Pharmacological Identification of P2X1, P2X4 and P2X7 Nucleotide Receptors in the Smooth Muscles of Human Umbilical Cord and Chorionic Blood Vessels

Abstract

To ascertain the role of extracellular adenosine 5′-triphosphate (ATP) receptors in human placenta circulation, we identified and pharmacologically characterized the P2X receptor population in its superficial vessels. Total RNA was extracted from segments of chorionic and umbilical arteries and veins of terminal placentae delivered by vaginal or Caesarian births. Polymerase chain reaction (PCR), followed by sequencing of the products, identified the presence of P2X 1, 4, 5, 6, and 7mRNAs in smooth muscle from chorionic and umbilical arteries and veins. Umbilical vessels proximal to the fetus expressed the same population of P2X subtypes, except for the P2X5, but additionally expressed the P2X2. Rings of chorionic vessels contracted upon addition of nucleotides and analogs with the following relative rank order of potencies in arteries and veins: α,β-methyleneATP>β,γ-methyleneATP>PNP>ATP=diBzATP>2-MeSATP>ADP>AMP; in umbilical vessels α,β-methyleneATP was at least 100-fold more potent than ATP. Nucleotide potency was less than that of PGF2α or endothelin-2, but had the same magnitude as serotonin. ATP-desensitized receptors evidenced cross desensitization to α,β-methyleneATP, 2-MeSATP and diBzATP, effect not observed when desensitization was elicited by α,β-methyleneATP, confirming the presence of various P2X receptor subtypes in the smooth muscles of these vessels. The vasocontractile efficacy of α,β-methyleneATP was unaltered by endothelium removal, while that of ATP was significantly attenuated and those elicited by 2-MeSATP were blunted, indicating the presence of additional endothelial nucleotide receptors. These results suggest that P2X receptors participate in the humoral regulation of placental blood flow.