Renal apical membrane cholesterol and fluidity in regulation of phosphate transport

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Abstract
Renal proximal tubule cells adapt to dietary phosphate (Pi) restriction by increasing Pi transport independent of parathyroid hormone, vitamin D metabolites or serum Ca2+. To determine the underlying cellular mechanism(s), brush border (BBM) and basolateral membranes (BLM) were isolated from growing male rats fed a synthetic diet containing variable levels of Pi (0.1-1.4%). Dietary Pi restriction was without effect on either BBM or BLM total lipid P, individual phospholipid species, or BLM Na+-K+-ATPase specific activity. Dietary Pi restriction of (0.1 vs. 1.0%) did cause a significant reduction in BBM but not BLM cholesterol (0.45 vs. 0.41 .mu.mol/mg protein). Brush border membrane cholesterol was inversely correlated with the tubular reabsorption of Pi (r = 0.77, P < 0.01) over a broad range (99.9-46.2%). Arrhenius analysis of 2 intrinsic BBM enzymes revealed a significant reduction in the breakpoint temperature for alkaline phosphatase but no change for Mg+-ATPase. Fluorescence polarization studies showed increased BBM inner core fluidity due to an alteration in neutral lipids but not phospholipid, fatty acid or protein membrane components. The BBM can regulate its cholesterol content independent of the BLM. Adaptation to dietary Pi restriction involves a reduction in BBM cholesterol, which may be mediated by an increase in membrane fluidity.

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