Vitamin D(3) increased intestinal Na/Pi-IIb and CYP27B1 mRNA level in rats fed low-phosphorus diets.

The objective of the study was to determine the role of vitamin D(3) (VD(3)) in regulating adaptation and mechanism of rats to low-phosphorus (P) diets. Rats were assigned to 4 diets containing 0.2%, 0.4%, 0.6%, or 0.8% P consisting of 5 replicate cages with 6 rats per replicate cage and fed for 7 days. Four rats from each replicate cage were treated with ethane-1-hydroxy-1,1-diphosphonicacid, tetrasodium salt (EHDP) and 2 rats remained untreated. Twelve hours prior to preparation on d 7, two of the EHDP-treated rats received an intraperitoneal injection of VD(3) [1,25-(OH)(2)D(3)] at 600 ng per kg body weight, while two rats did not receive the injection. Rats that did not receive VD(3) injection had decreased (P < 0.001) P absorption, but injection of VD(3) resulted in increased (P < 0.001) absorption. The effect of VD(3) injection was greater (P < 0.001) for rats fed 0.2% P diet than rats fed 0.8% P diet in ileum. Sodium dependent phosphate cotransporter type ⠡b (Na/Pi-II b) and 25-hydroxyvitamin D 1-α hydroxylase (CYP27B1) mRNA level showed the same trend with P absorption. Serum concentration of VD(3) and 1α-hydroxylase activity in rats fed 0.2% P diet were lower than those fed 0.8% P diet. The injection of VD(3) increased (P < 0.001) serum concentration of VD(3) and 1α-hydroxylase activity. Thus, VD(3) increased Na/Pi-IIb and CYP27B1 mRNA level and improved serum concentration of VD(3) and 1α-hydroxylase activity in rats fed low-P diets.