Effects of long-term treatment with the allosteric enhancer, PD81,723, on Chinese hamster ovary cells expressing recombinant human A1 adenosine receptors.

In this study, desensitization and down-regulation of A1 adenosine receptors (A1AR) by the allosteric enhancer PD81,723 (PD) and by N6-cyclopentyladenosine (CPA) were investigated after 24-hr pretreatment of CHO-K1 cells stably expressing recombinant human A1AR. Pretreatment with 20 microM PD and 10 microM CPA caused a 1.5- and 4.0-fold, respectively, desensitization (reduced potency of CPA to lower cAMP). Pretreatment with PD and/or CPA did not modify the acute effect of PD to increase (5-fold) the potency of CPA. Radioligand binding was used to measure receptor down-regulation in cell membranes and in intact cells. Pretreatment of cells with PD had no effect on the number of membrane binding sites for the agonist [125I] N6-(3-iodo-4-aminobenzyl) adenosine or for the antagonist, [3H]8-cyclopentyl-1,3-dipropylxanthine, but the binding of these radioligands to intact cells was modestly reduced (20-37%), possibly reflecting an effect of pretreatment on receptor subcellular distribution. Pretreatment of cells with CPA produced large ( > 40%) reductions in the binding of radioligands to both membranes and intact cells. Pretreatment of cells with CPA also increased the number of presumed internalized receptors measured as [3H]8-cyclopentyl-1,3-dipropylxanthine binding sites in intact cells insensitive to blockade by the charged antagonist 8-sulfophenyltheophylline. The relatively small degree of functional desensitization and down-regulation of A1AR caused by long term exposure of cells to PD is considered to be encouraging in terms of the therapeutic potential of the allosteric enhancer class of compounds.