Fig. 3.

Positive feedback loop between RKIP and PKA leads to enhanced contractility in cardiac myocytes. (A) Cardiac myocytes were adenovirally transduced with WT or S51V RKIP, and treated with or without TPA (PMA). Cell lysates were immunoblotted for RKIP or pS153 RKIP. (B) Cardiac myocytes were adenovirally transduced with WT or S51V RKIP and then treated with or without isoproterenol. Cell lysates were immunoblotted for RKIP or pS153 RKIP. (C) Cardiac myocytes were adenovirally transduced and treated with isoproterenol as in B. For the control (Con), LacZ was adenovirally transduced. Contractility was measured as described in Materials and Methods. Mean values from five (A) or six (B) independent experiments were plotted, and P values were obtained using a one-way ANOVA and Sidak’s post hoc test (WT + PMA vs. V51 + PMA: P = 0.0178 and WT + ISO vs. V51 + ISO: P = 0.0016). (C) Mean values from 20 replicates of five independent experiments were plotted, and P values were obtained using a one-way ANOVA and Tukey’s post hoc test (*P < 0.01 vs. Con-Iso and ^#P < 0.0001 as indicated). (D) Scheme depicting positive feedback loop between RKIP and PKA. RKIP phosphorylated at S153 by PKC inhibits GRK2, reducing down-regulation of the β-adrenergic receptor (β-AR). β-AR activates PKA, which then phosphorylates RKIP at S51 leading to enhanced phosphorylation by PKC at S153.