Recovery of Ca2+ current, charge movements, and Ca2+ transients in myotubes deficient in dihydropyridine receptor beta(1) subunit transfected with beta(1) cDNA

The Ca2+ currents, charge movements, and intracellular Ca2+ transients of mouse dihydropyridine receptor (DHPR) beta(1)-null myotubes expressing a mouse DHPR beta(1) cDNA have been characterized. In beta(1)-null myotubes maintained in culture for 10-15 days, the density of the L-type current was similar to 7-fold lower than in normal cells of the same age (I-max was 0.65 +/- 0.05 pA/pF in mutant versus 4.5 +/- 0.8 pA/pF in normal), activation of the L-type current was significantly faster ((t)au activation at +40 mV was 28 +/- 7 ms in mutant versus 57 +/- 8 ms in normal), charge movements were similar to 2.5-fold lower (Q(max) was 2.5 +/- 0.2 nC/mu F in mutant versus 6.3 +/- 0.7 nC/mu F in normal), CA(2+) transients were not elicited by depolarization, and spontaneous or evoked contractions were absent. Transfection of beta(1)-null cells by lipofection with beta(1) cDNA reestablished spontaneous or evoked contractions in similar to 10% of cells after 6 days and similar to 30% of cells after 13 days. In contracting beta(1)-transfected myotubes there was a complete recovery of the L-type current density (I-max was 4 +/- 0.9 pA/pF), the kinetics of activation (tau activation at 40 mV was 64 +/- 5 ms), the magnitude of charge movements (Q(max) was 6.7 +/- 0.4 nC/mu F), and the amplitude and voltage dependence of Ca2+ transients evoked by depolarizations. Ca2+ transients of transfected cells were unaltered by the removal of external Ca2+ or by the block of the L-type Ca2+ current, demonstrating that a skeletal-type excitation-contraction coupling was restored. The recovery of the normal skeletal muscle phenotype in beta(1)-transfected beta-null myotubes shows that the beta(1) subunit is essential for the functional expression of the DHPR complex.