Differential regulation of skeletal muscle L-type Ca2+ current and excitation-contraction coupling by the dihydropyridine receptor beta subunit

The dihydropyridine receptor (DHPR) of skeletal muscle functions as a Ca2channel and is required fdr excitation-contraction (EC) coupling. Here we s how that the DHPR beta subunit is involved in the regulation of these two f unctions, Experiments were performed in skeletal mouse myotubes selectively lacking a functional DHPR beta subunit, These beta-null cells have a, low- density:L-type current,:a low density of charge movements, and: lack EC cou pling. Transfection of beta-null cells with cDNAs encoding for either the h omologous beta(1a) subunit or the cardiac- and brain-specific beta(2a) subu nit fully restored the L-type Ca2+ current (161 +/- 17 pS/pF and 139 +/- 9 pS/pF, respectively, in 10 mM Ca2+). We compared the Boltzmann parameters o f the CB2+ conductance restored by beta(1a) and beta(2a), the kinetics of a ctivation of the Ca2+ current, and the single channel:parameters estimated by ensemble variance analysis and found them to be indistinguishable. In co ntrast, the maximum density of charge movements in cells expressing beta(2a ) was significantly lower than in cells expressing beta(1a) (2.7 +/- 0.2 nC /mu F and 6.7 +/- 0.4 nC/mu F, respectively). Furthermore, the amplitude of Ca2+ transient measured by confocal line-scans of fluo-3 fluorescence in v oltage-clamped: cells were 3- to 5-fold lower in myotubes:expressing beta(2 a). In summary, DHPR complexes that included beta(2a) or beta(1a) restored L-type Ca2+ channels. However, a DHPR complex with beta(1a) was required fo r complete restoration of charge movements and skeletal-type EC coupling, T hese results:suggest that the beta(1a) subunit participates in key: regulat ory events required for the EC, coupling function of the,DHPR.