PREFERENTIAL ACCUMULATION IN-VIVO OF 24R,25-DIHYDROXYVITAMIN D-3 IN GROWTH-PLATE CARTILAGE OF RATS

Vitamin D-3 is metabolized in vivo through 25-(OH)D-3 (25D) to both 1 alpha,25-(OH)(2)D-3 (1,25D) and 24R,25-(OH)(2)D-3 (24,25D). Whereas it is assumed that this metabolism occurs primarily in the kidney, recen t studies show that there are extrarenal 1 alpha- and 24R-hydroxylase activities as well, and in chondrocytes, these enzymes are regulated b y hormones and growth factors. Furthermore, chondrocytes from the rest ing zone of growth plate cartilage are a target cell population for 24 ,25D action, suggesting that this vitamin D metabolite may be targeted to this tissue in vivo. To test this hypothesis, 30 normal male Sprag ue Dawley rats (120 +/- 20 g) were divided into three groups of eight animals each, and a control group of six animals, and fed ad libitum f or 2 wk, a standard rat chow (Teklad LM-485), which contained 3 IU vit amin D-3/g. The rats were then injected im daily at 9:00 AM, for 4 con secutive d, with 0.1 mL of either [H-3]-25D, [H-3]-1,25D or [H-3]-24,2 5D. Each dose contained 13 pmol of hormone (0.36 mu Ci/dose). The dist ribution of these metabolites was assessed in tibial bone (B) followin g ablation of the bone marrow, articular cartilage from the tibia (AC) , costochondral growth plate cartilage (GC), serum (S), small intestin e (I), and kidney (K). The use of high specific activity tritiated vit amin D metabolites facilitated determining tissue localization and fur ther metabolism without perturbation of the body pools of each major m etabolite. Accumulation of [H-3]-1,25D or [H-3]-24,25D in each tissue was compared to circulating serum levels. In rats dosed with [H-3]-25D , the tissue:serum ratios for 1,25D were 4.1 (AC), 35.4 (GC), 1.3 (B), 0.7(K), and 3.0 (I); and tissue:serum ratios for 24,25D were 1.6 (AC) , 9.9 (GC), 0.04 (B), 0.2 (K), and 0.4 (1). In rats dosed with [H-3]-2 4,25D alone, GC was the only tissue to accumulate the administered met abolite at a concentration significantly higher than that of serum. Si milarly, in rats dosed with [H-3]-1,25D alone, GC was the only tissue to accumulate 1,25D at a concentration higher than that of serum. Thes e results demonstrate, for the first time, that under in vivo conditio ns, CC specifically accumulates 24,25D and 1,25D. This suggests that g rowth plate may be a target organ for these two hormones.