Adenosine A(2A) receptors in portal hypertension: their role in the abnormal response to adenosine of the cranial mesenteric artery in rabbits

Abstract

1. Adenosine is a regulator of mesenteric vasodilation involved in auto-regulation and post-prandial hyperemia, but the adenosine receptor subtype involved in this relaxant effect is poorly characterized. We have now pharmacologically characterized this receptor in rabbit mesenteric arteries and investigated how this adenosine receptor response changes in portal hypertensive animals since the adenosine response is decreased. 2. The closest non-metabolisable adenosine analogue, 2-chloroadenosine (CADO), the mixed A(1)/A(2) receptor agonist, 5'-ethylcarboxamidoadenosine (NECA), and the selective A(2A) receptor agonist, 2-[4-(2-p-carbonyethyl)phenylamino]-5'-N-ethylcarboxamidoadenosine (CGS 21680) (1 pM -- 1 mM) relaxed noradrenaline pre-contracted arteries with a rank order of potency of CGS 21680 (EC(50)=20 nM) > or = NECA (60 nM)>>CADO (640 nM). 3. The selective A(2A) receptor antagonist, 4-(2-[7-amino-2-(2-furyl)-[1,2,4]-triazolo[2,3-a][1,3,5]-triazin-5-ylamino]ethyl)phenol (ZM 241385, 100 nM), shifted to the right the CADO concentration-response curve. 4. In portal hypertensive animals, there was mainly a decreased potency but also a decreased efficacy of all tested adenosine agonists compared to normal animals. Concomitantly, there was a decreased adenosine plasma level and a decreased binding density of [(3)H]-CGS 21680 and [(3)H]-ZM 241385 to mesenteric artery membranes from portal hypertensive compared to normal rabbits. 5. These results indicate that A(2A) receptor activation is required for the adenosine-induced mesenteric relaxation and that the decreased density of A(2A) receptors may contribute to the decreased relaxation induced by adenosine of mesenteric arteries in portal hypertensive animals.

Figure 1

Relative potency of adenosine receptor agonists to relax rabbit cranial mesenteric arteries. The mesenteric artery rings were pre-contracted with noradrenaline (10 μM) and the functionality of the endothelium was verified by testing the response to acetylcholine. Then, cumulative concentration-response curves with increasing concentrations of the selective A_2A receptor agonist, CGS 21680, of the mixed A₁/A₂ receptor agonist, NECA, or with the closest non-metabolizable adenosine analogue, CADO, were carried out in (a). In (b), cumulative concentration-response curves with increasing concentrations of CADO were performed either in the absence or in the presence of the selective A_2A receptor antagonist, ZM 241385 (100 nM). The data are mean±s.e.mean of 5 – 6 experiments.

Figure 2

Modification of the potency of adenosine receptor ligands in cranial mesenteric arteries from portal hypertensive compared to control rabbits. In (a) and (b) are shown contractile recordings illustrating a cumulative concentration-response curve of the effect of CGS 21680 in cranial mesenteric arteries from control (a) and portal hypertensive rabbits (b). The mesenteric artery rings were pre-contracted with noradrenaline (10 μM, as shown by the dashed arrows in a and b). Then, a cumulative concentration-relaxant response to increasing concentration of CGS 21680 was carried out, as indicated by the upper bar in (a) and (b), and the recordings correspond to the last 30 s of recording out of each of the 5 min of recording after starting the superfusion of each concentration of CGS 21680, whose concentration is presented (as −log concentration) above each arrow. In (c) are shown the average results in preparations of control (open symbols, dashed lines) and portal hypertensive (filled symbols, filled lines) rabbits of the cumulative concentration-response curves with increasing concentrations of the selective A_2A receptor agonist, CGS 21680, with the mixed A₁/A₂ receptor agonist, NECA, and with the closest non-metabolizable adenosine analogue, CADO. The data are mean±s.e.mean of 5 – 6 experiments.

Figure 3

Average saturation curves (a) and corresponding Scatchard plots (b) of the binding of the A_2A receptor antagonist, ZM241385, to whole membranes of mesenteric arteries from control (open symbols and dashed lines) and portal hypertensive rabbits (filled symbols and filled lines). The ordinates in (a) represent the specific binding of [³H]-ZM 241385 on subtraction of non-specific binding, determined in the presence of 2 μM 8-{4-[(2-aminoethyl)amino]carbonylmetyloxyphenil} xanthine, from total binding. The curves in (a) were generated from the average binding parameters obtained on fitting by non-linear regression assuming a single binding site, whereas the curves in (b) correspond to linearizations of the data points. Results are mean±s.e.mean of 4 – 5 experiments. The s.e.mean values are not presented in the Scatchard plots for the sake of simplicity.