Phosphate transport across the basolateral membrane from rat kidney cortex: sodium-dependence?

Area of Science:

• Renal Physiology
• Membrane Transport
• Cell Biology

Background:

• Basolateral membrane vesicles are crucial for understanding solute transport in renal proximal tubules.
• Previous studies suggested sodium-dependent phosphate transport in these vesicles, but contamination was a concern.

Purpose of the Study:

• To clarify the mechanism of phosphate and D-glucose transport across the basolateral membrane of rat renal proximal tubules.
• To investigate the role of sodium gradients and potential brush border membrane contamination in phosphate transport.

Main Methods:

• Isolation of basolateral membrane vesicles from rat renal proximal tubules using Percoll-centrifugation.
• Further purification of the basolateral membrane fraction using free-flow electrophoresis.
• Assay of enzyme markers for basolateral and brush border membranes to assess purity.
• Measurement of phosphate and D-glucose transport under various conditions (e.g., sodium gradients).

Main Results:

• Percoll-isolated vesicles showed sodium-gradient-stimulated phosphate transport, but D-glucose transport was minimally affected.
• Free-flow electrophoresis separated basolateral and brush border markers, correlating sodium-dependent phosphate transport with brush border markers.
• Sodium-independent, carrier-mediated D-glucose uptake followed basolateral markers, while sodium-dependent D-glucose uptake followed brush border markers.
• Sodium-dependent phosphate uptake was inhibited by D-glucose, an effect reversed by phlorizin.

Conclusions:

• The observed sodium-dependent phosphate uptake in the Percoll-basolateral membrane fraction is attributed to brush border membrane contamination.
• Phosphate translocation across the true basolateral membrane is carrier-mediated and independent of sodium.
• D-glucose transport mechanisms differ between the brush border (sodium-dependent) and basolateral (sodium-independent) membranes.