8-Cyclopentyl-1,3-dipropylxanthine (DPCPX) - a selective high affinity antagonist radioligand for A1_1 adenosine receptors

The properties of 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) as an antagonist ligand for A$_1$ adenosirre receptors were examined and conipared with other radioligands for this receptor. DPCPX competitively antagonized both the inhibition of adenylate cyclase activity via A$_1$ adenosirre receptors and the stimulationvia A$_2$ adenosirre receptors. The K$_i$-values of this antagonism were 0.45 nM at the A$_1$ receptor of rat fat cells, and 330 nM at the A$_2$ receptor of human platelets, giving a more than 700-fold A$_1$-selectivity. A similar A$_1$-selectivity was determined in radioligand binding studies. Even at high concentrations, DPCPX did not significantly inhibit the soluble cAMPphosphodiesterase activity of human platelets. [$^3$H]DPCPX (105 Ci/mmol) bound in a saturable manner with high affinity to A$_1$ receptors in membranes of bovine brain and heart, and rat brain and fat cells (K$_D$ -values 50-190 pM). Its nonspecific binding was about 1% of total at K$_D$ , except in bovine myocardial membranes (about 10%). Binding studies with bovine myocardial membranes allowed the analysis of both the high and low agonist affinity states of this receptor in a tissue with low receptor density. The binding properties of [$^3$H]DPCPX appear superior to those of other agonist and antagonist radioligands for the A$_1$ receptor