Characterization of P2Y receptors mediating ATP induced relaxation in guinea pig airway smooth muscle: involvement of prostaglandins and K+ channels

In airway smooth muscle (ASM), adenosine 5′-triphosphate (ATP) induces a relaxation associated with prostaglandin production. We explored the role of K+ currents (IK) in this relaxation. ATP relaxed the ASM, and this effect was abolished by indomethacin. Removal of airway epithelium slightly diminished the ATP-induced relaxation at lower concentration without modifying the responses to ATP at higher concentrations. ATPγS and UTP induced a concentration-dependent relaxation similar to ATP; α,β-methylene-ATP was inactive from 1 to 100 μM. Suramin or reactive blue 2 (RB2), P2Y receptor antagonists, did not modify the relaxation, but their combination significantly reduced this effect of ATP. The relaxation was also inhibited by N-ethylmaleimide (NEM; which uncouples G proteins). In myocytes, the ATP-induced IK increment was not modified by suramin or RB2 but the combination of both drugs abolished it. This increment in the IK was also completely nullified by NEM and SQ 22,536. 4-Amynopyridine or iberiotoxin diminished the ATP-induced IK increment, and the combination of both substances diminished ATP-induced relaxation. The presence of P2Y2 and P2Y4 receptors in smooth muscle was corroborated by Western blot and confocal images. In conclusion, ATP: (1) produces relaxation by inducing the production of bronchodilator prostaglandins in airway smooth muscle, most likely by acting on P2Y4 and P2Y2 receptors; (2) induces IK increment through activation of the delayed rectifier K+ channels and the high-conductance Ca2+-dependent K+ channels, therefore both channels are implicated in the ATP-induced relaxation; and (3) this IK increment is mediated by prostaglandin production which in turns increase cAMP signaling pathway.