Involvement of Intracellular Stores in the Ca2+ Responses to N-Methyl-D-aspartate and Depolarization in Cerebellar Granule Cells

The [Ca2+]i of cerebellar granule cells can be increased in a biphasic manner by addition of NMDA or by depolarization (induced by elevating the extracellular K+ level), which both activate Ca2+ influx. The possibility that these stimuli activate Ca2(+)-induced Ca2+ release was investigated using granule cells loaded with fura 2-AM. Dantrolene, perfused onto groups of cells during the sustained plateau phase of the [Ca2+]i response to K+ or NMDA, was found to reduce the response to both agents in a concentration-dependent manner. Preincubation with thapsigargin (10 microM) substantially reduced the plateau phase of the [Ca2+]i response to K+ and both the peak and plateau phases of the NMDA response. Preincubation with ryanodine (10 microM) also reduced both the K(+)-evoked plateau response and both phases of the NMDA response. Neither had a consistent effect on the peak response to K+. The effects of thapsigargin and ryanodine on the NMDA response were partially additive. These results demonstrate that in cerebellar granule cells a major component of both K(+)- and NMDA-induced elevation of [Ca2+]i appears to be due to release from intracellular stores. The partial additivity of the effects of thapsigargin and ryanodine suggests that these agents affect two overlapping but nonidentical Ca2+ pools.