Figure 2. Ca²⁺-release properties in permeabilized R23-11 cells heterologously expressing IP₃R1 (A) and [Δ1–225]-IP₃R1 (B).

Free [Ca²⁺] was measured using fluo-3 fluorescence. After permeabilization of the plasma membrane with 50 μM digitonin, the cells accumulated Ca²⁺ in their intracellular stores, resulting in a decrease in medium-free [Ca²⁺]. After stabilization of the signal, adenophostin A (concentrations are shown in the Figures) was added. At the end of each experiment, 8 μM A23187 was added to estimate the total releasable amount of Ca²⁺. Results are typical for four independent experiments.