Interaction of CaMKII and Epac1 with β-arrestins. (A) WT, β-arrestin1 (β-Arr1) KO, and β-arrestin2 KO mice were administered with saline (NS), 5 mg/kg ISO, or 2.5 mg/kg 8-CPT by i.v. infusion for 5 min. LV homogenates were immunoprecipitated with either anti-CaMKII or anti-Epac1 antibodies. Immunoprecipitated proteins were analyzed by Western blotting using the specific antibodies (n = 5). (B) HEK-293 cells stably expressing WT–β₁-AR were transiently transfected with CaMKII-δC and Flag-Epac1, along with β-arrestin1– (left) or β-arrestin2–YFP (right). Serum-starved cells were stimulated with 10 µM ISO for the indicated times at 37°C. Cell lysates were immunoprecipitated with either anti-CaMKII or anti-Epac1 antibodies. Immunoprecipitated proteins were analyzed by Western blotting using the specific antibodies (n = 5). ISO stimulation resulted in a time-dependent increase in the association of CaMKII and Epac1 with β-arrestins. IP, immunoprecipitation.