P2-Receptor-mediated inhibition of noradrenaline release in the rat pancreas

Abstract

The aim of the study was to find out whether, and if so through which receptors, nucleotides modulate the release of noradrenaline in the rat pancreas. Segments of the pancreas were preincubated with [³H]-noradrenaline, superfused with medium containing desipramine (1μM) and yohimbine (1μM), and stimulated electrically, in most experiments by 60 pulses/1Hz.

The adenosine A₁-receptor agonist N⁶-cyclopentyl-adenosine (CPA; EC₅₀ 32nM), the non-subtype-selective adenosine receptor agonists adenosine (EC₅₀ 15μM) and 5’-N-ethylcarboxamidoadenosine (NECA; EC₅₀ 135nM), and the nucleotides ATP (EC₅₀ 13μM), adenosine-5’-O-(3-thiotriphosphate) (ATPγS; EC₅₀ 19μM) and adenosine-5’-O-(2-thiodiphosphate) (ADPβS; EC₅₀ 16μM) decreased the evoked overflow of tritium. The adenosine A_2A-agonist 2-p-(2-carboxyethyl)-phenethylamino-5’-N-ethylcarboxamido-adenosine (CGS 21680) caused no change. The concentration-response curve of CPA was shifted to the right by the A₁-antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX 10nM; pK_d 9.1) but, like the concentration-response curve of adenosine, hardly affected by the P2-receptor antagonist cibacron blue 3GA (30μM). Combined administration of a high concentration of DPCPX (1μM) and 8-phenyltheophylline (10μM) abolished the effects of CPA and NECA. The concentration-response curves of ATP and ADPβS were shifted to the right by both DPCPX (10nM; pK_d 8.7 and 8.9, respectively) and cibacron blue 3GA (30μM; pK_d 5.0 and 5.2, respectively). The antagonist effects of DPCPX (10nM) and cibacron blue 3GA (30μM) against ATP were additive in a manner compatible with the blockade of two separate receptors for ATP. In the presence of the high concentration of DPCPX (1μM) and 8-phenyltheophylline (10μM), ATP and ADPβS still decreased evoked tritium overflow, and this decrease was attenuated by additional administration of cibacron blue 3GA (30μM). The P2-antagonists cibacron blue 3GA, reactive blue 2, reactive red 2, and to a limited extent also suramin and 8-(3,5-dinitro-phenylenecarbonylimino)-1,3,5-naphthalenetrisulphonate (XAMR0721), increased the evoked overflow of tritium by up to 114%. Pyridoxalphosphate-6-azophenyl-2’,4’-disulphonate (PPADS) caused no change. The results indicate that the postganglionic sympathetic axons of the rat pancreas possess A₁-adenosine and P2-receptors. Both receptors mediate an inhibition of noradrenaline release. The presynaptic P2-receptors are activated by an endogenous ligand, presumably ATP, during appropriate trains of action potentials. This is the first demonstration of presynaptic P2-receptors at postganglionic sympathetic neurons that are located in prevertebral ganglia.