Functional characterization of thapsigargin and agonist-insensitive acidicCa2+ stores in Drosophila melanogaster S2 cell lines

The role of acidic intracellular calcium stores in calcium homeostasis was investigated in the Drosophila Schneider cell line 2 (S2) by means of free cytosolic calcium ([Ca2+](i)) and intracellular pH (pH(i)) imaging together with measurements of total calcium concentrations within intracellular com partments. Both a weak base (NH4Cl, 15 mM) and a Na+/H+ ionophore (monensin , 10 mu M) evoked cytosolic alkalinization followed by Ca2+ release from ac idic intracellular Ca2+ stores. Pretreatment of S2 cells with either thapsi gargin (1 mu M), an inhibitor of endoplasmic reticulum Ca2+-ATPases, or wit h the Ca2+ ionophore ionomycin (10 mu M) was without effect on the amplitud e of Ca2+ release evoked by alkalinization. Application of the cholinergic agonist carbamylcholine (100 mu M) to transfected S2-DM1 cells expressing a Drosophila muscarinic acetylcholine receptor (DM1) emptied the InsP(3)-sen sitive Ca2+ store but failed to affect the amplitude of alkalinization-evok ed Ca2+ release. Glycyl-L-phenylalanine-beta-naphthylamide (200 mu M), a we ak hydrophobic base known to permeabilize lysosomes by osmotic swelling, tr iggered Ca2+ release from internal stores, while application of brefeldin A (10 mu M), an antibiotic which disperses the Golgi complex, resulted in a smaller increase in [Ca2+](i). These results suggest that the alkali-evoked calcium release is largely attributable to lysosomes, a conclusion that wa s confirmed by direct measurements of total calcium content of S2 organelle s. Lysosomes and endoplasmic reticulum were the only organelles found to ha ve concentrations of total calcium significantly higher than the cytosol. H owever, NH4Cl (15 mM) reduced the level of total calcium only in lysosomes, Depletion of acidic Ca2+ stores did not elicit depletion-operated Ca2+ ent ry. They were refilled upon re-exposure of cells to normal saline ([Ca2+](o ) = 2 mM), but not by thapsigargin-induced [Ca2+](i) elevation in Ca2+-free saline.