Separate P2T and P2U purinergic receptors with similar second messenger signaling pathways in UMR-106 osteoblasts

UMR-106 rat osteogenic sarcoma cells express two calcium signaling P2 purinergic receptors. One is a P2U receptor with EC50's for adenosine triphosphate (ATP) and uridine triphosphate (UTP) of 2.6 and 2.4 microM, respectively. The other is a novel P2T receptor for adenosine diphosphate (ADP) (EC50 0.4 microM), adenosine 5'-O-(2-thiodiphosphate). (EC50 5 microM), 2-methylthio ATP (EC50 0.2 microM), and 2-methylthio ADP (EC50 0.04 microgram M). Responses to these ligands are desensitized by ADP but not by UTP. Responses to UTP and ATP are desensitized by UTP but not by ADP or adenosine 5'-O-(2-thiodiphosphate). 2-ChloroATP interacts with both receptors at high concentrations but with only the P2T receptor at low concentrations (EC50 0.05 microM). The weak platelet P2T receptor antagonist AMP blocks this P2T and not the P2U receptor. Addition of ATP after UTP desensitization of P2U receptors inhibits subsequent responsiveness to ADP but evidence for rapid conversion of ATP to ADP complicates interpretation of this apparent antagonism of P2T receptors by ATP. A subpassage of UMR-106.P135 cells lose P2U but retain P2T ligand responsiveness. Activation of either P2 receptor increases cellular IP3 concentrations in UMR-106 cells. Neither receptor can activate divalent cation entry as evidenced by their lack of effect on Mn++ quenching of fura-2 fluorescence. Neither receptor can modify parathyroid hormone receptor-mediated elevation of cellular cyclic AMP. This receptor for ADP demonstrates many of the same characteristics of the ADP receptor which we have previously reported as a P2T receptor expressed in K562 and Dami cells.