Uncaging of IP₃ does not elicit appreciable calcium release in adult single fibers. (A) Pseudocolor images of FluoForte Ca²⁺-dependent fluorescence from C2C12 myoblasts at various time points in response to photorelease of caged IP₃. The shown images were obtained at 5.7 and 12.9 s after photo liberating IP₃ from its caged precursor. Traces show the time course of the relative percentage change in fluorescence (Δf/f₀) caused by intracellular Ca²⁺ increase for four different regions of interest (identified by the colored boxes). (B) Changes in [Ca²⁺]_i after photorelease of caged IP₃ in four FDB fibers. A minimal signal was detected in 11 out of 35 fibers. (C) Quantification of the peak relative increase of [Ca²⁺]_i after photorelease, as measured by the change in FluoForte intensity, in C2C12 muscle cells (n = 85 cells). (D) Same as C for adult muscle fibers isolated from 4-mo-old CD1 mice (left; n = 35) and 5-wk-old BALB/c mice (right; n = 24). The difference between fibers incubated with caged IP₃ and control fibers is not statistically significant.