P2-RECEPTOR-MEDIATED INHIBITION OF NORADRENALINE RELEASE IN THE RAT PANCREAS

The aim of the study was to find out whether, and if so through which receptors, nucleotides modulate the release of noradrenaline in the ra t pancreas. Segments of the pancreas were preincubated with [H-3]-nora drenaline, superfused with medium containing desipramine (1 mu M) and yohimbine (1 mu M), and stimulated electrically, in most experiments b y 60 pulses/1 Hz. The adenosine A(1)-receptor agonist N-6-cyclopentyl- adenosine (CPA; EC50 32 nM), the non-subtype-selective adenosine recep tor agonists adenosine (EC50 15 mu M) and 5'-N-ethylcarboxamidoadenosi ne (NECA; EC50 135 nM), and the nucleotides ATP (EC50 13 mu M), adenos ine-5'-O-(3-thiotriphosphate) (ATP gamma S; EC50 19 mu M) and adenosin e-5'-O-(3-thiodiphosphate) (ADP beta S; EC50 16 mu M) decreased the ev oked overflow of tritium. The adenosine A(2A)-agonist yl)-phenethylami no-5'-N-ethylcarboxamido-adenosine (CGS 21680) caused no change. The c oncentration-response curve of CPA was shifted to the right by the A(1 )-antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX 10 nM; pK(d) 9. 1) but, like the concentration-response curve of adenosine, hardly aff ected by the P2-receptor antagonist cibacron blue 3GA (30 mu M). Combi ned administration of a high concentration of DPCPX (I mu M) and 8-phe nyltheophylline (10 mu M) abolished the effects of CPA and NECA. The c oncentration-response curves of ATP and ADP beta S were shifted to the right by both DPCPX (10 nM; pK(d) 8.7 and 8.9, respectively) and ciba cron blue 3GA (30 mu M; PKd 5.0 and 5.2, respectively). The antagonist effects of DPCPX (10 nM) and cibacron blue 3GA (30 mu M) 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 (1 mu M) and 8-phenyltheophylline (10 mu M), ATP and ADP beta S still decreased evoked tritium overflow, and this decrease was attenuated b y additional administration of cibacron blue 3GA (30 mu M). The P2-ant agonists cibacron blue 3GA, reactive blue 2, reactive red 2, and to a limited extent also suramin and ylenecarbonylimino)-1,3,5-naphthalenet risulphonate (XAMR0721), increased the evoked overflow of tritium by u p to 114%. Pyridoxalphosphate-6-azophenyl-2',4'-disulphonate (PPADS) c aused no change. The results indicate that the postganglionic sympathe tic axons of the rat pancreas possess A(1)-adenosine and P2-receptors. Both receptors mediate an inhibition of noradrenaline release. The pr esynaptic P2-receptors are activated by an endogenous ligand, presumab ly ATP, during appropriate trains of action potentials. This is the fi rst demonstration of presynaptic P2-receptors at postganglionic sympat hetic neurons that are located in prevertebral ganglia.