Figure 8. Relation between Ca²⁺ store depletion and store-operated Ca²⁺ entry.

A, a cell was stimulated in Ca²⁺-free conditions with two different ATP concentrations, 10 μM and 100 μM, to assess the degree of store depletion produced by the lower dose. Application of 100 μM ATP after the 10 μM ATP application triggered a small [Ca²⁺]_i transient indicating that stores were significantly depleted. Restoration of extracellular Ca²⁺ to 2 mM did not trigger any [Ca²⁺]_i elevation. B, the same cell was stimulated twice more with 100 μM ATP in Ca²⁺-free extracellular solution. Despite the fact that the second application of ATP did not produce any measurable Ca²⁺ transient, restoration of Ca²⁺ to the bath still did not result in [Ca²⁺]_i elevation. C, finally, the same cell was first challenged by 100 μM ATP (in Ca²⁺-free conditions) and then 0.1 μM TG was applied. Following stimulation with TG restoration of extracellular Ca²⁺ caused a large [Ca²⁺]_i transient, indicating the activation of store-operated Ca²⁺ entry.