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

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Abstract
The plasma disappearance rate of 1,25‐dihydroxyvitamin D3 [1,25‐(OH)2 D3] 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.