8) after the addition of 5% CO2/24 mM HCO3-. Upon a change in pH(o) to 7.0 in the presence of CO2/HCO3-, aNa(i) decreased by an average of 2 +/- 1.1 mM (n = 5); in CO2/HCO3--free saline external acidification produced a transient increase in aNa(i). It is concluded that, in the presence of CO2/HCO3-, the rate of intracellular acidification in glial cells is dominated by an outwardly directed, electrogenic Na+-HCO3- cotransport. Neurons, which do not possess this cotransporter, acidify at much lower rates under similar conditions. Since the Na+-HCO3- cotransporter is reversible, glial cells might play a crucial role in dampening ("muffling") acid and base transients in the extracellular spaces of the nervous system by active, Na+- dependent secretion and uptake of HCO3-.