Patlak J B, Ortiz M
J Gen Physiol. 1986 Feb;87(2):305-26. doi: 10.1085/jgp.87.2.305.
Na+ currents were measured during 0.4-s depolarizing pulses using the cell-attached variation of the patch-clamp technique. Patches on Cs-dialyzed segments of sartorius muscle of Rana pipiens contained an estimated 25-500 Na+ channels. Three distinct types of current were observed after the pulse onset: a large initial surge of inward current that decayed within 10 ms (early currents), a steady "drizzle" of isolated, brief, inward unitary currents (background currents), and occasional "cloudbursts" of tens to hundreds of sequential unitary inward currents (bursts). Average late currents (background plus bursts) were 0.12% of peak early current amplitude at -20 mV. 85% of the late currents were carried by bursting channels. The unit current amplitude was the same for all three types of current, with a conductance of 10.5 pS and a reversal potential of +74 mV. The magnitudes of the three current components were correlated from patch to patch, and all were eliminated by slow inactivation. We conclude that all three components were due to Na+ channel activity. The mean open time of the background currents was approximately 0.25 ms, and the channels averaged 1.2 openings for each event. Neither the open time nor the number of openings of background currents was strongly sensitive to membrane potential. We estimated that background openings occurred at a rate of 0.25 Hz for each channel. Bursts occurred once each 2,000 pulses for each channel (assuming identical channels). The open time during bursts increased with depolarization to 1-2 ms at -20 mV, whereas the closed time decreased to less than 20 ms. The fractional open time during bursts was fitted with m infinity 3 using standard Na+ channel models. We conclude that background currents are caused by a return of normal Na+ channels from inactivation, while bursts are instances where the channel's inactivation gate spontaneously loses its function for prolonged periods.
使用膜片钳技术的细胞贴附模式，在0.4秒的去极化脉冲期间测量Na⁺电流。牛蛙缝匠肌经铯透析节段上的膜片含有估计25 - 500个Na⁺通道。在脉冲开始后观察到三种不同类型的电流：脉冲开始后10毫秒内衰减的大的初始内向电流激增（早期电流）、稳定的孤立短暂内向单一电流“细雨”（背景电流）以及偶尔的数十至数百个连续单一内向电流“暴雨”（爆发）。在-20 mV时，平均晚期电流（背景电流加爆发电流）为早期电流峰值幅度的0.12%。85%的晚期电流由爆发通道携带。所有三种电流类型的单位电流幅度相同，电导为10.5 pS，反转电位为+74 mV。从一个膜片到另一个膜片，三种电流成分的大小是相关的，并且所有成分都通过缓慢失活而消除。我们得出结论，所有三种成分均归因于Na⁺通道活性。背景电流的平均开放时间约为0.25毫秒，每个事件通道平均开放1.2次。背景电流的开放时间和开放次数对膜电位均不敏感。我们估计每个通道背景开放的发生率为0.25 Hz。每个通道每2000个脉冲出现一次爆发（假设通道相同）。在-20 mV时，爆发期间的开放时间随着去极化增加到1 - 2毫秒，而关闭时间减少到小于20毫秒。爆发期间的开放时间分数用标准Na⁺通道模型的m∞3进行拟合。我们得出结论，背景电流是由正常Na⁺通道从失活状态恢复引起的，而爆发是通道失活门长时间自发失去功能的情况。
