Modulation of the cloned skeletal muscle L-type Ca²⁺ channel by anchored cAMP-dependent protein kinase

Ca²⁺ influx through skeletal muscle Ca²⁺ channels and the force of contraction are increased in response to β-adrenergic stimulation and high- frequency electrical stimulation. These effects are thought to be mediated by cAMP-dependent phosphorylation of the skeletal muscle Ca²⁺ channel. Modulation of the cloned skeletal muscle Ca²⁺ channel by cAMP-dependent phosphorylation and by depolarizing prepulses was reconstituted by transient expression in tsA-201 cells and compared to modulation of the native skeletal muscle Ca²⁺ channel as expressed in mouse 129CB3 skeletal muscle cells. The heterologously expressed Ca²⁺ channel consisting of α₁, α₂δ, and β subunits gave currents that were similar in time course, current density, and dihydropyridine sensitivity to the native Ca²⁺ channel. cAMP dependent protein kinase (PKA) stimulation by Sp-5,6-DCI-cBIMPS (cBIMPS) increased currents through both native and expressed channels two- to fourfold. Tail currents after depolarizations to potentials between -20 and +80 mV increased in amplitude and decayed more slowly as either the duration or potential of the depolarization was increased. The time- and voltage-dependent slowing of channel deactivation required the activity of PKA, because it was enhanced by cBIMPS and reduced or eliminated by the peptide PKA inhibitor PKI (5-24) amide. This voltage-dependent modulation of the cloned skeletal muscle Ca²⁺ channel by PKA also required anchoring of PKA by A-Kinase Anchoring Proteins because it was blocked by peptide Ht 31, which disrupts such anchoring. The results show that the skeletal muscle Ca²⁺ channel expressed in heterologous cells is modulated by PKA at rest and during depolarization and that this modulation requires anchored protein kinase, as it does in native skeletal muscle cells.