STIMULATION BY ALKYLXANTHINES OF CHLORIDE EFFLUX IN CFPAC-1 CELLS DOES NOT INVOLVE A(1) ADENOSINE RECEPTORS

A series of 8-substituted derivatives of 1,3,7-alkylxanthines was synt hesized as potential activators of chloride efflux from a human epithe lial cell line (CFPAC) expressing the cystic fibrosis transmembrane re gulator (CFTR) Delta F508 mutation. Their interactions with rat brain A(1) and A(2a) receptors were also studied in radioligand binding expe riments, Substitution was varied at the xanthine 1-, 3-, 7- and and 8- positions. 1,3-Dipropyl-8-cyclopentylxanthine (CPX) stimulated Cl- eff lux in the 10(-8) M range, with a maximal effect reaching 200% of cont rol and diminishing at higher concentrations. The potent adenosine ant agonist ]carbonyl]methyl]oxy]phenyl]-1,3-dipropylxanthine, nonselectiv e at human A(1) and A(2a) receptors, was inactive in Cl- efflux. 1,3-D iallyl-8-cyclohexylxanthine (DAX) was highly efficacious in stimulatin g chloride efflux with levels reaching >300% of control, although micr omolar concentrations were required. 1,3,7-Trimethyl-8-(3-chlorostyryl )xanthine, an A(2a)-selective adenosine antagonist, was only weakly ac tive. Caffeine, which acts as an nonselective adenosine antagonist in the range of 10(-5) M, was active in Cl- efflux in the low nanomolar r ange but with low efficacy. Thus, among the xanthine derivatives of di verse structure, there was no correlation between potency in Cl- efflu x and adenosine antagonism. Poly(A)+ RNA isolated from CFPAC-1 cells s howed no hybridization to a human A(1) receptor cDNA probe, excluding this receptor as a mediator of CPX-elicited Cl- efflux, Thus, this act ion of xanthines in stimulating Cl- efflux in CFPAC cells, which expre ss a defective CFTR, represents a novel site of action apparently unre lated to adenosine receptors.