Using sequential short interval tetracycline labeling, Tam et al. found an increase in dose-dependent stimulation of bone apposition rate in rats. Intact bovine parathyroid hormone [bPTH(1-84)] and a synthetic human parathyroid hormone [hPTH(1-34)] were tested. They confirmed that the 1-34 region of the hormone is responsible for the bone anabolic effect of PTH. They also determined that the anabolic effects of PTH that allow it to stimulate the formation of new bone function independently of the resorptive effects of the hormone.

Daily injection of PTH increased bone apposition rate without an increase in bone resorption, resulting in a net increase in trabecular bone volume. Continuous administration of PTH increased both bone formation and resorption, with a net decrease in trabecular bone volume.

Tam CS, Heersche JNM, Murray TM and Parsons JA. Parathyroid hormone stimulates the bone apposition rate independently of its resorptive action: Differential effects of intermittent and continuous administration. Endocrinology 1982;110:506-512.