In a second set of experiments (Figure ), forskolin was used as a pre-stimulation agent increasing the starting level of cAMP prior to modulation by IBMX and/or hPP. After initial forskolin addition at t = 0 min (vertical line 1) and a corresponding drop of all curves to -600 Ω, the forskolin concentration of 0.4 µM was maintained throughout the rest of the experiment. Again, hPP addition (vertical line 2) led to increasing impedance values that became stationary at 500 Ω -very similar to the results recorded for hPP in absence of forskolin. Subsequent addition of IBMX reduced impedance back to values recorded after forskolin addition (black curve). Simultaneously adding a mixture of hPP and IBMX again had no effect. The impedance level after forskolin addition was unchanged. IBMX addition after preexposure to forskolin resulted in an impedance decrease of additional 200 Ω (red curve), converging with the impedance level as observed for IBMX without forskolin pre-treatment (Fig. ). A plausible explanation is that cAMP levels were initially increased by forskolin activation of AC with hydrolysis still active. When hydrolysis is also inhibited by addition of IBMX, the supposedly maximum level of cAMP is reached. However, this time, hPP addition induced the impedance to recover to the forskolin-only plateau compensating for the IBMX effect. This observation is difficult to explain but must be due to the much higher activity of AC after forskolin stimulation rather than without. Moreover, it is naive to expect a linear correlation between cAMP levels and impedance along an extended concentration range which makes data interpretation at the upper and lower limits of the range of observed impedance changes more prone to errors. Nevertheless, the pool of our data with specific pharmacological modulators of cAMP illustrates compellingly that cAMP signaling in CHO cells is readily monitored by impedance measurements of the adherent cells. Using these pharmacological modulators, however, requires liquid handling and washing cycles which will affect time resolution and foster off-target effects. These phenomena are efficiently suppressed with photoswitchable cAMP mimics.