Adenosine receptors, cystic fibrosis, and airway hydration

Adenosine (Ado) regulates diverse cellular functions in the lung through its local production, release, metabolism, and subsequent stimulation of G-protein-coupled P1 purinergic receptors. The A(2B) adenosine receptor (A(2B)AR) is the predominant P1 purinergic receptor isoform expressed in surface airway epithelia, and Ado is an important regulator of airway surface liquid (ASL) volume through its activation of the cystic fibrosis transmembrane conductance regulator (CFTR). Through a delicate balance between sodium (Na(+)) absorption and chloride (Cl(-)) secretion, the ASL volume is optimized to promote ciliary activity and mucociliary clearance, effectively removing inhaled particulates. When CFTR is dysfunctional, the Ado/A(2B)AR regulatory system fails to optimize the ASL volume, leading to its depletion and interruption of mucociliary clearance. In cystic fibrosis (CF), loss of CFTR function and resultant mucus stasis leaves the lower airways susceptible to mucus obstruction, chronic bacterial infection, relentless inflammation, and eventually panbronchiectasis. Adenosine triphosphate (ATP) also regulates transepithelial Cl(-) conductance, but through a separate system that relies on stimulation of P2Y(2) purinergic receptors, mobilization of intracellular calcium, and activation of calcium-activated chloride channels (CaCCs). These pathways remain functional in CF, and may serve a protective role in the disease. In this chapter, we will review our current understanding of how Ado and related nucleotides regulate CFTR and Cl(-) conductance in the human airway, including the regulation of additional intracellular and extracellular signaling pathways that provide important links between ion transport and inflammation relevant to the disease.