Molecular Origin of the L-Type Ca super(2+) Current of Skeletal Muscle Myotubes Selectively Deficient in Dihydropyridine Receptor beta sub(1a) Subunit

The origin of I sub( beta null), the Ca super(2+) current of myotubes from mice lacking the skeletal dihydropyridine receptor (DHPR) beta sub(1a) subunit, was investigated. The density of I sub( beta null) was similar to that of I sub(dys), the Ca super(2+) current of myotubes from dysgenic mice lacking the skeletal DHPR alpha sub(1S) subunit (-0.6 plus or minus 0.1 and -0.7 plus or minus 0.1 pA/pF, respectively). However, I sub( beta null) activated at significantly more positive potentials. The midpoints of the G sub(Ca)-V curves were 16.3 plus or minus 1.1 mV and 11.7 plus or minus 1.0 mV for I sub( beta null) and I sub(dys), respectively. I sub( beta null) activated significantly more slowly than I sub(dys). At +30 mV, the activation time constant for I sub( beta null) was 26 plus or minus 3 ms, and that for I sub(dys) was 7 plus or minus 1 ms. The unitary current of normal L-type and beta sub(1)-null Ca super(2+) channels estimated from the mean variance relationship at +20 mV in 10 mM external Ca super(2+) was 22 plus or minus 4 fA and 43 plus or minus 7 fA, respectively. Both values were significantly smaller than the single-channel current estimated for dysgenic Ca super(2+) channels, which was 84 plus or minus 9 fA under the same conditions. I sub( beta null) and I sub(dys) have different gating and permeation characteristics, suggesting that the bulk of the DHPR alpha sub(1) subunits underlying these currents are different. I sub( beta null) is suggested to originate primarily from Ca super(2+) channels with a DHPR alpha sub(1S) subunit. Dysgenic Ca super(2+) channels may be a minor component of this current. The expression of DHPR alpha sub(1S) in beta sub(1)-null myotubes and its absence in dysgenic myotubes was confirmed by immunofluorescence labeling of cells.