6533b827fe1ef96bd1286565

RESEARCH PRODUCT

Purinoceptor-mediated modulation by endogenous and exogenous agonists of stimulation-evoked [3H]noradrenaline release on rat iris.

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To investigate whether endogenous purinoceptor agonists affect the sympathetic neurotransmission in the rat isolated iris, and to classify the purinoceptors modulating exocytotic [3H]-noradrenaline release, we have determined the effect of adenosine receptor antagonists on, and the relative potency of selected agonists in modulating, the field stimulation-evoked (3 Hz, 2 min) [3H]-noradrenaline overflow. In addition, the apparent affinity constants of 8-phenyltheophylline (8-PT) and 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) in antagonizing the prejunctional effects of purinoceptor agonists were estimated. The relatively A1-selective DPCPX 10 and 100 nmol/l increased the evoked [3H]-noradrenaline overflow by about 25%–35%a indicating a minor inhibition of evoked release by endogenous purinoceptor agonists probably via an A1 adenosine receptor. Whereas the A1/A2-antagonist 8-PT failed to increase the evoked [3H]-noradrenaline overflow in the absence of exogenous agonists (without or with dipyridamole 1 pmol/l present), the relatively A2-selective antagonist CP-66,713 (4-amino-8-chloro -1-phenyl(1,2,4)triazolo(4,3-a)quinoxaline) 100 nmol/l decreased it by 20%–30% in the absence and continuous presence of DPCPX. This may be compatible with a minor A2-mediated facilitation by an endogenous purinoceptor agonist. All exogenous agonists tested (except UTP 100 μmol/l) inhibited the evoked [3H]-noradrenaline overflow. The relative order of agonist potency (IC4o, concentration in μmol/l for inhibition of evoked release by 40%) was CPA (N6-(cyclopentyl)adenosine, 0.004) > R-PIA (R(−)N6-(2phenylisopropyl)adenosine, 0.066) = CHA (N6-(cyclohexyl)adenosine, 0.082) > NECA (N5-(ethyl-carboxamido)adenosine 0.44) > ADO (adenosine, 4.1). ATP was n early equipotent with ADO. Maximum inhibition was 70%–80% and similar for all agonists. Adenosine deaminase 1 u/ml failed to affect the ATP-induced, but abolished the adenosine-induced prejunctional inhibition. The adenosine uptake inhibitor S-p-nitrobenzyl-6-thioguanosine (NBTG) failed to enhance the potency of ADO and ATP. The A1-selective antagonist DPCPX 10 nmol/l did not reduce the ATP potency indicating an effect of ATP per se not mediated via an A1 purinoceptor. Prejunctional affinity constants of 8-PT were 6.07 when tested against adenosine (in the presence of dipyridamole), and 6.60 against CHA. The apparent -log KB of DPCPX tested against CPA was 9.71. The high DPCPX affinity is compatible with an A1 adenosine receptor mediating inhibition of sympathetic neurotransmission in rat iris. This receptor may not be the only prejunctional purinoceptor on rat iris sympathetic nerves. The receptor by which ATP acts prejunctionally in this tissue remains to be determined.