Intraneuronal Ca2+ stores act mainly as a 'Ca2+ sink' in cerebellar granule neurones.

The role of intracellular Ca2+ stores during depolarization-evoked Ca2+ signals in cultured cerebellar granule neurones was assessed by depletion of the stores with a specific inhibitor of the endoplasmic reticulum Ca2+ pump: thapsigargin (TG, 0.7 microM). The maximal rate of [Ca2+]i increase, measured as the first-order differential of the [Ca2+]i trace, increased directly in proportion to the increasing KCl concentration (in the range 20-90 mM) or, for a given KCl concentration (50 mM), to the levels of pre-stimulation [Ca2+i (range 25-100 nM Ca2+). Both these relationships were unaffected by TG. In contrast, the amplitude of the KCl-evoked response for KCl concentrations > 30 mM (inducing an increase in [Ca2+]i > 400 nM) was increased in the presence of TG.