Sodium-dependent phosphate transport across the apical membrane of alveolar epithelium in caprine mammary gland.

NaPi IIb cotransporter is expressed in various tissues including mammary glands of mice. The physiological role of NaPi IIb in lactating mammary glands is still unclear. Therefore, it was the aim of the study to detect and to localize NaPi IIb protein in lactating goat mammary glands by Western analysis and immunohistochemistry. Furthermore, Na(+)-dependent P(i) uptake into apical membrane vesicles isolated from goat milk was determined using rapid filtration technique. NaPi IIb protein could specifically be detected in the apical membranes of lactating alveolar epithelial cells. Na(+)-dependent P(i) uptake into apical membrane vesicles could be measured, which was inhibited by phosphonoformic acid. The kinetic parameters were V(max) with 0.9 nmol/mg protein/10 s and K(m) with 0.22 mmol/L for P(i) affinity, K(m) value for Na(+) affinity 11 mmol/L. Stoichiometry of this mammary gland Na(+)/P(i) transport across the apical membranes seemed to be 1:1 P(i):Na(+) without cooperativity in P(i) and Na(+) binding as assessed by Scatchard and Hill plots. These features of Na(+)/P(i) transport suggest that it could be mediated by NaPi IIb. The quantitative role of this P(i) transport which is directed from the alveolar lumen into the epithelial cell of goat mammary gland will be the topic of further investigations.