To study a potential reversibility of the cAMP-mediated signal response, cells were sequentially or simultaneously incubated with pharmacological modulators that have opposing effects on intracellular cAMP concentration. As shown in Figure , adding the endogenous ligand hPP to CHO Y4R cells (vertical line 1, 500 nM, black curve) led to an increase of 500 Ω, which is attributed to the (partial) inhibition of AC through inhibitory Gproteins and the corresponding decrease of cAMP. After blocking the PDE activity with IBMX (vertical line 2, 1 mM), impedance values returned to pre-stimulation levels of the resting cells. This suggests that equilibrium conditions were restored due to simultaneously inhibiting the formation and degradation of cAMP. After exposing the cell monolayer to a mixture of IBMX and hPP (same concentrations as above), the signal change was only marginal (blue curve, -200 Ω). This finding implies that simultaneous inhibition of synthesis and hydrolysis makes the resting state persist. An inverted sequence of substance addition -IBMX first and hPP second -leads to a significant drop in impedance of about 800 Ω due to the corresponding increase of cAMP. Lowered cAMP production upon hPP addition (vertical line 3) had no significant effect. AC is known for mechanisms of self-regulation that stop cAMP production when a threshold concentration is reached. This is presumably the case after IBMX addition. High levels of cAMP induce phosphorylation of AC by PKA, suppressing its catalytic activity. Thus, an additional inhibition of AC by GPCRmediated signaling on top of the shut down by phosphorylation is not measurable.