Induction of ornithine decarboxylase activity in isolated chicken osteoblasts by parathyroid hormone: The role of cAMP and calcium

We investigated the role of cAMP and Ca²⁺ as mediators in parathyroid hormone (PTH)-induced ornithine decarboxylase (ODC) activity in primary cultures of chicken osteoblasts. We present evidence that the induction of ODC activity by PTH is most likely a receptor-mediated process and that cAMP is a mediator. However, using three different approaches we have strong indications that cAMP is not the exclusive mediator of PTH-induced ODC activity. First, when the dose-response curve of PTH-induced ODC activity is compared with that of PTH-stimulated cAMP production, the ED₅₀ for cAMP production is about five times as high as that for the induction of ODC activity. Second, 1 mM 9-(tetrahydro-2-furanyl) adenise (SQ 22.536) almost completely inhibited PTH-stimulated cAMP production whereas there was only a small inhibitory effect on PTH-induced ODC activity. Third, some PTH fragments unable to stimulate cAMP production were still able to induce ODC activity. We therefore propose that apart from cAMP, an additional messenger, most likely Ca²⁺, must be present. Evidence for this concept are the observations that substances affecting extracellular and intracellular Ca²⁺ levels (EGTA, A23187, CoCl₂, verapamil) or antagonizing calmodulin (Trifluoroperazin, Compound 48/80) also strongly affect PTH-induced ODC activity. These effects could not be explained by a positive interaction of Ca²⁺ with the hormone-stimulated cAMP system as 2 mM EGTA strongly enhanced PTH-stimulated cAMP production but at the same time completely inhibited PTH-induced ODC activity. A similar dissociation between hormone-induced cAMP production and induction of ODC activity was found with the Ca²⁺-ionophore A23187 (10^-7M) which significantly inhibited PTH-stimulated cAMP production but strongly enhanced PTH-induced ODC activity. Our results suggest that intracellular Ca²⁺, and possibly calmodulin, in addition to cAMP, are involved in PTH-induced ODC activity in chicken osteoblasts. Most probably Ca²⁺ is the initial messenger and cAMP acts in a coordinate pattern as a synarchic messenger making the induction of ODC activity by PTH more sensitive to Ca²⁺. Furthermore, the present findings are in agreement with our concept of the existence of two receptors or two receptor-sites for PTH on osteoblasts. One receptor is coupled to the production of cAMP and is presumably activated when the first two amino-acids of the NH2-terminus of the hormone are present and the other, suggested to be responsible for the increase in intracellular Ca²⁺, is thought to be activated by a region of the hormone sequence between amino acid 3 and 34. Activation of both receptors by the intact hormone PTH(1-84) or its biological active fragment PTH(1-34) leads to the maximal induction of ODC activity.