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Functional interaction of CaV channel isoforms with ryanodine receptors studied in dysgenic myotubes

Ralph Peter Schuhmeier1, Elodie Gouadon, Daniel Ursu

  • 1Department of Applied Physiology, University of Ulm, Albert-Einstein-Allee 11, D-89069 Ulm, Germany.

Biophysical Journal
|January 1, 2005
PubMed

Insights

Calcium channel isoforms alpha(1C) and alpha(1A) exhibit similar Ca(2+) input flux in myotubes, differing from alpha(1S). Alpha(1C) shows Ca(2+)-induced Ca(2+) release, unlike alpha(1A), suggesting distinct coupling mechanisms.

Area of Science:

  • Molecular and Cellular Biology
  • Neuroscience
  • Physiology

Background:

  • L-type calcium channels, specifically Ca(V)1.1 (alpha(1S)) and Ca(V)1.2 (alpha(1C)), share targeting properties but differ in ryanodine receptor coupling within muscle cells.
  • The brain isoform, Ca(V)2.1 (alpha(1A)), does not target ryanodine receptors.

Purpose of the Study:

  • To investigate and compare the Ca(2+) input flux characteristics of Ca(V)1.1, Ca(V)1.2, and Ca(V)2.1 isoforms in skeletal muscle cells.
  • To elucidate the role of specific calcium channel isoforms in Ca(2+) release and entry mechanisms.

Main Methods:

  • Utilized voltage-clamp electrophysiology in alpha(1S)-deficient GLT myotubes expressing different Ca(2+) channel isoforms (alpha(1C), alpha(1A)).
  • Quantified Ca(2+) input flux, encompassing Ca(2+) entry and release.
  • Assessed the impact of sarcoplasmic reticulum depletion using cyclopiazonic acid and caffeine.

Main Results:

  • Ca(V)1.2 (alpha(1C)) and Ca(V)2.1 (alpha(1A)) isoforms displayed surprisingly similar Ca(2+) input flux amplitudes and kinetics, despite known differences in extracellular stimulation responsiveness.
  • The contribution of Ca(2+) entry to the total flux was significantly larger for alpha(1C) and alpha(1A) compared to alpha(1S)-expressing cells.
  • A distinct transient Ca(2+) flux component, sensitive to sarcoplasmic reticulum depletion, was observed in alpha(1C)-expressing cells but not in alpha(1A)-expressing cells, indicating Ca(2+)-induced Ca(2+) release is absent in the latter.
  • A chimeric channel (alpha(1)Aas(1592-clip)) incorporating the alpha(1S) targeting sequence showed a reduced transient flux component compared to alpha(1C), suggesting less effective triggering of Ca(2+) release.

Conclusions:

  • Despite similar global Ca(2+) influx, Ca(V)1.2 (alpha(1C)) and Ca(V)2.1 (alpha(1A)) isoforms differ in their ability to trigger Ca(2+)-induced Ca(2+) release in skeletal muscle cells.
  • The targeting sequence plays a role in the efficiency of Ca(2+) release, but other factors likely contribute to the distinct functional properties of these channel isoforms.

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