FSK evokes cAMP increase. Optical measurements of cAMP production in PDECs transfected with Epac1-camps, a FRET probe. (A) Time courses of YFP (dotted olive line) and CFP (dotted cyan line) fluorescence from a single cell treated with 20 μM FSK to stimulate adenylyl cyclase. When the FRET ratio (F_YFP/F_CFP; red line, plotted on a reversed axis) decreases, cytoplasmic cAMP concentration increases. (B) Mean normalized (Norm.) FRET ratio with 20 μM FSK (red line, n = 6) or 100 μM UTP (black line, n = 9). The gray bar indicates the duration of treatment with FSK or UTP in normal control solution. (C) The effect of 1 μM FSK on cAMP production in cells exposed to a solution free of Ca²⁺ (0 Ca²⁺, including 100 μM EGTA) in the presence 1 μM ionomycin for at least 5 min, and then treated with solution containing 2 mM Ca²⁺ (2 Ca²⁺, black bar). In this measurement, we used 1 μM FSK as in the later experiments. n = 5.