The effect of 22-oxacalcitriol on serum calcitriol

1,25-Dihydroxyvitamin D₃ (1,25D) regulates its own levels in circulation by affecting its rates of synthesis and degradation. 22-Oxacalcitriol (OCT), a vitamin D analog with low calcemic activity, decreases circulating PTH levels, one of the regulators of renal 1α-hydroxylase, and stimulates vitamin D degradation m vitro. The purpose of this study was to examine the effects of OCT administration on serum levels of 1,25D. In normal rats, OCT administration (4-200 ng, ip, daily for 5 days) caused a dose-dependent reduction in serum calcitriol levels. At a dose of 200 ng, OCT reduced serum 1,25D from 34.5 ± 2.7 to 10.9 ± 0.7 pg/ml (P ≤ 0.01) without significant changes in ionized Ca or phosphorus levels. The contribution of the suppression of PTH by OCT to the reduction of serum 1,25D was examined by administering OCT to parathyroidectomized (PTX) rats. Two hundred nanograms of OCT, ip, daily for 5 days significantly reduced serum calcitriol from 29.7 ± 7.6 to 9.1 ± 0.5 pg/ml (P < 0.01) in rats fed a normal calcium diet. Because OCT increased total calcium (TCa) in this group from 7.4 ± 0.1 to 9.5 ± 0.3 mg/dl, similar doses of OCT were given to PTX rats fed a calcium-deficient diet. OCT decreased 1,25D from 58.9 ± 8.9 to 10.3 ± 0.4 pg/ml and increased TCa from 4.8 ± 0.2 to 7.4 ± 0.1 mg/dl. Comparison of serum 1,25D for identical TCa levels in PTX rats (normal calcium diet controls vs. calcium-deficient diet, OCT-treated) clearly indicates that OCT per se reduced serum 1,25D. Further support for a direct effect of OCT was provided by studies in PTX rats fed a low phosphorus diet. OCT decreased serum 1.25D from 125.8 ± 15.6 to 10.9 ± 0.6 pg/ ml without significant changes in TCa. To further characterize the mechanisms involved in this effect, similar studies were performed in six normal dogs. Intravenous administration of 0.75 μg OCT every other day for 1 week decreased serum calcitriol from 25.4 ± 3.2 to 12.2 ± 1.3 pg/ml (P ≤ 0.002). Ionized Ca and phosphorus remained unchanged. Despite the short half-life of OCT in the circulation, 1,25D levels returned to basal concentrations 96 h after the last dose of OCT. Measurements of the MCR and production rate of calcitriol, using a single bolus injection of [³H]1,25D revealed that the decrease in serum calcitriol caused by OCT was the result of a suppressed production rate (from 0.23 ± 0.02 to 0.17 ± 0.02 μg/day; P ≤ 0.05) and a significant enhancement of MCR (from 6.7 ± 0.4 to 10.1 ± 1.3 ml/min; P < 0.05). The proportion of the injected [³H]1,25D converted to more polar water-soluble metabolites excreted in the urine in 10 h increased from a basal value of 0.69 ± 0.08% to 1.4 ± 0.13% (P ≤ 0.001) with OCT treatment. This supports an accelerated 1,25D breakdown. These results demonstrate that OCT blocks 1,25D production in normal animals by a mechanism independent of PTH suppression and accelerates 1,25D breakdown, with a concomitant reduction of serum calcitriol levels.