Stimulation of calcium uptake in cultured astrocytes by hypoosmotic stress — effect of cyclic AMP

To investigate the role of Ca 2+ in astrocyte volume regulation, we determined Ca 2+ fluxes following hypoosmotic stress and how these fluxes were modified by cyclic AMP. In isoosmotic conditions treatment with dibutyryl cyclic AMP (dBcAMP) caused almost a twofold increase in 45Ca 2+ uptake. Efflux studies of 45Ca 2+ in dBcAMP-treated cells showed three Ca 2+ compartments while only two Ca 2+ compartments were identified in non-dBcAMP-treated cells. Following hypoosmotic stress a twofold stimulation of 45Ca 2+ uptake occurred in both non-dBcAMP-treated and dBcAMP-treated astrocytes. Stimulation of Ca 2+ uptake begins at ∼ 270 mOsm and is half-maximally stimulated at ∼ 100 mOsm. This uptake is partly mediated through L-type ‘slow’ inactivating Ca 2+ channels. Hypoosmotic stress also induces the release of Ca 2+ from intracellular stores. The influx of extracellular Ca 2+ and efflux of intracellular Ca 2+ appear to be important factors in volume regulation after hypoosmotic stress. Cyclic AMP plays an important role in modulating hypoosmotically stimulated Ca 2+ uptake.