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

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Abstract
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.