Binding properties of plasma vitamin D-binding protein and intestinal 1,25-dihydroxyvitamin D 3 receptor in piglets with pseudo-vitamin D-deficiency rickets, type I: Treatment effects with pharmacological doses of vitamin D 3

Abstract

The effective treatment of the rachitic symptoms of pseudo-vitamin D-deficiency rickets, type I (PVDRI) by massive doses of vitamin D 3 was examined. For this purpose, the affinities and the maximum binding capacities ( B max) of the plasma vitamin D-binding protein (DBP) and of the intestinal 1,25-dihydroxyvitamin D 3 (1,25- (OH) 2D 3) receptor for vitamin D 3, 25-hydroxyvitamin D 3 (25-OHD 3) and 1,25-(OH) 2D 3, were investigated in normal piglets and in rachitic piglets that suffered from PVDRI. The piglets were 5 to 10 weeks old and of both sexes. The B max of plasma DBP for 25-OHD 3 was 6.77 ± 0.45 μ m for PVDRI piglets and 7.30 ± 0.41 μ m for control piglets and showed no differences between the two groups. Equilibrium association constants ( K a ) of DBP for 25-OHD 3 were 4.3 × 10 8 m −1 for PVDRI piglets and 4.0 × 10 8 m −1 for controls and showed also no differences between the two groups. Similarly the K a of DBP for 1,25-(OH) 2D 3 was also the same for rachitic and control piglets (1.45 × 10 7 and 1.54 × 10 7 m −1, respectively). Due to the lower circulating concentration of 1,25-(OH) 2D 3 in the plasma of rachitic piglets compared to that of controls its free metabolite index was significantly lower in rachitic (0.42 ± 0.05 × 10 −5) than in control piglets (3.63 ± 0.30 × 10 −5). The K d and B max of the intestinal nuclear receptor for 1,25-(OH) 2D 3 of rachitic and control piglets were 0.31 ± 0.05 and 0.33 ± 0.05 n m and 674 ± 103 and 719 ± 123 fmol/mg protein, respectively, and were also not different between the two groups of piglets. It was concluded from these observations that the rachitic symptoms of PVDRI piglets resulted solely from the lower free 1,25-(OH) 2D 3 concentration in plasma compared to that of normal piglets. The relative affinities of the intestinal 1,25-(OH) 2D 3 receptor for vitamin D 3 and 25-OHD 3 were also measured. It was found that 50% displacement of 1,25-(OH) 2D 3 from the intestinal receptor of PVDRI and control piglets required a 220,000- and 245,000-fold excess of the free concentration of vitamin D 3, respectively, and a 20- to 42- and 23- to 71-fold excess of the free concentration of 25-OHD 3, respectively. Treatment of PVDRI piglets with an effective dose of 7.5 × 10 4 IU of vitamin D 3 led to a transient 1073-fold excess of the free concentration of vitamin D 3 and a 59-fold excess of the free concentration of 25-OHD 3 over free 1,25-(OH) 2D 3 in plasma. Since a 59-fold excess of free 25-OHD 3 in plasma of treated piglets could cause at least 50% displacement of 1,25-(OH) 2D 3 from the intestinal 1,25-(OH) 2D 3 receptor it is suggested that after administration of pharmacological amounts of vitamin D 3 to PVDRI piglets both 25-OHD 3 and 1,25-(OH) 2D 3 acted upon the intestinal 1,25-(OH) 2D 3 receptor. The two metabolites probably exerted a coordinate effect in healing the rachitic symptoms.