Intestinal Na-[P.sub.i] cotransporter adaptation to dietary [P.sub.i] content in vitamin D receptor null mice

Recent studies suggest that vitamin D may play a role in intestinal [Na.sup.+]-dependent phosphate transport adaptation to variable levels of dietary [P.sub.i]. Therefore, the goal of the current study was to assess [Na.sup.+]-dependent [P.sub.i] cotransport activity in transgenic mice to determine whether vitamin D is an essential mediator of this process. Intestinal brush-border membrane (BBM), [Na.sup.+]-dependent [P.sub.i] cotransport activity was significantly decreased in vitamin D receptor (VDR) null [VDR (-/-)] mice compared with wild-type (VDR+/+) mice. While intestinal Na-[P.sub.i] cotransporter (type IIb) mRNA levels were similar in VDR (-/-) and VDR (+/+) mice, type IIb Na-[P.sub.i] cotransporter protein expression was markedly suppressed in VDR (-/-) mice compared with VDR (+/+) mice. Furthermore, Na-[P.sub.i] cotransport activity in renal BBM was similar in VDR (-/-) and VDR (+/+) mice, but type IIa Na-[P.sub.i] cotransporter protein expression was decreased in VDR (-/-) mice. After administration of a low-[P.sub.i] diet, type IIb protein expression was significantly increased in VDR (+/+) and VDR (-/-) mice, and type IIb protein expression was present in the intestinal BBM of VDR (-/-) mice. These data demonstrate that intestinal Na-[P.sub.i] cotransport adaptation to a low-[P.sub.i] diet occurs independently of vitamin D. dietary phosphate; phosphate transport; intestine