Figure 3. Equivalent dark light for dark PDE activities.

(A) Left, in a truncated L cone, GTP-independent and GTP-dependent (in the presence of 15-μM GTP) current decay (after brief exposure to 3-mM cGMP) was first measured in the dark. GTP-dependent current decay was then measured during exposure to steady light of 80 and 305 photons μm⁻² s⁻¹ at 620 nm, respectively. Right, the dark and light-induced GTP-dependent PDE activities are plotted against light intensities. A linear fit gives an abscissa intercept of −439 photons μm⁻² s⁻¹, corresponding to the equivalent dark light of spontaneous GTP-dependent PDE activity in this particular L cone of 847 equivalent isomerizations s⁻¹ (see Table S1 for effective collecting area). (B-D) Collected data from measuring the equivalent dark light of spontaneous GTP-dependent PDE activity in L cones (B), M cones (C), and S cones (D). Each set of symbols and linear fit represent a single cone. See also Table S1.