title: Cardiac-type EC-coupling in dysgenic myotubes restored with Ca2+ channel subunit isoforms alpha1C and alpha1D does not correlate with current density.
abstract: Ca(2+)-induced Ca(2+)-release (CICR)-the mechanism of cardiac excitation-contraction (EC) coupling-also contributes to skeletal muscle contraction; however, its properties are still poorly understood. CICR in skeletal muscle can be induced independently of direct, calcium-independent activation of sarcoplasmic reticulum Ca(2+) release, by reconstituting dysgenic myotubes with the cardiac Ca(2+) channel alpha(1C) (Ca(V)1.2) subunit. Ca(2+) influx through alpha(1C) provides the trigger for opening the sarcoplasmic reticulum Ca(2+) release channels. Here we show that also the Ca(2+) channel alpha(1D) isoform (Ca(V)1.3) can restore cardiac-type EC-coupling. GFP-alpha(1D) expressed in dysgenic myotubes is correctly targeted into the triad junctions and generates action potential-induced Ca(2+) transients with the same efficiency as GFP-alpha(1C) despite threefold smaller Ca(2+) currents. In contrast, GFP-alpha(1A), which generates large currents but is not targeted into triads, rarely restores action potential-induced Ca(2+) transients. Thus, cardiac-type EC-coupling in skeletal myotubes depends primarily on the correct targeting of the voltage-gated Ca(2+) channels and less on their current size. Combined patch-clamp/fluo-4 Ca(2+) recordings revealed that the induction of Ca(2+) transients and their maximal amplitudes are independent of the different current densities of GFP-alpha(1C) and GFP-alpha(1D). These properties of cardiac-type EC-coupling in dysgenic myotubes are consistent with a CICR mechanism under the control of local Ca(2+) gradients in the triad junctions.
authors.affiliation: Department of Biochemical Pharmacology, University of Innsbruck, Austria.
authors.forename: Nicole
authors.initials: N
authors.lastname: Kasielke
authors.forename: Gerald J
authors.initials: GJ
authors.lastname: Obermair
authors.forename: Gerlinde
authors.initials: G
authors.lastname: Kugler
authors.forename: Manfred
authors.initials: M
authors.lastname: Grabner
authors.forename: Bernhard E
authors.initials: BE
authors.lastname: Flucher
mesh_terms: D000200:Action Potentials / Q000502:physiology*; D000222:Adaptation, Physiological / Q000502:physiology; D002118:Calcium / Q000502:physiology; D015220:Calcium Channels; D020746:Calcium Channels, L-Type / Q000502:physiology*; D020013:Calcium Signaling / Q000502:physiology*; D002460:Cell Line; D006321:Heart / Q000502:physiology*; D008564:Membrane Potentials / Q000502:physiology*; D009119:Muscle Contraction / Q000502:physiology*; D018485:Muscle Fibers, Skeletal; D018482:Muscle, Skeletal / Q000502:physiology*; D016297:Mutagenesis, Site-Directed; D009154:Mutation; D009200:Myocardial Contraction / Q000502:physiology; D020033:Protein Isoforms; D021122:Protein Subunits; D011994:Recombinant Proteins / Q000378:metabolism; D012519:Sarcoplasmic Reticulum / Q000502:physiology; D013223:Statistics as Topic; D013329:Structure-Activity Relationship
chemical_list: D015220:Calcium Channels; D020746:Calcium Channels, L-Type; C419385:L-type calcium channel alpha(1C); D020033:Protein Isoforms; D021122:Protein Subunits; D011994:Recombinant Proteins; C093712:Cacna1d protein, rat; D002118:Calcium