Modulation of Basal and Peptide Hormone-Stimulated Na+ Transport by Membrane Cholesterol Content in the A6 Epithelial Cell Line

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Abstract
These studies examined the effect of altering plasma membrane cholesterol on basal Na+ flux as well as on the natriferic responses to the peptide hormones, insulin and anti-diuretic hormone (ADH) in the A6 model renal cell line. Membrane cholesterol concentrations were depleted or enriched using methyl-beta-cyclodextrin (MbetaCD) or a MbetaCD/cholesterol inclusion complex respectively. Effects of changes in the apical and basolateral plasma membranes were examined independently. Apical membrane cholesterol removal or supplementation had no effect on the basal Na+ transport rate. Short-term apical membrane cholesterol supplementation also had no effect on insulin-stimulated Na+ transport or on the initial phase of the ADH response. Interestingly, the additional apical membrane cholesterol had an inhibitory effect on the ADH response after 30 minutes. Apical membrane cholesterol depletion partially inhibited the responses to both insulin and ADH. Conversely, supplementation of basolateral cholesterol caused a significant increase in basal Na+ flux. Removal of cholesterol from the basolateral plasma membrane caused a decrease in basal Na+ flux with a time course analogous to channel turnover and completely inhibited peptide hormone responses. None of the changes in membrane cholesterol content decreased transcellular resistance. These results indicate an important role for membrane cholesterol content in the regulation of ENaC-mediated Na+ uptake.