Histomorphometric evaluation of the effects of low dose parathyroid hormone administration on cortical bone remodeling in adult dogs.

The effects of low dose levels of parathyroid hormone (PTH) on cortical bone remodeling of adult dogs were evaluated under conditions of normal turnover and accelerated remodeling produced by periosteal elevation. Six female dogs were administered PTH (2.5 units per kg. per day) intravenously in equally divided doses 6 hours apart for 60 days. The level and dose schedule of PTH used in this study resulted in a significant elevation in blood calcium above baseline values only at experimental day 5, no change in serum phosphorus and magnesium, and a transient increase in urinary hydroxyproline excretion on day 10. Accelerated cortical bone remodeling was induced by surgical elevation of periosteum of the 9th rib prior to PTH administration. Static and dynamic bone changes were analyzed using histomorphometric methods following tetracycline and DCAF ((2,4 bis) N, N'-di-(Carboxymethyl) aminomethyl fluorescein) in vivo double labeling. The results of this study suggested that the response of cortical bone to PTH was dependent upon the existing remodeling rate. PTH increased the radial closure rate and decreased the osteon formation time in unaltered cortical bone. These findings suggested that the initial anabolic effect of PTH may be due to increased bone formation at the osteoblastic level in existing Haversian units. In ribs with periosteal elevation, PTH decreased the circumference and thickness of osteoid seams, and there was a trend toward prolonged osteon formation time. The number of bone resorption sites was greater than the number of formation sites. PTH administration did not significantly change activation frequency or remodeling in cortical bone of adult dogs under conditions of normal turnover or accelerated remodeling. The accelerated remodeling produced by periosteal elevation appeared to mask certain responses to cortical bone to low dose levels of PTH.