Loss of F-actin dynamics abrogates sustained Ca²⁺ signaling by perturbing Ca²⁺ release from ER stores. (A) GFP-actin Jurkat T cells were pretreated with 25 µM Y-27632 or left untreated and loaded with fura-2 before plating on OKT3- or poly-l-lysine (PLL)–coated coverglasses in 2 mM Ca²⁺. (B) Cells were pretreated as in A and allowed to interact with the stimulatory surfaces for 5 min. DMSO or JAS was then added to the imaging chamber, and the response was measured for another 5 min. Similar results were obtained from three independent experiments. (C and D) Cells were pretreated as in B. 1 µM Tg was added to the dishes to induce Ca²⁺ release from ER stores.