Role of Troponin I Phosphorylation in Protein Kinase C-mediated Enhanced Contractile Performance of Rat Myocytes

Our goal was to define the role of phosphorylated cardiac troponin-I in the adult myocyte contractile performance response to activated protein kinase C. In agreement with earlier work, endothelin enhanced both adult rat myocyte contractile performance and cardiac troponin-I phosphorylation. Protein kinase C participated in both responses. The role of cardiac troponin-I phosphorylation in the contractile function response to protein kinase C was further investigated using gene transfer into myocytes of troponin-I isoforms/mutants lacking one or more phosphorylation sites previously identified in purified cardiac troponin-I. Sarcomeric replacement with slow skeletal troponin-I-abrogated protein kinase C-mediated troponin-I phosphorylation. In functional studies, endothelin slowed relaxation in myocytes expressing slow skeletal troponin-I, while the relaxation rate increased in myocytes expressing cardiac troponin-I. Based on these results, acceleration of myocyte relaxation during protein kinase C activation largely depended on cardiac troponin-I phosphorylation. Experiments with troponin-I isoform chimeras provided evidence that phosphorylation sites in the amino portion of cardiac troponin I-mediated the protein kinase C acceleration of relaxation. The cardiac troponin-I Thr-144 phosphorylation site identified in earlier biochemical studies was not significantly phosphorylated during the acute contractile response. Thus, amino-terminal protein kinase C-dependent phosphorylation sites in cardiac troponin-I are likely responsible for the accelerated relaxation observed in adult myocytes.