Differential regulation of skeletal muscle L-type Ca super(2+) current and excitation-contraction coupling by the dihydropyridine receptor beta subunit

The dihydropyridine receptor (DHPR) of skeletal muscle functions as a Ca super(2+) channel and is required for 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 sub(1a) subunit or the cardiac- and brain-specific beta sub(2a) subunit fully restored the L-type Ca super(2+) current (161 plus or minus 17 pS/pF and 139 plus or minus 9 pS/pF, respectively, in 10 mM C super(2+)). We compared the Boltzmann parameters of the Ca super(2+) conductance restored by beta sub(1a) and beta sub(2a), the kinetics of activation of the Ca super(2+) 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 sub(2a) was significantly lower than in cells expressing beta sub(1a) (2.7 plus or minus 0.2 nC/ mu F and 6.7 plus or minus 0.4 nC/ mu F, respectively). Furthermore, the amplitude of Ca super(2+) transient measured by confocal line-scans of fluo-3 fluorescence in voltage-clamped cells were 3- to 5-fold lower in myotubes expressing beta sub(2a). In summary, DHPR complexes that included beta sub(2a), or beta sub(1a), restored L-type Ca super(2+) channels. However, a DHPR complex with beta sub(1a) was required for complete restoration of charge movements and skeletal-type EC coupling. These results suggest that the beta sub(1a) subunit participates in key regulatory events required for the EC coupling function of the DHPR.