Dirksen R T, Beam K G
Department of Anatomy and Neurobiology, College of Veterinary Medicine and Biomedical Sciences, Colorado State University, Fort Collins 80523, USA.
J Gen Physiol. 1996 Jun;107(6):731-42. doi: 10.1085/jgp.107.6.731.
Skeletal and cardiac dihydropyridine receptors function both as voltage-dependent L-type calcium channels (L-channels) and as critical proteins that trigger calcium release from the sarcoplasmic reticulum in muscle. In spite of these similarities, skeletal L-channels exhibit a markedly slower activation rate than cardiac L-channels. We investigated the mechanisms underlying this difference by comparing the unitary behavior of L-channels in cell-attached patches of dysgenic myotubes expressing skeletal, cardiac, or chimeric dihydropyridine receptors. Our results demonstrate that ensemble averages activate rapidly for the purely cardiac dihydropyridine receptor and approximately five times more slowly for L-channels attributable to the purely skeletal dihydropyridine receptor or a chimeric dihydropyridine receptor in which only the first internal repeat and all of the putative intracellular loops are of skeletal origin. All of the constructs studied similarly exhibit a brief (2-ms) and a long (> or = 15-ms) open time in the presence of Bay K 8644, neither of which depend significantly on voltage. In the absence of Bay K 8644, the fraction of total open events is markedly shifted to the briefer open time without altering the rate of ensemble activation. Closed time analysis of L-channels with cardiac-like, rapid activation (recorded in the presence of dihydropyridine agonist) reveals both a brief (approximately 1-ms) closed time and a second, voltage-dependent, long-lasting closed time. The time until first opening after depolarization is three to six times faster for rapidly activating L-channels than for slowly activating L-channels and depends strongly on voltage for both types of channels. The results suggest that a voltage-dependent, closed-closed transition that is fast in cardiac L-channels and slow in skeletal L-channels can account for the difference in activation rate between these two channels.
骨骼和心脏二氢吡啶受体既作为电压依赖性L型钙通道（L通道）发挥作用，又作为触发肌肉肌浆网释放钙的关键蛋白。尽管存在这些相似之处，但骨骼L通道的激活速率明显慢于心脏L通道。我们通过比较表达骨骼、心脏或嵌合二氢吡啶受体的发育不全肌管的细胞贴附片中L通道的单一行为，研究了这种差异背后的机制。我们的结果表明，对于纯心脏二氢吡啶受体，总体平均值快速激活；而对于归因于纯骨骼二氢吡啶受体或仅第一个内部重复序列和所有假定的细胞内环均源自骨骼的嵌合二氢吡啶受体的L通道，激活速率则慢约五倍。在存在Bay K 8644的情况下，所有研究的构建体均类似地表现出短暂（2毫秒）和长（≥15毫秒）的开放时间，且两者均不显著依赖于电压。在不存在Bay K 8644的情况下，总开放事件的比例明显转向较短的开放时间，而不改变总体激活速率。对具有类似心脏的快速激活的L通道（在二氢吡啶激动剂存在下记录）的关闭时间分析显示，既有短暂（约1毫秒）的关闭时间，又有第二个电压依赖性的持久关闭时间。快速激活的L通道去极化后至首次开放的时间比缓慢激活的L通道快三至六倍，且对于两种类型的通道均强烈依赖于电压。结果表明，心脏L通道中快速而骨骼L通道中缓慢的电压依赖性关闭-关闭转变可以解释这两种通道激活速率的差异。
