A2A adenosine-receptor-mediated facilitation of noradrenaline release in rat tail artery involves protein kinase C activation and βγ subunits formed after α2-adrenoceptor activation

This work aimed to investigate the molecular mechanisms involved in the interaction of alpha-(2)-adrenoceptors and adenosine A(2A)-receptormediated facilitation of noradrenaline release in rat tail artery, namely the type of G-protein involved in this effect and the step or steps where the signalling cascades triggered by (alpha 2)-adrenoceptors and A(2A)-receptors interact. The selective adenosine A(2A)-receptor agonist 2-p-(2-carboxy ethyl) phenethylamino-5'-N-ethylcarboxamidoadenosine (CGS 21680; 100 nM) enhanced tritium overflow evoked by trains of 100 pulses at 5 Hz. This effect was abolished by the selective adenosine A(2A)-receptor antagonist 5-amino-7-(2-phenyl ethyl)-2-(2-furyl)-pyrazolo-[4,3-e]-1,2,4-triazolo [1,5-c]pyrimidine (SCH 58261; 20 nM) and by yohimbine (I RM). CGS 21680-mediated effects were also abolished by drugs that disrupted G(i/o)-protein coupling with receptors, PTX (2 mu g/ml) or NEM (40 mu M), by the anti-Gs, peptide (2 mu g/ml) anti-Gp, peptide (10 mu g/ml) indicating coupling of A(2A)-receptors to G(s alpha) and suggesting a crucial role for Gp, subunits in the A(2A)-receptor-mediated enhancement of tritium overflow. Furthermore, phorbol 12-myristate 13-acetate (PMA; 1 mu M) or forskolin (1 mu M), direct activators of protein kinase C and of adenylyl cyclase, respectively, also enhanced tritium overflow. In addition, PMA-mediated effects were not observed in the presence of either yohimbine or PTX. Results indicate that facilitatory adenosine A(2A)-receptors couple to G(s alpha) subunits which is essential, but not sufficient, for the release facilitation to occur, requiring the involvement of G(i/o)-protein coupling (it disappears after disruption of G(i/o)-protein coupling, PTX or NEM) and/or Gp, subunits (anti-G(beta gamma)). We propose a mechanism for the interaction in study suggesting group 2 AC isoforms as a plausible candidate for the interaction site, as these isoforms can integrate inputs from G(s alpha) subunits (released after adenosine A(2A)-receptor activation; pri me- activation), G(beta gamma) subunits (released after activation of G(i/o)-protein coupled receptors) which can directly synergistically stimulate the prime-activated AC or indirectly via Gp, activation of the PLC-PKC pathway. (C) 2007 Elsevier Ltd. All rights reserved.