Forskolin affects light-induced steady PDE activity, β_s. (A) β_s of a single rod in normal (○) and forskolin-containing (•) solutions versus light background intensity in log scale. (B) Averaged ratio β_{s forsk}/β_{s norm} versus light background intensity in log scale (14 cells). Error bars represent SEM. The straight line is drawn by a least-square procedure with parameters y = 0.11*ln(x) + 1.56. It has no mechanistic meaning and is only used to show general trend of the data.