Stimulation by Alkylxanthines of Chloride Efflux in CFPAC-1 Cells Does Not Involve A₁ Adenosine Receptors

A series of 8-substituted derivatives of 1,3,7-alkylxanthines was synthesized as potential activators of chloride efflux from a human epithelial cell line (CFPAC) expressing the cystic fibrosis transmembrane regulator (CFTR) ∆F508 mutation. Their interactions with rat brain A₁ and A_2a receptors were also studied in radioligand binding experiments. Substitution was varied at the xanthine 1-, 3-, 7- and 8-positions. l,3-Dipropyl-8-cyclopentylxanthine (CPX) stimulated Cl^- efflux in the 10^-8 M range, with a maximal effect reaching 200% of control and diminishing at higher concentrations. The potent adenosine antagonist 8-[4-[[[[(2-aminoethyl)amino]carbonyl]methyl]oxy]phenyl]-l,3-dipropylxanthine, nonselective at human A₁ and A_2a receptors, was inactive in Cl^- efflux. l,3-Diallyl-8-cyclohexylxanthine (DAX) was highly efficacious in stimulating chloride efflux with levels reaching >300% of control, although micromolar concentrations were required. l,3,7-Trimethyl-8-(3-chlorostyryl)xanthine, an A_2a-selective adenosine antagonist, was only weakly active. Caffeine, which acts as an nonselective adenosine antagonist in the range of 10^-5 M, was active in Cl^- efflux in the low nanomolar range but with low efficacy. Thus, among the xanthine derivatives of diverse structure, there was no correlation between potency in Cl^- efflux and adenosine antagonism. Poly(A)= RNA isolated from CFPAC-1 cells showed no hybridization to a human A₁ receptor cDNA probe, excluding this receptor as a mediator of CPXelicited Cl^- efflux. Thus, this action of xanthines in a defective CFTR, represents a novel site of action apparently unrelated to adenosine receptors.