Marcotti Walter, Johnson Stuart L, Kros Corné J
School of Life Sciences, University of Sussex, Falmer, Brighton BN1 9QG, UK.
J Physiol. 2004 Jun 1;557(Pt 2):613-33. doi: 10.1113/jphysiol.2003.060137. Epub 2004 Apr 2.
Ca(2+)-activated K(+) currents were studied in inner hair cells (IHCs) of mature mice. I(K,f), the large-conductance Ca(2+)-activated K(+) current (BK) characteristic of mature IHCs, had a fast activation time constant (0.4 ms at -25 mV at room temperature) and did not inactivate during 170 ms. Its amplitude, measured at -25 mV, and activation time constant were similar between IHCs in the apical and basal regions of the cochlea. I(K,f) was selectively blocked by 30 nm IbTx but was unaffected by superfusion of Ca(2+)-free solution, nifedipine or Bay K 8644, excluding the direct involvement of voltage-gated Ca(2+) channels in I(K,f) activation. Increasing the intracellular concentration of the Ca(2+) chelator BAPTA from 0.1 mm to 30 mm reduced the amplitude of I(K,f) at -25 mV and shifted its activation by 37 mV towards more depolarized potentials. A reduction in the size of I(K,f) and a depolarizing shift of its activation were also seen when either thapsigargin and caffeine or ryanodine were added intracellularly, suggesting that I(K,f) is modulated by voltage-dependent release from intracellular Ca(2+) stores. Mature IHCs had a small additional Ca(2+)-activated K(+) current (I(K(Ca))), activated by Ca(2+) flowing through L-type Ca(2+) channels. This current was still present during superfusion of either IbTx (60 nm) or apamin (300 nm) but was abolished in Cs(+)-based intracellular solution or during superfusion of 5 mm TEA, suggesting the presence of an additional BK-channel type. Current clamp experiments at body temperature show that I(K,f), but not I(K(Ca)), is essential for fast voltage responses of mature IHCs.
在成熟小鼠的内毛细胞（IHC）中研究了Ca(2+)激活的K(+)电流。I(K,f)是成熟IHC特有的大电导Ca(2+)激活的K(+)电流（BK），具有快速激活时间常数（室温下-25 mV时为0.4 ms），且在170 ms内不发生失活。在-25 mV测量时，其幅度以及激活时间常数在耳蜗顶部和基部区域的IHC之间相似。I(K,f)被30 nm的IbTx选择性阻断，但不受无Ca(2+)溶液、硝苯地平或Bay K 8644灌流的影响，排除了电压门控Ca(2+)通道直接参与I(K,f)激活的可能性。将Ca(2+)螯合剂BAPTA的细胞内浓度从0.1 mm增加到30 mm，可降低-25 mV时I(K,f)的幅度，并使其激活向更去极化电位偏移37 mV。当细胞内加入毒胡萝卜素和咖啡因或ryanodine时，也观察到I(K,f)大小减小及其激活的去极化偏移，这表明I(K,f)受细胞内Ca(2+)储存的电压依赖性释放调节。成熟的IHC还有一个小的额外Ca(2+)激活的K(+)电流（I(K(Ca))），由通过L型Ca(2+)通道流动的Ca(2+)激活。在60 nm的IbTx或300 nm的蜂毒明肽灌流期间，该电流仍然存在，但在基于Cs(+)的细胞内溶液中或5 mm TEA灌流期间被消除，提示存在另一种BK通道类型。体温下的电流钳实验表明，I(K,f)而非I(K(Ca))对成熟IHC的快速电压反应至关重要。
