Relation of Endogenous Parathyroid Secretion to3H-Cytidine Incorporation Into Bone Cells

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Abstract
This study was concerned with the effect of endo-genous parathyroid hormone on incorporation of tritiated cytidine and thymidine into RNA and DNA extracted from bone. Hormone secretion was stimulated by the technique of peritoneal lavage in rats. Following lavage, metaphysial and diaphysial sections of the femur were incubated separately in serum containing the 2 tagged precur-sors. The specific activities of RNA and DNA extracted from these bone chips were compared for control and parathyroidectomized rats, both lavaged and nonlavaged. The effect of the hormone was studied both as related to the time of response and to the type of bone affected. The specific activity of the RNA extracted from the metaphysis of the parathyroidintact rat increased after 20 min. of lavage, reaching a maximum increase of 50% by the 8th hr. The increase in the specific activity of the DNA fraction was delayed until the 4th hr., but doubled by the 8th hr. Parathyroidectomy alone produced a small increase in the specific activity of RNA, had no effect on that of DNA, and in neither case was influenced further by peritoneal lavage. In the diaphysis, RNA specific activity showed immediate but variable decrease following lavage. This effect was reversed after the 4th hr., after which time the increase in specific activity paralleled that of the metaphysis. The specific activity of the DNA extracted from the diaphysis paralleled that of the metaphysis at all time periods. These data demonstrate that endogenous parathyroid hormone may have contrasting effects on the incorporaton of H3-cytidine into RNA depending upon the area of bone and the duration of the hormonal stimulation, but that the major effect of the hormone is to increase incorporation of this precursor into RNA. The hormone also increased thymidine incorporation into bone DNA following a lag period of approximately 4 hr.