Regulation of Intestinal Phosphate Transport II. Metabolic acidosis stimulates Na+-dependent phosphate absorption and expression of the Na+-Pi cotransporter NaPi-IIb in small intestine

During metabolic acidosis, Pi serves as an important buffer to remove protons from the body. Pi is released from bone together with carbonate buffering protons in blood. In addition, in the kidney, the fractional excretion of phosphate is increased allowing for the excretion of more acid equivalents in urine. The role of intestinal Pi absorption in providing Pi to buffer protons and compensating for loss from bone during metabolic acidosis has not been clarified yet. Inducing metabolic acidosis (NH4Cl in drinking water) for 2 or 7 days in mice increased urinary fractional Pi excretion twofold, whereas serum Pi levels were not altered. Na+-dependent Pi transport in the small intestine, however, was stimulated from 1.89 ± 3.22 to 40.72 ± 11.98 pmol/mg protein (2 days of NH4Cl) in brush-border membrane vesicles prepared from total small intestine. Similarly, the protein abundance of the Na+-dependent phosphate cotransporter NaPi-IIb in the brush-border membrane was increased 5.3-fold, whereas mRNA levels remained stable. According to immunohistochemistry and real-time PCR NaPi-IIb expression was found to be mainly confined to the ileum in the small intestine, and this distribution was not altered during metabolic acidosis. These results suggest that the stimulation of intestinal Pi absorption during metabolic acidosis may contribute to the buffering of acid equivalents by providing phosphate and may also help to prevent excessive liberation of phosphate from bone.