Bone Resorption in Tissue Culture. Factors Influencing the Response to Parathyroid Hormone *

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Abstract
Parathyroid hormone was found to stimulate resorption in 72-hour cultures of the bone shafts to radius and ulna from 19-day rat embryos. Parathyroid activity was assayed by measuring the release of previously incorporated Ca45 from such bone explants. The effects of varying calcium and phosphate concentration, pH and oxygen tension have been measured. When different samples of human and rat serum were used to make up the media, variations in bone resorption were encountered which could not be ascribed to variations in parathyroid activity. Vitamin A was also found to stimulate bone resorption. Cortisol had no direct effect on bone resorption but inhibited the effect of vitamin A completely and the effect of parathyroid hormone partially. Triiodothyronine and vitamin D had no direct effects. The response to parathyroid hormone was diminished when the culture medium was made with serum from vitamin D-deficient rats, compared with medium made with serum from vitamin D-treated rats.