Figure 7. Dissociation between Ca²⁺ release and activation of capacitative Ca²⁺ entry.

A, glioma cells were stimulated by different concentrations of ATP in the absence of extracellular Ca²⁺, extracellular Ca²⁺ was then restored to 2 mM. The amplitude of [Ca²⁺]_i rise in response to Ca²⁺ restoration was determined as the amplitude of the capacitative Ca²⁺ entry-induced [Ca²⁺]_i transient, Δ[Ca²⁺]_SOC. The traces are representative for four independent experiments, 157 cells. B, from the experiments shown in A, the dose-response curve, relating ATP concentration to normalized Δ[Ca²⁺]_SOC, was constructed. The apparent K_D was 63 mM, and the Hill coefficient was 3.2. The numbers in parentheses indicate the percentage of cells which responded with [Ca²⁺]_i transients resulting from store-operated Ca²⁺ entry. C, from the same experiments the mean values of Δ[Ca²⁺]_SOC were calculated for responsive cells only. The dose-response curve has an apparent K_D of 24 mM and a Hill coefficient of 6.5.