Desmedt L, Simaels J, Van Driessche W
Laboratory for Physiology, KULeuven, Belgium.
J Gen Physiol. 1993 Jan;101(1):85-102. doi: 10.1085/jgp.101.1.85.
This study deals with the effect of mucosal UO2(2+) on the Ca(2+)-blockable, poorly selective cation channels in the apical membrane of frog skin and toad urinary bladder. Our data show that UO2(2+) inhibits the Na+ currents through the amiloride-insensitive cation pathway and confirm a previously described stimulatory effect on the amiloride-blockade Na+ transport. Noise analysis of the Ca(2+)-blockable current demonstrates that the divalent also depresses the low-frequency Lorentzian (fc = 11.7 Hz) in the power density spectrum (PDS) and reveals the presence of high-frequency relaxation noise (fc = 58.5 Hz). The action of UO2(2+) is not reversed upon washout and is not accompanied by noise, typically induced by reversible blockers. The divalent merely depresses the plateau of the low-frequency Lorentzian, demonstrating a decrease in the number of conductive cation channels. Similarly, with mucosal K+ and Rb+, UO2(2+) also unmasks the high-frequency Lorentzian by depressing the noise from the slowly fluctuating cation channels (type S). In all experiments with mucosal Cs+, the PDS contains high-frequency relaxation noise (fc = 75.1 Hz in Rana temporaria, and 65.4 Hz in Rana ridibunda). An effect of UO2(2+) on the Cs+ currents and Lorentzian plateaus could not be demonstrated, suggesting that this monovalent cation does not pass through type S channels. Experiments with the urinary bladder revealed only a UO2(2+)-insensitive pathway permeable for Na+, K+, Rb+, and Cs+. We submit that in frog skin two cation-selective channels occur, distinguished by their spontaneous gating kinetics, their sensitivity to UO2(2+), and their permeability for Cs+. In toad urinary bladder, only one kind of cation-selective channel is observed, which resembles the UO2(2+)-insensitive channel in frog skin, with fast open-closed kinetics (type F).
本研究探讨了黏膜UO2(2+)对蛙皮和蟾蜍膀胱顶端膜中可被Ca(2+)阻断的、选择性较差的阳离子通道的影响。我们的数据表明，UO2(2+)通过amiloride不敏感的阳离子途径抑制Na+电流，并证实了先前描述的对amiloride阻断的Na+转运的刺激作用。对可被Ca(2+)阻断的电流进行噪声分析表明，这种二价离子还会压低功率密度谱（PDS）中的低频洛伦兹峰（fc = 11.7 Hz），并揭示出高频弛豫噪声（fc = 58.5 Hz）的存在。UO2(2+)的作用在冲洗后不会逆转，且不会伴随通常由可逆性阻断剂诱导产生的噪声。这种二价离子仅仅压低了低频洛伦兹峰的平台期，表明传导性阳离子通道的数量减少。同样，对于黏膜K+和Rb+，UO2(2+)也通过压低来自缓慢波动阳离子通道（S型）的噪声来揭示高频洛伦兹峰。在所有使用黏膜Cs+的实验中，PDS都包含高频弛豫噪声（在泽蛙中fc = 75.1 Hz，在食用蛙中fc = 65.4 Hz）。未证实UO2(2+)对Cs+电流和洛伦兹峰平台期有影响，这表明这种单价阳离子不会通过S型通道。对膀胱的实验仅揭示了一条对Na+、K+、Rb+和Cs+通透的对UO2(2+)不敏感的途径。我们认为，在蛙皮中存在两种阳离子选择性通道，它们在自发门控动力学、对UO2(2+)的敏感性以及对Cs+的通透性方面有所不同。在蟾蜍膀胱中，仅观察到一种阳离子选择性通道，它类似于蛙皮中对UO2(2+)不敏感的通道，具有快速的开闭动力学（F型）。
