Figure 1.

A TG-insensitive Ca²⁺ store in Jurkat T cells. (A–C) fura-2–loaded cells were treated with 1 μM TG in Ca²⁺-free Ringer's solution to deplete TG-sensitive stores (open bar). Ca²⁺ was subsequently readded to elicit capacitative Ca²⁺ entry (filled bar). Where indicated, 4 μM ionomycin was applied in Ca²⁺-free Ringer's solution to assay the content of intracellular stores. Each trace represents the average response of ∼300 cells. (A) Readdition of 0.5 mM Ca²⁺ is not sufficient to load the TG-insensitive store. After exposure to 0.5 mM Ca²⁺, ionomycin releases a small amount of Ca²⁺ (solid trace) that is not significantly greater than the amount released before Ca²⁺ readdition (dotted trace). (B) Readdition of 2 mM Ca²⁺ loads a TG-insensitive store that is releasable by ionomycin. Two experiments are superimposed, showing that the content of the TG-insensitive store and the low Ca²⁺ plateau decline in parallel. (C) The TG-insensitive store is loaded during long (1,000 s; solid trace) and short (100 s; dotted trace) exposure to 2 mM Ca²⁺. In each case, after removal of Ca²⁺ _o, [Ca²⁺]_i returns to baseline with a biphasic time course. The slow recovery phase is lengthened by increasing the loading period.