Astrocytes fail to regulate intracellular pH at moderately reduced extracellular pH.

Microspectrofluorometry was used to study the regulation of intracellular pH (pHi) in 2'-7'-bis (carboxyethyl-)-5,6-carboxyfluorescein (BCECF)-loaded astrocytes. They rapidly regulated an acid transient induced by an NH4+ prepulse. This back regulation was blocked by removal of Na+, or by addition of amiloride, but was also inhibited when extracellular pH (pHe) was lowered. Furthermore, when cells were exposed to HEPES buffer with reduced or increased pHe, pHi changed in parallel. Thus, although the cells possess an efficient H+ extrusion mechanism they fail to regulate pHi to a normal value unless pHe is held constant. The results challenge the concept of a H+ regulatory site at the internal side of the exchanger regulating pHi to a constant value.