MECHANISMS OF INTRACELLULAR CALCIUM REGULATION IN ADULT ASTROCYTES

Microfluorimetric techniques were used to measure changes in intracell ular calcium in astrocytes cultured from the forebrain of the adult ra t. Application of ATP consistently raised intracellular calcium. The r esponse persisted in the absence of extracellular calcium, but then qu ickly declined upon repeated agonist application. Thapsigargin abolish ed responses to nucleotides following depletion of the endoplasmic ret icular calcium stores. Calcium release was inhibited by caffeine, but was dramatically increased through inositol phosphate receptor sensiti zation by the sulphydryl reagent thimerosal. Responses to repeated nuc leotide applications resulted in a gradual decline of peak calcium con centrations, suggesting a (post)receptor-mediated desensitization or g radual depletion of the internal calcium stores. Subsequent applicatio n of ionomycin suggested intracellular calcium depletion as the releva nt mechanism. Depletion of the internal calcium stores with ATP, ionom ycin or thapsigargin failed to reveal a calcium influx pathway. These results suggest that the capacitative mechanism of calcium entry does not operate in response to nucleotide receptor activation in these cel ls, and that the immediate refilling of the internal calcium stores is primarily determined by re-uptake of cytosolic calcium into the endop lasmic reticulum. A complete refilling of this calcium store by extrac ellular calcium may be a much slower process. Control of these signal transduction pathways is crucial to the maintenance of the calcium/ene rgy homeostasis of the adult astrocyte in the central nervous system.