Parathyroid hormone-related protein and Indian hedgehog control the pace of cartilage differentiation

Introduction Parathyroid hormone (PTH) controls calcium homeostasis through its actions on kidney and bone to raise blood calcium. Calcium, in turn, suppresses PTH secretion from the parathyroid gland. This negative feed-back loop (Fig. 1) serves to maintain the constancy of blood calcium from minute to minute. The receptor that mediates the actions of PTH is a G-protein-linked receptor that also mediates actions of parathyroid hormone related protein (PTHrP). The normal functions of PTHrP are still incompletely understood, but are likely to include predominantly paracrine actions (Broadus & Stewart 1994). PTHrP is synthesized by many tissues from the earliest stages of development and throughout life, although the blood levels of PTHrP are usually quite low. PTH/PTHrP receptors are often found near the sites of synthesis of PTHrP, and PTHrP has been shown to act on tissues in which PTHrP is synthesized. How one receptor can mediate the calcium homeostatic