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 Ca ²⁺ 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 α_1s 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 α₂δ ₁ dihydropyridine receptor subunit is physiologically necessary for sustaining Ca²⁺ transients in response to prolonged depolarization or repeated trains of action potentials.