Acute and chronic changes in cholesterol modulate Na-Picotransport activity in OK cells

Abstract

We previously showed an inverse correlation between membrane cholesterol content and Na-Picotransport activity during the aging process and adaptation to alterations in dietary Piin the rat (Levi M, Jameson DM, and van der Meer BW. Am J Physiol Renal Fluid Electrolyte Physiol 256: F85–F94, 1989). The purpose of the present study was to determine whether alterations in cholesterol content per se modulate Na-Picotransport activity and apical membrane Na-Piprotein expression in opossum kidney (OK) cells. Acute cholesterol depletion achieved with β-methyl cyclodextrin (β-MCD) resulted in a significant increase in Na-Picotransport activity accompanied by a moderate increase in apical membrane Na-Piprotein abundance and no alteration of total cellular Na-Piprotein abundance. Conversely, acute cholesterol enrichment achieved with β-MCD/cholesterol resulted in a significant decrease in Na-Picotransport activity with a moderate decrease in apical membrane Na-Pi protein abundance and no change of the total cellular Na-Piprotein abundance. In contrast, chronic cholesterol depletion, achieved by growing cells in lipoprotein-deficient serum (LPDS), resulted in parallel and significant increases in Na-Picotransport activity and apical membrane and total cellular Na-Piprotein abundance. Cholesterol depletion also resulted in a significant increase in membrane lipid fluidity and alterations in lipid microdomains as determined by laurdan fluorescence spectroscopy and imaging. Chronic cholesterol enrichment, achieved by growing cells in LPDS followed by loading with low-density lipoprotein, resulted in parallel and significant decreases in Na-Picotransport activity and apical membrane and total cellular Na-Piprotein abundance. Our results indicate that in OK cells acute and chronic alterations in cholesterol content per se modulate Na-Picotransport activity by diverse mechanisms that also include significant interactions of Na-Piprotein with lipid microdomains.