ONTOGENY OF INTESTINAL PHOSPHATE ABSORPTION IN RABBITS

The ontogeny of intestinal phosphate transport was examined in brush-border membrane vesicles prepared from 2-wk, 4-wk, 6-wk, and 3-mo-old rabbits. At all four ages, vesicles prepared from the duodenum demonstrated sodium-phosphate cotransport in that uptake in the presence of a sodium gradient was significantly greater than uptake in the presence of a potassium gradient. While sodium-independent phosphate uptake was comparable at all ages, sodium-phosphate cotransport was greatest in the youngest animals and declined with increasing age. Peak phosphate uptake was 10-fold the equilibrium value at 2 wk and declined to 3-fold in adults. Maximal transport capacity of sodium-phosphate cotransport fell from 2,292 +/- 161 at 2 wk to 286 +/- 12 pmol.mg-1.10 s-1 at 3 mo, whereas the Michaelis constant did not change with age, varying between 0.032 and 0.054 mM. At all ages, uptake was half-maximally stimulated between 30 and 50 mM sodium, and Hill coefficients were between 1.5 and 2. Generation of inside negative diffusion potentials did not significantly enhance phosphate uptake at any age. These data suggest that throughout development, the transport of each phosphate molecule across the rabbit duodenal brush-border membrane is coupled to the transport of two sodium molecules. There were distinct developmental changes in the distribution of the sodium-phosphate cotransport along the length of intestine. In 2-wk old animals, sodium-phosphate cotransport was present in the duodenum, proximal and distal jejunum, and proximal ileum. In 4-wk-old animals, sodium-phosphate cotransport was present throughout the duodenum and proximal and distal jejunum, and in 6-wk and 12-wk-old animals, sodium-phosphate cotransport was observed only in the duodenum and proximal jejunum.