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 bas eline myofibrillar Ca2+ sensitivity of tension development than nonfailing hearts. Phosphorylation of cardiac troponin I (TnI) by cAMP-dependent prote in kinase (PKA) decreases the affinity of the troponin complex for Ca2+, th us altering the Ca2+ sensitivity of force production. We tested the hypothe sis that PKA-dependent TnI phosphorylation is altered in the failing human heart and investigated changes in PKA regulatory subunits as a potential me chanism. Methods and Results-Using in vitro back-phosphorylation with [gamma-P-32]AT P, we demonstrated a significant (P<0.05) approximate to 25% reduction in b aseline PKA-dependent TnI phosphorylation in human hearts with dilated card iomyopathy (DCM) compared with nonfailing (NF) human hearts. There was no s ignificant difference in cAMP content or maximal PKA activity between DCM a nd NF hearts, but expression of the regulatory subunits of PKA-I (RI) and P KA-II (RII) was significantly decreased in DCM versus NF hearts (RI by appr oximate to 40%, P<0.05; RII by approximate to 30%, P<0.01). Conclusions-PKA activity is regulated at the substrate level through intera ctions of PKA regulatory subunits with A-kinase anchoring proteins. The red uced baseline PKA-dependent phosphorylation of TnI in DCM may be due to dec reased expression of RI and RII and consequently reduced anchoring of PKA h oloenzyme, These findings provide new evidence of deficiencies in downstrea m regulation of the beta-adrenergic pathway in the failing human heart and may account for increased baseline myofibrillar Ca2+ sensitivity.