Fig. 4. The effect of uncaging GTP--S on the cytosolic Ca²⁺. (A) Glial cell injected with the fluorescent Ca²⁺ indicator Oregon Green BAPTA-1 and caged GTP--S showed a transient increase in cytosolic Ca²⁺ (F) and a sustained membrane hyperpolarization following a first UV pulse (UV), while a second UV pulse remained without effect. Membrane depolarization induced by raising the extracellular K⁺ concentration from 4 to 40 mmol l^–1 induced a Ca²⁺ transient of similar amplitude. (B) Effect of UV pulse and 40 mmol l^–1 K⁺ on a cell not injected with GTP--S. (C) Summary of cytosolic Ca²⁺ changes as shown in A and B. (D) Changes in cytosolic Ca²⁺ and membrane potential following uncaging of GTP--S and during membrane depolarization in 40 mmol l^–1 K⁺ after depletion of the intracellular Ca²⁺ stores with cyclopiazonic acid (CPA, 20 µmol l^–1). While high-K⁺ still evoked a Ca²⁺ rise, the uncaging of GTP--S did not change the cytosolic Ca²⁺ after store depletion (E) but still elicited the sustained membrane hyperpolarization. (F) Injection of the Ca²⁺ chelator BAPTA into the glial cell did not suppress the membrane hyperpolarization following the uncaging of GTP--S by a UV pulse. ^**P<0.01.