Bone Remodeling and Calcium Metabolism

Abstract

Bones are constantly dissolved by osteoclasts and remineralized by osteoblasts in response to mechanical forces. Osteoclasts possess an acidic compartment and pass demineralized bone products to the periosteum (Sect. 1). They develop in stress-induced bony microcracks and are activated by differentiation factors secreted by osteoblasts, especially after menopause. Menopausal osteoporosis is controlled by drugs that are a stable form of pyrophosphate (bisphosphonate) or cathepsin K inhibitors (Sect. 2). The calcium ion concentration of blood is raised by parathyroid hormone and a vitamin D derivative called calcitriol. Parathyroid hormone causes kidneys to excrete phosphate, retain calcium, and activate calcitriol production (Sect. 3). Calcitriol induces calcium transporter proteins in osteoclasts and intestinal epithelium, where they move calcium from bone or diet into blood (Sect. 4). The chapter concludes with a discussion of calcitonin which lowers blood calcium concentrations by reversing parathyroid hormone effects on the kidney and inhibiting osteoclast activity (Sect. 5).