Selectivity and activity of adenine dinucleotides at recombinant P2x 2 and P2Y 1 purinoceptors

1 Adenine dinucleotides (AP x A, × = 2–6) are naturally‐occurring polyphosphated nucleotidic substances which are found in the CNS and are known to be released in a calcium‐dependent manner from storage vesicles in brain synaptosomes. The selectivity and activity of adenine dinucleotides for neuronally‐derived recombinant P 2 purinoceptors were studied using P2x 2 and P2Y 1 subtypes expressed in Xenopus oocytes. 2 For the P2Y 1 subtype derived from chick brain, AP 3 A was equipotent and as active as ATP (EC 50 values: 375 ± 86 nM and 334 ± 25 nM, respectively). Ap4A was a weak partial agonist and other dinucleotides were inactive as agonists. None of the inactive dinucleotides were antagonists nor modulated the acitivity of AP 3 A and ATP. 3 For the P2X 2 subtype derived from rat PC12 cells, AP 4 A was as active as ATP but less potent (EC 50 values: 15.2 ± 1 μ m and 3.7 ± 0.7 μ m , respectively). Other adenosine dinucleotides were inactive as either agonists or antagonists. 4 AP 5 A (1–100 nM) potentiated ATP‐responses at the P2x 2 subtype, showing an EC 50 of 2.95 ± 0.7 nM for this modulatory effect. AP 5 A (10 nM) shifted the concentration‐response curves for ATP to the left by one‐half log 10 unit but did not alter the Hill co‐efficient for ATP (n H = 2.1 ± 0.1). AP 5 A (10 nM) failed to potentiate AP 4 A‐responses but did enhance the efficacy of the P 2 purinoceptor antagonist, suramin, by 12 fold at the P2x 2 subtype. 5 In conclusion, the results show that ionotropic (P2x 2 ) and metabotropic (P2Y 1 ATP receptors which occur in the CNS are activated selectively by naturally‐occurring adenine dinucleotides which are known to be released with nucleotides from storage vesicles. The observed potentiation of P2x 2 ‐responses by AP 5 A, where co‐released with ATP by brain synaptosomes, may have a functional bearing in purinergic signalling in the CNS.