Release inhibitory receptors activation favours the A(2A)-adenosine receptor-mediated facilitation of noradrenaline release in isolated rat tail artery

Abstract

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 [H-3]-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 muM) 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 muM). 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 PM) 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.