Theoretical estimation of the capacity of intracellular calcium stores in the Bergmann glial cell

We propose a mathematical model for calcium dynamics ([Ca2+]i) during metabotropic activation of a specialized astrocyte, the cerebellar Bergmann glial cell. The model adequately describes the experimentally observed behaviour of the prototype in response to single and repetitive metabotropic stimuli and to the inhibition of Ca2+ uptake into the store. By means of the model, the capacity of the intracellular calcium store for two types of calcium buffer was estimated. The estimated buffer capacity of the store lies within the following intervals: (0.8-15.5).10-19 mol calmodulin, and (0.6-12.3).10-19 mol calbindin. This result reveals, that, in the store of a small Ca2+-containing compartment optically detected in the Bergmann glial cell process after electrical stimulation of parallel fibres, the amount of releasable Ca2+ does not exceed 25,000 ions. The quantitative estimates were obtained from experimentally based theoretical relationships between the capacity and volume of the store and parameters of the cytoplasmic calcium buffer. In these relationships, the estimated store capacity was proportional to the total buffer concentration, inversely proportional to the constant of buffer affinity for calcium and was smaller for a greater relative store volume.