Developmental changes in different types of ion channels of mammalian skeletal muscle. - Tight-seal whole-cell clamp study-

用全细胞电压钳技术研究了小鼠趾短屈肌单纤维钙通道Na^+电流和内向整流K^+电流I（K，inward）的发育性变化，在出生后第1天（P1）分离的肌纤维中，出现了瞬时性、I（Ca，T）和持续性I（Ca，S）型电流。在P1时，I（Ca，T）的平均比振幅为2.7 A/F（-30 mV，30 mM [Ba]o），随后逐渐下降，到P17时无法检测到。I（Ca，S）的平均比振幅从P1时的6.9 A/F（+20 mV）增加至P30时的27.7 A/F。在P4、P8或P17失神经支配肌纤维不能阻止I（Ca，T）的消失。Geographutoxin II（GTX）是一种来自蜗牛的肽毒素，它将电压敏感性Na电流分为两类：GTX敏感的I（Na，s）和-不敏感的I（Na，i）。电压依赖性激活 关于我们 I（Na，i）的失活发生在比I（Na，s）负10 ~ 20 mV的膜电位下。在大于P8的小鼠纤维中，I（Na，i）的衰减速度比I（Na，s）慢。然而，在年轻小鼠的纤维中，两种电流以相似的速度衰减。I（Na，S）的平均电导率（g_<Na>（？））在前20天增加了15倍。平均比g_<Na>of I（Na，i）在前8天保持在0.15mS/ F（5 mM [Na]o）的水平，然后逐渐下降，变得不能被P16检测到。在P12失神经支配的肌纤维中，<Na>I（Na，i）的g_（12）随后增加，而总g_（12）的增加<Na>小于正常。相反，在P4失神经支配的纤维中，总g_（max）增加<Na>超过正常，直到P12，并且g_<Na>（max）为I（Na，i）增加超过P0时所见的水平。在P0和P4（20 mM [K]o）的纤维和培养的小鼠肌管（100 mM [K] o）中未观察到I（K，向内）。但在P12及以上的纤维中，I（K，内向）的瞬时和时间依赖性成分明显。I（K，内向）在P20时达到最高水平。当肌纤维失神经支配在P4，出生后增加的两个组成部分的I（K，向内）主要是防止。在P20时失神经支配的肌纤维，术后2d内I（K，内向）下降至P4时失神经支配肌纤维的水平。I（Ca，T）和I（Na，i）在出生后最初几周内消失，I（K，内向）出现，I（Ca，S）和I（Na，s）在此期间增加。I（Na，i）的消失过程和I（K，内向）的出现过程需要神经支配的存在，而I（Ca，T）的消失过程则不需要。少

The whole-cell voltage-clamp technique was used to examine developmental changes in different types of ba currents through ca channels, Na^+ currents and inward rectifier K^+ current, I(K,inward), in single fibres acutely isolated from mouse flexor digitorum brevis muscles.The muscle fibres isolated on day 1 postnatal (P1) showed transient, I(Ca,T), and sustained I(Ca,S)types of currents. The mean specific amplitude of I(Ca,T) was 2.7 A/F (-30 mV, 30 mM [Ba]o) at P1, decreased progressively afterwards, becoming undetectable by P17. The mean specific amplitude of I(Ca,S) increased from 6.9 A/F (+20 mV) at P1 to 27.7 A/F at P30. Denervating muscle fibres at P4, P8 or P17 did not prevent the disappearance of I(Ca,T). The increase of I(Ca,S) was suppressed by the denervation.Geographutoxin II (GTX), a peptide toxin from the marine snail, distinguishes two distinct types of voltage sensitive Na currents; GTX-sensitive I(Na,s) and -insensitive currents, I(Na,i). Voltage-dependent activation … More and inactivation of I(Na,i) occurred at membrane potentials 10 to 20 mV more negative than those for I(Na,s). I(Na,i) in fibres from mice older than P8 decayed more slowly than I(Na,s). However, in fibres from younger mice, The two currents decayed with similar speed. the mean specific conductance (g_<Na>) of I(Na,s) increased 15-fold in the first 20 days. The mean specific g_<Na> of I(Na,i) remained at a level of 0.15 mS/ F (5mM [Na]o) for the first 8 days, then decreased progressively, becoming undetectable by P16. In muscle fibres denervated at P12, g_<Na> for I(Na,i)increased afterwards, whereas an increase of the total g_<Na> was less than normal. By contrast, in fibres denervated at P4, the total g_<Na> increased more than normal until P12, and g_<Na> for I(Na,i) increased above the level seen at P0.I(K,inward) was not observed in fibres of P0 and P4 (20 mM [K]o) and in cultured mouse myotubes (100 mM [K]o). However, instantaneous and time-dependent components of I(K,inward) were cleary seen in fibres of P12 and older. I(K,inward) reached its maximal level by P20. When muscle fibres were denervated at P4, the postnatal increase of the two components of I(K,inward) was mostly prevented. When muscle fibres were denervated on P20, I(K,inward) decreased to the level of P4-denervated fibres in 2 days after the operation.Conclusions. I(Ca,T) and I(Na,i) disappear and I(K,inward) appears in the first few weeks after birth.I(Ca,S) and I(Na,s) increase during this period. The disappearing process of I(Na,i) and appearing process of I(K,inward) require the presence of innervation, whereas the disappearing process of I(Ca,T)does not. Less