Fig. 5.

Determination of Ca²⁺ levels in response to weak illumination in normal Ringer’s solution.a, A WT photoreceptor was first illuminated for 500 msec with a dim LED stimulus (∼2000 photons/sec) generating an inward current of ∼500 pA amplitude (dotted trace). The Ca²⁺ level reached during this period (530 nm) was then determined during the latent period of the response to a saturating UV measuring stimulus (50 msec, 3 × 10⁷ effective photons). The Ca²⁺ signal (solid trace) is replotted on an expanded time base below. b, Ca²⁺ levels obtained from 19 cells (filled squares), as in Figure 5a, plotted against the total charge flowing during the 500 msec adapting step. Open square, “Dark” Ca²⁺ concentration determined identically, but without preillumination with the LED (mean ± SD of 12 cells). The data have been fitted by a regression line of slope 2.7 nm/pC with an intercept (dark resting Ca²⁺ level) of 161 nm.Triangles represent data determined using measurements of light-induced Ca²⁺ rises in ora orninaE flies with small amounts of residual rhodopsin (Fig. 6).