Intestinal phosphate absorption is mediated by multiple transport systems in rats

Abstract

Apical inorganic phosphate (Pi) transport in the small intestine seems to be mainly mediated by the sodium/Pi cotransporter NaPi2b. To verify this role, we have studied the combined effects of pH, phosphonoformate, and Pi deprivation on intestinal Pi transport. Rats were fed, ad libitum, three fodders containing 1.2, 0.6, or 0.1% Pi for 1, 5, or 10 days. Pi deprivation (0.1%) increased both sodium-activated and sodium-independent Pi transport in brush-border membrane vesicles from the duodenum and jejunum for all three times. Alkaline pH inhibited Pi transport, despite the increasing concentration of (Formula Present) (NaPi2b substrate), whereas acidity increased transport when the concentration of the PiT1/PiT2 substrate, (Formula Present), was at its highest. The effect of Pi deprivation was maximal at acid pH, but both basal and upregulated transport were inhibited (70%) with phosphonoformate, an inhibitor of NaPi2b. PiT2 and NaPi2b protein abundance increased after 24 h of Pi deprivation in the duodenum, jejunum, and ileum, whereas PiT1 required 5-10 days in the duodenum and jejunum. Therefore, whereas transporter expressions are partially correlated with Pi transport adaptation, the pH effect precludes NaPi2b, and phosphonoformic acid precludes PiT1 and PiT2 as the main transporters. Transport and transporter expression were also inconsistent when feeding was limited to 4 h daily, because the 1.2% Pi diet paradoxically increased Pi transport in the duodenum and jejunum, but NaPi2b and PiT1 expressions only increased with the 0.1% diet. These findings suggest the presence of a major transporter that carries H2PO4- and is inhibited by phosphonoformate. The combined effects of dietary inorganic phosphate (Pi) content, pH, and phosphonoformate inhibition suggest that the resulting apical Pi transport in the small intestine cannot be fully explained by the presence of NaPi2b, PiT1, or PiT2. We provide evidence of the presence of a new sodium-coupled Pi transporter that uses H2PO4-as the preferred substrate and is inhibited by phosphonoformate, and its expression correlates with Pi transport in all assayed conditions.