Intracellular pH regulation in cultured astrocytes from rat hippocampus. I. Role of HCO3/-

Abstract

We studied the regulation of intracellular pH (pH(i)) in single cultured astrocytes passaged once from the hippocampus of the rat, using the dye 2',7'-biscarboxyethyl-5,6-carboxyfluorescein (BCECF) to monitor pH(i). Intrinsic buffering power (β1) was 10.5 mM (pH unit)-1 at pH(i) 7.0, and decreased linearly with pH(i); the best-fit line to the data had a slope of - 10.0 mM (pH unit)-2. In the absence of HCO3/-, pH(i) recovery from an acid load was mediated predominantly by a Na-H exchanger because the recovery was inhibited 88% by amiloride and 79% by ethylisopropylamiloride (EIPA) at pH(i) 6.05. The ethylisopropylamiloride-sensitive component of acid extrusion fell linearly with pH(i). Acid extrusion was inhibited 68% (pH(i) 6.23) by substituting Li+ for Na+ in the bath solution. Switching from a CO2/HCO3/- -free to a CO2/HCO3/- -containing bath solution caused mean steady state pH(i) to increase from 6.82 to 6.90, due to a Na+ -driven HCO3/- transporter. The HCO3/- -induced pHi increase was unaffected by amiloride, but was inhibited 75% (pH(i) 6.85) by 400 μM 4,4'- diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), and 65% (pH(i) 6.55- 6.75) by pretreating astrocytes for up to ~6.3 h with 400 μM 4-acetamide- 4'-isothiocyanatostilbene-2,2'-disulfonic acid (SITS). The CO2/HCO 3/- - induced phi increase was blocked when external Na+ was replaced with N- methyl-D-glucammonium (NMDG+). In the presence of HCO3/-, the Na+ - driven HCO3/- transporter contributed to the pH(i) recovery from an acid load. For example, HCO3/- shifted the plot of acid-extrusion rate vs. pH(i) by 0.15-0.3 pH units in the alkaline direction. Also, with Na-H exchange inhibited by amiloride, HCO3/- increased acid extrusion 3.8-fold (pH(i) 6.20). When astrocytes were acid loaded in amiloride, with Li+ as the major cation, HCO3/- failed to elicit a substantial increase in pH(i). Thus, Li+ does not appear to substitute well for Na+ on the HCO3/- transporter. We conclude that an amiloride-sensitive Na-H exchanger and a Na+-driven HCO3/- transporter are the predominant acid extruders in astrocytes.