Figure 6. Mechanism of action of vitamin D₃ metabolites. Proposed pathways of action of 1,25-(OH)₂D₃ in growth zone chondrocytes (GC) and 24,25-(OH)₂D₃ in resting zone chondrocytes (RC). Phospholipase C (PLC), Phospholipase A₂ (PLA₂), arachidonic acid (AA), cyclooxygenase-1 (Cox-1), PGE₂ EP receptor (EP), phospholipase D (PLD), protein kinase C (PKC), protein kinase A (PKA).