Characterization of P2 receptors for purine and pyrimidine nucleotides in human placental cotyledons

The aim of this study was to characterize P2 receptors in the arterial vascular bed of human perfused placental cotyledons. Vasoconstrictor responses to bolus injections of purine and pyrimidine nucleotides were tested at basal tone, and vasodilator responses in preparations with tone raised by perfusion with prostaglandin F 2α (PGF 2α ; 10–50n m ). At basal tone, bolus injections of the P2X‐selective agonist α,β‐methylene ATP (α,β‐meATP; 0.5–500nmol) elicited dose‐dependent vasoconstriction. ATP (0.005–5μmol) also elicited dose‐dependent vasoconstriction, but was less potent than α,β‐meATP. Vasoconstriction was also elicited by other nucleotides, but only at the highest dose tested (5μmol): UTP >CTP=ITP ( n =6). GTP and TTP did not cause vasoconstriction. Constrictor responses to bolus injections of α,β‐meATP were resistant to desensitization and were not significantly affected when carried out in the presence of 1μ m α,β‐meATP added to the perfusate. However, responses to bolus injections of α,β‐meATP were partially blocked by perfusion with 10μ m α,β‐meATP. In contrast, responses to ATP and UTP were unaffected by 10μ m α,β‐meATP. The P2X receptor antagonist pyridoxalphosphate‐6‐azophenyl‐2′,4′‐disulphonic acid (PPADS; 10 and 100μ m ) had no significant effect on vasoconstriction mediated by α,β‐meATP and ATP. Removal of the endothelium had no significant effect on constrictor responses to α,β‐meATP, ATP and UTP. Inhibition of nitric oxide (NO) synthesis with N G ‐nitro‐ l ‐arginine methyl ester ( l ‐NAME; 100μ m ) had no significant effect on vasoconstriction to ATP and α,β‐meATP. In preparations with tone raised with PGF 2α (10–50n m ) vasodilatation was elicited by nucleotides with the following order of potency: 2MeSATP=ADP >>ATP >UTP >CTP=GTP=ITP=TTP. pD 2 values were: 2MeSATP, 10.03±0.26 ( n =7); ADP, 9.97±0.40 ( n =5); ATP, 8.89±0.18 ( n =7); UTP, 7.79±0.35 ( n =7). Maximal responses to 2MeSATP and ADP were similar and were approximately 40% greater than maximal responses to ATP and UTP. Vasodilator responses to nucleotides were abolished by l ‐NAME (100μ m ) and by removal of the endothelium. In conclusion, contractile responses mediated by α,β‐meATP and ATP in human placental smooth muscle are resistant to desensitization and insensitive to PPADS and, thus, show a dissimilar pharmacological profile to the classic smooth muscle P2X 1 receptor. There may be two subtypes of smooth muscle P2 receptor based on differential antagonism of α,β‐meATP and ATP with α,β‐meATP. A smooth muscle P2 receptor mediates vasoconstriction to UTP, and may indicate a further subtype. Endothelium‐dependent, NO‐dependent, vasodilatation to 2MeSATP and ADP may be mediated by P2Y 1 receptors, while endothelial P2Y 2 receptors are likely to mediate NO‐dependent relaxation to ATP and UTP.