Physiological evidence for a P2Y receptor responsive to diadenosine polyphosphates in human lung via Ca2+ release studies in bronchial epithelial cells

P2Y(2) receptors that an activated by the extracellular nucleotides ATP or UTP mediate Cl- secretion via an increase in [Ca2+](i) (intracellular calci um concentration). Therefore, in the lung of patients suffering from cystic fibrosis, inhalation of aerosolized UTP offers a way to circumvent the def ect in Cl- secretion by the cystic fibrosis transmembrane conductance regul ator. A possible alternative for the relatively unstable UTP in inhalation therapy is the more resistant diadenosine tetraphosphate (Ap(4)A). In human and rat lung membranes, Ap(4)A binds to P2 receptor sites coupled to G pro teins. Here, we showed that Ap(4)A caused an increase in [Ca2+](i) with an EC50 of 17 muM in human bronchial epithelial cells (HBE1). The [Ca2+](i) ri se evoked by ATP and UTP was completely, but that induced by Ap(4)A only pa rtially, caused by release of Ca2+ from internal stores. Moreover, the pote ncy of Ap(4)A to mobilize Ca2+ was lower than that of ATP and UTP (EC50 1.5 and 1.8 muM, respectively), and the maximal increase in [Ca2+](i) was cons iderably smaller than that after ATP or UTP. In accordance with our previou s results providing evidence for a common binding site for various diadenos ine polyphosphates in lung membranes, all Ap(n)A analogues tested (n = 3 to 6) caused a comparable [Ca2+](i) increase. Homologous or heterologous pres timulation largely diminished the increase in [Ca2+](i) found after a secon d pulse of either UTP or Ap(4)A. Although specific binding characteristics and functional responses of Ap(4)A on lung cells are in favor of a distinct receptor for Ap(4)A, the cross-talk between UTP and Ap(4)A in HBE1 cells a nd the only slight differences in Ca2+ mobilization by ATP or UTP and Ap(4) A render it impossible at this point to state unequivocally whether there e xists a distinct P2Y receptor specific for diadenosine polyphosphates in lu ng epithelia or whether Ap(4)A activates one of the nucleotide receptors al ready described. (C) 2001 Elsevier Science Inc. All rights reserved.