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

The dihydropyridine receptor (DHPR) of skeletal muscle functions as a Ca2+ channel and is required fdr excitation-contraction (EC) coupling. Here we show that the DHPR beta subunit is involved in the regulation of these two functions, 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 coupling. Transfection of beta-null cells with cDNAs encoding for either the homologous beta(1a) subunit or the cardiac- and brain-specific beta(2a) subunit 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 of the CB2+ conductance restored by beta(1a) and beta(2a), the kinetics of activation of the Ca2+ current, and the single channel:parameters estimated by ensemble variance analysis and found them to be indistinguishable. In contrast, 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 voltage-clamped: cells were 3- to 5-fold lower in myotubes:expressing beta(2a). 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 for complete restoration of charge movements and skeletal-type EC coupling, These results:suggest that the beta(1a) subunit participates in key: regulatory events required for the EC, coupling function of the,DHPR.