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Assessment of Myofilament Ca2+ Sensitivity Underlying Cardiac Excitation-contraction Coupling
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Assessment of Myofilament Ca2+ Sensitivity Underlying Cardiac Excitation-contraction Coupling

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Effects of calmodulin and okadaic acid on myofibrillar Ca2+ sensitivity in cardiac myocytes

Nobuhiro Suematsu1, Shinji Satoh, Yasuko Ueda

  • 1Department of Cardiovascular Medicine, Graduate School of Medical Sciences, Kyushu University, Fukuoka, Japan.

Basic Research in Cardiology
|May 11, 2002
PubMed

Insights

Cardiac myosin light chain 2 (MLC2) phosphorylation partially regulates cardiac muscle contraction. Calmodulin and okadaic acid increased myofibrillar Ca2+ sensitivity, suggesting MLC2

Area of Science:

  • Cardiovascular Physiology
  • Muscle Contraction Regulation
  • Biochemistry

Background:

  • Phosphorylation of 20 kD regulatory myosin light chain (MLC20) is crucial for smooth muscle contraction.
  • The troponin complex has been traditionally considered the primary regulator of myofibrillar Ca2+ sensitivity in cardiac muscle.
  • The specific role of cardiac regulatory myosin light chain 2 (MLC2) phosphorylation in cardiac contraction remains less understood.

Purpose of the Study:

  • To investigate the role of cardiac regulatory myosin light chain 2 (MLC2) phosphorylation in regulating cardiac muscle contraction.
  • To determine the effects of calmodulin and okadaic acid on myofibrillar Ca2+ sensitivity in cardiac muscle.

Main Methods:

  • Utilized beta-escin-skinned cardiomyocytes from Wistar rat hearts.
  • Assessed myofibrillar Ca2+ sensitivity via pCa50 values derived from pCa-tension relationships.
  • Measured changes in the phosphorylation of myofibrillar regulatory proteins, including MLC2 and troponin I.

Main Results:

  • Calmodulin significantly increased myofibrillar Ca2+ sensitivity (pCa50 from 5.92 to 6.03).
  • Okadaic acid (a protein phosphatase inhibitor) significantly increased myofibrillar Ca2+ sensitivity (pCa50 from 5.96 to 6.11).
  • Okadaic acid treatment significantly increased MLC2 phosphorylation (from 12% to 31%) without altering troponin I phosphorylation.

Conclusions:

  • MLC2 phosphorylation plays a partial role in regulating myofibrillar Ca2+ sensitivity in cardiac muscle.
  • Calmodulin and protein phosphatase inhibition (via okadaic acid) enhance cardiac myofibrillar Ca2+ sensitivity, partly through MLC2 phosphorylation.

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