In a study of the rat intestinal Pi transport system, an activator protein for rat Na/Pi co-transport system (PiUS) was isolated and characterized. We also investigated the effects of restriction of vitamin D and Pi (two of the most important physiological and pathophysiological regulators of Pi absorption in the small intestine) on intestinal Pi transport activity and the expression of Na/Pi co-transporters that are expressed in rat small intestine. Rat PiUS encodes a 424-residue protein with a calculated molecular mass of 51463 Da. The microinjection of rat PiUS into Xenopus oocytes markedly stimulated Na+-dependent Pi co-transport activity. In rats fed with a low-Pi diet, Na+-dependent Pi co-transport activity was increased approx. 2-fold compared with that of rats fed a normal Pi diet. Kinetic studies demonstrated that this increased activity was due to an elevation of Vmax but not Km. The PiUS mRNA levels showed an approximate doubling in the rats fed with the low-Pi diet compared with those fed with the normal Pi diet. In addition, after the administration of 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] to vitamin D-deficient animals, the Pi uptake was significantly increased in the Na+-dependent component in the brush border membrane vesicle (BBMV) at 24 and 48 h. In addition, we found a further high-affinity Na/Pi co-transport system in the BBMV isolated from the vitamin D-replete animals. The levels of type III Na/Pi co-transporter PiT-2 mRNA were increased 24 and 48 h after 1,25-(OH)2D3 administration to vitamin D-deficent animals, whereas PiUS and the type IIb Na/Pi co-transporter mRNA levels were unchanged. In conclusion, we first cloned a rat activator protein, PiUS, and then studied its role along with that of other type III Na/Pi co-transporters. PiUS and PiT-2 might be important components in the regulation of the intestinal Pi transport system by Pi restriction and 1,25-(OH)2D3.

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