(A) Cytosolic extracts prepared from FRTL5 cells were immunoprecipitated with splicevariant-specific antibodies against PDE4D3 (α-4D3), PDE4D8 (α-4D8) and PDE4D9 (α-4D9), a mixture of the three antibodies (α-4D3/8/9), the pan-specific PDE4D antibody M3S1 (α-4DPAN) and IgG as a control. Both the immunoprecipitation (IP) pellets, as well as the depleted immunoprecipitation supernatants, were separated by SDS/PAGE and PDE4D variants were detected by Western blotting using a pan-specific PDE4D antibody (α-4DPAN). (B) Quiescent FRTL5 cells were incubated for 15 min with 10 nM of TSH (+TSH) or without TSH (−TSH) before harvesting the cells. Cytosolic fractions were then subjected to immunoprecipitations using variantspecific PDE4D antibodies. The PDE activities of the immunoprecipitation pellets were determined at 1 μM cAMP. The results shown in the graph are corrected for the unspecific pull-down of PDE activity with purified non-immune IgG, and represent the means±S.E.M. of 4 experiments.