The Ca2+ Channel α2δ-1 Subunit Determines Ca2+ Current Kinetics in Skeletal Muscle but Not Targeting of α1S or Excitation-Contraction Coupling*

Auxiliary channel subunits regulate membrane expression and modulate current properties of voltage-activated Ca2+ channels and thus are involved in numerous important cell functions, including muscle contraction. Whereas the importance of the α1S, β1a, and γ Ca2+ channel subunits in skeletal muscle has been determined by using null-mutant mice, the role of the α2δ-1 subunit in skeletal muscle is still elusive. We addressed this question by small interfering RNA silencing of α2δ-1 in reconstituted dysgenic (α1S-null) myotubes and in BC3H1 skeletal muscle cells. Immunofluorescence labeling of the α1S and α2δ-1 subunits and whole cell patch clamp recordings demonstrated that triad targeting and functional expression of the skeletal muscle Ca2+ channel were not compromised by the depletion of the α2δ-1 subunit. The amplitudes and voltage dependences of L-type Ca2+ currents and of the depolarization-induced Ca2+ transients were identical in control and in α2δ-1-depleted muscle cells. However, α2δ-1 depletion significantly accelerated the current kinetics, most likely by the conversion of slowly activating into fast activating Ca2+ channels. Reverse transcription-PCR analysis indicated that α2δ-1 is the exclusive isoform expressed in differentiated BC3H1 cells and that depletion of α2δ-1 was not compensated by the up-regulation of any other α2δ isoform. Thus, in skeletal muscle the Ca2+ channel α2δ-1 subunit functions as a major determinant of the characteristic slow L-type Ca2+ current kinetics. However, this subunit is not essential for targeting of Ca2+ channels or for their primary physiological role in activating skeletal muscle excitation-contraction coupling.