Apparent affinity of some 8-phenyl-substituted xanthines at adenosine receptors in guinea-pig aorta and atria.

1 Some 8-phenyl-substituted, 1,3 dipropyl xanthines have previously been demonstrated to have a 20-400 fold greater affinity for A1 binding sites in rat CNS membranes than for A2 adenosine receptors in intact CNS cells from guinea-pigs. In the present study these compounds (1,3, dipropyl-8-phenylxanthine: DPPX; 1,3 dipropyl-8-(2 amino-4-chlorophenyl) xanthine: PACPX; 8-(4-(2-amino-ethyl)amino) carbonyl methyl oxyphenyl)-1,3-dipropylxanthine: XAC; and D-Lys-XAC) together with two that have not been reported to exhibit A1-receptor selectively (8-(p-sulphophenyl)theophylline: 8-PST; 8-(4-carboxy methyl oxyphenyl)-1,3-dipropylxanthine: XCC) have been evaluated as antagonists of the effects of 2-chloroadenosine in two isolated cardiovascular tissues. 2 The isolated tissues used were guinea-pig atria (bradycardic response) and aorta (relaxation), which are thought to possess A1 and A2 adenosine receptors, respectively. 3 All the xanthines antagonized responses evoked by 2-chloroadenosine in both tissues but did not affect responses evoked by acetylcholine (atria) or sodium nitrite (aorta). 4 The xanthines, 8-PST, XAC, D-Lys XAC, XCC and DPPX appeared to be competitive antagonists of the effects of 2-chloroadenosine, as Schild plot slopes did not differ significantly from unity. The 1,3-dipropyl substituted compounds had pA2 values from 6.5 to 7.4 and were more potent than the 1,3 dimethyl substituted 8-PST (pA2 4.9 to 5). 5 For individual xanthines, there was no difference between pA2 values obtained in the atria and in the aorta. 6 Responses evoked by 2-chloroadenosine in atria and aorta were antagonized to a similar degree by PACPX (1 microM). The agonist dose-ratio evoked by 10 microM PACPX was no greater than that evoked by 1 microM of the xanthine in both tissues. This was probably a consequence of the limited aqueous solubility of PACPX. 7. These results fail to demonstrate A, receptor selectivity for DPPX, XAC, D-Lys XAC or PACPX in tissues from the guinea-pig. The A, selectivity previously found for these compounds may therefore be due to their high affinity for binding sites in rat CNS cell membranes.