Release inhibitory receptors activation favours the A2A-adenosine receptor-mediated facilitation of noradrenaline release in isolated rat tail artery.

1. Interactions between A(2A)-adenosine receptors and alpha(2)-, A(1)- and P2- release-inhibitory receptors, on the modulation of noradrenaline release were studied in isolated rat tail artery. Preparations were labelled with [(3)H]-noradrenaline, superfused with desipramine-containing medium, and stimulated electrically (100 pulses at 5 Hz or 20 pulses at 50 Hz). 2. Blockade of alpha(2)-autoreceptors with yohimbine (1 microM) increased tritium overflow elicited by 100 pulses at 5 Hz but not by 20 pulses at 50 Hz. 3. The selective A(2A)-receptor agonist 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine (CGS 21680; 1-100 nM) enhanced tritium overflow elicited by 100 pulses at 5 Hz. Yohimbine prevented the effect of CGS 21680, which was restored by the A(1)-receptor agonist N(6)-cyclopentyladenosine (CPA; 100 nM) or by the P2-receptor agonist 2-methylthioadenosine triphosphate (2-MeSATP; 80 microM). 4. CGS 21680 (100 nM) failed to increase tritium overflow elicited by 20 pulses at 50 Hz. The alpha(2)-adrenoceptor agonist 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline (UK 14304; 30 nM), the A(1)-receptor agonist CPA (100 nM) or the P2-receptor agonist 2-MeSATP (80 microM) reduced tritium overflow. In the presence of these agonists CGS 21680 elicited a facilitation of tritium overflow. 5. Blockade of potassium channels with tetraethylammonium (TEA; 5 mM) increased tritium overflow elicited by 100 pulses at 5 Hz to values similar to those obtained in the presence of yohimbine but did not prevent the effect of CGS 21680 (100 nM) on tritium overflow. 6. It is concluded that, in isolated rat tail artery, the facilitation of noradrenaline release mediated by A(2A)-adenosine receptors is favoured by activation of release inhibitory receptors.