Characterization of a diadenosine tetraphosphate-receptor distinct from the ATP-purinoceptor in human tracheal gland cells

Human submucosal tracheal glands are now believed to play a major role in t he physiopathology of cystic fibrosis, a genetic disease in which ATP is us ed as a therapeutic agent. However, actions of ATP on tracheal gland cells are not well known. ATP binds to P2 receptors and induced secretory leucocy te protease inhibitor (SLPI) secretion through formation of cyclic adenosin e monophosphate and mobilization of intracellular [Ca2+]. Since diadenosine polyphosphates (ApnA) are also endogenous effecters of P2 receptors, we in vestigated their effects in a cell line (MM39) of human tracheal gland cell s. Diadenosine tetraphosphates (Ap4A) induced significant stimulation (+ 50 +/- 12%) of SLPI secretion and to a similar extent to that of ATP (+ 65 +/ - 10%). No significant effects were observed with diadenosine triphosphate (Ap3A), diadenosine pentaphosphate (Ap5A), ADP and 2-methylthio-adenosine t riphosphate (2-MeS-ATP). Since Ap4A was weakly hydrolyzed (< 2% of total), and the hydrolysis product was only inosine which is ineffective an cells, this Ap4A effect was not due to Ap4A hydrolysis in ATP and adenosine monoph osphate (AMP). A mixture of Ap4A and ATP elicited only partial additive eff ects on SLPI secretion. ADP was shown to be a potent antagonist of ATP and Ap4A receptors, with IC(50)s of 0.8 and 2 mu M, respectively. 2-MeS-ATP als o showed antagonistic properties with IC(50)s of 20 and 30 mu M for ATP- an d Ap4A-receptors, respectively. Single cell intracellular calcium ([Ca2+](i )) measurements showed similar transient increases of [Ca2+](i) after ATP o r Ap4A challenges. ATP desensitized the cell [Ca2+](i) responses to ATP and Ap4A, and Ap4A also desensitized the cell response to Ap4A. Nevertheless, Ap4A did not desensitize the cell [Ca2+](i) responses to ATP. In conclusion , both P2Y2-ATP-receptors and Ap4A-P2D-receptors seem to be present in trac heal gland cells. Ap4A may only bind to P2D-receptors whilst ATP may bind t o both Ap4A- and ATP-receptors. (C) 1999 Elsevier Science B.V. All rights r eserved.