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Temporal dissociation of frequency-dependent acceleration of relaxation and protein phosphorylation by CaMKII

Sabine Huke1, Donald M Bers

  • 1Department of Physiology, Stritch School of Medicine, Loyola University Chicago, 2160 South First Ave, Maywood, IL 60153-5500, USA.

Journal of Molecular and Cellular Cardiology
|January 24, 2007
PubMed

Insights

Frequency-dependent acceleration of relaxation (FDAR) is crucial for heart function at high heart rates. This study found that CaMKII phosphorylation of SR proteins does not mediate FDAR, despite CaMKII activation during pacing.

Area of Science:

  • Cardiovascular Physiology
  • Molecular Cardiology
  • Cardiac Excitation-Contraction Coupling

Background:

  • Frequency-dependent acceleration of relaxation (FDAR) is vital for ventricular diastolic filling during elevated heart rates.
  • Previous research linked FDAR to sarcoplasmic reticulum (SR) function and identified its modulation by phosphatases and Ca2+/calmodulin-dependent protein kinase II (CaMKII).
  • CaMKII activity exhibits frequency dependence, suggesting a potential role in FDAR.

Purpose of the Study:

  • To investigate the hypothesis that CaMKII-mediated phosphorylation of SR Ca2+-handling proteins, specifically Phospholamban (PLB) and the Ca2+ release channel (RyR), underlies FDAR.
  • To elucidate the molecular mechanisms governing FDAR in rat ventricular myocytes.

Main Methods:

  • Isolated rat ventricular myocytes loaded with fluo-4 were subjected to rapid pacing (0.1 Hz to 2 Hz).
  • Intracellular Ca2+ ([Ca2+]i) dynamics, including the decline phase (tau), were measured to quantify FDAR.
  • Phosphorylation levels of PLB (at Thr-17) and RyR (at Ser-2814), as well as CaMKII autophosphorylation, were assessed under various conditions, including phosphatase inhibition.

Main Results:

  • FDAR was observed abruptly upon increasing pacing frequency from 0.1 Hz to 2 Hz, with a significant decrease in the [Ca2+]i decline tau (42+/-3%).
  • While minor increases in PLB and RyR phosphorylation were detected, their time course was delayed relative to FDAR, and the magnitude was small (<5% for PLB, ~8% for RyR).
  • Pacing alone minimally increased CaMKII activity and autophosphorylation; however, inhibiting phosphatase 1 during pacing significantly enhanced CaMKII autophosphorylation and PLB/RyR phosphorylation.

Conclusions:

  • FDAR is not mediated by the phosphorylation of PLB or RyR by CaMKII.
  • Although CaMKII is activated by myocyte pacing, phosphatases effectively counteract its action, limiting substrate phosphorylation and preventing sustained CaMKII autophosphorylation.
  • These findings suggest that phosphatases play a critical role in regulating CaMKII activity and preventing global CaMKII effects during rapid heart rates, thereby influencing FDAR.

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