P2Y receptor specific for diadenosine tetraphosphate in lung: selective inhibition by suramin, PPADS, Ip5I, and not by MRS-2197

Extracellular dinucleotides, which act as signaling molecules in a variety of systems, may regulate fluid homeostasis in the human lung by activation of a specific P2Y receptor subtype. Previously, we presented evidence for a G protein-coupled P2Y receptor with high affinity for dinucleotides in both rat and human lung tissue. In a human bronchial epithelial cell line (HBE-1), diadenosine polyphosphates (Ap(n)A, n=2-6) increase intracellular Ca(2+). The aim of the present work was to find additional evidence that, in these cells, the receptors selectively activated by diadenosine polyphosphates are distinct from already known P2Y receptors, which are activated by the mononucleotides ATP or UTP. We tested antagonists suitable to classify P2Y receptor subtypes. The P2Y(1) receptor-selective antagonist 2'-deoxy-N(6)-methyl adenosine 3',5'-diphosphate (MRS-2197) did not affect Ca(2+) mobilization induced by diadenosine tetraphosphate (Ap(4)A). However, suramin, pyridoxal phosphate-6-azo(benzene-2,4-disulfonic acid) (PPADS) and diinosine pentaphosphate (Ip(5)I) inhibited the Ca(2+) response by 96%, 92% and 32%, respectively. Moreover, these results were confirmed by assessing the specific binding of [3H]Ap(4)A to membranes from human and rat lung. Suramin (100 microM), PPADS (400 microM) and Ip(5)I (200 microM), reduced [3H]Ap(4)A binding in lung membrane preparations by 66%, 77% and 80%, respectively. The Ap(4)A-induced Ca(2+) response in HBE-1 cells was inhibited to a much greater extent by these antagonists than the ATP- or UTP-evoked Ca(2+) rise. Thus, Ap(4)A in lung epithelial cells also activates a still unidentified P2Y receptor that is specific for dinucleotides over mononucleotides.