Figure 4.

Bulk [Ca²⁺]_i is neither necessary nor sufficient to dictate CREB signal strength. (A) Despite Cd²⁺ effects on Ca²⁺ flux, release from intracellular stores might maintain bulk [Ca²⁺]. (B) Fura-2 Ca²⁺ imaging experiment. Various [K⁺]_o with and without Cd²⁺ were applied as indicated. (C) Increased [Ca²⁺]_i plotted against Ca²⁺ flux (from Fig. 3). Δ[Ca²⁺]_i data from 10–23 cells and three to five experiments. Linear fit correlation coefficient equals 0.995 (P < 0.0005). (D) Log–log plot of CREB signal strength versus Δ[Ca²⁺]_i for the indicated conditions. Solid lines are linear fits through the data points collected with varying [K⁺]_o or [Cd²⁺]. (E) Possible gating-driven conformational changes working in concert with elevated bulk Ca²⁺. (top) Hypothetical conformational change repositions a local signaling determinant (circle). When combined with high local Ca²⁺ elevations, strong signaling to the nucleus is engaged (green). In the Ca²⁺-free solution (0 Ca²⁺), the conformational change remains, but the signaling determinant is not activated (red). (bottom) Can conformational change contribute to signaling in conjunction with increases in bulk Ca²⁺. (F) Nuclear pCREB levels from cells stimulated for 10 s in 2 mM Ca²⁺ (high local Ca²⁺) or for 15 s in the presence of caffeine with 0 Ca²⁺ (bulk Ca²⁺). Error bars represent SEM.