Anabolic actions of PTH in the skeletons of animals.

A brief historical perspective reviews studies that tested the hypotheses that PTH induces an anabolic effect in bone, and that the gain in trabecular bone was not at the expense of cortical bone. As PTH reduces the risk of fracture in humans with osteoporosis, the myths that postulated cortical bone porosity and increased bone turnover might increase fracture risk, are examined in the light of data from animals with osteonal bone. These show that PTH "braces" the bone by immediately stimulating bone formation at modeling and remodeling sites. Increased porosity is a late event, occurring close to the neutral axis of bone where detrimental effects on biomechanical strength are unlikely. PTH increases bone mass by stimulating modeling in favor of bone formation, and restructures bone geometry via more extensive remodeling. Cell and genetic events induced in bone by PTH have been studied in rats and are time- and regimen-dependent. In addition to the stimulation of gene expression for matrix proteins, early genes upregulated by once daily PTH are those associated with matrix degradation and induction of osteoclastic resorption, indicative of possible mechanisms by which PTH may increase bone turnover. Boneforming surfaces are increased due to increased numbers of newly differentiated osteoblasts and retention of older osteoblasts by inhibition of apoptosis. After stopping treatment, the number of osteoblasts is quickly reduced and bone turnover returns to that of controls, slowing both bone formation and resorption. The increased proportion of bone undergoing PTH-induced remodeling requires maturation and completion of mineralization. These responses may explain the delay in reversal of gains in bone mass and biomechanical properties for at least two turnover cycles following withdrawal in large animal models. Thus, the skeletal benefits of PTH extend beyond the active treatment phase.