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Troponin I in the murine myocardium: influence on length-dependent activation and interfilament spacing

John P Konhilas1, Thomas C Irving, Beata M Wolska

  • 1Program in Cardiovascular Sciences, Department of Physiology and Biophysics, Section of Cardiology, University of Illinois at Chicago, College of Medicine, Chicago, IL 60612, USA.

The Journal of Physiology
|February 4, 2003
PubMed

Insights

Cyclic AMP-dependent protein kinase (PKA) enhances length-dependent activation in the heart by phosphorylating contractile proteins. However, this effect is diminished when troponin-I lacks PKA sites, suggesting complex interactions in cardiac muscle mechanics.

Area of Science:

  • Cardiovascular Physiology
  • Muscle Contraction
  • Molecular Cardiology

Background:

  • Cyclic AMP-dependent protein kinase (PKA) phosphorylates cardiac contractile proteins, troponin-I (TnI) and myosin binding protein C (MyBP-C), impacting myofilament Ca2+ sensitivity.
  • The influence of sarcomere length (SL) on Ca2+ sensitivity, known as length-dependent activation (LDA), following PKA phosphorylation remains unclear.

Purpose of the Study:

  • To elucidate the role of PKA-dependent phosphorylation of TnI and MyBP-C in cardiac LDA.
  • To investigate the impact of PKA on myofilament lattice spacing and its relation to LDA.

Main Methods:

  • Examined LDA in skinned myocytes from non-transgenic (NTG) and slow skeletal troponin-I (ssTnI-TG) transgenic mice.
  • Assessed myofilament lattice spacing using X-ray diffraction in skinned cardiac trabeculae from NTG and ssTnI-TG models.
  • Applied PKA treatment to both myocyte and trabeculae preparations.

Main Results:

  • In NTG myocytes, PKA decreased Ca2+ sensitivity but enhanced LDA; ssTnI-TG myocytes showed increased Ca2+ sensitivity and reduced LDA, unaffected by PKA.
  • PKA phosphorylation increased lattice spacing in NTG trabeculae but decreased it in ssTnI-TG trabeculae.
  • MyBP-C phosphorylation by PKA did not alter calcium responsiveness but decreased myofilament lattice spacing.

Conclusions:

  • PKA-dependent phosphorylation enhances LDA in native cardiac muscle.
  • Replacing cardiac TnI with ssTnI alters Ca2+ sensitivity and reduces LDA.
  • LDA is not solely explained by changes in myofilament lattice spacing following PKA treatment.

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