RAT HIPPOCAMPAL ASTROCYTES EXHIBIT ELECTROGENIC SODIUM-BICARBONATE COTRANSPORT

1. We probed for the expression of electrogenic Na+/HCO3- co-transport in cultured mammalian astrocytes by recording voltage and current cha nges induced by bath application of HCO3-, with the use of patch-clamp electrophysiology. Application of 25 mM HCO3-, at a constant pH(0), t o astrocytes bathed in a nominally HCO(3)(-)free solution, produced an abrupt and reversible change in membrane potential ranging from +3 to -30 mV [-11.8 +/- 9.34 (SD) mV]; 55% of cells showed relatively large hyperpolarizing responses (-18.8 +/- 6.23 mV), whereas 45% showed onl y small shifts in membrane potential (range of -5 to +3 mV; -1.9 +/- 1 .96 mV). 2. The size of the HCO3--induced hyperpolarization was strong ly related to the cell's initial resting membrane potential in HCO3--f ree solution; the larger responses were seen in cells with relatively low resting membrane potentials (-48.5 +/- 9.4 mV),and the smaller res ponses were seen in cells with more negative potentials (-68.1 +/- 6.5 mV). The membrane potentials of hippocampal astrocytes were highly va riable in HCO3--free solution (range -38 to -80 mV; -60.9 +/- 12.53); this variability was greatly reduced in HCO3--containing solution (ran ge -59 to -82 mV; -68.5 +/- 4.8). 3. The magnitude of the HCO3--induce d response was less strongly correlated with cell input resistance, wh ich was higher in the large responder cells than in the small responde rs. However, the differences in input resistance were insufficient to account for the different HCO3--induced responses observed. 4. In the presence of extracellular Ba2+, which by blocking K+ conductance depol arized cells by 30-50 mV, cells that initially showed a small response , showed a large and completely reversible hyperpolarization (-18.4 +/ - 6.13 mV) to application of 25 mM HCO3-. 5. In Na+-free solution, the HCO3--induced hyperpolarization was reduced by 66%, and the response was not sustained, as in Na+-containing solution. Removal of extracell ular Cl- had no effect on the HCO3- response. The stilbene derivative 4,4'-diisothiocyano-2,2'-stilbene disulfonate (DIDS), a blocker of ani on transport, eliminated the HCO3--induced hyperpolarization. Blockers of Na+/K+ ATPase and Na+-H+ exchange were without effect. These obser vations indicated the presence of an electrogenic Na+/HCO3- co-transpo rter in hippocampal astrocytes. 6. Voltage-clamp recording demonstrate d that the HCO3--induced hyperpolarization was caused by outward curre nts averaging 335 +/- 104 pA. The reversal potential of the HCO3--indu ced current ranged between -80 and -99 mV with an average = -86.1 +/- 6.2 mV. On the basis of the reversal potential of the HCO3--induced re sponse, and knowledge of the transmembrane gradients for HCO3- and Na, it was calculated that the co-transporter has an apparent HCO3-:Nastoichiometry of 2:1. 7. It is concluded that hippocampal astrocytes e xhibit electrogenic Na+/HCO3- co-transport. This transporter may play an important role in regulation of intracellular pH, depolarization-in duced alkalinization and intracellular Na+ homeostasis.