Metabolism and Excretion of3H-1,2 5-(OH)2-Vitamin D3in Healthy Adults*

Abstract

The synthesis of very high specific activity 25-OH-vitamin D3 (78 Ci/mmol) has made possible the study of the metabolism and plasma disappearance of 3H after a single dose of 3H-1,25-(OH)2-D3 in quantities that are only 10-20% of the endogenous plasma pool. Seven healthy adults who were given doses of 1,25-(OH)2-D3 ranging 30-2300 pmol were studied. Plasma disappearance was rapid with only 14 .+-. 2% of administered 3H remaining in the plasma pool 4 h after labeling. Plasma metabolite profiles during the 1st 4 h showed only 1,25-(OH)2-D3. Significant amounts of other metabolites were detected. The 6 day cumulative excretion of 3H in urine and feces (virtually all associated with metabolites of 1,25-(OH)2-D3) averaged 16 .+-. 3% and 49 .+-. 11% of the dose, respectively. Compartmental analysis of the isotope data for 2 subjects who received the smallest doses of 1,25-(OH)2-D3 indicated that endogenous renal 1,25-(OH)2-D3 synthesis rates approximate 0.8-2.4 nmol/day (0.3-1.0 .mu.g/day).