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 extracellular environment, where they serve as ligands to P2 receptors, Here we report that the localized release of these nucleotides can sensitize osteoblasts to the activity of systemic factors. We have investigated the ability of parathyroid hormone (PTH), a principal regulator of bone resorption and formation, to potentiate signals arising from nucleotide stimulation of UMR-106 clonal rat osteoblasts, PTH receptor activation alone did not lead to [Ca2+](i) elevation in these cells, indicating no G(q) coupling, however, activation of G(q)-coupled P2Y(1) receptors resulted in characteristic [Ca2+](i) release. PTH potentiated this nucleotide-induced Ca2+ release, independently of Ca2+ influx, PTH-(1-31), which activates only G(s), mimicked the actions of PTH-(1-34), whereas PTH-(3-34), which only activates G(q), was unable to potentiate nucleotide-induced [Ca2+](i) release. Despite this coupling 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 forskolin effectively potentiated nucleotide-induced [Ca2+](i) release, however, further experiments proved that cyclic monophosphates were not involved 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 responsive Ca/cAMP response element of the c-fos promoter alone was effective at driving this synergistic gene expression. These findings demonstrate that nucleotides can provide a targeted response to systemic factors, such as PTH, and have important implications for PTH-induced signaling in bone.