Figure 4.

Nuclear and cytosolic Ca²⁺ signals induced by photorelease of caged InsP₃. Cells were microinjected with NPE-InsP₃ plus fluo-3, then InsP₃ was liberated in a controlled fashion by flash photolysis as cells were monitored by time-lapse confocal microscopy. (A) Liberation of 0.2 μM InsP₃ results in a small increase in nuclear Ca²⁺, and an even smaller increase in cytosolic Ca²⁺. (B) Liberation of 9 μM InsP₃ results in much greater increases in nuclear and cytosolic Ca²⁺, where the magnitude of the signal is greater in the cytosol than in the nucleus. (C) Summary of flash photolysis studies. Individual data points show the peak nuclear vs. cytosolic fluorescence (relative to baseline) in each of 60 observations. The identity line is shown for reference, to illustrate where data points would lie if nuclear and cytosolic fluorescence were equal. The regression curve for these data is: (% increase in cytosol) = 1.27 × (% increase in nucleus) − 39.2 (R² = 0.92; P < 0.00001), which demonstrates that nuclear Ca²⁺ increases more than cytosolic Ca²⁺ for small Ca²⁺ signals, but that cytosolic Ca²⁺ increases more than nuclear Ca²⁺ for larger Ca²⁺ signals (main graph). Note that nuclear Ca²⁺ signals predominate when the Ca²⁺ transient is <140% of baseline (see Inset).