Adenosine is a purine nucleoside that regulates cell function through G protein-coupled receptors that activate or inhibit adenylyl cyclase. Based on the understanding that cAMP regulates alveolar epithelial active Na ؉ transport, we hypothesized that adenosine and its receptors have the potential to regulate alveolar ion transport and airspace fluid content. Herein, we report that type 1 (A1R), 2a (A2aR), 2b (A2bR), and 3 (A3R) adenosine receptors are present in rat and mouse lungs and alveolar type 1 and 2 epithelial cells (AT1 and AT2). Rat AT2 cells generated and produced cAMP in response to adenosine, and micromolar concentrations of adenosine were measured in bronchoalveolar lavage fluid from mice. Ussing chamber studies of rat AT2 cells indicated that adenosine affects ion transport through engagement of A1R, A2aR, and/or A3R through a mechanism that increases CFTR and amiloride-sensitive channel function. Intratracheal instillation of low concentrations of adenosine (<10 ؊8 M) or either A2aR-or A3R-specific agonists increased alveolar fluid clearance (AFC), whereas physiologic concentrations of adenosine (>10 ؊6 M) reduced AFC in mice and rats via an A1R-dependent pathway. Instillation of a CFTR inhibitor (CFTRinh-172) attenuated adenosine-mediated down-regulation of AFC, suggesting that adenosine causes Cl ؊ efflux by means of CFTR. These studies report a role for adenosine in regulation of alveolar ion transport and fluid clearance. These findings suggest that physiologic concentrations of adenosine allow the alveolar epithelium to counterbalance active Na ؉ absorption with Cl ؊ efflux through engagement of the A1R and raise the possibility that adenosine receptor ligands can be used to treat pulmonary edema.active sodium transport ͉ adenosine receptors ͉ cystic fibrosis transmembrane conductance regulator P ulmonary edema is due to increased fluid flux into the airspace and impairment of the active Na ϩ transport that clears it (1-4). A variety of approaches to improve alveolar epithelial cell active Na ϩ transport for purposes of accelerating alveolar fluid clearance (AFC) have been explored in experimental systems. Of particular interest are receptor-ligand interactions that increase cAMP production in alveolar epithelial cells. Adenosine is a purine nucleoside that signals through four distinct G protein-coupled receptors, type 1 (A 1 R), type 2a (A 2a R), type 2b (A 2b R), and type 3 (A 3 R). In most cell systems, the A 1 R and A 3 R receptors inhibit adenylyl cyclase and/or lead to signaling through inositol-3-phosphate and phospholipase C. Engagement of type 2 receptors activates adenylyl cyclase by means of Gs␣ and increases cAMP levels. The ability of adenosine receptors (ARs) to couple to adenylyl cyclase led us to hypothesize that ARs might participate in regulation of alveolar epithelial active Na ϩ transport. We approached this hypothesis in rats and mice by testing whether adenosine and its receptors are present in the distal airspace and whether they affect AFC in vivo and vectorial Na ϩ...

Davis IC, Xu A, Gao Z, Hickman-Davis JM, Factor P, Sullender WM, Matalon S. Respiratory syncytial virus induces insensitivity to ␤-adrenergic agonists in mouse lung epithelium in vivo. Am J Physiol Lung Cell Mol Physiol 293: L281-L289, 2007. First published April 13, 2007; doi:10.1152/ajplung.00458.2006 is the most common cause of bronchiolitis in infants and children worldwide. We wished to determine whether intratracheal administration of ␤-agonists improved alveolar fluid clearance (AFC) across the distal respiratory epithelium of RSV-infected mice. Following intranasal infection with RSV strain A2, AFC was measured in anesthetized, ventilated BALB/c mice by instillation of 5% BSA into the dependent lung. We found that direct activation of protein kinase A by forskolin or 8-bromo-cAMP increased AFC at day 2 after infection with RSV. In contrast, short-and long-acting ␤-agonists had no effect at either day 2 or day 4. Insensitivity to ␤-agonists was not a result of elevated plasma catecholamines or lung epithelial cell ␤-adrenergic receptor degradation. Instead, RSV-infected mice had significantly higher levels of phosphorylated PKC in the membrane fractions of their lung epithelial cells. In addition, insensitivity to ␤-agonists was mediated in a paracrine fashion by KC (the murine homolog of CXCL8) and reversed by inhibition of either PKC or G protein-coupled receptor kinase 2 (GRK2). These results indicate that insufficient response to ␤-agonists in RSV may be caused, at least in part, by impaired ␤-adrenergic receptor signaling, as a consequence of GRK2-mediated uncoupling of ␤-adrenergic receptors from adenylyl cyclase. paramyxovirus; protein kinase C; G protein-coupled receptor kinase 2; CXCL8 RESPIRATORY SYNCYTIAL VIRUS (RSV) is the most common cause of lower respiratory tract disease in infants and children worldwide (44), is a frequent initiator of acute asthma exacerbations in young children, and has a disease impact comparable to that of nonpandemic influenza A in the elderly (8). Approximately 2-3% of all cases of RSV bronchiolitis result in severe hypoxia or a need for parenteral fluid supplementation that necessitates hospitalization (44). ␤-agonists are frequently used to treat RSV bronchiolitis, primarily because of their perceived ability to relax airway smooth muscle and cause bronchodilation, with the ultimate aim of alleviating hypoxemia. However, it is not clear that these drugs are clinically effective: meta-analyses have shown little or no overall benefit, regardless of viral bronchiolitis severity (15,20). Their lack of efficacy has not been explained, although it has often been ascribed to difficulties associated with drug delivery to the small airways of young infants, particularly in the presence of bronchoconstriction and inflammatory exudates or airway obstruction (28). ␤-agonists increase total body oxygen consumption, thereby increasing oxygen demands in infants hospitalized for respiratory compromise, and can potentially exacerbate ventilation-perfusion mismatch by inducing vaso...