Functional properties of heteromeric P2X(1/5) receptors expressed in HEK cells and excitatory junction potentials in guinea-pig submucosal arterioles

P2X receptors are ATP-gated cation channels; they form as homomers or heter omers from a family of seven related subunits. In particular, heteromeric c hannels comprising P2X(2) and P2X(3) subunits, or P2X(1) and P2X(5) subunit s, show distinctive physiological and pharmacological properties in heterol ogous expression systems. There is substantial evidence that one of the nat ive P2X receptors in sensory neurones corresponds to the P2X(2/3), heterome r, but there is no evidence for P2X(1/5), heteromers in native tissue. We r ecorded currents in response to activation of heteromeric P2X(1/5), recepto rs expressed in HEK293 cells to characterize further their functional prope rties. The ATP concentration-response curve had a threshold concentration o f 1 nM, and a Hill slope of one. TNP-ATP was a weak partial agonist, and a non-competitive antagonist which inhibited maximal ATP currents by 60%. Inc reasing or decreasing pH from 7.3 shifted the ATP concentration-response cu rves to the right by fivefold and decreased the maximum current by 40%. Cal cium permeability was lower than that observed for other P2X receptors (P-C a/P-Na ratio=1.1). The nanomolar sensitivity of this receptor revealed a st eady release of ATP from HEK293 cells, providing an extracellular concentra tion which ranged from 3 to 300 nM. Noradrenaline (0.3-30 mu M) increased A TP-evoked currents by 35%; this facilitation occurred within 20 ms. We also recorded excitatory junction potentials (EJPs) from guinea-pig submucosal arterioles. EJPs were inhibited by suramin and PPADS (IC(50)s of 0.2 mu M a nd 20 mu M) hut TNP-ATP (0.1-10 mu M) inhibited EJPs by <30%. Noradrenaline (0.3-30 mu M in the presence of phentolamine and propranolol) decreased EJ Ps in control preparations but facilitated EJPs by 5-20% in submucosal arte rioles from reserpinized guinea-pigs. These properties are discussed in rel ation to P2X receptors underlying EJPs at autonomic neuroeffector junctions . (C) 2000 Elsevier Science BN. All rights reserved.