Intracellular acidification and Ca2+ transients in cultured rat cerebellar astrocytes evoked by glutamate agonists and noradrenaline

The effect of different neurotransmitters on the intracellular pH (pH_i) and intracellular calcium (Ca²⁺_i) was studied in cultured astrocytes from neonatal rat cerebellum, using the fluorescent dyes 2,7′-bis(carboxyethyl)-5,6-carboxy-fluorescein (BCECF) and Fura-2. Application of glutamate or kainate (100 μM) in a HEPES-buffered, CO₂/HCO₃^?-free saline induced a decrease in pH_i and an increase in Ca²⁺_i. Amplitude and time course of the pH_i and Ca²⁺_i transients were different. Glutamate and kainate evoked a mean acidification of 0.22 ± 0.05 (n = 29) and 0.20 ± 0.04 (n = 12) pH units, respectively. The changes in pH_i and Ca²⁺_i induced by kainate, but not by glutamate, were inhibited by 6-cyano-7-dinitroquinozalin-2,3-dion (CNQX; 50 μM). In order to elucidate the mechanism of the agonist-induced acidification, whether the pH_i changes were secondary to the Ca²⁺ rises was tested. In the absence of extracellular Ca²⁺, the kainate-induced Ca²⁺_i transient was suppressed, while the intracellular acidification was only reduced by 13%. Removal of extracellular Ca²⁺ reduced the glutamate-induced pH_i change by 8%, while the second component of the Ca²⁺_i transient was abolished. Application of trans-(±)-1-amino-(1S,3R)-cyclopentadicarboxylic acid (t-ACPD, 100 μM), a metabotropic glutamate receptor agonist, and of noradrenaline (20 μM) evoked a Ca²⁺_i increase, but no change of pH_i. D-aspartate, which has a low affinity to glutamate receptors, but is known to be transported by the glutamate uptake system in some astrocytes, evoked an intracellular acidification, similar to that induced by glutamate, but no Ca²⁺_i transient. The results suggest that the kainate-induced acidification is only partly due to the concomitant Ca²⁺_i rise, while the glutamate/aspartate-induced acidification is mainly due to the activation of the glutamate uptake system. © 1995 Wiley-Liss, Inc.