P-2 PURINERGIC RECEPTORS POTENTIATE PARATHYROID-HORMONE RECEPTOR-MEDIATED INCREASES IN INTRACELLULAR CALCIUM AND INOSITOL TRISPHOSPHATE IN UMR-106 RAT OSTEOBLASTS

The PTH receptor has been cloned and shown to activate both adenylate cyclase and phospholipase C. Evidence exists that both signaling pathw ays are important for mediating the net physiological effects of this hormone on bone remodeling. We have shown previously that UMR-106 oste oblastic sarcoma cells express two calcium-signaling P-2 purinergic re ceptors, a P-2U and a unique P-2T receptor. Neither receptor modulates PTH receptor-mediated activation of adenylate cyclase. We now report that stimulation of either P-2 receptor will, however, potentiate the magnitude of the calcium signal observed after subsequent addition of human (h) PTH-(1-34) to fluo-3-loaded UMR-106 cells. Results from expe riments with staurosporine and phorbol 12-myristate 13-acetate argue a gainst a role for protein kinase C as a mediator of this potentiating effect of P-2 receptor ligands. The P-2 receptor-mediated intracellula r calcium elevation itself cannot account for the potentiating mechani sm, because addition of ionomycin will not replicate the effect of P-2 receptor ligands on hPTH-(1-34) signaling. Addition of EGTA after exp osure to P-2 ligands does not prevent the potentiation of hPTH-(1-34), indicating that P-2 ligands potentiate the release of intracellular c alcium after PTH receptor stimulation. Inositol trisphosphate producti on is potentiated in response to hPTH-(1-34) after first priming [H-3] inositol-labeled cells with a P-2 agonist. We conclude that UMR-106 ce lls express PTH receptors that are capable of activating adenylate cyc lase, but may be unable to activate phospholipase C until cells receiv e a signal as a consequence of P-2 receptor activation. The nature of the signal is unclear, but appears not to be mediated by either calciu m or protein kinase C.