Heat Shock Protein Synthesis and Cytoskeletal Rearrangements Occur Independently of Intracellular Free Calcium Increases in Drosophila Cells and Tissues

Heat shock causes significant changes in intracellular free calcium ([ Ca²⁺] _i) which occur rapidly following temperature elevation. The resting level of free calcium in single Drosophila melanogaster larval salivary gland cells measured with the fluorescent indicator fura-2 is 198 ± 31 nM (n = 4). It increases approximately 10-fold to 1870 ± 770 nM (n = 4), during a heat shock. When salivary glands are incubated in calcium-free, EGTA-buffered medium the resting free calcium is reduced to 80 ± 7 nM (n = 3) and heat shock results in a 4-fold increase in free calcium to 353 ± 90 nM (n = 3). Drosophila Kc cells show a heat shock-induced increase in [ Ca²⁺] _i from 118.4 ± 2 nM (n = 11) to 323 ± 18 nM. Procedures were devised to block the effects of heat shock on the increase in intracellular calcium and assess its role in the induction of heat shock proteins and in the stress-induced rearrangement of the vimentin cytoskeleton. We report here that changes in [ Ca²⁺] _i are not required for a complete induction of the heat shock response or for the collapse of the vimentin cytoskeleton.