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

全细胞电压钳技术被用来检查不同类型的ba电流通过ca通道，Na^+电流和向内整流K^+电流，I（K，向内），在单个纤维中从小鼠指屈肌短肌急性分离的发育变化。出生后第1天（P1）分离的肌纤维显示瞬时、I（Ca，T）和持续I（Ca，S）型电流。P1时I（Ca，T）的平均比振幅为2.7 A/F (-30 mV, 30 mM [Ba]o)，之后逐渐减小，P17无法检测到。I（Ca，S）的平均比振幅从P1时的6.9 A/F （+20 mV）增加到P30时的27.7 A/F。P4， P8或P17的去神经肌纤维不能阻止I（Ca，T）的消失。I（Ca，S）的增加受到去神经支配的抑制。地理毒素II （GTX）是一种来自海蜗牛的肽毒素，可区分两种不同类型的电压敏感Na电流；gtx敏感电流I（Na,s）和-不敏感电流I（Na， I）。在膜电位负10 ~ 20 mV时，I（Na， I）的激活和失活比I（Na,s）的激活和失活多。年龄大于P8的小鼠纤维中I（Na， I）的衰变速度比I（Na,s）慢。然而，在年轻老鼠的纤维中，两种电流以相似的速度衰减。I（Na,s）的平均比电导（g_<Na>）在前20天增加了15倍。I（Na， I）的平均特异性g_<Na>在前8天保持在0.15 mS/ F （5mM [Na]o）的水平，随后逐渐下降，P16无法检测到。在P12失神经肌纤维中，I（Na， I）的g_<Na>增加，而总g_<Na>的增加低于正常水平。相比之下，在P4失神经纤维中，g_<Na>的总量比正常增加，直到P12，并且g_<Na> （Na， I）高于P0的水平。在P0和P4纤维（20 mM [K]o）和培养小鼠肌管（100 mM [K]o）中未观察到I（K，向内）。然而，在P12及更老的纤维中可以清楚地看到I（K，向内）的瞬时和时间依赖性成分。I（K，向内）在P20时达到最大值。当肌纤维在P4去神经时，出生后I（K，向内）的两种成分的增加基本被阻止。当P20去神经肌纤维时，I（K，向内）在术后2天降至p4去神经纤维水平。I（Ca，T）和I（Na， I）消失，I（K，内向）出现在出生后的最初几周。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