FUNCTIONAL-HETEROGENEITY OF CALCIUM-RELEASE BY INOSITOL TRISPHOSPHATEIN SINGLE PURKINJE NEURONS, CULTURED CEREBELLAR ASTROCYTES, AND PERIPHERAL-TISSUES

Purkinje neurones of the cerebellar cortex are rich in receptors for t he Ca-mobilizing second messenger inositol trisphosphate (InsP3) in as sociation with intracellular Ca stores. Cytosolic Ca ions are importan t in regulating neuronal excitability but it has proved difficult to d emonstrate InsP3-evoked release of Ca in mammalian central neurones di rectly. Intracellular release Of InsP3 by flash photolysis of caged In sP3, combined with whole-cell patch clamp and microspectrofluorimetry of Ca indicators, allows comparison of InsP3-evoked Ca release in sing le Purkinje cells in cerebellar slices with the same process in cultur ed astrocytes and peripheral tissues. In astrocytes, hepatocytes, exoc rine cells, and vascular endothelium, minimal Ca release from stores r equires photorelease of InsP3 at concentrations of 0.2-0.5 muM, and ma ximal efflux as judged by the rate of increase of Ca concentration is seen with 5-10 muM InsP3. In contrast in Purkinje cells, InsP3 concent rations of greater-than-or-equal-to 9 muM were required to produce min imal Ca release from stores under the same conditions, and Ca efflux i ncreased with InsP3 concentrations up to 70-80 muM. Furthermore, the r ate 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 t he difference. The results show a functional difference in InsP3-evoke d Ca release between Purkinje cells and peripheral tissues.