α2δ1 Dihydropyridine Receptor Subunit Is a Critical Element for Excitation-Coupled Calcium Entry but Not for Formation of Tetrads in Skeletal Myotubes

It has been shown that small interfering RNA (siRNA) partial knockdown of the α2δ1 dihydropyridine receptor subunits cause a significant increase in the rate of activation of the L-type Ca2+ current in myotubes but have little or no effect on skeletal excitation-contraction coupling. This study used permanent siRNA knockdown of α2δ1 to address two important unaddressed questions. First, does the α2δ1 subunit contribute to the size and/or spacing of tetradic particles? Second, is the α2δ1 subunit important for excitation-coupled calcium entry? We found that the size and spacing of tetradic particles is unaffected by siRNA knockdown of α2δ1, indicating that the visible particle represents the α1s subunit. Strikingly, >97% knockdown of α2δ1 leads to a complete loss of excitation-coupled calcium entry during KCl depolarization and a more rapid decay of Ca2+ transients during bouts of repetitive electrical stimulation like those occurring during normal muscle activation in vivo. Thus, we conclude that the α2δ1 dihydropyridine receptor subunit is physiologically necessary for sustaining Ca2+ transients in response to prolonged depolarization or repeated trains of action potentials.