Parathyroid hormone stimulates dentin and bone apposition in the thyroparathyroidectomized rat in a dose-dependent fashion

Summary

The dose-dependency of the effects of parathyroid hormone (PTH) on bone and dentin apposition after both intermittent and continuous administration of the hormone was investigated. The purpose was to compare the sensitivity of these two mineralizing tissues to parathyroid hormone and to provide additional information regarding the direct effect of PTH on mineralized tissue formation. Adult rats were thyroparathyroidectomized and 5 groups of 4 or 5 rats each were given daily subcutaneous injections with different doses of bovine parathyroid hormone. Five more groups of 4 rats each were administered equivalent dosages by means of a continuous infusion pump implanted subcutaneously. An additional group of 4 rats served as controls. All animals were labeled with tetracycline injected on days 7, 9, and 11. The animals were killed on day 12 and blood samples were collected for serum calcium determination. The lower metaphysis of the femur and the left and right mandibles were dissected out, and undecalcified sections of plastic-embedded tissues were prepared. The distances between the three tetracycline bands were measured to determine the amount of bone or dentin formation. Results indicated that both dentin and bone apposition increased with higher dosages of hormone. No overall effect of the method of administration was evident. For both methods, bone apposition showed a more pronounced increase over the control levels than dentin apposition. This suggests that, although both osteoblasts and odontoblasts appear to respond directly to PTH, differences do exist in the magnitude and dose-dependency of the response. No causal relationship was found between increases in serum calcium levels and either bone or dentin apposition at the lower dose levels.