Prolonged stimulation of bone resorption by parathyroid hormone (PTH) or 25-hydroxycholecalciferol (HCC) in cultures of fetal rat long bone shafts does not require their continuous presence. After 6 to 8 hr of treatment with these agents, the subsequent resorptive response was as great as with continuous exposure. This phenomenon has been termed induction. Bones treated for only 2 hr with PTH and washed with culture medium or normal guinea pig serum also showed prolonged resorption, but this could be abolished by washing with antibovine PTH guinea pig serum. Doses of PTH and 25-HCC that were ineffective when given separately could induce when given together. The subsequent resorptive response was diminished when the calcium or phosphate concentration in the medium was reduced during induction with PTH. Induction was blocked by low concentrations of actinomycin, mithramycin, cycloheximide, and camptothecin; however, this was often associated with irreversible toxicity to bone resorbing cells. Two-hour induction is associated with an increase in incorporation of labeled uridine or orotic acid into RNA. Vitamin A did not induce, and prostaglandin E1 was a weak inducer. Thyrocalcitonin inhibited resorption but had little effect on induction. Dibutyryl cyclic 3’,5’-adenosine monophosphate and theophylline neither induced nor enhanced the response to PTH (Endocrinology90: 744, 1972)
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Raisz et al. (1972) studied this question.