Strube C, Beurg M, Sukhareva M, Ahern C A, Powell J A, Powers P A, Gregg R G, Coronado R
Department of Physiology, University of Wisconsin Medical School, Madison, Wisconsin 53706, USA.
Biophys J. 1998 Jul;75(1):207-17. doi: 10.1016/S0006-3495(98)77507-1.
The origin of Ibetanull, the Ca2+ current of myotubes from mice lacking the skeletal dihydropyridine receptor (DHPR) beta1a subunit, was investigated. The density of Ibetanull was similar to that of Idys, the Ca2+ current of myotubes from dysgenic mice lacking the skeletal DHPR alpha1S subunit (-0.6 +/- 0.1 and -0.7 +/- 0.1 pA/pF, respectively). However, Ibetanull activated at significantly more positive potentials. The midpoints of the GCa-V curves were 16.3 +/- 1.1 mV and 11.7 +/- 1.0 mV for Ibetanull and Idys, respectively. Ibetanull activated significantly more slowly than Idys. At +30 mV, the activation time constant for Ibetanull was 26 +/- 3 ms, and that for Idys was 7 +/- 1 ms. The unitary current of normal L-type and beta1-null Ca2+ channels estimated from the mean variance relationship at +20 mV in 10 mM external Ca2+ was 22 +/- 4 fA and 43 +/- 7 fA, respectively. Both values were significantly smaller than the single-channel current estimated for dysgenic Ca2+ channels, which was 84 +/- 9 fA under the same conditions. Ibetanull and Idys have different gating and permeation characteristics, suggesting that the bulk of the DHPR alpha1 subunits underlying these currents are different. Ibetanull is suggested to originate primarily from Ca2+ channels with a DHPR alpha1S subunit. Dysgenic Ca2+ channels may be a minor component of this current. The expression of DHPR alpha1S in beta1-null myotubes and its absence in dysgenic myotubes was confirmed by immunofluorescence labeling of cells.
对缺乏骨骼肌二氢吡啶受体（DHPR）β1a亚基的小鼠肌管的Ca2+电流Ibetanull的起源进行了研究。Ibetanull的密度与缺乏骨骼肌DHPRα1S亚基的发育不全小鼠肌管的Ca2+电流Idys相似（分别为-0.6±0.1和-0.7±0.1 pA/pF）。然而，Ibetanull在明显更正的电位下激活。Ibetanull和Idys的GCa-V曲线中点分别为16.3±1.1 mV和11.7±1.0 mV。Ibetanull的激活明显比Idys慢。在+30 mV时，Ibetanull的激活时间常数为26±3 ms，而Idys的为7±1 ms。在10 mM细胞外Ca2+中，根据+20 mV时的平均方差关系估计的正常L型和β1缺失型Ca2+通道的单通道电流分别为22±4 fA和43±7 fA。这两个值均明显小于发育不全的Ca2+通道在相同条件下估计的单通道电流84±9 fA。Ibetanull和Idys具有不同的门控和通透特性，表明这些电流背后的大部分DHPRα1亚基不同。Ibetanull被认为主要起源于具有DHPRα1S亚基的Ca2+通道。发育不全的Ca2+通道可能是该电流的次要成分。通过细胞免疫荧光标记证实了β1缺失型肌管中DHPRα1S的表达以及发育不全肌管中其缺失。
