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 (pHi) 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 microM) in a HEPES-buffered, CO2/HCO3(-) -free saline induced a decrease in pHi and an increase in Ca2+i. Amplitude and time course of the pHi and Ca2+i transients were different. Glutamate and kainate evoked a mean acidification of 0.22 +/- 0.05 (n = 29) and 0.20 +/- 0.09 (n = 12) pH units, respectively. The changes in pHi and Ca2+i induced by kainate, but not by glutamate, were inhibited by 6-cyano-7-dinitroquinozalin-2,3-dion (CNQX; 50 microM). In order to elucidate the mechanism of the agonist-induced acidification, whether the pHi 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 pHi 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 microM), a metabotropic glutamate receptor agonist, and of noradrenaline (20 microM) evoked a Ca2+i increase, but no change of pHi. 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.