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Articles in PresS, published online ahead of print September 11, 2002
Am J Physiol Renal Physiol, 10.1152/ajprenal.00216.2002
Submitted on June 6, 2002
Accepted on September 4, 2002
1 Laboratory of Physiology and Physiopathology, Universite Libre de Bruxelles, Brussels, Belgium
* To whom correspondence should be addressed. E-mail: sohraby{at}ulb.ac.be.
The activity of epithelial sodium selective channels is modulated by various factors, with growing evidence that membrane lipids also participate in the regulation. In the present study, Triton-X extracts of whole cells and of apical membrane-enriched preparations from cultured A6 renal epithelial cells were floated on continuous sucrose density gradients. Na+ channel protein, probed by immunostaining of western blots, was detected in the high-density fractions of the gradients (between 18 and 30% sucrose), which contain the detergent-soluble material but also in the lighter, detergent-resistant 16% sucrose fraction. Single amiloride-sensitive Na+ channel activity, recorded after incorporation of reconstituted proteoliposomes into lipid bilayers, was exclusively localized in the 16% sucrose fraction. In accord with other studies, high and low density fractions of sucrose gradients likely represent membrane domains with different lipid content. However, exposure of the cells to cholesterol-depleting or sphingomyelin-depleting agents did not affect transepithelial Na+ current, single Na+ channel activity, or the expression of the sodium channel protein. This is the first reconstitution study of native epithelial sodium channels, which suggests that functional channels are compartmentalized in discrete domains within the plane of the apical cell membrane.
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