Ins(1,4,5)P3 induces Ca2+ release from brain microsomes loaded either by the Ca2+ ATPase or by the Na+/Ca2+ exchanger

Ca2+ stores in the chick embryo retina cells

The Ca2+ stores of digitonin permeabilized chick embryo retina cells in culture were characterized, by using the fluorescence of Fluo-3 potassium salt to follow continuously the free [Ca2+] in the medium. After ATP dependent Ca2+ accumulation, the Ca2+ release was induced by several agents; 10 microM cyclic-ADP-ribose (cADPR), 40 microM Ins (1,4,5)P3 10 microM thapsigargin (Th), 25 microM ionomycin (Ion), 15 microM CCCP together with 4.5 micrograms/ml oligomycin (CCCP/Olig), 50 microM arachidonic acid (AA). Neither Ins(1,4,5)P3 nor cADPR were able to mobilize Ca2+ from internal stores in these cells, but Th and AA were effective in releasing Ca2+. Four major Ca2+ stores in chick embryo retina cells were distinguished: i) the thapsigargin sensitive Ca2+ store, most likely the ER; ii) the Ca2+ store sensitive to oligomycin and CCCP, most likely the mitochondrial Ca2+ store, iii) an AA sensitive Ca2+ store, which is distinct from the previous two; and, iv) the Ca2+ store only sensitive to ionomycin. The capacities of these different Ca2+ stores of the chick embryo retina cells, relative to the total intracellular stores, are: 63.3%, 14.1%, 8.2%, for the ER, the mitochondrial and for the AA sensitive Ca2+ stores, respectively.