Protein Kinase A (PKA)-Dependent Troponin-I Phosphorylation and PKA Regulatory Subunits Are Decreased in Human Dilated Cardiomyopathy

Background —Most studies indicate that failing human hearts have greater baseline myofibrillar Ca 2+ sensitivity of tension development than nonfailing hearts. Phosphorylation of cardiac troponin I (TnI) by cAMP-dependent protein kinase (PKA) decreases the affinity of the troponin complex for Ca 2+ , thus altering the Ca 2+ sensitivity of force production. We tested the hypothesis that PKA-dependent TnI phosphorylation is altered in the failing human heart and investigated changes in PKA regulatory subunits as a potential mechanism. Methods and Results —Using in vitro back-phosphorylation with γ- 32 PATP, we demonstrated a significant ( P <0.05) ≈25% reduction in baseline PKA-dependent TnI phosphorylation in human hearts with dilated cardiomyopathy (DCM) compared with nonfailing (NF) human hearts. There was no significant difference in cAMP content or maximal PKA activity between DCM and NF hearts, but expression of the regulatory subunits of PKA-I (RI) and PKA-II (RII) was significantly decreased in DCM versus NF hearts (RI by ≈40%, P <0.05; RII by ≈30%, P <0.01). Conclusions —PKA activity is regulated at the substrate level through interactions of PKA regulatory subunits with A-kinase anchoring proteins. The reduced baseline PKA-dependent phosphorylation of TnI in DCM may be due to decreased expression of RI and RII and consequently reduced anchoring of PKA holoenzyme. These findings provide new evidence of deficiencies in downstream regulation of the β-adrenergic pathway in the failing human heart and may account for increased baseline myofibrillar Ca 2+ sensitivity.