Age-dependent regulation of rat intestinal type IIb sodium-phosphate cotransporter by 1,25-(OH)2 vitamin D3

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Abstract
The current studies were designed to characterize type IIb sodium-inorganic phosphate (Pi) cotransporter (NaPi-IIb) expression and to assess the effect of 1,25-(OH)2 vitamin D3 on NaPi-IIb gene expression during rat ontogeny. Sodium-dependent Pi absorption by intestinal brush-border membrane vesicles (BBMVs) decreased with age, and NaPi-IIb gene expression also decreased proportionally with age. 1,25-(OH)2 vitamin D3 treatment increased intestinal BBMV Pi absorption by ∼2.5-fold in suckling rats and by ∼2.1-fold in adult rats. 1,25-(OH)2vitamin 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, 1,25-(OH)2 vitamin 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 1,25-(OH)2 vitamin D3 treatment. In conclusion, we demonstrate that the age-related decrease in intestinal sodium-dependent Pi absorption correlates with decreased NaPi-IIb mRNA expression. Our data also suggest that the effect of 1,25-(OH)2 vitamin D3 on NaPi-IIb expression is at least partially mediated by gene transcription in suckling rats.

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