Table 1.

Affinity, expressed as K_i values in nM, of adenosine receptor agonists and antagonists at central A₂-adenosine receptors in rabbit (vs [³H]XAC) and in rat (vs [³H]NECA) striatum, unless noted

Compound	Ki (nM) at rabbit A2-adenosine receptors	Ki (nM) at rat A2-adenosine receptors	Ratio‡
Antaqonists
(1) N,Nα-Me2-XAC	5.3±1.2	5.0	0.94
(2) Nα-Me XAC	7.0±0.63	9.3*	1.33
(3) p-DITC-XAC	7.2±1.6	321	46
(4) XAC	7.5±0.46	63	8.4
(5) m-DITC-XAC	15.6±1.6	343	22
(6) N-Ac-XAC	21.1±2.9	530	25
(7) 2-thio-CPX	30.7±5.9	314	10.2
(8) CPX	179±11.8	340	1.90
(9) XCC	196±31	2200	11.2
(10) DPSPX	277±79.2	11,000*	39.7
(11) PSPT	30,090±1504	15,300	0.51
(12) Theophylline	38,250±244	25,300	0.66
(13) 8-p-Sulfophenylcaffeine	> 100,000	150,000*
Agonists
(14) NECA	70.8±5.2	10.3	0.15
(15) CGS21680	79.1±4.7	15	0.19
(16) APEC	169±34.7	5.7 12†	0.034
(17) PAPA-APEC	279±52.0	28†	0.10
(18) ADAC	860±48.5	210	0.24
(19) R-PIA	3216±253	124 410†	0.039
(20) S-PIA	4688±159	1820 3020†	0.39

Binding of [³H]XAC (1.0 nM) was carried out for 60 min at 24°C, in the presence of 0.05 M Tris buffer at pH 7.4, 10 mM MgCl₂ and 50 mM CPX. K_i values are the mean ± SD of at least 3 experiments done in triplicate. K_i values in the rat are from Bruns et al. (1986), Daly and Jacobson (1989), Jacobson el al. (1989a, 1989c) and Shamim et al. (1989). ND = not determined.

^*Versus NECA stimulation of adenylate cyclase in rat PC 12 pheochromocytoma cell membranes.

^†Versus binding of [³H]CGS21680 in rat striatum.

^‡Ratio of K_i-values at rat A₂-receptors vs K_i-values at rabbit A₂-receptors.