Effects of Ca2+ sensing receptor activation in the growth plate.

The Ca2+-sensing receptor (CaR) is a G protein-coupled receptor expressed in many mammalian tissues, including the long bone's growth plate. CaR knockout mice exhibit growth retardation, suggesting that CaR may promote skeletal growth. However, the complex phenotype of these knockout mice, which includes hyperparathyroidism, hypercalcemia, and hypophosphatemia, may confound the effects of CaR activation. To determine whether CaR regulates growth plate chondrogenesis and longitudinal bone growth, we chose an organ culture model. Fetal rat metatarsal bones (dpc 20) were cultured in serum-free medium for 7 days in the presence or absence of NPS-R-568, a CaR agonist. The addition of 10 nM NPS-R-568 increased the cumulative longitudinal growth of the metatarsal explants. To explore the underlying mechanisms, we then assessed the effects of NPS-R-568 on growth plate chondrocyte hypertrophy/differentiation and chondrocyte proliferation. After 7 days in culture, NPS-R-568 increased the height of the growth plate hypertrophic zone and the expression of collagen X, a marker of growth plate chondrocyte differentiation (assessed by immunohistochemistry). NPS-R-568 also induced a significant increase of the height of the growth plate proliferative zone and of the total thymidine incorporation in the metatarsal bone. In conclusion, our findings suggest that the activation of CaR in the growth plate accelerates longitudinal bone growth by stimulating growth plate chondrogenesis.