FIG. 5.

Refractory Ca²⁺ signaling wanes with longer periods of toxin exposure. Primary T cells were pretreated (or untreated) with LLO (0.25 μg/ml) for 4 h or 18 h. To evaluate the intracellular Ca²⁺ stored in such cells, cells were labeled with Indo 1-AM and washed thoroughly as described in Materials and Methods and then resuspended in Ca²⁺-free medium before they were restimulated with 0.25 μg/ml LLO (the arrow indicates the point of stimulation). It is, however, worth pointing out that with our culture conditions, the indicated T cells showed no appreciable proliferation. Thus, with or without cellular growth, cells do recover from the indicated toxin effects.