An analysis of the role of serum in parathyroid hormone-induced bone resorption in tissue culture.

THE stimulation of bone resorption by parathyroid hormone can be quantitated in tissue cultures of embryonic bone [20], using paired limb bones from a single embryo in which radioactive calcium (45Ca) has previously been incorporated. The ratio of 45Ca release from the parathyroid hormonetreated bones to that from the control bones is a sensitive measure of parathyroid effect. This technique has been applied to a study of the mechanism of action of parathyroid hormone and of other factors affecting bone resorption and also has been used to assay for parathyroid hormone activity in biological fluids [S, 21-231. The major disadvantages of this system are the variability of the response [al] and the difficulty in analyzing for small biochemical changes. Because our preliminary observations suggested that some of these disadvantages might be ascribable to the high concentration of serum (50 per cent) used in the culture medium, the objective of the present study was to find the simplest and chemically best defined medium in which bone resorption could be stimulated by parathyroid hormone. Chemically defined media without serum proved to be inadequate to support parathyroid hormone-induced bone resorption; therefore the minimum serum concentration required for a parathyroid effect was determined and the constituent of serum necessary for this response identified. The critical factor occurs in the albumin fraction of serum. This fraction appears to contain a non-parathyroid factor which can both support parathyroid-stimulated bone resorption and enhance 45Ca release from control bones.