The enterohepatic physiology of 24,25-dihydroxyvitamin D3

Products of intravenously administered [3H]24,25-dihydroxyvitamin D3 are excreted in the bile of normocalcemic, vitamin D-replete rats. Within 3 and 24 hr, 7.3% +/- 1.4 and 18.5% +/- 1.4 (mean +/- S.D.) of the administered dose appears in the bile. Three hours after the instillation of the biliary products of [3H]24,25-dihydroxyvitamin D3 into the duodena of other rats. 7.6% +/- 2.05 of the administered dose appears in newly secreted bile; at 24 hr 21.1% +/- 7.8 of the instilled dose is present in newly secreted bile. These data suggest that products of 24,25-dihydroxyvitamin D3 are excreted in bile and undergo an enterohepatic circulation. The metabolites of 24,25-dihydroxyvitamin D3 excreted in bile are much more polar than the parent sterol as assessed by silicic acid chromatography and by high-performance liquid chromatographic techniques. The products are retained on DEAE-cellulose in the presence of methanol are eluted upon the addition of acetic acid to the eluting solvent. The data support the existence of a quantitatively important enterohepatic circulation of polar metabolites of [3H]24,25-dihydroxyvitamin E3; disturbances in this metabolic pathway could help explain the pathogenesis of hepatic and intestinal osteodystrophy.