Figure 4.

The steady-state distribution of Ca_i in a cross section of a photoreceptor cell is homogeneous. (a) Raw intensity image (gray scale) of 100 successive images averaged during the steady state. The colored squares indicate the regions of interest quantitatively analyzed in b and c; red, rhabdomere; green, center of cell; blue, side of cell opposite the rhabdomere; yellow, background, outside the cell that was injected with the Ca²⁺ indicator. (b and c) Normalized fluorescence traces (the first value obtained after turning on the light was used for normalization; arrows). The normalized fluorescence rises sharply in the rhabdomere (red line) and displays a large transient. After the decay of the transient, the normalized fluorescence shows the same values, independent of location of the region of interest. The distribution of Ca_i during the steady state thus is homogeneous. The traces are averages of five experiments.