MOLECULAR-ORIGIN OF THE L-TYPE CA2-MUSCLE MYOTUBES SELECTIVELY DEFICIENT IN DIHYDROPYRIDINE RECEPTOR BETA(1A) SUBUNIT( CURRENT OF SKELETAL)

The origin of I-beta null, the Ca2+ current of myotubes from mice lack ing the skeletal dihydropyridine receptor (DHPR) beta(1a) subunit, was investigated. The density of I-beta null was similar to that of I-dys , the Ca2+ current of myotubes from dysgenic mice lacking the skeletal DHPR alpha(1S) subunit (-0.6 +/- 0.1 and -0.7 +/- 0.1 pA/pF, respecti vely). However, I-beta null activated at significantly more positive p otentials. The midpoints of the G(Ca)-V curves were -16.3 +/- 1.1 mV a nd 11.7 +/- 1.0 mV for I-beta null and I-dys, respectively. I-beta nul l activated significantly more slowly than I-dys. At +30 mV, the activ ation time constant for I-beta null was 26 +/- 3 ms, and that for I-dy s was 7 +/- 1 ms. The unitary current of normal L-type and beta(1)-nul l Ca2+ channels estimated from the mean variance relationship at +20 m V in 10 mM external Ca2+ was 22 +/- 4 fA and 43 +/- 7 fA, respectively . Both values were significantly smaller than the single-channel curre nt estimated for dysgenic Ca2+ channels, which was 84 +/- 9 fA under t he same conditions. I-beta null and I-dys have different gating and pe rmeation characteristics, suggesting that the bulk of the DHPR alpha(1 ) subunits underlying these currents are different. I-beta null is sug gested to originate primarily from Ca2+ channels with a DHPR alpha(1S) subunit. Dysgenic Ca2+ channels may be a minor component of this curr ent. The expression of DHPR alpha(1S) in beta(1)-null myotubes and its absence in dysgenic myotubes was confirmed by immunofluorescence labe ling of cells.