Acceleration of activation and inactivation by the β subunit of the skeletal muscle calcium channel

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Abstract
THE L-type voltage-dependent calcium channel is an important link in excitation-contraction coupling of muscle cells1 (reviewed in refs 2 and 3). The channel has two functional characteristics: calcium permeation and receptor sites for calcium antagonists. In skeletal muscle the channel is a complex of five subunits, α1, α2, β, γ and δ (ref. 4). Complementary DNAs to these subunits have been cloned and their amino-acid sequences deduced5–8. The skeletal muscle α1 subunit cDNA expressed in L cells manifests as specific calcium-ion permeation, as well as sensitivity to the three classes of organic calcium-channel blockers9,10. We report here that coexpression of the α1, subunit with other subunits results in significant changes in dihydropyridine binding and gating properties. The available number of drug receptor sites increases 10-fold with an α1 β combination, whereas the affinity of the dihydropyridine binding site remains unchanged. Also, the presence of the β subunit accelerates activation and inactivation kinetics of the calcium-channel current.