Adenosine receptors on human airway epithelia and their relationship to chloride secretion

1 We have characterized an adenosine receptor subtype present in human airway epithelial cells by measuring the changes in the intracellular levels of adenosine 3′:5′‐cyclic monophosphate (cyclic AMP) and the rate of transepithelial Cl− secretion. 2 Primary cultures of human nasal epithelium obtained from excised surgical airway epithelial tissues and the cell lines BEAS39 and CF/T43 derived from human airway epithelium were grown on plastic dishes and labelled with [3H]‐adenine for measurement of intracellular cyclic AMP accumulation. Primary cultures were loaded with the calcium indicator fura‐2 to measure [Ca2+]i and studied as polarized, ion transporting epithelia on collagen matrix supports for measurement of Cl− secretion. 3 Adenosine analogues stimulated cyclic AMP accumulation with a rank order of potency characteristic of an A2‐receptor: 5‐N‐ethyl‐carboxamidoadenosine (NECA) > adenosine > R‐phenylisopropyladenosine (R‐PIA), 6‐N‐cyclopentyladenosine (CPA)>S‐PIA. NECA increased cyclic AMP accumulation in normal and cystic fibrosis (CF) primary cells as well as in the CF/T43 and BEAS39 cell lines with K0.5 values ranging from 0.3 to 3 μm. Preincubation with NECA resulted in the homologous desensitization of airway epithelial cells. The effect of NECA was specifically inhibited by the adenosine receptor antagonist, aminophylline, in a competitive manner. 4 The A1‐adenosine receptor agonists CPA and R‐PIA did not inhibit isoprenaline‐stimulated cyclic AMP accumulation in CF/T43 cells, and potentiating effects of the adenosine analogues were observed on forskolin‐stimulated cyclic AMP accumulation. Adenosine analogues did not cause significant changes in intracellular Ca2+ ([Ca2+]i) in airway epithelium. 5 Adenosine analogues, applied to either the serosal or mucosal side of the polarized amiloride pretreated primary cultures, induced changes in Isc with a rank order of potency of agonists similar to that observed for stimulation of cyclic AMP accumulation. Intracellular microelectrode studies indicated that the locus of action was the apical membrane Cl− conductance. Adenosine failed to stimulate Cl− secretion in CF airway epithelium. 6 These results provide evidence for the existence of an A2‐adenosine receptor that modulates intracellular levels of cyclic AMP in human airway epithelium. Activation of this receptor might lead to stimulation of Cl− secretion in amiloride pretreated normal but not CF cells.