Parathyroid hormone potentiates nucleotide-induced [Ca2+](i) release in rat osteoblasts independently of G(q) activation or cyclic monophosphate accumulation - A mechanism for localizing systemic responses in bone

The regulation of tissue turnover requires the coordinated activity of both local and systemic factors. Nucleotides exist transiently in the extracell ular environment, where they serve as ligands to P2 receptors, Here we repo rt that the localized release of these nucleotides can sensitize osteoblast s to the activity of systemic factors. We have investigated the ability of parathyroid hormone (PTH), a principal regulator of bone resorption and for mation, to potentiate signals arising from nucleotide stimulation of UMR-10 6 clonal rat osteoblasts, PTH receptor activation alone did not lead to [Ca 2+](i) elevation in these cells, indicating no G(q) coupling, however, acti vation of G(q)-coupled P2Y(1) receptors resulted in characteristic [Ca2+](i ) release. PTH potentiated this nucleotide-induced Ca2+ release, independen tly of Ca2+ influx, PTH-(1-31), which activates only G(s), mimicked the act ions of PTH-(1-34), whereas PTH-(3-34), which only activates G(q), was unab le to potentiate nucleotide-induced [Ca2+](i) release. Despite this couplin g of the PTHR to G(s), cAMP accumulation or protein kinase A activation did not contribute to the potentiation, 3-Isobutyl-1-methylxanthine, but not f orskolin effectively potentiated nucleotide-induced [Ca2+](i) release, howe ver, further experiments proved that cyclic monophosphates were not involve d in the potentiation mechanism. Costimulation of UMR-106 cells with P2Y(1) agonists and PTH led to increased levels of cAMP response element-binding protein phosphorylation and a synergistic effect was observed on endogenous c-fos gene expression following costimulation, In fact the calcium respons ive Ca/cAMP response element of the c-fos promoter alone was effective at d riving this synergistic gene expression. These findings demonstrate that nu cleotides can provide a targeted response to systemic factors, such as PTH, and have important implications for PTH-induced signaling in bone.