Fig. 3.

Demonstration that multiple parallel adaptive feedback loops exist in the Ca²⁺ signaling system. The error bars in all plots show SE. (A and B) Effect of RNAi knockdown on cytosolic and ER Ca²⁺ levels (A) and protein levels (B). Knockdown of YFP or GL3, proteins that are not present in S2R+ cells, were used as controls. (C) The small changes in STIM and SERCA and PMCA protein concentration correlated with Ca²⁺ level changes, but data in B are not sufficient to determine whether the changes were regulated just by ER or just by cytosolic Ca²⁺. (D and E) Effect of treating cells for 24 h with low (2.5 mM EGTA plus 2.5 mM Mg²⁺) and high (10 mM Ca²⁺) external Ca²⁺ and thapsagargin addition (1 μM) on cytosolic and ER Ca²⁺ levels (D) and protein levels (E). (F) Summary of the three identified feedback loops.