Effects of long-term infusion of physiologic doses of 1-34 PTH on bone

It has been suggested that small doses of PTH could exert an anabolic effect on bone and could be beneficial in the management of bone diseases secondary to low bone formation. The effects of 20 wk of continuous infusion of 0.05 U.kg^-1.h^-1 of 1-34 PTH on cellular, structural, and dynamic parameters of bone were studied in inbred beagles. This physiologic or near physiologic dose of PTH caused a small but significant rise in the concentration of serum calcium and a significant increment in the plasma concentration of 1,25(OH₂D. In addition, the sustained infusion of PTH was associated with a significant increment in volume and surface density of osteoid without a change in bone mass. Mineralization of osteoid was not altered as evidenced by normal double tetracycline uptake and normal osteoid seam thickness. The enhanced osteoid production was not due to augmented bone formation by individual osteoblasts or basic remodelling units but rather to increased activation frequency resulting in an increased number of remodeling units. There was also augmented bone resorption at the tissue level. The data indicate that small doses of PTH do not have an anabolic effect on the skeleton because they are not associated with increased net bone formation. Rather, PTH administration is associated with an increase in coupled bone turnover.