INOSITOL TRISPHOSPHATE AND RYANODINE RECEPTORS SHARE A COMMON FUNCTIONAL CA2+ POOL IN CEREBELLAR PURKINJE NEURONS

Changes in the intracellular free calcium concentration ([Ca2+](i)) co ntrol many important processes in excitable and nonexcitable cells. In cerebellar Purkinje neurons, increases in [Ca2+](i) modulate excitabi lity by turning on calcium-activated potassium and chloride conductanc es, and modifying the synaptic efficacy of inhibitory and excitatory i nputs to the cell. Calcium release from the intracellular stores plays an important role in the regulation of [Ca2+](i). Purkinje neurons co ntain both inositol trisphosphate (InsP(3)) and ryanodine (Ry) recepto rs. With the exception of the dendritic spines, where only InsP(3) rec eptors are found, InsP(3) and Ry receptors are present in the entire c ell. The distribution of the two calcium release channels, however, is not uniform, and it has been suggested that InsP(3) and Ry receptors use separate Ca2+ pools. The functional properties of InsP(3) and Ry C a2+ pools were investigated by flash photolysis and single-cell micros pectrofluorimetry. It was found that depletion of ryanodine-sensitive Ca2+ stores renders InsP(3) incapable of releasing more Ca2+ from the stores. Abolishing calcium-induced calcium release by blocking ryanodi ne receptors with ruthenium red did not have a significant effect on I nsP(3)-evoked Ca2+ release. It is concluded that InsP(3) receptors use the same functional Ca2+ pool as that utilized by Ry receptors in Pur kinje neurons.