Previous studies show activation of P2Y₂ receptors by UTP inhibits Na⁺ absorption in airway epithelial cells (Inglis et al. 1999, 2000; Ramminger et al. 1999) which may provide a method to control the Na⁺ hyper-absorption which occurs in the lungs of cystic fibrosis patients. Preliminary studies in H441 cells show UTP inhibits, whereas ATP unexpectedly stimulates Na⁺ absorption. As P2Y₂ receptors are equally sensitive to both, there must be another receptor which underlies the effects of ATP. Further experiments suggested the non P2Y₂ mediated response of ATP may be a result of ATP hydrolysis resulting in the stimulation of adenosine receptors. The aim of this study was to investigate the pharmacological basis of the effect of adenosine in H441 cells. H441 cells were grown on Costar snapwells in RPMI-1640 media supplemented with dialysed FBS and 0.2μM dexamethasone. Once confluent, cells were mounted onto Ussing chambers, bathed in physiological saline and gassed with 13% O₂ at 37°C. Basal values of potential difference, short circuit current (I_sc) and transepithelial resistance were 11.7±1.1mV, 44.6±3.3μAcm^-2 and 257.8±5.7Ωcm², respectively (n=240). Apical adenosine (200μM) evoked a slow increase in I_sc (60.54±.5μA cm^-2) compared to pre-stimulated I_sc (39.5±4.5μA cm^-2, n=8, p<0.05). This increase in I_sc is associated with an increase in apical sodium conductance (G_Na), stimulated cells (354.9±41.3μS cm^-2) vs. control cells (213.5±36.5μS cm^-2, n=6, p<0.001). To establish which adenosine receptor mediates the response, cells were subjected to a range of adenosine agonists and antagonists. Concentration curves for adenosine and three adenosine receptor agonists, SPA (A1), CGS 21680 (A2_A) and IB-MECA (A3) were constructed. From these curves the EC₅₀ was calculated, adenosine 1.6±0.7μM, SPA 1.5±0.3μM, CGS 21680 8.4±0.9μM and IB-MECA 6.2±1.5μM, therefore rank order of potency is adenosine ≈ SPA > CGS 21680 ≈ IB-MECA. As none of these agonists are more potent than adenosine it may be that the adenosine response is mediated by A2_B receptors, however this is difficult to confirm as there are no commercially available A2_B receptor agonists. We therefore investigated the effects of adenosine receptor antagonists (DPCPX (A1), ZM 241385 (A2_A) and MRS 1706 (A2_B)) on I_sc of cells pre-stimulated with maximal adenosine. DPCPX had no effect, whereas ZM241385 and MRS1706 both appear to displace adenosine from its receptor, although the effects of these two antagonists are indistinguishable. These data suggest a role for A2_A and/or A2_B receptors in the stimulation of Na⁺ absorption in response to apical adenosine in H441 cells.