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 approximately 7-fold lower than in normal cells of the same age (Imax 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 (tau activation at +40 mV was 28 +/- 7 ms in mutant versus 57 +/- 8 ms in normal), charge movements were approximately 2.5-fold lower (Qmax was 2.5 +/- 0.2 nC/microF in mutant versus 6.3 +/- 0.7 nC/microF in normal), Ca2+ 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 approximately 10% of cells after 6 days and approximately 30% of cells after 13 days. In contracting beta 1-transfected myotubes there was a complete recovery of the L-type current density (Imax was 4 +/- 0.9 pA/pF), the kinetics of activation (tau activation at +40 mV was 64 +/- 5 ms), the magnitude of charge movements (Qmax was 6.7 +/- 0.4 nC/microF), 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.
Beurg
M
M
Department of Physiology, University of Wisconsin School of Medicine, Madison, USA.
Sukhareva
M
M
Strube
C
C
Powers
P A
PA
Gregg
R G
RG
Coronado
R
R
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000096:biosynthesis / Q000172:deficiency* / Q000502:physiology*; D020746:Calcium Channels, L-Type; D002122:Calcium Chloride / Q000494:pharmacology; D002478:Cells, Cultured; D004553:Electric Conductivity; D005333:Fetus; D046911:Macromolecular Substances; D008564:Membrane Potentials / Q000187:drug effects; D051379:Mice; D018345:Mice, Knockout; D009119:Muscle Contraction; D018482:Muscle, Skeletal / Q000166:cytology / Q000196:embryology / Q000502:physiology*; D018408:Patch-Clamp Techniques; D011994:Recombinant Proteins / Q000096:biosynthesis / Q000378:metabolism; D014162:Transfection
D015220:Calcium Channels; D020746:Calcium Channels, L-Type; D046911:Macromolecular Substances; D011994:Recombinant Proteins; D002122:Calcium Chloride; D002118:Calcium