Fig. 4. Differential regulation of SRF and NFAT in IP₃R null DT40 cells. (A) Absence of intracellular Ca²⁺ response to BCR stimulation in IP₃R^–/– DT40 cell. IP₃R^–/– DT40 cells were stimulated with anti-IgM. One mM EGTA and 1 mM Ca²⁺ treatment was added at the indicated times. Representative trace of three independent experiments. (B) SRF but not NFAT is partially activated in IP₃R null mutant DT40 cells. SRF and NFAT transcriptional activity were determined in WT and IP₃R^–/– DT40 cells. (C) SRF transcriptional activity was determined in IP₃R^–/– DT40 cells stimulated with or without anti-IgM in the presence of the indicated concentration of EGTA or equimolar concentration of EGTA and Ca²⁺. *P < 0.05 versus control without EGTA. (D) SRF transcriptional activity was determined in IP₃R^–/– DT40 cells stimulated with anti-IgM in the absence or presence of indicated concentrations of NiCl₂.