Extraction of vitamin D metabolites by bones of normal adult dogs

Vitamin D3 uptake by the isolated perfused rat liver from lipoprotein fractions is separate from cholesterol and triglyceride uptake

Using the isolated perfused rat liver we examined the uptake of [14C] or [3H] vitamin D3 and [14C] triglycerides or [3H] cholesterol by the liver of normal rats, from different lipoprotein fractions, as measured by disappearance from the perfusate. When incorporated into chylomicrons only 43% of the vitamin D3 remained in the perfusate at 60 min (i.e. 57% uptake) as compared to 68% of the triglycerides (i.e. 32% uptake). When added on very low density lipoproteins (VLDL) at 60 min 37 +/- 3% (n = 6) of the vitamin D3, 38 +/- 2% of the cholesterol (n = 3) (P NS), and 83 +/- 4% of the triglycerides (n = 3) remained in the perfusate (P less than 0.0005 for cholesterol:triglycerides and vitamin D3:triglycerides). For high density lipoprotein fraction (HDL) isolated perfused livers were studied with and without albumin present in the perfusate. Without albumin 19 +/- 8% (n = 3) of the vitamin D3 and 43 +/- 8% (n = 3) of the cholesterol remained in the perfusate at 60 min. The results with albumin present were 40 +/- 1% (n = 5) of the vitamin D3 and 63 +/- 4% (n = 5) of the cholesterol at 60 min (P less than 0.0005). The cholesterol:cholesterol ester ratio of the VLDL fraction was 8.8:1 and of the HDL fraction 1:1.4. There was no metabolism of vitamin D3 during the 1 h perfusion. These results suggest that vitamin D3 is in equilibrium with the lipoprotein surface, and the hepatic uptake of vitamin D is a surface phenomenon independent of lipoprotein metabolism.

Restricted transport of vitamin D and A derivatives through the rat blood-brain barrier

The present studies measure the transport of retinol, retinoic acid, 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3], and 25-hydroxyvitamin D3 [25-(OH)D3] through the rat brain capillary endothelial wall, i.e., the blood-brain barrier (BBB). The vitamin A and D derivatives bind both to albumin and to specific high-affinity binding proteins in plasma. In the presence of physiologic concentrations of plasma proteins, the extraction by brain of all four compounds was 5% or less.