24R,25-Dihydroxyvitamin D3, also known as 24(R),25(OH)2D3, is a vitamin D metabolite; a dihydroxylated form of the seco-steroid. With the identification of a target cell, the growth plate resting zone (RC) chondrocyte, studies indicate that there are specific membrane-associated signal transduction pathways that mediate both rapid, nongenomic, and genomic responses of RC cells to 24(R),25(OH)2D3. These studies indicate that 24(R),25(OH)2D3 plays an important role in endochondral ossification by regulating less mature chondrocytes and promoting their maturation in the endochondral lineage. 24(R),25(OH)2D3 binds RC chondrocyte membranes with high specificity, increasing protein kinase C (PKC) activity. The effect is stereospecific; 24R,25(OH)2D3, but not 24S,25(OH)2D3, causes the increase, indicating a receptor-mediated response. Phospholipase D-2 (PLD2) activity is increased, resulting in increased production of diacylglycerol (DAG), which in turn activates PKC. 24(R),25(OH)2D3 does not cause translocation of PKC to the plasma membrane but activates existing PKCα. There is a rapid decrease in Ca2+ efflux, and the influx is stimulated. 24(R),25(OH)2D3 also reduces arachidonic acid release by decreasing phospholipase A2 (PLA2) activity, thereby decreasing the available substrate for prostaglandin production via the action of cyclooxygenase-1. PGE2 that is produced acts on the EP1 and EP2 receptors expressed by RC cells to downregulate PKC via protein kinase A, but the reduction in PGE2 decreases this negative feedback mechanism. Both pathways converge on MAP kinase, leading to new gene expression. One consequence of this is the production of new matrix vesicles containing PKCα and PKCγ, and an increase in PKC activity. The chondrocytes also produce 24(R),25(OH)2D3, and the secreted metabolite acts directly on the matrix vesicle membrane. Only PKCγ is directly affected by 24(R),25(OH)2D3 in the matrix vesicles, and activity of this isoform is inhibited. This effect may be involved in the control of matrix maturation and turnover. 24(R),25(OH)2D3 causes RC cells to mature along the endochondral developmental pathway, where they become responsive to 1α,25(OH)2D3 and lose responsiveness to 24(R),25(OH)2D3, a characteristic of more mature growth zone (GC) chondrocytes. 1α,25(OH)2D3 elicits its effects on GC through different signal transduction pathways than those used by 24(R),25(OH)2D3 (PMID: 11179745).