Abstract
1. The pharmacological actions of adenosine and adenosine 5’-triphosphate (ATP) are reviewed. The current knowledge of purinoceptor subtypes involved in mediating the actions of adenosine and ATP are summarised and the factors affecting the actions of purines are discussed. 2. The ontogeny of responses to adenosine, ATP and related analogues was studied in the rat duodenum, urinary bladder and vasa deferentia. In rat duodenum and rat urinary bladder responses to adenosine and ATP mediated by P1- and P2-purinoceptors respectively were present as early as postnatal day 2, the earliest day tested. In rat vasa deferentia responses to adenosine and ATP were not observed until postnatal day 15. 3. In rat duodenum adenosine and adenosine 5'-(β,γ-methylene) triphosphonate (AMPPCP) were inhibitory, their potency being less than the adult at 5 days, lowest at 10 days and highest at 25 days. The effects of adenosine and AMPPCP were mediated by P1-purinoceptors as their responses were antagonized by 8-(p-sulphophenyl)theophylline (8-SPT). The P1-purinoceptor action of AMPPCP was not due to its degradation to adenosine as it was resistant to degradation in this tissue. 4. The adenosine analogues, N6-cyclopentyladenosine (CPA), 5'-N-ethyl-carboxamidoadenosine (NECA) and 2-p-((carboxyethyl)phenethylamino)-5'-carbo-xamidoadenosine (CGS21680) were also inhibitory on rat duodenum, the order of potency of the adenosine agonists was NECA > CPA > AMPPCP = adenosine > CGS21680, and 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) antagonized CPA and AMPPCP at a concentration of InM whereas equivalent antagonism of NECA and adenosine required a concentration of 1μM. This suggests the presence of a mixture of A1 and A2 receptors in this tissue, with CPA and AMPPCP acting on the A1 and NECA and adenosine acting on the A2 receptors. The low potency of CGS21680 in this tissue suggests that the A2 receptor is of the A2b subtype. 5. In rat duodenum low concentrations of ATP were inhibitory at every age studied and its potency increased with age, however higher concentrations of ATP (3μM and above) were excitatory until day 15 after which no contractions to ATP were seen. Both relaxations and contractions were mediated by P2Y-purinoceptors as 2-methylthioadenosine 5'-triphosphate (2MeSATP) was much more potent than ATP or adenosine 5'-(α,β-methylene) triphosphonate (AMPCPP) at mediating these effects. These ATP-induced contractions were not inhibited by indomethacin (25μM) or by tetrodotoxin (1μM) and are therefore not due to prostaglandin synthesis or to ATP-induced release of transmitter substances from nerves. 6. In rat urinary bladder adenosine was inhibitory and was more potent in the neonate than in the adult. The adenosine analogues NECA, CPA and CGS21680 were also inhibitory with an order of potency of NECA » adenosine > CPA = CGS21680. A concentration of DPCPX of 1μM was required to antagonize responses to NECA and adenosine suggesting the presence of A2 receptors in this tissue. The low potency of CGS21680 would suggest that the A2 receptors are of the A2b subtype. 7. ATP and AMPPCP were excitatory on rat urinary bladder, their actions mediated by P2X-purinoceptors. The potency of the nucleotides initially increased with age but then declined, being highest between postnatal days 10 and 25. The ATP- and AMPPCP-induced contractions were not enhanced by DPCPX (1μM) which suggests that in this tissue AMPPCP was acting only via P2-purinoceptors and had no P1 agonist activity. That AMPPCP was active on the A1 receptors in the duodenum but inactive on the A2 receptors in the bladder implies that it has selectivity for the A1 subtype. 8. The changes in potency for adenosine and ATP in both rat duodenum and urinary bladder are not due to changes in the rate of degradation of the purines as both the rate and pattern of breakdown was similar in adult and neonatal animals. 9. In rat vasa deferentia adenosine inhibited nerve-mediated contractions from postnatal day 15 (the earliest day at which nerve-mediated responses could be observed), and its potency decreased with age. The P1-purinoceptor agonists CPA and NECA also inhibited nerve-mediated contractions with an order of potency of CPA = NECA > adenosine. InM DPCPX antagonized the inhibitory effect of CPA whereas 1uM DPCPX was required to antagonize responses to adenosine and NECA suggesting that CPA acts via A1 and NECA and adenosine act via A2 receptors. CPA was inactive as an inhibitor of contractions induced by exogenous ATP or noradrenaline (10uM) whereas adenosine and NECA inhibited these contractions with NECA > adenosine at mediating this effect. These results show that the rat vas deferens contains both presynaptic A1 receptors and postsynaptic A2 receptors, and that adenosine acts on the latter population to inhibit nerve-mediated contractions. 10. ATP contracted rat vasa deferentia from postnatal day 15 and its potency increased with age up to day 25. The development of the response to ATP was similar to nerve-mediated contractions, which were also observed at day 15 and increased with age. In contrast, contractions to noradrenaline were present from day 10 (the earliest day tested) and decreased with age.