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MEMBRANE TRANSPORTERS, ION CHANNELS, AND PUMPS

Membrane cholesterol extraction decreases Na⁺ transport in A6 renal epithelia

¹Laboratory of Physiology, Hasselt University, Diepenbeek, Belgium; ²Laboratory of Physiology, K. U. Leuven Campus Gasthuisberg O & N, Leuven, Belgium; ³Laboratory of Biophysics, International Centre of Biodynamics, Bucharest, Romania; ⁴Department of Health and Environmental Physics, Horia Hulubei National Institute for Physics and Nuclear Engineering, Bucharest, Romania

Submitted 12 June 2005 ; accepted in final form 9 August 2005

In this study, we have investigated the dependence of Na⁺ transport regulation on membrane cholesterol content in A6 renal epithelia. We continuously monitored short-circuit current (I_sc), transepithelial conductance (G_T), and transepithelial capacitance (C_T) to evaluate the effects of cholesterol extraction from the apical and basolateral membranes in steady-state conditions and during activation with hyposmotic shock, oxytocin, and adenosine. Cholesterol extraction was achieved by perfusing the epithelia with methyl--cyclodextrin (mCD) for 1 h. In steady-state conditions, apical membrane cholesterol extraction did not significantly affect the electrophysiological parameters; in contrast, marked reductions were observed during basolateral mCD treatment. However, apical mCD application hampered the responses of I_sc and G_T to hypotonicity, oxytocin, and adenosine. Analysis of the blocker-induced fluctuation in I_sc demonstrated that apical mCD treatment decreased the epithelial Na⁺ channel (ENaC) open probability (P_o) in the steady state as well as after activation of Na⁺ transport by adenosine, whereas the density of conducting channels was not significantly changed as confirmed by C_T measurements. Na⁺ transport activation by hypotonicity was abolished during basolateral mCD treatment as a result of reduced Na⁺/K⁺ pump activity. On the basis of the findings in this study, we conclude that basolateral membrane cholesterol extraction reduces Na⁺/K⁺ pump activity, whereas the reduced cholesterol content of the apical membranes affects the activation of Na⁺ transport by reducing ENaC P_o.

epithelial Na⁺ channel; Na⁺-K⁺-ATPase activity; short-circuit current; methyl--cyclodextrin; channel open probability

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