Abstract
In high-resistance, salt-absorbing epithelia the apical amiloride-sensitive Na+ channel is the key site for regulation of salt and water balance. The saturation of macroscopic Na+ transport through these channels was investigated using A6 epithelial monolayers. The relation between transepithelial Na+ transport (I Na) and apical Na+ concentration ([Na+]ap) under short-circuit conditions was studied. Michaelis-Menten analysis of the saturable short-circuit current (I sc) yielded an apparent Michaelis-Menten constant (K m I) of 5 mmol/l and a maximal current (I max) of 8 μA/cm2. The microscopic parameters underlying I Na, namely the single-channel current (i) and the open channel density (N o), were investigated by the analysis of current fluctuations induced by the electroneutral amiloride analogue CDPC (6-chloro-3,5-diaminopyrazine-2-carboxamide). A two-state model analysis yielded the absolute values of i (0.18 ± 0.01 pA) and N o (65.38 ± 9.57 million channels/cm2 of epithelium) at [Na+]ap = 110 mmol/l containing 50 μmol/l CDPC. Our data indicate that in A6 cells both i and N o depend on [Na+]ap. Between 3 and ≈ 20 mmol/l the density of conducting pores, N o, decreases sharply and behaves again as an almost [Na+]ap-independent parameter at higher [Na+]ap. The single-channel current clearly saturates with an apparent Michaelis-Menten constant, K m i, of ≈ 17 mmol/l. Thus, the [Na+]ap dependence of N o as well as the limited transport capacity of the amiloride-sensitive Na+ channel are both responsible for the saturation of I Na.
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Received: 2 June 1997 / Received after revision: 12 November 1997 / Accepted: 10 December 1997
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Smets, I., Zeiske, W., Steels, P. et al. Na+ dependence of single-channel current and channel density generate saturation of Na+ uptake in A6 cells. Pflügers Arch 435, 604–609 (1998). https://doi.org/10.1007/s004240050560
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DOI: https://doi.org/10.1007/s004240050560