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 expres sing a mouse DHPR beta(1) cDNA have been characterized. In beta(1)-nul l 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 s ame 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 fas ter ((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 n ormal), CA(2+) transients were not elicited by depolarization, and spo ntaneous or evoked contractions were absent. Transfection of beta(1)-n ull cells by lipofection with beta(1) cDNA reestablished spontaneous o r evoked contractions in similar to 10% of cells after 6 days and simi lar 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 transfec ted cells were unaltered by the removal of external Ca2+ or by the blo ck of the L-type Ca2+ current, demonstrating that a skeletal-type exci tation-contraction coupling was restored. The recovery of the normal s keletal muscle phenotype in beta(1)-transfected beta-null myotubes sho ws that the beta(1) subunit is essential for the functional expression of the DHPR complex.