Adenosine A1 receptors in mammalian brain: Species differences in their interactions with agonists and antagonists

Abstract

AbstractThe binding of the selective ligand [3H]cyclohexyladenosine (CHA) to adenosine A1 receptors was studied in brain membrane from six species; cow, rabbit, rat mouse, human, and guinea pig. Saturation analysis gave evidence for a single binding site in each species. The number of binding sites (2.6–4.7 pmol/mg protein), based on saturation analysis, was virtually identical in each species. The Kd values were, however, different; whereas rat, rabbit, and mouse A1 receptors had similar affinities (Kd=1–2 nM), the binding site in bovine tissue had three to four times greater affinity (Kd=0.59 nM). Adenosine binding sites in human and guinea pig brain had two to three times less affinity, with Kd values 3.7 and 6.6 nM, respectively, than those in rat, rabbit, and mouse brain. Examination of the binding of five agonists, the R and S diastereomers of N6‐phenylisopropropyladenosine (PIA), CHA, 5′N‐ethylcarboxamido adenosine (NECA), and 2‐chloroadenosine, and of five xanthine antagonists, PACPX [1,3,dipropyl‐8‐(2‐amino‐4‐chloro)phenylxanthine], 1,3 diethyl‐8‐phenylxanthine (DPX), 8‐phenyltheophylline (8PT), 8‐parasulfophenyl‐theophylline (8PST), and theophylline, showed that across the species N6‐substituted adenosine analogs showed consistent differences in rank order activity. R‐PIA ≥ CHA >S‐PIA, with the compounds being most active in bovine tissue and least active in guinea pig, showing an 18‐fold difference. In contrast, the 2‐substituted analog, 2‐chloroadenosine was most active in guinea pig and human brain membranes, and least active in rat and bovine brain. The 5′‐substituted analog NECA had similar activity in all species studied. PACPX was the most potent of the xanthines studied, being most active in bovine brain tissue and least active in guinea pig; the difference in activities was 393‐fold. The interaction of the remainder of the xanthines studied was complex. In general, the rank order of potency was PACPX > DPX=8 phenyltheophylline > 8PST >> theophylline. However, in human and rabbit brain 8PT was more active than DPX, and in guinea pig 8PT was twofold less active than DPX. In addition, in human and guinea pig brain, 8PST had comparable activities. These differences in pharmacological profiles may be attributable to differences in the adenosine systems studied as previously noted by Murphy and Snyder [Mol. Pharmacol. 22:250–257, 1982], which may in turn reflect distinct differences in A1 receptors or the presence of different subtypes of adenosine receptor in the various species. While these interspecies studies need to be extended to both A2 receptors and to coupled, i.e., adenylate cyclase, systems, they underline the need for care in choosing the mammalian species in which structure activity relationships are generated. Some caution is necessary in the use of bovine brain tissue in development programs targeted towards therapeutic entities in the adenosine A1 antagonist area.