SUBCELLULAR CALCIUM OSCILLATORS AND CALCIUM INFLUX SUPPORT AGONIST-INDUCED CALCIUM WAVES IN CULTURED ASTROCYTES

We have analysed Ca2+ waves induced by norepinephrine in rat cortical astrocytes in primary culture using fluorescent indicators fura-2 or f luo-3. The temporal pattern of the average [Ca2+](i) responses were he terogeneous from cell to cell and most cells showed an oscillatory res ponse at concentrations of agonist around EC(50) (200 nM). Upon recept or activation, Ca2+](i) signals originated from a single cellular locu s and propagated throughout the cell as a wave. Wave propagation was s upported by specialized regenerative calcium release loci along the le ngth of the cell. The periods of oscillations, amplitudes, and the rat es of [Ca2+](i) rise of these subcellular oscillators differ from each other. These intrinsic kinetic properties of the regenerative loci su pport local waves when stimulation is continued over long periods of t ime. The presence of local waves at specific, invariant cellular sites and their inherent kinetic properties provide for the unique and repr oducible pattern of response seen in a given cell. We hypothesize that these loci are local specializations in the endoplasmic reticulum whe re the magnitude of the regenerative Ca2+ release is higher than other regions of the cell. Removal of extracellular Ca2+ or blockade of Ca2 + channels by inorganic cations (Cd2+ and Ni2+) during stimulation of adrenergic receptors alter the sustained plateau component of the [Ca2 +](i) response. In the absence of Ca2+ release, due to store depletion with thapsigargin, agonist occupation alone does not induce Ca2+ infl ux in astrocytes. This finding suggests that, under these conditions, receptor-operated Ca2+ entry is not operative. Furthermore, our experi ments provide evidence for local Ca2+ oscillations in cells which can support both wave propagation as well as spatially discrete Ca2+ signa lling.