Inositol trisphosphate and ryanodine receptors share a common functional Ca²⁺ pool in cerebellar Purkinje neurons

Changes in the intracellular free calcium concentration ([Ca²⁺](i)) control many important processes in excitable and nonexcitable cells. In cerebellar Purkinje neurons, increases in [Ca²⁺](i) modulate excitability by turning on calcium-activated potassium and chloride conductances, and modifying the synaptic efficacy of inhibitory and excitatory inputs to the cell. Calcium release from the intracellular stores plays an important role in the regulation of [Ca²⁺](i). Purkinje neurons contain both inositol trisphosphate (InsP₃) and ryanodine (Ry) receptors. With the exception of the dendritic spines, where only InsP₃ receptors are found, InsP₃ and Ry receptors are present in the entire cell. The distribution of the two calcium release channels, however, is not uniform, and it has bean suggested that InsP₃ and Ry receptors use separate Ca²⁺ pools. The functional properties of InsP₃ and Ry Ca²⁺ pools were investigated by flash photolysis and single-cell microspectrofluorimetry. It was found that depletion of ryanodine-sensitive Ca²⁺ stores renders InsP₃ incapable of releasing more Ca²⁺ from the stores. Abolishing calcium-induced calcium release by blocking ryanodine receptors with ruthenium red did not have a significant effect on InsP₃-evoked Ca²⁺ release. It is concluded that InsP₃ receptors use the same functional Ca²⁺ pool as that utilized by Ry receptors in Purkinje neurons.