Numann R, Hauschka S D, Catterall W A, Scheuer T
Department of Pharmacology, University of Washington, Seattle, Washington 98195.
J Neurosci. 1994 Jul;14(7):4226-36. doi: 10.1523/JNEUROSCI.14-07-04226.1994.
Adult vertebrate skeletal muscle sodium channels are responsible for the spread of excitation from the end-plate through the muscle membrane and transverse tubular system that ultimately leads to contraction. These channels can be distinguished from other sodium channels by their sensitivity to both mu-conotoxin and TTX. The mouse satellite muscle cell line MM14 expresses only TTX- and mu-conotoxin-sensitive sodium channels having the physiological characteristics of adult skeletal muscle channels in both undifferentiated myoblasts and differentiated myotubes. Using undifferentiated and differentiated MM14 cells as well as primary cultures of rat skeletal muscle, we have examined modulation of adult skeletal muscle sodium channels by activators of protein kinase C (PKC). Stimulation of PKC by 1-oleoyl-2-acetyl-sn-glycerol (OAG) slows sodium current macroscopic inactivation rate by up to 70% and reduces the peak sodium current as much as 88%. Single-channel analysis reveals prolonged single channel openings and greatly increased probability of multiple channel openings during sustained depolarizations. These effects are due to PKC activation since they are blocked by a specific peptide inhibitor of PKC. The two effects of OAG are sequential. Low OAG concentrations can cause slowed macroscopic sodium current inactivation in the absence of peak current reduction, and intermediate concentrations of OAG cause slowing of inactivation followed by reduction of peak current. The separation of these two effects indicates that PKC modulation of the skeletal muscle sodium channel may occur by phosphorylation at two independent sites. PKC modulation of muscle sodium channels is expected to have important effects on muscle excitability and resultant contractile activity. Detection of adult skeletal muscle ion channels in replicating MM14 cells suggest that satellite cells may express a distinct subset of muscle-specific genes prior to activation of the terminal differentiation program.
成年脊椎动物骨骼肌钠通道负责将兴奋从终板通过肌膜和横管系统进行传播，最终导致肌肉收缩。这些通道可通过它们对μ - 芋螺毒素和河豚毒素（TTX）的敏感性与其他钠通道区分开来。小鼠卫星肌细胞系MM14在未分化的成肌细胞和分化的肌管中仅表达对TTX和μ - 芋螺毒素敏感的钠通道，这些通道具有成年骨骼肌通道的生理特性。我们使用未分化和分化的MM14细胞以及大鼠骨骼肌原代培养物，研究了蛋白激酶C（PKC）激活剂对成年骨骼肌钠通道的调节作用。1 - 油酰 - 2 - 乙酰 - sn - 甘油（OAG）刺激PKC可使钠电流宏观失活速率减慢高达70%，并使钠电流峰值降低多达88%。单通道分析显示，在持续去极化期间，单通道开放时间延长，多通道开放的概率大大增加。这些效应是由于PKC激活所致，因为它们被PKC的一种特异性肽抑制剂所阻断。OAG的两种效应是相继发生的。低浓度的OAG可在不降低峰值电流的情况下导致宏观钠电流失活减慢，中等浓度的OAG则先导致失活减慢，随后使峰值电流降低。这两种效应的分离表明，PKC对骨骼肌钠通道的调节可能通过在两个独立位点的磷酸化发生。PKC对肌肉钠通道的调节预计会对肌肉兴奋性和由此产生的收缩活动产生重要影响。在复制的MM14细胞中检测到成年骨骼肌离子通道，这表明卫星细胞在终末分化程序激活之前可能表达一组独特的肌肉特异性基因。
