P2X receptors are ATP-gated cation channels; they form as homomers or heteromers from a family of seven related subunits. In particular, heteromeric channels comprising P2X(2) and P2X(3) subunits, or P2X(1) and P2X(5) subunits, show distinctive physiological and pharmacological properties in heterologous expression systems. There is substantial evidence that one of the native P2X receptors in sensory neurones corresponds to the P2X(2/3) heteromer, but there is no evidence for P2X(1/5) heteromers in native tissue. We recorded currents in response to activation of heteromeric P2X(1/5) receptors expressed in HEK293 cells to characterize further their functional properties. The ATP concentration-response curve had a threshold concentration of 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%. Increasing or decreasing pH from 7.3 shifted the ATP concentration-response curves to the right by fivefold and decreased the maximum current by 40%. Calcium permeability was lower than that observed for other P2X receptors (P(Ca)/P(Na) ratio=1.1). The nanomolar sensitivity of this receptor revealed a steady release of ATP from HEK293 cells, providing an extracellular concentration which ranged from 3 to 300 nM. Noradrenaline (0.3-30 microM) increased ATP-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 microM and 20 microM) but TNP-ATP (0.1-10 microM) inhibited EJPs by <30%. Noradrenaline (0.3-30 microM in the presence of phentolamine and propranolol) decreased EJPs in control preparations but facilitated EJPs by 5-20% in submucosal arterioles from reserpinized guinea-pigs. These properties are discussed in relation to P2X receptors underlying EJPs at autonomic neuroeffector junctions.