Functional heterogeneity of calcium release by inositol trisphosphate in single Purkinje neurones, cultured cerebellar astrocytes, and peripheral tissues

Purkinje neurones of the cerebellar cortex are rich in receptors for the Ca-mobilizing second messenger inositol trisphosphate (InsP₃) in association with intracellular Ca stores. Cytosolic Ca ions are important in regulating neuronal excitability but it has proved difficult to demonstrate InsP₃-evoked release of Ca in mammalian central neurones directly. Intracellular release of InsP₃ by flash photolysis of caged InsP₃, combined with whole-cell patch clamp and microspectrofluorimetry of Ca indicators, allows comparison of InsP₃-evoked Ca release in single Purkinje cells in cerebellar slices with the same process in cultured astrocytes and peripheral tissues. In astrocytes, hepatocytes, exocrine cells, and vascular endothelium, minimal Ca release from stores requires photorelease of InsP₃ at concentrations of 0.2-0.5 μM, and maximal efflux as judged by the rate of increase of Ca concentration is seen with 5-10 μM InsP₃. In contrast in Purkinje cells, InsP₃ concentrations of ≥9 μM were required to produce minimal Ca release from stores under the same conditions, and Ca efflux increased with InsP₃ concentrations up to 70-80 μM. Furthermore, the rate of increase and size of the Ca concentration in Purkinje cells are 10- to 30-fold greater than in astrocytes and peripheral tissues. The InsP₃ sensitivity was not affected by changing exogenous cytosolic Ca buffering, suggesting that endogenous Ca binding cannot account for the difference. The results show a functional difference in InsP₃-evoked Ca release between Purkinje cells and peripheral tissues.