Effects of adenosine A2 receptor agonists on nucleoside transport.

A series of adenosine A2 receptor agonists were examined for their ability to activate adenosine A2 receptors and inhibit nucleoside transport. A2 receptor activation was measured by the ability of these adenosine agonists to relax porcine coronary smooth muscle, where the compounds varied in their EC50 values from 4.5 nM (CGS 21680A (2-[p-(2-carboxyethyl) phenylethylamino]-5'-N-ethylcarboxamidoadenosine)] to 3.6 microM (CGS 23321 [2 alpha,3 alpha-dihydroxy-1 beta-hydroxymethyl-4 beta-(2-phenylamino-9- adenyl)-cyclopentane]). Nucleoside transport was measured as the nitrobenzylthioinosine-sensitive cellular accumulation of [3H]uridine into guinea pig erythrocytes at 22 degrees C. The initial velocity of transport was dependent on substrate concentration and a substrate-velocity curve yielded a Km of 78 +/- 16 microM and a Vmax of 0.31 +/- 0.049 mmol/l of cell water per hr (mean +/- S.D., n = 4). Dipyridamole, a known potent inhibitor of nucleoside transport, blocked cellular [3H]uridine accumulation with an EC50 of 29.4 nM. Whereas a number of the adenosine agonists tested showed little or no inhibition of nucleoside transport, CV 1808 (2-phenylaminoadenosine) inhibited transport with an EC50 of 140 nM. In addition, two carbocyclic derivatives of CV 1808, CGS 23321 and CGS 23302 [(-)2S,3R-dihydroxy-4R-hydroxymethyl-1R-[2-(p-ethoxycarbonyl)- phenylamino-9-adenyl]-cyclopentane) inhibited nucleoside transport with respective EC50 values of 366 and 168 nM. The data suggest that these compounds have a different structure-activity relationship for adenosine A2 receptors and for the site mediating nucleoside transport inhibition.