Li J Q, Prod'hom B, Kucera P
Institut de Physiologie, Faculté de médecine, Université de Lausanne, Switzerland.
Pflugers Arch. 1994 Dec;429(2):183-92. doi: 10.1007/BF00374311.
In the chick embryo (20 h incubation, gastrula stage), the apical membrane of the ectodermal cells shows a high density of a non-selective cation channel which is blocked by very low extracellular Ca2+ concentrations. Properties of this channel were studied at the single-channel level using the patch-clamp technique in the cell-attached mode. With 1 mmol/l Ca2+ in the pipette, only outward current was present and the channel conductance measured at +120 mV was 25.5 pS. In the absence of Ca2+, also inward current through the channel was observed. The conductances measured at -50 mV were 49.5 pS with Na+ as the charge carrier, 72.5 pS with K+, 49.1 pS with Cs+, and 18.5 pS with Li+. The conductance measured at +80 mV was around 23 pS in all four cases. The reversal potential was similar (around 25 mV) for all four ions, which indicates a poor selectivity of the channel. In the absence of Ca2+ and the presence of 1 mmol/l ethylenebis(oxonitrilo)tetraacetate (EGTA), the kinetics of the channel were characterized by bursts of the order of seconds. During a burst, the channel flickered between one open and one closed level. The open time was constant between -30 mV and -80 mV, while the closed time decreased with hyperpolarization. The open time varied according to the permeant ion (K+ < Na+ = Cs+ < Li+). Extracellular Ca2+ blocked the inward current in a voltage-dependent manner. The Kd values, 1 mumol/l at -30 mV and 3.2 mumol/l at -80 mV, indicate that Ca2+ ions exit the channel toward the intracellular side. A weak voltage dependency of the association rate constant suggests that the Ca(2+)-binding site is close to the outside mouth. Extracellular Ca2+ was much less efficient at blocking the outward current (Kd about 1 mmol/l at 80 mV). Tetracaine, but not uraniumdioxide, decreased the opening probability of the channel. The embryonic channel shows similarities with the Ca(2+)-blockable, poorly selective channel described in the epithelium of toad urinary bladder.
在鸡胚（孵化20小时，原肠胚阶段）中，外胚层细胞的顶端膜显示出高密度的非选择性阳离子通道，该通道可被极低的细胞外Ca2+浓度阻断。使用膜片钳技术在细胞贴附模式下，在单通道水平研究了该通道的特性。移液器中含有1 mmol/l Ca2+时，仅存在外向电流，在+120 mV时测得的通道电导为25.5 pS。在无Ca2+的情况下，也观察到通过该通道的内向电流。在 -50 mV时测得的电导，以Na+作为电荷载体时为49.5 pS，以K+时为72.5 pS，以Cs+时为49.1 pS，以Li+时为18.5 pS。在所有四种情况下，在 +80 mV时测得的电导约为23 pS。所有四种离子的反转电位相似（约25 mV），这表明该通道的选择性较差。在无Ca2+且存在1 mmol/l乙二胺四乙酸（EGTA）的情况下，通道的动力学特征为持续数秒的爆发。在一次爆发期间，通道在一个开放水平和一个关闭水平之间闪烁。开放时间在 -30 mV至 -80 mV之间恒定，而关闭时间随着超极化而减少。开放时间根据通透离子而变化（K+ < Na+ = Cs+ < Li+）。细胞外Ca2+以电压依赖性方式阻断内向电流。Kd值在 -30 mV时为1 μmol/l，在 -80 mV时为3.2 μmol/l，表明Ca2+离子从通道向细胞内侧流出。结合速率常数的弱电压依赖性表明Ca(2+)结合位点靠近外侧口。细胞外Ca2+阻断外向电流的效率要低得多（在80 mV时Kd约为1 mmol/l）。丁卡因而非二氧化铀降低了通道的开放概率。胚胎通道与蟾蜍膀胱上皮中描述的可被Ca(2+)阻断、选择性较差的通道具有相似性。
