A guanine nucleotide-independent inwardly rectifying cation permeability is associated with P2Y₁ receptor expression in Xenopus oocytes

The functional properties of the G protein-coupled P2Y₁ receptor were investigated in Xenopus oocytes. Incubation of oocytes expressing either the human or turkey P2Y₁ receptor with adenine nucleotide agonists resulted in an increase in Cl^- current and activation of a novel cation current with an inwardly rectifying current-voltage relationship. Activation of either the human P2Y₂ (P(2U)-purinergic) or M1 muscarinic receptor expressed in oocytes resulted in an increase in Cl^- current similar to that observed in P2Y₁ receptor-expressing oocytes but had no effect on cation current. P2 receptor agonists stimulated both the cation current and Cl^- current in P2Y₁ receptor-expressing oocytes with EC₅₀ values and an order of potency (2- methylthioadenosine diphosphate > 2-methylthioadenosine triphosphate (2MeSATP) > ATP > UTP) that were similar to those previously observed for activation of phospholipase C in 1321N1 human astrocytoma cells stably expressing the human or turkey P2Y₁ receptor. The P2Y receptor antagonists suramin and pyridoxal phosphate 6-azophenyl-2'-4'-disulfonic acid both shifted to the right the concentration-response relationship for 2MeSATP for stimulation of oocyte currents. Although injection of oocytes with either GDPβS (guanyl-5'-yl thiophosphate) or GTPγS (guanosine 5'-3-O- (thio)triphosphate) resulted in loss of adenine nucleotide-promoted Cl^- channel activation, neither guanine nucleotide altered the 2MeSATP-stimulated cation current. These data are consistent with the view that activation of the novel cation current by the P2Y₁ receptor does not involve a G protein. Tail current analysis of the novel P2Y₁ receptor-associated cation conductance revealed that the open channel current-voltage relationship was outwardly rectifying with a reversal potential of -38 mV for the turkey P2Y₁ receptor and -36 mV for the human P2Y₁ receptor. Replacement of Na⁺ with K⁺ ions in the bathing solution produced a shift in reversal potential to near zero mV, but significant outward rectification remained. The cation current was not permeable to either Ca²⁺ or Ba²⁺ and exhibited steady- state inactivation at holding potentials below -60 mV. These results indicate that the P2Y₁ receptor exhibits both metabotropic properties and a novel G protein-independent ionotropic response when expressed in Xenopus oocytes.