Er. Oconnor et al., RAT HIPPOCAMPAL ASTROCYTES EXHIBIT ELECTROGENIC SODIUM-BICARBONATE COTRANSPORT, Journal of neurophysiology, 72(6), 1994, pp. 2580-2589
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.