Cholesterol modulates rat renal brush border membrane phosphate transport

Area of Science:

• Nephrology
• Cell Biology
• Biochemistry

Background:

• Dietary phosphate deprivation and aging correlate with reduced renal proximal tubular brush border membrane (BBM) cholesterol, decreased BBM fluidity, and impaired sodium gradient-dependent phosphate (Pi) transport (Na-Pi cotransport).
• The precise role of cholesterol in regulating this transport process remains unclear.

Purpose of the Study:

• To investigate whether in vitro enrichment of renal BBM with cholesterol directly modulates Na-Pi cotransport activity.

Main Methods:

• Renal BBM were enriched with cholesterol in vitro at 12 mol% and 24 mol% increments.
• Na-Pi cotransport activity was measured.
• BBM fluidity was assessed using fluorescence anisotropy of diphenylhexatriene (rDPH).
• Experiments were conducted at varying temperatures and with BBM from rats fed a low Pi diet.

Main Results:

• Cholesterol enrichment dose-dependently decreased Na-Pi cotransport activity (2,000 in control vs. 900 pmol/5 s/mg BBM protein at +24% cholesterol).
• This decrease was paralleled by a dose-dependent increase in fluorescence anisotropy (decrease in BBM fluidity).
• Increased temperature enhanced Na-Pi cotransport, while cholesterol enrichment still reduced transport at equivalent fluidity.
• In vitro cholesterol enrichment reversed adaptive increases in Na-Pi cotransport and fluidity in BBM from rats on a low Pi diet.

Conclusions:

• Cholesterol directly modulates renal BBM Na-Pi cotransport activity.
• In vivo alterations in BBM cholesterol content are likely important in regulating renal tubular Pi transport.