Modulation of apical Na+/Pi cotransporter type IIb expression in epithelial cells of goat mammary glands.

Recently, the type II sodium-dependent phosphate cotransporter NaPi IIb, which mediates the intestinal phosphate transport, was detected in the apical membranes of caprine mammary gland epithelial cells. Regulatory influences of developmental stages, dietary phosphorus (P) supply and hormones like calcitriol are well described for the intestinal NaPi IIb. Therefore, it was the aim of this study to examine the influence of involution and dietary P restriction on the expression of mammary gland NaPi IIb and of vitamin D receptor (VDR). During involution both, NaPi IIb and VDR, showed an initial increase of expression. This resulted in a delayed response on protein level. Dietary P restriction resulted in a decrease of mRNA expression which was not reflected on protein level. Influenced expression pattern, at least on mRNA level, indicate that mammary gland NaPi IIb is a regulated phosphate transporter which might have an important role especially during involution. Coexpression pattern with VDR provides an indication that calcitriol could be the modulator of these adaptive responses to involution and dietary P restriction. Therefore, a physiological meaning of NaPi IIb in mammary gland epithelia during processes of cell regeneration has to be considered.