Age-Dependent Regulation of Rat Intestinal Sodium-Phosphate Cotransporter (NaPi-IIb) by 1,25-(OH)2 Vitamin D3

The current studies were designed to characterize type IIb sodium-phosphate cotransporter (NaPi-IIb) expression and to assess the effect of 1,25-(OH)2 vitamin D3 (vit-D3) on NaPi-IIb gene expression during rat ontogeny. Sodium-dependent Pi absorption by intestinal brush-border membrane vesicles (BBMV) decreases with age, and NaPi-IIb gene expression also decreases proportionally with age. Vit-D3 treatment increased intestinal BBMV Pi absorption by ~2.5 fold in suckling rats and by ~2.1 fold in adult rats. Vit-D3 treatment also increased NaPi-IIb mRNA abundance by ~2 fold in 14-day-old rats, but had no effect on mRNA expression in adults. Furthermore, in rat intestinal epithelial (RIE) cells, vit-D3 increased NaPi-IIb mRNA abundance, an effect that was abolished by actinomycin D. Additionally, human NaPi-IIb gene promoter activity in transiently transfected RIE cells showed ~1.6-fold increase after vit-D3 treatment. In conclusion, we demonstrate that the age-related decrease in intestinal Na/Pi absorption correlates with decreased NaPi-IIb mRNA expression. Our data also suggest that the vit-D3 effect on NaPi-IIb expression is at least partially mediated by gene transcription in suckling rats.