ISO and 8-CPT activate β-arrestin–dependent CaMKII activity. (A and B) 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. (A) LV homogenates were immunoblotted with anti-pCaMKII and anti-CaMKII antibodies. The CaMKII activation was quantified, expressed as fold increase over NS-treated WT mice, and shown as mean ± SEM (n = 5). (B) CaMKII kinase activity was determined under Ca²⁺-dependent and -independent conditions. The CaMKII activity was quantified, expressed as fold increase over NS-treated WT mice, and shown as mean ± SEM (n = 5). (A and B) *, P < 0.01 versus NS-treated WT mice; ^#, P < 0.01 versus ISO-treated WT mice; ^‡, P < 0.01 versus 8-CPT–treated WT mice. (C and D) HEK-293 cells stably expressing WT–β₁-AR were transfected with CaMKII-δC and Flag-Epac1 alone (Mock) or with siRNAs targeting β-arrestin1, β-arrestin2, β-arrestin1/2, or control siRNA. Serum-starved cells were stimulated at 37°C with 10 µM ISO (C) or 5 µM 8-CPT (D). Cell lysates were then immunoprecipitated with anti-CaMKII antibody before blotting with anti-pCaMKII and anti-CaMKII antibodies. The CaMKII activation was quantified, expressed as fold increase over nonstimulated mock cells, and shown as mean ± SEM (n = 5). Inhibited ISO- and 8-CPT–mediated CaMKII activation were observed in cells transfected with β-arrestin siRNA. *, P < 0.01 versus ISO-stimulated control siRNA; ^#, P < 0.05 versus 8-CPT–stimulated control siRNA. IP, immunoprecipitation; T-CaMKII, total CaMKII.