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, recent studies show that there are extrarenal 1 alpha- and 24R-hydroxylase activities as well, and in chondrocytes, these enzymes are regulated by hormones and growth factors. Furthermore, chondrocytes from the resting 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 Sprague 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 for 2 wk, a standard rat chow (Teklad LM-485), which contained 3 IU vitamin D-3/g. The rats were then injected im daily at 9:00 AM, for 4 consecutive d, with 0.1 mL of either [H-3]-25D, [H-3]-1,25D or [H-3]-24,25D. Each dose contained 13 pmol of hormone (0.36 mu Ci/dose). The distribution of these metabolites was assessed in tibial bone (B) following ablation of the bone marrow, articular cartilage from the tibia (AC), costochondral growth plate cartilage (GC), serum (S), small intestine (I), and kidney (K). The use of high specific activity tritiated vitamin D metabolites facilitated determining tissue localization and further metabolism without perturbation of the body pools of each major metabolite. 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]-24,25D alone, GC was the only tissue to accumulate the administered metabolite at a concentration significantly higher than that of serum. Similarly, 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. These results demonstrate, for the first time, that under in vivo conditions, CC specifically accumulates 24,25D and 1,25D. This suggests that growth plate may be a target organ for these two hormones.
