Effect of dietary mineral and vitamin D content and parathyroidectomy on the plasma disappearance rate of 1,25-dihydroxyvitamin D3 in rats.

The plasma disappearance rate of 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] was determined in rats after a single intravenous injection of the tritiated hormone. Tritiated 1,25-(OH)2D3 (120 Ci mmol-1) was administered to rats at a dose of 400 000 dpm kg-1 body weight and the animals were bled between 0 and 8 hours. The dose was estimated to produce negligible perturbations in endogenous plasma levels of 1,25-(OH)2D3. The plasma disappearance of 3H-1,25-(OH)2D3 occurred in two phases. The second phase plasma half-life of 1,25-(OH)2D3 in immature 25- to 35-day-old animals (4.7 hours) was significantly shorter than the second phase plasma half-life of maturing 49- to 65-day-old animals (8.0 hours). Phosphorus deprivation for 12 days significantly prolonged the second phase plasma half-life of 1,25-(OH)2D3 from a control value of 4.9 hours to 10.4 hours. Parathyroidectomy, regardless of the plasma calcium concentrations, shortened the second phase plasma half-life of 1,25-(OH)2D3 from control values of 9.1 hours to 5.0 hours. Calcium deprivation for 7 days did not alter the second phase plasma half-life of 1,25-(OH)2D3. Vitamin D deprivation for 5 weeks increased the second phase plasma half-life from 11.0 to 19.9 hours but the difference was not significant.