Effect of troponin I Ser23/24 phosphorylation on Ca2+-sensitivity in human myocardium depends on the phosphorylation background

J Mol Cell Cardiol. 2010 May;48(5):954-63. doi: 10.1016/j.yjmcc.2010.01.002. Epub 2010 Jan 15.


Protein kinase A (PKA)-mediated phosphorylation of Ser23/24 of cardiac troponin I (cTnI) causes a reduction in Ca(2+)-sensitivity of force development. This study aimed to determine whether the PKA-induced modulation of the Ca(2+)-sensitivity is solely due to cTnI phosphorylation or depends on the phosphorylation status of other sarcomeric proteins. Endogenous troponin (cTn) complex in donor cardiomyocytes was partially exchanged (up to 66+/-1%) with recombinant unphosphorylated human cTn and in failing cells similar exchange was achieved using PKA-(bis)phosphorylated cTn complex. Cardiomyocytes immersed in exchange solution without complex added served as controls. Partial exchange of unphosphorylated cTn complex in donor tissue significantly increased Ca(2+)-sensitivity (pCa(50)) to 5.50+/-0.02 relative to the donor control value (pCa(50)=5.43+/-0.04). Exchange in failing tissue with PKA-phosphorylated cTn complex did not change Ca(2+)-sensitivity relative to the failing control (pCa(50)=5.60+/-0.02). Subsequent treatment of the cardiomyocytes with the catalytic subunit of PKA significantly decreased Ca(2+)-sensitivity in donor and failing tissue. Analysis of phosphorylated cTnI species revealed the same distribution of un-, mono- and bis-phosphorylated cTnI in donor control and in failing tissue exchanged with PKA-phosphorylated cTn complex. Phosphorylation of myosin-binding protein-C in failing tissue was significantly lower compared to donor tissue. These differences in Ca(2+)-sensitivity in donor and failing cells, despite similar distribution of cTnI species, could be abolished by subsequent PKA-treatment and indicate that other targets of PKA are involved the reduction of Ca(2+)-sensitivity. Our findings suggest that the sarcomeric phosphorylation background, which is altered in cardiac disease, influences the impact of cTnI Ser23/24 phosphorylation by PKA on Ca(2+)-sensitivity.

Publication types

  • Research Support, N.I.H., Extramural

MeSH terms

  • Analysis of Variance
  • Blotting, Western
  • Calcium / metabolism*
  • Carrier Proteins / metabolism
  • Humans
  • Myocardium / metabolism*
  • Myocytes, Cardiac / metabolism
  • Phosphorylation
  • Sarcomeres / metabolism
  • Troponin I / metabolism*


  • Carrier Proteins
  • Troponin I
  • myosin-binding protein C
  • Calcium